SCR Power Control Systems

Zeefax Authorisation


To Whom It May Concern

Following the obsolescence of the IMP product line in July 2010, Mobrey Measurement continued to offer service support and supply of spares.

We have been reviewing on-going support of IMP’s to ensure that we offer the best solution to our customers. We have concluded that the best solution is to transfer the service and support to an external company better suited to the role.

This letter is to inform you that in order to better serve our customers, we are transferring, on an exclusive basis, the IMP support and spares supply to Zeefax Limited in the UK.

Zeefax has for many years been an important Value Added Retailer (VAR), selling and supporting IMP’s in the UK and in the America’s. As a VAR, Zeefax always offered solutions to meet the varied needs of the Industry. They offer a range of expertise and knowledge covering a wide range of disciplines. They have been in the Data Acquisition and Condition Monitoring Business for almost 20 years and will continue to offer professional solutions in the future.

From the 1st of May 2012 all new repair enquiries and orders for IMP spares should be sent to Zeefax Limited at the following address:

Zeefax Limited Zeefax House 320 King Street London W6 0RR

Contact: Mrs Amanda Philpott on 020 8748 7808, email:

To allow Zeefax to sustain and provide this support, Mobrey has  transferred to Zeefax all manufacturing and assembling documentation relating to the IMP products, including Manufacturing drawings, Circuit diagrams, Assembly procedures, Calibration procedures and Maintenance manual. All and any unique test equipment, jigs or fixtures relating to the calibration of the IMP products have also been handed to Zeefax. All this will enable Zeefax to support existing products but also to continue manufacture and supply new IMP range of products to customers.

We are also pleased to announce that Malcolm Bloxham, whom many of you will already know through previous work with Solartron and Mobrey, will be working for Zeefax in the continued support of the IMP product range. Malcolm will play a vital part in the successful transfer of the IMP repair business having over 20 years experience in all of the former Data Acquisition products previously sold under the Solartron Instruments brand name.

If you have any questions on the above, please contact either Mobrey Measurement or Zeefax through the following contacts:-


To download a certified copy of this agreement, please on the link opposite, or for further help or information, please contact us using one of the available options.



Warm Up Period

he time required after energizing a device before its rated performance characteristics start to apply.

Waterfall plot

A series of spectral maps taken at regular intervals of time, regularly spaced shaft speeds, etc. Early maps move down the display, followed by later maps, something like the flow of a waterfall.


Nature’s mechanism for transporting energy without transporting matter.


A presentation or display of the instantaneous amplitude of a signal as a function of time, as on an oscilloscope or oscillograph. In the time domain.


As used for shock testing on shakers, are half-sine windowed (amplitude modulated) sinusoids of specific frequencies, with the window chosen so the resulting waveforms have an odd number of half cycles. Usually, ten to thirty wavelets, each with different basis frequencies, e.g., 10 Hz to 2000 Hz, and different numbers of half cycles, e.g., three to 31 or so, are superimposed to synthesize a transient oscillatory signal (pulse) with a prescribed Shock Response Spectrum that can be used as a reference waveform for a shaker shock test.


That property of an object that can be weighed, as on a scale; the gravitational force on an object.

Weighting (Acoustic)

Emphasis or attenuation applied to sound measurements at certain frequencies. C weighting is essentially flat. A weighting attempts to compensate for the non-constant sensitivity of human hearing at certain frequencies.

White random vibration

That broad-band random vibration in which the power spectral density (PSD (ASD) is constant over a broad frequency range.


A “weighting function” applied to time-data prior to performing an FFT. Rectangular, Hann, Flat top, DP7, BH4, Force and Response.


Rotation about the vertical axis.

Zwicker loudness

A sound measurement methodology. Loudness is measured in linear units called sones as opposed to decibels.

Zero-G drift

The amount that a sensor’s signal shifts over some temperature range.

Zero-G output

The output that is read when the sensor is not accelerating.


See peak, as in peak value. Half of the peak-to-peak value.



Test, analyze and fix. See Accelerated Life Testing.


Selecting or altering test procedures, conditions, values, tolerances, measures of failure, etc., to simulate or exaggerate the environmental effects of one or more forcing functions.

Test Type

The mode of measurement operation

Temperature coefficient

The amount of drift, in percent of full scale output, that might result from a 1[DEGREE]C change in ambient temperature.

Temperature range

The temperatures between which a sensor will operate accurately.

Total Harmonic Distortion (THD)

For a signal, the ratio of the sum of the powers of all harmonic frequencies above the fundamental frequency to the power of the fundamental frequency.

Thermal cycling

Subjecting a product to predetermined temperature changes, between hot and cold extremes.


An electrical device used for temperature measurement.


An electrical device used for temperature measurement. Two dissimilar metals joined together, making a continuous loop.


The smallest change in a measured variable that gives a measurable change in output signal.

Thrust position

Location in direction of a shaft centerline. See axial position.

Thump Signal

A “velocity and displacement compensated” half-sine pulse. This transient is a classic shock-test acceleration waveform.

Time Averaging

In a DSA, averaging of time records that results in reduction of asynchronous components.

Time Constant

The interval needed for an instrument’s output to move 63% of its ultimate shift as a result of a step change in its input.

Time constant

The interval needed for an instrument’s output to move 63% of its ultimate shift as a result of a step change in its input.

Time Record

In a DSA, the sampled time data converted to the frequency domain by the FFT. Most DSAs use a time record of 1024 samples.


A switch that causes the opposite value. For example, a switch that is “on,” when toggled, becomes “off.”

Torsional Vibration

Amplitude modulation of torque measured in degrees peak-to-peak referenced to the axis of shaft rotation.

Tracking filter

A narrow band-pass filter whose center frequency follows an external synchronizing signal.

Tracking Filter

A low-pass or band-pass filter which automatically tracks the input signal. A tracking filter is usually required for aliasing protection when data sampling is controlled externally.

Transducer (or pickup or sensor)

A device which converts some mechanical quantity into an electrical signal. Less commonly, the reverse conversion./ A device for translating the magnitude of one quantity into another quantity./ An element or device which receives information in the form of one quantity and converts it to information in the same or an other quantity or form. Primary elements and transmitters are also referred to as transducers.


A mathematical operation that converts a function from one domain to another domain with no loss of information. For example, the Fourier transform converts a function of time into a function of frequency.


The behavior of a variable during transition between two steady-states.

Transient Vibration

Temporarily sustained vibration of a mechanical system. It may consist of forced or free vibration or both. Typically this is associated with changes in machine operating condition such as speed, load, etc. / Transient vibration Short-term vibration of a mechanical system.


In steady-state vibration, Tr is the non-dimensional ratio of response motion/input motion: two displacements, two velocities or two accelerations. The maximum Tr value is the mechanical “Q” of a system. At resonance, Tr is maximum.


A transducer which responds to a measured variable by means of a sensing element, and converts it to a standardized transmission signal which is a function only of the values of the measured variable.

Transverse sensitivity

The unfortunate sensitivity of a sensor (or pickup or transducer) in a direction perpendicular to the advertised or stated sensitive axis. Also called cross-axis or lateral sensitivity.

Transverse Sensitivity

See Cross-Axis Sensitivity.


Any event which can be used as a timing reference. In a DSA, a trigger can be used to initiate a measurement.

Time Span (Tspan)

The time duration (second) of each frame or capture window that is numerically equal to Block Size x dT.

Tuneable filter

A filter whose cut-off frequencies are adjustable, either manually or under remote electrical control.


Ultrasound is sound with a frequency greater than the upper limit of human hearing, this limit being approximately 20 kilohertz (20,000 hertz). SignalCalc analyzers provide measurements will into the ultrasonic range, as high as 97 kHz.


Every measurement is expressed as a multiple or fraction of dome appropriate, well-defined unit quantity such as centimeter, volt, etc.

Unbalance (imbalance)

Unequal mass distribution on a rotor. The mass centerline does not coincide with the rotation or geometric centerline.

Undamped Natural Frequency

The same as the natural frequency of a structure.

Uniform Window

In a DSA, a window function with uniform weighting across the time record. This window does not protect against leakage, and should be used only with transient signals contained completely within the time record.

In the FFT Analyzer, the uniform, or rectangular, window does not modify the signal amplitude at all. It is also called rectangular weighting, or uniform weighting, and is selected when the signal to be Analyzed is a transient rather than a continuous signal.


Every measurement is expressed as a multiple or fraction of dome appropriate, well-defined unit quantity such as centimeter, volt, etc.

Upper range limit (URL)

The highest value of the measured variable that a device can be adjusted to measure. (This value corresponds to the top of the range.) Also Universal Resource Location on the web.

Upper range value (URV)

The highest value of the measured variable that a device is adjusted to measure. (This value corresponds to the top of the span.)


Universal Serial Bus. USB data acquisition devices are ideal for a variety of applications from simple data logging to embedded OEM systems. The SignalCalc ACE powered by Quattro is an example of a USB data acquisition product.


Unit under test. See also DUT or device under test (being tested).


Any condition which is measured, controlled (directly or indirectly) or manipulated.


A quantity which has both magnitude and direction (phase).


Rate of change of displacement with time, usually along a specified axis; it may refer to angular motion as well as to axial motion.

Rate of change of displacement with time. A vector quantity that specifies the time rate of change of displacement with respect to a reference frame. If the reference frame is not inertial, the velocity is often designated relative velocity

Velocity Transducer

An electrical/mechanical transducer who’s output is directly proportional to the velocity of the measured unit. A velocity transducer consists of a magnet suspended on a coil, surrounded by a conductive coil. Movement of the transducer induces movement in the suspended magnet. This movement inside the conductive coil generates an electrical current proportional to the velocity of the movement. A time waveform or a Fourier transform of the current will result in a velocity measurement. The signal can also be integrated to produce a displacement measurement.

Velocity limit

A limit on the rate of change, which a particular variable may not exceed.


Mechanical oscillation or motion about a reference point of equilibrium. / A periodic motion or oscillation of an element, device, or system.

Any periodic process, especially a rapid linear motion of a body about an equilibrium position. SignalCalc analyzers and SignalStar controllers require transducers to convert the mechanical vibrations to electrical signals for analysis or control.

Vibration Analysis

The study of a continuing periodic change in the magnitude of a displacement with respect to a specified central reference. Various measurements for vibration analysis are provided as part of all SignalCalc systems.

Vibration machine (or exciter or shaker)

A device which produces controlled and reproducible mechanical vibration for the vibration testing of mechanical systems, components and structures.

Vibration meter

An apparatus (usually an electronic amplifier, detector and readout meter) for measuring electrical signals from vibration sensors. May display displacement, velocity and/or acceleration.


A device which produces controlled and reproducible mechanical vibration for the vibration testing of mechanical systems, components and structures.


Also called mechanomyography or acoustic myography. The measurement of muscle vibration when a muscle contracts.

Viscous Damping

Damping that is proportional to velocity. Viscous damping is used largely for system modeling since it is linear.

Volt (V)

The electrical potential difference between two points in a circuit. One volt is the potential needed to move one coulomb of charge between two points while using one joule of energy.


Sampling rate

The number of readings an A/D converter takes per second or per minute.

Sampling period

The time interval between observations.

Scale factor

The factor by which the number of scale divisions indicated or recorded by an instrument should be multiplied to compute the value of the measured variable.


The process of stressing products so that defective units can be identified, then repaired or replaced. A winnowing out.

Sensing element

The element which is directly responsive to the value of the measured variable.


Having to do with earth motion, as earthquakes. A kind of sensor that depends upon the inertia of an internal mass to generate a signal, as an accelerometer or velocity pickup. / Refers to an inertially referenced measurement or a measurement relative to free space.

Seismic Transducer

A transducer that is mounted on the case or housing of a machine and measures casing vibration relative to free space. Accelerometers and velocity transducers are seismic.

Self-induced vibration

Also called self-excited vibration, results from conversion of non-oscillatory energy into vibration, as wind exciting telephone wires into mechanical vibration.

Sensitivity. Of a mechanical-to-electrical sensor or pickup, the ratio between electrical signal (output) and mechanical quantity (input).


The minimum change in a physical variable to which an instrument can respond; the ratio of the change in output magnitude to the change of the input which causes it after the steady-state has been reached.


An element or device which detects a variable by receiving information in the form of one quantity and converts it to information in the form of that or an other quantity. (see also Transducer)


A variable, expressed in the same units as the measurement, which sets either the desired target for a controller, or the condition at which alarms or safety interlocks are to be energized.


An electro-mechanical or electro-hydraulic machine, usually operating under closed-loop-control, which simulates the vibration signatures experienced in real life situations. Used in accelerated life testing for equipment and systems subject to vibration in normal operating conditions.

Shielded twisted pair (STP)

Cable construction that includes an external grounded shield as well as twisting on a regular basis to help minimize noise interferences.

Shock Machine or Shock Test Machine

An electro-mechanical or electro-hydraulic device for subjecting equipment to controlled and reproducible mechanical shock pulses.

Shock pulse

An event that transmits kinetic energy into a system in a relatively short interval compared with the system’s greatest natural period. A natural decay of oscillatory motion follows. The event is usually displayed as a time history, as on an oscilloscope.

Shock pulse, classical

An ultra-simple shock pulse, one that is never seen in the “real world” yet is often specified in test requirements. Such pulses contain much more low-frequency energy than do “real world” shock events. Such pulses are said to be “mathematically tractable”. They have been studied to death. Examples include the half-sine pulse, the square pulse, the triangular pulse, the trapezoid and the saw tooth. The first four, which are symmetrical, are being specified less with each passing year. Only the saw tooth is required in MIL-STD-810F.

Shock pulse, haversine

A practical variation on the obsolete half-sine pulse, whose abrupt transitions at beginning and end cannot be achieved in test labs. Practical testing requires some rounding, and the result in called a haversine pulse. Another definition: an inverted cosine offset by half its amplitude. A continuous haversine resembles a sine wave.

Shock response spectrum (or SRS)

A plot of maximum responses of SDoF systems vs. their natural frequencies, as they respond to an applied shock.

SI (Systeme International)

Decimal system of units

SI Exponents

SI Units



A variable that carries information about another variable that it represents. Signal-to-noise ratio: Ratio of signal amplitude to noise amplitude. The ratio of the overall RMS signal level to the RMS noise level, expressed in dB. For sinusoidal signals, amplitude may be peak or rms. Single-ended (SE): An analog input that is measured with respect to a common ground.

Signal Amplifier (see also Signal conditioner)

An amplifier following a sensor, which prepares the signal for succeeding amplifiers, transmitters, readout instruments, etc. May also supply sensor power.

Signal Conditioner

A device placed between a signal source and a readout instrument to change the signal. Examples: attenuators, preamplifiers, charge amplifiers.


Signal-to-Noise Ratio (SNR)

The ratio of the amplitude of the desired signal to the amplitude of noise signals at a given point in time. The larger the number the better.


Term usually applied to the vibration frequency spectrum which is distinctive and special to a machine or component, system or subsystem at a specific point in time, under specific machine operating conditions, etc. Used for historical comparison of mechanical condition over the operating life of the machine.

Simple harmonic motion

Periodic vibration that is a sinusoidal function of time.

Sine Output

Single-degree-of-freedom system (SDOF)

Single-Ended Input

An analog input measured with respect to a common ground.

Slew rate

The maximum rate at which an instrument’s output can change by some stated amount.

Slow Roll Speed

Low rotative speed at which dynamic motion effects from forces such as imbalance are negligible.



see Signal-to-Noise Ratio.

Solid-state sensor

One which has no moving parts.

Soft failure

A product under test ceases to operate correctly, but resumes correct operation when the stressing environment is eased. Differs from hard failure.


(1.) An oscillation in pressure, capable of evoking the sensation of hearing. (2.) The sensation of hearing

Sound intensity

In a specified direction, the average rate of sound energy flow through a unit area perpendicular to that direction.

Sound level

The quantity in dB measured by a standardized Sound Level Meter. The reading is 20 log10 of the ratio between a given sound pressure and 20 micro-pascals.


A D/A converter output channel

Source follower

A device for converting a high impedance electrical signal to low impedance. Also referred to as an “impedance converter.” Generally has a voltage gain of unity.

Source Impedance

The combination of resistance and reactance that a source presents to the input terminals of a measuring instrument.


A flake or chip of metal; removed from one of the races of a rolling-element bearing. That bearing is nearing the end of its useful life.


The algebraic difference between the upper and lower range values expressed in the same units as the range.


Documents (the USA military service had as many as 28,000 specifications) that describe products or services, for the purpose of procurement. Differs from standards.

Spectral map

A three-dimensional plot of amplitude (Z axis) vs. time (or shaft speed) (Y axis) vs. frequency (X axis) / A three-dimensional plot of the vibration amplitude spectrum versus another variable, usually time or rpm. / A three-dimensional plot of amplitude (Z axis) vs. time (or shaft speed) (Y axis) vs. frequency (X axis).


The plural of Spectrum


See also frequency spectrum. The spectrum is the result of transforming a time domain signal to the frequency domain. It is the decomposition of a time signal into a collection of sine waves.

Spectrum Analyzer

A spectrum analyzer is a device used to examine the spectral composition of some electrical, acoustic, or optical waveform. Often, it measures the power spectrum. SignalCalc systems are an example of a modern high speed digital spectrum analyzer. / An instrument which displays the frequency spectrum of an input signal, usually amplitude vertical vs. frequency horizontal. / An instrument which displays the frequency spectrum of an input signal.


A sound resembling that of an un-lubricated hinge. A sound made by PVC or other material rubbing on glass, i.e. a windshield.

Squeak & Rattle

A term given to annoying noises such as those in a vehicle caused by plastic parts rubbing etc.

Stable Averaging

(Linear) An averaging method wherein all constituents are equally weighted; the amplitude is continuously normalized to the number of constituents currently captured.


The ability of an instrument or sensor to maintain a consistent output when a constant input is applied. Steady-state: A characteristic of a condition, such as value, rate, periodicity, or amplitude, exhibiting only negligible change over an arbitrary long period of time.


Documents (the USA military services had as many as 1,700 standards) that prescribe engineering disciplines, impose particular management practices, reporting and auditing requirements. Differ from specification.

Standard deviation

A statistical term: the square root of the variance , i.e., the square root of the mean of the squares of the measured deviations from the mean value.

Standing wave

A wave that is characterized by lack of vibration at certain points (nodes), between which are areas of maximum vibration (antinodes). Standing waves are produced at certain forcing frequencies when the resulting resonant vibratory response is confined within boundaries, as in the vibrating string of a musical instrument or the wing of an airplane or the whip antenna on your automobile. Also called “stationary wave”.


Stationary Signal

A stationary signal is a signal whose average statistical properties over a time interval of interest are constant, and it may be deterministic or not. In general, the vibration signatures of rotating machines are stationary.

Steady state vibration

Periodic vibration for which the statistical measurement properties (such as the peak, average, RMS and mean values) are constant.

Stepped Sine

Used for measuring the stability of closed loop servo systems and is one of many different techniques for structural excitation and frequency response function measurement.

Stepped sine testing

Sine shaking in a series of dwells. Frequency is incrementally increased or decreased.

Step stressing

Increasing stresses in a series of pre-selected increments.


The ratio of force (or torque) to deflection of a spring-like element. / The spring-like quality of mechanical and hydraulic elements to elasticity deform under load. / The ratio of change of force (or torque) to the resulting change in deflection of a spring-like element. It is the opposite of compliance.


The physical deformation, deflection, or change in length resulting from stress (force per unit area). / The ratio of the change in length to the initial unstressed reference length of an element under stress.

Strain-gauge transducer

A changing-resistance sensor whose signal depends upon sensitive element deformation. In an unbounded wire strain-gauge accelerometer, inertia affects a mass supported by ni-chrome wires; the wires change resistance in proportion to acceleration. The term may include piezoresistive accelerometers.

Strain gauge

Sensor whose resistance varies with applied force. A measuring element for converting force, pressure, tension, weight, etc., into a change in electrical resistance.


Intensity of applied load, usually at the site of a failure.

Stress Screening

A modern electronics production tool for precipitating latent defects such as poorly-soldered connections. Utilizes random vibration + rapid temperature ramping.

Structural Modification

Mathematically determining the effect of changing the mass, stiffness, or damping of a structure and determining its new modal parameters. A modal analysis provides, in essence, a mathematical model of the structure. This model can be manipulated to determine the effect of modifications to the structure. The modal model can be generated either experimentally or using a finite element program.


A sinusoidal quantity having a frequency that is an integral sub-multiple (x1/2, x1/3, etc.) of a fundamental (x1) frequency. / Sinusoidal quantity of a frequency that is an integral sub-multiple of a fundamental frequency.


Components of a vibration signal whose frequency is less than 1´ shaft speed. / Component(s) of a vibration signal which has a frequency less than shaft rotative frequency.

Synchronous Sampling

In a Dynamic Signal Analysis, it refers to the control of the effective sampling rate of data; which includes the processes of external sampling and computed re-sampling used in order tracking.

Supply current

The typical current that must be supplied to a sensor (along with the supply voltage).

Swept Sine

A sine signal of linearly varying frequency.

Swept-sine testing

Sine shaking whose frequency is smoothly and continuously varied. Commonly required for sequentially identifying resonances. By contrast, see stepped sine testing.


Vibration components (on rotating machinery) that are related to shaft speed. / An event or action that is synchronized to a reference clock.

Synchronous Averaging

A type of signal averaging where successive records of the time waveform are averaged together. This is also know as time domain averaging. The important criterion is that the start of each time record must be triggered from a repetitive event in the signal, such as once per rev. The triggering assures that the phase of the waveform components that are synchronized with the trigger are the same in each record. Then in the averaging process, these in-phase components will overwrite in the input buffer and will remain in memory, while the rest of the signal components will gradually average out because of their random relative phases. The technique is excellent for extracting signals from noisy environments.

Synchronous sampling

Control of a computer’s rate of data sampling to achieve order tracking.


A composite of equipment and skills, and techniques capable of performing or supporting an operational role, or both. A complete system includes all equipment, related facilities, material, software, services, and personnel required for its operation and support to the degree that it can be considered self-sufficient in its intended operational environment.

System effectiveness

(a) For repairable systems and items: the probability that a system can successfully meet an operational demand within a given time when operated under specified conditions.

(b) For “one-shot” devices and non-repairable items: the probability that the system will operate successfully when called upon to do so under specified conditions.



A direction perpendicular to a shaft’s centerline.

Radial Position

The average location, relative to the radial bearing centerline, of the shaft dynamic motion.

Radial Vibration

Shaft dynamic motion or casing vibration which is in a direction perpendicular to the shaft centerline.

Radio frequency

The frequency range between ultrasonic and infrared. AM broadcast frequencies range from 540 to 1,800 kHz, while FM broadcasts from 88 to 108 MHz.

Radio frequency interference (RFI)

Noise induced upon signal wires by ambient radio-frequency electromagnetic radiation with the effect of obscuring the instrument signal.

Random Noise

A random Signal Generator output exhibiting uniform spectral density over Fspan, Gaussian amplitude distribution and a crest factor of 5.


Broad-band random mixed with random narrowband tones.

Random vibration

(See Probabilistic vibration.) One whose instantaneous magnitudes cannot be predicted. Adjective “Gaussian” applies if they follow the Gaussian distribution. May be broad-band, covering a wide, continuous frequency range, or narrow band, covering a relatively narrow frequency range. No periodic or deterministic components.


A statement of the upper and lower limits over which an instrument works satisfactorily. / The region between the limits within which a quantity is measured, received or transmitted. It is expressed by stating the lower and upper range values in engineering units.

Rate gyro

A kind of gyroscope that measures rotational velocity (degrees or radians per second) around a fixed axis.


A sound exemplified by shaking a steel can full of steel nuts and bolts.

Real or Normal Modes

In a real mode, all points on the structure reach a maximum or a minimum value at the same time and all pass through equilibrium at the same time.

Real Part

A plot of the real part of the frequency response function versus frequency. For a single degree of freedom, the magnitude is zero at the damped natural frequency.

Real-Time Analyzer

See Dynamic Signal Analyzer.

Real-time closed loop control

Real-time closed loop control resembles iterative closed loop control but continuously modifies drive signals throughout the test.

Real-Time Rate

For a DSA, the broadest frequency span at which data is sampled continuously. Real-time rate is mostly dependent on FFT processing speed.

Rectangular Window

See Uniform Window.


The existence of more than one means for accomplishing a given function. Each means of accomplishing the function need not necessarily be identical. The two basic types of redundancy are active and standby.

Active Redundancy

Redundancy in which all redundant items operate simultaneously.

Standby Redundancy

Redundancy in which some or all of the redundant items are not operating continuously but are activated only upon failure of the primary item performing the function(s).

Relative Motion

Vibration measured relative to a chosen reference. Displacement transducers generally measure shaft motion relative to the transducer mounting.


The likelihood or probability that an equipment will “do its job” for a specified length of time (say 1000 hours) under specified circumstances (such as cycling in a specified manner over certain temperature limits, experiencing a particular vibration spectrum, etc.). Reliability defined in this way can be determined experimentally. Take 1000 units. Operate them under specified conditions. At the end of 1000 hours, how many are still operating correctly?

MIL-STD-721C offers (1) The duration or probability of failure-free performance under stated conditions. If you want to be complete, add (2) The probability that an item can perform its intended function for a specified interval under stated conditions. (For non-redundant items this is equivalent to definition (1). For redundant items this is equivalent to definition of mission reliability.)

Atomica (formerly says “The extent to which an experiment, test, or measuring procedure yields the same results on repeated trials”. This definition is found in a number of dictionaries.

A student dictionary just says “dependable” when defining “reliable”.

The probability that a device will perform its objective adequately for the period of time specified, under the operating conditions specified.

Reliability engineering

Reliability engineering is the doing of those things which insure that an item will perform its mission successfully. The discipline of reliability engineering consists of two fundamental aspects:

(1) paying attention to detail

(2) handling uncertainties

Remote terminal unit (RTU)

Industrial control and data collection device similar to a PLC but designed for remote data collection, transfer and communication via wire-based or radio telemetry links to DCS or computer systems.


(1) The maximum deviation from the mean of corresponding data points taken under identical conditions. (2) The maximum difference in output for identically-repeated stimuli (no change in other test conditions). Do not confuse with accuracy. / The ability of a transducer or readout instrument to reproduce readings when the same input is applied repeatedly. / The maximum difference between output readings when the same input is applied consecutively. This is the closeness of the agreement among consecutive measurements of the output for the same value of the input under the same operating conditions, approaching from the same direction. It is usually measured as non repeatability and expressed as in percent of span.

Repetitive shock machine

A platform to which products (to be tested or screened) are attached. Often this platform forms the bottom surface of a thermal test chamber. Pneumatic vibrators are attached to the bottom of the platform, causing it to vibrate, usually simultaneously in several axes.


Testing that reproduces a specified desired motion history (time domain) or waveform.

Residual Terms

Terms added to a curve fit algorithm to take into account the effects of modes outside the range being fitted. These terms consist of a mass term on the low frequency end and a stiffness term on the high.


The smallest change in stimulus that will produce a detectable change in the instrument output. / The smallest change in input which produces a detectable change in the output. This is the smallest increment of change that can be detected by a measurement system. Resolution can be expressed in bits, in proportions, in percent of actual reading or in percent of full scale. For example, a 12-bit system has a resolution of one part in 4,096 or 0.0244% of full scale. / The smallest change in input that will produce a detectable change in an instrument’s output. Differs from precision in that human capabilities are involved.

Resolution Bias Error

See Picket Fence Effect.


Forced vibration of a true SDoF system causes resonance when the forcing frequency equals the natural frequency, when any forcing frequency change decreases system response. (See also critical frequency) Therefore resonance represents maximum sprung mass response, if forcing frequency is varied while input force is held constant. More complex systems have many resonances. / The condition of vibration amplitude and phase change response caused by a corresponding system sensitivity to a particular forcing frequency. A resonance is typically identified by a substantial amplitude increase, and related phase shift. / A condition of oscillation caused by a small amplitude of periodic input has a frequency approaching one of the natural frequencies of the driven system.


The vibratory motion or force that results from some mechanical input.

Response signal

The signal from a “response sensor” measuring the mechanical response of a mechanical system to an input vibration or shock.

Response Spectrum

The frequency response function, also called the response spectrum, is a characteristic of a system that has a measured response resulting from a known applied input. In the case of a mechanical structure, the frequency response is the spectrum of the vibration of the structure divided by the spectrum of the input force to the system. To measure the frequency response of a mechanical system, one must measure the spectra of both the input force to the system and the vibration response, and this is most easily done with a dual-channel FFT Analyzer. Frequency response measurements are used extensively in modal analysis of mechanical systems.

Response time

An output expressed as a function of time, resulting from the application of a specified input under specified operating conditions.

Response Window

A decaying exponential Window of adjustable Time Constant used on the motion signal in an impact Modal test.


Continued oscillation after an external force or excitation is removed, as after a guitar string is plucked.

Rise time

The time required for the output of a transducer to rise from 10% to 90% of its final value, as it responds to a step change in the measurand.

RMS or Root-Mean-Square value

The square root of the time-averaged squares of a series of measurements. Refer to a textbook on electrical engineering. In the exclusive case of a sine wave, the RMS value, is 0.707 the peak value.

RMS value

See Root Mean Square.

RMS Responding

A measurement equal to the RMS value of the input signal for all waveforms within the specified frequency range and crest factor limit.


Rotation about the axis of linear motion.

Rolling Element (anti-friction) Bearing

Bearing whose low friction qualities derive from rolling elements (balls or rollers), with little lubrication. A shaft rotates on rollers or balls.

Root cause analysis

Determining what actually caused a failure, as opposed to what appears to have been the cause. Usually requires the services of a specialized failure analysis lab.


The roots of the characteristic equation are complex and have a real and imaginary part. The real part describes the damping (decay rate) of the system and the imaginary part describes the oscillations or damped natural frequency of the system.

Root Mean Square (RMS)

Square root of the arithmetical average of a set of squared instantaneous values. DSAs perform RMS averaging digitally on successive vibration spectra. / The square root of the average of the squares (root-mean-square) of the instantaneous values. It is the square root of the arithmetical mean of the squares.

Rotor, Flexible

A rotor which operates close enough to, or beyond its first bending critical speed for dynamic effects to influence rotor deformations. Rotors which cannot be classified as rigid rotors are considered to be flexible rotors.

Rotor, Rigid

A rotor which operates substantially below its first bending critical speed. A rigid rotor can be brought into, and will remain in, a state of satisfactory balance at all operating speeds when balanced on any two arbitrarily selected correction planes.

RPM Spectral Map

A spectral map of vibration spectra versus rpm.

Runout Compensation

Electronic correction of a transducer output signal for the error resulting from slow roll runout.



A measured quantity.

Parameter Estimation

An educated estimate of a varying parameter, sometimes used in calculations to produce simulations of the resultant output.

Patent defect

A flaw (in a part or assembly) and/or workmanship that has failed under test or screen. See Latent Defect.


In Vibration Testing, the total weight of all components mounted on the shaker table, including the derive under test (DUT), fixture and head expander (extender).


Extreme value of a varying quantity, measured from the zero or mean value. Also, a maximum spectral value.

Peak Hold

In a DSA, a type of averaging that holds the peak signal level for each frequency component.

Peak Pick

A parameter estimation technique where the peak value of the imaginary part of the frequency response function is used to estimate the mode shape value at that point. The phase is given by its sense (positive or negative). This method is also known as quad picking since the value is being picked off the imaginary or quadrature part of the frequency response function.

Peak-to-peak value

The algebraic difference between extreme values (as D = 2X).

Performance-based requirements (specifications)

Requirements that describe what the product should do, how it should perform, the environment in which it should operate, and interface and interchangeability characteristics. They should not specify how the product should be designed or manufactured.


The interval of time over which a cyclic vibration repeats itself. / The time required for a complete oscillation or for a single cycle of events. The reciprocal of frequency.


The interval of time over which a cyclic vibration repeats itself.

Periodic vibration

(See also Deterministic vibration.) An oscillation whose waveform regularly repeats. Compare with probabilistic vibration.


At right angles (90°) to a given line or plane.


(Of a periodic quantity), the fractional part of a period between a reference time (such as when displacement = zero) and a particular time of interest; or between two motions or electrical signals having the same fundamental frequency. / A measurement of the timing relationship between two signals, or between a specific vibration event and a keyphasor pulse. / A time-based relationship between a periodic function and a reference.

Phase reference probe

A device for giving a once-per-shaft-revolution signal.

Phase (time lag or lead)

The difference in time between two events such as the zero crossing of two waveforms, or the time between a reference and the peak of a waveform. The phase is expressed in degrees as the time between two events divided by the period (also a time), times 360 degrees.

Phase shift

The angle in degrees between an energizing voltage waveform and an output signal waveform.

Picket Fence Effect

The FFT spectrum is a discrete spectrum, containing information only at the specific frequencies that are decided upon by setting the FFT Analyzer analysis parameters. The true spectrum of the signal being Analyzed may have peaks at frequencies between the lines of the FFT spectrum, and the peaks in the FFT spectrum will not be at exactly the correct frequencies. This is called Resolution Bias Error, or the Picket Fence Effect. The name arises because looking at an FFT spectrum is something like looking at a mountain range through a picket fence.


See transducer.


Any material which provides a conversion between mechanical and electrical energy. For a piezoelectric crystal, if mechanical stresses are applied on two opposite faces, electrical charges appear on some other pair of faces.

Piezoelectric (PE) transducer

One which depends upon deformation of its sensitive crystal or ceramic element to generate electrical charge and voltage. Many present-day accelerometers are PE.

Piezoresistive (PR) transducer

One whose electrical output depends upon deformation of its semiconductor resistive element, offering greater resistance change than does the wire of a strain-gage transducer, for a given deformation.


Rotation in the plane of forward motion, about the left-right axis. In music, relates to frequency.

Plastic Reinforcement

(example: inside a box section acting as a beam) raises local stiffness without adding much weight.


Per MIL-STD-810, any vehicle, surface or medium that carries an equipment. For example, an aircraft is the carrying platform for internally-mounted avionics equipment and externally-mounted stores. The land is the platform for a ground radar set, and a man for a hand-carried radio.

Point and Direction

A book-keeping facility to identify the degrees-of-freedom involved in every Modal measurement.

Polar Plot

Polar coordinate representation of the locus of the 1x vector at a specific lateral shaft location with the shaft rotational speed as a parameter.


In electricity, the quality of having two charged poles, one positive and one negative.


A communications connection on an electronic or computer-based device.

Power Spectrum

See Auto Spectrum.

Power spectral density or PSD

Describes the power of random vibration intensity, in mean-square acceleration per frequency unit, as g²/Hz or m²/s³. Acceleration spectral density or ASD is preferred abroad.

Power supply

A separate unit or part of a system that provides power (pneumatic, electric, etc.) to the rest of the system.


The smallest distinguishable increment (almost the same meaning as resolution); deals with a measurement system’s possible or design performance.

Preload, Bearing

The dimensionless quantity that is typically expressed as a number from zero to one where a preload of zero indicates no bearing load upon the shaft, and one indicates the maximum preload (i.e., line contact between shaft and bearing).

Preload, External

Any of several mechanisms that can externally load a bearing. This includes ` soft” preloads such as process fluids or gravitational forces as well as “hard” preloads from gear contact forces, misalignment, rubs, etc.

Preview Average

In SignalCalc Dynamic Signal Analyzers: A facility to view the most recently captured signals before they are (conditionally) added to an average.

Probabilistic vibration

(As compared to Deterministic vibration), one whose magnitude at any future time can only be predicted on a statistical basis.

Probability distribution

An evaluation of the magnitude of events, stresses, strengths, etc. They don’t all have the same value. Often they are distributed in some sort of “bell shaped” graph of percentage vertical vs. value horizontal. Most are clustered around the mean value.


A machinery-mounted sensor (usually internal). May sense vibration.

Programmable logic controller (PLC)

Computer based industrial monitoring and control package with applications mostly in the areas of safety, sequential or logical operations, where control actions are based on equipment and alarm status.

Proof of screen

A process aimed at showing that a screen is effective in identifying existing defects in a product yet that the screen does not damage good products.

Proportional control

A control mode which generates an output correction in proportion to the error (the process variable’s deviation from set point).Proportional-integral-derivative (PID): Also referred to as a 3-mode controller, combining proportional, integral, and derivative control actions.


See Oscillator/Demodulator


Direction perpendicular to the shaft centerline.

Proximity sensor

Usually a displacement sensor for measuring the varying distance between a housing and a rotating shaft.

Pulse Sensor

A sensor which detects the presence or absence of material or an object (usually ferrous metal).


Power Spectral Density. The power of random vibration intensity in mean-square acceleration per frequency unit, as g²/Hz or m²/s³.

Pseudo Random Noise

A single frame of Random Noise that is exactly repeated every Time Span. Used to separate non-linear effects from noise contamination.


(Sharpness of Resonance) (Amplification factor) A measure of the sharpness of a resonance or frequency. The mechanical gain at resonance.

Quadrature motion

(Or side or lateral motion or crosstalk), any motion perpendicular to the reference axis. Shakers are supposed to have zero quadrature motion.

Quadrature Response

Another name for the imaginary part of the frequency response function.

Quadrature sensitivity

(Or side or lateral motion or crosstalk sensitivity) of a vibration sensor is its sensitivity to motion perpendicular to the sensor’s principal axis. Commonly expressed in % of principal axis sensitivity.


A quasi-periodic signal is a deterministic signal whose spectrum is not a harmonic series, but nevertheless exists at discrete frequencies. The vibration signal of a machine that has non-synchronous components resembles a quasi-periodic signal. In most cases, a quasi-periodic signal actually is a signal containing two or more different periodic components.


A slowly changing signal, usually associated with parameters such as Case Expansion.


Narrow band Analysis


Natural environments

Conditions occurring in nature, not caused by any equipment; effects are observed whether an equipment is at rest or in operation.

Natural frequency

The frequency of an undamped system’s free vibration; also, the frequency of any of the normal modes of vibration. Natural frequency drops when damping is present / The frequency of free vibration of a system. The frequency at which an undamped system with a single degree of freedom will oscillate upon momentary displacement from its rest position.

Nodal Point

A point of minimum shaft deflection in a specific mode shape. May readily change location along the shaft axis due to changes in residual imbalance or other forcing function, or change in restraint such as increased bearing clearance.


A point or line on a vibrating structure that remains stationary.


The total of all interferences in a measurement system, independent of the presence of signal. / Any component of a transducer output signal that does not represent the variable intended to be measured. / Any undesirable electrical signal, from external sources such as AC power lines, motors, electrical storms, radio transmitters, as well as internal sources such as electrical components. Non-linearity: The deviation from the best fit straight line that passes through zero. Normal-mode rejection ratio: The ability of an instrument to reject electrical interference across its input terminals, normally of line frequency (50-60 Hz).

Noise Floor

The minimum discernible signal that can be detected by a receiver.

Non Linearity

The deviation from a best fit straight line of true output vs. actual value being measured. / The deviation from a best fit straight line of true output vs. actual value being measured.


Minimum spectral value, at a natural frequency. Also, the deliberate reducing of a portion of a test spectrum (random vibration testing).

Nyquist Criterion

Requirement that a sampled system sample at a frequency greater than twice the highest frequency to be measured.

Nyquist Frequency

Digital signal processing requires analogue to digital (A to D) conversion of the input signal. The first step in A to D conversion is sampling of the instantaneous amplitudes of signal at specific times determined by the sampling rate. If the signal contains any information at frequencies above one-half the sampling frequency, the signal will not be sampled correctly, and the sampled version of the signal will contain spurious components. This is called aliasing. The theoretical maximum frequency that can be correctly sampled is equal to one-half the sampling rate, and is called the Nyquist frequency. In all digital signal-processing systems, including FFT Analyzers, the sampling rate is made to be significantly greater than twice the highest frequency present in the signal in order to be certain the aliasing will not occur.

Nyquist Plot

A plot of real versus imaginary spectral components that is often used in servo analysis. Should not be confused with a polar plot of amplitude and phase of 1x vibration.

A plot of the real part versus the imaginary part of the frequency response function. For a single-degree-of-freedom system, the Nyquist plot is a circle. The Nyquist plot is representation of a frequency response function by graphing the “real” part versus the “imaginary” part. In the Nyquist plot, a resonance is displayed as a circle, but without indication of its.

Nyquist theorem

This law is the basis for sampling continuous information. It states that the frequency of data sampling should be at least twice the maximum frequency at which the information might vary. This theorem should be observed in order to the preserve patterns in the information or data, without introducing artificial, lower frequency patterns.


The interval between two frequencies differing by exactly 2:1.


Original Equipment Manufacturer.

Ohm meter

A device used to measure electrical resistance. One-to-one repeater: A diaphragm operated device which detects the process pressure and generates an air (or nitrogen) output signal of equal pressure.

Oil Whirl / Oil Whip

An unstable free vibration whereby a fluid-film bearing has insufficient unit loading. Under this condition, the shaft centerline dynamic motion is usually circular in the direction of rotation. Oil whirl occurs at the oil flow velocity within the bearing, usually 40 to 49% of shaft speed. Oil whip occurs when the whirl frequency coincide with (and becomes locked to) a shaft resonant frequency. (Oil whirl and whip can occur in any case where fluid is between two cylindrical surfaces. )

Open Loop Control

In vibration control, provides pre-computed or preconceived drive signals to the exciter system without modifying or refining those signals based on observation of the resulting motion. See also Closed Loop.

Operational environment

The aggregate of all external and internal conditions (such as temperature, humidity, radiation, magnetic and electric fields, shock vibration, etc.) either natural or man made, or self-induced, that influences the form, operational performance, reliability or survival of an item.

Operational limit

The extremes beyond which a product is not expected to operate.

Optical isolation

Two networks or circuits in which an LED transmitter and receiver are used to maintain electrical discontinuity between the circuits.


The path of a shaft centerline during rotation. The orbit is usually observed on an oscilloscope connected to x- and y-axis displacement sensors. Sometimes called a Lissajous pattern. / The path of the shaft centerline motion during rotation. The orbit is observed with an oscilloscope connected to x and y-axis displacement transducers. Some dual-channel DSAs also have the ability to display orbits.


Vibration at multiples of a machine’s shaft speed. See also First order vibration.

Order tracking

Control of a computer’s data sampling rate, such that display of vibrations at multiples of the shaft’s rotating speed (´1, ´2, ´3, etc.) are enhanced and more easily identified and measured..


Variation with time of a quantity such as force, stress, pressure, displacement, velocity, acceleration or jerk. Usually implies some regularity (as in sinusoidal or complex vibration).


A signal conditioning device that sends a radio frequency signal to an eddy-current displacement probe, demodulates the probe output, and provides output signals proportional to both the average and dynamic gap distances.

Output settling time

Time required for the analog output voltage to reach its final value within specified limits.

Output signal

A signal delivered by a device, element or system.

Output slew rate

Maximum rate of change of analog output voltage from one level to another.

Overall RMS Level

A measure of the total RMS magnitude within a specified frequency range.


In Dynamic Signal Analysis: The maximum percent (of Tspan) of adjacent measurement overlap that is permitted when the process runs faster than real-time; the amount of measurement redundancy desired.

Overlap Processing

In Dynamic Signal Analysis: The windowing applied to a time signal stored in a buffer before being processed to form the spectrum causes data at the beginning and end of the time records to be represented at the wrong amplitude values, creating errors in the spectral amplitude levels. Overlap processing allows processing of the entire contents of the input buffer, more frequently than the time taken for the buffer to completely refresh (Tspan). Overlap processing is desirable when using a Hanning Window because it ensures against loss of data for parts of the signal that occur near the beginning and end of the window. An overlap of 66.7% will completely correct for amplitude errors caused by the Hanning window.


That part of the stroke which falls between the end of the calibrated range and the travel stop.


The process of writing new data over existing data. This option causes you to lose the information that has been written over.


Machinery health monitoring (MHM)

The current machine condition (signature) compared with an earlier condition used as an indicator to detecting potential failure before it occurs. (Also see Condition Monitoring)


Slight changes in the dimensions of iron or steel components resulting from changes in the magnetic fields acting on these components.


A physical property, dynamically computed as acceleration divided by force. Statically computed as W (which can be measured on a butcher scale) divided by the acceleration due to gravity. Ordinary structures are not pure masses as they contain reactive elements, i.e. springs and damping.


A value intermediate between quantities under consideration. A shaker’s mean acceleration must be zero – no steady-state acceleration. But a vehicle can have steady-state motion.

Mean-Time-Between-Failure (MTBF)

A basic measure of reliability for repairable items: The mean number of life units during which all parts of the item perform within their specified limits, during a particular measurement interval under stated conditions.


A basic measure of reliability for non-repairable items: The total number of life units of an item divided by the total number of failures within that population, during a particular measurement interval under stated conditions.

Measurement Parameters

A control group controlling Triggering, Averaging and Auto-Ranging.

Mechanical impedance

The ratio of force to velocity, where the velocity is a result of that force only. Its reciprocal, mobility, is today more favored.

Mechanical failure

A malfunction consisting of cracking, excessive displacement, misalignment, loosening, etc.

Mechanical Runout

An error in measuring the position of the shaft centerline with a displacement probe that is caused by out-of-roundness and surface imperfections.

MEMS – Micro Electro-mechanical Structures

Extremely small devices utilizing both electrical and mechanical properties.


Length or displacement equal to 10E-6 meter. One micron = 0.04 mil or 0.00004 inch.

Micrometer (MICRON)

One millionth (.000001) of a meter. (1 micron = 1 x 10E-6 meters = 0.04 mils.)


An instrument which converts a relatively small dynamic pressure change into an electrical signal. See transducer.


One thousandth (0.001) of an inch. (1 mil = 25.4 microns.) / Length or displacement equal to 0.001 inch or 25.4 x10 e-6 m.


Used to describe the angle of one shaft centerline to the other. It is equivalent to milliradians. It can also be expressed as rise/run (1 unit = 17.45 mils/inch), as long as the rise is measured in mils and the run is measured in inches.

Milliamp (mA)

One thousandth of an ampere.

Millivolt (mV)

One thousandth of a volt.

Mission Profile

A time-phased description of the events and environments an item experiences from initiation to completion of a specified mission, to include the criteria or mission success or critical failures.

Mission Reliability

The measure of the ability of an item to perform its required function for the duration of a specified mission profile. Mission reliability defines the probability that the system will not fail to complete the mission, considering all possible redundant modes of operation.

Mobility – Velocity / Force

Mobility is the inverse of mechanical impedance. It is a measure of the ease with which a structure is able to move in response to an applied force, and varies it with frequency. The vibration measured at a point on a machine is the result of a vibratory force acting somewhere in the machine. The magnitude of the vibration is equal to the magnitude of the force times the mobility of the structure. From this it follows that the amplitude of the destructive forces acting on a machine are not determined directly by measuring its vibration if the mobility of the machine is not known. For this reason, it is a good idea to measure the mobility at the bearings of a machine in order to find out the levels of the forces acting on the bearings due to imbalance or misalignment.

Modal analysis

The process of breaking complex structural motion into individual vibration modes. Resembles frequency domain analysis that breaks complex vibration down to component frequencies. / The process of breaking complex vibration into its component modes of vibration, very much like frequency domain analysis breaks vibration down to component frequencies.

A process of determining the natural frequencies, damping factors, and mode shapes for a structure. This is usually done either experimentally through frequency response testing or mathematically using finite element analysis. Complex structural motion is reduced to individual vibration modes. The process of determining a set of generalized coordinates for a system such that the equations of motion are both inertially and elastically uncoupled.


A characteristic pattern in a vibrating system. All points reach their maximum displacements at the same instant.

Mode Shape

The resultant deflected shape of a rotor at a specific rotational speed to an applied forcing function. A three-dimensional presentation of rotor lateral deflection along the shaft axis.

Modulation, Amplitude (AM)

The process where the amplitude of a signal is varied as a function of the instantaneous value of another signal. The first signal is called the carrier, and the second signal is called the modulating signal. Amplitude modulation produces a component at the carrier frequency, with adjacent components (sidebands) at the frequency of the modulating signal.

Modulation, Frequency (FM)

The process where the frequency of the carrier is determined by the amplitude of the modulating signal. Frequency modulation produces a component at the carrier frequency, with adjacent components (sidebands) at the frequency of the modulating signal.

Modulus of Elasticity E (static)

The initial slope of the stress vs strain curve, where Hooke’s Law applies, before the elastic limit is reached. Typical values are 30,000,000 pounds/square inch for steel and about 10,000,000 for aluminum.


Abbreviation for Mean (or average) time between failures.


Mean-Time-To-Failure. A basic measure of reliability for nonrepairable items; the total number of life units of an item divided by the total number of failures within that population.

Multiple-Degree-of-freedom-system (MDOF)

An N-degree-of-freedom system is a system whose position in space can be completely described by N coordinates or independent variables.

Multiplexer (Mux)

A switching device that sequentially connects multiple inputs or outputs in order to process several signal channels with a single A/D or D/A converter. / A device that selects multiple inputs into an aggregate signal.


The factor reflecting the sensitivity of a transducer in volts (or parts of a volt) per given engineering unit.



In Dynamic Signal Analyzers, the LastH file contains the last system transfer function used by the test for the equalization of the drive signal.

Latent defect

A flaw (in a part or assembly) and/or workmanship that is dormant, not immediately apparent visually or by electrical test, yet can result in failure. See Patent Defect.

Lateral Location

The definition of various points along the shaft axis of rotation.

Lateral sensitivity

Sensitivity of a transducer in a direction perpendicular to the normal measurement axis. See also transverse sensitivity.

Lateral Vibration

See Radial Vibration.

Layout File

In SignalCalc Dynamic Signal Analyzers: A binary file within a Test or Run folder that defines all graphical parameters for a specific (user named) arrangement of display windows.

Layout Manager

In SignalCalc Dynamic Signal Analyzers: A control dialog affecting all (named) Layouts used within a Test or Run; facilitates creation, deletion, renaming and automatic usage.


Smearing of frequency components caused during the process of computing the FFT from a digitized block of time data. Weighting or Windowing functions such as Hanning reduce or remove leakage).


Liquid Crystal Display


An unfortunate result of sampling with finite intervals. Results in smearing of frequency components. Improved by windowing (e.g. Hanning). / In DSAs, a result of finite time record length that results in smearing of frequency components. Its effects are greatly reduced by the use of weighted window functions such as flat top and Hanning.


Light emitting Diode.


The (usually base 10) logarithm of the ratio between a quantity and a reference quantity. For acoustic measurements the reference quantity is 20 micro-pascals. For acceleration measurements, one reference is 1 micro-g.

The level of a signal is its amplitude, alternatively the amplitude expressed on a decibel scale relative to a reference value.

Life Cycle Testing

Subjecting products to stresses similar to those anticipated in actual service while collecting engineering data related to life expectancy, reliability, specification compliance, or product improvements. Usually aimed at determining the products’ mean time between failures or MTBF.

Life cycle history

A time history of events and conditions associated with an item of equipment from its release from manufacturing to its removal from service. The life cycle should include the various phases that an item will encounter it its life, such as: handling, shipping and storage prior to use; mission profiles while in use; phases between missions, such as standby time or storage, transfer to and from repair sites and alternate locations; and geographical locations of expected deployment.

Life units

A measure of use duration applicable to the item. Measures include time, cycles, distance, rounds fired, attempts to operate, etc.


In Vibration Control: The ability to limit the Control spectrum based on a measurement signal approaching a Limit spectrum. Limiting is done on a spectral line basis.

Line Spacing

In an FFT spectrum, the frequency difference between two adjacent bin centers or lines.


Having or being a response or output that is directly proportional to the input.

Linear Averaging

See Time Averaging.

Linear system

A system is linear if its magnitude of response is directly proportional to its magnitude of excitation, for every part of the system.


The closeness of a calibration curve to a specified straight line, preferably passing through zero. Commonly specified as a % of full scale. / The response characteristics of a linear system remain constant with input level. That is, if the response to input a is A, and the response to input b is B, then the response of a linear system to input (a + b) will be (A + B). An example of a non-linear system is one whose response is limited by mechanical stop, such as occurs when a bearing mount is loose. / The closeness to which a curve approximates a straight line or the deviation of an instrument’s response from a straight line.


Common term used to describe the filters of a DSA (e.g., 400 line analyzer).

Limit Control Spectrum

In Vibration Control, the ability to limit the Control spectrum based on a measurement signal approaching a Limit spectrum. Limiting is done on a spectral line basis.


Liquid nitrogen, often used for rapid cooling of environmental test chambers.


A function the raises a number by an exponent to produce a given number. For example, the logarithm of 100 to the base 10 is 2.

Longitudinal Vibration

Longitudinal or Compressional or Axial Vibration is in the same direction as the transfer of energy. Transverse Vibration is perpendicular to that direction.


The human ranking of an auditory sensation, usually in terms ranging from soft to loud, expressed in sones (not in decibels).

Low-Pass Filter

A filter whose transmission band extends from dc to an upper cutoff frequency.

A filter that passes signals with less than 3 dB attenuation up to its cut-off frequency, and attenuates the signal above that frequency. The attenuation slope is called the roll off, q.v. An anti-aliasing filter is an example of a low pass filter.



Integral Electronics Piezoelectric transducer, using a constant current source as the conditioning medium.

Imaginary Part

A plot of the imaginary part of the frequency response function versus frequency. For a single-degree-of-freedom system, the magnitude is a maximum or minimum at the damped natural frequency.


Unequal distribution of weight or mass on a rotor. The geometric center of the mass is not where it should be: along the shaft centerline.


Unequal radial weight distribution on a rotor system; a shaft condition such that the mass and shaft geometric centerlines do not coincide.


A collision between masses.

Impact test (bump test)

A broad frequency range of structural responses is caused by a deliberate impact.

Impact Test

Response test where the broad frequency range produced by an impact is used as the stimulus. Sometimes referred to as a bump test.

Impact Testing – Hammer Test

A method of measuring the frequency response function of a structure by hitting it with a calibrated hammer and measuring the system’s response. The impact hammer is instrumented with a force transducer to measure the input force pulse while the response is typically measured using an accelerometer. The impact imparts a force pulse to the structure that excites it over a broad frequency range.

Impedance, Mechanical

The mechanical properties of a machine system (mass, stiffness, damping) that determine the response to periodic forcing functions.

The mechanical impedance of a point on a structure is the ratio of the force applied to the point to the resulting velocity at the point. It is a measure of how much a structure resists motion when subjected to a given force, and it is the reciprocal of mobility. The mechanical impedance of a structure varies in a complicated way as frequency is varied. At resonance frequencies, the impedance will be low, meaning very little force can be applied at those frequencies.

Impedance, electrical

The opposition to the flow of AC current, the equivalent of the resistance in DC circuits. Its unit is the ohm. The impedance of an AC circuit is one ohm if a potential difference of one volt creates a current flow of one ampere in it.


The property by which an electromotive force (emf) is induced in a conductor when the magnetic field is changing about it. This is usually caused by changes in the current flow in the circuit or in a neighboring circuit.


To convert and use a file created by another program.


The integral of force over a time interval.

Impulse Response

The response of a system to an impulse as input signal. The output then produces the impulse response that is the time domain equivalent to the Frequency Response Function, FRF.

Impulse Signal

A sharp “spike” positioned 5% from the beginning of a sample block; produces a near-flat spectrum to Fspan.


A control setting that allows a Test to produce a sequence of incrementally number Run folders, one for each run of the Test.

Induced environments

Conditions generated by operating some equipment, as opposed to natural environments.

Inertance (or accelerance)

The ratio of acceleration to force.


Motion that is referenced to free space or to a fixed point in space. A sensor (such as an accelerometer) which measures such motion.

Input Channels Parameters

In SignalCalc Dynamic Signal Analyzers: Controls for each input channel:

Influence Coefficients

Mathematical coefficients that describe the influence of system loading on system deflection.


The mechanical motion, force or energy applied to a mechanical system, e.g. the vibratory input from shaker to test item. Or an electrical signal, e.g. from an oscillator to the power amplifier driving a shaker.

Input control signal

Originates in a control sensor; sometimes selected between or averaged between several sensors. Used to regulate shaker intensity. (May originate in a force sensor for force-controlled testing.)

Input Impedance

The shunt resistance and capacitance (or inductance) as measured at the input terminals, not including effects of input bias or offset currents.

Input/output (I/O)

The analog or digital signals entering or leaving a DCS or other central control or computer system involving communications channels, operator interface devices, and/or data acquisition and control interfaces.


Representation inversely related to time rate of change. Example: integrating velocity yields displacement. In a computer, this is accomplished by dividing the velocity signal by jw, where w is frequency multiplied by 2p. / A process producing a result that, when differentiated, yields the original quantity. Integration of acceleration, for example, yields velocity. Integration is performed in a DSA by dividing by jw, where w is frequency multiplied by 2þ. (Integration is also used to convert velocity to displacement).


Circuitry which converts an acceleration signal to a velocity signal or a velocity signal to a displacement signal. / Circuitry which converts an acceleration signal to a velocity signal or a velocity signal to a displacement signal.


The severity of a vibration or shock. Nearly the same meaning as Amplitude, defined earlier, but less precise, lacking units.

Intrinsically safe

Equipment or wiring which is incapable of releasing sufficient electrical or thermal energy to ignite a hazardous mixture of hydrocarbon vapors and air. In such equipment the electrical energy is limited so that it can not generate a spark or otherwise ignite a flammable mixture.


Formerly the Instrument Society of America, now referred to as the International Society for Measurement & Control.


A reduction in motion severity, usually by a resilient support. A shock mount or isolator attenuates shock. A vibration mount or isolator attenuates steady-state vibration.

A reduction in motion severity, usually by a resilient support. A shock mount or isolator attenuates shock. A vibration mount or isolator attenuates steady-state vibration. Isolators are used on electrodynamic shakers to reduce transmission of vibration energy into the floor.


Sometimes called glass itch or lace itch. An automotive sound caused by metal sliding on glass.

Iterative closed loop control

Pre-calculated, then modified drive signals based upon resulting motion, in order to better match measured with desired motions. Evaluation and modifications take place after each excitation, repeating until the match is acceptable.


The rate of change of acceleration with time.


Specific portions of the shaft surface from which rotor applied loads are transmitted to bearing supports.


A signal used in rotating machinery measurements, generated by a sensor that observes a once-per-revolution event. Used in phase measurements for analysis and for balancing. Bentley-Nevada trade name / A signal used in rotating machinery measurements, generated by a transducer observing a once-per-revolution event. The keyphasor signal is used in phase measurements for analysis and balancing. (Keyphasor is a Bently Nevada trade name.)


g (G)

The acceleration produced by Earth’s gravity. By international agreement, the value for 1 gravitational unit is 9.80665 m/s² = 386.087 in/sec² = 32.1739 ft/sec². / The value of acceleration produced by the force of gravity. / The acceleration produced by the force of gravity, which varies with the latitude and elevation of the point of observation. By international agreement, the value 980.665 cm/sec’ = 386.087 in/sec’ = 32.1739 ft/sec’ has been chosen at the standard acceleration due to gravity.

g units or gravitational units.

A way to express an acceleration, in terms of a ratio. Divide a given acceleration by the appropriate value (9.80665 m/s² or 386.087 in/sec² or 32.1739 ft/sec²).

Gain (magnitude ratio)

For a linear system or element, the ratio of the magnitude (amplitude) of a steady-state sinusoidal output relative to a causal input. In an electrical circuit, it is the amount of amplification used and is sometime expressed in decibels, dB.

Gain accuracy

Measure of deviation of the gain (of an amplifier or other device) from the ideal gain.

Gain, dynamic

For a sinusoidal signal, the magnitude ratio of the steady-state amplitude of the output signal to the amplitude of the input.

Gain, static

The ratio of change of steady state value to a step change in input, provided that the output does not saturate.


The electrical neutral line having the same potential as the surrounding earth; the negative side of a direct current power system; the reference point for an electrical system.


1 gal (seems to mainly be used in Japan) is an acceleration of 1 cm/sec². A more or less typical earthquake measures around 250 gal near the epicenter, so 400 gal is sometimes specified for earthquake testing. 980.6 gal = 1g.

Gate Encoders

Pulse generator for measuring slide gate or valve positions.

Gate Monitors

Device for monitoring slide gate or valve position.

Gear mesh frequency

A potential vibration frequency on any machine employing gears. Multiply the number of teeth on a gear times its RPM, then divide by 60 / A potential vibration frequency on any machine that contains gears; equal to the number of teeth multiplied by the rotational frequency of the gear.

Gear tooth Sensor

Inductive sensor for measuring rotational speed.

Generalised Coordinates

The minimum number of independent coordinates necessary to completely describe a systems position constitutes a set of generalized coordinates. For an N degree of freedom system, N generalized coordinates are required.

Graduation mark

The marks that define the scale intervals on a measuring instrument are known as graduation marks.

Graph Attributes Dialog

In SignalCalc Dynamic Signal Analyzers: A control dialog that determines the appearance of a graph window.

Graph Template

A user-stored graphic format that is used when opening a New Graph; provides a desired graphic appearance with minimum control interaction.

Ground Loop

A current loop created when a signal source and a signal measurement device are grounded at two separate points on a ground bus through which noise currents flow. These currents generate voltage drops between the two ground connections, which cause measurement errors.

Hall effect sensor

A sensor used to detect rotating magnets and send square wave pulses back to a speed switch, rate meter or tachometer.


Highly accelerated life test. See accelerated life test.

Hamming Window

Named after its originator, the Hamming window is a Hanning window sitting on top of a small rectangular pedestal. Its function is similar, but has its first side lobes 42 dB down, whereas the Hanning window’s first side lobes are only 32 dB down. Thus the Hamming has better selectivity for large signals, but it suffers from the disadvantage that the rest of the side lobes are higher, and in fact fall off slowly at 20 dB per octave like those of the rectangular window. The Hamming window had some advantage in the days when FFT Analyzers only had 50 dB or so of dynamic range, but today it is essentially obsolete.

Hand Held Tachometer

A hand held device used to measure rates such as revolutions per minute, inches per mi9nute and feet per minute.

Hanning Window

DSA window function that provides better frequency resolution than the flat top window, but with reduced amplitude accuracy. / The standard Window for general spectrum analysis of continuous signals and all random signals.

Hard failure

A product under test ceases to work correctly. It does not resume correct operation, even when the stressing environment is eased. Differs from soft failure.


A sinusoidal quantity having a frequency that is an integral multiple (x2, x3, etc.) of a fundamental (x1) frequency. / Frequency component at a frequency that is an integer multiple of the fundamental frequency.

Harmonic Distortion

In the output signal of a device, distortion caused by the presence of frequencies not present in the input signal.


Distortion (usually non-harmonic) on a signal. May be viewed on an oscilloscope trace. (slang).


Highly accelerated stress screening. See environmental stress screening (ESS).

Heavy Spot

The angular location of the imbalance vector at a specific lateral location on a shaft. The heavy spot typically does not change with rotational speed.

HELP (button)

Pops up helpful information about the current dialog box or area.

hertz / Hz

Lower-case h, but abbreviated Hz. The unit of frequency. Formerly cps for cycles per second. / Unit of frequency, defined as one cycle per second.

High-Pass Filter

A filter with a transmission band starting at a lower cutoff frequency and extending to (theoretically) infinite frequency.

High Spot

The angular location on the shaft directly under the vibration transducer at the point of closest proximity. The high spot can move with changes in shaft dynamics (e.g., from changes in speed).


The inverse of the system transfer function.


A Test type providing single channel amplitude domain functions including Probability Density (PDH) and Cumulative Density (CDF).


Also called deadband. That portion of a measuring system’s response where a change in input does not produce a change in output. / Non-uniqueness in the relationship between two variables as a parameter increases or decreases. Also called deadband, or that portion of a system’s response where a change in input does not produce a change in output. / The property of an element or sensor, that its output is dependent not only on the value of the input, but on the direction of the current traverse. (The reading of the same value differs as a function of whether the measurement is rising or falling.)

Hysteresis Damping (Hysteric Damping, Structural Damping)

Energy losses within a structure that are caused by internal friction within the structure. These losses are independent of speed or frequency of oscillation but are proportional to the vibration amplitude squared.



The event, or inoperable state, in which any item or part of an item does not, or would not, perform as previously specified.

Failure analysis

Subsequent to failure, the logical systematic examination of an item, its construction, application, and documentation to identify the failure made and determine the failure mechanism and its basic course.

Failure catastrophic

A failure that can cause loss.

Failure effect

The consequence(s) a failure mode has on the operation, function, or status of an item. Failure efforts are classified as local effect, next higher level, and end effect.

Failure mechanism

The mechanical, chemical, physical or other process that results in failure.

Failure mode analysis

A procedure aimed at determining why a failure occurred.

Failure mode and effects analysis (FMEA)

A procedure by which each potential failure mode is a system is analyzed to determine the results to effects thereof on the system and to classify each potential failure mode according to its severity.

Failure rate

The total number of failures within an item population, divided by the total number of life units expended by that population, during a particular measurement interval under stated condition.

Fatigue life

The amount of time under defined operational conditions that a product is expected to survive.


Immediate cause of failure (e.g. maladjustment, misalignment, defect, etc.)

FFT or Fast Fourier Transform

A popular computer method of shifting data from the time domain to the frequency domain. / A computer (or microprocessor) procedure for calculating discrete frequency components from sampled time data. A special case of the discrete Fourier transform where the number of samples is constrained to a power of 2. / A fast Fourier transform (FFT) is an efficient algorithm to compute the discrete Fourier transform (DFT) and its inverse. SignalCalc analyzers and SignalStar controllers use both FFT and DFT algorithms in various applications.

FFT Analyser

The FFT Analyzer is a device that uses the FFT algorithm to calculate a spectrum from a time domain signal, See also Fast Fourier Transform and Dynamic Signal Analyzer.

FFT Analysis

In general, FFT analysis is used to refer to any and all types of Digital Signal Analysis (DSA) which included such measurements as: linear spectrum, power spectrum, energy spectrum, cross power spectrum, transfer function, frequency response function, coherence, inverse FFT, impulse response, auto- and cross-correlation, histogram, spectrogram and others. SignalCalc analyzers and SignalStar controllers use various combinations of these to provide user friendly solutions for many applications.

FFT Library

A collection of FFT algorithms and other signal processing tools that can be manipulated by the user to perform custom signal processing. The SignalCalc analyzers and SignalStar controllers have optional packages that allow user control of the Data Physics hardware for custom applications.

FFT Power Spectrum

In the frequency domain, this is the square of FFT´s magnitude. The power spectrum answers the question, “Which frequencies contain the signal’s power?” The answer is in the form of a distribution of power values as a function of frequency. SignalCalc systems are an example of a modern high speed digital spectrum analyzer which can provide a power spectrum in real time.

Feedback Signal

Feedback signal is a response measurement that is used in a closed-loop control process.

File Extension

A three letter addition to a file name that usually identifies the file type and appears after the file name, separated by a period.


An electronic device to pass certain frequencies (pass band) but block other frequencies (stop band). Classified as low-pass (high-stop), high-pass (low-stop), band-pass or band-stop. / Electronic circuitry designed to pass or reject a specific frequency band.

Finite Element Modeling (Finite Element Analysis, FEA)

A computer-aided design technique for predicting the dynamic behavior of a possible future mechanical system.

Finite Element Modeling

A computer aided design technique for predicting the dynamic behavior of a mechanical system prior to construction. Modeling can be used, for example, to predict the natural frequencies of a flexible rotor.

First order vibration

Rotating machine vibration caused by shaft unbalance. Frequency in hertz (Hz) is calculated by shaft RPM/60. Also called 1x vibration. Additional orders, 2x, 3x …. 36x, etc. are caused by other mechanisms


The intermediate structure that attaches a device under test (DUT) to a shaker or shock test machine.

Flat Top Filter

DSA window function which provides the best amplitude accuracy for measuring discrete frequency components.

In signal processing, a window function is used to shape the time record to provide a correction for the basic limitations of the FFT. The flat top window is used to make accurate measurements of the amplitude of periodic signals. SignalCalc systems provide for a number of window functions.

Fluid-Film Bearing

A bearing which supports the shaft on a thin film of oil. The fluid-film layer may be generated by journal rotation (hydrodynamic bearing), or by externally applied pressure (hydrostatic bearing).


Failure Mode and Effects Analysis. A procedure by which each potential failure mode of a system is analyzed to determine the effects on the system and classify each potential failure mode according to its severity.

Forced vibration

The vibratory motion of a system caused by some mechanical excitation. If the excitation is periodic and continuous, the response motion eventually becomes steady-state. / The oscillation of a system under the action of a forcing function. Typically forced vibration occurs at the frequency of the exciting force. / The vibratory motion of a system caused by some mechanical excitation. If the excitation is periodic and continuous, the response motion eventually becomes steady-state.

Force Window

A rectangular Window of adjustable Width used on the impact force signal in a Modal Analysis. Since the duration of the actual impact is usually very short relative to the overall digitized time sample, the frequency response function of the force signal can have a low signal to noise ratio. The force window does not alter the actual force pulse but minimizes the noise in the rest of the data block giving a much improved signal to noise ratio.

Forced Response Analysis (Forced Response Simulation)

Mathematically calculating the system response to an arbitrary forcing function using modal analysis data as the system model.

Forcing frequency

In sinusoidal vibration testing or resonance searching, the frequency at which a shaker vibrates.

Forcing function

A climatic or mechanical environmental input to an item of equipment that affects its design, service life or ability to function. (Also referred to as an environmental condition or an environmental stress.


See Fast Fourier Transform or FFT. Term honors mathematician Baron Jean Baptiste Joseph Fourier 1768-1830.

Fourier Analysis

Fourier analysis is another term for spectrum analysis, although it generally refers to analysis using an FFT Analyzer.

Fourier Transform



The maximum load an equipment can stand before failure (malfunction, irreversible loss of performance or structural damage) occurs.

Fragility test

Expensive but highly useful dynamic tests of several samples (to account for variations in tolerances, material properties and manufacturing processes) at potentially destructive frequencies, to determine fragility.


A time domain “capture window” containing Block Size samples of the input signals.

Free vibration

Free vibration occurs without forcing, as after a reed is plucked./ Vibration of a mechanical system following an initial force—typically at one or more natural frequencies.


The reciprocal of the period T in seconds (of a periodic function) (1/T). Usually given in hertz (Hz), meaning cycles per second (cps). / The repetition rate of a periodic event, usually expressed in cycles per second (Hz), revolutions per minute (rpm), or multiples of a rotational speed (orders). Orders are commonly referred to as 1x for rotational speed, 2x for twice rotational speed, etc. / The number of cycles over a specified time period over which an event occurs. Normally expressed in cycles per second (hertz, Hz).

Frequency Analysis

The application of a Dynamic Signal Analyzer (DSA) to help understand the meaning of an electrical signal derived via a transducer form a physical process. SignalCalc systems are an example of a modern high speed DSA that can be used for frequency analysis.

Frequency Analyser

A frequency analyzer is a device used to examine the spectral composition of some electrical, acoustic, or optical waveform. Often, it measures the power spectrum. SignalCalc systems are an example of a modern high speed digital frequency analyzer.

Frequency Domain

A plot of frequency vs. amplitude, called a spectrum, and the spectrum is in the frequency domain.

Frequency range – see bandwidth

The frequency range selected or available for measurements.

Frequency response

The portion of the frequency spectrum over which a device can be used, within specified limits of amplitude error. / The amplitude and phase response characteristics of a system. / The frequency-dependent characteristics that determines the phase and amplitude relationship between sinusoidal input and output.

Frequency Response Function (FRF)

A characteristic of a system that has a measured response resulting from a known applied input. In the case of a mechanical structure, the frequency response is the spectrum of the vibration of the structure divided by the spectrum of the input force to the system. To measure the frequency response of a mechanical system, one must measure the spectra of both the input force to the system and the vibration response, and this is most easily done with a dual-channel FFT Analyzer. Frequency response measurements are used extensively in modal analysis of mechanical systems. The frequency response function is actually a three-dimensional quantity, consisting of amplitude vs. phase vs. frequency. The so-called Bode plot consists of two curves, one of amplitude vs. frequency and one of phase vs. frequency. Another way to look at the frequency response function is to resolve the phase portion into two orthogonal components, one in-phase part (called the real part), and one part 90 degrees out of phase (called the imaginary part). / The amplitude and phase response characteristics of a system.

Frequency Response Curve

A graph of amplitude/phase vs. frequency indicating the response of a device to different frequencies. Frequency Response Functions (FRF) measurements are provided as part of all SignalCalc systems.

Frequency Response Matrix

For an N degree of freedom system, it is an N x N symmetrical matrix whose elements are the frequency response functions between the various points on the structure. Rows correspond to response points and columns to excitation points. For example, H23 is the frequency response with excitation at point 3 and response at point 2. The matrix is redundant, that is, by knowing any row or column, the other elements of the matrix can be computed.

Fundamental frequency

The number of hertz or cycles per second of the lowest-frequency component of a complex, cyclic motion. (See also Harmonic and Sub-harmonic.)

Frequency spectrum

A description of the resolution of any electrical signal into its frequency components, giving the amplitude (sometimes also phase) of each component.


The span of frequency covered in a spectrum; numerically equal to Lines multiplied by DF.

Fundamental Frequency

The lowest frequency component of a complex, cyclic signal.

Fundamental mode of vibration

That mode having the lowest natural frequency


Earthquake (Earth Quake)

An earthquake is a tremor of the earth’s surface usually triggered by the release of underground stress along fault lines. This release causes movement in masses of rock and resulting shock waves. A valuable tool for the analysis of earthquakes is Shock Response Analysis (SRS) which is provided as an option to SignalCalc systems and as a control option for the SignalStar controllers.

Eccentricity, mechanical

The variation of shaft surface radius when referenced to the shaft’s true geometric centerline. Also called Out-of-roundness.

Eccentricity Ratio

The vector difference between the bearing centerline and the average steady-state journal centerline.

Eddy current

Electrical current generated (and dissipated) in a conductive material (often a rotor shaft) when it intercepts the electromagnetic field of a displacement or proximity probe. / Electrical current which is generated (and dissipated) in a conductive material in the presence of an electromagnetic field.

Eddy Current Probe

A non-contact electrical device that measures the displacement of one surface relative to the tip of the probe. Construction consists of an electrical coil of various lengths and diameters. This coil located in the tip of the probe is energized producing an electrical field around the tip of the probe. When a conductive surface is placed in the field and the distance from the probe is noted, variations in this gap can be determined by the variations in the voltage flow to the probe tip.

Effective Mass

The frequency response function of force/acceleration.


The roots of the characteristic equation.

Eigenvalue Problem

The mathematical formulation and solution of the characteristic equation is called the Eigenvalue problem.


The mode shape vectors.

Electrical Runout

An error signal that occurs in eddy current displacement measurements when shaft surface conductivity varies.

Electromotive force

Force that causes the movement of electricity, such as potential difference of voltage. A measure of voltage in an electrical circuit.

Engineering Units (EU)

In a DSA, refers to units that are calibrated by the user (e.g., in/s, g’s). / Units that are decided upon by an individual user or by agreement among users. Examples include inches/second, mm/s, g, Hz, Tu, etc. / The units in which a measurement is made; for instance acceleration may be expressed in g, velocity may be expressed in milimeters per second.

External Sampling

In a DSA, refers to control of data sampling by a multiplied tachometer signal. Provides a stationary display of vibration with changing speed.


High resolution pulse generators.


The aggregate of all external and internal conditions (such as temperature, humidity, radiation, magnetic and electric fields, shock vibration, etc.) either natural or man made, or self-induced, that influences the form, performance, reliability or survival of an item.

Environmental engineering specialist

One whose principal work assignment lies in the technical area of natural and induced environments and their relation to military equipment. A person who has expertise in measuring and analyzing field environmental conditions, formulating environmental test criteria, specifying laboratory simulation of environments, and evaluating the effects of environments on equipment.

Environmental stress screening (ESS)

A post-production process in which 100% of produced units are subjected to stresses more severe than anticipated in service. The object is to precipitate latent defects into recognizable failures, so that that particular unit does not proceed further in production nor reach the customer.

A process in which products from the production line are subjected to thermal and / or vibration stresses to reduce the likelihood of early life field failures by forcing them to occur before final test in the factory.

Environmental testing

Subjecting a sample of products to a simulation of anticipated storage, transport and service environments (such as vibration, shock, temperature, altitude, humidity, etc.)

Equal Loudness Curves

Graphs of pure tone (constant or steady) sound pressure levels (labeled as to loudness level in phons) vs. frequency, with each graph representing equal loudness.


The difference between the indicated and the true values of a variable being measured. / The difference between the measured signal value or actual reading and the true (ideal) or desired value.

Essential machinery

See Critical Machinery.


The voltage or current applied to a transducer. / An external force (or other input) applied to a system that causes the system to respond in some way.

Exponential Averaging

A continuous averaging method wherein the most recent constituent is most influential and old information decays away exponentially with time.

Exponential Response Window

A windowing function for minimizing leakage in lightly damped structures. Typically used in Modal measurements made with an impact hammer. In a lightly damped structure, oscillations may not die out within the sampled time data block, Tspan, which results in leakage error. An exponential window adds damping to the time signal to force it to die out within the time T, thus minimizing leakage.


To create a file containing information in a format that can be used in a different program.

Export Folder

A folder, analogous to a Run folder, containing Signals in application-specific formats including ASCII, Standard Data Format (SDF), Universal File Format (UFF), ME Scope and SMS Star modal.

Extremal Control

A control strategy which selects the largest response at each frequency from the measured reference spectrum at control points to generate a combined spectrum to be used in the control loop.


D/A converter

A device that converts a digital signal (discrete values) into an analog voltage.

Damped Natural Frequency

The damped natural frequency is the frequency at which a damped system will oscillate in a free vibration situation.


Adheres or is sprayed onto relatively thin metal panels so as to extract energy when panels flex. Lowers “Q” of each panel mode. Goal: to reduce air low (over the panel)-induced vibration and radiated noise.


Dissipation of oscillatory or vibratory energy with motion or with time. Critical damping Cc is that value of damping that provides most rapid response to a step function without overshoot. Damping ratio is a fraction of Cc. / The quality of a mechanical system that restrains the amplitude of motion with each successive cycle. Damping of shaft motion is provided by oil in bearings, seals, etc. The damping process converts mechanical energy to other forms, usually heat. /  The suppression of oscillation. The viscosity of a fluid is used in viscous damping, while the induced current in electrical conductors is used to affect magnetic damping.

Damping, Critical.

The smallest amount of damping required to return the system to its equilibrium position without oscillation.

Damping Factor. (Damping Ratio)

The ratio of actual damping in a system to its critical damping

Dead band

The range through which an input can be changed without causing an observable response.


The interval between two frequencies which differ by exactly 10:1.

Decibel (dB)

Ratios of identical quantities are expressed in decibel or decibel or dB units. The number of dB is ratio against some standard or reference value in terms of the base 10 logarithm of that ratio. In measuring acoustic or vibration power (as in PSD or ASD of random vibration), the number of dB = 10 log10 P/Po. Po, the reference level, equals 0 dB. In measuring the more common voltage-like quantities such as acceleration, the number of dB = 20 log10 E/Eo Eo, the reference level, equals 0 dB.

A logarithmic representation of amplitude ratio, defined as 20 times the base ten logarithm of the ratio of the measured amplitude to a reference. DbV readings, for example, are referenced to 1 volt rms. Db amplitude scales are required to display the full dynamic range of a DSA.

The decibel is a unit which denotes the magnitude of a quantity with respect to an arbitrarily established reference value of the quantity, in terms of the logarithm (to the base 10) of the ratio of quantities. For example, in electrical transmission circuits, a value of power may be expressed in terms of a power level in decibels. Ratios of quantities can expressed in decibels ( dB). For acoustic or power quantities dB = 10 log10 P1/P2. P2 (the reference level), equals 0 dB. For voltage units such as acceleration, dB = 20 log10 V1/V2 V2, (the reference level), equals 0 dB.


A selection automatically used by the system in the absence of a choice made by the user.

Degrees of freedom (Mechanical)

In mechanics, the total number of directions of motion (of all the points being considered) on a structure being modeled or otherwise evaluated. Example: a platform experiencing heave, fore-and-aft, left-and-right motions as well as roll, pitch and yaw, is said to have six degrees of freedom. In random vibration measurement, twice the number of averages being used. / A phrase used in mechanical vibration to describe the complexity of the system. The number of degrees of freedom is the number of independent variables describing the state of a vibrating system.

The number of degrees of freedom of a mechanical system is equal to the minimum number of independent coordinates required to define completely the positions of all parts of the system t any instant of time. In general, it is equal to the number of independent displacements that are possible.

Degrees of Freedom (Statistical)

A statistical term expressing the confidence in an estimated measurement. In a random vibration signal, 1 Average = 2 DOF)


A measure of the degree to which an item is operable and capable of performing its required function at any (random) time during a specified mission profile, given item availability at the start of the mission. (This definition is significantly different from the definition of dependability used by most other US and international organizations dealing with reliability e.g., IEC and SAE. E.g. IEC 50 Chapter 191: “The collective term used to describe the availability performance and its influencing factors: reliability performance, maintainability performance and maintenance support performance.” As such, its use is restricted to general descriptions in non-quantitative terms).

Dependability is related to reliability; the intention was that dependability would be a more general concept then reliability.

Design limit

The operational limit of a product, beyond which it not required to function properly.

Design ruggedization

See Accelerated Life Testing.


A type of signal whose spectrum consists of a collection of discrete components, as opposed to a random signal, whose spectrum is spread out or “smeared” in frequency. Some deterministic signals are periodic, and their spectra consist of harmonic series. Vibration signatures of machines are in general deterministic, containing one or more harmonic series, but they always have non- deterministic components, such as background noise.

Deterministic vibration

A vibration whose instantaneous value at any future time can be predicted by an exact mathematical expression. Sinusoidal vibration is the classic example. Complex vibration is less simple (two or more sinusoids).

A vibration whose instantaneous value at any future time can be predicted by an exact mathematical expression. Sinusoidal vibration is the classic example. Complex vibration is less simple (two or more sinusoids).


Open low level network that provides connections between industrial devices

Dialog Box

A window that appears during the operation of the program that requires you to make a choice or enter information.

Differential Inputs

Two inputs, where the measured signal is the difference between them. Any voltage common to both is rejected. Differential inputs can reduce noise picked up by the signal leads. / Two inputs, where the measured signal is the difference between them. Any voltage common to both is rejected. Differential inputs can reduce noise picked up by the signal leads.


Representation in terms of time rate of change. Example: differentiating velocity yields acceleration. In a computer, this is accomplished by multiplying the velocity signal by jw, where w is frequency multiplied by 2p. / Representation in terms of time rate of change. For example, differentiating velocity yields acceleration. In a DSA, differentiation is performed by multiplication by jw, where w is frequency multiplied by 2þ. (Differentiation can also be used to convert displacement to velocity.)


Digital instrumentation consists of devices that convert analogue signals into a series of numbers through a sampling process and an analogue to digital converter. They then perform operations on the numbers to achieve such effects as equalization, data storage, data compression, frequency analysis, etc. This process in general is called digital signal processing. It is characterized by several advantages and disadvantages. One advantage is that the converted signals can be manipulated, transformed and copied without introducing any added noise or distortion. The disadvantage is that the signal representation may not be truly representative of the original signal.

Digital Converter

Converts an analog signal into a digital signal.

Digital Filter

A filter which acts on data after it has been sampled and digitized. Often used in DSAs to provide anti-aliasing protection after internal re-sampling.

Digital Sensor

A sensor that puts out a square wave signal.

Digital Spectrum Analyser

A spectrum analyzer is a device used to examine the spectral composition of some electrical, acoustic, or optical waveform. Often, it measures the power spectrum. A digital spectrum analyzer uses the Fast Fourier transform (FFT), a mathematical process that transforms a waveform into the components of its frequency spectrum. SignalCalc systems are an example of a modern high speed digital spectrum analyzer.

Digital Tachometer

A panel mountable meter with LED display, programmable for any engineering rate.

DIN Rail

Metal rail used in control cabinets to mount devices.


With reference to a spectrum, discrete means consisting of separate distinct points, rather than continuous. An example of a discrete spectrum is a harmonic series. An FFT spectrum, which consists of information only at specific frequencies (the FFT lines), is actually discrete regardless of the input signal. For instance, the true spectrum of a transient is continuous, and the FFT of a transient appears continuous on the screen, but still only contains information at the frequencies of the FFT lines. The input signal to an FFT Analyzer is continuous, but the sampling process necessary to implement the FFT algorithm converts it into a discrete form, with information only at the specific sampled times.

Discrete Fourier Transform

A procedure for calculating discrete frequency components (filters or lines) from sampled time data. Since the frequency domain result is complex (i.e., real and imaginary components), the number of points is equal to half the number of samples.

The mathematical calculation that converts, or “transforms” a sampled and digitized waveform into a sampled spectrum. The fast Fourier transform, or FFT, is an algorithm that allows a computer to calculate the discrete Fourier transform very quickly. See also Fast Fourier Transform.


The change in distance or position of an object relative to a reference. / Displacement specifies change of position, or distance, usually measured from mean position or position of rest. Usually applies to axial, less often to angular motion.

Displacement Transducer

A transducer whose output is proportional to the distance between it and the measured object (usually the shaft).

Distortion (Electrical)

In electronic measurements, distortion is any unwanted signal; e.g. amplifiers may generate unwanted signals.  (Usually expressed as Total Harmonic Distortion) the amount by which the signal deviates from its pure form.

Distortion (mechanical)

In mechanics, any unwanted motion. Structural response at harmonics or sub-harmonics of a forcing frequency. If sinusoidal motion were desired at a fundamental frequency, distortion is any motion at harmonics or sub-harmonics of that frequency, or any mechanical “hash” (perhaps due to parts colliding). Dither: A useful oscillation of small magnitude, introduced to overcome the effects of friction, Hysteresis, or clogging.

Distributed control system (DCS)

Typically a large-scale process control system characterized by a distributed network of processors and I/O subsystems that encompass the functions of control, user interface, data collection, and system management.


A domain is a set of coordinates in which a mathematical function resides. A waveform, for instance, has dimensions of amplitude and time, and it is said to exist in the time domain, while a spectrum has dimensions of amplitude and frequency, and is said to exist in the frequency domain.

Double Click

The act of pressing the left mouse button twice in rapid succession


Slow variation of a performance characteristic such as gain, frequency, or power output; for instance, due to temperature or aging. Usually, drift only is significant when measuring low-level signals (a few milli-volts) over long periods of time or in difficult environmental conditions. / Undesired change in the input-output relationship over a period of time.


See Dynamic Signal Analyzer.


Digital Signal Processor. A microprocessor optimized for digital signal manipulations.

Driving Point Measurement

A frequency response measurement where the excitation point and direction are the same as the response point and direction.

DT (Delta Time)

The time resolution (second); the time difference between adjacent signal samples.

Dual Probe

A transducer set consisting of displacement and velocity transducers. Combines measurement of shaft motion relative to the displacement transducer with velocity of the displacement transducer to produce absolute motion of the shaft.

Dual Voting

Concept where two independent inputs are required before action (usually machine shutdown) is taken. Most often used with axial position measurements, where failure of a single transducer might lead to an unnecessary shutdown.

Duration of a shock pulse is how long it lasts. For “classical” pulses, time is usually measured between instants when the amplitude is greater that 10% of the peak value.


A measure of useful life (a special case of reliability).


Device under test. See also UUT or unit under test (being tested).

Dynamic motion

Movement, as compared with nonmoving or static position. Dynamic motion is sensed with displacement or velocity pickups or with accelerometers. / Vibratory motion of a rotor system caused by mechanisms that are active only when the rotor is turning at speeds above slow roll speed.

Dynamic Range

The ratio of a specified maximum level of a parameter, such as power, current, voltage, or frequency, to the minimum detectable value of that parameter. / Ratio of the largest to smallest signal level a circuit can handle, normally expressed in dB.

Dynamic Signal Analyzer (DSA)

Vibration analyzer using digital signal processing and the Fast Fourier Transform (FFT) to display vibration frequency components. May also display the time domain and the phase spectrum, and can  usually interfaced to a computer.

Dynamic Stiffness

The frequency response function of force/displacement.



An orderly procedure or test for determining the sensitivity of a transducer or instrument as a function of frequency, temperature, speed etc. Known values of the measured variable are applied to the transducer or instrument, and output readings are varied or adjusted. During the process, values are compiled, and deviation charts are produced, so that the reading can be correlated to the actual values being measured.


To ascertain that the output of a device properly corresponds to the information it is measuring, receiving or transmitting. This might involve the location of scale graduations, adjustment to bring the output within specified tolerance or ascertaining the error by comparing the output to a reference standard.

Calibration curve

A graphical representation of the calibration report, which report can be in the form of a table or chart.

Calibration cycle

The application of known values of the measured variable and the recording of the corresponding output readings over the range of the instrument in both ascending and descending directions.

Calibration traceability

The relationship of the calibration process to the calibration steps performed by a national standardizing laboratory.

Campbell diagram

A mathematically-constructed diagram used to check for coincidences of vibration sources (1x imbalance, 2x misalignment, etc. shaft speed) with rotor natural frequencies, resulting in rotor resonances. It plots frequency vs. RPM, with plot size growing with increasing amplitude. Sometimes called an interference diagram.

The form of the diagram is a rectangular plot of resonant frequency (y-axis) vs. excitation frequency (x-axis). Also known as an interference diagram.

Cascade Plot

See Spectral Map.


A localized low-pressure condition which can occur in liquid handling machinery (e.g. centrifugal pumps pr compressors)  where a system pressure decrease in the suction line or in the pump inlet line, lowers the fluid pressure, and vaporization occurs (cavities or bubbles) within the liquid, for example around a propeller or impeller. As the bubbles collapse, the resulting mixed flow may produce unwanted sound and/or vibration. May be destructive.

Center Frequency

For a band-pass filter, the center of the transmission band.


The forward Fourier transform of a spectrum. (the spectrum of a spectrum), It is useful in many types of signal analysis. Periodicities, or repeated patterns, in a spectrum will be sensed as one or two specific components in the Cepstrum. If a spectrum contains several sets of sidebands or harmonic series, they can be confusing because of overlap. But in the Cepstrum, they will be separated in a way similar to the way the spectrum separates repetitive time patterns in the waveform. Gearboxes and rolling element bearing vibrations lend themselves especially well to Cepstrum analysis.


A sensor (or pickup or transducer) with its associated signal conditioner and monitor (for observing the signal) and recorder (for storing the signal).

Channel Parameters

A control group affecting Input channel settings and Signal Generator outputs.

Characterization is aimed at providing the shaker control system with needed information about the testing system and the test article. Pretest characterization (at reduced intensity), repeated several times – iteration – is commonly performed. Increasingly, adaptive characterization – modifying the control system, based upon observing responses, is being performed during the test.

Charge amplifier

An amplifier which converts a very high impedance charge signal (as from a piezoelectric charge coupled accelerometer) into a low impedance output voltage i.e. a charge-to-voltage converter. This makes calibration much less dependent on cable capacitance.

Circuit card / Circuit Board

A flat board that holds chips and other components on the top side and has printed electrically conductive paths in multiple layers for the components on its bottom side.

Closed Loop Control

Responses are measured and fed back to the control system so as to refine or modify drive signals in order to bring responses closer to the reference or desired motions. See iterative closed loop control.

Closed loop motor controller

PID Controller used for precise control of multiple motor.

Coefficient of Thermal Expansion

The constant value or factor of expansion of a material for a given increase in temperature, divided by the length of the material. This is different for each material.


An effective means of measuring the similarity of vibration at two locations, giving insight into possible cause and effect relations.It is the ratio of the coherent output power between channels in a dual-channel Digital Signal Analyzer (DSA).


Another name for the real part of the frequency response function.

Common mode rejection

The ability of a circuit to discriminate against a common mode voltage.

Common mode voltage

A voltage of the same polarity on both sides of a differential input relative to ground.


A term applied to calibration (e.g. of an accelerometer) in which sensitivity is tested against a standard. See also Absolute Calibration.


The reciprocal of stiffness, i.e. displacement divided by force.

Condition monitoring (CM)

The measurement, recording and analysis of machinery parameters (such as acceleration) to determine machinery health.

The current machine condition or signature, is compared with earlier condition or fingerprint, usually taken when a machine was new or newly refurbished. It is used as an indicator for detecting potential failure before it occurs. Also called Machinery Health Monitoring.

Constant bandwidth filter

A band-pass filter whose bandwidth is independent of center frequency. Filters simulated digitally by an FFT process are constant bandwidth.

Constant percentage filter / Constant percentage bandwidth

A band-pass filter whose bandwidth is a constant percentage of the center frequency of that same filter (¼ x, ½ x etc).

Contact Bounce

The intermittent opening of relay contacts during closure.

Control Potentiometers

Multi-turn potentiometers with programmable display.


A Test type providing Auto and Cross Correlation functions to measure cause/effect in the time domain


Device used to totalize production volumes

Critical Damping

The smallest amount of damping required to return a system to its equilibrium condition without oscillating.

Critical frequency

A particular resonant frequency (see resonance) at which damage or degradation in performance is likely.

Critical machines

Machines in a process or power plant which are vital to continued operation. These machines are usually un-spared. Machine Health should be monitored continuously.

Critical speeds

Any rotating speed which results in high vibration amplitudes. Often these are speeds which correspond to system natural frequencies.

Critical Speed Map

A rectangular plot of system natural frequency (y-axis) versus bearing or support stiffness (x-axis).

Cross-axis sensitivity

A measure of off-axis response of velocity and acceleration transducers (see also Transverse Sensitivity/)

Crossover frequency

In sinusoidal vibration testing, the unique forcing frequency at which the required displacement yields the desired acceleration and vice versa.


Interference or noise in a sensor or channel, coming from another sensor or channel.


One complete sequence of values of a periodic quantity – for exampl,e a sine wave processing from zero to 360°.



A device, usually filled with foam or fiber, which is used to block noise from entering or exiting an confined space.

Balancing (mechanical)

A procedure for adjusting the radial mass distribution of a rotor so that the mass centerline approaches the rotor geometric centerline, thereby reducing the vibratory forces generated by rotation.

Balancing Resonance Speed(s)

A rotative speed that corresponds to a natural resonance frequency.

Balanced Condition

For rotating machinery, a condition where the shaft geometric centerline coincides with the mass centerline.

Band-Pass Filter

A filter with a single transmission band extending from lower to upper cutoff frequencies. The width of the band-pass filter (band width) is determined by the separation of frequencies at which amplitude is attenuated by 3 dB (0.707).

Bandwidth (Measurement)

The frequency range (usually stated in hertz or Hz) within which a measuring system can accurately measure an amplitude quantity.

Bandwidth (Digital Filters)

The spacing between frequencies at which a band-pass filter attenuates the signal by 3 dB. In a digital signal analyzer, the measurement bandwidth is equal to [(frequency span)/(number of filters) x (window factor)]. Window factors are: 1 for uniform , 1.5 for Hanning, and 3.63 for flat top (Uniform, Hanning and Flat Top are digital filter shapes).

Baseline spectrum

A vibration spectrum taken when a machine is in good working condition (new or just overhauled), which is then used as reference for future monitoring or analysis.

Bearing (rolling Element)

A Rolling element bearing has four parts: an inner race, an outer race, balls or rollers, and a cage to maintain the proper separation of the rolling elements.

Bearing (sleeve)

A sleeve bearing is a cylinder of alloy metal surrounding the rotating shaft. Contact between the shaft and sleeve is prevented by a lubrication film.

Beat Frequency

Where two cyclic components are close together in frequency they combine in such a way that their sum will vary in amplitude at a rate equal to the difference in frequency between the two components. This phenomenon is known as beating, and its frequency is known as the beat frequency.


Refers to a more or less persistent tendency for the measurements, as a group, to be too large or too small.

Biased Hall Effect Sensor

Sensor that operates with either magnets or ferrous metal targets, Generates a square wave output.

Bins (lines)

In an FFT spectrum, the individual frequencies at which the amplitudes are calculated.


Short for binary digit. A number expressed in binary notation utilizes the digits 1 and 0, and these are called bits. Any number can be expressed with combinations of them.

Blade Passing Frequency

A potential vibration frequency on any bladed machine (turbine, axial compressor, fan, etc.). It is represented by the number of blades times shaft-rotating frequency.

Block Size

The number of samples used in a Digital Signal Aanalyser (DSA) to compute the Fast Fourier Transform (FFT). Also the number of samples in a DSA time display. Most DSAs use a block size of 1024. Smaller block size reduces frequency resolution.


Rectangular coordinate plot of 1x component amplitude and phase (relative to a keyphasor) vs. running speed / Magnitude and phase of vibration in a machine plotted against speed.


Vibration (or other) signals which are unfiltered. Signals at all frequencies contribute to the measured value.

Bode plot

The magnitude of vibration at 1x shaft speed, also its phase relative to the key phaser, both plotted against running speed.


In Rotating Machinery. A shaft condition in which the geometric shaft centerline is not straight.Can be caused due to inadequate baring during cool down.


Common abbreviations for ball pass frequency of defects on outer and inner bearing races, respectively.


Impressions made by bearing rolling elements on the bearing race; typically caused by external vibration when the shaft is stationary.


Vibration (or other) signals which are unfiltered. Signals at all frequencies contribute to the measured value.


A memory location in a computer or digital instrument that is set aside for temporarily storing digital information while it is waiting to be processed.


A sound exemplified by loose power transformer laminations (dominated by 120 Hz where the power frequency is 60 Hz).



A term applied to calibration (e.g. of an accelerometer) based upon the primary standards of mass, length and time.

Absolute Accuracy

A measure of the uncertainty of an instrument reading compared to that of a primary national traceable standard.

Absolute Displacement

Displacement of an object relative to a fixed point in space. A signal from an accelerometer integrated to provide a displacement signal, provides an indication of absolute displacement. By contrast, an eddy current probe fastened to a housing, measuring displacement on a shaft is measuring relative displacement.

Absolute Vibration

Vibration of an object relative to a fixed point in space. Seismic sensors (accelerometers and velocity pickups) measure absolute vibration. Contrasts with relative vibration as measured by Eddy Current Probes.


A device capable of soaking up vibration.


Acceleration is rate of change of velocity with time (denoted as the first derivate of velocity – δv/δt or the second derivative of displacement – δ2x/δt2), and is usually specified along an axis, usually expressed in g or gravitational units (1g = 32.17 ft/s/s = 9.81 m/s/s), but may also be seen as ft/s/s or meters/s/s. It may also refer to angular motion usually expressed in radians. Acceleration measurements are usually made with accelerometers.


A sensor or transducer whose output is directly proportional to acceleration, used for converting mechanical movement into an electrical signal. Two common types are piezoresistive and piezoelectric. Most commonly used is piezoelectric crystals to produce output.


The capability of an instrument to indicate the true value. Do not confuse with inaccuracy (sum of Hysteresis + non-linearity + drift + temperature effect, etc.) nor with repeatability.

A/D Converter / ADC

Analo to Digital COnverter. A device that changes an analog signal such as voltage or current into a digital signal (consists of discrete data values).


A spectrum analysis problem resulting from sampling broadband data at too low a frequency. According to Nyquist, in a sampled data system, the analog input must be sampled at a rate of at least twice the maximum frequency component within the sampled signal, to avoid loss of data and to avoid aliasing. At sampling frequencies of less than this, spurious frequencies appear within the resultant spectrum, manifesting as low frequencies. In digital signal analyzers, aliasing is avoided by applying very steep anti-aliasing filters (low pass) with corner frequencies at 1/2 the sample rate.


A desired machinery condition, in which the axes of components of a machine are adjusted so as to be co-linear, parallel or perpendicular.

Ambient environment / Ambient Conditions

The conditions (e.g. temperature and humidity) characterizing the air or other medium that surrounds materiel.

Amplification Factor (Q) (Synchronous)

A measure of the sharpness of a resonance frequency or of susceptibility of a rotor to vibration amplitude when rotational speed is equal to the rotor natural frequency (implies a flexible rotor). For imbalance type excitation, synchronous amplification factor is calculated by dividing the amplitude value at the resonant peak by the amplitude value at a speed well above resonance (as determined from a plot of synchronous response vs. rpm).


The magnitude of dynamic motion or vibration; the y-axis of the vibration time waveform; the maximum value of a quantity; the measurement of energy or movement in a vibrating object. Amplitude is usually expressed in terms of peak-to-peak, zero-to-peak, or rms. For pure sine waves only, these are related as follows: rms = 0.707 x zero-to-peak; peak-to-peak = 2 x zero-to-peak.


Relating to a mechanism in which data is represented by continuously variable physical quantities. Quantities in two separate physical systems having consistently similar relationships to each other are called analogous. One is then called the analogue of the other. The electrical output of a transducer is an analogue of the vibration input of the transducer as long as the transducer is not operated in the non-linear (overloaded) range. This is in contrast to a digital representation of the vibration signal, which is a sampled and quantized signal consisting of a series of numbers, usually in binary notation.

Analog to Digital Conversion

The process of sampling an analogue signal produces a series of numbers, which is the digital representation of the same signal. The sampling frequency must be at least twice as high as the highest frequency present in the signal to prevent aliasing errors.

Analog Sensor

A sensor that puts out a voltage or current

Analog Tachometer

Panel Meter with needle indicator

Anti-aliasing filter

A low pass filter designed to stop frequencies higher than the ½ the sample rate, in order to minimize aliasing.


An acronym that stands for American Standard Code for Information Interchange. ASCII defines a standard for representing characters on computers.

Asymmetrical Support

Rotor support system that does not provide uniform restraint in all radial directions. This is typical for most heavy industrial machinery where stiffness in one plane may be substantially different than stiffness in the perpendicular plane. Occurs in bearings by design, or from preloads such as gravity or misalignment.


Vibration components that are not related to rotating speed (also referred to as non-synchronous).




The capability of an instrument to switch among ranges automatically. Ranges usually are in decade steps.


In the exclusive case of a pure sine wave, the average value is 0.636 x peak value.


Summing and suitably dividing several similar measurements to improve accuracy or to lessen any asynchronous components. Refer to definitions of RMS, time, and peak-hold averaging.


In the same direction as the shaft centre line.

Axial (thrust) Position

The average position, or change in position, of a rotor in the axial direction with respect to some fixed reference position. Ideally the reference is a known position within the thrust bearing axial clearance or float zone, and the measurement is made with a displacement transducer observing the thrust collar.


In this section, we have provided a Turbine Supervisory Glossary of terms and expressions used along with their description.

We are always willing to include additional terms so please let us know if we have missed anything which yoi feel should be included.

Calibration and Repair

As well as providing a single source for all your transducer and instrumentation needs, Zeefax also provides a range of support and after sales services aimed at maintaining the health and integrity of  Machine Health Monitoring and Turbine Supervisory systems.

These services are not only available to existing customers to whom we may have sold complete systems, but also to any instrumentation user who may require support and assistance from time to time with eddy probe and sensor calibration, parameter setting, fault finding and replacement.

We believe that the most important parts of any system are the transducers, and that they should be maintained and checked periodically to ensure continued good calibration and operation.

To this end, Zeefax provide a periodic re-calibration, repair and replacement service, intended to ensure that the most important part of any system – the probes – remain in good working order.

Outage Support

The University of Life teaches us that, ironically, the most important things to do are often the hardest things to do – just try asking any politician about this! Similarly, experience in the field of working on turbine supervisory instrumentation projects, often shows that it is the most important parameters to be measured which are the hardest to get right. Even more strange is that these parameters are at best, given scant consideration, and at worst, they are overlooked altogether until the last moment, just before the turbine is being closed up, or just as it is about to be run, when it is often then too late.

Thrust, Differential Expansion and Case Expansion can have such an impact on the efficient operation and on the general health of the equipment being monitored that it is clearly an anomaly that these parameters are probably the least understood of the TSI parameters. The reasons for this are probably many, not least that the other main parameters of Vibration, Temperature and Valve Position are easier to set up, and are well established and documented. Other historical and anecdotal evidence points to a general scarcity of knowledge and understanding as to the benefits and pitfalls associated with measuring these parameters.

Lack of ownership of the supervisory system is often cited as the reason for this, with no clear responsibility being taken either for the collation of relevant information or for making appropriate allowance in the outage schedule. This is particularly the case for the differential expansion, the setting and checking of which is necessarily time consuming and often requires mechanical and electrical support from outage engineers.

This paper is an attempt to clarify the requirements and to highlight the pitfalls associated with measuring Thrust, Differential Expansion and Case Expansion, which will, it is hoped, lead to a more proactive approach towards them, thereby ensuring that they are adequately provided for in any project schedule.

Mounting Hardware

In supervisory systems provided by Zeefax, it is common for Sensor Mounting Hardware to be designed and manufactured specifically to meet the needs of the particular installation.

This is done to ensure that any mounting hardware supplied by us fully satisfies the needs of each project, but it should also be self evident that a series of standard designs and techniques are used or modified to complete the task.

Single and Dual Probe Mounting Hardware

The single sensor version of the probe holder will typically support a non-contact eddy current probe.

The probe is threaded into the variable length support tube – commonly called a stinger – which is inserted deep into the machine so that the probe tip is held close to and can observe the surface of the rotating shaft. The stinger itself is threaded through a base plate, which is held above the machine surface by an adaptor or spacer ring. The probe/stinger/base plate assembly is carefully mounted onto the machine first – taking care not to touch any part of the machine with the probe tip – and the probe is gaped and locked into place using the lock nuts provided.

Externally, there is a can or cover piece which encloses and protects any internal wiring and probe connections, and an threaded adapter is provided to facilitate conduit attachment and termination if needed. When probe set-up is complete, the body is fixed in place and the top cover seals the unit.

In some situations, and particularly when retro fitting new probes onto older machines where shaft riders or other transducers may have been removed, spot faces or flat surfaces may already be present on the bearing or machine case. In this eventuality, additional adapter pieces may be needed, and these can be designed to suit by Zeefax.

However in some cases it may be prudent to make use of existing shaft rider support tubes and assemblies to minimize any additional machining re-work required on and inside bearing cases for example. In this case, the existing components of the shaft rider assembly may be be re-engineered to accept any new probe stingers or holders which may be needed to complete the installation, and once again Zeefax can provide the mechanical design and engineering support to complete these types of modifications.

In a modified shaft rider assembly, the stinger is typically held in place using a pair of lock nuts intended to maintain the probe-to-shaft gap, and ‘O’ rings can be designed in to provide seals against the oil pressure inside the bearing case. Usually, the original top covers can be used with re-engineered base plates to provide conduit attachment points.

For installations requiring absolute vibration measurement, or where two sensors are required for other purposes, a dual probe holder will normally be fitted.

The dual probe holder provides a single housing for a non-contact eddy current probe and a seismic transducer – either a velometer or an accelerometer. In this way, both sensors can be installed at the same point on the bearing or machine case, enabling relative, seismic and correct absolute vibration measurements to be made if desired. The seismic transducer typically fits directly to the base plate, threaded onto a flat spot face to ensure excellent vibration transmission to the sensing element.

The VM600 online vibration monitoring systems typically used by Zeefax includes features which can combine and correctly phase match the two signals – one displacement and one either velocity or acceleration – thereby giving a measure of the absolute vibration of the monitored shaft within free space.

As before, the dual probe holder mounts directly onto the machine case using a variable length adapter tube if necessary, and provides mechanical protection for the probes and connections as well as a device for connecting conduit in the same way as for the single probe holder.

Single or Dual Probe holders are available to suit almost any installation, and are often fully customized to suit each particular application, depending on height restrictions, mounting arrangements, insertion length, cable conduit types etc. Zeefax provide a design, build, calibrate and installation service for supervisory transducers, as well as periodic re-calibration, repair and replacement service to ensure that the most important part of any system – the probes – remain in calibration and in good working order. To compliment any sensor installation, fixtures, junction boxes and equipment cabinets are often required, to mount and mechanically protect drivers and modules and to ensure that the components stand up to the rigors each installation. Zeefax will design and build all parts of the system to ensure compatibility, reliability and trouble free operation.

Brackets, adapters and miscellaneous sensor mounting hardware

Based on a detailed survey of locations, available access and parameters to be monitored, the selection and mounting of sensors is a critical part of any system design, and careful attention to detail will result in well mounted sensors providing good quality data at the right time and at the right price.

Sensors and Instruments

transducers 1

The quality of a monitoring system lies in the accuracy and reliability of the measurements, and at Zeefax, we believe that you only ever get quality data out of a system if you put quality signals in to a system. Therefore, selection of the right transducer or sensor is paramount to the success of a system.

Reliability Begins with Accurate Signals

We supply the best Transducer and Sensor solutions, carefully selected to fit each application and intended to measure almost any type of machinery parameter, such as vibration, position, rotational speed, temperature, casing growth, rotor bow, valve position etc.

Transducer systems supplied by Zeefax will typically include all of the items necessary to complete the installation.

We can supply:-

  • Transducers and Sensors, including such as
    • Thermocouples
    • RTDs
    • Proximity Probes / Eddy Current Probes
    • Velocity Transducers
    • Accelerometers
    • Valve Position Transducers / LVDTs
    • Case Expansion Transducers
    • Dynamic Power Transducers
  • Cabling Systems
    • Customized cabling and connection systems, either standard or designed and manufactured by Zeefax to meet the specific needs and wants of the actual installation.
  • Junction and Marshaling boxes
    • Designed and built to meet the needs of the system, typically enclosing termination points, power supplies and other electronic modules needed for the system to operate correctly and reliably.
  • Signal Conditioners
    • Depending on the selection of sensors used and on the environment into which the components will be installed, additional signal conditioning, galvanic isolation or monitoring instrumentation may be included.
Custom Software Development

We have taken on the challenge of seamlessly combining diverse off-the-shelf products, often ‘glued’ together with in-house developed and customized software and hardware products, to provide fully integrated and guaranteed solution based systems.

We have been providing data acquisition solutions for more than 20 years, and in that time, we have created a wide range of software tools and products. By using the latest software development tools and adhering to the newest communication and data sharing techniques and specifications, we can ensure that systems provided by us yield the right data, in the right format, in good time.

As well as custom software development, our many years of experience in the industrial, process and power environments has resulted in a series of software products, spearheaded by our flagship data acquisition product – PowerPAC™.


35951C/51C Reed Relay IMP/IMC

In this simple concept, plant parameters are measured by intelligent, multi-channel modules, called Isolated Measurement Pods (IMPs) or rack mounted mulit-channel cards called Isolated Measurement Cards (IMCs); these are all linked together by a low cost twisted pair communications network called S-Net. The cable is connected through a special interface to the host computer, which has data acquisition software to configure and control the IMPs/IMCs and to store and display the data.

A wide range of precise process parameter measurements can be made using built-in sensor energisation, integral signal conditioning and a 16 bit ADC; signal noise is kept to an absolute minimum by using selectable integration times, and drift correction is automatically applied between scans.

The 35951C Isolated Measurement Pods (IMP) and the 359551C Isolated Measurement Cards (IMC) both use reed-relay switching, which makes them ideal for applications requiring high inter-channel isolation; they are capable of measuring up to 250 volts with the optional high voltage connector.

The IMP is housed in a NEMA4/IP55 environmentally sealed package which can be installed directly onto the plant or machinery being monitored, making installation very cost effective, and data and power are supplied from the single twisted pair cable, which can be up to 1500 metres in length. The IMC is designed for use where very high density measurement is required, and up to 14 cards may be housed in a single 5U Eurorack – then yielding up to 280 channels per rack; the rack is in turn housed in and supported by a secondary cabinet or enclosure. Both of these devices us a multi-channel removable connector block which makes installation and exchange simple.