No clamping, no rolling out, maximum speed with low operating costs: the advantages of the so-called Flexfeed process can be summarized in just a few words. The company behind this invention is Tecodrive, a successful spin-off from the Institute of Forming Technology and Forming Machines at Leibnitz University Hannover. The potential behind Flexfeed becomes clear if you take a look at the sheet metal processing operations of the Arnsberg-based company Trilux, for example: the lighting experts punch reflector sheets from thin aluminum for high-quality lighting systems. The punching machine used for this purpose previously worked with a classic roller feed with 200 strokes per minute. However, the fact that a higher cycle rate was not possible was not due to the punch itself - it could easily have achieved double that. "Rather, it was the roller feed that limited productivity," explains Dr. Olaf Marthiens, Managing Partner of Tecodrive.

The two stators generate the powerful magnetic field required for the new process in the feed unit, which in turn is controlled by a Kollmorgen servo controller.

© Torsten Sienk

Today, the lighting manufacturer uses the Flexfeed from Tecodrive - with amazing results. The company now runs at twice the speed and can finally make full use of the performance of the sheet metal processing machines. "Our brief at the start of the project was to achieve 400 strokes per minute - which we did," says Hans-Jörg Lindner, also Managing Partner of Tecodrive, looking back and adds: "With existing processes, we usually achieve an increase by a factor of two."

The contact-free technology behind Flexfeed conveys the material absolutely gently, without lateral forces and without the formation of belt waves at maximum speed. And this is how it works: As with linear direct drive technology, the contactless feed also uses the principle of magnetic attraction or repulsion. To achieve this, two stators are arranged opposite each other in the device, which is just under one meter long. They generate a magnetic traveling wave in the metal strip between them, which is used for the feed thanks to the resulting Lorenz force. The operating principle in the Flexfeed can be compared to an asynchronous linear drive in which the metal sheet takes on the role of the short-circuit rotor. However, this detail poses a number of challenges for the control technology, which ultimately form the core of the patented process.

New control procedure developed

The implementation of the flexfeed method required the realization of a new control method: The picture shows the acceleration and deceleration ramp of a positioning cycle.

© Torsten Sienk

"I had to write a program with which the phase angle of the current and the amplitude could be changed," says Dr. Olaf Marthiens, looking back on the beginnings of the development. The use of field-oriented vector control (FOC) did not work in this particular application because the constantly changing magnetic behavior of the material to be transported would make the control loop unstable. According to Marthiens, the FOC reacts very sensitively to the so-called rotor constant - i.e. the ratio of the inductance to the electrical resistance of the sheet metal to be conveyed. In a standard asynchronous motor, the rotor is mechanically unchanged and the rotor constant only depends on the temperature. And even that can 'drive an applicator to white heat' when using asynchronous motors in highly dynamic control applications.

The Flexfeed unit, on the other hand, had to be designed precisely to position as many different metal materials as possible in a processing machine in a highly dynamic and precise manner. As a result, the expected different thicknesses, widths and invisible changes in the material structure meant that new resistance and inductance values had to be expected time and time again - and therefore different rotor constants. Tecodrive has solved this problem together with its drive technology partner Kollmorgen to the effect that there is only one current and position controller in this application. In other words, there is no longer any field-oriented control because the S772 servo inverter used is programmed in such a way that the field is fixed. To determine the position of the sheet metal, Tecodrive uses measuring wheels developed in-house as feedback.

The speed of the control loop is remarkable, because the new process calculates the next setpoint directly from the actual position of the sheet metal in the magnetic field without any detours via a controller or the field bus. Dr. Olaf Marthiens explains: "We need absolute freedom from delay in order to be able to position with 10 µm accuracy even at a cycle rate of 4000 strokes per minute. The peak values are partly a result of the high computing performance of the S700 exciter and the overall very lean and clear programming."

Low energy consumption

Together, Tecodrive and Kollmorgen have therefore succeeded in designing a feed that achieves performance figures that would never be possible with rollers - if only because the masses cannot be adjusted so quickly. The feed, on the other hand, has no mass of its own because it only consists of a magnetic field. In simple terms, the process is reduced to switching the current on and off without a mass. In practice, this principle has further advantages: Switching is noiseless and free of mechanical stress. This aspect significantly increases the service life of the production equipment, particularly in high-performance operation, because there is no mechanical wear in the feed unit.

The Flexfeed is already being used successfully by a lighting manufacturer and an automobile manufacturer.

© Torsten Sienk

And there is another point that is currently making companies in the direct and indirect automotive sector sit up and take notice: The new development requires only a fraction of the energy that a roller feed consumes to supply it. "Although we have to use a 35 kW servo drive from Kollmorgen to achieve the necessary magnetization current, the pure active power is only 700 watts," says Dr. Olaf Marthiens. For his co-managing director Hans-Jörg Lindner, these advantages also speak in favor of leaving familiar paths in material feeding: "At Trilux, we have managed to turn two systems into one - with the same productivity and with significantly less space requirement and effort for the tools." The experience at Volkswagen in Hanover Stöcken has been similarly positive. There, too, the new technology has already significantly reduced set-up, maintenance and downtimes in the production of delicate radiator fins. Flexfeed is currently designed for all electrically conductive metals with strip thicknesses of up to 3 mm and strip widths of between 10 and 500 mm.

Author:
Dr. Arne Linder is Product Manager at Kollmorgen Europe.