Condition Monitoring Systems


Earthquake (Earth Quake)

An earthquake is a tremor of the earth’s surface usually triggered by the release of underground stress along fault lines. This release causes movement in masses of rock and resulting shock waves. A valuable tool for the analysis of earthquakes is Shock Response Analysis (SRS) which is provided as an option to SignalCalc systems and as a control option for the SignalStar controllers.

Eccentricity, mechanical

The variation of shaft surface radius when referenced to the shaft’s true geometric centerline. Also called Out-of-roundness.

Eccentricity Ratio

The vector difference between the bearing centerline and the average steady-state journal centerline.

Eddy current

Electrical current generated (and dissipated) in a conductive material (often a rotor shaft) when it intercepts the electromagnetic field of a displacement or proximity probe. / Electrical current which is generated (and dissipated) in a conductive material in the presence of an electromagnetic field.

Eddy Current Probe

A non-contact electrical device that measures the displacement of one surface relative to the tip of the probe. Construction consists of an electrical coil of various lengths and diameters. This coil located in the tip of the probe is energized producing an electrical field around the tip of the probe. When a conductive surface is placed in the field and the distance from the probe is noted, variations in this gap can be determined by the variations in the voltage flow to the probe tip.

Effective Mass

The frequency response function of force/acceleration.


The roots of the characteristic equation.

Eigenvalue Problem

The mathematical formulation and solution of the characteristic equation is called the Eigenvalue problem.


The mode shape vectors.

Electrical Runout

An error signal that occurs in eddy current displacement measurements when shaft surface conductivity varies.

Electromotive force

Force that causes the movement of electricity, such as potential difference of voltage. A measure of voltage in an electrical circuit.

Engineering Units (EU)

In a DSA, refers to units that are calibrated by the user (e.g., in/s, g’s). / Units that are decided upon by an individual user or by agreement among users. Examples include inches/second, mm/s, g, Hz, Tu, etc. / The units in which a measurement is made; for instance acceleration may be expressed in g, velocity may be expressed in milimeters per second.

External Sampling

In a DSA, refers to control of data sampling by a multiplied tachometer signal. Provides a stationary display of vibration with changing speed.


High resolution pulse generators.


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, performance, reliability or survival of an item.

Environmental engineering specialist

One whose principal work assignment lies in the technical area of natural and induced environments and their relation to military equipment. A person who has expertise in measuring and analyzing field environmental conditions, formulating environmental test criteria, specifying laboratory simulation of environments, and evaluating the effects of environments on equipment.

Environmental stress screening (ESS)

A post-production process in which 100% of produced units are subjected to stresses more severe than anticipated in service. The object is to precipitate latent defects into recognizable failures, so that that particular unit does not proceed further in production nor reach the customer.

A process in which products from the production line are subjected to thermal and / or vibration stresses to reduce the likelihood of early life field failures by forcing them to occur before final test in the factory.

Environmental testing

Subjecting a sample of products to a simulation of anticipated storage, transport and service environments (such as vibration, shock, temperature, altitude, humidity, etc.)

Equal Loudness Curves

Graphs of pure tone (constant or steady) sound pressure levels (labeled as to loudness level in phons) vs. frequency, with each graph representing equal loudness.


The difference between the indicated and the true values of a variable being measured. / The difference between the measured signal value or actual reading and the true (ideal) or desired value.

Essential machinery

See Critical Machinery.


The voltage or current applied to a transducer. / An external force (or other input) applied to a system that causes the system to respond in some way.

Exponential Averaging

A continuous averaging method wherein the most recent constituent is most influential and old information decays away exponentially with time.

Exponential Response Window

A windowing function for minimizing leakage in lightly damped structures. Typically used in Modal measurements made with an impact hammer. In a lightly damped structure, oscillations may not die out within the sampled time data block, Tspan, which results in leakage error. An exponential window adds damping to the time signal to force it to die out within the time T, thus minimizing leakage.


To create a file containing information in a format that can be used in a different program.

Export Folder

A folder, analogous to a Run folder, containing Signals in application-specific formats including ASCII, Standard Data Format (SDF), Universal File Format (UFF), ME Scope and SMS Star modal.

Extremal Control

A control strategy which selects the largest response at each frequency from the measured reference spectrum at control points to generate a combined spectrum to be used in the control loop.