On the Lisim lines produced by Brückner Maschinenbau over 700 clips pull the plastic film through the machine and simultaneously stretch the material in the machine and cross-machine directions. The clips are pulled by a magnetic field like the railway cars on a magnetic levitation train. Linear motors with 728 windings generate the moving magnetic field that makes this possible. Since recently, motion control is handled by drives from B&R’s ACOPOS series, which are synchronized via POWERLINK. With the transition to B&R technology, the Chiemgau-based machine manufacturer was able to reduce the cost of the drive equipment, significantly shorten the cycle time, simplify maintenance of the hardware
and software and replace proprietary drive and bus technology with an off-the-shelf solution.
At up to 6.6 m/s, hundreds of clips speed through the linear motor driven simultaneous stretching system – Lisim for short – on two opposing ring shaped rail lines. They grip the plastic film, which has been produced using an extruder and a casting roller, along its left and right edges and pull it through the system at increasing speed, stretching the film in the machine direction. The rails are not parallel; instead, the distance between them increases along the length of the line. This simultaneously stretches the film perpendicular to the machine direction with absolutely no scratches. During this stretching process the film is also passed through an oven. The high temperature binds the molecular structure and permanently defines the film’s physical properties, such as shrink, which were affected by the stretching. After completion of the stretching process, the clips release the film and travel empty back to the beginning of the system. A rack transports the film from the system where it is wound up after trimming off the unusable edges.
Only a certain number of clips are actually driven; hundreds of non-driven clips improve force distribution and prevent the film from sagging as it is stretched. The driven clips are designed as a secondary side (rotor) of the linear motor and are therefore equipped with permanent magnets on the traction surfaces. The linear motor is split to improve force distribution, with the primary elements (stators with windings) positioned symmetrically above and below the permanent magnets on the traction surfaces. The windings are distributed along the rail in zones of different lengths so that no more than one powered clip is present in each zone.
728 individually controlled windings
Each of the zones is controlled separately using an inverter. This new Lisim system from Brückner relies on B&R drive technology. The power for the 728 zones along the approximately 65 m long machine is supplied by 384 single and dual axis inverter modules in conjunction with 14 power supply modules from B&R’SACOPOS device series that are synchronized via POWERLINK. The electrical power must be higherin longer zones, so drive types ranging from 2.6 a to 22 a with a power rating of 5 kW must be used.
“The machine concept, which is based on linear motors, was invented by us over 15 years ago, but it is still state-of-the-art and is experiencing a renaissance because of the rapidly increasing demand by the packaging and flat panel display industry for film with special film properties. The drive technology thatwe’ve used up to now is getting on in years however,” says Günter Oedl, electrical engineering manager for automation and development at Brückner Maschinenbau regarding the change over to a B&R system. “It was also developed specifically for us and took advantage of a proprietary bus, so a new development would be difficult and expensive.”
Through the use of ACOPOS modules in combination with POWERLINK, the machine manufacturing company was able to replace the proprietary products with cost-effective standard solutions. The further development of these standard solutions is of course required and funded by all users.
Precise synchronization with POWERLINK
“This was possible because POWERLINK allows precise synchronization of hundreds of network nodes and simultaneously provides high data throughput.On one hand, we were able to reduce the cycle time significantly – it is now only 400 µs. On the other hand, we were able to move large chunks of software from the drives to a central drive controller,” explains Oedl. “This significantly simplifies servicing and maintaining the software.”
All of the 398ACOPOS modules (power supply modules and inverters)in the system are synchronized by 12 industrial computers from the aPC810 series. They are each equipped with three POWERLINK cards that control up to 13 modules. Using another POWERLINK card, the industry computers communicate with each other or with a higher level aPC810 that is running Brückner’s motion control software. The plant control system, which for example is responsible for controlling the oven, is connected to this B&R industrial computer using a Profibus interface.
POWERLINK’s short cycle times and minimumjitter allowed Brückner to position the zones very closely, as confirmed by the head of the electrical engineering development at the company, “The individual zones are grouped in a very homogeneous manner. The errortolerance is significantly less than a millisecond as stipulated by the application.” Advantageous for machine manufacturers is also the fact that POWERLINK is not a proprietary bus system; instead, it is an open source solution and is therefore supported by many producers. Thus, the number of solutions available is very large, and they include professional diagnostic tools.
Saving energy and space
Brückner managers were impressed by the wide range of products in the ACOPOS series and the possibility of controlling two axes or windings with one inverter, as Oedl states, “Instead of having to use 728 invertermodules, 398were sufficient to get the job done. This clearly reduced costs and saved space.”
The Brückner managers also paid special attention to the cooling technology. “With air cooling, the risk is that a clogged filter will not be exchanged in time, or worse, removed and no longer be replaced,” says Oedl. “This is why we wanted to go with cold plate cooling. B&R proved themselves to be very cooperative in preliminary discussions and developed an ideal solution.” Even later during the project, B&R took service aspects very seriously. The company replicated the entire drive system at their own technology centre in Eggelsberg and made it available for development activities.
This and further details regarding the ACOPOSinverter supported the decision to use the B&R drive technology: The device does not require heat conductive paste. “Changing an inverter during service work can now be done in half an hour and not an hour as was done previously,” says Oedl. Butthat’s not enough – for drives with a cold plate, heat is transported from the cabinet much more effectively with the help of water or oil.Peak temperatures and large temperature fluctuations can be avoided,whichextends the lifespanofthe electronics.If the coolantis fed into anexisting cooling system, higher overall efficiency is achieved compared to conventional cooling methods.
Closed loop control without sensors
The planned switch from open loop to closed loop control of the windings will resultina significant increase in energy savings. For this purpose, Brücknermustrely on a sensor free method because sensors cannot be mounted near the oven. “Thanks to sensorfree control, we are able to reduce the power used by each system by 100 to 150 kW,” says Oedl.
Another plus point ofthis control method is much higher damping compared to a weakly damped magnetic shaft mass system. Machines with sensor free control are therefore more resistantto disturbances such as process variations orfilm rips. “The advantages of the sensorfree closed loop control are clear. We therefore welcome the fact that we will be able to obtain ideal solutions from B&R in the future,” concludes Oedl.