Condition Monitoring Systems


Machinery health monitoring (MHM)

The current machine condition (signature) compared with an earlier condition used as an indicator to detecting potential failure before it occurs. (Also see Condition Monitoring)


Slight changes in the dimensions of iron or steel components resulting from changes in the magnetic fields acting on these components.


A physical property, dynamically computed as acceleration divided by force. Statically computed as W (which can be measured on a butcher scale) divided by the acceleration due to gravity. Ordinary structures are not pure masses as they contain reactive elements, i.e. springs and damping.


A value intermediate between quantities under consideration. A shaker’s mean acceleration must be zero – no steady-state acceleration. But a vehicle can have steady-state motion.

Mean-Time-Between-Failure (MTBF)

A basic measure of reliability for repairable items: The mean number of life units during which all parts of the item perform within their specified limits, during a particular measurement interval under stated conditions.


A basic measure of reliability for non-repairable items: The total number of life units of an item divided by the total number of failures within that population, during a particular measurement interval under stated conditions.

Measurement Parameters

A control group controlling Triggering, Averaging and Auto-Ranging.

Mechanical impedance

The ratio of force to velocity, where the velocity is a result of that force only. Its reciprocal, mobility, is today more favored.

Mechanical failure

A malfunction consisting of cracking, excessive displacement, misalignment, loosening, etc.

Mechanical Runout

An error in measuring the position of the shaft centerline with a displacement probe that is caused by out-of-roundness and surface imperfections.

MEMS – Micro Electro-mechanical Structures

Extremely small devices utilizing both electrical and mechanical properties.


Length or displacement equal to 10E-6 meter. One micron = 0.04 mil or 0.00004 inch.

Micrometer (MICRON)

One millionth (.000001) of a meter. (1 micron = 1 x 10E-6 meters = 0.04 mils.)


An instrument which converts a relatively small dynamic pressure change into an electrical signal. See transducer.


One thousandth (0.001) of an inch. (1 mil = 25.4 microns.) / Length or displacement equal to 0.001 inch or 25.4 x10 e-6 m.


Used to describe the angle of one shaft centerline to the other. It is equivalent to milliradians. It can also be expressed as rise/run (1 unit = 17.45 mils/inch), as long as the rise is measured in mils and the run is measured in inches.

Milliamp (mA)

One thousandth of an ampere.

Millivolt (mV)

One thousandth of a volt.

Mission Profile

A time-phased description of the events and environments an item experiences from initiation to completion of a specified mission, to include the criteria or mission success or critical failures.

Mission Reliability

The measure of the ability of an item to perform its required function for the duration of a specified mission profile. Mission reliability defines the probability that the system will not fail to complete the mission, considering all possible redundant modes of operation.

Mobility – Velocity / Force

Mobility is the inverse of mechanical impedance. It is a measure of the ease with which a structure is able to move in response to an applied force, and varies it with frequency. The vibration measured at a point on a machine is the result of a vibratory force acting somewhere in the machine. The magnitude of the vibration is equal to the magnitude of the force times the mobility of the structure. From this it follows that the amplitude of the destructive forces acting on a machine are not determined directly by measuring its vibration if the mobility of the machine is not known. For this reason, it is a good idea to measure the mobility at the bearings of a machine in order to find out the levels of the forces acting on the bearings due to imbalance or misalignment.

Modal analysis

The process of breaking complex structural motion into individual vibration modes. Resembles frequency domain analysis that breaks complex vibration down to component frequencies. / The process of breaking complex vibration into its component modes of vibration, very much like frequency domain analysis breaks vibration down to component frequencies.

A process of determining the natural frequencies, damping factors, and mode shapes for a structure. This is usually done either experimentally through frequency response testing or mathematically using finite element analysis. Complex structural motion is reduced to individual vibration modes. The process of determining a set of generalized coordinates for a system such that the equations of motion are both inertially and elastically uncoupled.


A characteristic pattern in a vibrating system. All points reach their maximum displacements at the same instant.

Mode Shape

The resultant deflected shape of a rotor at a specific rotational speed to an applied forcing function. A three-dimensional presentation of rotor lateral deflection along the shaft axis.

Modulation, Amplitude (AM)

The process where the amplitude of a signal is varied as a function of the instantaneous value of another signal. The first signal is called the carrier, and the second signal is called the modulating signal. Amplitude modulation produces a component at the carrier frequency, with adjacent components (sidebands) at the frequency of the modulating signal.

Modulation, Frequency (FM)

The process where the frequency of the carrier is determined by the amplitude of the modulating signal. Frequency modulation produces a component at the carrier frequency, with adjacent components (sidebands) at the frequency of the modulating signal.

Modulus of Elasticity E (static)

The initial slope of the stress vs strain curve, where Hooke’s Law applies, before the elastic limit is reached. Typical values are 30,000,000 pounds/square inch for steel and about 10,000,000 for aluminum.


Abbreviation for Mean (or average) time between failures.


Mean-Time-To-Failure. A basic measure of reliability for nonrepairable items; the total number of life units of an item divided by the total number of failures within that population.

Multiple-Degree-of-freedom-system (MDOF)

An N-degree-of-freedom system is a system whose position in space can be completely described by N coordinates or independent variables.

Multiplexer (Mux)

A switching device that sequentially connects multiple inputs or outputs in order to process several signal channels with a single A/D or D/A converter. / A device that selects multiple inputs into an aggregate signal.


The factor reflecting the sensitivity of a transducer in volts (or parts of a volt) per given engineering unit.