Velocity feedback measurement method

Abstract

To provide accurate measurement of rotational velocity and an adequate bandwidth for servo control as a function of the measurement over a wide range of velocities, the present invention utilizes unique speed computation algorithms based on a combination of fine and coarse shaft velocity measurements from a tachometer. The preferred embodiments utilize control signals derived from the velocity measurements to control motor speed, for example, to provide servo control of a digital tape drive. The preferred embodiments utilize an optical encoder coupled to a reel-shaft to generate tachometer pulses as the shaft rotates. The coarse measurement may actually take two different forms for different speed ranges. In one range, this measurement involves counting the number of pulses in a sampling interval. In the other range, the coarse measurement entails counting the number of sampling intervals per detection of a tachometer pulse. The fine velocity measurement is based on detection of differences in time delays between certain ones of the tachometer pulses and the edges of one or more of the sampling intervals. The delay detections may entail counting pulses of a clock signal, which has a rate substantially higher than the sampling rate, for periods between certain tachometer pulses and the edges of one or more of the sample intervals. The fine measurements provide a correction in the speed calculation for variations in phase relationship between the sampling and the tachometer pulses.