Research On The Flexible Bionic Robot Fish With Online Variable-Stiffness Driven By High-Frequency

When swimming,fish can adjust their own stiffness through muscles to change the natural frequency of its body,so as to match with the tail beat frequency,thereby them can achieve better swimming performance.Moreover,the tail beat frequency of some fish can reach 20 Hz or above,and the swimming speed can exceed 15 body length per second.It is because of these excellent regulatory capacity and swimming performance that fish attract many researchers.In this paper,the main joints of flexible robotic fish are realized by virtual rotation pair.The virtual rotation pair is based on the tensegrity structure,which is composed of two rigid platforms to realize the function similar to rotation pair through non-contact form,and this structure is consistent with the musculoskeletal system of human and animals.The variable-stiffness mechanism adopts the mechanically adjustable compliance and controllable equilibrium position actuator(MACCEPA).The joint with variable rotational stiffness is also realized by the virtual rotation pair.The steering gear swings to change the pre-tensioned force of the spring,so as to realize the online adjustment of the stiffness.The driving part transforms the continuous rotation of single motor into reciprocating swing to drive the fish body,and finally the flexible robotic fish can swing in the high-frequency range.Taking the geometry of bluefin tuna as the bionic object,the mathematical models of virtual rotation pair,variable-stiffness mechanism and high-frequency swing mechanism are established to analyze the stiffness characteristics,the relationship between pre-tensioned force and rotation stiffness,the relationship between motor driving angle and swing angle,and the relationship between torque.Finally,the geometric parameters of the mechanism are determined according to the shape of the fish body.The simulation model is established.Based on the large-amplitude elongatedbody theory of fish locomotion,hydrodynamics is added.The tail beat frequency range is set at 6.5 ? 12.5Hz.From the simulation results,it can be analyzed that the swimming speed of flexible robotic fish will have a local peak at a certain tail beat frequency,which is the natural frequency and increases with the increase of stiffness,It is verified that the fish can improve the swimming performance by changing the stiffness to match the tail beat frequency,but the overall trend of swimming speed is positively correlated with the tail beat frequency.At the end of this paper,the specific structure design and physical production of the flexible robotic fish are given,and the design scheme of the electric control system is introduced.The results show that: firstly,the simulation results are verified;Secondly,the swing amplitude and rotational stiffness of the driving joint are not the bigger the better;Finally,the minimum Strouhal number of flexible robotic fish is0.57,compared with 0.25?0.35 of fish,there is still a certain gap in swimming performance.