INTRODUCTION
Components that are applied in drive trains (e.g., gear wheels, bearings and axles) are subject to increasingly strong requirements, amongst others with regard to load transmission, noise reduction and weight [Goc03]. These requirements can only be met if a high accuracy is preserved concerning the workpiece geometry. In the production process of steel parts the geometry is accomplished in a process chain, which consists of various subsequent subprocesses. These sub-processes typically range from casting and forming to turning, heat-treatment and grinding. Besides defined modifications of the workpiece geometry and/or its material characteristics, each of these subprocesses can negatively affect the state of a workpiece to a greater or lesser extend, eventually contributing to a geometric distortion of the workpiece at the end of the production process. Additional finishing treatments, together with the loss of production, account for a large part of the production costs. In Germany alone, these costs add up to 0,85 billion Euro per year [Hof02]. In order to reduce distortion in the process chain, two basic approaches can be applied.
Conventional methods of operational Quality Management (QM) maintain given quality specifications by monitoring and optimising machine- and process capabilities. The occurring quality deviations are usually managed by a minimisation or homogenisation of disturbance factors. However, when it comes to the production of steel parts, these methods can only be applied to a limited extend [Som04, Erm08]. For one, the difficulty in understanding the mechanisms of distortion prevents that conventional QM can be deployed to a full extend. Secondly, even if the variation of certain parameters can be related to distortion, they often cannot be changed. For example, the measurement of certain material properties (grain structure, residual stress) may indicate distortion potential, however the distribution of these properties cannot be altered. A further
enhancement of the process capability in the production of steel parts requires that the minimisation of disturbances is complemented by a compensation of such influences. This means that the impact of the disturbance factors is measured and ‘counterbalanced’ by accordingly adjusting the actuating variables of a process. In order to enable the compensation of distortion in the standard process chain of steel parts, the Scientific Research Centre (Sonderforschungsbereich, SFB) 570 “Distortion Engineering” at the University of Bremen modified several sub-processes.
This thesis proposes control strategies for an optimal deployment of the newly achieved compensation potential. These include strategies for the (local) control of single sub-processes as well as for a coordinated compensation along the entire process chain. The proposed methods are implemented and verified upon the modified processes of the SFB 570. The presented work has been carried out at the Bremen Institute for Metrology, Automation and Quality Science (BIMAQ), in the framework of the subproject B6 “Control Strategies for the Compensation of Geometric Distortion of Parts in a Process Chain” of the SFB 570. The SFB 570 is funded by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG).
