In this paper, a rotational speed measuring system is proposed on the basis of a laser mouse sensor. The rotational speed of a motor can be determined by measuring the air gap change of a target disc mounted on the motor shaft with the laser mouse sensor ADNS-9800 of Avago / PixArt. By applying a Fast Fourier Transform algorithm, the fundamental frequency of the sampled data from the sensor can be extracted, which is directly proportional to the rotational speed. With this method, a measuring accuracy of 0.5% in speed range from 50 RPM to 6000 RPM can be achieved.
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