Strain-Gage-Based Transducers: Measurement of Force, Torque, and other Variables

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Strain-Gage-Based Transducers

The resistance strain gage is, in itself, a transducer, since it converts mechanical deformation, or strain, into a corresponding electrical signal. Beginning, however, with the initial motivation for its invention, the strain gage has always been applied not only to the measurement of strain for stress analysis purposes, but also as the sensing element in numerous types of transducers for measuring other mechanical variables such as force, torque, and pressure. This article deals with transducers in the latter sense, and is intended to provide guidance for designing and building a variety of simple transducers.

As pointed out by Robert Hooke in the 17th century, the deformation of a metal spring is directly proportional to the applied load. Although Hooke's measurements were of limited accuracy due to the instruments available in his time, it is a fact, nevertheless, that common structural metals such as steel and aluminum alloys are essentially linear in their elastic stress/strain characteristics. The foregoing applies, of course, only at stress levels below what is now referred to as the "proportional limit". A further restriction is that the overall deformation of the spring under the applied load is small enough that the spring geometry does not change sensibly. Otherwise, neither the stress nor the strain will be proportional to the load.