Servo control apparatus having function of sensorless controlled stop

A servo control apparatus for controlling a servomotor installed in a machine tool or an industrial machine, comprising:

• a sensor unit for detecting a speed of the servomotor and outputting the speed as speed feedback;
• an amplifier for driving the servomotor, and feeding back a current flowing through the servomotor;
• a magnetic pole position detector for detecting a magnetic pole position of the servomotor;
• a position controller for outputting a speed command based on a position command for the servomotor and a position of the servomotor calculated from the detected speed;
• a speed controller for outputting a current command based on the speed command and the detected speed;
• a current controller for outputting a voltage command based on the current command and the detected current;
• a speed estimator for calculating an estimated speed based on the voltage command;
• a magnetic pole position estimator for calculating an estimated magnetic pole position from the estimated speed;
• a sensor malfunction detector for detecting a malfunction in the sensor unit;
• a stop position command generator for generating a stop position command to stop the servomotor;
• a first switch for switching from the position command to the stop position command, when the sensor malfunction detector has detected the malfunction in the sensor unit;
• a second switch for switching from the magnetic pole position to the estimated magnetic pole position, when the sensor malfunction detector has detected the malfunction in the sensor unit;
• and a third switch for switching from the speed feedback to the estimated speed, when the sensor malfunction detector has detected the malfunction in the sensor unit.