Sliding-mode Formation Control For Multiple Robots Via Disturbance Observer

With the development of robot technique,the multi-robot cooperation has become an important research direction.Formation control is the foundation of the multi-robot cooperation problem,which is highly concerned by the domestic and foreign scholars.Consider the influences of load variations,parameter perturbations and the external disturbances,etc.,the formation control is difficult to achieve.Therefore,the multi-robot formation control problem with uncertainties has become a popular research topic.Disturbance observer is known as a powerful method to improve uncertain system robustness,which is widely used in the formation control against uncertainties in order to improve the control precision.Concerning the formation control problem of multi-robot,this paper proposes a sliding model control scheme based on disturbance observer,the main contents of this paper follows as :First of all,the nonholonomic characteristic of wheeled mobile robot is analyzed,and the kinematics and dynamics model of nonholonomic mobile robot is established.The analysis of common formation control method is given,and the dynamic model of leader-follower formation system is formulated.Then the uncertainties rooted in formation system are classified and analyzed.Secondly,the concept of disturbance observer(DOB)is analyzed,the standard and classic structure of disturbance observer is established,besides,its performance is analyzed.For a class of nonlinear systems,a disturbance rejection approach based on disturbance observer is proposed.The design principle of nonlinear disturbance observer(NDOB)is emphatically illustrated.The concept of observer error is introduced,besides,its convergence property is analyzed.Thirdly,consider the dynamics model of the formation system,in the matched condition,the conventional sliding mode control(SMC),the second sliding mode control(2nd SMC)and integral sliding mode control(ISMC)strategy are addressed.Both of the design processes and the selection principle of parameters of two controllers are illustrated,besides,the system stability is also touched.Simulation results demonstrate the effectiveness of these three proposed strategy and the invariance of sliding mode control.Finally,for the problem of the mismatched uncertainties cannot be suppressed by the invariance of SMC,and the chattering phenomenon exist in the SMC,a control strategy based on the nonlinear disturbance observer is presented.Provided theuncertainties have unknown boundaries,a nonlinear disturbance observer is adopted to observe the uncertainties.By selecting some parameters,the observer error is of exponential convergence.For the formation system with the observer,anl sliding mode controller is designed.In the sense of Lyapunov,the formation system can be asymptotically stabilized.The simulation results verify that the proposed method can suppress the mismatched uncertainties successfully,the chattering can be effectively alleviated,and the control performance can be improved.