Control of sheet-metal forming processes with piezoactuators in smart structures

The most important project in sheet metal forming is streamlining the material flow since each rejects increases production costs. Using the multipoint cushion device together with an elastic blankholder makes it possible to actively manipulate the material flow in the flange range. This allows major enhancements in the deformation ratio, especially with the novel high strength materials in car body production. State-of-the-art is multiple draw pins to initiate the force on selected points on the blankholder. Admittedly, the cushion plate does not allow optimum force allocation because it is situated between hydraulic pressure rollers and draw pins. Replacing selected draw pins with piezoactuators for generating high forces allows systematic control of the force progression at critical fort-ning areas during sheet draw-in. The system, consisting of the piezostack actuator, dynamometer and components for force initiation, was built as a compact unit with low resilience with the intension of using the inherent sensory properties of the piezostack actuator to measure force. Applying this principle throughout allows a reduction of hydraulic components which eventually lead to a less expensive one-point cushion device. Initial finding have already been arrived at in the context of a research project at the Fraunhofer Institute for Machine Tools and Forming Technology in Chemnitz, Germany in cooperation with a partner from the automobile industry. A draw pin was replaced ad hoc with a highly durable piezoactuator integrated in a force control cycle. The force progression during the sheet draw-in could be accurately adjusted according to a predetermined master curve. The master curve was taken up in the unregulated process and represents the quality criteria of a formed useable part. The real-time MATLAB Simulink XPC-Target simulation tool was used to develop an adjustment strategy that connects the specific signals of the press control (such as the tappet path, the die cushion position and the die cushion force) with the reference force (i.e., the master curve) and the actual force of the piezoactuator.