If you have ever looked at an alarming crack in a house wall after a full structural survey has been undertaken, you may have noticed a number of what looks like pieces of tape across the crack. this tell-tail piece of material, normally a form of paper, is the simplest form of strain gauge and it is placed there to monitor movement of the crack over time. If the paper rips apart, the crack is moving. With a number of these strateigically-placed strain gauges, the structural engineer can obtain quite a lot of information just by inspection. More sophisticated strain gauges measure the width of the crack and this information can be collected digitally.
For measurement of small changes in length (or strains) where the material is not disrupted as the brick wall was, the change as measured by a strain gauge is not visible to the eye, this is the basis for all force sensors. To briefly explain the physics, for a homogeneous material such as steel alloy, aluminium, other metals and ceramics, there is a fixed and proportional relationship between stress to which the material is subjected and the strain that occurs. This is known as Young’s Theorem and the constant for the relationship between stress and strain is known as Young’s Modulus. Each material has a known Young’s Modulus provided that the material has not been stressed beyond its ‘elastic limit’ which is usually close to but not coincident with its ‘yield point’.
Typical metal foil strain gauge element
Most commercially available force sensors consist of 4 piezo-resistive strain gauges bonded to a specially machined steel billet and arranged in a diamond along the axis of the force to be measured. These strain gauges are wafer thin and comprise a specially printed circuit that changes it’s electrical resistance and the four gauges are wired as a Wheatstone Bridge to measure the changes in resistance.
Thus the force sensor can be calibrated in force or torque units depending on the design of the steel body. It can be compressive as in pancake or load cells, tensile loads as in the ‘Z’ load Cell such as can be found in craneage, or torsion force as can be found in dynamometers; the applications are legion.