In sheet metal forming technology stamping machines are mainly used for an economical production of sheet metal workpieces. Apart from increasing the stroke rates of currently more than 3000 min-1, which can be achieved with modern high-performance stamping machines, the demands on the periphery of the plant are rising as well. In particular, this concerns the material feeding systems used for a reliable feed of the sheet metal. The current technology is based primarily on the roll and gripper feed. Here the sheet metal is clamped between the grippers or rollers with a high contact pressure, which is required for a slip-free operation. To avoid an external damage of the surface or a roll out of the sheet, the clamping forces may not be increased indefinitely. In addition, contamination of the sheet metal or the elements of the feeding system should be excluded in order to avoid a permanent damage of the system and related maintenance costs. This means that the feed rates of previous feeding systems, currently up to 2000 min-1, cannot be further increased, so that the performance potential of modern high-performance presses with large stroke rates cannot be exhausted. Thus the development of feeding systems in sheet metal processing with significantly higher forces is required.As part of a research project at the IFUM, facilitated by the German Machine Tools' Association (VDW), a novel method has been developed in which the sheet metal is fed completely without contact by means of electromagnetic forces. No mechanical elements are required for clamping the sheet metal, so that the inertia of the system can be reduced significantly. Thus higher dynamic properties of the feeder can be realized. The principle is based on the asynchronous linear motor with eddy current runner in a double cam arrangement. This feeder basically consists of two primary components, comprised of a laminated iron package and a three-phase winding. The primaries are symmetrically fixed positioned to compensate the forces of attraction in ferromagnetic materials as well as the repulsive forces in paramagnetic sheet metals such as aluminium or copper. The electrical conductive sheet metal acts as a secondary part and is located in the air gap between the two primary components. Thus the sheet is kept suspended in the air gap a damage to the sheet metal surface is prevented. Therefore surface-finished metal sheets can also be fed with high speed rates. The force initiation is performed entirely contactless to the sheet metal with the three-phase winding in the primaries which induce a sinusoidal magnetic traveling field in the air gap. During operation eddy currents are induced in the metal strip due to the speed of the traveling magnetic field relative to the sheet. By the interaction between the magnetic field and the eddy currents an advancing force is applied to the sheet metal according to the Lorentz law.For the design and optimization of the electromagnetic feeder extensive simulation-based studies have been performed using a parameterized finite element model. For this purpose the development of a three-dimensional model was necessary to represent the eddy currents in the sheet metal. The main subjects of the investigations were in particular the optimization of the iron core, the winding distribution and also to ensure an acceptable temperature in the primaries and the sheet metal during continuous operation. The studies show that, depending on the sheet material applied, very high feed forces can be achieved. For sheet metals with a width of about 100 mm more than 1000 N can be achieved by means of the electromagnetic feeding system. Compared to current mechanical feeders the forces can be more than doubled.To validate the simulation results and test the functional ability a demonstrator of the electromagnetic feeder was designed and manufactured. Due to the simulation-based optimization of the feeding system an external cooling is not required. The control of the feeder is realised via a conventional frequency converter, with which the voltage can be controlled in its amplitude and frequency, and thus indirectly the sheet metal position. The first experimental investigations were carried out on a specially designed force test bench. The results show a very good correlation obtained by simulation and the experimental measured feed forces. Future work objectives are to identify the feed characteristics and limitations as well as the implementation of a robust control algorithm for a reliable positioning of the sheet metal.
Novel analysis methods of dynamic properties for vehicle pantographs