A wireless strain sensing system utilizing passive, wireless, physically small and light weight sensors is desirable for measuring strain in harsh environments such as jet engine compressor and turbine blades. A cluttered and time varying environment results in high loss, blockage, multipath and modulation of the electromagnetic wave. Also, temperature changes affect the sensitivity of the strain measurement. Isolating the information signal from the reverberations in the environment requires time delays in the order of 100s of ns for jet engine environment. Therefore, a wireless strain gauge system that utilizes surface acoustic wave (SAW) strain sensors was studied and tested.
SAW strain sensors are designed to operate at 2.45GHz. Electron beam lithography is used to achieve minimum required feature size at this frequency. The fabrication process is outlined and scanning electron microscope images of some results are given.
A transceiver circuit is designed and constructed. The circuit is tested in free space, in the presence of signal blockage and a time varying channel. Measurements are shown to be in good agreement with predicted data. Sources of errors in the setup are identified to be leakage from transceiver circuit switches and bounce waveforms from the transceiver antenna.
A General Electric J85 jet engine compressor section is analyzed for signal propagation characteristics. Minimum frequency that can propagate through the compressor section is determined to be 5.2GHz. Measurements are done to show that circumferential polarization propagates stronger than radial inside the compressor section. An analytical approximation for the compressor section is generated by modeling compressor section blades as rectangular waveguides. Good agreement on cutoff frequency is achieved for circumferential polarization with the analytical predictions and measurement.
SAW temperature and strain sensors are measured in comparison to traditional gauges. This concept can be generalized to measuring many different physical quantities wirelessly without disturbing the operation of the equipment.