Simulation And Control Of Tethered Underwater Vehicle's Motion

As an important part of ocean engineering equipment,tethered underwater vehicle,its maneuvering performance and trajectory tracking control are always hot topics among underwater engineering technology.This paper presents the numerical simulation of umbilical cable motion,tethered underwater vehicle maneuvering and its trajectory tracking control under different velocities of uniform steady current,and the simulation results are compared and analyzed.In the study of umbilical cable motion,the finite difference method model is improved to consider the effects of uniform steady flow.And the least square method is adopted to solve the nonlinear equations obtained by discretization.To verify the validity and reliability of the improved model,the numerical solution and the analytical one are compared in the case of flow and movement in the same direction when underwater vehicle is horizontally moving forward with a constant velocity.The agreement between two solutions not only confirms the validity and reliability of the improved model but also proves the validity of the numerical scheme.Through comparison and analysis of simulation results between different current velocities,it indicates that current direction and velocity value can have significant effects on cable motion.When the current direction is positive,the cable has a positive offset,current direction negative,the cable a negative offset.And the greater the current velocity value,the bigger the offset of cable.When umbilical cable moves against current,the greater the current velocity value,the bigger the cable tension,and the longer the cable length immersed in water.When umbilical cable moves following current,the greater the current velocity value,the smaller the cable tension,and the shorter the cable length immersed in water.The umbilical cable can generate force and moment acting on underwater vehicle to stop the change of underwater vehicle's motion state.In the study of tethered underwater vehicle's maneuvering,based on the six degrees of freedom motion model,hydrodynamic coefficients obtained by PMM test,umbilical cable force numerically calculated by the improved finite difference method model,the maneuvering of tethered underwater vehicle is simulated with the thruster rotation speed given.And the influences of different uniform steady current velocity and umbilical cable are analyzed.The results show that the current velocity can significantly change the velocity and Euler angle of underwater vehicle,the umbilical cable force value and direction.Although the umbilical cable is connected to the gravity center of underwater vehicle,only generating force to stop the change of vehicle's motion state,and not generating moment,but the acting force will change the vehicle's linear velocity which can change the hydrodynamic loads generating by vehicle itself,and result in perturbation motion of tethered underwater vehicle with a time-varying amplitude.In tethered underwater vehicle's trajectory tracking control study,kinetmatics backstepping controller and dynamics sliding mode controller are designed.Bio-inspired neural dynamic model is introduced to smooth the output speed jump generating by backstepping controller,because of a larger tracking error.Meanwhile the Lyapunov stability is verified.With the parameters of controller set by trial calculated results,the simulation of broken line path tracking and spiral path tracking is carried out.And the impact on tracking effect by current and umbilical cable is analyzed.The results indicate that the bio-inspired neural dynamic back-stepping sliding mode controller is able to achieve tethered underwater vehicle's trajectory tracking better.Only at the initial stage and discrete trajectory inflection point,is there a larger tracking error and thruster force.In the rest of time,it can always achieve better steady state tracking.Uniform steady current can not cause the increase of the tracking error of the controller,when the velocity of current is low,the effects of current on the propeller thrust output is not obvious.But the umbilical cable will increase the controller's tracking error,especially in the spiral path tracking.The tracking errors in the x and y direction are larger and the influence is significant.