Servo control apparatus having function of obtaining frequency characteristics of machine on line

A servo control apparatus that is a control apparatus for a machine tool having a feed axis driven by a servomotor comprising:

• a speed command generator for generating a speed command value for the servomotor;
• a torque command generator for generating a torque command value for the servomotor;
• a speed detector for detecting the speed of the servomotor;
• a speed control loop including the speed command generator, the torque command generator, and the speed detector;
• a sinusoidal sweep input unit for performing a sinusoidal sweep on the speed control loop;
• and
• a frequency characteristics calculator for estimating the gain and phase of speed control loop input and output signals from the output of the speed control loop when a sinusoidal disturbance is inputted to the speed control loop of the control device,
• wherein the frequency characteristics calculator expresses the output of the speed control loop as the Fourier series having an arbitrary number of terms using a disturbance input frequency from the sinusoidal sweep input unit as a fundamental frequency, and calculates the amplitude and phase of a fundamental component of the Fourier series in order to calculate frequency characteristics on line,
• wherein the frequency characteristics calculator verifies a convergence of the output of the speed control loop to a steady state by monitoring energy of one period of a sinusoidal wave on a periodic basis, and the sinusoidal sweep input unit continues inputting the sinusoidal wave at a constant frequency, until the frequency characteristics calculator determines that the output of the speed control loop has reached the steady state.