Piezoelectric Accelerometer: Principle of Operation & Applications

An accelerometer is a sensor that is designed to measure acceleration or rate of change of speed due to motion, vibration e.g. from rotating equipment and impact events such as the deployment of an automobile airbag. Accelerometers are usually mechanically attached or bonded to an object or structure for which acceleration is to be measured. The accelerometer detects acceleration along one axis and is insensitive to motion in orthogonal directions. Sensing element may be made up of strain gauges or piezoelectric elements (which is our focus of discussion in this article).

Piezoelectric crystal is a material whose deformation results in charge polarization across the crystal. Contrariwise, the application of an electric field to a piezoelectric material results in deformation. The piezoelectric crystal material is used in the manufacture of some of the highest quality accelerometers used in various acceleration and vibration measurement applications.

A typical piezoelectric accelerometer consists of a crystal in contact with a mass, supported in housing by a spring as demonstrated in the figure below:

The function of the preload spring is to help keep the mass in contact with the crystal and to keep the crystal in compression which can aid in prolonging its life. The crystal itself also has stiffness that contributes to the overall stiffness of the system. In addition to the natural damping properties inherent in the crystal, additional damping is sometimes incorporated such as filing the housing with oil.

Piezoelectric Accelerometer Operation

In reference to the figure above, when the supporting object experiences acceleration, relative displacement occurs between the case/housing and the mass due to the inertia of the mass. The resulting strain in the piezoelectric crystal causes a displacement charge between the crystal conductive coatings as a result of the piezoelectric effect. The output of the accelerometer is attached to a charge amplifier, which converts the displacement charge on the crystal to a voltage that can be measured. Most accelerometers are calibrated in millivolts per g (acceleration due to gravity) for specified charge amplifier.

Generally, piezoelectric accelerometers can’t measure constant or slowly changing accelerations, because the crystals can only measure a change in force by sensing a change in strain. However piezoelectric accelerometers are excellent for dynamic measurements such as vibration and impacts.

It is important to note that an accelerometer employing a piezoelectric crystal requires no external power supply. Furthermore, take note of the fact that, the accelerometer measures acceleration only in the direction in which it is mounted i.e. along the axis of the spring, mass and crystal.

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