Research On Three-dimensional Path-following Backstepping Control Of Underactuated Unmanned Underwater Vehicle

Due to the lack of additional thrusters in sway and heave dirctions of underactuatedunderwater vehicle(UUV), the numbers of control inputs for the system is less than thedegrees of freedom related to three-dimensional motion, which doesn’t satisfies the Brocekettnecessary condition, and can’t achieve stabilization through designing contious time-invariantfeedback control law. The model of UUV exhibits severely nonlinearities and couplingcharacteristic between each degree of freedom when moving in three-dimensional space,meanwhile the model also has parameter uncertainties induced by the surroundingviscousfluid, which all brought the difficulties to the research on control technology for UUV.The main research content of this thesis focuses on the solutions for diving control andthree-dimensional path-following control problems, and the main works has been carried outas follows:Firstly, towards the diving control problem of an UUV, backstepping method based onfeedback gain is developed for the controller design, part of nonlinear terms can be eliminatedby properly selecting the controller parameters, which leads a simplification of virtual control,and the resulted controller has PID gain tuning function. Robustness analysis is carried out forthe closed-loop system in the presence of model uncertaintieswith the Lyapunov stabilitytheorem and the boundary of the convergence region of tracking errors for the system is givenout.Secondly, to deal with the model uncertainties in diving control problem for UUV, aneural network based dynamic surface control method is proposed, the derivative of virtualcontrol is obtained through first-order filter, which can avoid the “computation explosive”when directly computing the analystic derivative of virtual control. A neural network adaptivecontroller is designed to compensate the model uncertainties and external disturbance basedon Lyapunov stability theorem.Thirdly, to deal with the three-dimensional straight-line path-following control problemof underactuated unmanned underwater vehicle (UUV). The parameterized path equation iscomputed for space straight-line with the given coordinate informations of discreteway-points. Then, the three-dimensional path tracking error equation is established inbody-fixed frame based on ‘virtual guidance’ for UUV. The controller design process isdivided into kinematic and dynamic two parts with backstepping method, which leads asimplication for the design procedure. Robust control terms are designed based on Lyapunov theorem and asympotic stability can be guaranteed for the closed-loop system. The pathswitching strategy is used to drive the UUV to follow discontinuous straight-line path.Forthly, to simplify the three-dimensional path-following controller, given thethree-dimensional path tracking error equation, the backstepping methodsbased on feedbackgain is used for the controller design which avoids the complex form of the virtual controlwhen designing the guidance laws based on line-of-sight method. Additional integrator termsare introduced to enhance the tracking performance for the control system in steady phase.Finally, be differ from obtaining the derivative of virtual control through numericaldifferentiation process in dynamic surface control, towards the three-dimensionalpath-following problem, The backstepping method based on second-order filter is proposedfor the controller design. The virtual control is selected as the reference input of the filter, thefiltered signal of virtual control and its derivative is obtained through the integration processrather than differentiation process, which avoids the the complexity caused by needing tocompute the derivatives of the virtual control and also reduces the measurement noiseinfluence that act on the controller. Additional tracking error compensation loop is designedbased on the Lyapunov stability theory to gurarantee the tracking precision of the filter for thereference input. The numerical results are performed to illustrate the effectiveness of theproposed filter and control scheme.