Research On A Pneumatic Soft Gripper With Jointed Endoskeleton Structure

Modern industrial production lines require an efficient,versatile,and flexible gripper with sufficient grasping force to grasp fragile and delicate objects with irregular shapes and masses safely and reliably.Although various soft grippers have the advantages of flexibility and adaptability to irregular shapes,they still have a general problem of insufficient grasping force.Most of the soft grippers are made of pure soft materials.The property of low rigidity determines the small grasping force,and various unexpected deformations such as radial expansion and irregular deformations will occur.To solve this problem,a novel pneumatic soft gripper with jointed endoskeleton structure is proposed.A series of research on the soft gripper is carried out,including structure design,simulation,mathematical modeling,performance test,position and output force control,and grasping strategy.To overcome the soft gripper's insufficient grasping force,a design idea of a pneumatic soft gripper with jointed endoskeleton is proposed,which structurally separates the functions of actuation and force bearing of the soft gripper.The outer part of the soft finger act as the finger muscles,which uses a soft silicone rubber layer with circumferentially wound fibers.It can generate axial stretching and achieve a soft bending deformation action.A series of rigid units jointed by hinges are implanted inside the finger to act as the endoskeleton.This new structure gives consideration to both rigidity and flexibility,and avoids the inherent defects of the soft muscles that bear both the actuation and the bearing function.It can improve grasping ability while maintaining the flexibility of the soft gripper.Through the research on the structure of the endoskeleton unit and the single finger joint,the three-segment finger's overall structure design is determined.To make the soft gripper have a proper position feedback function,the flexible bending sensor is embedded in the finger.And a three-finger prototype of the soft gripper is successfully fabricated on this basis.The finger bending motion and dynamics model of the novel soft gripper with rigid endoskeleton and soft material are established.Firstly,based on the strain energy function Yeoh model of the hyperelastic body and the theory of continuum mechanics,the theoretical bending model of single finger joint by the extension of the soft structure under the action of air pressure is built.Then the finger is simplified as a series system,and the kinematic model is obtained.Furthermore,the finger joint is equivalent to elastic element,damping element,and bending driving element,and the nonlinear dynamic model of the finger is established with the gas thermodynamics theory.Then,the simulation research is carried out.The simulation data of the finger angle response and other basic characteristics are obtained,which lays a foundation for the subsequent gripper movement control.In order to give full play to the functional characteristics of the novel pneumatic soft gripper with jointed endoskeleton in practical application,two grasping strategies,shape adaptive grasping and grasping force adjustment,which are suitable for pneumatic soft gripper with endoskeleton,are studied.The shape adaptive grasping includes four grasping modes: envelope mode,pinch mode,hook mode,and clamp mode.According to the shape and size of the object to be grasped,the corresponding pre-grasping configuration of each mode is built,and the finger control strategy based on the pre-grasping bending angle of each finger segment is given.The reduced-order sliding mode variable structure controller is used to control the bending angle.The strategy of grasping force adjustment is similar to those occasions with special requirements for grasping force.Therefore,the grasping matrix is determined according to the relevant geometric equations,and the fuzzy PID parameter self-tuning controller is used to control the grasping force.For the particularly refinement grasping force demand,a pressure compensation algorithm based on the real-time detection of contact force is also proposed.And the preload air pressure value of each finger segment is estimated to carefully grasp the fragile and easily damaged objects.Bsed on the above work,the basic characteristics test and grasping experiment of the developed soft gripper were carried out.The basic characteristics of fingers,such as bending angular displacement,open-loop response time of inflation and deflation,fingertip output force and bending stiffness,were obtained,which are consistent with the theoretical analysis.The control strategy of shape adaptability and grasping force adjustment were tested,and the effects are good.The results show that the maximum grasping mass of the gripper is 3.5kg,which is 2.5 times higher than that of the soft gripper reported in the literature.The grasping scale ranges from 20 mm to 260 mm,and the grasping response time of the smallest scale is112 ms.The research results indicate that the novel pneumatic soft gripper with jointed endoskeleton structure prosed in this paper has achieved the expected goal of maintaining flexibility and significantly improving grasping force,which promising in future development and applicationThe research results indicate that the novel pneumatic soft gripper with jointed endoskeleton proposed in this paper is successful and promising in future development and application.