Feature Extraction for Advanced Control of Flexible Forming Processes

Flexible forming processes are characterized by the implementation of shape-independent tools. However, varying properties of semi-finished goods in conjunction with the lack of a forming tool technology also lead to an insufficient reproducibility of the products. In order to overcome this problem a closed-loop control system for the on-line-capable process control was worked out for laser forming of extrusion parts. A preprocessor models the forming process by means of several forming stages. Areas which are relevant for the control loop are determined in the set geometry of the forming stages. They are digitized by means of a 3D sensor during the controlled process. The determination of geometrical deviations is based on the feature extraction in these areas. The use of form features speeds up the geometrical data processing thus allowing an online process control. Furthermore, a manufacture adhering closely to specifications without computing complete surface geometries is possible. 1 Flexible forming with the laser beam As product cycles are becoming ever shorter, new manufacturing and rapid prototyping technologies are required. An example of application in automotive engineering are space frames, which aim at a weight reduction of car bodies on the basis of aluminium extrusion parts [1,2]. In the case of small batch productions and prototypes laser forming can be implemented for the forming of the extrusion parts. Thus extrusions which differ in their geometry can be formed within short developing and manufacturing [3,4]. Additionally, this process chain is considerably more cost-saving for small batch productions as tools no longer have to be manufactured. Laser forming of extrusion parts is based on the introduction of local stresses into the workpiece. The laser beam heats the material locally, causing thermal stresses until the flow limit is reached. The surrounding material prevents a lateral expansion, which finally leads to the plastic forming. By irradiating a wedge-shaped zone, as illustrated in figure 1, the asymmetry of the heating and cooling causes the stresses to remain within the workpiece after cooling [5,6]. This results in a forming of the extrusion part (figure 1). The forming direction is orientated towards the laser beam. Larger bending angles can be achieved by several irradiation steps either at the same or at adjoining positions (figure 1, at the right). In the case of extrusion parts, both the residual stress in the extrusions and the time period between the individual irradiation steps significantly affect the reproducibility of the process [7]. The bending angles vary, even if aluminium extrusion parts of Figure 1: laser forming of profiles