Motion Control For Multifunction Autonomous Underwater Vehicle

An Autonomous Underwater Vehicle (AUV) works in the complex ocean environment. So it is a seriously nonlinear system with strong coupling in every freedom, and it is difficult to get its precise movement equations.The sliding mode controller (SMC) doesn't need the precise mathematics models to design the controller. SMC has the advantage of strong robustness and it is suitable to control AUV.This paper is mainly about the SMC for a controllable regular nonlinear system based on multifunction autonomous underwater vehicle. A decentralized control strategy is selected for the controllable regular nonlinear system, and changes the nonlinear into linear reduced form. The pole setting method is applied to designing the switching function. Approach rate strategy is introduced to eliminate chattering. The AUV position controller is designed through changing AUV dynamic model with 5 degrees of freedoms into controllable regular form.The switching surface designed by the pole setting method could lead to slow convergence of the system around the origin. Therefore the polyline switching surface is presented. To increase the switching surface slope in the origin area can accelerate the convergence of system to origin. If the slope of the switching surface is set 0 and the integral is added, velocity control can be realize without changing the mathematic model when the control system slide on this switching surface. Setting the position of start and end of the velocity control can realize the given position velocity control. The simulation result shows that SMC with polyline switching surface converges faster than the tradition one. Velocity control is quite precise, and overshoot and steady state error are relatively small. The velocity control at given position is achieved.Weakening the chattering of sliding mode control is the main task of the SMC. In this paper, a sliding region is defined, and an equivalent control in the sliding regionis given. The sliding region is used to weaken the chattering of sliding mode control. After entering the sliding region, the system produces low frequency and small amplitude oscillation under the equivalent control. The simulation experiment shows that the sliding region can solve the chattering problem effectively.