Nonlinear Control Strategy For Autonomous Underwater Vehicle’s Steering Motion

Autonomous underwater vehicle (AUV)’s dynamic system has strong external disturbances, nonlinearity, coupling feature, underactuated and a large number of uncertainties in water coefficients. Therefore, the control of AUV belongs to a class of typical nonlinear control research category. The research of AUV’s nonlinear control problem has important significance both in theory and practice. Therefore, based on the nonlinear adaptive robust control theory, this paper discusses the problem of nonlinear control of AUV to improve dynamic quality of AUV’s system. Based on the nonlinear system of AUV, this paper discuss the path following control of single AUV and distributed formation control of muti-AUVs, the research work are as follows:Firstly, for the problem of an AUV deep-set motion control, a nonholonomic motion model is set up and an adaptive output feedback controller based on backstepping technique is proposed. The precision of deep-set motion can be satisfied by the controller in lack of vertical thrusters and only with main thruster and level rudders. In addition, backstepping technique combinated with neural network adaptive laws not only can compensate the nonlinear part and the uncertainty parameters of the AUV, but also can make the controller robust to the external disturbance.Secondly, the path following control of AUV moving in shallow water area is discussed. In the body coordinate frame, the path following error dynamics of AUV in set up and an adaptive output feedback controller based on neural network is proposed. The significant disturbance due to wave motion will be introduced into the measurable output signals and will influence the actual position and attitude of AUV. If the wave disturbance enter into the control loop, it will cause increase of the output control, reduction of the control accuracy, waste of fuel and wear of actuators in serious situation. According to this problem, the proposed controller can filter out the disturbance from the output signals and make them unable to enter control loop. This control method does not need the accurate model of system. Simulation experiment results show that the proposed controller has good adaptability for viscous damping coefficients and unknown nonlinear structure and strong robustness for uncertain additional quality and changing disturbance.Thirdly, for the path following control of AUV, the Serret-Frenet coordinate frame is introduced. In the Serret-Frenet coordinate frame, the path following error dynamics of AUV in set up and a nonlinear robust adaptive controller is proposed. The control method not only can stabilize the closed-loop system, but also fully considered the parameters’ time-varying properties which cause the difference between the nominal value and the actual value of the parameters in AUV’s dynamic system. The difference is called parameter perturbation value. The control accuracy will be improved by the proposed robust adaptive control law which is used to compensate parameters’perturbation. By introducing the line-of-sight (LOS) angle, the movement behavior of AUV can be changed according to the target. The AUV can move to the path fast by the LOS navigation.Finally, based on the path following control of a single AUV, the formation coordinated control of muti-AUVs is studied in this paper. The proposed control method based on the Lyapunov stability theory and graph theory. The control method is not only applicable to communication unimpeded, but also applicable to communication failures and delays on the performance of the overall AUV formation. The impact of communication failures and delays on the overall system performance is analyzed. Conditions are derived under which the cooperation errors dynamics is stable. Finally, the theoretic proof and simulation results show that the proposed control method is effective on formation coordinated control of muti-AUVs.