Dataforth Application Note: Strain Gage Signal Conditioner


May 2018, MARIETTA, GA ~

Preamble
The most dominant application of bridge circuits is in the instrumentation arena. Within these applications, the strain gage as a resistive element sensitive to displacement has been used extensively for a century. Today, process variables such as load weight, pressure, flow, distance, motion, vibration, etc., employ strain gage bridge circuits as the fundamental sensor device. In addition, commercially available individual strain gage elements are combined with bridge circuit signal conditioning modules (SCMs) in custom instrumentation applications, such as structural beam deflections, internal strain within concrete structures, etc. There are scores of strain gage manufacturers with a wide spectrum of configurations for various applications. The reader is encouraged to examine strain gage manufacturers’ product specifications for more in-depth information on strain gage construction and specification details.

Examining all types of individual strain gage element configurations is beyond the scope of this application note; however, the behavior fundamentals of strain gages interfaced to Dataforth’s strain gage bridge signal conditioning modules will be examined. The effects of line resistance, strain gage parameter tolerances, and methods of excitation are included.

This application note is based on the basic bridge circuit fundamentals published in Dataforth’s Application Note AN117 “Basic Bridge Circuits,” which should be considered prerequisite reading; see Reference 1.

Review Conductive Resistance
The reader is encouraged to visit Dataforth’s Application Note AN105 (Reference 2) for the simple derivation of conductive current. Conductive current, “i,” is defined as coulombs/second, the flow of charged particles as shown in Equations 1 and 2.

The expression for resistance, Equation 2, shows that conductive resistance depends on geometry (length and area) and the molecular structure quantities: “n,” density of electrons available for motion; “q,” charge of an electron, and “ì ,” mobility of electrons. Electron charge “q” is constant; “free” electrons “n” are dependent upon temperature and internal molecular structure, and mobility “ì ,” the relative ease with which electrons can navigate through a molecular structure, is very sensitive to temperature, material composition, and structure deformity. Mathematically (a little calculus here), the change in conductive resistance is described in Equation 3, which again says resistance change is a function of both geometry and electron behavior.

Basic Strain Gage
The following resistive strain gage element fundamentals are based on the simple element geometry of length, “L,” and cross sectional area A=x*y where the x and y dimensions are much less than L. See Figure 1.

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