Performance Analysis And Application Of Parallel Manipulators With PP Joints

The considered laser facility is the most advanced one with the maximum energy and highest power in China,whose main function is for the research about inertial confinement fusion.A lot of optical mechanical modules are distributed through the laser path in the laser facility,and these modules are very clean,heavy and expensive.Moreover,their installation is a precision assembly process with blind source and long stroke.For this reason,a special equipment is needed to install and uninstall these modules.Aiming at this requirement,a precision and compliant pose adjustment system is developed based on the investigation of the facility's characteristics.The core component of this system is a 6-DOF hybrid mechanism,which is made of a 3-(?)PR planar parallel manipulator(PM)and a 3-P(?)S spatial PM.The 3-(?)PR PM used there can adjust the planar pose of the modules,while the 3-P(?)S PM can adjust the horizontal pose.For the 3-(?)PR PM,there are two typical configurations,namely the ?-shape one and the U-shape one.The main difference between the two configurations is the shape of the base platform,which will affect the mechanisms' performance.The kinematic models are established through vector method,upon which the inverse position and singularity are analyzed.Based on the performance evaluation of dexterity,velocity and stiffness,the deciding factors which will affect the performance distribution are found,the optimum regions are also obtained.The results show that ?-shape one has better dexterity and stiffness performance,while the U-shape one has advantages on velocity and workspace performance.In order to improve the actuation performance of the considered mechanism,a 4-(?)PR redundant PM is proposed by adding a (?)PR limb to the U-shape 3-(?)PR PM.The DOF,inverse position and singularity are analyzed,and the optimum region is achieved based on the performance evaluation.The dynamic model is established through Lagrange formulation,and a case study for the analysis of dynamic behavior is conducted.Performance comparison between the redundant one and another two non-redundant PMs is presented,while the simulation results reveal that redundant actuation can improve the mechanism's dexterity,stiffness and actuation performance.The 3-PPS PM also possesses two typical configurations,namely the 3-(?)PS one and the 3-P(?)S one.The inverse position,singularity,velocity and performance evaluation are analyzed.However,the results show that there is no difference between the two PMs on the considered performance indices.Moreover,a new performance index named dynamic manipulability(DM)is introduced and analyzed.The results reveal that the 3-(?)PS one owns greater DM performance near the zones far away from singularities,while their DM performance are very similar under load.Aiming to find a 3-(?)PR PM with greater working performance,a parametric model of general 3-(?)PR PM is presented.The kinematic model is established with shape parameters and motion variables.With the model,the inverse kinematic analysis is carried out,upon which the global dexterity performance and transmission quality are evaluated and analyzed,and some corresponding optimal configurations are obtained.The analysis also reveals a new singularity named ‘shape singularity',which is a type of architecture singularity associated with special shape designs of base and mobile platforms that lead to singularity spanning in the whole workspace.The definition of shape singularity is provided,and its geometry and algebra properties are analyzed,the approaches to solve shape singularity are also presented.To extend this concept from planar PMs to spatial PMs,a parametric model of general 3-(?)PS spatial PM is established,upon which shape singularity is verified.Usually,singularity will exert negative effect on mechanisms' working performance,which should be avoided in design.However,shape singularity can be taken into practical application for its special properties.The application of shape singularity in adjustable compliance mechanism design is illustrated,the mechanism can switch from high stiffness mode to high compliance mode,so it is able to complete the task of precision positioning and compliant assembly respectively.In light of this method,optimum design of the 6-DOF hybrid mechanism is presented.This case provides a potential application for shape singularity,which is also a new clue for rethinking common singularity.