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# A

Absolute

A term applied to calibration (e.g. of an accelerometer) based upon the primary standards of mass, length and time.

Absolute Accuracy

A measure of the 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.

Absolute Vibration

Vibration of an object relative to a fixed point in space. Seismic sensors (accelerometers and velocity pickups) measure absolute vibration. Contrasts with relative vibration as measured by Eddy Current Probes.

Absorber

A device capable of soaking up vibration.

Acceleration

Acceleration is rate of change of velocity with time (denoted as the first derivate of velocity – δv/δt or the second derivative of displacement – δ2x/δt2), and is usually specified along an axis, usually expressed in g or gravitational units (1g = 32.17 ft/s/s = 9.81 m/s/s), but may also be seen as ft/s/s or meters/s/s. It may also refer to angular motion usually expressed in radians. Acceleration measurements are usually made with accelerometers.

Accelerometer

A sensor or transducer whose output is directly proportional to acceleration, used for converting mechanical movement into an electrical signal. Two common types are piezoresistive and piezoelectric. Most commonly used is piezoelectric crystals to produce output.

Accuracy

The capability of an instrument to indicate the true value. Do not confuse with inaccuracy (sum of Hysteresis + non-linearity + drift + temperature effect, etc.) nor with repeatability.

A/D Converter / ADC

Analo to Digital COnverter. A device that changes an analog signal such as voltage or current into a digital signal (consists of discrete data values).

Aliasing

A spectrum analysis problem resulting from sampling broadband data at too low a frequency. According to Nyquist, in a sampled data system, the analog input must be sampled at a rate of at least twice the maximum frequency component within the sampled signal, to avoid loss of data and to avoid aliasing. At sampling frequencies of less than this, spurious frequencies appear within the resultant spectrum, manifesting as low frequencies. In digital signal analyzers, aliasing is avoided by applying very steep anti-aliasing filters (low pass) with corner frequencies at 1/2 the sample rate.

Alignment

A desired machinery condition, in which the axes of components of a machine are adjusted so as to be co-linear, parallel or perpendicular.

Ambient environment / Ambient Conditions

The conditions (e.g. temperature and humidity) characterizing the air or other medium that surrounds materiel.

Amplification Factor (Q) (Synchronous)

A measure of the sharpness of a resonance frequency or of susceptibility of a rotor to vibration amplitude when rotational speed is equal to the rotor natural frequency (implies a flexible rotor). For imbalance type excitation, synchronous amplification factor is calculated by dividing the amplitude value at the resonant peak by the amplitude value at a speed well above resonance (as determined from a plot of synchronous response vs. rpm).

Amplitude

The magnitude of dynamic motion or vibration; the y-axis of the vibration time waveform; the maximum value of a quantity; the measurement of energy or movement in a vibrating object. Amplitude is usually expressed in terms of peak-to-peak, zero-to-peak, or rms. For pure sine waves only, these are related as follows: rms = 0.707 x zero-to-peak; peak-to-peak = 2 x zero-to-peak.

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 Sensor

A sensor that puts out a voltage or current

Analog Tachometer

Panel Meter with needle indicator

Anti-aliasing filter

A low pass filter designed to stop frequencies higher than the ½ the sample rate, in order to minimize aliasing.

ASCII

An acronym that stands for American Standard Code for Information Interchange. ASCII defines a standard for representing characters on computers.

Asymmetrical Support

Rotor support system that does not provide uniform restraint in all radial directions. This is typical for most heavy industrial machinery where stiffness in one plane may be substantially different than stiffness in the perpendicular plane. Occurs in bearings by design, or from preloads such as gravity or misalignment.

Asynchronous

Vibration components that are not related to rotating speed (also referred to as non-synchronous).

Attenuate

Reduce.

Auto-ranging

The capability of an instrument to switch among ranges automatically. Ranges usually are in decade steps.

Average

In the exclusive case of a pure sine wave, the average value is 0.636 x peak value.

Averaging

Summing and suitably dividing several similar measurements to improve accuracy or to lessen any asynchronous components. Refer to definitions of RMS, time, and peak-hold averaging.

Axial

In the same direction as the shaft centre line.

Axial (thrust) Position

The average position, or change in position, of a rotor in the axial direction with respect to some fixed reference position. Ideally the reference is a known position within the thrust bearing axial clearance or float zone, and the measurement is made with a displacement transducer observing the thrust collar.