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A method for improving the accuracy of a parallel manipulator with full-circle rotation is systematically investigated in this work via kinematic analysis, error modeling, sensitivity analysis, and tolerance allocation. First, a kinematic analysis of the mechanism is made using the space vector chain method. Using the results as a basis, an error model is formulated considering the main error sources. Position and orientation error-mapping models are established by mathematical transformation of the parallelogram structure characteristics. Second, a sensitivity analysis is performed on the geometric error sources. A global sensitivity evaluation index is proposed to evaluate the contribution of the geometric errors to the accuracy of the end-effector. The analysis results provide a theoretical basis for the allocation of tolerances to the parts of the mechanical design. Finally, based on the results of the sensitivity analysis, the design of the tolerances can be solved as a nonlinearly constrained optimization problem. A genetic algorithm is applied to carry out the allocation of the manufacturing tolerances of the parts. Accordingly, the tolerance ranges for nine kinds of geometrical error sources are obtained. The achievements made in this work can also be applied to other similar parallel mechanisms with full-circle rotation to improve error modeling and design accuracy.

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Purpose The purpose of this paper is to develop a means of the kinematic calibration of a parallel manipulator with full-circle rotation. Design/methodology/approach An error-mapping model based on the space vector chain is formulated and parameter identification is proposed based on double ball-bar (DBB) measurements. The measurement trajectory is determined by the motion characteristics of this mechanism and whether the error sources can be identified. Error compensation is proposed by modifying the inputs, and a two-step kinematic calibration method is implemented. Findings The simulation and experiment results show that this kinematic calibration method is effective. The DBB length errors and the position errors in the end-effector of the parallel manipulator with full-circle rotation are greatly reduced after error compensation. Originality/value By establishing the mapping relationship between measured error data and geometric error sources, the error parameters of this mechanism are identified; thus, the pose errors are unnecessary to be measured directly. The effectiveness of the kinematic calibration method is verified by computer simulation and experiment. This proposed calibration method can help the novel parallel manipulator with full-circle rotation and other similar parallel mechanisms to improve their accuracy.

A five-axis computer numerical control hybrid machine tool composed mainly of a three-RPS parallel spindle head with one translation and two rotations is investigated; because of its unique machining characteristics, this machine tool has been applied to high-efficiency aerospace monolithic component processing, and its trajectory control is one of the core technologies underpinning its efficacy. This research investigates a new type of five-axis hybrid machine tool which is loaded with a three-RPS parallel spindle head and finds a cutter orientation interpolation algorithm which can ensure its high-precision, and high-speed, operation in machining processes. First, the kinematic model of the hybrid machine tool is established using the vector chain method. Using this model, the virtual reality mapping relationship between the control axis space and the operating space is established. Then a control algorithm governing two interpolation strategies in the operation and joint spaces is proposed; this method disperses the cutter vector in the workspace. Through the mapping relationship, the point data array in the joint space can be obtained from the machining data which are calculated from the mapping model, and then the improved three B-spline interpolation method is used in the joint space. Thereafter, the post-machining module outputs the interpolation data to the motor, and the moving platform can realise the desired tool path. Finally, using the hybrid machine tool for the machining of an 'S' test-piece, the results prove the feasibility and effectiveness of the interpolation algorithm. The experiment indicates that the interpolation algorithm can be used in this kind of parallel machine tool control system.

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