Feed-system autotuning of a CNC machining center: Rapid system identification and fine gain tuning based on optimal search

We report an autotuning technique for feed systems of a CNC machine tool using a least-square parametric system identification, a frequency-domain design method, and a fine-tuning method based on an optimal search algorithm. The feed system of a movable-column-type vertical machining center has a large moving mass because spindle and z-axis servo systems are housed in the column. Therefore, perturbation signal is carefully designed. Using a reasonably “smooth” multiharmonic signal, system identification is completed rapidly (8 s) without causing excessive vibration or violating travel limits. Accurate information on the plant dynamics is obtained up to 30 Hz. Feed systems (i.e., x, y, z axis) are modeled as 3rd-order transfer functions in a discrete domain, and compared with the identification results obtained using a Gaussian random sequence and a frequency-domain system-identification method. A proportional (P) controller is designed using numerical search in frequency domain that maximizes the tracking bandwidth and still keeps the system well damped. The frequency response is improved compared to that of a pole-placement method (ζ = 0.707). P controllers of all the three axes that minimize contour error for three-dimensional a 20-mm-diameter circular trajectory are fine-tuned using a fast optimal-search method (440 s). The contour error is significantly improved (average error of 2.25 μm), compared to the results of the pole-placement method (37.89 μm) and the frequency domain design method (12.37 μm) when feed rate is 0.5 m/min. The calculated stability margins of the controller gains are satisfactory.