Research On Robustness Enhancement Strategy For Induction Motor Drive System Based On High-order Sliding-mode Control

Induction motor(IM)drive system based on vector control has a widespread usage in advanced manufacturing,transportation and daily life.With the development of industry technology,the robustness of IM driving system to disturbance needs to be improved for the application in various harsh operation environment.Generally,the disturbance of IM system comes from two aspects: one is the internal disturbance caused by the uncertainty of the IM parameters.The other one comes from the changes of load,operating conditions and mechanical and electrical components in the system.To improve the performance of IM system under complex conditions with internal and external disturbances,high-order sliding mode control(SMC)is adopted to redesign the current controller and speed controller.Conventional SMC can improve the robustness of IM current controller,but it will result in serious chattering problem.Although the the first-order sliding mode method such as reaching law or boundary layer method can effectively reduce the chattering,it cannot achieve static-errorless convergence.Thus,a nonlinear integral SMC strategy is proposed.First,an integral sliding mode surface based on nonlinear function is designed,which gives the capability of disturbance compensation to the sliding mode surface.Thus,the steady-state error convergence of the stator current can be realized,and effectively avoid the transient performance deterioration caused by the sudden increase of the conventional integral sliding mode surface.Secondly,by introducing the information of the sliding surface into the power reaching law,a first-order SMC with adjustable gain is formed,which can effectively reduce the chattering of the system while solving the steady-state error problem.Although the nonlinear integral sliding mode control can achieve steady-state errorless convergence and chattering suppression of the current loop,the method based on the first-order SMC law essentially sacrifices robustness to suppress chattering.Thus,high-order SMC is needed to address the shortcomings of the first-order SMC.Super-twisting sliding mode control(ST-SMC)is a typical high-order SMC,which can also achieve static-errorless convergence and chattering suppression.It also has the advantages of simple parameter tuning method and maintaining the robustness of traditional sliding mode.To accelerate the convergence rate of current loop,an improved fast ST-SMC based on fast power reaching law is proposed.Through the comparative analysis of the convergence time,it is proved that the proposed algorithm can effectively speed up the convergence speed of the system.The proposed fast ST-SMC can theoretically improve the fast convergence of current loop.However,the disadvantage of conventional ST-SMC,the limited anti-disturbance capability has not been solved effectively.To address this problem,a ST-SMC based on convergence trajectory optimization is proposed.Firstly,the conventional ST-SMC is analyzed from the point of convergence trajectory.The results show that the convergence trajectory will shift and cause the convergence time delay under the system disturbance.Thus,a nonlinear sliding mode surface is designed by using the ideal ST-SMC convergence trajectory.Then,the disturbance compensation control term is constructed by using the designed sliding mode surface,and it is introduced into the control law of the conventional ST-SMC.Through the analysis of the theoretical and simulation results,the proposed method can effectively eliminate the influence of disturbance,force the system to converge along the ideal trajectory,eliminate the convergence time delay and optimize the convergence trajectory.Compared with the conventional ST-SMC,the proposed method can effectively improve the anti-disturbance capability of current loop and improve the system transient performance.Finally,the experiment is carried out on the 3.7KW IM experimental platform,which verifies the superiority of the proposed method.In the IM speed control system,the conventional PI speed controller has poor robustness to external load disturbance.When the external load torque sudden changes,the speed fluctuation is large,which leads to the daguration of speed tracking performance.To improve the robustness of speed controller against load disturbance,a second-order terminal sliding mode control strategy with adaptive gain is designed.Through the analysis of the convergence trajectory of the system,it can be proved that the proposed method can use an integral gain to represent the robustness of the system against load disturbance.Furthermore,to improve the robustness against load disturbance and reduce the fluctuation of output current,this paper proposes an adaptive control gain,which increases the control gain in the transient process to improve the robustness and decreases the control gain in the steady process to suppress chattering.Finally,the whole control law will be cascaded with anti-windup term and finally output as torque current.The experimental results show that the method can effectively improve the robustness of speed loop against torque disturbance.