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Influence of kinematic redundancy on the singularity-free workspace of parallel kinematic machines
Jens KOTLARSKI, Bodo HEIMANN, Tobias ORTMAIER
Front. Mech. Eng.. 2012, 7 (2): 120-134.
https://doi.org/10.1007/s11465-012-0321-8
In this paper the effect of kinematic redundancy in order to reduce the singularity loci of the direct kinematics and to increase the operational, i.e., singularity-free, workspace is demonstrated. The proposed approach consists of additional prismatic actuators allowing one or more base joints to move linearly. As a result, a selective reconfiguration can be performed in order to avoid singular configurations. Exemplarily, kinematically redundant schemes of four structures, the 3RRR, the 3RPR, the 6UPS, and the 6RUS, are considered. The relationship between the redundancy and the operational workspace is studied and several analysis examples demonstrate the effectiveness of the proposed concept. Furthermore, the additional benefit of an increasing number of redundant actuators is discussed.
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Modular design of typical rigid links in parallel kinematic machines: Classification and topology optimization
Xinjun LIU, Xiang CHEN, Zhidong LI
Front Mech Eng. 2012, 7 (2): 199-209.
https://doi.org/10.1007/s11465-012-0315-6
Due to the demand of reconfigurable system in parallel kinematic machines (PKMs), modular design technology is significant and necessary. However, in earlier research, the core joint modules have been concerned about rather than the customized link modules. The modular design to the typical customized links from the point of seeking optimal structures with best mechanical performances is analyzed and processed in two steps: classification and optimization. Firstly, a brief introduction to the current research status and the aims of this paper are outlined. And then, how the typical customized links classified is proposed. Next, the technology method and the iterative formula derivation process of topology optimization are described in detail. Finally, calculation models for each group of classified ones are set up and their optimal structures are achieved through topology optimization technique. The results provide useful references for reconfigurable and modular design in engineering cases.
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Parasitic rotation evaluation and avoidance of 3-UPU parallel mechanism
Haibo QU, Yuefa FANG, Sheng GUO
Front Mech Eng. 2012, 7 (2): 210-218.
https://doi.org/10.1007/s11465-012-0317-4
Based on the prototype of 3-UPU (universal-prismatic-universal joint) parallel mechanism proposed by Tsai [1], the parasitic rotation evaluation is performed and calculated the bound of instability of SNU Seoul National University 3-UPU parallel mechanism. Through analysis of the terminal constraint system of the 3-UPU parallel mechanism, the equation about the parasitic rotation and limited clearance is presented. Then the norm of possible parasitic rotation is employed to evaluate the mechanism stability with limited clearance. The higher this number the worst is the pose, the lower it is the best it is. And the contour atlas of parasitic rotation is obtained, which can be used for further analysis and design. With the practice experiment result of SUN 3-UPU parallel mechanism, we find it’s bound of instability, which indicates there will appear the parasitic rotation when the number exceeds the bound. Finally, the method for avoidance of possible parasitic motions is presented by adding redundantly actuated limbs.
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