Strain Gauge



The strain gauge is a piezoresisitve element which changes its resistance on application of pressure. This change of resistance is proportional to the strain applied on it due to the pressure change. So basically, it uses a property of materials(wire,foil or a film) that they tend to change their electrical resistance on applying pressure.
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The electrical resistance of an material is directly proportional to its length and inversely to its cross-sectional area. This implies that when a tensile deformation or stretching of material takes place its electrical resistance increases as its length increases and cross-sectional area decreases simultaneously. Consecutively, if it is compressed, its electrical resistance decreases. This change in resistance is very small. A Wheatstone Bridge network can be used to sense its resistance change.

              




Construction:
Typically, a strain gauge houses a zigzag pattern or parallel lines of a conductor in an insulated flexible backing as shown in the image above. This arrangement of conductor facilitates multiplicity of the applied strain over the effective length of the conductor in a small area.
Earlier, metals were used as strain gauge elements. Many metals exhibit good strain characteristics within their specified elastic limits. However, they are subject to fatigue after frequent cycles of tension and compression. They also start 'flowing' or showing plastic deformation when subjected to strain beyond their elastic limits. This is a common problem in metallic band strain gauges.
Nowadays, these metal band strain gauges are replaced by silicon ones which have higher fatigue resistance and high linearity. Another major advantage of silicon is seen in actual applications when a sensor fails, it needs to be replaced unlike the metal ones which continue functioning in that state.
The strain gauge is attached to the object whose strain needs to be measured using suitable adhesive.

Types:

There are various types of strain gauges. The one shown in the image above is a foil type bonded strain gauge. The one shown in the image below is the wire type bonded strain gauge. In bonded strain gauges, the thin ware or the foil is connected to a paper tissue or a thin film support.



An unbonded strain gauge is the one in which a resistance wire is stretched an attached between two insulated points placed on the ends of two different attached but non-coplanar frames which can slide on each other when subjected to tension or compression. When motion occurs between the two frames due to force, the strain on the resistance wire changes, thus changing its resistance.
The types discussed above were all uniaxial strain gauges. Another two types of strain gauges are biaxial and triaxial. The biaxial type uses 2 strain gauges and can be used to measure strain in 2 directions, mostly in 90 degrees. The triaxial type of strain gauge can measure strain in 3 direction and uses 3 strain gauges mounted in 45 deg. or 60 deg. as per requirement. This type of strain gauge is used in applications where we need to identify magnitude as well as direction of the strain. The image below shows a tri-axial strain gauge mounted in 45 degrees.



The thickness of these types of strain gauges is around 0.2 mm.
Working:
An excitation voltage is required for the working of the strain gauge circuit, which is typically 5V or 12V. This voltage is applied to the input terminals of the Wheatstone's Bridge. Output Voltage, which is in the range of a few millivolts is taken from the output leads. An amplification and offset circuit is needed to get the reading accurate.