On Robust Trajectory Tracking Control For Nonholonomic Wheeled Mobile Robots

As an important branch of robots,wheeled mobile robots have better flexibility and wider workspace than traditional industrial robots,so they are widely used in industrial,agricultural production,education,entertainment,service and even military fields.Wheeled mobile robot is composed of robot body,perception system,drive system and control system.It is a highly automated system that integrates perception,decision-making and control.The so-called trajectory tracking control is to design a tracking control law by using the robot's state information detected by sensors to achieve the tracking of a given trajectory.Trajectory tracking is the prerequisite for mobile robots to achieve complex tasks,and it is also one of the important problems to be solved in realizing the control of mobile robots.However,the wheeled mobile robot model has the characteristics of under-actuation,non-linearity,strong coupling and multiple-input multiple-output.In addition,the mobile robot system is susceptible to uncertainties such as parameter perturbation,ground friction,unmodeled dynamics and external disturbances,which pose a certain challenge for the precise tracking control of the wheeled mobile robots.In this dissertation,advanced control methods such as sliding mode control,guaranteed cost control,extended state observer and disturbance observer technology are used to study the robust tracking control of nonholonomic wheeled mobile robots with unmodeled dynamics,external disturbances,wheel skidding and control input signal constraints,and some effective tracking control strategies are proposed.The main research work and results of this dissertation are as follows:(1)An anti-saturation adaptive sliding mode control method is proposed for robust trajectory tracking control of nonholonomic wheeled mobile robots with input saturation constraints.Firstly,a nonlinear extended state observer is designed to estimate system uncertainties such as parameter perturbations and external disturbances.Then,based on the estimation of the nonlinear extended state observer an anti-saturation adaptive sliding mode controller is designed.Thus,the adverse effects of parameters perturbation and external disturbance for the control performance can be eliminated,the saturation constraint of the actuator on the control input signal can be satisfied,and the outputs of the system can accurately track the desired signals.(2)Aiming at the trajectory tracking control problem of nonholonomic wheeled mobile robot with model parameter perturbation and external bounded disturbance,a full-order sliding mode control method based on the extended state observer is proposed.To facilitate the design of the extended state observer,the coupled system dynamics model is transformed into two independent subsystems by coordinate transformation.And then,the model parameter perturbation and the external bounded disturbance are extended to a new state variable,and for observing the system parameter perturbation and the external bounded disturbance,the extended state observers are designed for two subsystems respectively.Finally,the full-order sliding mode controller is designed by using the estimated value of the extended state observer.The simulation results show that the proposed control method can effectively solve the outstanding chattering problem of control signal in traditional sliding mode control,and improve the system tracking control effect.(3)The problem of robust finite-time tracking control for nonholonomic wheeled mobile robots is studied.The nonlinear extended state observer is used to accurately estimate the total disturbance of the system,thus the robustness of the tracking control performance is improved.The gains of the nonlinear extended state observer are determined by pole placement technique,which simplifies the parameter tuning process of the nonlinear extended state observer.Based on the kinematics model of the system,a generalized speed controller is designed to ensure the finite time convergence of pose tracking errors.Finally,by combining the non-linear extended state observer and terminal sliding mode control technology,the finite-time controller of system dynamic model is designed to ensure the finite-time convergence of speed tracking errors.(4)The optimal guaranteed cost control for trajectory tracking of nonholonomic wheeled mobile robots with model parameter uncertainties and control input constraints is studied.According to the relative position and attitude relationship between actual robot and virtual robot,the dynamic model of system trajectory tracking error is established.The non-linear dynamic model of trajectory tracking error is transformed into the linear form model by employing the method of equilibrium point linearization.The existence condition and design method of the optimal guaranteed cost controller are given by means of linear matrix inequality(LMI)method.(5)The trajectory tracking control of nonholonomic wheeled mobile robots with longitudinal and lateral wheel skidding is studied,and a robust tracking controller design method based on disturbance observer is presented.Firstly,the model of the nonholonomic wheeled mobile robot system considering the longitudinal and lateral skidding of wheels is established.Then a robust tracking controller based on disturbance observer is designed,and the stability of the closed-loop system is proved by Lyapunov stability theory.Finally,the simulation study is carried out for two different situations of the desired signal.The simulation results show the effectiveness of the designed control method.Finally,the main research work of robust trajectory tracking control for nonholonomic wheeled mobile robots is summarized,and the future research issues are pointed out.