Page 164 - MEMS Mechanical Sensors

CHAPTER 7
Force and Torque Sensors

Professor Barry E. Jones and Dr. Tinghu Yan

7.1 Introduction

In a highly mechanized world, force and torque are among the most important of all
measured quantities [1–4]. They play a significant role in products from weighing
machines and load cells used in industrial and retailing applications, to automotive
and aerospace engines, screw caps on medicine bottles, and nut and bolt fasteners.
Forces and torques can range from greater than 10 kN to less than 1 µN, and from
50 kNm to below 1 Nm, respectively. Measurement accuracy levels required can
vary widely from, say, 5% to better than 0.01% of full scale ranges, depending on
the application. Hysteresis and nonlinear effects in the mechanical structures of
measuring devices need to be small, and measurement resolutions need to be high.
Measurement devices need to be robust to withstand changing environmental
influences such as temperature, vibration, and humidity, and they must also provide
reliable measurement over long periods of time. Mechanical interfacing of the
devices can be difficult and can influence final measurement. The forces and torques
may change rapidly, and so the devices must have adequate frequency and transient
responses.
There are several methods to measure forces and torques. Often, the force to be
measured is converted into a change in length of a spring element. The change in
dimensions is subsequently measured by a sensor, for example, a piezoresistive, a
capacitive or a resonant sensor.
It is not so surprising, therefore, that most force and torque measurement
devices utilize the long and well-established resistance strain gauge technology.
Unfortunately, the metallic resistance strain gauge is relatively insensitive such that
in use it is normal to obtain only several millivolts of analog voltage before amplifi-
cation, and the gauges must not be significantly overstrained. The rangeability and
overloading capabilities are seriously restricted. Also, the gauges consume relatively
high electrical power (e.g., 250 mW).
In general, measurement instrumentation now needs smaller sensing devices of
lower power consumption and with greater rangeability and overload capabilities.
Greater compatibility with digital microelectronics is highly desirable. Noncontact
and wireless operation is sometimes needed, and in some cases batteryless devices
are desirable. Production of measurement devices using metallic resistance strain

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