Method and apparatus for sensing current position in position control

Abstract

A method and apparatus for sensing the current position of a movable element in a position control system which includes first and second control devices each having a 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. The position control system also includes 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 element being sensed by one control device when the other control device is connected to the speed control circuit. The method includes steps of storing the errors which prevail in the respective storage means just prior to switching from the first control device to the second control device, returning the movable element to the position which prevailed just prior to the switching operation by supplying the speed control circuit with an offset voltage corresponding to an arithmetic difference found by subtracting the stored error from the error storage means of said second control device, from the error in the error storage means of the second control device after the switching operation when the movable element has stabilized, updating the data in a current position register of the first control device on the basis of an arithmetic difference found by subtracting the stored error from the error storage means of the first control device, from the data in the error storage means of the first control device while the motor controlled by the second control device, and revising the data in the error storage means of the first control device on the basis of the arithmetic difference.