Adaptive Nonlinear Sliding Mode Contour Control for Direct Drive XY Motion Platform Based on Reference Adjustment Contour Error

The direct drive XY motion platform can realize the movement of multi-axis high-end CNC machine tools in the XY coordinate system. It has the advantages of fast response speed and high machining accuracy, which is the key to achieving precise contour control. However, the direct drive XY motion platform uses two permanent magnet linear synchronous motors as the power source. The nonlinear dynamics in the system will directly affect the motion of the platform without attenuating the transmission links. In addition, the matching and coordination of the two feed drive axes in the motion process will also reduce the contour machining accuracy of the system. The precision direct drive XY motion platform is vulnerable to the influence of the system uncertainty dynamics and the dual axis coordination matching when performing the machining tasks. Therefore, an adaptive nonlinear sliding mode contour control (ANSMCC) method is proposed based on the reference adjusted contour error (RACE) model and uncertainty compensator (UC). Firstly, the dynamic model of the direct drive XY motion platform is established, including uncertainty factors such as parameter variations, external disturbances, and nonlinear friction forces. Next, the traditional method of regarding the normal component of the tracking error as an approximate contour error is not applicable to the large curvature contour machining path. A RACE model is established to calculate a more accurate contour error using coordinate transformation and equivalent contour error theory. Since the direct drive XY platform servo system will be affected by the nonlinear dynamics in the operation process, the ANSMCC method and UC method are designed to control the linear reference model and the nonlinear dynamics. Based on the designed RACE model, an ANSMCC is designed. The nonlinear sliding mode surface is introduced in ANSMCC to replace the traditional linear sliding mode surface, guaranteeing the system has a fast dynamic response speed while enhancing the contour tracking accuracy. Meanwhile, regarding the uncertain dynamics in the system, the UC method and linear sliding mode controller are designed to reduce the error between the reference model and the actual object and compensate for the influence of uncertain dynamics on the contour accuracy. Finally, three comparative experimental conditions, including no-load low-speed condition, no-load high-speed condition, and load condition, are conducted under the three contour control methods of adaptive sliding mode contour control (ASMCC), and ANSMCC based on RACE and UC. In order to ensure the fairness of the comparison experiments, all parameters in the experiment are repeatedly debugged, and appropriate parameters are selected to meet the dynamic and static performance of the system. The proposed ANSMCC based on the RACE and UC methods can decrease the contour error of the direct drive XY motion platform to −2.5~2.5 μm. Compared with the other two methods, contour tracking accuracy is significantly improved, which can meet the contour machining accuracy requirements in practical applications. The following conclusions can be drawn from the experimental analysis: (1) Compared with the traditional method of approximating the contour error by the normal component of the tracking error, the RACE model is more suitable for the reference trajectory with large curvature, which is simple and accurate; (2) The ANSMCC method can improve system response speed and contour machining accuracy by introducing a nonlinear sliding surface; (3) UC can compensate for the influence of uncertain dynamics on the system. Thus the direct drive XY motion platform servo system has strong robustness. © 2023 Chinese Machine Press. All rights reserved.