Research On Accurate Pitching Trajectory Control Methods Of Robotic Fish Driven By Pectoral And Caudal Fin

Bionic robot fish is one of the important directions of underwater robotics research,which has a wide range of application prospects in the fields of water environment monitoring,resource exploration,underwater reconnaissance and military strikes,underwater archaeology and fishery protection,etc.,and is much favored and concerned by researchers.Among them,fish motion is the result of multi-fin synergy,so revealing the mechanism of pectoral fin and flexible body/tail fin synergistic propulsion of robot fish,hydrodynamic characteristics and designing the trajectory tracking control method in complex environment are the key problems of motion control of bionic robot fish.In this paper,we investigate the problem of accurate control of the pitching motion of the bionic machine fish when the3-degree-of-freedom pectoral fins,flexible body and tail fins are propelled synergistically under the disturbance of the external environment such as water turbulence and waves by using the homemade 3-degree-of-freedom pectoral fins propulsion bionic machine fish in the laboratory,and the main contents are as follows:(1)Based on the analysis of morphological characteristics and movement patterns of pectoral and caudal fin-propelled fish by biomechanists,a hydrodynamic model of a3-degree-of-freedom pectoral fin and flexible body/caudal fin co-propelled machine fish was established.The kinematic model of the bionic machine fish is established in the defined coordinate system by determining the equations of motion of the pectoral fin of the machine fish through the study of the fish swimming mechanism and the extraction of characteristic data;the 3-degree-of-freedom pectoral fin pushing and swinging propulsion force,pushing and swinging additional mass force and swinging back resistance,the 3-joint flexible body/tail fin swinging propulsion force and the morphological resistance to the fish body parts are analyzed by using the calculus integration method,so as to construct the pectoral fin and flexible body/ This model is the basis for the subsequent research on the accurate control of the trajectory of the fish.(2)Based on the obtained hydrodynamic model,a dual closed-loop attitude control algorithm is designed to track the pitching trajectory of the fish.The mapping relationship between the pectoral fin,flexible body/tail fin co-propulsion and the pitch angle of the machine fish is constructed by taking the pectoral fin rocker offset angle as the control parameter;and the dual closed-loop control algorithm is designed by taking the attitude control as the inner loop and the position control as the outer loop.The dynamic performance of the inner loop affects the outer loop,so the inner loop converges faster than the outer loop by using a sliding mode controller to ensure the overall stability of the closed-loop control system.The experimental results show that the machine fish takes 12 hours to complete the preset sinusoidal trajectory motion,and the maximum and average errors of its trajectory are0.078 m and 0.052 m respectively,which verifies the stability and high efficiency of the double closed-loop algorithm in the machine fish pitch trajectory tracking control.(3)A 3-degree-of-freedom pectoral fin and flexible body/tail fin co-propulsion model including parameters such as angle of attack and thrust of the fish body was established through numerical computational fluid dynamics simulations,and a fuzzy adaptive sliding mode machine fish depth control algorithm was designed.The mapping relationship between the angle of attack of the fish body,the thrust applied and the phase difference of the pectoral fin rotation is established by fitting the CFD simulation data with the least squares method;and a fuzzy controller is used to compensate and correct the uncertainty terms in the dynamics model of the machine fish in real time and adjust the sliding mode gain,while the sliding mode controller effectively reduces the system jitter and the final system converges to the desired depth in a more stable manner.The experimental results show that the fish has a large deviation only in the initial moment of motion,and the trajectory of the fish is smooth during the whole motion,and the maximum depth deviation is only 0.08 m.The steady-state error is kept within 0.04 m.The designed fuzzy adaptive controller can realize the constant depth motion control of the fish.(4)A hydrodynamic model of the machine fish considering the disturbance factors in the water,waves and other external environmental disturbances is established,and a neural network-based adaptive sliding mode control method for the spatial motion of the machine fish is designed with the phase difference of the flapping wing angle and the flexible body/tail fin offset angle as the control variables.In order to solve the complexity of fluid motion and the influence of the machine fish’s own deformation on the kinetic parameters,a radial basis function neural network is used for online approximation of the uncertainty terms and water flow disturbance terms in the kinetic model,and the adaptive sliding mode controller is combined to adjust the control parameters in real time.The experimental results show that the machine fish has a large error only in the initial stage during the spiral downward motion,and the trajectory of the machine fish and the desired trajectory basically match after about 8s,which verifies the effectiveness of the method described in this paper.