Solving Tough Strain Gauge Problems

June 1, 2006 By: H. Philip White, IOSelect Inc. Sensors

"What was old is new again" perfectly describes the use of carrier frequency technology in strain gauge measurements. And this approach takes the sting out of problems encountered when using strain gauges.

Before the digital technology explosion of the 1970s and 1980s, carrier frequency (AC) excitation of strain gauge–type devices (e.g., load cells and pressure sensors) was quite common, especially for applications characterized by varying temperatures and long measurement lead lengths. As digital technology progressed, it became possible to place the strain gauge signal conditioning close to the actual sensor. This allowed standard DC excitation to become equally accurate and much less expensive.

Over time, and to no seasoned engineer's surprise, measurement precision, stability, and speed requirements have increased. AC excitation never disappeared but was relegated to tough specialty applications that demanded high-performance capabilities and justified the steeper cost.

Today, that premium has been reduced to the point where AC technology is attractive for cost-sensitive industrial measurement applications.

Modern digital technology has brought the additional benefits of high resolution, high sampling rates, flexible signal processing, and numerous data transfer options. The resulting new products can be used in applications in which they are cost-effective and can be installed without changing existing transducers or control systems, and where standard DC technologies just can't deliver the results.

An Overview of Strain Gauges

If a strip of conductive metal is stretched, it will become thinner and longer. These changes result in an increase of electrical resistance from one end to the other. Conversely, if a strip of conductive metal is compressed, it will thicken, shorten, and decrease in resistance. If these stresses are kept within the elastic limit of the conductive metal strip (i.e., no permanent deformation occurs), the strip can be used as a measuring element for physical force, and the amount of applied force can be determined by measuring the change in resistance.

This type of device is called a strain gauge and is typically used to measure stresses generated in material. Most modern strain gauges are made by plating a thin layer of metal onto a polymer substrate. The device is bonded to the surface of the item where strain or deflection is to be measured (Figure 1). One common application is aircraft component testing, where strain gauge strips are glued to structural members or other critical components of an airframe to measure stress.

Figure 1. An example of a strain gauge bonded to a sample undergoing tensile-strength testing
Figure 1. An example of a strain gauge bonded to a sample undergoing tensile-strength testing

But applications using strain gauges do not have to be exotic. For example, electronic scales typically use a strain gauge to measure the weight of objects on the scale. Also, many pressure sensors use an engineered diaphragm with a strain gauge to electrically determine the applied pressure. In addition, one of the most common ways engineers and technicians encounter strain gauges is by using load cells. A load cell is just a strain gauge applied to a metal bar with known characteristics inside a pre-engineered package to measure a specific range of applied force.

Although the resistance of the strain gauge varies with the force applied, it does not vary much (typically only a fraction of 1% over the full force-rated range of the gauge). Therefore, the measurement instrument must be precise. To measure these small changes, a resistor-bridge (sometimes called a Wheatstone bridge) measurement technique is used. In many applications, a full-bridge configuration is used because it provides the most robust measurement, but it requires four strain gauges (Figure 2). In other applications, half- and quarter-bridge circuits can be used. In these applications, fewer strain gauges are deployed, and the others are replaced by appropriate resistors in the measurement instrument. This is called bridge completion.

Figure 2. Typical strain gauge
Figure 2. Typical strain gauge

DC excitation (the battery in Figure 2) can be easily replaced with an AC signal source. This is possible because the measurement circuit is simply a bridge of resistors, and it operates just as well with AC or DC voltage.

Measurement Instruments

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