Study On A Modular Symmetric 3-R(SRS)RP Multi-loop Manipulator

With the successive launch and implementation of major aerospace projects such as manned spacecraft,space stations,moon and Mars exploration,earth observation,and space scientific research in China,there is an urgently increasing demand for aerospace mechanisms achieving missions of large scale and on-orbit manipulation in space.In the future space missions,the aerospace mechanisms should have the ability to carry out large-scale manipulations,such as space garbage recycling,the orbit correction and maintenance of out-of-control satellites,and the implementation of space offensive and defense.However,the current research and technology on space manipulations turns out to be problematic.For example,the rigid robotic arms lacks enough mobility;the variable geometry truss plays complex structure and actuation;The serial-parallel manipulators show low overall rigidity.Therefore,there is an urgent need to study a large-scale,multiple mobility,high-rigidity,and deformable space truss manipulator system.In this dissertation,multi-loop mechanisms with coupled kinematic chains are employed for the research.Inspired by the biological and cell theory,a kinematic pair,a kinematic subchain,and a kinematic chain are seen as cells,as well as,are described by the screw algebra.Then a cell-division method,respectively,for mobility and singularity analyses of the multi-loop mechanisms with coupled kinematic chains is proposed,in light of the celldivision process in cell theory.Subsequently,the cell-division mobility analysis method is used to analyze the mobility of three representative multi-loop mechanisms with coupled kinematics chains,to verify the correctness and universality of the method.A symmetric 3-R(SRS)RP multi-loop mechanism with larger ratio of rigidity and mass,compared to the parallel mechanisms,is represented,where R denotes revolute joint,S spherical joint,and P prismatic joint.Multiple modules of the same are connected in series to construct a modular multi-loop mechanism manipulator,which is capacity of higher rigid than the articulated robotic arms,being suitable for constructing large-scale arms with a foldable function.A rope-bar structure,additionally,is employed to enhance the rigidity.Besides,a novel spherical joint mechanism is proposed,then being employed to construct an SRS compound hinge composed of two concentric spherical joints and a revolute joint with its axis passing through the center.With this design,an ideal node of the structure is to be realized,while enhancing the motion accuracy and simplifying kinematics and dynamics models.The geometric,mobility,and singularity properties are analyzed.It can be seen that:(1)the mechanism has three degrees of freedom(dof),which are the rotational dof around two intersecting axes in the midplane and a translational dof along the normal of the same plane;(2)the upper and lower units undergoes symmetric motions respect with the midplane under any postures;(3)the singular postures are avoided due to the prismatic joints.Furthermore,the kinematics properties of the mechanism is analyzed and shows that:(1)The mechanism can theoretically achieve a completely folding posture and a maximum rotation of 180°;(2)The motions in the workspace are always continuous.Through the kinematics and workspace analysis of the modular multiloop manipulator,it can be known that the arm has excellent folding and bending motion performance;the workspace of the three-modular manipulator is approximately a ball thereby the arm achieving a good operating range.The 3-R(SRS)RP multi-loop mechanism has multiple single-dof joints(revolute and prismatic joints),while three actuators are needed.It has 84 possibilities of actuation modes,mainly four type of 3R,2R1 P,1R 2P,and 3P.This work then establishes an actuation-mode optimization method,including generalized driving force uniform distribution criterion,power consumption uniform distribution criterion and actuation strategy criterion.The actuation modes of the mechanism of interest are analyzed.It is found that the 3R actuation mode is the optimum choice in the power-distribution criterion,but from the perspective of the actuation-strategy criterion,the 3P actuation mode is the optimal solution.Then,a optimal method for the manipulator is represented;the actuation mode of a three-modular system is determined.That is,the module closed to the base platform uses the 3R actuation mode,while the other two modules apply 3P.In addition,the dynamics model of the three-modular manipulator is established and then used to calculate the generalized driving force array for the design of the prototype and motor selection.A three-modular manipulator prototype experimental test system is build for a test verification.Then,the motion properties,kinematics characteristics,power consumption,and generalized actuation forces tests are conducted;respectively verify the correctness of the configuration properties,kinematic characteristics,actuation mode optimization,and dynamics analyses of the multi-loop mechanical system in this work.This proves that,therefore,the modular 3-R(SRS)RP multi-loop mechanism manipulator has capacity of large-scale,high rigidity,enough mobility,and deformability,thereby having potentials for space applications.In conclusion,this work contributes ideas to the theory and method of designing largescale intelligent structures in our country,as well as,provides theoretical and technical support for the capture of non-cooperative targets and large-scale flexible manipulation in aerospace.