Research On Motion Control Of Ball Screw Drives Based On Sliding Mode Control

Ball screw is the key functional component of CNC machine tool feed drives,and high performance motion control algorithm of ball screw drives is the key to realize high-speed and high-precision machining of CNC machine tool.Sliding mode control is widely used in the motion control of ball screw drives for its strong robustness,fast-respondability and convenience of physical realization.Conventional sliding mode control can achieve high tracking accuracy,but it will induce chattering phenomenon,and external disturbance will affect the tracking accuracy and stability of the system.In addition,the transient response characteristics of the control system can be further improved.Aiming at the above problems,novel sliding mode controllers were designed to solve the shortcomings of conventional sliding mode control and further improve the tracking performance of ball screw drives.The main research contents of this thesis are as follows:(1)The structure and working principle of the experimental system were introduced.Rigid body model,two degrees of freedom flexible body model and Stribeck friction model of ball screw drives were established.On this basis,P-PI cascade controller and sliding mode controller were designed.Tracking performance of the control system was simulated and experimented.The experimental results show that the sliding mode control has higher tracking accuracy and stronger robustness than P-PI cascade control,but there exists chattering phenomenon in the control voltage signal of sliding mode control.(2)Aiming at the chattering phenomenon in conventional sliding mode control,an improved variable power reaching law was proposed,and it was proved that the sliding mode function has better motion quality when approaching the sliding mode surface.Controllers with known and unknown disturbance supremum were designed respectively based on the rigid body model and the flexible body model.The simulation and experimental results show that the sliding mode control method based on proposed variable power reaching law can effectively eliminate the chattering and steady-state error at the uniform speed stage and improve the tracking accuracy,it has strong robustness to external disturbances meanwhile.(3)Aiming at the limitation of friction feedforward compensation,a nonlinear disturbance observer was designed,which could accurately observe external disturbances.It is proved that the observer satisfies the exponential convergence condition.The sliding mode control method based on variable power reaching law was improved.The disturbances in the control law were changed to the observed disturbances.The simulation and experimental results show that the sliding mode control method based on disturbance observer can compensate the external disturbances more accurately,enhance the anti-disturbance ability of the control system,further improve the tracking accuracy,and eliminate the chattering in the control voltage signal.(4)Based on the composite nonlinear feedback,a nonlinear sliding mode function was designed to change the damping ratio of closed-loop control system,which improved the transient response characteristics of the system.It is proved that the tracking error of state variables can converge gradually on the nonlinear sliding mode surface.Combined with the variable power reaching law and the disturbance observer,a sliding mode controller based on nonlinear sliding mode function was designed,and the matrices to be designed in the nonlinear sliding mode function were obtained through pole placement principle.The simulation and experimental results show that the sliding mode control method based on the nonlinear sliding mode function can dynamically adjust damping ratio of the system,improve fast-respondability of the system without overshoot,and make the closed-loop control system have higher tracking accuracy and servo bandwidth.