Development of digital speedometer using magnetoresistive sensor

We have indigenously developed a micro-controlled based digital speedometer using commercial GMR based magnetic sensors Non-Volatile Electronics Inc. In this technique, even number of NdFeB hard magnets of diameter (1.5 mm) and length (3 mm) were fixed on the top of an aluminium plate in a circular fashion. Successive magnets were placed with top faces having opposite polarity to form a multipolar magnetic ring. This magnetic encoder arrangement was then mounted concentrically on a rotating shaft, whose rotating speed was to be measured. Magnetic sensor having two separate sensing elements to measure the field gradients was connected to fixed frame and then placed on the top of the multipolar magnetic ring with adjustable air gap. The air gap was varied from 0.5mm to 4mm for achieving higher signal was observed to be ac with amplitude of 400 mV (p-p) and the sectorial periods of 2π/N, where N is the number of magnets. This signal was then amplified using a preamplifier circuit with a gain of 10 and then converted to pulse signal using a diode considering positive half of the cycle. The whole signal conditioning circuit was developed indigenously in the laboratory. The pulse output from the diode was then fetched to the micro-controller circuit for displaying the various parameters in the display unit. With entering the diameter of the wheel used in vehicle and the number of magnets in the multipolar ring, the actual speed of the vehicle can be displayed