This glossary defines relevant engineering terms and user interface terms for Data Physics products. |
|
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. |
| Acoustic | Sound, the sense of hearing, or the science of sound. SignalCalc Analyzers provide a number of optional packages that are tailored to acoustic applications. SignalCalc systems provide a number of options for acoustic analysis. |
| 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. |
| Analog | Relating to a mechanism in which data is represented by continuously variable physical quantities. Quantities in two separate physical systems having consistently similar relationships to each other are called analogous. One is then called the analogue of the other. The electrical output of a transducer is an analogue of the vibration input of the transducer as long as the transducer is not operated in the non-linear (overloaded) range. This is in contrast to a digital representation of the vibration signal, which is a sampled and quantized signal consisting of a series of numbers, usually in binary notation. |
| Analog to Digital Conversion | The process of sampling an analogue signal produces a series of numbers, which is the digital representation of the same signal. The sampling frequency must be at least twice as high as the highest frequency present in the signal to prevent aliasing errors. |
| Analog Spectrum Analyzer | A spectrum analyzer is a device used to examine the spectral composition of some electrical, acoustic, or optical waveform. Often, it measures the power spectrum. An analog spectrum analyzer uses either a variable band pass filter whose mid-frequency is automatically tuned (shifted, swept) through the range of frequencies of which the spectrum is to be measured or a super heterodyne receiver where the local oscillator is swept through a range of frequencies. In general analog spectrum analyzers are used for very high frequency analysis. |
| 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. |
| 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. |
| DFT | In mathematics, the discrete Fourier transform (DFT), sometimes called the finite Fourier transform, is a transform widely employed in digital signal processing. It is used to identify the frequencies contained in a sampled signal. The DFT can be computed efficiently in practice using a fast Fourier transform (FFT) algorithm. SignalCalc analyzers and SignalStar controllers use both FFT and DFT algorithms in various applications. |
| 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. |
| Digital Spectrum Analyzer | A spectrum analyzer is a device used to examine the spectral composition of some electrical, acoustic, or optical waveform. Often, it measures the power spectrum. A digital spectrum analyzer uses the Fast Fourier transform (FFT), a mathematical process that transforms a waveform into the components of its frequency spectrum. SignalCalc systems are an example of a modern high speed digital spectrum analyzer. |
| 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 positin. 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) o display time, order, frequency and phase components of signals |
| Dynamic Stiffness | The frequency response function of force/displacement. |
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E Earthquake (earth quake) |
An earthquake is a tremor of the earth's surface usually triggered by the release of underground stress along fault lines. This release causes movement in masses of rock and resulting shock waves. A valuable tool for the analysis of earthquakes is Shock Response Analysis (SRS) which is provided as an option to SignalCalc systems and as a control option for the SignalStar controllers. |
Eccentricity, mechanical |
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, humidiy, 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. |
| 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 or Fast Fourier Transform | A fast Fourier transform (FFT) is an efficient algorithm to compute the discrete Fourier transform (DFT) and its inverse. SignalCalc analyzers and SignalStar controllers use both FFT and DFT algorithms in various applications. |
| FFT 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 Analysis | In general, FFT analysis is used to refer to any and all types of Digital Signal Analysis (DSA) which included such measurements as: linear spectrum, power spectrum, energy spectrum, cross power spectrum, transfer function, frequency response function, coherence, inverse FFT, impulse response, auto- and cross-correlation, histogram, spectrogram and others. SignalCalc analyzers and SignalStar controllers use various combinations of these to provide user friendly solutions for many applications. |
| FFT Library | A collection of FFT algorithms and other signal processing tools that can be manipulated by the user to perform custom signal processing. The SignalCalc analyzers and SignalStar controllers have optional packages that allow user control of the Data Physics hardware for custom applications. |
| FFT Power Spectrum | In the frequency domain, this is the square of FFT´s magnitude. The power spectrum answers the question, "Which frequencies contain the signal's power?" The answer is in the form of a distribution of power values as a function of frequency. SignalCalc systems are an example of a modern high speed digital spectrum analyzer which can provide a power spectrum in realtime. |
| 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 | In signal processing, a window function is used to shape the time record to provide a correction for the basic limitations of the FFT. The flat top window is used to make accurate measurements of the amplitude of periodic signals. SignalCalc systems provide for a number of window functions. |
| 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. |
| 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 Analysis | The application of a Dynamic Signal Analyzer (DSA) to help understand the meaning of an electrical signal derived via a transducer form a physical process. SignalCalc systems are an example of a modern high speed DSA that can be used for frequency analysis. |
| Frequency Analyzer | A frequency analyzer is a device used to examine the spectral composition of some electrical, acoustic, or optical waveform. Often, it measures the power spectrum. SignalCalc systems are an example of a modern high speed digital frequency analyzer. |
| Frequency Domain | A plot of frequency vs. amplitude, called a spectrum, and the spectrum is in the frequency domain. |
| Frequency range | The frequency range selected or available for measurements. |
| Frequency Response Curve | A graph of amplitude/phase vs. frequency indicating the response of a device to different frequencies. Frequency Response Functions (FRF) measurements are provided as part of all SignalCalc systems. |
| Frequency Response | (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 | |