POWERLINK at the pulse of the printing industry
Increasing complexity, growing numbers of axes and pressure to maximize both speed and precision are the challenges facing builders of printing and binding machines. These applications demand dynamic reactions and rigidly synchronized axes. The combination of cross traffic capabilities, slave multiplexing and the freedom to put together any topology make POWERLINK an ideal communication standard for print media production.
Competition is fierce in the printing industry, with individual printers vying to print ever smaller batches on office machines. There is enormous pressure to deliver impeccable quality at lower and lower costs. There is limited room in the budget for misprints and waste. This applies not only to the print process itself, but also to post press processing, cutting, folding, binding and packaging.
Equipment suppliers for the printing industry have responded by developing faster and faster machines with more comprehensively automated processes all the way from printing to processing and delivery. They are also designing more versatile printing equipment that allows for the high levels of utilization necessary to make it profitable. This includes the flexibility to print substrates of varying thickness, texture and rigidity – from plastic film several micrometers thick to paper in the millimeter range. After all, printed materials include more than magazines like the one you're reading right now. There's also the ad on the bus down the street, the bag of frozen peas in your freezer and the money in your wallet.
Exploding numbers of axes
One consequence of these developments has been a rapidly increasing number of movement axes. It's not uncommon to find individual machines with fifty axes or large-scale operations in excess of two hundred. A series of up to eighteen different color stations needs to be perfectly coordinated. The slightest variation in the positioning of any one color degrades the overall image quality. Large deviations, as shown in the image, result in costly waste. Yet even the tiniest flaw can cause soft edges or distort color gradients.
The challenge would be considerably more easily surmountable if the properties of the substrate remained constant throughout the entire process. However, even the printing process itself changes the characteristics of the paper or film. Tension and slack due to irregularities in winding and unwinding impact the absolute position of the partially printed image. To account for this, sensors throughout the system detect the current position using registration marks and make corrections for each subsequent processing step.
These high-precision positioning functions are a vital element throughout every aspect of print media production – pre press, press and post press – and demand absolute coordination of all axes involved. This synchronization is generally derived from a single master axis, making the transfer bandwidth of the communication medium a decisive factor. Today's printers are achieving unprecedented speeds, and registration mark sensors have sample rates as low as two microseconds. At this level, when drives need to be addressed and queried sequentially, even Ethernet is too slow.
Three qualities of the POWERLINK real-time protocol for Ethernet can help alleviate this time pressure. One of them is its cross traffic capability, which means that every drive on the network receives data from an intelligent network node simultaneously and directly, without having to pass through the master. This allows them to react with no delay.
The second accelerating mechanism within the protocol is slave node multiplexing. Since the response from the drives is not nearly as time critical as the synchronization signal, is isn't necessary for every node to respond within the same cycle. Communication cycle times can therefore be kept short, even with extremely large numbers of axes.
This makes it possible to design printing machines where complexity, speed and precision are not mutually exclusive, but instead can all be maximized at the same time.
Another characteristic of POWERLINK with particular advantages for printing technology is that it is completely independent of the network topology. This allows cable lengths and signal paths to be optimized in large machines. It is also possible to construct networks where all nodes remain accessible and operation can continue, even in the event of a broken cable. Above all, however, it is easy to design very modular automated printing solutions. Individual color stations or entire post press machines can be designed as self-contained units that can be plugged together to form flexible production lines that can easily be expanded or rearranged.
"Right now there is no other communication protocol on the industrial automation market that better meets the demands of print media production machines," says Dr. Robert Kickinger, mechatronic technologies manager at B&R. "With increasing complexity and growing numbers of axes, a trend toward higher speeds and greater precision, as well as a need for fast and safe reactions to errors, POWERLINK is the ideal standard for this industry."