Bioinspired Closed-loop CPG-based Control of a Robotic Manta for Autonomous Swimming

Fish in nature exhibit a variety of swimming modes such as forward swimming,backward swimming,turning,pitching,etc.,enabling them to swim in complex scenes such as coral reefs.It is still difficult for a robotic fish to swim autonomously in a confined area as a real fish.Here,we develop an untethered robotic manta as an experimental platform,which consists of two flexible pectoral fins and a tail fin,with three infrared sensors installed on the front,left,and right sides of the head to sense the surrounding obstacles.To generate multiple swimming modes of the robotic manta and online switching of different modes,we design a closed-loop Central Pattern Generator(CPG)controller based on distance information and use a combination of phase difference and amplitude of the CPG model to achieve stable and rapid adjustment of yaw angle.To verify the autonomous swimming ability of the robotic manta in complex scenes,we design an experimental scenario with a concave obstacle.The experimental results show that the robotic manta can achieve forward swimming,backward swimming,in situ turning within the concave obstacle,and finally exit from the area safely while relying on the perception of external obstacles,which can provide insight into the autonomous exploration of complex scenes by the biomimetic robotic fish.Finally,the swimming ability of the robotic manta is verified by field tests.

Effect of Active–Passive Deformation on the Thrust by the Pectoral Fins of Bionic Manta Robot

Bionic manta underwater vehicles will play an essential role in future oceans and can perform tasks,such as long-duration reconnaissance and exploration,due to their efficient propulsion.The manta wings’deformation is evident during the swimming process.To improve the propulsion performance of the unmanned submersible,the study of the deformation into the bionic pectoral fin is necessary.In this research,we designed and fabricated a flexible bionic pectoral fin,which is based on the Fin Ray®effect with active and passive deformation(APD)capability.The APD fin was actively controlled by two servo motors and could be passively deformed to variable degrees.The APD fin was moved at 0.5 Hz beat frequency,and the propulsive performance was experimentally verified of the bionic pectoral fins equipped with different extents of deformation.These results showed that the pectoral fin with active–passive deformed capabilities could achieve similar natural biological deformation in the wingspan direction.The average thrust(T)under the optimal wingspan deformation is 61.5%higher than the traditional passive deformed pectoral fins.The obtained results shed light on the design and optimization of the bionic pectoral fins to improve the propulsive performance of unmanned underwater vehicles(UUV).

Rate-Dep endent Characteristic of Relaxation Time of Concrete

This study focused on the rate-dependent relaxation time of concrete using the split Hopkinson pressure bar test and the ultrasonic technique.Based on the experimental results,and considering the viscous effect in concrete,the viscoelastic stress-strain relationship was established in terms of the principles of irreversible thermodynamics and the generalized Onsager's principle,providing the convergence of an iterative scheme in terms of the Lipschitz's condition.It was found that the relaxation time of concre te depends on the strain rate.Furt her more,considering the state equations and damage evolution,a nonlinear viscoelastic constitutive model of concrete coupling damage was proposed.Compared with the experimental results,this model could better characterize the strengthening and softening characteristics of concrete prior to and beyond the strength limit.Further analysis indicated that the viscous effect is dominantly caused by the C-S-H in concrete.