Condition Monitoring Systems



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.