TABLE OF CONTENTS Ken Watkins, PCB Piezotronics, Inc.
Measurement of a force, load, or a weight can be accomplished by a multitude of different sensors. While other technologies exist, the most commonly used sensors are generally based on either piezoelectric quartz crystal or strain gage sensing elements, which will be the focus of this chapter.
Before proceeding, it is important to recognize the difference between force, load, and weight, as these terms are often incorrectly used interchangeably with one another.
Force: The measurement of the interaction between bodies.
Load: The measurement of the force exerted on a body.
Weight: The measurement of gravitational forces acting on a body.
Quartz force sensors are ideally designed and suited for the measurement of dynamic oscillating forces, impact, or high speed compression/tension forces. The basic design utilizes the piezoelectric principle, where applied mechanical stresses are converted into an electrostatic charge that accumulates on the surface of the crystal. See Figure 11.2.1.
The quartz crystals of a piezoelectric force sensor generate an electrostatic charge only when force is applied to or removed from them. In other words, if you apply a static force to a piezoelectric force sensor, the electrostatic charge output initially generated will eventually leak away and the sensor output ultimately will return to zero.
The rate at which the charge leaks back to zero is exponential and based on the sensor s discharge time constant (DTC). DTC is defined as...
