Determination of accuracy contour and optimization of workpiece positioning for robot milling

Abstract

Workpiece positioning into a machine's workspace has become a simple task. Advanced CNC machines are equipped with standardized clamping systems, allowed workpiece dimensions are listed in the machine's documentation and tolerance levels of the end produced parts are known. This gives users plenty of information and good confidence that they are choosing the best machine for a specific task. For more universal machines like industrial robots this is not the case. Due to their flexibility industrial robots can be an alternative to specialized CNC machines, but when a specific task should be executed, important information is missing. For a standard industrial robot the mechanisms layout, its dimensions and its reachable workspace is known, but accuracy levels over the robot's workspace are not. If a workpiece should be milled within certain accuracy limits the robot's documentation offers no information on how close it can be located to the borders of the robot's workspace. This article deals with the mentioned problem with a novel methodology. Based on experimental data we found that a standard 6 DOF industrial robot's reachable workspace can be divided into two regions, one with suitable milling accuracy and another with rapidly decreasing milling accuracy. To isolate the suitable accuracy region a regional non-dominated sorting algorithm was developed and an accuracy contour separating the regions was extracted. In the second part of the article a genetic search algorithm based on regional non-dominated sorting was applied to find the biggest arbitrary shaped workpiece's size, position and orientation in the suitable milling accuracy region of the robot's workspace.