Piezoelectric accelerometer utilizes the phenomenon of Piezoelectricity in measuring vibration and acceleration of an object or a structure. The Principle of Piezoelectric Effect revolves around inducing a potential difference across a solid material when a mechanical stress is applied to it. In fact, the literal meaning of Piezoelectricity is electricity resulting from pressure and latent heat. An electric charge is generated as output in response to Vibrations. Similar to how other accelerometers operate, Piezoelectric Accelerometer has a seismic mass attached to a spring or cantilever beam. Before getting converted into an electrical energy, the acceleration is converted into a force or displacement. This is done by a mass-spring system built inside the accelerometer. In the accelerometer, on experiencing a physical force, the seismic mass loads the piezoelectric element and an electrostatic force or a voltage is generated in lieu of the force experienced on the piezoelectric element.
The main driving element in a Piezoelectric Accelerometer is the Piezoelectric Ceramic. On one side of the ceramic, the accelerometer body is attached, while on the other side of it, a seismic mass is attached. When the accelerometer experiences a vibration, a force is acted upon the seismic mass and Piezoelectric Ceramic. As a result of this applied force, a charged output is generated. There are basically three ways in which Piezoelectric Accelerometers are configured.
- Shear Mode: The piezoelectric Ceramic is positioned to be perpendicular to the base of the accelerometer. This causes the Piezoelectric Ceramics to go through shear deformation. The Ceramics are made to isolate from the base to reduce the temperature sensitivity. Although this combination promises a high performance, the sensitivity-to-mass ratio, which is managed by using a charge amplifier.
- Compression Mode: The Piezoelectric Ceramic is directly aligned with the Seismic Mass and the base. Because of erroneous results produced by such kind of combination, this is rarely used, except in high shock applications. Provides moderate to high sensitivity-to-mass ratio.
- Piezoelectric bender: This type of combination provides the most suitable sensitivity-to-mass ratio and doesn’t even require power to operate. These are, however, very fragile and highly temperature sensitive. Hence, these are rarely used for accurate testing.
Types of Piezoelectric Accelerometers
Charge type piezoelectric accelerometer
These types of accelerometers measure charge generated by crystal element operating in either shear or compressed mode. These are ideal to be used in high temperature (up to 260 ˚C) or high radiation environments. These accelerometers do not require any integral electronics, but have amplifying electronics mounted several feet away from the source of local heat or radiation. The charge-type accelerometers are self-generating. These accelerometers are ideal to be used in structural testing, machine monitoring and measuring shocks and vibrations in vehicles where there is an exposure to a high temperature.
IEPE Accelerometer (Integrated Electronics Piezoelectrics Accelerometer)
IEPE accelerometers are the sensors that generates electrical output that is in proportion to the applied acceleration. Integrated Electronic Circuits are used in Accelerometers to convert high impedance output voltage to low impedance of about 100 Ω. These low-impedance signals can be readily transmitted over to any data acquisition system or readout device via any wire/cable. Now-a-days, Piezoelectric sensors have this functionality of charge amplifier and voltage amplifier inbuilt in them. Low impedance Signals ensures that the signal is transmitted without any loss in quality even across long-length cables.
The IEPE sensors work with a constant current i.e. between 2 to 20 mA. This constant current supply results in a positive bias voltage as output, typically between 8 V to 12 V. IEPE circuits have the following advantages:
- Cost effective, since the expensive cables and signal conditioners are cut out.
- Fixed sensitivity is ensured which is independent of the cable length and quality.
- Due to their ability of giving very accurate measures, they are widely used for many testing purposes.
Uses and Applications
Condition Monitoring and Fault Diagnosis in Machines
Checking the overall vibrations in a machine will tell you about its impending problems. A full-machine frequency analysis can give more information about the machine’s health.
Aerospace and Flight Test
The very main motive behind using a Piezoelectric Accelerometer in aircrafts or an engine is purely to detect faults and prevent any potential damage incurred by these heavy machineries. They are used to measure the vibration on gas-turbine engines in the fight and test cells.
Defense and Military Testing
Used in voluminous amounts in the military for vibration test in high temperature gas turbine and helicopter vibration monitoring. Piezoelectric accelerometers provide a wide range of usable frequency response to several thousand hertz which make them one of the most cost-effective vibration monitoring solution available to military applications.
Aerodynamic shock tube and Seismograph
Piezoelectric accelerometers are used in the study of high-speed explosions and blast-waves propagation in high-end laboratories. They may also be used in rocket and nuclear applications.