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This
glossary defines relevant engineering terms and user interface
terms for Data Physics products. |
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A
A/D Converter |
A
device that changes an analogue signal into a digital signal |
| Absolute Accuracy |
The
accuracy 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. |
| Accelerance (Inertance) |
Acceleration
divided by force. |
| Accelerated Life
Testing |
An
activity during development of a new product. Prototypes are subjected
to stress levels (including vibration, usually random) that are
much higher than those anticipated in the field. The purpose is
to identify failure-prone, marginally-strong elements by causing
them to fail. Those elements are strengthened and tests are continued
at higher levels. Sometimes called Test, Analyze & Fix (TAAF)
testing. |
| Accelerated Stress
Testing |
A
post-production activity on a sampling (100% at first) of units.
The intent is to precipitate hidden or latent failures caused by
poor workmanship and to prevent flawed units from reaching the next
higher level of assembly or the customer. Intensity is typically
half that achieved in accelerated life testing. |
| Acceleration |
The
rate of change of velocity with time |
| Accelerometer |
A
transducer for measuring vibration in acceleration units. |
| Accuracy |
The
degree of closeness to a true value. Usually expressed as a percentage
or ratio relative to the reference value. |
| Active Redundancy |
Redundancy
in which all redundant items operate simultaneously. |
| ADC Indicators |
In
SignalCalc Dynamic Signal Analyzers: A control group that sets and
indicates the Full Scale range of each Input Channel in Volts (zero-to-peak). |
| Aggravated Test |
A
test in which one or more conditions are set at a more stressful
level that the test item will encounter in the field, in order to
reduce test time or assure a margin of safety. |
| Algorithm |
A
specific procedure for solving mathematical problems. An FFT is
an algorithm. |
| Aliasing |
A
phenomenon caused by sampling analogue data at too-low a frequency.
It results in a digital reconstruction of the original signal at
a false, lower frequency. This causes higher frequency signals to
appear in a spectrum at lower frequencies (Aliasing terms). When
analogue signals are digitized, the analogue input must be sampled
at a rate at least twice the bandwidth of the signal to avoid loss
of data (Nyquist Theorem). |
| Alignment |
A
procedure in which the axes of components of a machine are adjusted
so as to be perfectly true with each other. |
| Ambient environment |
The
surrounding conditions (e.g. temperature and humidity) |
| Amplification Factor
(Q) |
(Q)
(Sharpness of resonance) A measure of the sharpness of a resonance
or frequency. The mechanical gain at resonance. |
| Amplitude |
The
y-axis of the vibration time waveform. The maximum value of a quantity.
The measurement of energy or movement in a vibrating object. |
| Analogue |
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.
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| Analogue 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. |
| Angular rate sensor |
A
sensor that measures rotational velocity (degrees or radians per
second) around its sensitive axis. |
| Anti-aliasing filter |
A
low pass filter designed to stop frequencies higher than the required
Fspan |
| Append |
In
SignalCalc Dynamic Signal Analyzers: To add additional measurements
to an exiting Run or Export folder. During an Append, signal processing
controls are locked to match the initial data. |
| ASCII |
An
acronym that stands for American Standard Code for Information Interchange.
ASCII defines a standard for representing characters on computers. |
| Attenuate |
Reduce. |
| Auto Correlation |
Auto
correlation is a time-domain function that is a measure of how much
a signal shape, or waveform, resembles a delayed version of itself.
It is closely related to the Cepstrum, q.v. The numerical value
of auto correlation can vary between zero and one. A periodic signal,
such as a sine wave has an auto correlation that is equal to one
at zero time delay, zero at a time delay of one-half the period
of the wave, and one at a time delay of one period; in other words,
it is a sinusoidal waveform itself. Random noise has an auto correlation
of one at zero delay, but is essentially zero at all other delays.
Auto correlation is sometimes used to extract periodic signals from
noise. Certain dual-channel FFT Analyzers are able to measure auto
correlation. |
| Auto Power Spectrum |
A
Test type providing single channel time/frequency functions including
frequency domain power averages. |
| Auto Spectral Density
(ASD) |
The
measure of acceleration per Hz of analysis bandwidth. See Power
Spectral Density (PSD). The area under ASD curve is defined as the
grms of acceleration. |
| Auto spectrum (power
spectrum) |
A
spectral display of the power (voltage squared) at each frequency.
Phase is ignored. |
| Auto ranging |
The
capability of an instrument to switch among ranges automatically. |
| Average Responding |
A
measurement proportional to the average of the absolute values of
all input waveforms within a specified frequency range. |
| Averaging |
Summing
and dividing several like measurements to improve accuracy or to
reduce asynchronous components. |
| A-weighting |
In
acoustic analysis, a filter to approximately compensate for the
non-flat frequency response of human hearing, in order to get numbers
approximating human response. See Equal Loudness Curves. |
| Axial |
The
axis along the centerline of a shaft. |
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B
Balancing |
For
a mechanical system: Adjusting the distribution of mass in a rotating
element, to reduce vibratory forces generated by rotation. |
| Band-pass filter |
A
wave filter that has a single transmission band extending from a
lower cut-off frequency greater than zero to a finite upper cut-off
frequency. |
| Bandwidth |
The
frequency range selected or available for measurements. |
| Baseline spectrum |
A
vibration spectrum taken when a machine is in good working condition
(new or just overhauled), 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 the beat frequency. |
| Bins (Lines) |
In
an FFT spectrum, the individual frequencies at which the amplitudes
are calculated |
| Bit |
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 |
The
frequency on a bladed machine such as a turbine or fan at which
blades pass a given point. (number of blades multiplied by shaft
speed) |
| Block Size |
The
(binary) number of time samples acquired in each frame or capture
window |
| Bode plot |
Magnitude
and phase of vibration in a machine plotted against speed. |
| Bow |
A
shaft condition (rotating machinery) in which the shaft centre line
is not straight. |
| Breakpoint |
In
vibration control: One of the frequencies at which amplitude is
defined in order to specify the shape of the reference spectrum. |
| Broadband |
Vibration
(or other) signals which are unfiltered. Signals at all frequencies
contribute to the measured value. |
| Buffer |
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. |
| Bump Test: |
Repeated
shock applied to a product using a bump test machine or Shaker system. |
| Burn-in |
To
assist in preventing early life field failures, electronic products
are continuously powering a product, often at constant elevated
temperature for a specified period before shipment. |
| Burst Random Signal |
A
burst of (true) Random signal gated to be less than Tspan
in duration, useful as a stimulus for Modal Testing. |
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C
Campbell diagram |
(Interference
diagram) A form of display used in machine analysis to show response
amplitude as a function of speed for rotational orders. |
| Cavitation |
A
localized low-pressure vaporization condition (cavities or bubbles)
within a liquid, as around a propeller or within a pipe or in a
pump's suction line. Bubble collapse creates unwanted sound and/or
vibration. May be destructive. |
| Cepstrum |
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. |
| Channel Parameters |
In
SignalCalc Dynamic Signal Analyzers: A control group affecting Input
channel settings and Signal Generator outputs. |
| Charge amplifier |
An
amplifier used to condition piezo electric charge coupled transducers.
The charge input signal is converted into an output voltage. |
| Chirp Signal |
A
low crest factor transient used for (selected) broad-band testing.
A rapid frequency sweep between two limits, accomplished in less
than Tspan |
| Circle Fit |
A
single-degree of freedom curve fitting routine that tries to fit
a mode to a circle (Nyquist plot of a single-degree of freedom system).
The modal coefficient is determined by the diameter of the circle
and the phase by its location relative to the imaginary axis. For
a real mode, it should be either completely above or completely
below the imaginary axis. |
| Clipboard |
A
temporary storage area used by Windows programs for holding text
or graphics. |
| 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. |
| 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. |
| Coherence |
A
measure of the similarity of vibration at two locations. |
| Co-Incident |
Another
name for the real part of the frequency response function. |
| Complex Modes |
The
points on a structure which have varying phase relationships between
them at a natural frequency. This is unlike a real mode where the
phase between points is either 0° or 180°. |
| Compliance |
The
reciprocal of stiffness, (displacement divided by force). |
| Condition monitoring
(CM) |
The
current machine condition (signature) compared with an earlier condition
used as an indicator to detecting potential failure before it occurs.
Also called machinery health monitoring. |
| Confidence level |
Defines
the degree of spread of a series of measurements around the true
value. For example, one may estimate with 90% confidence (confidence
level) that an averaged power spectral density estimate is within
2 dB (the confidence interval) of the true power spectral density
function. |
| Constant percentage
filter |
A
band-pass filter whose bandwidth is maintained at a constant proportion
to the centre frequency. |
| Constant-bandwidth
filter |
A
band-pass filter whose bandwidth is independent of centre frequency.
In an FFT analysis the effective filters are constant bandwidth.
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| Control Panel |
In
SignalCalc Dynamic Signal Analyzers: An area on the user interface
screen consisting of test information and command buttons for test
operation. |
| Correlation |
A
Test type providing Auto and Cross Correlation functions to measure
cause/effect in the time domain. |
| Critical Damping |
The
smallest amount of damping required to return a system to its equilibrium
condition without oscillating. |
| Critical frequency |
A
resonant frequency at which damage or degradation in performance
is likely. |
| Critical speeds |
Any
rotating speed which results in high vibration amplitudes. Often
these are speeds which correspond to system natural frequencies.
See Campbell Diagram. |
| Cross Correlation |
Cross
correlation is a measure of the similarity in two time domain signals.
If the signals are identical, the cross correlation will be one,
and if they are completely dissimilar, the cross correlation will
be zero. |
| Cross-axis sensitivity |
Sensitivity
of a transducer in a direction perpendicular to the normal measurement
axis. |
| Crossover frequency |
In
a swept sine vibration test, the frequency at which the units of
displacement, velocity or acceleration yields the same test level. |
| Cross-talk |
Pickup
in one channel coming from another channel. |
| Cycle |
The
complete sequence of values of a periodic quantity that occur during
a period. |
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D
D/A converter |
A
device that converts a digital signal into an analogue signal. |
| Damped Natural
Frequency |
The
damped natural frequency is the frequency at which a damped system
will oscillate in a free vibration situation. |
| Damping |
The
dissipation of energy with time or distance. |
| Damping Factor |
(Damping
Ratio) The ratio of actual damping in a system to its critical damping |
| Decade |
The
interval between two frequencies which differ by exactly 10:1. |
| DeciBel (dB) |
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. |
| Default |
In
SignalCalc Dynamic Signal Analyzers: A selection automatically used
by the system in the absence of a choice made by the user. |
| Default Folder |
In
SignalCalc Dynamic Signal Analyzers: A directory of "starting point"
signal processing Setup and graphical layout files initially copied
into a New Test folder. |
| Degrees of Freedom
(Mechanical) |
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 signal 1 Average = 2 DOF) |
| Delimited |
A
character or combination of characters used to separate one item
or set of data from another. For example, in comma delimited records,
a comma is used to separate each field of data. |
| Design limit |
The
operational limit of a product, beyond which it not required to
function properly. |
| Detector |
An
electronic circuit that determines the amplitude level of a signal
in accordance with certain rules. The simplest type of detector
consists of a resistor and a capacitor, and it measures the average
value of a fluctuating DC signal. A more complex but much more useful
type of detector is an RMS detector. RMS detectors are used because
they are proportional to the power or energy present in the signal
or a vibration. |
| Deterministic |
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). |
| DF (Delta Frequency) |
The
nominal frequency resolution (Hz) of a spectrum; the difference
between adjacent frequency points; numerically equal to 1/Tspan. |
| 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. |
| Differentiation |
Representation
in terms of time rate of change. Differentiating velocity yields
acceleration. |
| Digital |
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. |
| Discrete |
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 |
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. |
| Displacement |
A
vector quantity that specifies the change of position of a body
or particle and is usually measured from the mean position or position
of rest. |
| Distortion Electrical |
(Usually
expressed as Total Harmonic Distortion) the amount by which the
signal deviates from its pure form. |
| Distortion Mechanical |
Structural
response at harmonics or sub-harmonics of a forcing frequency. |
| Domain |
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. |
| Drift |
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
millivolts) over long periods of time or in difficult environmental
conditions. |
| Driving Point Measurement |
A
frequency response measurement where the excitation point and direction
are the same as the response point and direction. |
| DSP |
Digital
Signal Processor |
| DT (Delta Time) |
The
time resolution (second); the time difference between adjacent signal
samples. |
| Durability |
A
measure of useful life (a special case of reliability). |
| 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. |
| DUT |
Device
under test. |
| Dynamic Compliance |
See
compliance. |
| Dynamic motion |
Movement,
as compared with non-moving or static position. Dynamic motion is
sensed with displacement or velocity pickups or with accelerometers. |
| Dynamic Range |
The
ratio (usually expressed in dB) of the maximum level to the minimum
detectable value. |
| Dynamic Signal
Analyzer DSA |
A
system using digital signal processing and the Fast Fourier Transform
(FFT) to display time, order, frequency and phase components of
signals |
| Dynamic Stiffness |
The
frequency response function of force/displacement. |
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E
Eccentricity, mechanical |
Variation
of shaft surface radius when referenced to the shaft's true geometric
centre line. Out-of-roundness. |
| 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. |
| Eigenvalue |
The
roots of the characteristic equation. |
| Eigenvalue Problem |
The
mathematical formulation and solution of the characteristic equation
is called the Eigenvalue problem. |
| Eigenvector |
The
mode shape vectors. |
| Engineering Units |
(EU)
The units in which a measurement is made; for instance acceleration
may be expressed in g, velocity may be expressed in milimeters
per second. |
| Engineering Units
Table |
In
SignalCalc Dynamic Signal Analyzers: A control table defining all
available choices of input and output Engineering Units; contains
unit name and definition in terms of nine SI dimensions. |
| 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, performance, reliability or survival of an item. |
| Environmental stress
screening (ESS) |
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 |
Simulating
the vibration, shock, noise, thermal, humidity etc. environment
in the lab. |
| 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. |
| Error |
The
difference between the indicated and the true values of a variable
being measured. |
| Excitation |
An
external force (or other input) applied to a system that causes
the system to respond in some way. |
| 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. |
| Exponential Averaging |
A
continuous averaging method wherein the most recent constituent
is most influential and old information decays away exponentially
with time. |
| Export |
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. |
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F
Failure |
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 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. |
| Fast Fourier Transform
(FFT) |
The
FFT is an algorithm, or digital calculation routine, that efficiently
calculates the discrete Fourier transform from the sampled time
waveform. In other words it converts, or "transforms" a signal from
the time domain into the frequency domain. See also DFT. |
| Fatigue life |
The
amount of time under defined operational conditions that a product
is expected to survive. |
| Fault |
Immediate
cause of failure (e.g. maladjustment, misalignment, defect, etc.)
|
| Feedback signal |
A
response measurement that is used in a closed-loop control process. |
| FFT Analyzer |
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 or Fast Fourier
Transform |
A
method of computing frequency domain data from time domain data. |
| 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. |
| Filter |
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. |
| Finite Element
(FEA) |
A
computer-aided design technique for predicting the dynamic behavior
of a mechanical system. |
| 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 |
| Fixture |
The
intermediate structure that attaches a device under test (DUT) to
a shaker or shock test machine. |
| Flat top Window |
A
specialized Window for the analysis of periodic signals from which
you need to extract very precise amplitudes (at the cost of spectral
resolution). |
| FMEA |
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. |
| Folder |
Another
name for a directory or sub-directory in computers with Windows. |
| 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. |
| 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. |
| 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.
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| Fourier |
Mathematician
Baron Jean Baptiste Joseph Fourier 1768-1830. (See also FFT) |
| Fourier Analysis |
Fourier
analysis is another term for spectrum analysis, although it generally
refers to analysis using an FFT Analyzer. |
| Fourier Transform |
See
FFT |
| Frame |
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. |
| Frequency |
The
reciprocal of the period T in seconds (of a periodic function) (1/T).
Usually given in hertz (Hz), meaning cycles per second (cps). |
| Frequency Domain |
A
plot of frequency vs. amplitude, called a spectrum, and the spectrum
is in the frequency domain. |
| Frequency range |
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. |
| Frequency Response |
(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). |
| 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. |
| Frequency spectrum |
A
description of the resolution of any electrical signal into its
frequency components, giving the amplitude (sometimes also phase)
of each component. |
| Fspan |
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 |
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G
g |
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. |
| 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 |
| Generalized 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. |
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H
HALT |
Highly
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. |
| Hann Window |
(Hanning)
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. |
| Hardware |
The
physical parts of the computer or system that you can touch. |
| Harmonic |
A
sinusoidal quantity having a frequency that is an integral multiple
of a fundamental frequency. |
| Harmonic Distortion |
In
the output signal of a device, distortion caused by the presence
of frequencies not present in the input signal. |
| Harmonics |
also
called a harmonic series, are components of a spectrum that are
integral multiples of the fundamental frequency. A harmonic series
in a spectrum is the result of a periodic signal in the waveform.
Harmonic series are very common in spectra of machinery vibration.
|
| HASS |
Highly
accelerated stress screening |
| Help button |
Pops
up helpful information about the current dialog box or area. |
| Hertz |
The
unit of frequency. Also expressed as cps (cycles per second.) |
| High-Pass Filter |
A
filter that passes signal frequencies above a specific, or cut off,
frequency. They are used in instrumentation to eliminate low-frequency
noise, and to separate alternating components from direct (DC) components
in a signal. |
| Hinv |
The
inverse of the system transfer function. |
| Histogram |
A
Test type providing single channel amplitude domain functions including
Probability Density (PDH) and Cumulative Density (CDF). |
| Hotspots |
Areas
on the graph where you can click with the mouse to bring up the
appropriate Graph Attributes dialog box. |
| Hysteresis |
Also
called dead band. That portion of a measuring system's response
where a change in input does not produce a change in output. |
| Hysteresis Damping
(Hysteretic 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.
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I
IEPE (ICP) |
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. |
| Imbalance |
Unequal
distribution of weight or mass on a rotor. The geometric centre
of the mass is not where it should be: along the shaft centerline.
|
| Impact test |
In
modal analysis a form of structural excitation often using an impact
hammer. (A hammer with an integrated force transducer to measure
the impact) |
| 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 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. |
| Import |
To
convert and use a file created by another program. |
| Impulse |
The
integral of force over a time interval. |
| Impulse Response |
The
response of a system to a unit impulse or Dirac's delta function.
The Fourier transform of the impulse response is the frequency response
function. |
| Impulse Signal |
A
sharp "spike" positioned 5% from the beginning of a sample block;
produces a near-flat spectrum to Fspan. |
| Increment |
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 (Accelerance) |
Acceleration
divided by force. |
| Inertially-referenced |
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:
|
| Input Impedance |
The
shunt resistance and capacitance (or inductance) as measured at
the input terminals. |
| Integration |
Representation
inversely related to time rate of change. (integrating velocity
yields displacement). |
| Integrator |
Circuitry
which converts an acceleration signal to a velocity signal or a
velocity signal to a displacement signal. |
| Isolation |
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. |
| Itch |
Sometimes
called glass itch or lace itch. An automotive sound caused by metal
sliding on glass. |
| Iterative closed
loop control |
pre-calculates
drive signals but then modifies those 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. |
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| Key phasor |
A
once-per-revolution pulse. Used in phase measurements for analysis
and for balancing.. |
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L
LastH |
In
SignalCalc 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. |
| Lateral sensitivity |
Sensitivity
of a transducer in a direction perpendicular to the normal measurement
axis. |
| 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. |
| Leakage |
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). |
| Level |
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
a product 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 units |
A
measure of use duration applicable to the item. Measures include
time, cycles, distance, rounds fired, attempts to operate, etc. |
| Limiting |
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. |
| Linear |
Having
or being a response or output that is directly proportional to the
input. |
| Linear System |
A
system is linear if for every element in the system the response
is proportional to the excitation. |
| Linearity |
The
closeness of a calibration curve to a specified straight line, preferably
passing through zero. Commonly specified as a % of full scale. |
| Lines |
The
measured frequency points in a spectrum |
| LN2 |
Liquid
nitrogen, often used for rapid cooling of environmental test chambers. |
| Logarithmic |
A
function the raises a number by an exponent to produce a given number.
For example, the logarithm of 1 00 to the base 1 0 is 2. |
| Loudness |
The
human ranking of an auditory sensation, usually in terms ranging
from soft to loud, expressed in sones. |
| Low Pass Filter |
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. |
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M
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 called machinery health monitoring. |
| Magnetostriction |
Slight
changes in the dimensions of iron or steel components resulting
from changes in the magnetic fields acting on these components. |
| Mass |
Acceleration
divided by force. |
| Mean |
A
value intermediate between quantities under consideration. |
| 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. |
| Mean-Time-To-Failure |
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 |
Force
/ Velocity, where the velocity is a result of that force only. See
Mobility |
| MEMS - Micro Electro-Mechanical
Structures |
Extremely
small devices utilizing both electrical and mechanical properties.
|
| Micron |
Length
or displacement equal to 10-6 meter. One micron = 0.04 mil or 0.00004
inch. |
| Microphone |
A
transducer used to convert acoustic dynamic pressure change into
an electrical signal. |
| Mil |
Length
or displacement equal to 0.001 inch. |
| Mils/Inch |
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. |
| Mission Profile |
A
time-phased description of the events and environments an item experiences
from initiation to completion of a specified mission. |
| 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 |
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. |
| Mode |
A
characteristic pattern in a vibrating system. All points reach their
maximum displacements at the same instant. |
| Mode Shape |
The
relative position of all points on a structure at a given natural
frequency. |
| 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
aluminium. |
| Monotonic |
Sequentially
increasing in frequency. Each breakpoint in the table must be at
a higher frequency than the previous breakpoint. |
| MTBF |
Abbreviation
for Mean (or average) time between failures. |
| MTTF |
Mean-Time-To-Failure.
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. |
| 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.
|
| MUX |
Multiplexer.
A device that selects multiple inputs into an aggregate signal. |
| mV/EU |
The
factor reflecting the sensitivity of a transducer per given engineering
unit. |
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N
Narrow band Analysis |
See
FFT. |
| 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 any of the modes of vibration of a structure or system.
Damping lowers the natural frequency. |
| Node |
A
point or line on a vibrating structure that remains stationary.
|
| Noise |
Any
unwanted signal. Can be random or periodic. |
| Noise Floor |
The
level at which signals cannot be discerned due to the noise floor
(electronic hash) in a measuring system. |
| Non linearity |
The
deviation from a best fit straight line of true output vs. actual
value being measured. |
| Notch |
In
Vibration Control: The process of manually or by control techniques,
reducing certain frequencies in a vibration test. Often used as
a form of protection to prevent over-test of sensitive items. |
| 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 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. |
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O
Octave |
The
interval between two frequencies differing by exactly 2:1. |
| OEM |
original
equipment manufacturer. |
| Open Loop control |
In
Vibration Control: Pre-computed 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. |
| Orbit |
The
path of a shaft centre line during rotation. The orbit is usually
observed on an oscilloscope connected to x- and y-axis displacement
sensors. Sometimes called a Lissajous pattern. |
| Order |
In
Machine Vibration Analysis: Vibration at multiples of a machine's
shaft speed. |
| Order tracking |
In
Machine Vibration Analysis: Analysis of machine order amplitude
and phase as a function of speed. (See order) |
| Oscillation |
Variation
with time of a quantity. Usually implies some regularity (as in
sinusoidal or complex vibration). |
| Overall RMS Level |
A
measure of the total RMS magnitude within a specified frequency
range. |
| Overlap |
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. |
| Overwrite |
The
process of writing new data over existing data. This option causes
you to lose the information that has been written over. |
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P
Parameter |
A
value or option associated with a specific field in the dialog
box. |
| Parameter Estimation |
The
process of evaluating and curve fitting frequency response functions
in order to estimate modal parameters. |
| Patent defect |
A
flaw (in a part or assembly) and/or workmanship that has failed
under test or screen. See Latent Defect. |
| Payload |
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). |
| Peak |
maximum
value of a varying quantity. Peak is also a form of spectral averaging
where the maximum level at each frequency is stored and presented
as the spectrum. |
| Peak Hold |
An
"averaging" method that retains the maximum positive value encountered
at each time or frequency point; a "roof extrema". |
| 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 |
| Period |
The
interval of time over which a cyclic vibration repeats itself. |
| Periodic |
A
signal is periodic if it repeats the same pattern over time. The
spectrum of a periodic signal always contains a series of harmonics.
|
| Periodic vibration |
(See
also Deterministic vibration.) An oscillation whose waveform regularly
repeats. Compare with probabilistic vibration. |
| Perpendicular |
At
right angles (90°) to a given line or plane. |
| Phase |
(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. |
| 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 reference
probe |
A
transducer giving an output once per shaft revolution. |
| 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. |
| Piezoelectric (PE)
transducer |
A
transducer using deformation of a crystal or ceramic element to
generate electrical charge and voltage. |
| Piezoresistive
(PR) transducer |
A
transducer using deformation of its semiconductor resistive element
to generate electrical output. |
| Pitch |
Rotation
in the plane of forward motion, about the left-right axis. |
| Platform |
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. |
| Power Spectral
Density |
(PSD)
The power of random vibration intensity, in mean-square acceleration
per frequency unit, as g2/Hz or m2/s3. |
| Power Spectral
Density (PSD) |
A
function of frequency whose integral over any frequency interval
represents the spectral content over that frequency interval. The
limiting mean-square value |
| 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 |
Vibration
such as random, where 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. |
| 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. |
| Proximity sensor |
A
displacement sensor for measuring the varying distance between two
points |
| Pseudo Random Noise |
A
single frame of Random Noise that is exactly repeated every Tspan.
Used to separate non-linear effects from noise contamination. |
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Q
Q |
(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. |
| Quasi-Periodic |
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. |
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R
Radial |
A
direction perpendicular to a shaft's centerline. |
| Random Noise |
A
random Signal Generator output exhibiting uniform spectral density
over Fspan, Gaussian amplitude distribution and a crest
factor of 5. |
| Random vibration |
Non
periodic or deterministic vibration where magnitude cannot be predicted.
Random vibration is usually termed Gaussian if it follows a Gaussian
distribution. |
| Random-on-Random |
Broad-band
random mixed with random narrowband tones. |
| Range (EU) |
The
(zero-to-peak) Full Scale for an Input Channel, expressed in the
engineering units of the transducer (signal source). |
| Rate gyro |
A
kind of gyroscope that measures rotational velocity (degrees or
radians per second) around a fixed axis. |
| 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. |
| Rectangular Window |
In
an FFT Analyzer, the rectangular window is actually no window at
all. It is also called rectangular weighting, or uniform weighting,
and is used when the signal to be Analyzed is either a transient
or is exactly symmetrical within the Tspan. |
| Reliability |
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? |
| Repeatability |
The
consistency (or variation) of readings and results between consecutive
sets of measurements. It has nothing to do with accuracy. |
| Repetitive shock
machine |
A
machine in which products (to be tested) are attached to a platform
(table). 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. |
| Replication |
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. |
| Resolution |
Usually
a measure of the number of samples in a block of data or the number
of frequency points in an FFT analysis. |
| Resolution Bias
Error |
See
Picket Fence Effect. |
| Resonance |
A
vibration of large amplitude in a mechanical or electrical system
caused by a relatively small periodic stimulus of the same or nearly
the same period as the natural vibration period of the system. The
frequency of any of the natural modes of vibration of a structure
or system. |
| Response |
The
vibratory motion or force resulting from a mechanical or electrical
input. |
| 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 Window |
A
decaying exponential Window of adjustable Time Constant used on
the motion signal in an impact Modal test. |
| Ringing |
Continued
oscillation after an external force or excitation is removed. |
| Rise time |
(For
a vibration transducer) 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. |
| RMS (Root-Mean-Square) |
The
root of the mean of the square of a quantity. In a sine wave the
RMS value is 0.707, for a square wave the value is 1. |
| Roll |
Rotation
about the axis of linear motion. |
| Rolling element |
(anti-friction)
bearings 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. |
| Roots |
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. |
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S
Sampling
Parameters |
A
control group that sets the interrelated parameters Fspan,
Lines, dF, Tspan, Block size and dF. |
| Sampling rate |
The
number of measurements an A/D converter takes per second. |
| Screening |
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. |
| Seismic |
Measurements
and activity concerning earth motion, such as earthquakes. |
| Selectivity |
Selectivity
is a measure of the narrowness of a band pass filter. The greater
the selectivity, the narrower (or more selective) the filter. |
| Self-induced vibration |
Also
called self-excited vibration, results from conversion of non-oscillatory
energy into vibration. |
| Sensitivity |
Of
a mechanical-to-electrical sensor or pickup, the ratio between electrical
signal (output) and mechanical quantity (input).. |
| Sensor |
Any
device that translates the magnitude of one quantity into another
quantity. Three of the most common transducers used in vibration
measurements are accelerometer, velocity transducer, and eddy current
probe. |
| Session Trigger |
In
SignalCalc Dynamic Signal Analyzers: A single trigger condition
awaited from an input channel; once acquired, data is collected
in Free Run without need for further triggers. |
| Shaker |
A
vibration machine or device for subjecting a mechanical system to
controlled and reproducible mechanical vibration. |
| Sharpness of Resonance |
(Q)
(Amplification factor) A measure of the sharpness of a resonance
or frequency. The mechanical gain at resonance. |
| Shock machine |
Or
shock test machine, a device for subjecting a system to controlled
and reproducible mechanical shock pulses. |
| Shock pulse, classical |
Classical
shock pulses include half-sine, square, triangular, trapezoid and
sawtooth. |
| Shock pulse, haversine |
An
inverted cosine offset by half its amplitude. |
| Shock Response
Spectrum (SRS), |
A
spectrum consisting of the maximum responses of an array of SDOF
systems vs. their natural frequencies, as they respond to an applied
shock. |
| Shock test machine |
A
mechanical device for generating mechanical shock pulses. |
| SI (Systeme International) |
An
internationally accepted system of measurement based upon nine precisely
defined metric units. |
| SI Exponents |
Integer
exponents of (S1) mass, length, time, etc. that define the dimensions
of an EU entered to the Engineering Unit Table. |
| SI Units |
Kilogram
(mass), meter (length), second (time), Ampere (current), Kelvin
(temperature), candela (light intensity), mole (molecular atomic
weight), radian (plane angle), steradian (solid angle) |
| Sidebands |
Spectral
components that are the result of amplitude or frequency modulation.
The frequency spacing of the sidebands is equal to the modulating
frequency, and this fact is used in diagnosing machine problems
by examining sideband families in the vibration spectrum. For instance,
a defective gear will exhibit sidebands spaced apart at the gear
rpm around the gear mesh frequency. |
| Signal |
In
vibration analysis, a signal is an electric voltage or current which
is analogue of the vibration being measured. |
| Signal conditioner |
An
amplifier which prepares a transducer signal for use by other instrumentation
such as FFT analysis, vibration control etc. |
| Signal Generator |
A
device or system output from a Dynamic Signal Analyzer producing:
Sine, Swept Sine, Chirp, Random, Pseudo Random, Burst Random, Thump
and Impulse signals. |
| Signal Map |
In
SignalCalc Dynamic Signal Analyzers: A control window that determines
what signals are stored where and in what format. |
| 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. |
| Simple harmonic
motion |
Periodic
vibration that is a sinusoidal function of time. |
| Sine Output |
A
Signal Generator output signal of selectable frequency, amplitude
and phase. |
| Single-Degree-of-Freedom
System (SDOF) |
A
system whose position in space can be completely described by one
coordinate. |
| Single-Ended Input |
An
analogue input measured with respect to a common ground. |
| Six Degree of Freedom
machines |
Repetitive
Shock Machines. That class of non stationary random vibration producing
machines that are air driven impact hammers o excite a vibrating
plate onto which products are either attached directly or fixtured.
The resulting excitation is a combination of impulse and table modal
response, or an inter-modulation of both |
| Slew rate |
The
maximum rate at which an instrument's output can change by some
stated amount. |
| Soft failure |
A
product under test ceases to operate correctly, but resumes correct
operation when the stressing environment is eased. Differs from
hard failure. |
| Solid-state sensor |
One
which has no moving parts. |
| Sone |
A
unit of loudness. |
| Sound |
An
oscillation in pressure, capable of evoking 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 20log10 of the ratio between a given sound
pressure and 20 micro pascals. |
| Source |
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. |
| Spall |
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. |
| Spectra |
The
plural of spectrum. |
| Spectral map |
A
three-dimensional plot of amplitude (Z axis) vs. time (or shaft
speed) (Y axis) vs. frequency (X axis) |
| 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 converts a signal from the time domain into the
frequency domain. |
| Squeak |
In
Automotive sound quality, an annoying sound, A sound resembling
that of an unlubricated hinge. |
| 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. |
| Standard deviation |
The
square root of the mean of the squares of the measured deviations
from the mean value. |
| Standards |
Documents
that prescribe engineering disciplines, impose particular management
practices, reporting and auditing requirements. |
| Standing wave |
A
wave that is characterized by lack of vibration at certain points
(nodes), between which are areas of maximum vibration (antinodes).
|
| Stationarity |
A
property of probabilistic vibration if the PSD and the probability
distribution remain constant. |
| 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. |
| Step stressing |
Increasing
stresses in a series of pre-selected increments. |
| 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. |
| Stiffness |
The
ratio of force (or torque) to deflection of a spring-like element. |
| Strain-gage transducer |
A
changing-resistance sensor whose signal depends upon sensitive element
deformation. |
| Stress |
Intensity
of applied load, usually at the site of a failure. |
| Stress Screening |
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. |
| 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. |
| Sub synchronous |
Frequencies
in a vibration spectrum that are lower than the fundamental frequency.
Components of a vibration signal whose frequency is less than once
per rev shaft speed. |
| Sub-harmonic |
A
sinusoidal quantity having a frequency that is an integral sub-multiple
of a fundamental frequency. |
| Supply current |
In
ICP (IEPE) Transducers: 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
vibration testing where frequency is smoothly and continuously varied
with a constant or profiled controlled amplitude. |
| Synchronous |
(Rotating
machinery) Vibration components related to shaft speed. |
| Synchronous Average |
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. |
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T
TAAF |
Test,
Analyze and fix. See Accelerated Life Testing. |
| Tailoring |
(Test
Tailoring) 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 |
| THD |
(Total
Harmonic Distortion). 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 pre-determined temperature changes, between hot and
cold extremes. |
| Thermistor |
An
electrical device used for temperature measurement. |
| Thermocouple |
An
electrical device used for temperature measurement. Two dissimilar
metals joined together, making a continuous loop. |
| Threshold |
The
smallest change in a measured variable that gives a measurable change
in output signal. |
| Thrust position |
Location
in direction of a shaft centre line. See axial position. |
| Thump Signal |
A
"velocity and displacement compensated" half-sine pulse. This transient
is a classic shock-test acceleration waveform. |
| 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. |
| Toggle |
A
switch that causes the opposite value. For example, a switch that
is "on," when toggled, becomes "off." |
| Tracking filter |
A
narrow band-pass filter whose centre frequency follows a synchronizing
signal. |
| Transducer |
A
device which converts shock or vibratory motion into an electrical
signal that is proportional to a parameter of the experienced motion. |
| Transfer Function |
The
output to input relationship of a structure. Mathematically it is
the Laplace transform of the output divided by the Laplace transform
of the input. |
| Transform |
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. |
| Transient vibration |
Short-term
vibration of a mechanical system. |
| Transmissibility |
The
non-dimensional ratio of response / input. The maximum Transmissibility
value is the mechanical "Q" of a system. |
| Transverse sensitivity |
Sensitivity
of a transducer in a direction perpendicular to the normal measurement
axis. |
| Trigger |
A
facility to synchronize the start of data acquisition. |
| 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. |
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U
Unbalance (imbalance) |
Unequal
mass distribution on a rotor. The mass centre line does not coincide
with the rotation or geometric centre line. |
| Undamped Natural
Frequency |
The
same as the natural frequency of a structure. |
| Uniform window |
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. |
| Unit |
Every
measurement is expressed as a multiple or fraction of dome appropriate,
well-defined unit quantity such as centimeter, volt, etc. |
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V
Velocity |
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. |
| Vibration |
Mechanical
oscillation or motion about a reference point of equilibrium. |
| Vibration meter |
A
device for measuring electrical signals from vibration sensors.
May display displacement, velocity and/or acceleration. |
| Vibrator |
A
device which produces controlled and reproducible mechanical vibration
for the vibration testing of mechanical systems, components and
structures. |
| Viscous Damping |
Damping
that is proportional to velocity. Viscous damping is used largely
for system modeling since it is linear. |
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W
Waterfall plot |
A
spectral maps with multiple FFT traces plotted on a graph regular
intervals of time, speed or other periodic parameter. |
| Waveform |
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. |
| Weight |
That
property of an object that can be weighed, as on a scale; the gravitational
force on an object. |
| Weighting |
A
"weighting function" applied to time-data prior to performing an
FFT. Rectangular, Hann, Flat top, DP7, BH4, Force and Response. |
| Weighting (Acoustic) |
An
artificial curve applied to acoustic measurements across the frequency
spectrum. C weighting is essentially flat. A weighting attempts
to compensate for the non-constant sensitivity of human hearing
at certain frequencies. |
| Weighting 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.
See Equal Loudness Curves. |
| White random vibration |
Broad-band
random vibration with constant power per bandwidth. |
| Window |
A
"weighting function" applied to time-data prior to performing an
FFT. Rectangular, Hann, Flat top, DP7, BH4, Force and Response. |
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Y
Yaw |
Rotation
about the vertical axis. |
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Z
Zwicker loudness
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A
sound measurement in linear units (sones).
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