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Vertical motion control of underwater robot based on hydrodynamics and kinematics analysis

Depth control of underwater robot is of vital importance, especially when it needs to work at specific depths under the water. However, vertical motion control of underwater robot is nonlinear and it is difficult to make the robot stop at the specific depth for the first time it reaches the depth for the existence of inertia. The paper proposes a vertical motion control strategy combining hydrodynamics with kinematics analysis, to make sure that the underwater robot can just stop at the specific depth with its velocity nearly zero at the same time. At first, the hydrodynamics analysis of underwater robot is finished using FLUENT to obtain the relationship between water resistance and speed of the underwater robot in heave direction. Then, the error of the hydrodynamics analysis is compensated through field experiments using least square method with the help of depth gauge. After that, the control strategy of thrust from the two vertical propellers is proposed using kinematical analysis of underwater robot, according to the difference between the current depth and the target depth. Simulation and field experiments verify the effectiveness of the proposed vertical motion control strategy.