Sensor Sense: Piezoelectric Force Sensors

Some materials generate an electric charge when placed under mechanical stress. For example, a 2-kN force properly applied to a cubic-centimetersized quartz crystal produces over 12.5 kV. Voltage created by an applied stress is called piezoelectricity. Unlike strain gages that can measure static forces, piezoelectric force sensors are mostly used for dynamic- force measurements such as oscillation, impact, or highspeed compression or tension. Any force applied to the piezoelectric sensing element produces a separation of charges within the atomic structure of the material, generating an electrostatic output voltage. The polarity of the voltage generated depends on the atomic structure of the material and the direction in which the force is applied. However, any leakage path lets electrons redistribute across the material, dropping the voltage output back to zero. Internal leakage paths are formed by impurities within the crystal while external paths are created by the electronics used to measure the voltage generated. All leakages must be considered to determine the discharge time constant (DTC). The DTC typically follows an exponential curve similar to an RC time constant and is used to determine the sensor's lowest frequency response. In a typical quartz-based force sensor, a charge-collection electrode is sandwiched between two quartz-crystal elements. The quartz elements are oriented to supply the same polarity voltage to the electrode when compressed, while the opposite polarity is applied to