Study On Non-stationary Motion Simulation And Auto Matic Control Of Trawl System

Pelagic fishery is the emphasis for marine fisheries development in China,it is also one of the measures to build a marine powerful nation.Trawl systems have the following characteristics: wide operation areas,abundant types of fishing,large amount of fishing and ease of operation,for these reasons trawl fishing has become an important way of operations in deep-sea fishing practices.At present,the trawl systems in our country is relatively backward,most fishing boats are dilapidated with a low automatic degree.Hence,it is important to study the movement characteristics and the control theory of trawl system.The study of trawl systems is also helpful for other studies of underwater towed systems.Trawl systems belong to a kind of complex underwater towed system and to study their kinetic characteristics,a generalized underwater towed system is regarded as the research object in the first place.The kinetics equations of towing rope are establish based on the dynamic equilibrium conditions of towing infinitesimal section and the motion continuity conditions.The change of cable length can be simulated through the way of adding some virtual points.The dynamic model of underwater towed-body is obtained based on the six degrees of freedom motion equation of underwater robot.Through the coupling boundary conditions,the complete motion mathematical model of underwater towed system is established.In order to solve the proposed dynamics equations of towing rope,two numerical solution methods are adopted,which are finite difference box method and general ? method.The accuracy and stability of these two method are analyzed for comparison.Synthesizing the advantages of these two methods and considering the characteristics of the number of unknown variables changing in the dynamic equations of time-varying length cable,a new numerical discrete method is presented.For the underwater towed-body,a numerical method similar to the form for towed rope is employed to proceed the numerical discrete in time direction.The solving program for time-varying length underwater towed system is made in the Matlab software.Finally,a simulation comparison proves that this scheme can filter high-frequency perturbation effectively and has faster computing speed and better convergence than finite difference method.For the whole movement modeling approach of midwater trawl systems.At first,a geometry modeling method for the shape of trawl net is presented,which can calculate the nets appearance quickly.Then based on the dynamic model of underwater towed system,regard otter board as the towed-body,regard trawl net as a concentrated force,modeling all the subsystems and boundary conditions.Through the above stages,a motion mathematical model of midwater trawl system is established.The comparison of simulation calculation and sea test results shows the validity of the proposed model.Otter board is the key equipment to hold the horizontal opening of trawl net.The six degree of automatic movement of otter board is studied.Since the hydrodynamic performance of otter board can direct influence the spreading effect of trawl net,the hydrodynamic calculation method for otter board is introduced detailedly in this chapter.The steady hydrodynamic forces of otter board are solved based on similarity theorem,and the transient hydrodynamic forces are solved based on Vortex Lattice Method.With the movement model,a dual-loop control system for the motion attitude control of otter board is designed based on S plan control method.Several simulation experiments been carried out in the end.For three-dimensional trajectory tracking of midwater trawl systems,a new mathematical model of the trawl system while considering the horizontal expansion effect of two otter boards is presented based on the Newton Euler method.Subsequently,an active path tracking strategy of the trawl system based on the backstepping method is proposed.The non-strict feedback characteristic of the proposed model employs a control allocation method and several parallel nonlinear controllers to eliminate the high order state variables.Based on the Lyapunov Stability Theory,the stability analysis proves that the proposed controller can maintain the stability of the trawl system even with the presence of external disturbances.The simulation results illustrate that the improved backstepping controller is effective for three-dimensional trajectory tracking of the midwater trawl system.