Application Solutions
Norske Skog - Automation makes the papers
The first Rockwell Automation ControlLogix dc drive system in Australia is bringing much needed reliability and stability to the winders at the Norske Skog paper mill in Tasmania.
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Norske Skog has three paper mills in the region - Boyer, Albury and Kawerau - servicing 95 per cent of the Australian and New Zealand markets. The Boyer mill employs over 400 people and constitutes the chief export industry in the district. At its heart are two paper machines, almost six metres wide, each making paper at the rate of 1180 metres per minute (mpm). Completing the process, two dedicated winders convert the massive 'jumbo' rolls from the paper machines into a range of sizes that suit both shipping and customers.
In 2001, Norske Skog project engineer Adam Tassell devised a way of bringing digital control to the winders, without the usual costs of a traditional upgrade. Based on a Rockwell Automation drives solution that used its 'next generation' control platform ControlLogix, the upgrade program schedule ingeniously fitted all installation work into the plant's existing maintenance schedule.
Winders and blowups
A paper winder is a complicated machine that unwinds a paper machine jumbo, cuts the paper to make rolls of different sizes, and discharges tightly-wound customised rolls. The operational profile is demanding - it must be able to rapidly accelerate and decelerate the heavy rolls (a new jumbo weighs 15 tonnes), as well as maintain speeds up to 2500 mpm, while the diameters of the rolls are changing.
 A Wartsila two-drum winder unwinds a 'jumbo' roll. |
According to Tassell, the winders were experiencing frequent blowups. Repairing the break and re-threading the winder took 20 minutes each time, and when it occurred too often, it would threaten the ability of the winder to keep up with paper production. "Sometimes this would cause us to have to shut the paper machine down - when that happens, it essentially costs us A$15 thousand an hour - something we try to avoid at all costs," he explains.
Maintaining fine tension control on the winders was a constant struggle at the Boyer mill, given its legacy logic control technology and 1980s - vintage analog control. Drift in analog component settings and the lack of diagnostic information was making fault-finding time-consuming and difficult. Frequently the problems were beyond the expertise of Boyer staff, and it was necessary to bring in a technical support engineer from Finland, at considerable expense.
Non-traditional considerations
In late 2000, Tassel investigated a number of options that would bring stability to the drive system at low cost. The traditional method for winder upgrades is to carry them out as part of a complete mechanical upgrade of the paper machines, and generally involves the change-out of all winder motors and drives. Typically involving a total paper machine shut down of up to seven days, the cost for such an upgrade can be in the order of A$15 million.
Given their robust design, there was no need to change the mechanical architecture of the winders - a reliable, cost-efficient alternative for the 'analog to digital' conversion of the winder was required. One that would be easy to operate, maintain and support. In addition, given the mill's busy schedule, strong consideration was given to solutions that involved minimal downtime for the paper machines.
The team at Norske Skog decided to use a non-traditional approach. Tassell felt much could be gained by making use of the Boyer Mill's ten hours per month downtime scheduled for each paper machine. "I saw it as a question of how many problems can you get in a ten hour period. Not very many, but it's long enough to do one drive at a time," says Tassell. Digital control, he believed, could be realised 'on the run', rather than scheduling a single lengthy (and costly) shutdown.
A solution unwinds
In 2001, Tassell and Norske Skog coordinator, John Meehan, were given the go-ahead and A$4.5 million to upgrade the winder control system, in conjunction with a separate material upgrade to the winder drums. Rockwell Automation's Drive Systems team was commissioned to work with Tassell and his team and together they developed an ingenious 'drive-by-drive' scheme to convert the winder from analog to digital control.
Rockwell Automation's solution was based around its 'next generation' control platform ControlLogix - the company's new, flexible control architecture that allows multiple types of control, networks and I/O to be combined in a single system.
Tassell and the Rockwell Automation Drive Systems team set about to replace Norske Skog's legacy logic and analog control system with two ControlLogix units linked via the peer-to-peer communications network, ControlNet. The first unit would be responsible for drive control, the second for hydraulic and logic control. The company's PC-based 'SIGMA server' human-machine interface would provide a huge range of continuous data trending, diagnostics and history.
One hurdle existed. While the change to ControlLogix would ensure the latest technology, at the time of the order in January 2001, it was not yet available in Australia as a DC drive controller. To overcome this, Rockwell implemented a transitional phase, using its well established drive control system AutoMax.
 Norske Skog project engineer, Adam Tassell - "The operators were saying it was the best it's been in twenty years." |
The transition of the winder's drives to AutoMax was successfully completed in October 2001. The stability of the new system was excellent. "When we finally got everything onto AutoMax, the performance was unbelievable. The operators were saying it was the best it's been in twenty years," says Tassell.
By working in with Norske Skog's planned shutdown schedule, the length of paper mill shut time has been dramatically reduced. Avoiding a traditional winder shut and opting for the staged Rockwell Automation-engineered solution saved the paper manufacturer A$5 million.
Similarly, as a result of the more stable and serviceable digital control system, the mill now enjoys a dramatic reduction in winder maintenance. Winder problems no longer waste maintenance money nor tie up people's time. Local rather than overseas expertise can now be used for control system maintenance, saving the paper manufacturer AS200,000 per year.
The changeover to digital control has had an additional side-benefit. This is the speed improvement of the winder from 2300mpm to 2500mpm. The winders must always stay ahead of the 1180mpm speed of the paper machines, so the change allows the winders to catch up faster, following a shutdown. "The old system was limited electrically to control to 2300mpm. Both AutoMax and ControlLogix are able to control up to the full speed, allowing the plant to fully use the capacity the machine was designed for," says John Dunn, senior project engineer for Drive Systems, Rockwell Automation.
Progress with the Norske Skog winder project is now in its final stage, as the mill drives are swung over from AutoMax to ControlLogix. The new DC drive control has made both winders much more stable and reliable. Ultimately, the upgrade has transformed the deteriorating winder control into state-of-the-art technology, allowing Norske Skog to meet confidently the demands of the Australian and New Zealand print media.
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