A comprehensive characterization of a linear deformation sensor for applications in triaxial compression tests

A linear deformation transducer (LDT), which will be applied to triaxial tests for determining deformation behavior of soils, was designed. It, which is made from a thin and flexible steel strip and a strain gage (KFG-5-120-C1-16L1M2R) bonded on the strip using an adhesive, was characterized systematically. Soil deformation was simulated by applying a certain force to the steel strip. The deformed steel strip was converted into a resistance by the strain gage. A full Wheatstone bridge was used to change the resistance into a small analog voltage. The small analog voltage was then amplified by an instrumentation amplifier. This analog voltage was finally processed by a digital data acquisition system consisting of a 12-bit analog to digital converter (ADC) and a microcontroller. It was found that the Wheatstone bridge output voltage obtained by applying a positive change in deformation (the present deformation is bigger than the previous one) is the same as that achieved by its negative change. When an input voltage was supplied to the bridge, the strain gage resistance reduces slightly with the maximum reduction about 1 ohm, from its static resistance of 119.6±0.4 ohm, depending on the force applied to the steel strip and the input voltage supplied to the bridge. For achieving the maximum deformation of 15 mm, the instrumentation amplifier used the supply voltage of 12 V and the gain resistor of 240±1% to get an amplification or gain of 204.8.