Batteryless sensorless bicycle speed recorder with hub dynamo and STT-MRAM

A batteryless sensorless bicycle speed recorder system with a hub dynamo that functions as both a power source and a speed sensor has been developed. The hub dynamo produces a voltage waveform with more than ten AC cycles per rotation of the bicycle wheel, which enables precise determination of speed and acceleration. The data is stored in a spin-transfer-torque magnetoresistive random-access memory (STT-MRAM) that has perpendicular magnetic tunnel junctions (MTJs) and an infinite rewriting capability. For writing operation, the MRAM employs an intermittent write operation, which involves high-speed writing with a duty cycle of less than 0.01%. This reduces the average power dissipation of the MRAM to that of the standby mode. A road test showed that a fabricated speed recorder mounted on a bicycle stored and reproduced accurate values of a large acceleration and its duration when the brakes were suddenly applied while the bicycle was being ridden.