Method of sensing current position in position control system

Abstract

A method of sensing the current position of a movable element in a position control system which includes first and second control devices each having command pulse generating means and error storage means for computing and storing an error between a number of command pulses and a number of feedback pulses indicative of an amount of motor movement, a speed control circuit for driving and controlling the motor on the basis of the error in the storage means, and a switching circuit for selectively connecting the first and second control devices to the speed control circuit, the current position of the movable elements being sensed by the first control device when the second control device is connected to the speed control circuit. The method includes the following steps, when the second control device is connected to the speed control circuit, of reading and storing, as a steady deviation, the error in the error storage means of the first control device after the motor is rotated in accordance with the magnitude of drift of a closed loop formed by the second control device. The data in a current position register of the first control device is then updated on the basis of an arithmetic difference found by subtracting the stored steady deviation from the data in the error storage means of the first control device and revised in the error storage means of the first control device on the basis of the arithmetic difference.