Transducers: Accelerometers

Accelerometers are by far the most common and versatile types of transducers in use. The seismic, or piezoelectric, accelerometer produces a output charge when held against a vibrating surface due to the characteristics of the piezoelectric disks that are inside the transducer. These transducers contain no moving parts and as such are quite rugged. The charge that is generated through the compression and expansion of the piezoelectric disks is proportional to the amount of vibration acceleration (force).  Generated voltage is very small - most models have built in hi-gain electronic amplifiers.  Output charge is proportional acceleration force accelerometer is exposed to. Accelerometers are the only transducer capable of reliably measuring high frequency vibration that is related to problems such as bearing defects and gear problems.  Accelerometers have fair to poor accuracy at low frequencies where acceleration amplitudes can become quite small even in the presence of high displacement amplitudes.  Some are somewhat susceptible to radio frequency interference  Some can have problems with thermal growth (putting the transducer on a very hot bearing) which can cause false amplitude values at low frequencies unless they are given time to thermally stabilize. Transducer should have sensitivity of 100 mv/G or higher. The more mv/G the accel generates, the more accurate it will be in the low frequency (< 120 cpm) roll-off ranges. One Design of a Piezoelectric  Accelerometer Recent years have seen the development of affordable accelerometers that are reasonably accurate to frequencies even well below 120 cpm. Low frequency will remain, however, a weakness of accelerometers due to the nature of acceleration amplitudes at low frequencies. Accelerometers are also useful because they are much more compact and lightweight than velocity transducers and therefore can be used in more environments and applications. They are also not as susceptible to magnetic fields generated by electric motors or other magnetic sources as velocity transducers.

A final advantage of using an accelerometer is the ability to integrate the results in order to obtain velocity readings and double integrate the results in order to obtain displacement readings. This ability means that a single transducer can be used to register and quantify all three of the primary amplitude units of acceleration, velocity and displacement over a wide range of common frequencies.