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Proven Condition Monitoring Solutions

Accelerometer

Vibration is usually measured using piezo-electric transducers generally termed "accelerometers", the study of which can become a full time activity and is beyond the scope of this introduction.

Given that the choice of accelerometer should be made for a specific application, there are numerous variations on the market based on three fundamental designs, illustrated in Figure 1, where the crystal – in red – is in compression, shear and bending mode.

The compression accelerometer has the piezo-electric crystal sandwiched between the base and a seismic mass element, movement of the whole device causes the seismic mass to compress the crystal which responds by emitting a signal.

The flexural beam type depends upon the bending of the crystal over an anvil, or it may be cantilevered against the body of the device, either way the signal is produced by flexing the crystal.

The shear type mounts the crystal on a post surrounded by a peripheral mass, which induces a shear strain on the piezo-crystal.

Each has their own advantages and disadvantages:

Some accelerometers have built in pre-amplifiers and are referred to as ICP® types (Integrated Circuit Piezo-electrics) powered by a constant current. The presence of which effectively restrict their use to temperature environments below 120° C. The attempted use of these accelerometers in high temperatures will destroy the amplifiers internal components. It is worthwhile remembering that temperatures on a structure particularly an engine rise dramatically after shut-down, when cooling airflow is no longer available.

Frequency response is also important; all accelerometers have an internal resonant frequency which limits the dependable range over which their output is linear, a typical plot is shown in Figure 2.

Physical size and low mass are also important for some situations, clearly if space is restricted dimensionally smaller accelerometers may have to be selected. It is also important to consider mass as a heavy accelerometer plus mounting can modify the vibration characteristics that we are attempting to acquire.

The piezo-electric material used in an accelerometer has a structure which when strained causes the +’ve charge to be on one side of the crystal and the –‘ve on the other, see Figure 1. If an accelerometer is selected to be used in conjunction with a speed reference signal to acquire both phase and amplitude data it is important that the crystal is orientated similarly between accelerometers otherwise the phase will differ 180° between types. This can lead to incorrect diagnostic interpretation.

The design of the mounting of the piezo-crystal within the accelerometer case will determine the maximum amount of mechanical movement it can tolerate before it becomes overloaded. The signals from an overloaded accelerometer are distorted or 'saturated'. Information from saturated accelerometers is worthless for any analytical purpose.

All accelerometers are designed to measure vibration on a single axis. Movement in a transverse axis should produce no more than is quoted for in their specification for 'Transverse Sensitivity'. However if you are trying to measure vibration in an environment of high transverse movement the results should be treated with due caution.