The integrated linear and nonlinear motion control design for precise CNC machine tools

The error resources of precise motion control systems are basically categorized into linear and nonlinear effects. To pursue the motion precision of industrial CNC machine tools, This work proposes an integrated control structure with modular algorithms including both the linear control and the nonlinear compensation. In linear control design, This work applies three algorithms: (1) the feedforward control to deal with the tracking error, (2) the cross-coupled control (CCC) to reduce the contouring error, and (3) the digital disturbance observer (DDOB) to lessen the effects of modeling error and disturbance in real applications. Results indicate that the linear motion controller achieves both greatly improved tracking and contouring accuracy by reducing the servo lags and mismatched dynamics of different axes. However, the slip-stick effect due to friction still exists and cannot be removed by applying the linear motion controller only. This work further integrates the nonlinear compensator and develops friction estimation and compensation rules for CNC machine tools. The DSP microprocessors are suitable to implement all linear and nonlinear algorithms and the proposed controllers have been successfully applied to industrial CNC machine tools. Experimental results in various speed commands indicate that the proposed integrated controller reliably achieves 7 μm contouring accuracy in CNC motion control.