Study On Stability Of Hydraulic Propulsion System Of Deep-sea ROV And Its Control Methods

Hydraulic propulsion system is an important part of ROV(Remotely Operated Vehicle) control system, which characteristics have direct influence on the moving and working ability. Under the support of national 863 major project of 4500 meters class deep sea operational capability, the first hydraulic propulsion system in china used in 4500 meters class deep ROV-"Hama" was developed successfully. Its performance of all aspects met technical indexes and the ROV had completed its sea-trial mission in south China sea in April of 2014. As one of the key technologies, hydraulic propulsion technology helped "Hama" become one of "Ten Pieces of News in Science and Technology of Year 2014 in China"(rank 2). This dissertation mainly had studied on the stability of hydraulic propulsion system and its control methods. The main conclusions and research achievements were obtained as follows:(1) In order to solve the stability problem of propulsion system controlled by proportional valves, the influence of hydraulic parameters including the effective bulk modulus and the fluid viscosity on valve stability was studied. A drive-pressure and back-pressure independent regulation control algorithm was put forward to enhance its damping ratio and stability and the sea trial showed that this method was effective. The linearized model was analyzed, and the transfer functions of the main valve and the pilot valve were established near the thruster operation point. The influence of the hydraulic parameters on valve stability was studied by the simulation and experiments. This system can replace the control system with servo valves or variable displacement motors and it has been successfully applied in "Hama" ROV. This kind of system and the analytic technique can also be used in engineering machinery with large-inertia actuator.(2) An adaptive backstepping sliding mode control method was proposed based on the fact that underwater hydraulic propulsion system controlled by electro-hydraulic proportional valves has the control characteristics of strong nonlinearity of the hydraulic system, unknown hydrodynamic parameter, parametric uncertainties susceptible to temperature and pressure changes of the external environment. The controller combined with experimental identification method and the robust adaptive controller design method to analyse complex electro-hydraulic system and estimate unknown parameters, the globally asymptotic stability and boundedness of the controll system was guaranteed by using Lyapunov stability theory. A comparative study was conducted using the control method presented in this paper and PID controller. Both the simulation results and the test results indicate that the proposed controller with strong robustness can track the desired reference trajectory of thruster speed with satisfied dynamic performance and steady accuracy.(3) A control method was proposed which combined the nonlinear sliding model control and PID control to solve the strong nonlinearity of ROV motion control system. It was applied in the heading and depth control to reduce the negative influence of the rotational speed and thrust imblance of the thrusters. The 6 degree of freedom nonlinear model of ROV was established in MATLAB/Simulink and the simulations of various operation modes including surface and underwater modes were conducted. The simulation results indicated that the hybird controller can decrease the amplitude of attitude angle fluctuation and the position error caused by the rotational speed and thrust imblance of the thrusters. The nonlinear controller inside can make the thruster have a more smooth speed and a faster response, which will improve the dynamic response and control accuracy of the ROV essentially.