An amorphous alloy stress sensor for wireless, battery-free applications

Battery-free sensor systems would benefit from the availability of a stress or strain sensor that exhibits a large enough property change to allow simplification and power reductions in sensor interface and data transmission circuitry. A new sensor has been developed specifically for this purpose, which uses the large stress induced impedance changes exhibited by ribbons of amorphous magnetic alloy. In comparison to semiconductor strain gauges, which show a change in resistance of 15% when strained to their maximum recommended stress level, the amorphous alloy sensor demonstrates a change in inductance of 315%, when strained to its maximum working level. Although, amorphous magnetic alloys are inherently sensitive to external magnetic fields, a simple, biasing technique renders the stress-sensing device insensitive to modest field strengths. The amorphous magnetic alloys are produced in large volumes and realize an extremely low cost sensor. A low cost and low power analogue electrical circuit has been designed that, in combination with the amorphous alloy sensor, functions as a battery-free sensor \u27tag\u27. The sensor tag can transmit stress data to a transceiver system via an inductive link, negating the need for battery power or a hardwire connection. The system"s range is directly related to the transceiver and tag antenna dimensions; however a system with 20cm diameter antennas has been demonstrated operating over a range of up to 60cm. This range is achieved through the extremely low power demands of the sensor tag (<1mW). A demonstration unit has been developed for vehicle tyre pressure monitoring applications