TABLE OF CONTENTS Dr. Thomas Kenny, Department of Mechanical Engineering, Stanford University
Strain gages are used in many types of sensors. They provide a convenient way to convert a displacement (strain) into an electrical signal. Their output is actually a change of resistance. It can be converted to a voltage signal by connecting the strain gage(s) in a bridge configuration. A few sensors use only a single strain gage element in the bridge, along with three fixed resistors. Others use two strain gages and two fixed resistors, and most recent designs use four strain gages. The gages may be almost any material, but some materials are much more efficient strain gages than others. Proprietary metal alloys and semiconductor silicon are the most commonly used materials.
A piezoresistor is a device that exhibits a change in resistance when it is strained. There are two components of the piezoresistive effect in most materials the geometric component and the resistive component.
The geometric component of piezoresistivity comes from the fact that a strained element undergoes a change in dimension. These changes in cross-sectional area and length affect the resistance of the device.
A good example of the geometric effect of piezoresistivity is the liquid strain gage. A great many of these devices were in use years ago. Imagine an elastic tube filled with a conductive fluid, such as mercury. The resistance of the mercury in the tube can be measured with a pair of...
