Passive Wireless Vibration Sensors Based on the Amplitude Modulation of Harmonic Backscattering

This article presents a wireless passive vibration sensor based on harmonic backscattering. The adopted vibration transducer is a commercial piezoelectric cantilever. The cantilever is connected to the gate of an NMOS transistor, which controls the self-bias point of the Schottky diode responsible for the second-harmonic generation. This way, vibrations induce the amplitude modulation of the backscattered second harmonic. A technique is presented to retrieve the vibration information (both acceleration and frequency) from the acquired spectrum. The vibration frequency is computed as the average frequency difference between the carrier and the first sidebands, while the vibration acceleration is encoded in the duty cycle of the amplitude modulation of the backscattered harmonic. In this contribution, we demonstrate for the first time that this information can be retrieved from the magnitude ratio of the first and second sidebands of the acquired spectrum regardless of the tag-to-reader distance. A complete wireless prototype, working at
$f_{0}=1.5$
GHz, is manufactured and tested in a laboratory environment for tag-to-reader distances up to 1.3 m. Despite some sensor limitations, i.e., the nonlinear relationship between the duty cycle and the vibration acceleration and the dependence of the sensor sensitivity on the vibration frequency, excellent results are obtained for vibration frequencies around the natural oscillation frequency of the cantilever (i.e., 130 Hz), and for variable accelerations and distances, which testifies the high sensitivity and the robustness of the proposed solution.