An innovative Z-fold system from researchers at TU Berlin eliminates slow pick-and-place processes through the use of adaptive manufacturing technologies from Beckhoff
Lack of low-cost lithium-ion battery (LIB) cells could bring mass electromobility to a standstill. While EV advances continue to speed ahead, the battery manufacturing process has moved more slowly and driven cost – until now. Researchers at Technische Universität Berlin are using continuous Z-folding to replace the traditional pick-and-place movements, accelerating battery cell production. The eXtended Transport System (XTS) from Beckhoff has played a crucial role here.
The biggest challenge in the production of LIB cells is that the assembly of the electrode-separator composite, which consists of many layers, is very slow. The Z-folded structures destined for EVs previously required pick-and-place operations using robots or special kinematics – for each electrode. With up to 53 electrodes in a single 50 Ah LIB, it’s easy to see how the traditional manufacturing process can create gridlock.
Eliminating this productivity bottleneck is critical. But that’s easier said than done, according to Dr.-Ing. Arne Glodde, senior researcher at the TU Berlin’s Institute of Machine Tools and Factory Management (IWF), Department of Handling and Assembly Technology.
For example, handling and joining times can’t simply be reduced by faster braking and acceleration in the industrial robots. “That would impair positioning accuracy when placing the electrodes,” Glodde says. This would result in a lower degree of coverage in the cell array, at the expense of cell capacity, service life and even short circuits.
However, one thing was clear to the TU Berlin researches as they started down the road that would eventually lead to the new LIB production concept, as Glodde explains: “The previous process flow has reached its productivity limit in practical terms.”
Mechatronics speeds up battery stacking by 150%
The researchers at TU Berlin replaced the pick-and-place process with continuous material transport of the electrodes and separator. “This has enabled us to increase production by more than 150% compared to the current state-of-the-art process,” Glodde says. This increase in throughput means that, in the future, it will be possible to produce 250 cells in the time that it currently takes to produce 100.
The key innovation in this process is the patented Z-folding technique using the XTS linear transport system and its rotating grippers. Two of these systems with a total of 10 movers ensure continuous folding with precise positioning of the electrodes in the stack.
The electrodes are available as stacks of individual sheets, and the separator is available as belt material. Using a vacuum-based process, the flexible electrodes are each picked up by an individual roller and accelerated to the speed of the separator belt. During transportation, positions are detected and the electrode sheets are precisely aligned with the separator. The electrodes are then fixed alternately on the separator material as it is continuously conveyed.
This is followed by Z-folding. Here, the separator belt with the electrodes fixed to it feeds into the folding process from above via two guide rollers. Two commuter rollers deflect the belt material for this purpose and to facilitate alternating gripping from the rear with special grippers. These are mounted on the movers of the two interlocked XTS systems, and they guide the folds until the belt material is deposited on the folding table. Finally, a hold-down system secures the folds.
The XTS systems are mounted in a vibration-free structure that can be integrated into the individual sequence of any battery production system.
Above: See an early version of the concept presented at Hannover Messe 2019!
Adaptive manufacturing with XTS drives innovation
The unique functionality of XTS and PC-based control from Beckhoff enabled this massive leap in performance. “With its freely controllable movers, XTS brings our demanding motion sequences to life,” Glodde says.
The grippers must move in relation to each other to fold the material with adequate belt tension while coping with the stresses. While this may sound simple, it demands high-performance yet flexible automation technology:
TwinCAT 3 automation software calculates the position of a mover in relation to the position of the other movers precisely and in real time.
Accurately synchronizing the rollers over the shortest distance requires high dynamics.
The number of actively intervening movers must be able to adjust dynamically during the process.
To ensure the positions along the separator belt are approached at the right time, a high degree of repeatability is needed.
Load fluctuations require high-quality control.
The module-specific programming and synchronization of the controllers, using the multitasking capabilities in TwinCAT, also empowered the TU Berlin team during implementation, according to Mathias Arndt, Application Engineer at the Beckhoff office in Berlin. “To calculate and control all of the kinematics in real time, we had to ensure that the XTS servo axes can seamlessly switch from NC (point-to-point) operation to interpolation,” Arndt says.
Path planning for all axes is performed directly in the controller based on the kinematic calculation. The sequences in the other modules are coupled via virtual axes and corrected via super-positioning movements in relation to the other elements in the process. Plus, the fully integrated automation platform simplified implementation and synchronization of the six-axis robot kinematics that eject the folded electrode packages according to Glodde.
Achieving the fastest and most accurate position detection of objects is crucial for precision and repeatability. The researchers relied on XFC technology to provide this. “We use eXtreme Fast Control technology’s timestamping feature for calculating the positions,” Glodde explains. “With XFC, signal acquisition becomes independent of the PLC cycle.”
The advantage is that control technology is no longer the limiting factor in position detection. Instead, the current sensor technology with sampling rates of 62.5 kHz constitutes the limit here.
EV and battery production accelerates with intelligent transport
The XTS-equipped solution has cut processing time from over 1 second per fold to 0.7 s for large-format Z-folded electrode-separator composites. To create a fold in 0.7 s, the movers travel at a speed of about 600 mm/s. “Without the linear transport system from Beckhoff, we would not have been able to implement continuous Z-folding with this process control properly and achieve these significantly shorter processing times,” says Glodde.
However, the process is still far from reaching the limits of its potential. In the future, times of around 0.35 s per fold could be possible with XTS movers reaching speeds around 1,000 mm/s.
“Our standard XTS is designed to travel at up to 4 m/s, so it has enough capacity to accelerate the process even further,” says Guido Sieder, Sales Engineer at the Beckhoff.
“The mechanical properties of the separator material and the moments of inertia will probably always be the limiting factors in the long run,” Glodde adds.
The team at TU Berlin keeps working to push the boundaries of Z-folding further. “We see lower accuracy at higher speeds due to vibration and inertial forces. This could be counteracted with even stiffer guide rails or active grippers.”
Beckhoff has solutions for both: the XTS solution’s V-shaped guide system and No Cable Technology (NCT) for wireless transmission of communication and power to the movers broaden the scope for further optimization. Beyond making the folding processes faster and more flexible, the XTS technologies will help the Z-fold system support more electrode formats and dynamically compensate for errors in the feed.
“We are working to further develop our grippers and want to use NCT to replace the mechanical positive guidance with active control of the grippers,” Glodde says. “Furthermore, we also want to integrate a vision system into the system for quality assurance.”
Interested in revamping your EV automotive and battery production applications with linear transport technologies? Contact your local Beckhoff sales engineer today.
Philip Ehlers, Automotive Industry Manager, Beckhoff Automation LLC