Implementation and demonstration of characteristic diagram as well as structure model based correction of thermo-elastic tool center point displacements

It is a well-known problem of milling machines, that waste heat from motors, friction effects on guides and most importantly the milling process itself greatly affect positioning accuracy and thus production quality. Therefore, milling machines commonly use active cooling and lead time to reach thermal stability. A cheaper and more energy-efficient approach is to gather sensor data from the machine tool and the process and to use that information to predict and correct the resulting tool center point displacement. Two such approaches are the characteristic diagram based and the structure model based correction algorithms. The former algorithms use a selection of temperature sensors at critical locations on the machine tool to directly predict the tool center point displacement with characteristic diagrams. The latter algorithms use thermal and thermo-elastic models to obtain correction values through simulations in thermal real time. This paper will give a brief introduction into both correction principles followed by recent research progress made in the Collaborative Research Centers Transregio 96 project on thermo-energetic design of machine tools. Finally, the presented methods will be demonstrated in application and their effectiveness evaluated on a hexapod kinematics. Though this R&D demonstration platform is no typical milling machine, it serves well to illustrate the function principle and easy online-applicability of both methods.