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# C

Calibration

An orderly procedure or test for determining the sensitivity of a transducer or instrument as a function of frequency, temperature, speed etc. Known values of the measured variable are applied to the transducer or instrument, and output readings are varied or adjusted. During the process, values are compiled, and deviation charts are produced, so that the reading can be correlated to the actual values being measured.

Calibrate

To ascertain that the output of a device properly corresponds to the information it is measuring, receiving or transmitting. This might involve the location of scale graduations, adjustment to bring the output within specified tolerance or ascertaining the error by comparing the output to a reference standard.

Calibration curve

A graphical representation of the calibration report, which report can be in the form of a table or chart.

Calibration cycle

The application of known values of the measured variable and the recording of the corresponding output readings over the range of the instrument in both ascending and descending directions.

Calibration traceability

The relationship of the calibration process to the calibration steps performed by a national standardizing laboratory.

Campbell diagram

A mathematically-constructed diagram used to check for coincidences of vibration sources (1x imbalance, 2x misalignment, etc. shaft speed) with rotor natural frequencies, resulting in rotor resonances. It plots frequency vs. RPM, with plot size growing with increasing amplitude. Sometimes called an interference diagram.

The form of the diagram is a rectangular plot of resonant frequency (y-axis) vs. excitation frequency (x-axis). Also known as an interference diagram.

See Spectral Map.

Cavitation

A localized low-pressure condition which can occur in liquid handling machinery (e.g. centrifugal pumps pr compressors)  where a system pressure decrease in the suction line or in the pump inlet line, lowers the fluid pressure, and vaporization occurs (cavities or bubbles) within the liquid, for example around a propeller or impeller. As the bubbles collapse, the resulting mixed flow may produce unwanted sound and/or vibration. May be destructive.

Center Frequency

For a band-pass filter, the center of the transmission band.

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

A sensor (or pickup or transducer) with its associated signal conditioner and monitor (for observing the signal) and recorder (for storing the signal).

Channel Parameters

A control group affecting Input channel settings and Signal Generator outputs.

Characterization is aimed at providing the shaker control system with needed information about the testing system and the test article. Pretest characterization (at reduced intensity), repeated several times – iteration – is commonly performed. Increasingly, adaptive characterization – modifying the control system, based upon observing responses, is being performed during the test.

Charge amplifier

An amplifier which converts a very high impedance charge signal (as from a piezoelectric charge coupled accelerometer) into a low impedance output voltage i.e. a charge-to-voltage converter. This makes calibration much less dependent on cable capacitance.

Circuit card / Circuit Board

A flat board that holds chips and other components on the top side and has printed electrically conductive paths in multiple layers for the components on its bottom side.

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. See iterative closed loop control.

Closed loop motor controller

PID Controller used for precise control of multiple motor.

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

An effective means of measuring the similarity of vibration at two locations, giving insight into possible cause and effect relations.It is the ratio of the coherent output power between channels in a dual-channel Digital Signal Analyzer (DSA).

Co-incident

Another name for the real part of the frequency response function.

Common mode rejection

The ability of a circuit to discriminate against a common mode voltage.

Common mode voltage

A voltage of the same polarity on both sides of a differential input relative to ground.

Comparison

A term applied to calibration (e.g. of an accelerometer) in which sensitivity is tested against a standard. See also Absolute Calibration.

Compliance

The reciprocal of stiffness, i.e. displacement divided by force.

Condition monitoring (CM)

The measurement, recording and analysis of machinery parameters (such as acceleration) to determine machinery health.

The current machine condition or signature, is compared with earlier condition or fingerprint, usually taken when a machine was new or newly refurbished. It is used as an indicator for detecting potential failure before it occurs. Also called Machinery Health Monitoring.

Constant bandwidth filter

A band-pass filter whose bandwidth is independent of center frequency. Filters simulated digitally by an FFT process are constant bandwidth.

Constant percentage filter / Constant percentage bandwidth

A band-pass filter whose bandwidth is a constant percentage of the center frequency of that same filter (¼ x, ½ x etc).

Contact Bounce

The intermittent opening of relay contacts during closure.

Control Potentiometers

Multi-turn potentiometers with programmable display.

Correlation

A Test type providing Auto and Cross Correlation functions to measure cause/effect in the time domain

Counter

Device used to totalize production volumes

Critical Damping

The smallest amount of damping required to return a system to its equilibrium condition without oscillating.

Critical frequency

A particular resonant frequency (see resonance) at which damage or degradation in performance is likely.

Critical machines

Machines in a process or power plant which are vital to continued operation. These machines are usually un-spared. Machine Health should be monitored continuously.

Critical speeds

Any rotating speed which results in high vibration amplitudes. Often these are speeds which correspond to system natural frequencies.

Critical Speed Map

A rectangular plot of system natural frequency (y-axis) versus bearing or support stiffness (x-axis).

Cross-axis sensitivity

A measure of off-axis response of velocity and acceleration transducers (see also Transverse Sensitivity/)

Crossover frequency

In sinusoidal vibration testing, the unique forcing frequency at which the required displacement yields the desired acceleration and vice versa.

Cross-talk

Interference or noise in a sensor or channel, coming from another sensor or channel.

Cycle

One complete sequence of values of a periodic quantity – for exampl,e a sine wave processing from zero to 360°.