| In today’s intelligent era,more and more robots are used to help people complete some redundantg and dangerous work.As the end executive device in the overall structure of the robot,the flexible hand claw has a wide application prospect in grasping some fragile and vulnerable objects because it is made of soft materials,has the advantages of good flexibility and strong adaptive ability.Compared with electric drive and intelligent material drive,poeumatic drive has the characteristics of light weight,fast response speed,smooth movement and no pollution.In the study of pneumatic flexible gripper,many scholars are committed to improving its gripping ability and rigid and flexible structure.However,there are few simulation analyses on the contact dynamics of flexible gripper grasping objects,especially the research on multi-contact area retrieval and contact characteristics considering friction in the process of large deformation contact.This paper takes a kind of pneumatic flexible gripper sa the research object,analyzing the contact dynamic characteristics between each flexible finger and the object in the grasping process.The main contents are as follows:Firstly,the dynamic modeling and simulation analysis of the designed flexible finger were carried out by using the low-order three-dimensional two-node full-parameter beam element based on the Absolute Node Coordinate Formulation.The expressions of mass matrix,elastic force and generalized external force of flexible fingers under Neo-Hookean super elastic constitutive model are derived.According to the structural characteristics of the finger,the integral expression of the pressure on the inner surface of the flexible finger cavity was derived.The equivalent low-order full-parameter beam element model of a single pneumatic flexible finger was established,and the bending angle of the finger under different pressure was obtained.By comparing with the finite element simulation results and experimental data,the correctness of the established model is verified.Secondly,based on the search algorithm of the contact area between flexible beam-beams without friction,the non-connected multi-area search algorithm of the contact area between flexible beam-beams considering friction is proposed,and the expressions of the normal contact force and tangential friction on the contact surface are derived.By using the masterslave surface contact algorithm,Newton iteration method,orthogonal projection method and minimum distance criterion method,the surface-surface contact area between the flexible beam and beam is searched,and the non-connected multi-contact area under the orthogonal and oblique state of the flexible beam and beam is obtained.Using penalty function method,implicit Euler method and classical return mapping algorithm,the normal contact force and friction force in the face-surface contact region are solved.The equivalent plane-camber oblique contact characteristics when the pneumatic flexible fingers grasp objects are simulated,and the results of the example verify the correctness of the above method.Finally,the contact dynamics model of a flexible pneumatic gripper is established,and the contact characteristics of the flexible gripper in the process of grasping objects are simulated and analyzed.The dynamic equation of ANCF beam element was assembled,and the contact dynamic equation in the grasping process of the designed flexible pneumatic gripper was derived and solved.A numerical example of a flexible pneumatic gripper grasping a cylinder under the condition of ramp loading pressure is designed.The contact state,contact area and the change of friction force between the flexible fingers and the cylinder are analyzed.The theoretical foundation is laid for the characterization of the actual contact area and the friction force in the contact area during the process of grasping objects in the engineering example,which makes the theoretical pneumatic flexible gripper forward to the engineering application. |
