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.