| A compact wireless strain sensor using a Frequency Modulation (FM) technique is proposed. Frequency Modulation with both a Voltage Controlled Oscillator (VCO) and a single transistor circuit is explained. Motivation behind this research is a need to design a sensor body, as compact as possible. The sensor employs a conventional resistive strain gauge such that the resistance changes as the measured structure is mechanically strained. When the strain gauge is connected to a frequency modulation circuit, the change in the resistance appears as a frequency shift in the output signal that is transmitted wirelessly to a remote station. The carrier frequency of square wave output in the VCO demodulator circuit depends on the input voltage and carrier frequency of sinusoidal output in a single transistor modulator circuit depends on the oscillation circuit parameters of the modulator. Due to the change in input voltage to modulation circuit, width of the depletion layer inside the transistor changes which appears as a shift in frequency output. The received signal is then demodulated and the buried strain measurement data can be retrieved. Using this technique, a dynamic strain measurement is possible without any wire connection between the sensing point and the measurement equipment. The results are experimentally derived using a gauge mounted on a cantilever beam. Due to the compactness, these kinds of sensors can be placed in remote places like helicopter blades, fans on a turbine engine, etc. |
