Research On Structural Synthesis Of Single-loop Parallel Mechanism Based On Multi-DOF Drive Unit

The traditional parallel mechanism is a multi-loop closed-chain structure constructed by multi limbs connected between the moving platform and the fixed base.Due to the multi limbs and multi-loop closed-chain structure,the parallel mechanism has the disadvantages of complex structure and small workspace,which affects the performance of the parallel mechanism.Simplifying the structure of the parallel mechanism and increasing the workspace becomes the key to improving the performance of the parallel mechanism.The multi-DOF single loop parallel mechanism is composed of two limbs.Its structure is simple,and its workspace is large.It can not only overcome the shortcomings of the traditional parallel mechanism but also retain its advantages.Therefore,the multi-DOF single-loop parallel mechanism can improve the kinematic and dynamic performance of the parallel mechanism.The research of the single-loop parallel mechanism is an important way to promote the application of the parallel mechanism in industrial and engineering fields.Therefore,developing the structure synthesis of multiDOF single-loop parallel mechanism and the designing new parallel mechanism,which play an important role in improving the performance of mechanism and improving the disadvantages of traditional parallel mechanism.Aiming at the problem of unsatisfactory drive of multi-DOF single-loop parallel mechanism,a new design method of the multi-DOF drive unit is proposed.Based on Lie group theory,the motion type of the multi-DOF drive unit is deduced by calculating the displacement subset corresponding to the joints and the kinematical chains.Then,using the principle of differential transmission,the multi-DOF drive unit is designed according to the motion types.Finally,the ideal drive units for the single-loop parallel mechanism,i.e.,2-DOF drive unit and 3-DOF drive unit,are obtained.Aiming at the problems of complex structure and a small workspace of traditional3-DOF parallel mechanism,a topological structure of 3-DOF single-loop parallel mechanism driven by 2-DOF drive unit is proposed.The construction method of this kind of single-loop parallel mechanism is studied.A class of 3-DOF single-loop parallel mechanisms based on a 2-DOF drive unit is synthesized by using the constraint synthesis method based on screw theory and virtual work principle.The 3-DOF single-loop parallel mechanism can be used as a leg mechanism.Using a single-loop parallel mechanism as the leg mechanism,an omnidirectional walking robot is designed.Through the kinematic analysis of the single-loop parallel mechanism,and the gait planning and motion simulation of the walking robot,the advantages of simple kinematics,large workspace of the single-loop parallel mechanism,and flexible motion of the walking robot are shown.Through the integrated design of the drive units of the two limbs of the 3-DOF single-loop parallel mechanism,a topological structure of novel 3-DOF single-loop parallel mechanism based on the 3-DOF drive unit is proposed.Based on the Lie group theory,the type synthesis method of the novel 3-DOF single-loop parallel mechanism is proposed,and a class of novel 3-DOF single-loop parallel mechanism with 3T,2T1 R and2R1T-motions driven by a 3-DOF drive unit is constructed.According to the motion characteristics of the novel 3-DOF single-loop parallel mechanism,it is used as a leg mechanism,and a class of walking robots that can realize high-speed motion and omnidirectional motion is designed.The performance of the novel single-loop parallel mechanism is compared with the classical parallel leg mechanism,and the application advantages of the novel single-loop mechanism as a leg mechanism are proved.Aiming at the problems of the complex structure of multi-limbs 3T1 R parallel mechanism and unsatisfactory drive unit of two limbs 3T1 R parallel mechanism,combined with the structural and kinematic characteristics of the multi-DOF drive unit,the topological structures of two kinds of 3T1 R single-loop parallel mechanism driven by a multi-DOF drive unit are proposed.The output link connection style which can realize full circle rotational motion is introduced,and then based on Lie group theory,the type synthesis method of 3T1 R single-loop parallel mechanism with large angle rotation is proposed,and two classes of 3T1 R single-loop parallel mechanisms with large angle or full circle rotation are synthesized.The large pose workspace performance of the 3T1 R single-loop parallel mechanism is verified by motion planning and task simulation.According to the requirements of the sorting robot and automatic assembly robot in the industrial production line,a CRH-RRC single-loop parallel robot with simple structure and 3T1 R motion is designed.The kinematic and dynamic models of the CRHRRC single-loop parallel mechanism are established,the position,velocity,acceleration,workspace,singularity and dynamic performance of the mechanism are analyzed,and the kinematic and dynamic performance is compared with the typical SCARA serial robot.