NEW TECHNOLOGY FEATURE
iRoll for intelligent parent roll build-up and tension control
By Tatu Pitkänen, R&D Manager, Metso Paper Inc., Järvenpää, Finland
Metso’s iRoll technology creates new possibilities to optimize the runnability properties of the web. The iRoll reel drum allows the measurement of both nip load profile and parent roll profile online. iRoll tension is a method of measuring web tension profiles online, without any separate scanning devices. The iRoll reel drum and iRoll tension also facilitate the online control of parent roll profiles and web tension profiles.
The iRoll system (Fig. 1) consists of a high-precision machined roll body with grooves for the sensors, force sensors mounted in a helical pattern, roll covers, measurement electronics, digital radio transmission free from disturbances, wireless power transmission, and a receiver system that is connected to the mill automation system.
Fig. 1 iRoll components
The iRoll system and its components
The special feature of the iRoll is that external measurement devices such as scanners are not needed. The iRoll is just like any other roll in the process, and this makes it possible to view how web tension or parent roll profile really affects the production line. Unlike additional measurement devices, the iRoll causes no errors in the properties being measured. Moreover, it may be located in various positions in the process, wherever the tension profile is critical or the roll profile needs to be measured (Fig. 2).
Fig 2. iRoll technology enables measurements in various positions.
The iRoll reel drum
For decades, papermakers have wanted to measure nip load profiles automatically, online, directly at the nip. The intelligent iRoll reel drum addresses this need.
Fig. 3 The iRoll reel drum installed in a paper machine reel.
The iRoll reel drum brings to the market a totally new system of online nip force profile measurements, providing valuable information about paper web properties. One goal is to replace the old system of manual parent roll hardness measurements in new reels and rebuilds.
The reeling nip load profile (Fig. 4) clearly shows the bumps and valleys that derive from variations in the paper thickness profile. The resolution of the profile measurement is excellent, since thousands of paper layers are reeled on top of each other in the parent roll, and thickness variations are cumulated and multiplied in the roll.
Fig. 4 Example of a reeling nip load profile that shows the bumps and valleys in the roll that derive from variations in the paper thickness profile.
The iRoll reel drum, combined with closed loop control for parent roll profiles using calender zone controls, calender induction profilers or coat weight profiles, for example, enables an immediate response to profile-related quality variations and significantly reduces the recovery time after grade changes.
Fig. 5 Bump test to increase roll hardness at the edges with SYM-rolls and iRoll response.
The above graphs in Fig. 5 illustrate a bump test and an iRoll measurement response (increasing roll hardness and diameter) to 50kN/m bumps with a SYM roll. The colour graph shows the iRoll profile map during the parent roll build up. The middle graph shows an individual iRoll profile measured at a paper amount of 122mm. The bottom graph shows the caliper profile measured by a Tapio tester. A 1mm increase in caliper increased the roll hardness at the edges significantly. At the same time the reeling nip load was reduced in the middle area. This shows the extremely high resolution of iRoll measurement: small changes in caliper accumulate into huge changes in reeling nip load profile, because thousands of layers are wound on top of each other.
Fig. 6 Bump test to reduce roll hardness at the edges with SYM-rolls and iRoll response.
The graphs in Fig. 6 illustrate a bump test where an increase in nip load profile (also reducing roll hardness and diameter) was targeted by a 50kN/m bump with a SYM roll. When the linear load was increased on the edge areas by changing the SYM roll load profile, the caliper was reduced. The same change can be seen in the iRoll profile. This time the highest nip load is located in the middle.
Fig. 7 iRoll measurement showing the response of various SYM-profiling shapes
The graphs in Fig. 7 summarise the previous bump tests and also a straight SYM roll profile test. The Tapio RQP hand-held hardness scanner profiles from each test roll are illustrated in the lower graph. The correlation between the iRoll measurement and the Tapio RQP can be clearly seen. The conclusion to be drawn from the test is that it is feasible to control calliper, and thus parent roll profile, with a SYM roll using iRoll profile measurement. This case also shows the effect on reeling broke: the more even the iRoll profile, the fewer reeling defects there are (e.g. crepe wrinkles or glossy areas). With iRoll reel drum measurement, it is possible to control calender zone controls, calender induction profilers, coat weight profiles etc. and thus optimise the efficiency of reeling, winding and paper press room runnability.
In the past, it has been difficult to control the thickness profiles of glossy grades, such as SC, due to difficulties in scanning thickness profile measurements. This has led to runnability problems in reeling and winding, due to bumpy rolls. iRoll reeling nip load profile measurement makes it possible to accurately control parent roll structure and thus ensure good runnability in winding. The Fig. 8 illustrates a basic concept for parent roll profile control. The roll hardness and diameter profile is measured through an iRoll reeling nip profile. The control system calculates the set point for SYM roll zone control and thus optimizes the roll structure. Gloss is controlled in the machine direction by calendering nip loads and in the cross direction by using steam boxes. The system may also be complemented with tension profile measurement. Thus, multivariate control can calculate an optimum for achieving the best combination of parent roll structure and tension profiles.
Fig. 8 Parent roll profile (hardness and diameter) control concept.
Fig. 9 Roll profile control system operator interface in an online calendering SC production line.
The iRoll not only facilitates control, but also reduces reeling problems. Since it monitors the reeling nip load profile at the nip, it reveals force peaks, discontinuities in reel build-up, skewed nip profiles and “carrot-shaped” parent rolls immediately. Force control problems caused by friction and wear in the machine parts are also exposed. This results in less broke from reeling-related quality defects.
Fig. 10 An iRoll reeling nip load profile during a reel build up. A force peak caused by calendering problems is visible at the edge.
The benefits of the iRoll reel drum for papermakers:
- Improved runnability in winders and printing
- Better wind-up control and online condition monitoring of the reeling nip
- Higher end product quality and runnability, a more saleable product
iRoll tension
Another application of the iRoll technology platform is the iRoll tension-measuring roll that monitors the full width tension profile continuously, online, and without using any separate scanning devices. It helps detect uneven tension profiles at an early stage in the paper machine, long before they would be revealed at the printing press. 
Fig 11 A bad tension profile in the parent roll is inherited by the customer rolls, and this leads to slack edges, wrinkles, web breaks, poor roll edge quality, winding problems and customer complaints.
Fig 12 Tension profile measurement after a machine calender.
Like the iRoll reel drum, the iRoll tension-measuring roll can be combined with closed loop control. There are multiple concepts, applicable in different cases, designed to control tension profiles and improve runnability. Two examples of tension profile control are illustrated in this paper.
The first alternative is based on traditional moisture profile control using the press section steambox as an actuator. Moisture profile measurement and control are applied as normal. Tension profile optimization is added to the system in cascade with moisture profile control. The tension profile is measured by an iRoll located in a suitable position and a set point profile is calculated for moisture profile control. The system is simple and easy to tune. Tension profile control has a certain window to operate in, and the moisture profile variation is limited. Thus, the tension profile may be optimized and the moisture profile is kept within its limits.
Fig 13 Cascade control for moisture and tension profiles.
Another possible concept for tension profile control is based on several actuators affecting the moisture and tension profile. The control system can be a true multivariate system with several inputs and outputs, or the tension and moisture profiles may be controlled separately. The Fig. 13 illustrates a system with separate controls. The tension profile is measured in a suitable location and the press section steam box is used for control purposes. The press section steam box responses well to the tension profile. A moisturizer system located at the drying section, for example, is used to adjust the moisture profile. Thus, the control system can operate more freely, with the press section steam box straightening the tension profile that may have caused an error in the moisture profile. The moisturizer, which has a small effect on the tension profile, but a big effect on moisture, is used to adjust the moisture profile.
Fig 14 Separate controls for moisture and tension profiles.
A cascade control system was built in a newsprint production line Fig 14 in control alternative no. 1. The tension profile was measured before the reel. The responses of the different actuators were studied. The responses triggered by the tension profile in the press section steam box, machine calender SYM roll profiling and the machine calender induction profiler could be measured by the iRoll. The press section steam box was chosen as the actuator due to its best response.
The control system was tuned so that it had a certain moisture profile window to operate in. The result was that, in the loose areas of the tension profile, an increase in tension level of up to 50% (100N/m) could be obtained. Generally, the tension profile could be straightened quite significantly within the moisture window. The main challenge in tuning the control system was shrinkage, since this had to be taken into account in finding the correct CD mapping for each profile and actuator.
Fig. 15 The effect of tension profile control.
The benefits of iRoll tension for papermakers:
- Improved paper and board machine runnability
- Improved runnability in winding
- Better printing machine runnability and printability
