Performance-Guaranteed Tracking And Formation Control Of Underwater Vehicles Subject To Unknown Disturbances

As an intelligent tool for ocean exploration,autonomous underwater vehicle(AUV)plays an important role in both civil and military applications,such as resource exploration,marine environment monitoring,military reconnaissance,etc.Efficiently accomplishing these tasks often requires precise motion of the AUV,which could be guaranteed by the high-control performance(e.g.,fast tracking and accurate trajectory tracking).Fast and accurate trajectory tracking capabilities play a vital role in the AUV control system and the advancement of autonomy in marine technology.However,the AUV is usually exposed to the environmental disturbances(e.g.,waves,and ocean currents).The external disturbances might deteriorate the control system performance,or/and result in large steady-state errors.Additionally,most of the existing AUV control literatures achieve the closed-loop system stability,while the transient performance of the control system subject to external disturbances has not been taken into consideration.In practical applications,the transient performance guarantee is also of great importance.The prescribed performance control(PPC)methodology,a flexible and powerful tool in the analysis of transient performance,could be employed to guarantee that the convergence rate of the tracking error is faster than a pre-specified value with the maximum overshoot less than a pre-specified constant.Therefore,the study of trajectory tracking control of AUV with prescribed performance has important practical value and theoretical meanings,especially when the external disturbances present in the AUV dynamics.Based on backstepping design procedure and Lyapunov stability theory,this thesis studies the tracking and formation control problem of AUV system with prescribed performance.The robust integral of the sign of the error(RISE)technique is used to design a continuous controller to force the output of the system to track the desired trajectory asymptotically.PPC methodology is employed to guarantee the steady-state and transient performance of the system.The main contents of the work are outlined as follows.(1)Considering the trajectory tracking control problem of a single underwater vehicle whose system dynamics are actually known,chapter 2 proposes to incorporate RISE feedback technique and PPC methodology into the tracking controller design to achieve asymptotic stability of the closed-loop system with guaranteeing prescribed transient performance.The stability analysis and prescribed performance guarantee are mathematically performed through Lyapunov stability theory.Moreover,an adaptive control gain(part of RISE feedback term)is adopted to replace the constant gain,which overcomes the control problems caused by direct measurement of disturbances as well as high gain,and reduces the conservativeness of controller design.Finally,numerical simulations on AUV are performed to demonstrate the effectiveness of the proposed design techniques.(2)Chapter 3 studies the trajectory tracking control problem of a single AUV system whose accurate dynamics could not be obtained a priori.The dynamic system of an AUV is time-varying,nonlinear,and often include factors that are difficult to model,e.g.,hydrodynamic coefficients and external disturbances such as waves,and ocean currents.When the modeling uncertainties and external disturbances present in the AUV dynamics,in general,traditional adaptive controllers typically result in uniformly ultimate boundedness,where the tracking errors cannot be stabilized asymptotically.In order to obtain an asymptotic tracking performance,a RISE feedback term and an adaptive feedforward term are introduced in our control design to compensate for the modeling uncertainties and external disturbances.Consequently,RISE-based adaptive controller with compensation of uncertainties and external disturbances is proposed such that the asymptotic tracking performance is achieved with guaranteeing prescribed transient behaviors.The performance of the proposed tracking controller is shown by simulation results.(3)Based on the work of Chapter 2 and Chapter 3,the RISE feedback technique and PPC methodology are further extended in Chapter 4,to address the leader-follower formation tracking control problem for a group of AUVs with guaranteeing prescribed transient performance.The fundamental idea of multiple AUVs coordination is to use relatively inexpensive,simple,and small marine vehicles instead of an expensive specialized vehicle to cooperatively perform the complex tasks that usually cannot be accomplished by a single vehicle even with sophisticated equipments.To compensate for the uncertainties and disturbances,this chapter presents Gaussian radial basis function(RBF)neural network(NN)approximators to estimate uncertain nonlinear dynamics,and applies RISE-based adaptive control technique to compensate for the NN approximation errors and external disturbances.In consideration of the limited underwater communication,a virtual leader is constructed between the leader and follower.Based on PPC methodology,RISE feedback technique,backstepping design procedure and Lyapunov stability theory,a decentralized formation tracking controller is designed such that(i)the relative distance errors asymptotically converge to zero;and(ii)the prescribed transient performance is guaranteed.Finally,simulation results show the performance of the proposed formation tracking controller.