Design And Experimental Study Of Fluid-Driven Bionic Soft Actuator With Variable Stiffness

In the past decade,soft robots have gradually become one of the research hotspots in the field of robotics.This is due to the fact that the main bodies of soft robots adopt soft materials(such as silicone rubber),flexible layers or membranes,which give them passive flexibility and flexible deformation ability.Soft robots have unique advantages in human-robot interaction,bionic structure design and exploration of complex and changing environments.But as the research proceeds,some shortcomings of soft robots have been revealed,such as small load capacity,poor posture maintenance,and low action frequency.Thus,the integration of variable stiffness materials or structures into soft robots becomes an ideal improvement scheme.Depending on the working conditions,soft robots can switch between soft and rigid states.This improved scheme combines the advantages of soft robots and traditional rigid robots.The key factor in the design of the variable stiffness soft robot is its power mechanism—the soft actuator.Therefore,a fluiddriven variable stiffness soft actuator is proposed in this dissertation,which mainly consists of a fiber-reinforced soft actuator that generates bending action and a bionic wire jamming layer with tunable stiffness.Specific studies and conclusions are as follows:(1)A fiber-reinforced structure with a rectangular cross-section is proposed as the pneumatic soft actuator scheme.Based on the moment balance equation,a theoretical model of bending is developed.Then,uniaxial tensile experiments and data fitting are performed to obtain the constitutive model property parameters of the rubber.For the issue of the complex and variable posture of the soft actuator under driving air pressure,the use of the finite element analysis method is proposed to optimize the actuator structure.The accuracy of the proposed theoretical model is verified by comparing the simulation results with the theoretical model.Finally,the effects of gravity and external structure on the posture and the end output force of the actuator are studied.(2)In order to solve the problem of large size and poor flexibility of the current variable stiffness structure,this dissertation is inspired by the action principle of human skeletal muscle and the structure of muscle fiber,and designed a bionic wire jamming structure.For the problems of local rearrangement and top gradient effect in the bending process,a reverse gradient trimming scheme is proposed,and a rectangular sealing membrane and a set of manufacturing processes are designed.By building a three-point bending experimental platform and a torsional experimental platform,the effects of material,negative pressure,and thickness on stiffness are revealed.The 8mm thick wire jamming structure is nearly 7 times stiffer when negative pressure is turned on.(3)In order to understand the stiffness change mechanism of the bionic wire jamming structure,a cantilever beam bending experiment is performed.Combining the transition turning point equation and the theoretical model of bending stiffness proposed in this work,the stiffness change of the jamming structure during the bending process is explained.In order to address the problem of multi-material combinations of soft actuators,a modular design scheme based on the Velcro structure is proposed to realize rapid detachment and assembly of soft actuators and variable stiffness layers.The bending angle and bending stiffness experiments are performed on the assembled variable stiffness soft actuator.The proposed variable stiffness soft actuator is compact,with flexible bending performance and a wide range of tunable stiffness,which is suitable for soft robotic manipulators,multi-legged crawling robots and medical assistive devices.(4)A soft robotic gripper and a set of wearable exoskeletons are fabricated as application examples.Through the gripping experiments of the soft robotic gripper,it is proved that the actuator designed in this work can significantly improve the gripping quality and maintain the flexible active/passive bending capability.Then a dynamic experimental platform is built to confirm that the variable stiffness layer can effectively suppress the soft actuator from shaking in motion and maintain the initial posture through acceleration and deceleration round-trip experiments.After that,the focus is on the gripping force test of the soft gloves in the wearable exoskeleton,and the results show that a single empty glove can grasp objects with a weight of more than 10 kg at high stiffness,which is more than twice the weight grasped at low stiffness.Finally,flexible and stretchable wires based on liquid metal are developed for powering electronic devices or data transmission.