Condition Monitoring Systems




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 catastrophic

A failure that can cause loss.

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.


Immediate cause of failure (e.g. maladjustment, misalignment, defect, etc.)

FFT or Fast Fourier Transform

A popular computer method of shifting data from the time domain to the frequency domain. / A computer (or microprocessor) procedure for calculating discrete frequency components from sampled time data. A special case of the discrete Fourier transform where the number of samples is constrained to a power of 2. / 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 Analyser

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 real time.

Feedback Signal

Feedback signal is a response measurement that is used in a closed-loop control process.

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.


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. / Electronic circuitry designed to pass or reject a specific frequency band.

Finite Element Modeling (Finite Element Analysis, FEA)

A computer-aided design technique for predicting the dynamic behavior of a possible future mechanical system.

Finite Element Modeling

A computer aided design technique for predicting the dynamic behavior of a mechanical system prior to construction. Modeling can be used, for example, to predict the natural frequencies of a flexible rotor.

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


The intermediate structure that attaches a device under test (DUT) to a shaker or shock test machine.

Flat Top Filter

DSA window function which provides the best amplitude accuracy for measuring discrete frequency components.

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.

Fluid-Film Bearing

A bearing which supports the shaft on a thin film of oil. The fluid-film layer may be generated by journal rotation (hydrodynamic bearing), or by externally applied pressure (hydrostatic bearing).


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.

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. / The oscillation of a system under the action of a forcing function. Typically forced vibration occurs at the frequency of the exciting force. / 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.

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.

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.


See Fast Fourier Transform or FFT. Term honors mathematician Baron Jean Baptiste Joseph Fourier 1768-1830.

Fourier Analysis

Fourier analysis is another term for spectrum analysis, although it generally refers to analysis using an FFT Analyzer.

Fourier Transform



The maximum load an equipment can stand before failure (malfunction, irreversible loss of performance or structural damage) occurs.

Fragility test

Expensive but highly useful dynamic tests of several samples (to account for variations in tolerances, material properties and manufacturing processes) at potentially destructive frequencies, to determine fragility.


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./ Vibration of a mechanical system following an initial force—typically at one or more natural frequencies.


The reciprocal of the period T in seconds (of a periodic function) (1/T). Usually given in hertz (Hz), meaning cycles per second (cps). / The repetition rate of a periodic event, usually expressed in cycles per second (Hz), revolutions per minute (rpm), or multiples of a rotational speed (orders). Orders are commonly referred to as 1x for rotational speed, 2x for twice rotational speed, etc. / The number of cycles over a specified time period over which an event occurs. Normally expressed in cycles per second (hertz, Hz).

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 Analyser

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 – see bandwidth

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. / The amplitude and phase response characteristics of a system. / The frequency-dependent characteristics that determines the phase and amplitude relationship between sinusoidal input and output.

Frequency Response Function (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). / The amplitude and phase response characteristics of a system.

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 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.

Fundamental frequency

The number of hertz or cycles per second of the lowest-frequency component of a complex, cyclic motion. (See also Harmonic and Sub-harmonic.)

Frequency spectrum

A description of the resolution of any electrical signal into its frequency components, giving the amplitude (sometimes also phase) of each component.


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