Vibration Detector Based on GMR Sensors

Up to now, vibrations have mostly been sensed by measuring displacement, velocity, and acceleration. The most common types of vibration sensors are piezoelectric, capacitive, null-balance, strain gage, optoelectronic, resonance beam, and piezoresistive. We present a low-cost and low-power vibration detector based on the measurement of magnetic field variations induced in a recent SS501 giant magnetoresistance (GMR) magnetic sensor, for which has never been applied. Vibrations on small ferromagnetic pieces disturb the Earth´s magnetic field. These weak perturbations can be detected and measured over the assumed constant Earth´s magnetic field, which is uniform over a wide area. A novel array configuration of three half-bridge GMR sensors powered in chain is used to measure magnetic field variations in the X-, Y-, and Z-axis with a very low power consumption. This paper includes a characterization of the novel GMR sensors and describes the practical design and implementation of a vibration detector. Two examples demonstrate the utility of these sensors: one for measuring the rotating speed of a small drilling machine and its vibrations at different speeds (in revolutions per minute) and the other one for measuring the characteristic 440-Hz vibration of a tuning fork (A key), which is used to tune musical instruments. The results are shown and discussed.