Design and Adaptive Terminal Sliding Mode Control of a Fast Tool Servo System for Diamond Machining of Freeform Surfaces

This paper presents the methodology for the design and control of a high-stiffness fast tool servo (FTS) system for practical application in diamond machining of freeform surfaces. The electromechanical model of the FTS system is established by combining the piezoelectric actuator and the flexure hinge mechanism for the formulation of the control methodology. In order to accommodate the system nonlinearity, cutting force disturbances as well as to realize fast and accurate tracking of designated tool path required for diamond machining of freeform surfaces, a novel adaptive terminal sliding mode (NATS) control strategy capable of online self-tuning of the system gain and reach time related parameters are developed. Stability analysis of the proposed NATS controller is performed. The convergence behavior of tracking error in the presence of model parameter variations and cutting force disturbance is analyzed. Experimental verification was conducted to validate the effectiveness and robustness of the NATS control. By comparing with other related control methods, it shows significantly improved performance. The percentage of tracking error was only 0.1% for the varying trajectories on machining water-drop surface. Taking advantage of the excellent tracking performances of the FTS prototype on a diamond turning machine, various freeform surfaces were successfully produced with high quality.