Research On Design Methods Of Flexible Grasping Devices With Variable Stiffness

A robot-gripper is an operating device installed at the end of the robot for grasping and clamping tasks.,which is widely used in material sorting,food packaging,workpiece assembling and other fields.Its grasping objects include objects of different sizes and weights,as well as other objects of different shapes,soft or hard material properties.However,for gripping these different objects,the traditional rigid grippers can only adapt to hard objects of different sizes through open-and-close motion of the gripper,but can not adapt to soft and fragile objects of complex geometry.Therefore,in order to widen new application field of the gripper,it is urgent to develop a novel variable stiffness gripper to achieve flexible grasping and clamping of objects of different shapes and materials.A novel double-clamping-claw gripper with variable stiffness is proposed in this paper.Each clamping-claw of the gripper is a Variable Stiffness Flexible Parallel Mechanism(VSFPM),which consists of four flexible limbs with symmetrical rotation.The gripper has two driving motors.One motor is employed to control the open-and-close motion of the gripper to adapt to the geometric size of the gripping objects,while the other is employed to control the rotation angle of the flexible limbs to change the stiffness of the clamping-claw in open-and-close direction,so as to realize flexible grasping with variable stiffness and improve the adaptability of the clamping-claw to the geometric shape and material properties of the gripping objects.This paper studies the configuration design of VSFPM,working principle analysis of stiffness variation,static modeling,performance analysis,optimization design,simulation experiment verification and prototype design of the whole gripper system.Firstly,according to the configuration design requirements of the new variable stiffness gripper,the overall design idea is determined.And based on stiffness projection method,the working principle of the VSFPM is analyzed.Thus,a novel variable stiffness gripper based on flexible parallel mechanism with symmetric rotary flexible limbs is proposed,and it is verified that the configuration design method of VSFPM is effective by ANSYS software.Secondly,based on adjoint transformation,the static model of VSFPM is established,so that the stiffness of the VSFPM for an arbitrary rotation angle of the rotary limb is obtained.The accuracy of the stiffness model is verified through Finite Element Analysis(FEA).In addition,the FEA results show that the proposed VSFPM has a wide stiffness changing range(0.178N/mm-9.663N/mm).Thirdly,through ANSYS Workbench software,the key performance indicators of VSFPM are analyzed,including fatigue life,parallel clamping performance and modal.The FEA results show that it has a longer fatigue life,better parallel clamping performance,and achieving parallel-clamping function.In addition,the modal analysis of the mechanism is carried out.These performance analysis results lay a foundation for later structure optimization design and stiffness control of VSFPM.Then,according to the mirror symmetry structure characteristics and specific application requirements,the design parameters 、 performance indicators and constraint conditions of VSFPM are determined.One single-objective optimization mathematical model with compound weighting factors is established.And a set of optimized parameters results of VSFPM sizes are obtained by choosing the appropriate optimization algorithm.Finally,according to the optimized parameters results of VSFPM sizes,the structure design of the new variable stiffness gripper is carried out.According to the requirements of grasping and clamping operation,the motor driver and controller are reasonably selected and two control schemes are proposed to realize the system integration of the flexible grasping device with variable stiffness.Through a series of experiments in grasping objects,it is fully demonstrated that the variable stiffness gripper prototype has excellent performance in grasping objects and stiffness changing.This paper provides a new approach for the design and analysis of flexible gripper with variable stiffness,and lays a theoretical foundation for the motion and stiffness control of the subsequent variable stiffness gripper.