Near-field interrogation of SAW resonators on rotating machinery
Surface acoustic wave (SAW) resonators electrically behave like LCR circuits, their frequency can be influenced by temperature, pressure and torque. When they are used for passive wireless sensing on rotating machinery, they can also be influenced by the angular variations of the coupling between the coupler elements and the receiving coupler element impedance. This parasitic frequency shift is known as the "pulling effect". In this paper, we present a capacitive coupler based on open coplanar strip lines for physical measurements on a small diameter rotating shaft. This approach allows a single 434 MHz resonator angular frequency pulling lower than 200 Hz (0.46 ppm) and 100 Hz (0.23 ppm) in a differential configuration. This is more than 10 times lower compared to frequency pulling obtained using couplers based on circular and electrically shorted transmission lines. RADAR-based interrogation, finite element method (FEM) simulation, coupler parameters and frequency pulling measurements results are presented to demonstrate the performances of the complete system.