Low-Cost RFID Threshold Shock Sensors

This paper describes a battery-free threshold shock sensor with wireless readout via RFID circuitry. The entire package is designed to provide low-cost sensors for monitoring shock loads in applications such as shipping. The sensor is based on a mechanical bistable mechanism laser-cut from a single Delrin layer which switches states when exposed to accelerations above a threshold. The sensor was tested using quasi-static accelerations in a centrifuge, pulsed accelerations from an impact event, and vibration (sinusoidal) accelerations on a shaker table. Measured threshold accelerations were similar in magnitude for the three load types, although they varied slightly due to dynamic effects. This phenomenon was further explored using a dynamic model of the sensor. Threshold accelerations in the range of 30-50 G´s were measured, and these values can be varied by changing the mass of the accelerometer´s outer frame. Wireless readout was successfully demonstrated by using the mechanical accelerometer as an electrical switch connected to the sensor input of an RFID chip.