Reference point return method

Claim 1

A reference point return method in a numerical control system having: a computerized numerical controller for generating a move command value ΔR n at predetermined time intervals ΔT, a digital servo-circuit for digitally servo-controlling a servomotor on the basis of the move command value ΔR n to transport a movable element, pulse generating means for generating a pulse whenever the servomotor rotates a predetermined amount and for generating a one-revolution signal whenever a predetermined amount of the pulses (N) are generated, first counting means for counting the pulses generated during the predetermined time interval so as to monitor an amount of travel ΔP n of the movable element during the predetermined time interval, second counting means for counting the pulses from generation of the one-revolution pulse until an initial one of the predetermined time intervals so as to monitor an amount of travel B of the movable element during the predetermined time interval, and a deceleration limit switch with normal and depressed states, said method comprising the steps of: (a) generating a reference point return command; (b) transporting the movable element in accordance with the reference point return command; (c) providing a command position R n at an initial time within each of the predetermined time intervals; (d) monitoring a present position P n of the movable element; (e) obtaining an error E n , which is the difference between (a) the commanded position R n and the present machine position Pn; (f) obtaining a value REF n corresponding to the sum of the error E n and the amount of travel B; (g) setting the commanded position to REF n during the predetermined time interval when the position at which the one-revolution signal is generated is zero; (h) updating REF n during each of the predetermined time intervals in accordance with the formula REF.sub.n +ΔR.sub.n →REF.sub.n (i) detecting a depressed state of the deceleration limit switch and the switch being restored from the depressed state; and (j) outputting, as a final commanded amount of travel, ΔR n obtained in accordance with the formula (N-REF.sub.n)→ΔR.sub.n using a commanded position REF n at an initial time after the deceleration limit switch is restored from the depressed state.