A Systematic Approach for Accuracy Design of Lower-Mobility Parallel Mechanism

Abstract

SUMMARY Geometric accuracy is a critical performance factor for parallel robots, and regardless of error compensation, accuracy design or tolerance allocation is another way to ensure the pose accuracy of a robot at design stage. A general method of both geometric error modeling and accuracy design of lower-mobility parallel mechanisms is presented. First, a general approach for error modeling of lower-mobility parallel mechanism is proposed based on screw theory, and then the geometric errors affecting the compensatable and uncompensatable accuracy of the end-effector are separated using the properties of dual vector space. The pose error aroused by compensatable geometric errors can be compensated via kinematic calibration, while the uncompensatable geometric errors should be minimized during the manufacturing and assembly processes. Based on that, the tolerance allocation method is presented, giving each uncompensatable geometric error a proper tolerance by the use of reliability theory. Compared with the traditional tolerance allocation method, the advantages of the proposed method are as follows: the number of geometric errors to be allocated is greatly reduced; the results of serialized tolerance allocation can be obtained according to different reliability indices of pose accuracy of end-effector for designers to choose; on the premise of guaranteeing the same pose accuracy of end-effector, the allocated tolerances are loose and easy to realize. Finally, the proposed methods are successfully applied to an R(2-RPS&RP)&UPS lower-mobility parallel robot, and the effectiveness and practicability of the proposed method are verified.

Analysis of coupling effects on hydraulic controlled 6 degrees of freedom parallel manipulator

“…2d and 2e and can be explained as above. The period of 0.122hz can be regarded as the kinematic coupling effects which can be assumed negligible, the reasons given by [3], and also due to stiffness, inertia effects and level of manufacturing accuracy. The simulations results as shown below are consistent with the analytical results presented in table 1.…”

Rigid-Flexible Modal Analysis of the Hydraulic 6-DOF Parallel Mechanism

“…In recent years, many studies have focused on the parallel mechanisms due to the advantages such as high payload to weight ratio, high structural rigidity and high dynamic capabilities [1][2][3][4]. With the rapid development of the parallel mechanisms in the direction of heavy load, high speed and light weight, many parallel mechanisms that were previously considered to be rigid have become flexible, and more and more attention is being paid to the dynamic characteristics and the control methods of rigid-flexible parallel mechanisms.…”