Research On Path Following And Coordinated Control For Second-order Nonholonomic AUVs

In the 21th Century, oceans will play a key role in the human society. With the emerging requirement on ocean exploration and exploitation, and the rapid development of Autonomous Underwater Vehicles (AUV), underactuated AUV becomes an important type of underwater vehicles as a significant tool for ocean engineering. Underactuaed AUV suffers from the second-order nonholonomic constraints, has strong nonlinear characteristics. Hence, research on the motion control for underactuaed AUV, is very interesting and valuable from both practical and therotical point of view.In this dissertation, the state of art of motion control on the second-order nonholonomic AUV has been reviewed, and the exsisting problem is posed. After that, the path following control of underactuated AUV is choosen as the core topic of control design, and the path following based coordinated formation control is also proposed later. The contents of the paper are as follows.Firstly, the nonholonomic system is introduced, the characteristics of path following and trajectory tracking are compared, and the suitable method to build the tracking error dynamics is chosen. The strategy of path following control is carefully analyzed based on the first-order nonholonomic mobile robot. In the Serret-Frenet frame, the virtual reference point on the path is highlighted which can collaborately help the convergence of robot towarding the path. With the extra control freedom from virtual point, the time-space decoupling path following controller rises up.Secondly, the modeling of AUV kinematics and dynamics is analyzed. Based on the similarity between the first and second order nonholonomic system, standing on the useful control strategy for the first-order nonholonomic mobile robot, the similar control design is proposed for the second-order nonholonomic AUV, relieving the limation of angular persistent excitation on some other controllers for path following. By introducing the line-of-sight (LOS) angle adapting with the curvature of curves to the AUV, which is originally used for straight line tracking of ships, and using separated LOS navigation and position control, the path following controller is simplified. Based on Lyapunov design and backstepping technique, the kinematic controller is extended to the dynamic controller. Moreover, the acceleration of AUV side-slip angle is deliberately tackled, such that the complete path following controller is obtained, and the high precision path following is guaranteed. Under the disturbance of ocean current, the principle of input-state-stability and the idea of directly rejecting disturbance are used to design the new path following controllers.Thirdly, under the constraints of limited communication distance and bandwidth of underwater acoustic communication, path following and rigid formation control is integrated, such that the coordinated formation of AUVs based on path following control is more feasible and valuable in practice. Using the methods of formation reference point and virtual structure, the paths are ready for coordinated AUVs with control flexibility. The time-space decoupling characteristic is also embodied in the coordinated controller design, which means each vehicle employs individual path following control to reach the desired spatial position and orientation, and the coordinated controller adjusting speeds of multiple AUVs can assure the formation structure. Later, two types of controller, i.e., leader-follower based controller and decentralized controller are proposed, to realize the path following based formation control of multiple AUVs. The single source and uni-direction of acoustic communication renders leader-follower structure, and the algebraic graph theory is used to build complex underwater communication network, which is helpful to design the decentralized controller. Moreover, the speed saturation of underwater vehicle has been considered in the multi-vehicle controller design. The revised controller is also designed to deal with the varied communication topology and the time-delay in the acoustic communication network, in case of the communication graph is uniformly connected in mean. After that, based on the sensor information supported by the underwater looking forward sonar, the trimmed line-of-sight angle is introduced and the principle of input-state-stability is used to propose the novel controller, in order to achieve the simultaneous path following and obstacle avoidance/collision free for multiple AUVs under formation control.Finally, the research work in the dissertation is concluded, and the innovative idea is highlighted. The future work on path following and coordinated control for the second-order nonholonomic AUV is also indicated.