Research On A Space Series-parallel Capturing Robotic Gripper Based On 3RRlS Metamorphic Unit

To meet the manipulation requirements of complex space missions,e.g.,space junk cleaning,satellite orbital transfer,on-orbit servicing of failure equipment,it is increasingly vital to study aerospace mechanisms with the capability of tracking and capturing non-cooperative targets in the complex space environment.Therefore,it is an urgent need to study a new type of space large-scale robotic gripper capturing mechanism with the advantages of large operating range,flexible mobility and high structural stiffness good adaptability for various targets.The 3RRlS metamorphic parallel mechanism(MPM)unit of space large-scale capturing robotic gripper(SLSCRG)for multitasking requirements is studied,based on the lockable spherical(l S)joint that can function as a Hooke(l S₁)or spherical(l S₂)joint.3RRlS MPM has four configurations(3RRlS₁,3RRlS₂,2RRl S₁-RRl S₂,and 2RRl S₂-RRl S₁),of which the DOF,overconstraint,singularity and parasitic motion are analyzed using screw theory.The results indicate that the mechanism motion screws can qualitatively represent the mechanism parasitic motions.Moreover,the position equation of 3RRlS MPM based on its geometric conditions is derived.The prototypes of l S metamorphic joint and 3RRlS MPM are conducted and experiments on the unlocking and locking of l S metamorphic joint and the metamorphic function of 3RRlS MPM are implemented.A novel unit-two-actuator n(3RRlS)metamorphic serial-parallel manipulator(MSPM)with multiple working conditions is constructed.First,.a limb combination coding method is proposed to describe the topological graph and determine the isomorphism of the n(3RRlS)MSPM.The isomorphism of all actuation distribution configurations(ADCs)and controllable metamorphic configuration(CMCs)of the3RRlS MPM,2(3RRlS)MSPM and 3(3RRlS)MSPM is determined.Secondly,a workspace transformation method with the advantages of simple operation and high efficiency is presented to generate the workspace of all CMCs of all ADCs,and the best actuation configuration scheme is optimized.Finally,the prototype of a three-knuckle robotic finger is conducted,and the position accuracy of the robotic finger is measured.The generalized Jacobian matrix,generalized Hessian matrix an d kinematic equations of the 3RRlS RPM and n(3RRlS)MSPM are established by recursive and inductive methods.Subsequently,the dynamic equations of the 3RRlS RPM and3(3RRlS)MSPM are successively derived using the principle of virtual work.A method for establishing the stiffness model of n(3RRlS)MSPM is proposed by combining the virtual joint method and matrix structural analysis.The theoretical and the finite-element-analysis deformations of the 3RRlS RPM and 3(3RRlS)MSPM are solved.By comparing the stiffness of 3(3RRlS)MSPM in different configurations,the influence of l S metamorphic joints on the stiffness of 3(3RRlS)MSPM is analyzed.Finally,the stiffness of the robotic finger in different configurations is measured,the results show that metamorphic joints can improve the stiffness in the directions of constrained DOFs.To stably capture non-cooperative targets in space,the capturing workspace and capturing statics of the 3(3RRlS)MSPM are analyzed and verified by Sim Mechanics simulation.Subsequently,the contact force is decomposed into two display expressions about joint torque and external force,and the judgment method of force closure considering passive force is obtained.Using a linear polyhedral cone instead of a nonlinear friction cone,the mathematical model of the capturing force optimization considering passive force is established.The force closure of two-finger and three-finger robotic grippers with fingertip capturing and constrained capturing is judged,and the capturing force is calculated and optimized.The results show that three-finger robotic gripper with constrained capturing achieves force-closed capture and has the best capture performance.Finally,the contact forces of the captured object are measured,which verifies the capturing force closure and the theoretical optimization results.In conclusion,the n(3RRlS)MSPM can realize their respective functions of the SLSCRG under three working conditions with the minimum number of actuators,which can reduce the weight of the SLSCRG and can gain benefits in multitasking adaptability,simplified control,precision and rigidity improvement.