Resistance strain gauge is a device that experiences a change in resistance when they are stretched or strained. They are able to detect very small displacements, usually in the range 0–50µm, and are typically used as part of other transducers.
Construction and Working (Wire Type)
Resistance strain gauge consists of a length of metal resistance wire formed into a zigzag pattern and mounted onto a flexible backing sheet. The wire is nominally of circular cross-section. As strain is applied to the gauge, the shape of the cross-section of the resistance wire distorts, changing the cross-sectional area. As the resistance of the wire per unit length is inversely proportional to the cross-sectional area, there is a consequential change in resistance. The input-output relationship of a strain gauge is expressed by the gauge factor, which is defined as the change in resistance (R) for a given value of strain (S), i.e.
Gauge factor = δR / δS
In use, resistance strain gauges are bonded to the object whose displacement is to be measured. The resistance of the gauge is usually measured by a DC bridge circuit and the displacement is inferred from the bridge output measured. The maximum current that can be allowed to flow in a resistance strain gauge is in the region of 5 to 50 mA depending on the type. Thus, the maximum voltage that can be applied is limited and consequently, as the resistance change in a resistance strain gauge is typically small, the bridge output voltage is also small and amplification has to be carried out. This adds to the cost of using strain gauges.
In recent years, wire-type gauges have largely been replaced by metal-foil types. Metal-foil types are very similar to metal-wire types except the active element consists of a piece of metal foil cut into a zigzag pattern. Cutting a foil into the required shape is much easier than forming a piece of resistance wire into the required shape, and this makes the devices cheaper to manufacture. A popular material in metal strain gauge manufacture is a copper–nickel–manganese alloy, which is known by the trade name of ‘Advance’.
Semi-conductor type strain gauge:
Semiconductor types have piezo-resistive elements. Compared with metal gauges, semi-conductor types have a much superior gauge factor (up to 100 times better) but they are more expensive. Also, whilst metal gauges have an almost zero temperature coefficient, semiconductor types have a relatively high temperature coefficient.
