There are a number of different accelerometer designs, each with their own characteristics, advantages, and disadvantages.
Vibration is usually measured using piezoelectric transducers, generally referred to as "accelerometers".
The right accelerometer should be selected for specific applications. There are numerous variations on the market, based on three fundamental designs (illustrated in Figure 1), where the crystal (in red) is in Compression Mode, Shear Mode, or Bending Mode.
The Compression Mode Accelerometer has the piezoelectric 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 Shear Mode Accelerometer has the piezoelectric crystal mounted on a post surrounded by a peripheral mass, which induces a shear strain on the crystal.
The Bending Mode ('flexural beam') Accelerometer is based on bending the piezoelectric crystal over an anvil; alternatively, it may be cantilevered against the body of the device - either way, the signal is produced by flexing the crystal.
Each type has its own advantages and disadvantages.
Some accelerometers have built-in pre-amplifiers and are referred to as ICP® types (Integrated Circuit Piezo-electrics). These are powered by a constant current, the presence of which effectively restricts their use to temperature environments below 120°C. The attempted use of these accelerometers in high temperatures will destroy the amplifier's internal components. It is worthwhile remembering that temperatures on any 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.
Physical size and low mass are also important for some situations; if space is restricted, smaller accelerometers may obviously 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 piezoelectric 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 charge on the other side. If accelerometers are intended for use in conjunction with a speed reference signal to acquire both phase and amplitude data, it is important to ensure that the crystal within each accelerometer has a similar orientation, otherwise the phase will differ 180° between types, which 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 that it can tolerate before it becomes overloaded. The signals from an overloaded accelerometer will be distorted or 'saturated'. Information from saturated accelerometers is worthless for any analytical purposes.
All accelerometers are designed to measure vibration on a single axis. Movement within a transverse axis should be no greater than quoted in the specification for 'Transverse Sensitivity'. If you are trying to measure vibration in an environment of high transverse movement, the results should be treated with due caution.
