Research On The Follow Control Strategy For An Underwater Moving Target

Underwater target follow is an important function of ship dynamic positioning system, which can manipulate mother ship on surface to follow the underwater target (ROV) automatically. This paper has done some researches on the follow control and strategy for underwater target, the contents include the following:First of all, it should build mathematical simulation models for study objects (mother ship, underwater target). The dynamic movement of mother ship is a very complex process, it being subjected to various interferences of environment (wind, wave, current). We should establish mathematical equations to simulate forces on the mother ship for these interferences. We would also like to build simulation model for onboard thruster system which are power source of mother ship. In the end, build whole mother ship movement model with the impacts of all factors. In addition, we need to design a simple movement model for underwater target, it is described to an object which moves random in the water.In addition, it must design a new underwater target follow strategy for the purpose of minimizing energy. The mother ship can track underwater target by the judgment of region. The follow control will be started when the position of target beyond the scope of specified region near the ship, until ship and target return to the appropriate relative location. In this case, the thrusters of ship will work discontinuously, that saves energy. Further more, because the transverse resistance of ship is far greater than the longitudinal resistance, so make sure the mother ship moves in longitudinal direction during the positioning control, which should be necessary reflected to follow strategy. For the situation of that the target is leaving from region at different border, it should make different methods to calculate the destination position and destination heading of mother ship, and control mother ship to its destination location.At last, It could establish a specific coordinate system, and compute the position error in longitudinal and transverse directions between current and destination location of mother ship by vectors, so does angle error of heading. Use the classic PD control law on sway and yaw motion of mother ship and achieve good control result. The longitudinal movement of mother ship has great inertia, so the surge control of mother ship has a certain extent lagging, and the movement of underwater target is active, this makes relationship between ship and its destination location more complex. To resolve this problem, we may introduce intelligent fuzzy control theory to the surge motion control. Along the longitudinal direction, to study the relative position and velocity relationship between ship and its destination location, and quantify the position error and its rate by fuzzy quantifying, then make fuzzy rules with kinematics knowledge and experience in ship dynamic positioning control. Finally, design intelligent fuzzy PD controller to improve control result with ship surge motion, and reduce the overshooting, which makes follow path smooth.Underwater target follow is a dynamic process, it's important to adjust controller that makes dynamic response better. The simulation result shows that the mother ship tracks underwater target successfully. The path of mother ship is shorter than target's, witch save energy efficiently.