Sensor Sense: Piezoelectric Force Sensors

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 the sensor housing. This assembly resides between two mounting disks held together by an elastic, beryllium-copper stud and then weld-sealed within the enclosure to prevent contamination. The stud preloads the quartz elements to assure all parts are in intimate contact and to provide good linearity and tensile-force measurements.

When a force is applied to the impact cap, the quartz elements generate an output voltage which can be routed directly to a charge amplifier or converted to a low-impedance signal within the sensor. The use of the direct sensor output demands that any connector, cable, and charge amplifier input must maintain a high insulation resistance on the order of >10≠″ Ω.

Low-impedance quartz sensors have an internal MOSFET amplifier. Its output is a low-impedance voltage signal that uses standard cabling. However, force sensors with internal amplifiers do require external power to operate the amp.

This cross-section diagram of a Series 208 Quartz Force Sensor from PCB Piezotronics Inc. of Depew, N.Y., shows the basic assembly of a piezoelectric sensor.