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Strain Gauge Force Sensor

By: Mel Joelle

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Loosely defined, a strain gauge is a device that takes advantage of the physical property of electrical conductance of a material, and its relationship to not only the electrical conductivity (a property of the material itself), but also the geometry of the conductor. When a conductive material is stretched to the limits of its elasticity (without breaking or deforming), its electrical resistance rises. On the other hand, a compressed conductive material (without being crushed or buckled) decreases in its electrical resistance. A strain gauge measures these properties in a long, thin conductive strip. With the right application and the proper adhesive, strains as great as 10% can be measured in, for instance, a metal beam.

FOIL GAUGES

Many applications use a foil strain gauge. Attached to a load cell, foil strain gauges stay stable over a period of many years. Thus they are a technology commonly used in the manufacture of pressure sensors for scales. Other materials might include thick film, metal film, bonded foil, silicon or polysilicon.

The foil material itself is usually made from a constantan alloy, which is designed to compensate for temperature effects on the resistance of the strain gauge. Measurements of small strain are often done with semiconductor (piezoresistor) strain gauges, which have a larger gauge factor than foil gauges. They also tend to be more expensive, more temperature-sensitive and more fragile. They are also much less accurate than foil gauges.

Other designs include:
• Mercury-in-rubber strain gauges. Used in medical applications, they consist of a small amount of liquid mercury inside a rubber tube. When the tubing stretches (i.e. around a toe or leg), it becomes longer and thinner and increases the electrical resistance of the mercury inside.
• Fiber optic sensing. By taking measurements along an optical fiber, sensors can detect changes in resistance.
• Capacitive strain gauges. These use a variable capacitor to indicate the amount of mechanical deformation and changes in electrical resistance.
• Mechanical strain gauges. Used in civil engineering, a mechanical strain gauge might have two halves of the device attached to opposite sides of a crack in a foundation or other structure. More sophisticated mechanical gauges include dial indicators or other mechanisms that will compensate for temperature shifts. These devices can measure movements as fine as .002 mm.
• For high precision applications, a new technology has been developed, known as Surface Acoustic Wave, or SAW load cells. These are typically twenty times more accurate than the best strain gauge sensors. They are also more rugged.

WHEATSTONE BRIDGES

Temperature changes will cause the specimen to change in size due to thermal expansion, which is then detected as a strain by the gauge, causing an anomaly in the readings. Temperature can also change the resistance of the leads and connecting wires.

Many strain gauges incorporate a Wheatstone bridge arrangement that allows compensation for temperature changes in the specimen under test and the strain gauge itself. Usually, two gauges are attached to the specimen and two are left unattached, not under strain, and at the same temperature as the specimen. Temperature effects on the lead wires can also be cancelled using a 3-wire bridge or 4-wire Kelvin connector.

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Click here to read the rest of Strain Gauge Force Sensor. If you enjoyed this article, you also might like our other stories about Industrial Scales.

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