6. Force transducers and impedance heads

A washer of piezoelectric material can easily be inserted into an interface between two structures (e.g. between a machine and its foundation) to measure the dynamic forces transmitted. It is usually prestressed by a central bolt. The forces transmitted are shared between the piezoelectric washer and this prestressing rod in exact proportion to their respective stiffnesses, which can be calibrated very precisely, enabling the total force to be determined. This device for measuring vibratory forces is known as a "load cell" or "load washer". They are generally made of piezoelectric quartz, for very high intrinsic stiffness, and measure dynamic forces only.

As with the construction of accelerometers, constructional details are essential to ensure good quality measurement. The surfaces involved must be ground and greased, and specially designed washers are often used....