Wireless read-out of a resonant temperature sensor over an acoustic channel

Abstract:
For saving weight and reducing resources, fiberreinforced polymers are commonly used for pressure tanks. To monitor process parameters inside the tank, wireless sensor systems are desired to avoid a reduction in structural integrity or hermeticity due to additional holes or wire feedthroughs through the shell of the tank. However, since carbon fiber reinforced polymers (CFRP) are electrically conductive, wireless sensor systems based on electromagnetic waves cannot be used in these applications. This contribution, therefore, presents a measurement technique to wirelessly read out a resonant sensor exploiting an acoustic communication channel through an electrically conductive transmission medium. This passive wireless sensor technology consists of a read-out unit and an analog sensor node located inside the tank. The sensor node comprises an electromechanical transducer and a resonant temperature sensor with a high mechanical quality factor. The sensor node is read out using a pulse-echo method in which the resonator is excited by a sinusoidal pulse from the readout unit. The decaying echo of the resonator is backscattered and the temperature-dependent resonance frequency can, thus, be extracted with a time domain separation in the read-out unit. We will show a wireless passive resonant temperature measurement with coaxially coupled piezoceramic ultrasonic transducers through a 10 mm thick CFRP plate with a software defined read-out unit and a temperature sensitive quartz crystal resonator. A typical temperature range for CFRP from -40 to 110 °C is demonstrated in the measurements.