Research On Adaptive Constraint Control Of Uncertain Nonlinear Systems

This dissertation studies the adaptive tracking control of uncertain nonlinear systems under several types of constraints under the framework of backstepping technique.As the control performance of the system becomes more and more stringent,in addition to ensuring the stability of the system,the designed controller also needs to consider the constraints related to external input,external output and the system itself.This dissertation focuses on high-order-free nonlinear systems and high-order nonlinear systems,and studies their adaptive control problems under several types of constraints(including actuator input saturation constraint,bandwidth constraints for controller transmission,system output constraints,and algorithm constraints by the system).The main research results of this dissertation are as follows:We propose an adaptive fuzzy dynamic surface control(DSC)scheme for singlelink flexible-joint robotic systems with input saturation.A smooth function is utilized with the mean-value theorem to deal with the difficulties associated with input saturation.An auxiliary first-order filter is used to solve the “explosion of complexity” problem existed in traditional backstepping design framework.It is proved that all the signals in the closed-loop system are semi-globally uniformly ultimately bounded.The main advantage of the proposed method is that only one adaptation parameter needs to be updated.Simulation results demonstrate the feasibility of the proposed scheme and the comparison results show that the improved DSC method can reduce the computational burden by almost two thirds in comparison with the standard DSC method.Based on the control of the above-mentioned single-link flexible joint robot,A new adaptive fuzzy command filtered control strategy is presented for the precise tracking control of the n-link flexible joint robotic system.Compared with existing backsteppingbased methods,the proposed scheme can not only overcome the so-called “explosion of complexity” problem,but also reduce filter errors by introducing an error compensation mechanism to improve the tracking control performance.The proposed scheme requires only one parameter and can ensure that the tracking error eventually converges to a small neighborhood near zero.The simulation results of a two-link robot system demonstrate the advantages of the design method in comparison with existing results,such as the backstepping method and the dynamic surface control(DSC)method.A novel event-triggered control framework is proposed for the nonlinear system constrained by network bandwidth in network environment.Unlike existing event-triggered schemes,the developed triggering mechanism only compares the difference between the signal to be transmitted and the holding signal.The proposed control method also considers the response of the triggering mechanism.Under the proposed event-triggered control framework,a fixed-time event-triggered control strategy is further proposed to solve the problem that the system convergence time is constrained by the initial value of the system.The proposed control method under the event-triggered framework can reduce the amount of control signals transmitted while ensuring that the system output tracking error converges to a small neighborhood close to zero within a fixed-time frame.The results of two examples,including comparative studies and a practical application,show the effectiveness of the proposed control method.For the tracking control problem of high-order nonlinear systems,most relevant results in the literature have two main restrictions: R1)some constraints need to be imposed on powers and growth conditions have to be established for the nonlinear functions of the system;R2)nonlinear bounding functions must be derived based on repeated derivations of virtual control variables during the design process.Based on the multiple power integration technique,a systematic tracking control scheme is proposed for a class of highorder uncertain nonlinear systems with time-varying asymmetric output constraints.The proposed control scheme eliminates the above two constraints.Moreover,with an improved time-varying nonlinear transformed function introduced for high-order systems,the proposed scheme can be regarded as a unified tool in the sense that it works no matter whether the constraint is symmetric or asymmetric,or even without constraint.The simulation results of two deterministic practical applications show that the proposed control scheme is effective not only for high-order systems,but also for high-order-free systems.