Condition Monitoring Systems


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.