Condition Monitoring Systems


Narrow band Analysis


Natural environments

Conditions occurring in nature, not caused by any equipment; effects are observed whether an equipment is at rest or in operation.

Natural frequency

The frequency of an undamped system’s free vibration; also, the frequency of any of the normal modes of vibration. Natural frequency drops when damping is present / The frequency of free vibration of a system. The frequency at which an undamped system with a single degree of freedom will oscillate upon momentary displacement from its rest position.

Nodal Point

A point of minimum shaft deflection in a specific mode shape. May readily change location along the shaft axis due to changes in residual imbalance or other forcing function, or change in restraint such as increased bearing clearance.


A point or line on a vibrating structure that remains stationary.


The total of all interferences in a measurement system, independent of the presence of signal. / Any component of a transducer output signal that does not represent the variable intended to be measured. / Any undesirable electrical signal, from external sources such as AC power lines, motors, electrical storms, radio transmitters, as well as internal sources such as electrical components. Non-linearity: The deviation from the best fit straight line that passes through zero. Normal-mode rejection ratio: The ability of an instrument to reject electrical interference across its input terminals, normally of line frequency (50-60 Hz).

Noise Floor

The minimum discernible signal that can be detected by a receiver.

Non Linearity

The deviation from a best fit straight line of true output vs. actual value being measured. / The deviation from a best fit straight line of true output vs. actual value being measured.


Minimum spectral value, at a natural frequency. Also, the deliberate reducing of a portion of a test spectrum (random vibration testing).

Nyquist Criterion

Requirement that a sampled system sample at a frequency greater than twice the highest frequency to be measured.

Nyquist Frequency

Digital signal processing requires analogue to digital (A to D) conversion of the input signal. The first step in A to D conversion is sampling of the instantaneous amplitudes of signal at specific times determined by the sampling rate. If the signal contains any information at frequencies above one-half the sampling frequency, the signal will not be sampled correctly, and the sampled version of the signal will contain spurious components. This is called aliasing. The theoretical maximum frequency that can be correctly sampled is equal to one-half the sampling rate, and is called the Nyquist frequency. In all digital signal-processing systems, including FFT Analyzers, the sampling rate is made to be significantly greater than twice the highest frequency present in the signal in order to be certain the aliasing will not occur.

Nyquist Plot

A plot of real versus imaginary spectral components that is often used in servo analysis. Should not be confused with a polar plot of amplitude and phase of 1x vibration.

A plot of the real part versus the imaginary part of the frequency response function. For a single-degree-of-freedom system, the Nyquist plot is a circle. The Nyquist plot is representation of a frequency response function by graphing the “real” part versus the “imaginary” part. In the Nyquist plot, a resonance is displayed as a circle, but without indication of its.

Nyquist theorem

This law is the basis for sampling continuous information. It states that the frequency of data sampling should be at least twice the maximum frequency at which the information might vary. This theorem should be observed in order to the preserve patterns in the information or data, without introducing artificial, lower frequency patterns.


The interval between two frequencies differing by exactly 2:1.


Original Equipment Manufacturer.

Ohm meter

A device used to measure electrical resistance. One-to-one repeater: A diaphragm operated device which detects the process pressure and generates an air (or nitrogen) output signal of equal pressure.

Oil Whirl / Oil Whip

An unstable free vibration whereby a fluid-film bearing has insufficient unit loading. Under this condition, the shaft centerline dynamic motion is usually circular in the direction of rotation. Oil whirl occurs at the oil flow velocity within the bearing, usually 40 to 49% of shaft speed. Oil whip occurs when the whirl frequency coincide with (and becomes locked to) a shaft resonant frequency. (Oil whirl and whip can occur in any case where fluid is between two cylindrical surfaces. )

Open Loop Control

In vibration control, provides pre-computed or preconceived drive signals to the exciter system without modifying or refining those signals based on observation of the resulting motion. See also Closed Loop.

Operational environment

The aggregate of all external and internal conditions (such as temperature, humidity, radiation, magnetic and electric fields, shock vibration, etc.) either natural or man made, or self-induced, that influences the form, operational performance, reliability or survival of an item.

Operational limit

The extremes beyond which a product is not expected to operate.

Optical isolation

Two networks or circuits in which an LED transmitter and receiver are used to maintain electrical discontinuity between the circuits.


The path of a shaft centerline during rotation. The orbit is usually observed on an oscilloscope connected to x- and y-axis displacement sensors. Sometimes called a Lissajous pattern. / The path of the shaft centerline motion during rotation. The orbit is observed with an oscilloscope connected to x and y-axis displacement transducers. Some dual-channel DSAs also have the ability to display orbits.


Vibration at multiples of a machine’s shaft speed. See also First order vibration.

Order tracking

Control of a computer’s data sampling rate, such that display of vibrations at multiples of the shaft’s rotating speed (´1, ´2, ´3, etc.) are enhanced and more easily identified and measured..


Variation with time of a quantity such as force, stress, pressure, displacement, velocity, acceleration or jerk. Usually implies some regularity (as in sinusoidal or complex vibration).


A signal conditioning device that sends a radio frequency signal to an eddy-current displacement probe, demodulates the probe output, and provides output signals proportional to both the average and dynamic gap distances.

Output settling time

Time required for the analog output voltage to reach its final value within specified limits.

Output signal

A signal delivered by a device, element or system.

Output slew rate

Maximum rate of change of analog output voltage from one level to another.

Overall RMS Level

A measure of the total RMS magnitude within a specified frequency range.


In Dynamic Signal Analysis: The maximum percent (of Tspan) of adjacent measurement overlap that is permitted when the process runs faster than real-time; the amount of measurement redundancy desired.

Overlap Processing

In Dynamic Signal Analysis: The windowing applied to a time signal stored in a buffer before being processed to form the spectrum causes data at the beginning and end of the time records to be represented at the wrong amplitude values, creating errors in the spectral amplitude levels. Overlap processing allows processing of the entire contents of the input buffer, more frequently than the time taken for the buffer to completely refresh (Tspan). Overlap processing is desirable when using a Hanning Window because it ensures against loss of data for parts of the signal that occur near the beginning and end of the window. An overlap of 66.7% will completely correct for amplitude errors caused by the Hanning window.


That part of the stroke which falls between the end of the calibrated range and the travel stop.


The process of writing new data over existing data. This option causes you to lose the information that has been written over.