The integral sliding mode control of a pneumatic force servo for the polishing process

The automation of the polishing process is essential to improve the production efficiency and the quality of the part surfaces. The control strategy, which involves the position/force control, is a big bottleneck for the automated polishing process. This paper focuses on the precise control of the pneumatic force in polishing. Firstly, a position/force decoupled polishing system is described, which consists of a tool-path control subsystem and a polishing force control subsystem based on a pneumatic force servo. Then, the dynamic model of the pneumatic force servo is identified by evaluating the system output while the input is provided by the Pseudo Random Binary Sequence (PRBS) signals. Based on this dynamic input-output model, a new integral sliding mode controller (ISMC), which is composed by equivalent control and switching control, is especially designed for the non-minimum phase system identified. The force control experiments show that the proposed controller is effective in suppressing the external disturbance of spindle rotation and model uncertainties due to the nonlinear characteristics of the pneumatic system; and the control accuracy of the polishing force is improved compared to the traditional PID controller and RST controller. Finally, the polishing experiment of a spherical surface with uniform material removal has been conducted and the polishing results show that the surface roughness greatly improved from 135.8 nm to 14.7 nm. This new integral sliding model control method is effective and has great potential application in the further industrial polishing force control.