Precision Requirements of Hexapod-Machines and Investigation Results

Abstract

Since the occurence of Parallel Kinematic Machines (PKMs) in 1994 their usage for different applications has increased. They can be applied for robot technology, for instance for the assembly, welding and measuring, as well as HSC applications for milling or for laser machining technology. There are different factors which influence the precision of these machines e.g. mechanical tolerances, kinematic errors in the actuators, elastic and thermal deformations. Due to these effects the mathematical model differs from the real system. For an error compensation it this necessary to know the behaviour of the single component parts. Components as joints and struts used in PKMs influence the static, dynamic and thermal behaviour of these machine tools. Some results of investigations concerning these influences are documented in this paper. Digests characterise some different joints and give a qualitative rating of the main parameters. Measurements were carried out and show in diagrams the stiffness of existing joints. A classification of struts points out their actuators and therewith the reachable precision. Investigations of the thermal behaviour of struts at a tripod prove the possible influence of temperature changes on the accuracy of PKMs with ball screw driven struts. To get an impression of the dynamic behaviour modal analysis is a convenient tool. Results of an exemplarily executed modal analysis show the mode shapes of the investigated PKM.