Condition Monitoring Systems




Integral Electronics Piezoelectric transducer, using a constant current source as the conditioning medium.

Imaginary Part

A plot of the imaginary part of the frequency response function versus frequency. For a single-degree-of-freedom system, the magnitude is a maximum or minimum at the damped natural frequency.


Unequal distribution of weight or mass on a rotor. The geometric center of the mass is not where it should be: along the shaft centerline.


Unequal radial weight distribution on a rotor system; a shaft condition such that the mass and shaft geometric centerlines do not coincide.


A collision between masses.

Impact test (bump test)

A broad frequency range of structural responses is caused by a deliberate impact.

Impact Test

Response test where the broad frequency range produced by an impact is used as the stimulus. Sometimes referred to as a bump test.

Impact Testing – Hammer Test

A method of measuring the frequency response function of a structure by hitting it with a calibrated hammer and measuring the system’s response. The impact hammer is instrumented with a force transducer to measure the input force pulse while the response is typically measured using an accelerometer. The impact imparts a force pulse to the structure that excites it over a broad frequency range.

Impedance, Mechanical

The mechanical properties of a machine system (mass, stiffness, damping) that determine the response to periodic forcing functions.

The mechanical impedance of a point on a structure is the ratio of the force applied to the point to the resulting velocity at the point. It is a measure of how much a structure resists motion when subjected to a given force, and it is the reciprocal of mobility. The mechanical impedance of a structure varies in a complicated way as frequency is varied. At resonance frequencies, the impedance will be low, meaning very little force can be applied at those frequencies.

Impedance, electrical

The opposition to the flow of AC current, the equivalent of the resistance in DC circuits. Its unit is the ohm. The impedance of an AC circuit is one ohm if a potential difference of one volt creates a current flow of one ampere in it.


The property by which an electromotive force (emf) is induced in a conductor when the magnetic field is changing about it. This is usually caused by changes in the current flow in the circuit or in a neighboring circuit.


To convert and use a file created by another program.


The integral of force over a time interval.

Impulse Response

The response of a system to an impulse as input signal. The output then produces the impulse response that is the time domain equivalent to the Frequency Response Function, FRF.

Impulse Signal

A sharp “spike” positioned 5% from the beginning of a sample block; produces a near-flat spectrum to Fspan.


A control setting that allows a Test to produce a sequence of incrementally number Run folders, one for each run of the Test.

Induced environments

Conditions generated by operating some equipment, as opposed to natural environments.

Inertance (or accelerance)

The ratio of acceleration to force.


Motion that is referenced to free space or to a fixed point in space. A sensor (such as an accelerometer) which measures such motion.

Input Channels Parameters

In SignalCalc Dynamic Signal Analyzers: Controls for each input channel:

Influence Coefficients

Mathematical coefficients that describe the influence of system loading on system deflection.


The mechanical motion, force or energy applied to a mechanical system, e.g. the vibratory input from shaker to test item. Or an electrical signal, e.g. from an oscillator to the power amplifier driving a shaker.

Input control signal

Originates in a control sensor; sometimes selected between or averaged between several sensors. Used to regulate shaker intensity. (May originate in a force sensor for force-controlled testing.)

Input Impedance

The shunt resistance and capacitance (or inductance) as measured at the input terminals, not including effects of input bias or offset currents.

Input/output (I/O)

The analog or digital signals entering or leaving a DCS or other central control or computer system involving communications channels, operator interface devices, and/or data acquisition and control interfaces.


Representation inversely related to time rate of change. Example: integrating velocity yields displacement. In a computer, this is accomplished by dividing the velocity signal by jw, where w is frequency multiplied by 2p. / A process producing a result that, when differentiated, yields the original quantity. Integration of acceleration, for example, yields velocity. Integration is performed in a DSA by dividing by jw, where w is frequency multiplied by 2þ. (Integration is also used to convert velocity to displacement).


Circuitry which converts an acceleration signal to a velocity signal or a velocity signal to a displacement signal. / Circuitry which converts an acceleration signal to a velocity signal or a velocity signal to a displacement signal.


The severity of a vibration or shock. Nearly the same meaning as Amplitude, defined earlier, but less precise, lacking units.

Intrinsically safe

Equipment or wiring which is incapable of releasing sufficient electrical or thermal energy to ignite a hazardous mixture of hydrocarbon vapors and air. In such equipment the electrical energy is limited so that it can not generate a spark or otherwise ignite a flammable mixture.


Formerly the Instrument Society of America, now referred to as the International Society for Measurement & Control.


A reduction in motion severity, usually by a resilient support. A shock mount or isolator attenuates shock. A vibration mount or isolator attenuates steady-state vibration.

A reduction in motion severity, usually by a resilient support. A shock mount or isolator attenuates shock. A vibration mount or isolator attenuates steady-state vibration. Isolators are used on electrodynamic shakers to reduce transmission of vibration energy into the floor.


Sometimes called glass itch or lace itch. An automotive sound caused by metal sliding on glass.

Iterative closed loop control

Pre-calculated, then modified drive signals based upon resulting motion, in order to better match measured with desired motions. Evaluation and modifications take place after each excitation, repeating until the match is acceptable.


The rate of change of acceleration with time.


Specific portions of the shaft surface from which rotor applied loads are transmitted to bearing supports.


A signal used in rotating machinery measurements, generated by a sensor that observes a once-per-revolution event. Used in phase measurements for analysis and for balancing. Bentley-Nevada trade name / A signal used in rotating machinery measurements, generated by a transducer observing a once-per-revolution event. The keyphasor signal is used in phase measurements for analysis and balancing. (Keyphasor is a Bently Nevada trade name.)