Adaptive Robust Second-order Sliding Mode Control Of A Novel Hybrid Robot For Automobile Electro-Coating Conveying

The automobile manufacturing industry is developing rapidly in recent years,which makes it urgent to improve the technology of modern automobile electrocoating.The conveying equipment plays an important role in the process of modern automobile electrocoating.However,most of the existing conveying equipments,such as RoDip,employ the cantilever beam structure,which makes their degree of flexibility and bearing capacity are poor.To deal with the issue,this paper introduces a novel hybrid robot for automobile electrocoating conveying by combining the series and parallel mechanisms.The designed hybrid robot is featured with high stiffness,heavy payload,and superior dynamic characteristics.The designed hybrid robot is a highly nonlinear and coupling multi-input and multi-output system.Compared to kinematic control,dynamic control for the hybrid robot is capable to achieve better control performances by taking the dynamic characteristics and the couplings into account.Therefore,this paper adopts the dynamic control method.As a matter of fact,the complex dynamic model of the hybrid robot is difficult to be built accurately,which results in the existence of modeling errors.In addition,the hybrid robot can be easily affected by various other uncertainties,including parameter uncertainties,frictions,external disturbances,etc.All these above uncertainties will cause adverse effects to the control performances of the hybrid robot.Considering that the super-twisting second-order sliding mode(SOSM)control not only inherits the robustness of the traditional sliding mode control,making the sliding variable and its derivative convergent in the presence of uncertainties,but also attenuates the chattering.Hence,the super-twisting SOSM control is suitable to be applied to the hybrid robot for automobile electrocoating conveying to deal with uncertainties.Generally,the super-twisting SOSM controller requires the control gains larger than the upper boundary of the total uncertainty,which ensures the stability of the control system.In fact,the boundary of the total uncertainty in the hybrid robot is usually unknown.Hence,the control gains should be selected as large as possible to counteract the total uncertainty under the worst working condition in the automobile electrocoating conveying process.However,too high control gains will intensify the chattering problem.To solve this problem,a novel gain adaptation law is proposed to dynamically adjust the gains of the controller,which can guarantee the stability of the hybrid robot system while selecting the control gain as small as possible,further weakening the chattering phenomenon.Besides,the robustness of the super-twisting SOSM controller is achieved at the price of sacrificing the nominal control performances.To handle this,a disturbance observer is designed in this paper to estimate the total uncertainty by using the information of the states and outputs,compensating the adverse effects of the total uncertainty actively by feedforward principle as a result.The outstanding benefit of the disturbance observer lies in that it can be designed separately from the original controller and can enhance the performances of the automobile electrocoating conveying without changing the original controller.Hence,a novel adaptive robust SOSM control scheme is proposed in this paper by integrating the disturbance observer.This paper expounds the developments of automobile electrocoating conveying and hybrid robot firstly.Besides,the analysis concerning the uncertainties in the hybrid robot and researches on kinematic and dynamic modelling of the hybrid robot is conducted to show the existing problems unsolved.In the following,inverse kinematic model and Jacobian matrix of the hybrid robot are established.According to the requirements of automobile electrocoating conveying technology and the parameters of the hybrid robot,the desired trajectories of the end effector of the hybrid robot is determined.On the basis of kinematic analysis,the dynamic model of the hybrid robot is constructed by using Lagrange method.Afterwards,an adaptive robust SOSM controller is designed in this paper to improve the control performances and counteract the adverse effects of uncertainties,including modelling error,friction,and external disturbance.Moreover,the disturbance observer is introduced to estimate and compensate the total uncertainty to further improve the robustness of the control system.The stability of the proposed control scheme is proved by using Lyapunov stability theorem.Further,by comparing with the fixed-gain super-twisting SOSM controller and the adaptive super-twisting SOSM controller without disturbance observer,several simulations are conducted to show that the adaptive robust SOSM control scheme obtains superior tracking performance and is able to attenuate the chattering phenomenon effectively in the presence of modelling error,frictions,and external disturbances.Then,according to the requirements of the hybrid robot for conveying,the hardware of the hybrid robot control system is built by using the distributed method of “host computer + slave computer”.The application of the host computer is developed based on the platform of MFC and Pcomm32 W.ddl.Besides,the motion control program of the slave computer is developed based on Prewin32Pro2.Finally,comparative experiments are taken for the hybrid robot prototype by comparing the designed adaptive robust SOSM control scheme with the adaptive super-twisting SOSM controller without disturbance observer,showing the effectiveness and superiority of the proposed control scheme.