Wireless Vibration Sensing Without a Battery

This paper presents the implementation and characterization of a low power wireless vibration sensor that can be powered by a flash light. The wireless system consists of two components, namely the wireless sensor node and the wireless interrogation unit. The wireless sensor node includes a wireless strain gauge that consumes around 6 mW, a signal modulation circuit, and a light energy harvesting unit. To achieve ultra-low power consumption, the signal modulation circuit was implemented using a voltage-controlled oscillator (VCO) to convert the strain gauge output to an intermediate frequency (IF) signal, which is then used to alter the impedance of the sensor antenna and thus achieves amplitude modulation of the backscattered antenna signal. A generic solar panel with energy harvesting circuit is used to power the strain sensor node continuously. The wireless interrogation unit transmits the interrogation signal and receives the amplitude modulated antenna backscattering, which can be down-converted to recover the IF signal. In order to measure the strains dynamically, a Phase Lock Loop (PLL) circuit was implemented at the interrogator to track the frequency of the IF signal and provide a signal that is directly proportional to the measured strain. The system features ultra-low power consumption, complete wireless sensing, solar powering, and portability. The application of this low power wireless strain system for vibration measurement is demonstrated and characterized.