Kinematic characterisation of hexapods for industry

Purpose – The purpose of this paper is to propose two simple tools for the kinematic characterization of hexapods. The paper also aims to share the experience of converting a popular commercial motion base (Stewart-Gough platform, hexapod) to an industrial robot for use in heavy duty aerospace manufacturing processes. Design/methodology/approach – The complete workspace of a hexapod is a six-dimensional entity that is impossible to visualize. Thus, nearly all hexapod manufacturers simply state the extrema of each of the six dimensions, which is very misleading. As a compromise, a special 3D subset of the complete workspace is proposed, an approximation of which can be readily obtained using a computer-aided design (CAD)/computer-aided manufacturing (CAM) software suite, such as computer-aided 3D interactive application (CATIA). While calibration techniques for serial robots are readily available, there is still no generally agreed procedure for calibrating hexapods. The paper proposes a simple calibration method that relies on the use of a laser tracker and requires no programming at all. Instead, the design parameters of the hexapod are directly and individually measured and the few computations involved are performed in a CAD/CAM software such as CATIA. Findings – The conventional octahedral hexapod design has a very limited workspace, though free of singularities. There are important deviations between the actual and the specified kinematic model in a commercial motion base. Practical implications – A commercial motion base can be used as a precision positioning device with its controller retrofitted with state-of-the-art motion control technology with accurate workspace and geometric characteristics. Originality/value – A novel geometric approach for obtaining meaningful measures of the workspace is proposed. A novel, systematic procedure for the calibration of a hexapod is outlined. Finally, experimental results are presented and discussed.