Startling possibilities offered by new webscanning techniques are among many temptations for today's papermakers to get automated

by Mark Lockie

Automation keeps an eye on the papermaking process

Automation has caused head counts to fall sharply in pulp and paper mills, but when it comes to making vital decisions, the human brain is not redundant, just yet. Actually, computers are making it easier for people to use their heads the way they were intended - to come up with solutions to problems and to innovate.

In the pulp and paper sector, this means that the responsibilities of operators will increase. At a recent Valmet Automation conference, the company's vice president of marketing, Antti Kaunonen, pointed out, "Automation is clearly superior to operators in terms of its calculation power, consistency and accuracy. However, if there is a problem, it is the operators with their adaptive and innovative intelligence that must take care of recovery to the normal state. We must always remember that automation is a tool of the organization and not its replacement."

One example of how automation has given operators more freedom to move freely around the mill to solve problems can be seen in Valmet's futuristic-looking Wearable XDi mobile control unit.

With the help of wireless technology, it allows the operator to have information at hand and is equipped with a telephone and a video camera. This is a departure from the past, where operators traditionally worked in teams of two to test one loop at a time for example. Wearable XDi can also provide the operator with all the manuals needed on location for maintenance.

Optical wizardry

What is clear is that automation can make the entire papermaking process run more smoothly and more profitably. According to one of Jaakko Pöyry's senior consultants, Jorma Henttonen, "We are now moving toward digital systems and in terms of quality it is similar to the difference between a vinyl record and a compact disc."

Automation suppliers recognize that most of their products must have a visible affect on the quality or cost of the end product. And bearing in mind that most mill managers will not consider making an investment if the payback time is more than two or three years, they have their work cut out.

Perhaps one of the most visible pieces of automation technology to emerge over the last year, with the potential to improve quality, is full-sheet web scanning. As Tampere University's professor of automation, Timo Nyberg, points out, "Full-sheet measurements have been an aim for a long time. I believe that optical scanning systems will be the best solution because you can measure moisture, thickness, ash content and a range of other properties."

Traditional mechanical scanners with on-line sensors provide less than 1% visibility of the sheet and because the scanning sensor is always measuring a diagonal strip of paper (as the sheet is moving in the machine direction (MD) and the scanner in the cross direction(CD)) it must calculate average measurements. At the end of last year, ABB Industrial Systems announced it had constructed a full-sheet imaging system, which could electronically scan the paper web at a rate of 1,000-2,000 measurements per second. The supplier named its product the AccuRay HyperScan and claims it gives papermakers the ability to view 100% of the sheet.

The pure detection of MD, CD and residual variability is expected to result in better control of the papermaking process, while the reduced need for profile averaging could make control actions up to 20 times faster. ABB claims that papermakers will have the ability to improve grade change responses by up to 35% over conventional mechanical scanners and recover faster from process upsets.

The basic system components of the system include a light source and cameras. As the light beam illuminates the full sheet, an array of highly-sensitive cameras measures the passage of near-infrared light through the sheet. The light intensity is automatically adjusted to accommodate varying sheet opacities and provides adequate camera light levels for sheets from 30-300 g/m.

The measurements this technology can make include basis weight and sheet formation properties, such as quantitative analysis of the weight distribution and flock size.

In on the act

The "holy grail" of full-sheet imaging has proved tempting for other paper industry suppliers and companies such as Honeywell-Measurex and Siemens are getting in on the act and developing their own particular versions.

Honeywell-Measurex's approach differs because its product is for the wet end of the paper machine. The company has called its system the SpectraFoil full-sheet measurement sensor, and it comes as part of the supplier's FullWeb Advanced Systems Technology (FAST) family. According to vice president of marketing at the company, Bob Bucher, "While scanning measurements at the dry end provide outstanding absolute accuracy, there is a significant process transport lag from the point of control to the point of measurement."

With the SpectraFoil, sensor elements are integrated into a foil, which is mounted on the fourdrinier table rails. Honeywell-Measurex believes this could allow basis weight changes in both machine and cross direction to be seen minutes before they are noticed at the paper reel. The sensors can also measure the drainage profile on the wire, which could give operators the opportunity to control sheet formation. For operators working on multiply machines, it could be possible to have control over each ply making up the paper, if a sensor is positioned a couple of meters after each headbox.

Also keen not to miss any of the action is Siemens. It has just brought onto the market a full-sheet measurement system, which it calls Qualimat. According to one of the company's research engineers, Herbert Furumoto, "So far we've sold one of these systems, but we've had interest from 33 other paper mills."

The Qualimat technology comprises up to several hundred sensors mounted on a frame above and below the paper web and it can measure paper quality parameters such as basis weight, moisture, caliper, ash content and coatings. "There are no moving parts and its sensors are based on Near Infrared Light," explains Furumoto. The machine forms an integral part of Siemens' quality control system, which is based on its new Simatic PCS 7 distributed control system.

Strong stuff

While accurate measurement of web characteristics on-line is possible for most paper properties, determination of one variable has eluded technologists for some time - paper strength. It is easy to understand why. After all, how can you measure the strength of a piece of paper without destroying it?

The tensile stiffness index (TSI) of paper is an important property of paper, affecting many applications. Testing has shown that there is a good correlation between the strength of paper and the TSI, so it has become an acceptable measure of a paper's strength properties. As TSI measurements can be obtained quicker than traditional destructive compression testing methods, it has become increasingly popular in paper mills.

Usually these measurements are made off-line in a paper mill's laboratory. But Swedish supplier, Lorentzen & Wettre, has announced a method for measuring TSI and tensile stiffness orientation on-line. The process involves using non-contact, ultrasonic measuring equipment and a transmitter. According to a product specialist at the company, Patrik Stolpe, "The signal is transmitted cordlessly to the receivers and then undergoes signal processing. The method is extremely simple."

The supplier believes that some of the advantages of the product will include reduced waste and shorter changeover times between different paper grades. An added consideration is that the more timely information could allow a mill to optimize quality, because a mill may be able to reduce tolerances.

Simple irony

New scanning techniques are very visible examples of new automation in the paper mill which help boost the quality of the end product. But automation isn't just about the big shiny new pieces of machinery. There are many other new products on the market which are helping in other ways. For example, great strides are still being made in the world of advanced controls such as fuzzy logic. This is notably true in pulp mill digester systems. Siemens for example has its CDOC (Continuous Digestor Optimal Control) and ABB its CLA2000 cooking liquor analyzer. Many suppliers also incorporate advanced controls into the workings of their mill-wide information systems (PPI March, p39).

Although there are many advanced controllers available, most control systems in mills today still rely on the humble PID controller with a control valve. The irony is that it could be with these simple controllers that the greatest steps forward are made by a pulp or paper mill. According to Nyberg, "The problem is, that even in technologically advanced Finland, many of the controllers are out of tune. I would suggest that the most profitable improvement that could be made in a mill's automation is to have all the systems tuned up."

Unfortunately comprehensive tuning of all control loops is very time consuming. Despite the fact that a normal PID controller only has 3-5 parameters to be tuned, it is still difficult to achieve good tuning, due to changes in operating point, non-linearities, load disturbances and noise.

Valmet Automation has released a package it calls TuneUp, which is specifically tailored for mills which use its Damatic systems. UPM-Kymmene's Tervasaari PM 6 in Finland is one mill which decided to use the supplier's package following significant fluctuations in the level control unit for a particular tank. According to control loop analysis, there was no apparent reason to suspect the related control valve was not operating properly, so controller tuning was performed based on a simple and fast automatic process experiment. The process was modeled and the controller's tuning parameters calculated by TuneUp. The fluctuations in level approximately halved following adjustment of the PID parameters.

The improvements seen by tuning up control loops are of great value, but as paper machines go faster and the pressure increases to produce high quality paper at high operating rates, the other automation developments also have a large part to play. Whether it is through better operator efficiency, in-depth web analysis, advanced control or any of the other numerous technologies being launched on the market.