Moog's Advanced Pressure Control Solution
The concept of the printed word on paper might not have changed much since the Gutenberg Bible was produced in 1455 but production techniques have changed fundamentally. The average person frequently encounters the incredible range of magazines available at their local newsagent today. This proliferation and the extremely high quality of paper and reproduction are the result of recent developments in digital layouts, printing, and paper making machinery. One of the companies that has contributed notably to this change is the Metso Corporation with its headquarters in Finland. Northern Europeans are in the forefront of both forest production and conservation, so it is no coincidence that one of the foremost paper machine manufacturers is located in this part of the world.
Although paper making techniques have been refined in the last 20 years, the basic concept is still the same. You take paper pulp and pass it through multiple rolls to squeeze out the liquid and then dry it to produce paper. Sounds easy! Well, modern paper machines such as the PM12 currently being installed at Kvarnsveden in Sweden are 300 meters (985 ft.) long, 70 meters (230 ft.) wide and 30 meters (99 ft.) high. It has a calender width of 11.3 meters (37.1 ft.) and a machine speed of 2,000 m. (6,560 ft.) per minute, which will make it the world's largest paper machine. When in operation at the end of 2005, PM12 will produce 420,000 tons of paper per year, which will increase the capacity at Kvarnsveden to over 1 million tons per year. This is equivalent to 2 kilograms (4.41 lbs.) of paper per European. It utilises 15,000 control elements handling 75,000 control signals and operates six shifts with only 9 operators per shift. This kind of machine is considered to be as technically sophisticated as a modern Jumbo Jet.
PM 12 utilises 8 high-precision SYM-CD rolls with narrow zone profiling to produce what the industry calls SC (Supercalender) paper with a higher gloss and smoother surface. CD stands for Cross Direction and the CD-roll is a device that corrects the cross directional errors on the paper web. The machine uses up to 76 hydraulic pistons placed inside each roll to exert forces at each load zone to maintain a controlled profile across the complete width of the roll. These forces are hydrostatically counterbalanced on the opposite side of the roll. The force exerted by each piston is controlled by a Moog D638 pQ Servovalve and the pressure in the counterbalance zones with D941pQ Servo-Proportional Valves. In addition, four Moog D638 pQ Servovalves control the hydrostatic bearings located at either end of each roll.
The term pQ is used to denote electrohydraulic valves that can be accurately controlled in both pressure and flow control modes, which are critical in this application. In normal operation the valves are controlled in pressure or force control mode but there are conditions in which it is necessary to open the rolls fast by selecting the flow control or "fail-safe" mode. One of these situations is the "fast break recovery" when the paper strip rips or breaks.
In order to produce high quality paper, cross directional profile of the paper web is measured and fed back to the machine controller which adjusts the pressure of each valve independently to achieve paper symmetry. An important feature of these valves is the internal pressure control loop. This uses a built-in powerful microprocessor and accurate pressure transducer together with the valve's high dynamics to ensure precise control down to very low pressures. The state of the art controller uses special control algorithms to simplify systems optimisation by reducing the multiple parameters that are the basis for hydraulic pressure control transfer functions, to a single variable.
Earlier paper machines used valves controlled by analogue signals but digital control provides many advantages. With 78 valves per roll and up to 600 valves per machine the needs for decentralised control, reduced wiring and improved diagnostics have meant that the valves now need to communicate with the machine controller via a fieldbus. The result of this change is continuous development of real time diagnostics, improved functionality, and unique preventative maintenance solutions. Just one example is the ability of the valve to monitor its own output and react in order to prevent damage to the machine. If the actual pressure is out of a pre-set "window" for more than 30 ms. the valve goes to a pre-defined safe position.
So next time you pick up a glossy magazine at your local newsstand you might reflect on the fact that it is the end result of a highly sophisticated process. It started with a tree and went through multiple phases of production using some of the most advanced motion control products in the world and was eventually distributed to your neighbourhood.
Peter Lillqvist is Area Manager for Moog Finland. He started with Moog in 1999 as an Application Engineer, and has been working the last four years as a System Sales Engineer. He has a Masters degree in Automation Engineering from the University of Tampere.