Research On The Nonlinear Characteristics Of A Hydraulic Propulsion System And Disturbance Rejection Control Of An ROV

This paper describes a study on improving the control performance of a deepsea working-class ROV(remotely operated vehicle)based on the characteristics of the propulsion system and the unique working environment of the ROV.There are two main factors that influence the control performance of a deep-sea working-class ROV:1)The characteristics of the propulsion system used in the ROV.The two main types of propulsion systems currently used in deep-sea working-class ROVs are hydraulic and electrical systems.Each type offers different advantages.This paper focuses primarily on a hydraulic propulsion system controlled by pressure difference,which is adjusted using proportional valves,and a hydraulic system controlled by displacement,which is adjusted using servo valves.The nonlinear nature of the two types of hydraulic propulsion systems is well-documented and has a significant effect on the control performance of the ROV.2)The external environmental factors acting on the ROV.Because of the unique operating environment,the following factors should be considered for the controller design of the deep-sea working-class ROV: 1.The ROV may carry different functional equipment depending on the operation type which will cause large variations in the mass and inertia of the ROV;2.The seawater density changes with the operational depth and area of the ROV,which can change the hydrodynamic coefficients associated with the ROV;3.The ROV may encounter strong currents that impose disturbing forces on the ROV;4.The umbilical cable connecting the ROV and the TMS(tether management system)produces a disturbing force that impacts the ROV.For this study,the parameter variations,current disturbance and umbilical cable disturbance are defined as disturbances acting on the ROV.The paper aims to improve the control performance of a deep-sea working-class ROV based on the characteristics of the propulsion system and anti-disturbance control.The solutions to address the nonlinear nature of the hydraulic propulsion system are as follows: 1.To improve the nonlinear characteristics of the propeller thrust when the thrust is small,the initial pressure is added to both ends of the constant displacement motor used in the propulsion system controlled by the pressure difference,which is adjusted using proportional valves if the input signal is small.A segmented linear function is used to approximate the small thrust response curve of the propulsion system in which the initial pressure is added and the large thrust response curve of a conventional propulsion system.2.A dead zone in thrust occurs when propulsion systems,which are controlled by displacement using servo valves,start from rest.The dead zone in thrust is reduced effectively by using an impulse control method.3.The maximum thrust when a propeller rotates in forward is not equal to the thrust when the propeller rotates in reverse.To account for this condition,an improved pseudo-inverse thrust allocation method and an equal-proportion thrust allocation method have been designed.The thrust can reach a maximum in both the forward and in the reverse directions,and the efficiency of the propulsion system can be increased by using these methods.4.To address the impact of the dead zone in thrust on the control performance of the ROV,dead zone correction methods that use the layout of thrusters and GAs(genetic algorithms)are proposed.The solutions to the ROV disturbances are as follows: The character of an FLS(fuzzy logic system)dictates that it can approach any nonlinear function with any precision,thus the disturbances acting on the ROV were approximated using an FLS.The results from numerical simulations and underwater experiments verified the effectiveness of the control law.