Position control apparatus and method

Claim 1

A position control apparatus of a closed-loop configuration for controlling the position of a movable machine element driven by a motor having a shaft and receiving position command pulses, comprising: a position detector provided on the movable machine element for detecting the position thereof, said position detector producing a position signal indicative of the detected position; a rotary encoder provided on the motor shaft for detecting the rotational speed of the motor, said rotary encoder generating pulses of a number proportional to the detected rotational speed, the total number of said pulses representing the position of the motor and the frequency thereof representing the actual speed of the motor; position control means, operatively connected to said position detector and said rotary encoder, for producing a control signal based upon the position signal from said position detector and the total number of pulses from said rotary encoder, said position control means comprising: an arithmetic unit, operatively connected to said rotary encoder and said position detector, for computing a first difference between the position signal and the total number of pulses; a first order lag circuit operatively connected to receive the first difference; an adder, operatively connected to said first order lag circuit and to receive the position command pulses, for summing the position command pulses and the first difference; and a first subtractor, operatively connected to said adder and said rotary encoder, for computing a difference between the sum produced by said adder and the total number of pulses, and outputting the difference as the control signal; and speed control means, operatively connected to said position control means, said rotary encoder and the motor, for producing a motor control signal based upon the control signal from said position control means and the frequency of the pulses from said rotary encoder, the motor being driven by said motor control signal to control the position of the movable machine element, said speed control means comprising: a frequency converter, operatively connected to said rotary encoder, for converting the frequency into an actual speed signal; and a second subtractor, operatively connected to said first subtractor, said frequency converter and the motor, for computing the difference between the control signal and the actual speed signal and producing the motor control signal.