Design And Experimental Study Of Cable Driven Flexible Spine Bionic Fish

As the oldest vertebrate,fish have excellent swimming characteristics.Compared with traditional propeller thrusters,biomimetic robotic fish has many advantages,such as high propulsion efficiency,good maneuverability and strong concealment.The research and development of new biomimetic robotic fish has attracted more and more attention from scholars from home and abroad.Biomimetic robotic fish can be divided into Anguilliform Mode,Carangiform Mode,Thunniform Mode according to their body shape and swimming form.The Thunniform Mode has the strongest comprehensive performance and is easy to realize in mechanical structure and motion control.In this paper,the Thunniform Mode is selected as the bionic object.Aiming at the swimming characteristics of this kind of fish and the current drawbacks in the mechanical structure of bionic robotic fish,the cable-driven flexible spine bionic fish is designed.Firstly,this paper analyzed and compared the driving mechanism of bionic robotic fish,and comprehensively considered the advantages and disadvantages of various driving mechanisms.Besides,the design objectives are formulated with the motion characteristics of Thunniform Mode being combined.According to the design objective and the design experience of the existing flexible bionic robot fish and hyper-redundant degree of freedom robot,the general plan of design is completed.Then,the mechanical structure of the cabledriven flexible spine bionic fish was designed from three aspects: head,body and fin.On the basis of structure design,the control system is designed according to the requirements of the selected control layer.Secondly,according to the structural characteristics of the flexible spine,the forward kinematics model of the flexible fish body is established by using D-H method based on piecewise constant curvature assumption.The mapping relationship between the input variable(the change of the length of the pull cable)and the output variable(the point of the center line of the fish body)is solved.The simulation results show that there is a linear relationship between the change of the length of the pull cable and the displacement of the point on the center line of the fish body.The fish body and tail fin are connected by hinges and controlled separately by two sets of steering engines.There is no coupling relationship between the two inputs.According to the geometric relationship of the structure at the hinge joint,the linear relationship between the length of the pull cable and the rotation angle of the tail fin in a certain range(greater than 0 degrees,less than 40 degrees)was solved by analytic method.Using the existing kinematics model of fish body and tail fin,the trajectory curve of fish body center line and tail fin moving in straight line and c-turning motion is simulated.The simulation curve and theoretical fish body wave curve are fitted to solve the fish body wave parameters c1,c2 and wavelength ? of the bionic fish designed in this paper.The simulation curve of fish body center line of cable-driven flexible spine mechanism is connected with the joint series machanism.The simulation curves of the fish body centerline and the fish body wave curves are fitted to calculate the envelope area.The comparison results show that the fitting degree of the joint series mechanism is higher under the condition of small swing,and the fitting degree of the cable-driven flexible spine mechanism is higher under the condition of large swing.Thirdly,on the basis of the known motion model,the concept of pre-strain is introduced to establish the bionic fish numerical model.A multi-physical field simulation software,is used to simulate the two-dimensional autonomous swimming process of bionic fish under different swing frequencies by numerical simulation method,and the swimming characteristics and hydrodynamic characteristics of bionic fish are solved.The simulation results show that the forward velocity in the x direction increases stepwise from 0,and the growth rate decreases gradually until it reaches the cruising state.It fluctuates with constant average velocity and constant amplitude,and the fluctuation period of velocity is 1/2 of the fish body's swing period.The velocity in the y direction deviates from the equilibrium position at the beginning of the motion,and then reaches the stable equilibrium position and becomes the average speed.The fluctuation period of velocity variation is consistent with that of fish body oscillation.With the increase of swing frequency,the speed and propulsion amplitude of bionic fish in cruising state increase,too.Finally,an experimental platform is built to verify the correctness of the kinematics model by measuring the pose of the bionic fish prototype.The underwater swimming test verifies the feasibility of the design scheme,the underwater sealing of the prototype and the rationality of the counterweight.