Patent ID: 8242734

Claim:
A controller for controlling a spindle connected to an induction motor via a belt by controlling the rotational velocity of the induction motor, the spindle having an encoder attached thereto for detecting the position of the spindle, the controller comprising: a spindle velocity detecting unit for detecting a spindle velocity from the number of feedback pulses of the encoder within a predetermined period of time; a first motor velocity estimating unit for obtaining a first estimated motor velocity of the induction motor from the spindle velocity detected by the spindle velocity detecting unit and a reduction ratio between the spindle and the induction motor; a motor velocity control unit for controlling the induction motor based on a velocity command given to the induction motor and the first estimated motor velocity obtained by the first motor velocity estimating unit; a slip detecting unit for detecting occurrence of slip of the belt based on the first estimated motor velocity obtained by the first motor velocity estimating unit or the spindle velocity detected by the spindle velocity detecting unit; and a switch unit for controlling input of the first estimated motor velocity to the motor velocity control unit based on a result of the belt slip detection from the slip detecting unit, wherein the switch unit is controlled so that the first estimated motor velocity will not be used for the control of the induction motor when slip of the belt is detected by the slip detecting unit, further comprising a second motor velocity estimating unit for calculating a second estimated motor velocity from the velocity command of the induction motor and a motor current flowing through the induction motor, wherein the slip detecting unit recognizes occurrence of slip of the belt when the difference between the first estimated motor velocity and the second estimated motor velocity exceeds a second predetermined value.