Average value tachometer for a disc drive servo and the like

Abstract

In a disc pack data storage system that has a plurality of discs and a plurality of heads for coaction with the discs, a tachometer for producing a signal indicative of the radial speed of the heads with respect to the discs. At least one of the disc surfaces is a servo surface and at least one of the heads is a servo head so that in response to radial movement of the servo head a triangular position signal is produced. Sampling pulses indicative of specific radial positions of the head are derived from the position signal so that the time required for the head to move between two positions represented by adjacent sampling pulses is inversely proportional to radial head speed. Control and computing circuitry measures the time between adjacent pulses by providing a clock pulse source and counting the number of clock pulses that occur between adjacent sampling pulses. There is circuitry for controlling the specific sampling pulses employed in accordance with radial head speed and the clock pulse source is frequency controlled to achieve accuracy at all radial speeds. A digital to analog converter converts the counted clock pulses to a voltage and amplifier circuitry derives from the voltage a speed signal indicative of radial speed. Circuitry active at very low speeds serves to combine with the speed signal a signal derived by integrating motor current so that an accurate tachometer signal is produced at all radial speeds.