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Vibration is most commonly measured using a ceramic piezoelectric sensor or accelerometer. An accelerometer is a sensor that measures the dynamic acceleration of a physical device as a voltage. Accelerometers are full-contact transducers typically mounted directly on high-frequency elements, such as rolling-element bearings, gearboxes, or spinning blades. These versatile sensors can also be used in shock measurements (explosions and failure tests) and slower, low-frequency vibration measurements. The benefits of an accelerometer include linearity over a wide frequency range and a large dynamic range.
Another sensor you can use to measure vibration is the proximity probe. Unlike accelerometers, which measure acceleration to determine vibration, proximity probes are noncontacting transducers that measure distance to a target. These sensors are almost exclusively used in rotating machinery to measure the vibration of a shaft. An example of a common application is machine monitoring and protection measurements for mechanical systems like turbo machinery. Because of the flexible fluid film bearings and heavy housing, vibrations do not transmit well to the outer casing, so you use proximity probes instead of accelerometers to directly measure shaft motion.
Most accelerometers rely on the use of the piezoelectric effect, which occurs when a voltage is generated across certain types of crystals as they are stressed. The acceleration of the test structure is transmitted to a seismic mass inside the accelerometer that generates a proportional force on the piezoelectric crystal. This external stress on the crystal then generates a high-impedance, electrical charge proportional to the applied force and, thus, proportional to the acceleration.
Piezoelectric or charge mode accelerometers require an external amplifier or inline charge converter to amplify the generated charge, lower the output impedance for compatibility with measurement devices, and minimize susceptibility to external noise sources and crosstalk. Other accelerometers have a charge-sensitive amplifier built inside them. This amplifier accepts a constant current source and varies its impedance with respect to a varying charge on the piezoelectric crystal. These sensors are referred to as Integrated Electronic Piezoelectric (IEPE) sensors. Measurement hardware made for these types of accelerometers provide built in current excitation for the amplifier. You can then measure this change in impedance as a change in voltage across the inputs of the accelerometer.
To learn more about these and other hardware considerations for accelerometer measurements, download the Engineer's Guide to Accurate Sensor Measurements.
