Research On Hydrodynamic Performance Of Swimming Paddle For Shoal Crab Robot And Its Floating Gait Planning Method

After a long period of natural selection and evolution,a biological sea crab,which widely inhabits on rock,shoal environment or high current area,can walk on land or seabed and can swim in water with the advantages of well flexibility,hidden performance and high energy utilization rate in low speed.Therefore,the movement mechanism of sea crab can be applied to micro amphibious bionic robot,which has an important research significance and practical value for exploring a new underwater propulsion method and improving the motion performance of the robot.For a sea crab,a pair of paddle-shaped swimming legs with multiple-actuated characteristics is the main driven joints,thus a variety of complex movements can be implemented underwater.However,few studies on the swimming paddle of crab are mainly concentrated on the movement process and physiological characteristics from the point of movement mechanism of crab.Considering that the movement mechanism of crab is not clear and the application of paddle-driven propulsion on underwater robot is still at the exploratory stage,the study on propulsion mechanism of swimming paddle is of significance.The main work here is completed from 3 aspects including theoretic calculation,numerical simulation and experimental research.Some research including the structural designing of the shoal crab robot,the analysis of propulsion mechanism of the swimming paddle,underwater floating gait planning,underwater swimming controlling method and the cruise performance test for robot is done.According to the analysis of the morphological structure and the movement characteristics of the sea crab,the structure scheme of the leg-paddle hybrid driven shoal crab robot is proposed.The structure includes the series-parallel hybrid walking leg,swimming paddle,buoyancy adjusting device,drag reduction shell,control box and body.The double propulsion device composed by walking leg and swimming paddle allows the robot to walk on land or seabed and swim in water.On this basis,the kinematic and dynamic performances of the shoal crab robot are analyzed and simulated.According to the joint angle constraint,the corresponding working space of the walking leg is calculated by search method.To investigate the hydrodynamic performance of the swimming paddle of the shoal crab robot,the three-dimensional flow field of the swimming paddle is simulated based on parallel CFD method.And the relationship between hydrodynamic forces of the paddle and reference point's motion trajectory and velocity angle of attack in lift-based mode and drag-based mode is also studied.The propulsion mechanism of the paddle in different motion modes is analyzed from different views including trajectory characteristics,load generation,vortex evolution,trailing vortex structure and pressure distribution.Moreover,the influences of structured parameters of the swimming paddle,time asymmetrical motion parameters,propulsion mode and the corresponding motion parameters on hydrodynamic performance and flow field structure are also studied.And then,the optimized parameters of 2 and 3 DOFs rigid swimming paddle in lift-based mode and drag-based mode are obtained.A motion control method based on the central pattern generator(CPG)is proposed in allusion to the floating movement of the shoal crab robot.The CPG neural network,which employs the modified nonlinear oscillator as rhythm signal generator is established by adjacent weak coupling method.Then the uniqueness and stability of a limit cycle of the oscillator model are validated.Therefore,the swimming forward gait,swimming backward gait,turning gait and snorkeling gait are proposed and planned in order to realize the three-dimensional underwater motion of the shoal crab robot by analyzing the hydrodynamic performance of the synergy propulsion of swimming paddles.And the CPG parametric database for the corresponding gait is built.The target point tracking problem of the shoal crab robot with floating gait is studied.Firstly,the nonlinear dynamic model of the shoal crab robot with floating gait is established by Newton-Euler method.And the motion performance of the robot is simulated so as to investigate the controlling method for robot underwater swimming.Secondly,a sliding model variable structure controller,whose variable is the phase difference between the flapping motion and feathering motion,is designed based on the exponential approach law to control the turning speed of the robot.The system is validated global asymptotic stable according to the Lyapunov direct method and the corresponding simulation and experimental study are carried out.The results show that the robot controlled by sliding mode control method exhibits well tracking capability for the target point and well robustness for the uncertainty of dynamic parameters and external disturbance.Experimental research on two swimming paddles is carried out to verify the correctness of the numerical simulation.Considering that the periodical variation of hydrodynamic and its unsteady characteristic,the hydrodynamic test system for rigid swimming paddles with 3 DOFs is set up in circulating water channel to measure the propulsion performance of swimming paddles.Then the results of the experiment and CFD numerical simulation are contrasted,and the generating of error is explained.Furthermore,the influences of the shape and stiffness of the paddle and coupled motion parameters of joints on the thrust and efficiency of swimming paddles are studied.Finally,the experimental researches on the swimming performance and motion control of the shoal crab robot prototype is conducted in open water environment.The straight-line swimming,turning and snorkeling performances of the prototype are tested.And the results validate the performance of multi-mode motion based on CPG model and the feasibility of the target point tracking algorithm.As a new type of biornic propulsion technology,leg-paddle hybrid driven method combines the walking leg and swimming paddle,with which the robot can choose walking or swimming mode automatically,according to shoal environment and operation task.Therefore,this research has important significance to improve the robot's adaptability for shoal environment and practicability.