Synchronized Sliding Mode Control With Nonlinear Disturbance Observer Of A Novel Hybrid Mechanism For Automobile Electro-coating Conveying

The conventional automobile electro-coating conveying systems,such as push bar conveyer and swing-rod conveyer,can not solve the problem of air bag on car roof.The advanced automobile electro-coating conveyers,such as Rodip and VarioShuttle,cannot bear heavy load because of the cantilever beam structures,which lead to the low flexibility of multi-vehicle mixed production,although the air bag on car roof can be eliminated.Fortunately,supported by National Natural Science Foundation of China(Grant No.51375210),a novel hybrid mechanism for automobile electro-coating conveying has been developed by the advantages of hybrid mechanism.The novel conveying mechanism not only eliminates the air bag on car roof,but also has the advantages of wide applicability and high capacity.Due to the hybrid structure,the novel conveying mechanism has multiple kinematic chains,and there are strong couplings among the chains.When the mechanism is running at high speed,the dynamic characteristics of the mechanism will have major impact on the control accuracy and control performance.Therefore,it is necessary to study on the dynamic control of the mechanism.In most dynamic control algorithms for the parallel/ hybrid mechanism,the control loop only receives the feedback signal from its corresponding kinematic chain without feedback signals from other kinematic chains.Thus,the error in the control loop of one kinematic chain is corrected by this loop only,while the others do not respond,which will result in tracking accuracy decline.Considering that the trajectory of the end-effector of the parallel/ hybrid mechanism is determined by all kinematic chains,kinematic chains should be controlled in a synchronous manner so that a desired trajectory can be followed with high tracking accuracy.Especially for the novel conveying mechanism,which is bilaterally symmetrical in mechanical structure,requires higher demands on the synchronization.The tracking accuracy will be reduced due to uncoordinated motions of kinematic chains.In some severe situations,damage to the manipulator mechanical structure may occur.In this paper,in order to improvethe synchronous coordination performance,the research on high control performance for the novel conveying mechanism will be carried out,by starting with synchronized control theory.In addition,there have always been various uncertainties,such as unmodelled dynamics,joint friction and unknown extern disturbances,in the actual control system.However,the existing researches on synchronization control can not effectively resist these uncertainties.Considering that the sliding mode control(SMC)is insensitive to unmodelled dynamics and external disturbance,the synchronization control will be combined with the SMC to to improve the synchronization performance and enhance the robustness of the system.In reality,however,the switching gains of SMC are often chosen to be large enough to cover a wide range of uncertainties.But such large switching gains may cause chattering phenomenon and actuator saturation.To solve the above problems,in order to improve the synchronization performance and enhance the disturbance-rejection performance and robustness of the system without chattering and actuator saturation,a synchronized sliding mode control with nonlinear disturbance observer(SSMC-NDO)is proposed in this paper.At first,the development situation of automobile electro-coating conveying equipments and hybrid mechanism are overviewed.And the researches on kinematics analysis,dynamic modeling and control strategies of the hybrid mechanism are also introduced.Secondly,the inverse kinematics model and the Jacobi matrix of the conveying mechanism are derived.At the same time,the desired trajectory of the end effector of the conveying mechanism is determined according to the process of automobile electro-coating.After that,based on the kinematics analysis,the dynamic model of Cartesian space and the dynamic model with lumped disturbances of joint space are formulated by the Lagrangian method,and the MATLAB simulation results verify the reliability of the models.Thirdly,in order to improve the synchronous coordination,the adjacent cross-coupling nonlinear proportion-derivative controller(ACCNPD)is proposed.The simulation results show that,compared with the normal NPD controller,the ACCNPD can reduce the synchronization error among the active joints and improve the system tracking accuracy effectively.And then,in order to solve the problem of poor robustness in the face of uncertainties,the adjacent cross-coupling sliding mode control(ACCSMC)is proposed by adopting the double power reaching law.The simulation results show that,compared with the ACCNPD,even in the case of uncertainties,the ACCSMC has higher tracking accuracy.Moreover,in order to solve the chattering problem of SMC and improve the disturbance-rejection ability of the control system,the synchronized sliding mode control with nonlinear disturbance observer(SSMC-NDO)is proposed,and the convergence of NDO is proved in theory when the lumped disturbances are not zero.According to the simulation results,the proposed synchronized sliding mode control with nonlinear disturbance observer(SSMC-NDO)not noly improve the synchronous coordination performance,robustness and the disturbance-rejection ability,but also restrain the chattering and actuator saturation problem effectively.Finally,according to the control requirements of the conveying mechanism,the experimental platform is established by the distributed control architecture of "PC+UMAC".And the high-accuracy motion control experiment carried out based on the experimental platform verifies the feasibility and effectiveness of the SSMC-NDO.