Theory and application of a combined self-tuning adaptive control and cross-coupling control in a retrofit milling machine

In this paper, the control plant is a ball screw actuated milling machine table which has been retrofit with DC servo motor control. The retrofit milling machine has nonlinear time-varying behaviors because of the effect of irregular Coulomb friction of the sliding surface and obvious backlash. Thus, it is difficult for a conventional digital controller to provide a high contouring accuracy as well as adequate disturbance rejection and parameter variation robustness. To improve contouring performance of the retrofitted milling machine, a self-tuning adaptive control strategy, combined with cross-coupled control of axial motion is proposed. The proposed control scheme has been verified as being internally stable. Experimental results show that this control method achieves satisfactory contouring accuracy, transient response, tracking and robustness under the influence of backlash on each axis and irregular Coulomb friction. This performance verifies the applicability of this economic approach to automation of traditional milling machines.