Method for controlling a servomotor

A method for controlling a servomotor by executing both a position loop processing and a speed loop processing, in a position and speed loop cycle, on the basis of a shift command supplied from a numerical control apparatus in each distribution cycle, comprising the steps of:

• (a) obtaining a smoothing data for each of a plurality of position and speed loop cycles by executing an advance smoothing processing including averaging a plurality of successive shift commands corresponding to said plurality of position and speed loop cycles;
• (b) calculating a position feedforward amount FFp according to the following equation:
• FFp=.alpha.{(1-k) SMD.sub.0 +k.SMD.sub.1 }
• where SMD.sub.0 represents the obtained smoothing data corresponding to a present said position and speed loop cycle of the position-speed loop processing, SMD.sub.1 represents the obtained smoothing data corresponding to a preceding said position and speed loop cycle of the position-speed loop processing, .alpha. represents a feedforward coefficient, and k represents a parameter which is determined to obtain desired dynamic characteristics of a machine to be driven with the servomotor in response to a shift command;
• (c) correcting a speed command obtained in the position loop processing based on the position feedforward amount FFp obtained in said step (b);
• and
• (d) transmitting said corrected speed command to the speed loop processing.