Optimization of a parallel manipulator for use as a guide gear in a sewing unit

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The outstanding properties of parallel structures depend very much on their geometry. An optimal and successful use of these structures is only possible if the geometric parameters are defined in such a way that they meet as far as possible all requirements of the handling task. The volume of the working area, the accuracy and the rigidity of the tool play a very important role here. While serial robots can perform successive optimization steps in which certain groups of parameters are treated, parallel robots cannot perform this procedure. The requirement to maximize a certain criterion, such as the volume of the working space, can result in solutions that have low stiffness due to the geometric relationships between the final effector and the drives. Classical optimization approaches based on the minimization of a target function are only partially suitable for the solution of this optimization problem. The resulting result reflects the weighting of the individual optimization goals and does not represent the optimal solution with regard to the defined criteria. Above all, this procedure does not guarantee that all requirements are met.

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The prototype of a parallel manipulator existing at the Institute of Mechanism Theory, Machine Dynamics and Robotics is used as a guide gear for the handling of a highly dynamic sewing head with a new sewing technology, see figure 1, for which the robot has to be optimized with regard to several criteria. Based on the requirements of the handling task, such as the size of the workpieces to be sewn, stitch speeds, etc., requirements with regard to the working area, rigidity and accuracy of the robot can be derived. The seams to be realized must be in the working area of the robot. In addition, the structure must be stiff enough so that the high-frequency process forces that occur do not cause deviations from the specified course of the seam, which are greater than the permissible limits.

In [1] and [2], a series of requirements for the parallel structure are derived from the handling task and formulated mathematically. Subsequently, a multi-criteria optimization is carried out to ensure compliance with all predefined criteria. In doing so, the volume of the working space (Figure 2), the maximum attainable speeds of the end effector, the accuracy (Figure 3), the rigidity of the structure, the singular points (Figure 4) and the restrictions in the passive joints are taken into account. In addition, restrictions are defined to obtain constructive results.

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[1] Nefzi, M; Riedel, M; Corves, B.: Dimensional Synthesis of a Parallel Manipulator with Five Degrees of Freedom. In: 16th IFToMM International Workshop on Robotics (RAAD 2007). 7-9 June, 2007, Ljubljana, Slovenia. ISBN: 978-961-243-067-2

[2] Nefzi, M; Riedel, M; Corves, B.: Optimal Design of a Parallel Manipulator for Use in a High-Speed Sewing Machine. To appear: In: 13th IEEE IFAC International Conference on Methods and Models in Automation and Robotics. 27-30 August, 2007, Szczecin, Poland



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