Research On Control And Hydrodynamic Response Of A Tethered Underwater Robot Under Multi-mechanism Manipulations

With the applications of tethered underwater robot more and more used in scientific research,ocean observation and operation and underwater engineering construction,the research on hydrodynamic and control characteristics under multi-mechanism control has attracted the attention of more and more researchers and engineering designers.It is difficult to establish an accurate analytical mathematical model for the tethered underwater robot due to its strong non-linear,strong coupling and time-varying characteristics.Therefore,on the basic of the existing governing equations of umbilical cable and motion equations of underwater robot,this dissertation firstly establish the hydrodynamic mathematical model of tethered underwater robot with considering the coupling relationship among duct propeller,underwater robot main body and the umbilical cable in combination with boundary conditions.In this hydrodynamic mathematical model,multiple sliding mesh technique is used to analyze the hydrodynamic characteristics of the underwater robot main body and the controlled propellers during the movement process.Secondly,in the design of the underwater robot controller,the feedforward and feedforward-feedback algorithms are applied on determining the control action of the umbilical cable;at the same time the incremental PID method is adopted to adjust the rotational speeds of propellers to produce desired thrusts at every moment according to the control requirements of the underwater robot trajectory and attitude.Then the two control algorithms are coupled into the hydrodynamic mathematical model of the tethered underwater robot system,thus a complete hydrodynamic and control model of tethered underwater robot system is constructed.Finally with this hydrodynamic and control model,the numerical simulations about trajectory tracking of the tethered underwater robot under pre-designated trajectory and attitudes commands;the relationship bwtween the rotational speed and thrust of propeller,the influences of the two control methods on the underwater robot system velocities,the control performance of the feedforward-feedback algorithm for the umbilical cable and the hydrodynamic responses are observed and analyzed.The movements of the underwater robot in water contain three dimensions and six freedom degrees.It makes the dynamic relationship of the underwater robot system rather complex due to the strong coupling characteristics among the robot main body,control propellers and the umbilical cable.In the past the umbilical cable system was usually simplify that only cable tension was considerated into the mataining length of the umbilical cable in the numerical simulation about heave movement of the underwater robot because of the differences of research backgrounds and interests among researchers;meanwhile the umbilical cable was assumed to be always stretched during the underwater robot moves,this would lead singular points occurring in the governing equations of the cable by using finite different calculation method which would interrupt numerical simulation.In order to make sure normal tasks in the practical Marine operation,the underwater length of umbilical cable could be adjusted to avoid its tension exceeds the destructive strength.In this paper the feedforward and feedforward-feedback strategies are applied on adjusting instantaneous length of the umbilical cable according to the movement state of the underwater robot;also the amplitude limit filtering method and average filtering method are applied on restraining the change range of the cable tension for avoiding chattering effect of the cable tension and maintaining the stability of the system and the continuity of the calculation.In order to meet the requirements of accurate trajectory tracking of the underwater robot,the rotational speeds of propellers can be operated to produce appropriate thrusts to assist the umbilical cable.In this paper the incremental PID algorithm is adopted to regulate the rotational speeds of the propellers in order to produce desired thrust according to the trajectory deviation and achieve the purpose of moving along the designated trajectory.The numerical simulations results show that the heave movement of the underwater robot is controlled mainly by adjusting length of the cable and the propulsion of propeller is auxiliary;while the horizontal trajectory of the robot is controlled primarily by the duct propellers equipped at both sides of the robot main body.For the heave movement,the feed-forward strategy is stable without overshoot nature and the relative error for adjusting length of the cable is less than 15%,while the displacement in vertical direction is limited;the feed-forward feedback control can be used for large displacement and the max relative error is less than 10% with more stable and accurate control performance.The maximum relative error about horizontal trajectory is about 10%.The amplitude limit filtering method and the average filtering method for restraining the chattering effect of the cable tension are effective and reasonable and ensuring the stability of the numerical calculation.The hydrodynamic responses of the underwater robot main body during the motion process are influenced mainly by the flow fields' changes due to the velocity of the underwater robot changes and the rotational speeds of propellers changes.