Characterization of Differentially Measured Strain Using Passive Wireless Surface Acoustic Wave (SAW) Strain Sensors

This paper presents characterization results from surface acoustic wave resonator (SAWR) strain sensors used to carry out wireless passive differential strain measurement. An elevated temperature SAWR strain sensor characterization data set is presented and the results used to demonstrate temperature compensation strategies and overall measurement channel signal sensitivity improvements with a differential SAW sensing element pair. The results are generated by a new wide bandwidth wireless interrogator capable of interrogating two SAWR sensors at high sample rates. Cross axis sensitivity caused by loading conditions generating strains in different directions to the desired strain condition is shown and a compensation strategy presented. The compensation method has reduced the cross axis sensitivity from 24% to 4% of full scale output (FSO). The elevated temperature characterization performance between 20 °C and 100 °C demonstrated a maximum hysteresis level of 0.49% of FSO between 400 με in compression and tension and an overall reduction in the apparent strain due to temperature from approximately 5.5% to 2.7% of FSO.