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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 ( |