Glossary

The vibration monitoring industry has specialized jargon that is commonly used when talking about vibration sensors, accelerometers and associated applications. It's important that you understand common terminology to communicate effectively with vibration monitoring, reliability, condition monitoring and process automation professionals. We hope you find our glossary useful to find specialized terms and definitions used across vibration, condition monitoring and predictive maintenance.

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Acceleration

A change in the velocity of a body particle with respect to time. The parameter that an accelerometer measures (dv/dt).  Usually given in terms of "g."  (see g).

Accelerometer

Any device which is used to measure acceleration.  A sensor or transducer. (See Piezoelectric Accelerometer, Charge Mode Accelerometer, Triaxial Accelerometer.)

Amplified Gain

Increase in signal due to the amplifier.

Amplitude

A measurement of the distance from the highest to the lowest excursion of motion. Measured in either a positive or negative direction.  (See Peak Amplitude, Peak to Peak Amplitude). The maximum percent of change of the sensitivity of an accelerometer over the amplitude range.

Acceleration Range

The maximum amplifier input signal amplitude under normal conditions before the amplifier begins to distort and become 1% nonlinear.

Back-to-Back Calibration

Comparison calibration accomplished by mounting the test accelerometer to a reference accelerometer so both are subjected to identical motion.

Balance-of-Plant

Balance-of-Plant (BoP) is a term given to all the infrastructural components. For example, it can refer to the supporting equipment and auxiliary components that are required to make a power plant or energy system (wind farm) operate.

Base Isolation

Accelerometer base electrically insulated from its case, output signal, and the test structure it is mounted to.

Base Strain Sensitivity

The parameter quantifying the unwanted output signal picked up by a motion transducer when its mounting surface is subjected to mechanical strains.

Bender Mode Accelerometer

An accelerometer design which stresses the piezoelectric element by bending it. This design is primarily used for low frequency, high sensitivity applications.

Bias Decoupling Capacitor

See Blocking Capacitor.

Bias Output Voltage

abr. (BOV).  syns. Bias Voltage, Rest Voltage. The DC voltage at the output of an amplifier on which the AC dynamic signal is superimposed.

Blocking Capacitor

A capacitor placed in series with the input of a signal conditioning or measurement device which blocks the DC Bias Voltage but passes the AC signal.

Broadband Noise

The total noise of an electronic circuit within a specified frequency bandwidth. The total noise of the electronic circuit, usually expressed in rms volts.

Broadband Vibration Level

The total vibration amplitude within a specified frequency bandwidth.

Capacitance

Is the ability of a body to store an electrical charge. A material with a large capacitance holds more electric charge at a given voltage, than one with low capacitance. Any object that can be electrically charged exhibits capacitance, however the concept is particularly important for understanding the operations of the capacitor, one of the three fundamental electronic components (along with resistors and inductors).

Case Isolation

Sensor case electrically insulated from output signal and the test structure.

Charge Amplifier

An amplifier which compensates its gain so that it conditions a charge signal independently of any capacitance parallel to the source of the charge signal: i.e. cable capacitance. A charge to voltage converter. A capacitive feedback amplifier which converts the high impedance output from a charge mode sensor to a low impedance voltage signal.

Charge Converter

Converts the high impedance output of a charge mode sensors to a low impedance voltage signal.

Charge Mode Accelerometer

Any piezoelectric accelerometer that does not contain an internal amplifier and produces a high impedance charge signal.

Charge Sensitivity

The amount of charge produced by a charge mode accelerometer per unit of acceleration and expressed as picocoulomb/g (pC/g) or pC/ms2 (see picocoulomb and g2).

Clipping

Output limitation when a signal amplitude exceeds the limits imposed by the amplifier and the supply voltage.

Compression Mode Accelerometer

An accelerometer design which stresses the piezoelectric element in the compressive direction: i.e. the electrode faces move toward and away from each other.

Condition Monitoring

Process of monitoring conditions in machinery (vibration) in order to identify a significant change which is indicative of a developing fault. It is a major component of predictive maintenance. Condition monitoring allows maintenance to be scheduled, or other actions to be taken to prevent failure and avoid its consequences. Condition monitoring has a unique benefit in that conditions that would shorten normal lifespan can be addressed before they develop into a major failure. Condition monitoring techniques are normally used on rotating equipment and other machinery (pumps, motors, engines, presses, compressors).

Current Regulating Diode

A semiconductor device which limits and regulates electrical current independent of voltage.

Discharge Time Constant (DTC)

Time required for a sensor or measuring system to discharge its signal to 37% of the original value from a step change of measurand. This time constant directly relates to the low frequency measuring capability for both transient and sinuoidal events. (It should not be confused with rise time which relates to high frequency responses.)

Displacement

symbol: x. The measured distance traveled by a point from its position at rest. Peak to peak displacement is the total measured movement of a vibrating point between its positive and negative extremes. Measurement units expressed as inches or milinches.

Displacement Sensor

A sensor whose output varies proportionally to the displacement aspect of motion.

Electromagnetic Interference (EMI)

A condition in which an electromagnetic field produces an unwanted signal.

Electrical Noise

Noise, or interference, can be defined as undesirable electrical signals, which distort or interfere with an original (or desired) signal. Noise could be transient (temporary) or constant. Unpredictable transient noise is caused by lightning. Constant noise can be due to the predictable 50 or 60 Hz AC 'hum' from power circuits or harmonic multiples of power frequency close to the data communications cable. Noise can be internal, generated from within the system or from an outside source (external). Typical sources of noise are devices, which produce quick changes (spikes) in voltage or current or harmonics, such as large electrical motors being switched on, fluorescent lighting tubes, solid-state converters or drive systems, lightning strikes, high-voltage surges due to electrical faults, welding equipment.

Electromagnetic Sensitivity

Electromagnetic sensitivity is the resultant output that is generated when an accelerometer is subjected to external AC magnetic fields. This output is considered noise and therefore contributes to measurement errors. A significant effort is made to avoid the use of magnetic material during the accelerometer product design phase in order to minimize the risk of electromagnetic sensitivity. Despite this, most accelerometers still exhibit some degree of electromagnetic sensitivity.

Enviroprene

Thermoplastic elastomer use on some Wilcoxon cables and connectors.

FireFET®

A high temperature amplifier for piezoelectric sensors trademarked by Wilcoxon Research, Inc.

FFT Analyzer

Fast Fourier Transform real time spectrum analyzer (see Spectrum Analyzer).

Flat Frequency Response

The frequency range of an accelerometer where the ratio of the electrical output to the vibration input deviates by less than ±5%, referenced at 100 Hz.

Flexural Mode Accelerometer

Structured with a crystal sensing element which is stressed in a bending mode due to vibratory motion.

g

Standard unit of acceleration equal to one earth’s gravity at mean sea level – one “g” equals 32.17 ft/sec2 or 9.807 m/s2.

Frequency Domain Measurement

Data taken in the time domain and converted to the frequency domain by applying the FFT.

Frequency Response

A measure of the change in a quantity, usually sensitivity, as a function of frequency.

Ground Isolation

Refers to a condition when the amplifier ground is electrically isolated from the test structure ground. See base isolation and case isolation.

Ground Loop

A condition where a sensor signal return lead or cable shield is grounded at both sensor as well as remote readout and where grounds are of differing electrical potential. Such conditions cause ” Ground Loop Noise” at frequencies between 50 and 60 Hz as well as multiples thereof.

High Impedance

Usually relates to a high value of resistance such as at the input to an amplifier or output of a charge mode piezoelectric sensor.

High Pass Filter

A filter which attenuates low frequencies and passes high frequencies.

Integration

The mathematical process where an accelerometer output is converted to the velocity. Double integration converts the output to displacement.

IsoRing®

A bolt through shear mode piezoelectric sensor designs that electrically, mechanically and thermally isolates the sensing element from the sensor housing. A registered trademark of Wilcoxon Research, Inc.

Lead-Zirconate-Titanate (PZT)

A piezoelectric ceramic material characterized by a very high activity (sensitivity), broad temperature range and long term stability.

Linearity

The closeness of a calibration curve to a specified straight line. Linearity is commonly specified as a % of full scale based on best straight line through zero.

Low Impedance

Usually relates to a low value of resistance such as the output of an amplifier of the sensor.

Low Pass Filter

A filter (mechanical or electrical) which attenuates high frequencies and passes low frequencies.

Loop Powered Sensors (LPSTM)

A 4-20 mA output vibration sensor containing a miniaturized transmitter. A trademark of Wilcoxon Research, Inc.

Mass Loading

The change in motion of a test structure due to the added weight of the accelerometer attached to it.

Mounting Stud

A threaded screw used to rigidly attach a motion sensor to the structure under test.

Mounting Torque

The optimum torque applied to the sensor when mounting with a threaded stud.

m/s2

Unit of acceleration in meters/second/second.

Natural Frequency

A frequency at which a system vibrates freely after excitation is removed.

NIST

National Institute of Standards and Technology (formerly NBS National Bureau of Standards) is a national standards lab which provides calibration certification for primary reference standards.

Noise

Any signal other than the true signal from the measurand.

Normalization

Refers to standardization of an accelerometer output to a specified value and tolerance, e.g 100 mV/g ñ2%.

Peak Amplitude

The amplitude of a cyclic quantity as measured from the rest value to the extreme maximum of the wave height, usually in the positive direction.

Peak-to-Peak Amplitude

The amplitude of a cyclic quantity as measured from the extreme minimum to the extreme maximum of the wave height.

Phase Shift

The difference in phase angle between the output and input as a function of frequency.

Picocoulomb

A measure of electrostatic charge and equal to 1×10–12 coulombs.

Piezoceramic

A class of specialty ceramics tailored to exhibit piezoelectric properties after processing.

Piezoelectric Accelerometer

A sensor which employs piezoelectric materials to transduce mechanical motion into an electrical signal proportional to the acceleration.

Piezoelectric Velocity Transducer (PVTTM)

A piezoelectric accelerometer with onboard signal integration into velocity.

Piezoelectricity

The property exhibited by some materials whereby a mechanical stress causes the material to produce an electric charge. Conversely, an electric field across a piezoelectric material will produce a mechanical strain.

PiezoFET®

Electronic amplifier circuit trademarked by Wilcoxon Research.

Polarity

The positive or negative output from an accelerometer corresponding to acceleration corresponding to the force applied.

Power Requirement

Electrical requirements, both DC voltage and current requirements, to properly power an accelerometer.

Process automation

Use of various control systems for operating equipment including machinery, processes in factories, and other applications with minimal or reduced human intervention. Some processes have been completely automated. Automation saves labor, energy and materials to improve quality, accuracy and precision.

Pyrostrain Effect

A property whereby a change in temperature strains the case of a transducer and produces a corresponding strain signal on the piezoelectric crystal.

QuickLINK®

Quick connect mounting hardware for attaching vibration sensors to test surfaces.

Repeatability

Providing the same output signal corresponding to the same measurand input under the same environmental conditions.

Resonance

symbol: fr. The frequency of a force driven system at which any incremental change in the frequency results in an increase in the signal amplitude. The frequency of simple harmonic excitation at which maximum amplitude occurs.

Resolution

Lowest level of discernible signal or measurand – Also referred to as “threshold.”

RFI

Abbreviation for Radio Frequency Interference.

Resonant Frequency

The frequency of simple harmonic excitation at which maximum amplitude occurs.

RMS (Root Mean Square)

The square root of an arithmetic average of squared values. It relates to energy and vibration damage potential.

Seismic Accelerometer

A high sensitivity sensor for measuring low level, low frequency motion of buildings, bridges and other structures.

Sensitivity

The output level as measured at 100 Hz at RT.

Shear Mode Accelerometer

An accelerometer design which stresses the piezoelectric element in the shear direction: i.e. the electrode faces move parallel to each other. (See Bender Beam Accelerometer, Compression Mode Accelerometer).

Shock Limit

The maximum amount of short duration mechanical shock that a sensor can be subjected to before the possibility of permanent damage can occur.

Spectrum Analyzer

An instrument for quantitative spectrum analysis of time varying signals, generally, displayed as amplitude vs. frequency.

Temperature Coefficient

The percent change in the sensitivity of a sensor as a result of a unit change in the operating temperature of the sensor; express as percent per degree: i.e., %/°F or %/°C.

Temperature Range

The temperature span, given by the temperature extremes, over which the sensor will perform without damage. Specifications within the temperature range may vary as a function of temperature.

Temperature Response

A measure of the change in a quantity, usually sensitivity, as a function of temperature.

Time Constant

See Discharge Time Constant.

Time Domain Measurement

Data taken continuously or periodically and processed as a function of time.

Transducer

Any device which converts one form of energy into another. An accelerometer converts mechanical acceleration into an electrical signal.

Transverse Sensitivity

syn. Cross Axis Sensitivity. The unwanted output signal of a motion sensor when subjected to motion perpendicular to the sensitive axis; usually expressed as a percent of the normal axis sensitivity.

Triaxial Accelerometer

A sensor which consists of three mutually perpendicular accelerometers.

Triboelectric Noise

Charge generated noise caused by frictional effects within the cable due to separation and motion between the dielectric and shield.

Velocity

symbol: v. The measure of the speed at which a mass is moving or vibrating; usually expressed in/sec or mm/sec.

Velocity Transducer

Any device which measures velocity.

VibraLINK®

Vibration sensor starter kit used to monitor vibration in machinery and the parts thereof: sensors, cables, junction boxes, etc. Trademarked by Wilcoxon Research, Inc.

Vibration

Motion in a mechanical system, resulting in various reversals in acceleration relative to a reference.

Vibration Analysis

Interpreting the vibration signature of plant equipment.

Vibration Limit

The maximum amount of vibration that a sensor can be subjected to before the possibility of permanent damage can occur.