Spindle orientation control method and apparatus

An apparatus for controlling the orientation of a spindle driven by a motor, which apparatus comprises:

• speed sensor means for producing an actual speed signal upon sensing the actual rotational speed of the motor;
• speed control circuit means for effecting control in such a manner that the error between the actual speed and a commanded speed approaches zero;
• and
• a spindle orientation control circuit which includes spindle orientation sensing means for generating a single position pulse whenever the spindle rotates by a predetermined angle, and a one-revolution pulse whenever the spindle makes one full revolution, spindle stopping position command means for commanding the stopping position of the spindle, position error signal generating means for generating a position error signal on the basis of the spindle orientation and commanded stopping position, means for sensing the direction of spindle rotation, and correction means for correcting the commanded stopping position in accordance with the direction of spindle rotation;
• the spindle being rotated at the commanded speed by said speed control circuit means, and the rotation of the spindle being controlled in such a manner that the position error signal approaches zero when said spindle orientation control circuit issues a command for stopping the spindle at a predetermined position, a correction being effected of a spindle stopping position error that results from a difference between the position of the leading edge of said one-revolution pulse during forward rotation of the spindle and the position of the trailing edge of said one-revolution pulse during reverse rotation.

A method for stopping a rotating spindle at a command position in a system which includes a motor to rotate the spindle, speed control circuit means for receiving an input signal and driving the motor so that its actual speed conforms to the input signal, and spindle position sensing means for generating a position pulse each time the spindle rotates through a predetermined angular increment and for generating an orientation pulse each time the spindle makes a complete revolution, the angular position of the spindle when an orientation pulse is generated being dependent upon the direction of rotation of the spindle, wherein the method comprises the steps of:

• reducing the level of the input signal applied to the speed control circuit means until the spindle is slowed to a predetermined speed;
• keeping the level of the input signal applied to the speed control circuit means constant after the predetermined speed is achieved in order to maintain the predetermined speed;
• sensing the direction of rotation of the spindle;
• initiating a count of position pulses when an orientation pulse is generated as the spindle rotates at the predetermined speed;
• counting a first quantity of position pulses while the spindle rotates at the predetermined speed to move the spindle to a predetermined angular deviation from the command position, the first quantity of pulses being corrected depending upon the direction of rotation of the spindle;
• counting a second quantity of position pulses corresponding in number to the predetermined angular deviation from the command position, the voltage of the input signal applied to the speed control circuit means being reduced as the second quantity of position pulses is counted so that the voltage substantially reaches zero when the second quantity is entirely counted.