Position control apparatus

A position control apparatus that controls a position of a movable part of a machine tool based on a position command value from an upper-level, numerical control apparatus, the position control apparatus comprising:

• a position control loop that acquires a position of the movable part, and that executes a position feedback using a value based on the acquired position of the movable part as a position feedback value;
• and
• a velocity control loop that calculates a torque command value for a motor, which drives the movable part via a drive force transmission system including a ball screw, based on a velocity command value calculated by the position control loop, and that controls a velocity of the motor, wherein
• the position control apparatus further comprises:
• an inversion detector that detects an inversion of a direction of the position command value;
• a storage unit that stores a deflection characteristic which is a correspondence relationship between the torque command value and an amount of deflection of the drive force transmission system;
• and
• an inversion compensation calculator that acquires the amount of deflection immediately before inversion by applying the deflection characteristic on a torque command value immediately before the inversion, that estimates the amount of deflection immediately after the inversion by applying the deflection characteristic on a value calculated by inverting a sign of the torque command value immediately before the inversion, and that calculates an inversion compensation amount which is a difference between the amount of deflection immediately before the inversion and the estimated amount of deflection, and wherein
• when the inversion detector detects the inversion of the direction of the position command value, the position is controlled using, as a new position command value, a value obtained by adding the inversion compensation amount to the position command value.