Servo controller having function for correcting amount of expansion/contraction of ball screw

A servo controller for controlling a feed shaft of a machine tool by using a servomotor connected to a ball screw, the servo controller comprising:

• a position command generating part which generates a position command value for the feed shaft;
• a position detecting part which detects an angular position of the servomotor;
• a velocity command generating part which generates a velocity command value for the servomotor based on the position command value generated by the position command generating part and a position detection value detected by the position detecting part;
• a velocity detecting part which detects a velocity of the servomotor;
• a torque command generating part which degenerates a torque command value for the servomotor based on the velocity command value generated by the velocity command generating part and a velocity detection value detected by the velocity detecting part;
• and
• an amount of position correction calculating part which calculates an amount of position correction for a movable body threadably engaged with the ball screw based on an amount of expansion/contraction of the ball screw, the amount of position correction being used to correct the position command value,
• wherein the amount of position correction calculating part calculates the amount of expansion/contraction of the ball screw based on a distance from the servomotor to the movable body and the torque command value generated by the torque command generating part, and calculates the amount of position correction based on the calculated amount of expansion/contraction of the ball screw.