Research On The Mechanical Properties And Design Methods Of The Flexible Joint Driven By Dielectric Elastomer

Compared with the rigid robots,the mechanical body of the soft robots is made of elastic soft materials.When it collides with the outside world,it can absorb some collision energy,effectively avoiding or reducing the damage to the outside world and itself.Therefore,the soft robots have greater advantages in the applications of human-computer interation.In addition,the soft robots can achieve continuous deformation and have better adaptability in difficult environments.Therefore soft robots have become a research hotspot in recent years.In this paper,based on the flexible driving technology required by soft robots,the acrylic dielectric elastomer film with high energy density and fast response speed is selected as the driving material to study the static,dynamic and bistable characteristics of the minimum energy structure joint with one degree of freedom.Based on these mechanical properties,a quantitative design method of the joint is proposed,and two types of soft robots were designed according to the design method.The details are as follows.In the analysis of static characteristics,since the dielectric elastomer film is a viscoelastic and hyperelastic material,it has a complex nonlinear constitutive relationship,and the film forms a saddle morphology when the flexible joint is bending,so it is difficult to establishes an accurate analytical model of statics and dynamics base on Newtonian mechanics.In this paper,based on the thermodynamic theory framework and the hyperelastic polymer mechanics theory,the finite element static model of the flexible joint is established by using the Yeoh strain energy density model and the ABAQUS software.The model was used to study the influence of the hollow shape of the flexible frame and the thickness of the film on the static performance of the flexible joint.In addition,the effects of different pre-stretching ratios on the static performance of the joint were also studied through experiments.About the dynamic characteristics,a macroscopic dynamic model of flexible joints considering system damping and air resistance is established according to Newtonian mechanics.Due to the difficulty of using the mathematical analytical to express the film torque,it is proposed to determine the torque of the film by experimental equivalent calibration and fitting.As the creep phenomenon of joint angle caused by viscoelasticity of film,the correction function is used to correct the dynamic torque of film obtained by the quasi-static test.According to the damped oscillation method,the powered on and powered off damping coefficients were obtained by the testing vibration curves.The solution process of the dynamic model was given through an example of a joint.In order to fully verify the correctness of the dynamic model,four variables with different size,pre-stretching ratio,voltage amplitude and voltage frequency were selected to complete eight groups dynamic model verification experiments.We found there are two steady states of the flexible joint in the static equilibrium state,which is called bistable characteristics.The reason of the bistable characteristics is analyzed through the change of the torque direction of the film when the joint is bending.A symmetric bistable structure that can achieve the same joint deformation angle at two steady states was proposed,that is stiffening frame-film-flexible frame-film-stiffening frame.Based on the free vibration curve when the joint is powered on,the trigger time between the two steady states was studied.In addition,using the bistable characteristic,the method of increasing the dynamic deformation amplitude of the joint by adjusting the square wave frequency and duty ratio was proposed and proved by experiments.Based on the static,dynamic and bistable characteristics of flexible joints,an evaluation index system for flexible joints is established,including static equilibrium deformation angle,dynamic deformation amplitude,maximum output torque,natural frequency,average power and average power density.The detailed meanings of each evaluation index are given.By analyzing the relationship among these evaluation indexes,it is found that the static equilibrium deformation angle and the maximum output torque are the two basic indexes of the joint.From the viewpoint of the elastic energy work of the dielectric elastomer film,it is inferred that the torque of the film to the flexible frame is proportional to its effective volume for any deformation angle of the joint,pre-stretching ratio of the film and shape of hollow area.The proportional relation was proved and verified through theory and experiments.Based on the relation,a flexible joint design method for static equilibrium deformation angle and maximum output torque is proposed.The detailed design process is presented,and the usability of the design method is verified by an example.Taking the flapping wing robot and the breaststroke robot as examples,we further verified the practical value of the flexible joint design method established in this paper.According to the action postures of the bird's wings in a flight cycle,a flapping wing robot composed of two fold-unfold joints and a flapping joint were designed.Through the demand analysis of the driving performance of each joint,the static equilibrium deformation angle and the maximum output torque design indexes of each joint were proposed.According to the joint design method established in this paper,the joint design was completed.The design indexes were verified by the finite element static model and experiment.The postures of the joints agreed with the bird's wing postures through the dynamic experiment.Similarly,the structure of the breaststroke robot was designed according to the action posture of the legs and feet in a swimming cycle during the breaststroke.The structure of the robot consists of two flexible joints for simulating human legs and two self-adaptive variable stiffness structures for simulating human feet.Through the demand analysis of the driving performance of the joint,the static equilibrium deformation angle and the maximum output torque design indexes of the joint were proposed.The joint was designed by using the design method established in this paper.The design indexes were verified by the finite element static model and experiment.