Measurement of Vibration and Acceleration. 

Vibration and shock are the two main physical characteristics that define the motion in structures and machine components. Vibration monitoring of important machines, such. as power station turbines and generators are very necessary as it gives an early wanting of impending conditions which may develop and lead to complete failure and destruction of the machines and other equipments. The quantities which are required to be measured for measurement of vibration are displacement, velocity, and acceleration but measurement of acceleration is preferred.

The most important transducer for measurement of vibration, shock, and general-purpose absolute motion is the accelerometer, which is commercially available in a wide variety of types and ranges in order to meet the requirements of diverse applications.

Seismic transducer. also known as a seismic accelerometer, is used for measuring acceleration. This transducer can be used in two different modes namely, 

(1) displacement mode and 

(2) acceleration mode. 

Various types of accelerometers used are potentiometric, LVDT, piezoelectric, strain gauge and null balance of servo type accelerometers.

Potentiometric accelerometer is the most simple type of accelerometer and is mainly used for slowly varying accelera­tions and low-frequency vibrations. Such accelerometers have the drawbacks of limited resolution and low natural frequency. The sliding contacts may also give errors.

LVDT  accelerometers are contactless devices,  have high natural frequency because of the smaller mass of core, offer low resistance to motion than the potentiometers, and are capable of much better resolution. Such accelerometers are used for steady­ state and low-frequency vibration measurements.

Piezoelectric accelerometers are quite small in size and are of very small weight (say 25 g) and have very high natural frequency (as high as 100 kHz). The crystal has high input impedance and so needs a high input impedance voltage monitoring system in order to avoid the loading effect.

Such accelerometers are useful for high input frequencies but their response for low frequencies is very poor. Hence such accelerometers should not be used where the input frequency is lower than 10 Hz.

Piezoelectric transducers are widely used for shock and vibration measurements.

Strain gauge accelerometers are of three types-unbonded strain gauge accelerometers, bonded strain gauge accelerometers, and semiconductor strain gauge accelerometers.

Unbonded strain gauge accelerometers employ the strain wires as the spring element and as the motion transducer. Such accelerometers are used for general-purpose motion measurement and for vibrations up to relatively high frequencies. The characteristics of bonded strain gauge accelerometers are simi­ lar to those of unbonded strain gauge accelerometers except for their large size and weight.

The semiconductor strain gauge accelerometers have many desirable properties such as a larger natural frequency of more than 10 kHz with a damping ratio of 0.06. The low damping ratio is because of material hysteresis and air drag and no intentional damping is used. The usable frequency range is from 0 to 2 kHz. Semiconductor strain gauges are widely used as strain sensors.