The Motion Control Of Nonholonomic Wheeled Mobile Robots

Nonholonomic constraints arise in many advanced robotic systems,such as wheeled mobile robots(WMR),multifingered robot hands,space manipulators,etc. Due to nonholonomic constraints and intrinsic nonlinearity of these systems,it's a challenging problem to solve control problems related to nonholonornic systems.As a result,the research in nonholonomic systems is of great importance both in theory and practice.WMR is a typical nonholonomic system.The control problems of WMR addressed in the literatures can be classified into three groups:path following, trajectory tracking,and point stabilization.This dissertation focuses on solving trajectory tracking and point stabilization problems of WMR.In practical applications,the control speed of WMR may be limited to obtain stable movement.In Chapter Three,based on kinematic model of WMR,two different controllers are synthesized for solving global trajectory tracking problems under control input saturation constraints.Firstly,a discontinuous feedback control law is designed using backstepping method,and it is proven that the tracking error globally uniformly asymptotically converge to zero.Secondly,a time-varying tracking controller is proposed based on Lyapunov direct method It is proved that the controller can guarantee the global uniform asymptotical stability of closed-loop system.Both controllers can satisfy input saturation constrains and track reference trajectories effectively.The influence of controller parameters on tracking performance is analyzed through simulation.Moreover,the dynamics of WMR can affect trajectory tracking performance in practical applications.Also,it is difficult to obtain the exact dynamic model of WMR due to parameter uncertainty and external disturbance.So,the trajectory tracking problems of WMR based on dynamic model with parameter uncertainty and external disturbance is studied in Chapter Four.First of all,a linear tracking error model coupling with nonlinear uncertain items is deduced using input-output feedback linearization method.Then,a robust trajectory tracking controller based on sliding mode control is synthesized to deal with parameter uncertainty of the model.The influence of parameters uncertainties on controller is also analyzed and a sufficient condition to guarantee the stability of closed-loop system is suggested.Second,a robust adaptive controller is developed to deal with both the parameter uncertainty and external bounded disturbance.As the gain of controller updates online adaptively, good tracking performance is achieved.One of the important characteristics of nonholonomic systems is that there are no smooth static or dynamic feedback control laws to stabilize the nonholonomic system, therefore,it's very difficult to solve point stabilization problem.In Chapter Five,a smooth time-varying controller which can exponentially stabilize all the states to the origin is proposed when some geometric parameters are unknown but bounded.The unknown parameters include the radius of wheels and the length of axle between them. The unknown distance d between the interested point C on the WMR and the center point of axle is estimated by adaptive laws.Moreover,a sufficient condition of the controller parameters is given for the robustness.