Instrumentation (AMME2700) Displacement Measurement with Multiple-Resistor Devices
Instrumentation (AMME2700) Strain Gage A strain gauge (also strain gage) is a device used to measure the strain of an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable adhesive. As the object is deformed, the foil is deformed, causing its electrical resistance to change. 6
Instrumentation (AMME2700) Gage Factor The resistance change of the strain gage is usually measured using a Wheatstone bridge, which is related to the strain by the quantity known as the gage factor (GF). 7 ΔR is the change in resistance caused by strain R is the resistance of the undeformed gauge is strain
Instrumentation (AMME2700) Common Strain Gage Arrangement 8
Instrumentation (AMME2700) Wheatstone Bridge 9
Instrumentation (AMME2700) Wheatstone Bridge Sensitivity For a single active arm (e.g. R 2 ): 10
Instrumentation (AMME2700) Applications of Strain Gage Stress and Strain Measurement The basic function of a strain gauge is to measure strain, and in turn stress. Strain is simply the change in length of a component divided by the original length of the component while experiencing an applied load. Stress is calculated by multiplying the strain by the modulus of elasticity for the specific material being measured. Load Measurement Strain gauges are integral elements of load cell transducers, which measure the magnitude of an applied tensile or compressive force. The strain gauges are installed on precisely machined steel components where the exact cross section is known, and therefore the strain measurement can be converted to a load measurement output. Pressure Measurement A pressure sensor uses the same strain gauge configuration and installation as a load cell, except that the sensor measures the applied load over a discrete area, or pressure.
Instrumentation (AMME2700) Example The strain gauges have been positioned so that the force will increase the resistance of two of the strain gages while decreasing the resistance of the other two. The strain gages have nominal resistances of R=120Ω. This resistance is expected to increase or decrease by no more than 2 Ω due to strain. As R varies, the output voltage is required to vary from -10V to +10V in order that it can be sampled by an A/D converter. Determine the amplifier gain, b, needed. 12 A/D