Machine Design

Sensor Sense: Resistive Strain Gages

Strain gages make up the fundamental sensing element for many different types of sensors including those for pressure, load, torque, and weight.

They operate on a simple physical property of materials: The measured resistance of a material depends on its length and cross-sectional area.

A material under an applied force is said to be under stress in the form of compression or tension. For example, a block of rubber holds a large weight. If the weight sits on top of the rubber block, it compresses the rubber. A weight suspended below the block of rubber forces the rubber to expand under tension. The amount that the rubber expands or compresses is called the strain of the material. The ratio of stress to strain is known as the modulus of elasticity or Young’s modulus.

Many materials lengthen or compress fairly linearly in response to an applied force. That is, a doubling of stress produces twice the strain. This response, known as Hooke’s Law, is valid when the applied force per unit area is not too great. A strain gage measures the amount of change in the material, which can then be used to calculate the stress or force applied. By changing the material or its crosssectional area, the same strain gage can measure forces from grams to tons.

A typical strain gage is smaller than a postage stamp and consists of a resistive foil pattern bonded to a backing material. The most common pattern is a long single conductor snaking back and forth in an S pattern, although other patterns are possible depending on the stress measured. The gage mounts in such a manner that any tension or compression on the underlying material extends or shortens conductor length. A tension stress lengthens the conductor and makes it thinner, raising its resistance. Conversely, a compression stress shortens and thickens the conductor, forcing its resistance to drop.

To sense the extremely small change in resistance — typically a total resistance change of only a few milliohms — the strain gage serves as an element in a Wheatstone bridge circuit. The simplest arrangement is the quarter-bridge, where the strain gage replaces only one resistance in the bridge. Other arrangements place additional strain gages in half-bridge and fullbridge configurations for added sensitivity and some types of error compensation.

Strain gages consist of a resistive conductor mounted to a backing material. When tension or compression is applied to the length of the gauge, the resistance of the conductor changes. Forces applied from the sides have little impact.

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