Experimental And Numerical Study On Subsea Propulsion For Shoal Crab Robot

As the marine science research and the ocean exploration becoming the hot issues all over the world,the technology of subsea robot develops rapidly.Actually,the usage of the current offshore unmanned platform which is composed of high speed unmanned ship,unmanned underwater vehicle and deep-sea space station can realize all-round coverage towards the ocean except the shoal between ocean and land.In fact,there exists many benthic and plankton in shoal environment which cause the unmanned device driven by propeller poor adaptability for shallow marine,surf zone,breaker zone and tidewater settlements.Therefore,it has a practical significance to develop an amphibious robot which is suitable for the shoal environment in order to improve the application range of the offshore unmanned platform.In this paper,a leg-paddle hybrid driven shoal crab robot employs six walking legs and two 3DOFs swimming paddles as its compound propulsion system is developed based on the prototype of Portunus with the movement characteristic of walking on land,crawling on seabed and swimming in water.It can reach the area where the traditional underwater or legged robots are disabled to realize the movement and operation,so as to implement the biological modeling design with respect to the configuration,shell structure,movement ability and method of sea crab.The hydrodynamics of 3DOFs swimming paddle is investigated numerically by parallel CFD method to investigate the influence of motion parameters of rigid swimming paddle on the propulsive performance and efficiency.Propulsive efficiency as the optimization criterion is used stepwise to determine the motion parameters in lift-based swimming mode and drag-based swimming mode.And the relationships between the motion trajectory of reference points,the angle of attack of velocity and hydrodynamic performance are studied in lift-based mode and drag-based mode respectively.In order to explore the underwater propulsion mechanism of rigid swimming paddle,the slice method combining with vorticity contours is employed in this paper to analyze the relationship between hydrodynamic performances,vortex structures,streamline and pressure distribution on the surface of swimming paddle.The propulsion mechanism of swimming paddle is investigated from the view of trajectory characteristics,thrust generation and flow field structure.Imitating the motion of biological sea crab,a series of floating gaits for shoal crab robot are put forward by means of proper planning the sculling motions of two swimming paddles in order to make the shoal crab robot swim underwater without replacing driven device which greatly improve the adaptability of robot in uneven environment.The mainly research of this paper are the propulsion strategies which comprise tail-swing gait,switch-flapping gait,lift-based efficiency optimized collaborative sculling gait and its alternate sculling gait,lift-based thrust optimized collaborative sculling gait and drag-based efficiency optimized collaborative sculling gait.Taking Hopf oscillators as the rhythm signal generator,the CPG network model for controlling the motion of swimming paddles is also designed to build the database of CPG network parameters for the floating gaits proposed.This work focus on the new way for underwater walking which includes the transverse skip-step gait and the longitudinal leap-floating gait integrated by walking legs and swimming paddles.And the extension definitions of gait parameters are proposed for describing the underwater gait.The dynamic models of skip phase in these two walking gaits are established and analyzed considering hydrodynamic force by Lagrange floating base method.The dynamic analysis of skip phase provides a basis for the optimal design of skip-step gait in future.These two gaits as the attempt for the underwater motion mode of multi-legged robots provide a new thought for the similar robots.The propulsion test platform of the swimming paddles and the verification platform for testing transverse skip-step gait are set up to carry out the comprehensive experiment research and a prototype of shoal crab robot is built for testing the floating ability of robot.The propulsion experiment of two swimming paddles in the Ship Maneuverability Laboratory(HEU)Water Tunnel are used to demonstrate fundamental properties of swimming paddle and make comparison with the results in numerical study.For further validating the swimming ability of robot,the prototype is tested by using the proposed floating gaits.The feasibility of transverse skip-step gait is also demonstrated in this paper in order to provide a reference for the further study on subsea walking gait.As a new bionic propulsion method,leg-paddle hybrid driven technology employs the compound propulsion strategy with walking legs and swimming paddles rather than their simple combination,by which the robot can choose motion state according to the shallow geomorphic and operation tasks,and this also provides a reference for the bionics designing of underwater robot.The results in this paper will be of great importance in improving the environmental suitability and practicability for shoal robots in shoal environment.