![]() ![]() For vibrations parallel to the polarization, the force acting upon the piezoelectric element is given by Newton’s second law of motion and will generate a proportional charge as shown earlier. In its simplest form, a piezoelectric accelerometer consists of a thin piezoelectric element with polarization perpendicular to the main surface, clamped between a so-called seismic mass or proof mass and a base plate. Often quartz is the piezoelectric material used in sensors intended for measuring force, torque, strain and pressure, especially when high stability and/or quasi-static measurements are important, but piezoelectric ceramics may also be chosen for a handful of these sensor types to take advantage of the superior charge output and shaping possibilities inherent to the piezoelectric ceramic.Īcceleration sensors and acoustic emission sensorsĪ sensor for measuring acceleration is called an accelerometer and can be seen as a force sensor with a mass attached to it. Piezoelectric sensors for extremely small pressure variations in the range of μbar are called piezoelectric microphones or – for underwater applications – piezoelectric hydrophones. It is common to classify these sensors according to the measured pressure, with low-pressure sensors measuring up to a few bar, and high-pressure sensors for pressures above 1 kbar. Similarly, pressure sensors are a special case of force sensors, generally using a membrane with a well-defined area. However, a few piezoelectric sensors measuring surface strain also exist. ![]() Most of these sensors are intended for measuring internal strain, longitudinal or transverse, and are designed as so-called strain pins to be inserted in a bore. They have the important advantage over strain gauges that they are very well suited for dynamical measurements. Piezoelectric strain sensors can be seen as force sensors with a design that emphasizes strain. Reaction torque sensors may also rely on the piezoelectric effect, using several shear-mode piezoelectric elements, whereas torque measurements on rotating shafts typically are done with strain gauges. The bimorph element consists of two piezoelectric plates, either poled in opposite directions (series configuration) or in the same direction (parallel configuration). Where d 33 is the piezoelectric charge coefficient.Īlternatively, a flexure mode design may be used for certain piezoelectric sensors, in which case a so-called bimorph element within the sensor is deflected by the input force. For a poled piezoelectric ceramic, the charge Q is proportional to the input force F according to the following equation It follows directly from the basic piezoelectric equations that a piezoelectric element will produce a charge when a force is applied parallel to the direction of polarization. Sensors for force, torque, strain and pressure The upper frequency limit is generally given by the natural frequency of the piezoelectric sensor, which in turn is determined by the design. As a rule of thumb, 10 Hz can be considered as the minimum frequency for piezoelectric ceramics. ![]() Typically, such piezoelectric sensors are less suitable for low-frequency signals due to charge dissipation. Through the direct piezoelectric effect, several mechanical quantities can be measured, including force and torque, strain, pressure, acceleration and acoustic emission. Sensors using the direct piezoelectric effect While the majority of piezoelectric sensors are based on the direct effect, some also exploit the converse effect as is the case with resonant piezoelectric sensors. Piezoelectric sensors utilize the direct piezoelectric effect to convert a mechanical input into an electric signal. ![]()
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