Actual Kinematics Performance Analysis For Lower DOF Parallel Manipulators

The symmetrical lower-mobility parallel mechanism,which characterizes identical limbs,symmetrical arrangement and isotropy,has great potentials in practical application.Particularly the symmetrical lower-mobility parallel mechanism,which characterizes identical limbs,symmetrical arrangement and isotropy,has great potentials in practical application.However,the parallel manipulator is sensitive to assembling and manufacturing due to characteristic of lower DOF parallel manipulator.The problem of constraint error of lower-mobility parallel mechanisms has not been well solved over many years.A universal and effective theory of error has not been established and consequently there is a difficulty to know the actual kinematic performance for lower DOF parallel manipulator.This paper focuses on constraint error of symmetrical lower-mobility parallel mechanisms and aims to establish a universal and effective theory and to analyze actual movement for lower DOF parallel manipulators.The main contributions are as follows:The concept and geometrical condition of common constraint is obtained using screw theory.Then the criterions of common constraint is proposed.A mobility criterion based on constraint analysis is proposed.The sublimb for the symmetrical lower-mobility parallel mechanism which characterizes identical limbs is analysed.Given the output maxtric and parasitic motion of parallel manipulator.Given a system of wrenches of constraint,the corresponding reciprocal basis screws are determined.Then,the joint screws of a limb are obtained by a linear combination of these basis screws.The condition which the parallel manipulator can moves according to anticipated aim is offered.From perspective of structure synthesis,certain special geometric constraints, such asjoint axes intersecting at one point or perpendicular to each other,are necessary in realizing the end-effector motion of kinematically parallel manipulators along individual motion axes.Those errors that violate the geometric constraint requirements are termed "constraint errors".The constraint errors usually are more troublesome than other manipulator errors because the motion characteristics of the manipulator may no longer exist and the kinematic models will be rendered useless due to these constraint errors.Therefore,identification and prevention of these constraint errors in initial design and manufacturing stage are of great significance.In this article,three basic types of constraint errors are identified,and an approach to evaluate the effects of constraint errors on characteristics of parallel manipulators is proposed.The parasitic motion and Jacobian matrix of lower DOF parallel manipulator is offered in the paper.A total theory for constraint error of lower DOF parallel manipulator is developed according to the principles discussed.This paper presents a method based on screw theory for the analysis of positioning accuracy in parallel manipulators with joint clearances.A general method is introduced, and a new analytical procedure is formulated which allows to determine analytically a sub-optimal estimation of the worst case condition for positioning accuracy.Moreover this procedure can determine exactly the worst-case angular accuracy in translating fully-parallel manipulators under the influence of joint clearances.The paper proposes a unified analytical methodology to identify the principal screws sysrems.The accuracy synthesis model has been formulated by means of the D-H theory.Based on the principle that primary errors exert the same effects on the driven and the concept—influence factors of the pose errors,two methods for pose error synthesis of parallel manipulators have been presented.