Method and apparatus for sensing current position in position control system

An apparatus for sensing a current position in a position control system of the type including first and second control devices, each having at least means for generating command pulses, error storage means for computing and storing an error between the number of command pulses and feedback pulses each of which is generated whenever a motor rotates by a predetermined amount and a current position register, a speed control circuit for driving and controlling the motor on the basis of the error stored in each of said error storage means and a switching circuit for connecting said first and second control devices to the speed control circuit, which apparatus updates the data in the current position register of which ever of the first and second control devices is not connected to said speed control circuit and enables the unconnected control device to sense the current position of a movable element driven by the motor under the control of the connected control device, said apparatus comprising:

• a second register, provided in said second control device and operatively connected to the error storage means therein, for storing the error which is stored in the error storage means of said second control device immediately before the connection to said speed control circuit is switched from said first control device to said second control device by said switching circuit;
• an arithmetic circuit, provided in said second control device and operatively connected to the error storage means and said second register therein, for computing an arithmetic difference by subtracting the error stored in said second register from the error stored in the error storage means of said second control device after the switching operation when the movable element has stabilized prior to being controlled by said second control device;
• means, provided in said second control device, for returning the movable element to the position which prevailed prior to the switching operation by supplying said speed control circuit with an offset voltage corresponding to the computed arithmetic difference;
• a first register, provided in said first control device and operatively connected to the error storage means therein, for storing the error stored in the error storage means of said first control device immediately before the switching operation;
• first updating means, provided in said first control device and operatively connected to said first register, the error storage means and the current position register therein, for updating the data in the current position register of said first control device, while said motor is under the control of said second control device, on the basis of an arithmetic difference found by subtracting the error in said first register from the error in the error storage means of said first control device;
• and
• first means, provided in said first control device and operatively connected to said first register and the error storage means therein, for revising the data in the error storage means of said first control device on the basis of said arithmetic difference.