Ruzomberok achieves excellent results from new control strategy, Solutions!, Online Exclusives, August 2005, Vol. 88(8)

A machine analysis conducted at the Ruzomberok paper mill in Slovakia found that changes in the calender and in the control strategy could substantially improve the quality of the paper with regard to thickness and gloss. To improve the “2 Sigma” values, the existing control software was changed to Profilmatic, the standard software for all Voith cross-profile control systems. A Voith ModuleTherm was also installed in the calender to further increase quality.

Fig.1 : The rebuilt PM 18 2 Sigma results
Voith Paper Automation achieved above-average 2 Sigma values in the thickness cross-profile of PM 18 in Ruzomberok with the commissioning of the Nipcorect and ModuleTherm with Profilmatic control. The Ruzomberok mill produces photocopy paper.

A machine analysis was conducted at Ruzomberok to increase the quality standards with regard to the paper profile. Technicians found that changes in the calender and in the control strategy could substantially improve the quality of the paper with regard to its thickness and gloss. These quality features then have a positive effect on the final printing properties of the paper. The thickness cross-profile is influenced by a Nipcorect roll in the calender.

For this paper quality, the mill installed an EcoCal hardnip calender after the dryer group and before the reel to optimize the thickness cross-profile. Until then, the Ruzomberok mill avoided profile variations at the edges on PM 18 by using an additional IR edge zone dryer. The previously achieved thickness cross-profiles were to be improved with the aid of a control system further developed by Voith.

In order to improve the 2 Sigma values, the existing control software was changed to Profilmatic, which is the standard software for all Voith cross-profile control systems. A Voith ModuleTherm was also installed in the calender to further increase quality.

Profilmatic as control software
On the basis of the cross-profiles measured by the scanner, Profilmatic calculates optimized control value adjustments for each control zone of the installed actuator system with the objective of minimizing fluctuations in the cross-profile. The papermaker can set the desired nominal profile individually. In this case, Profilmatic controls the Nipcorect roll and the ModuleTherm.
The Profilmatic control principle is optimally adjusted to the Nipcorect roll and ModuleTherm actuator systems. The Voith Profilmatic control software package can be configured for any actuator systems. It permits the effects of a paper machine’s different actuator systems to be adjusted to each other. At Ruzomberok, the adaptation of Profilmatic to the linear force model of the Nipcorect roll led to substantial improvements in the control range.

The control system is designed to compensate long-wave profile disturbances up to 115mm by pistons in the Nipcorect roll. Short-wave profile disturbances up to 38mm are equalized by fine adjustment of the local diameter of the Flexitherm™ roll wire to different zone surface temperatures, generated by the ModuleTherm.

The previously used controller has been replaced by the new Profilmatic. The previously employed IR edge dryer was put out of service and a ModuleTherm actuator was installed in the calender. In the first stage, the Profilmatic control system for the Nipcorect roll was put into operation. Technicians adapted the Profilmatic control system to the Nipcorect roll of the EcoCal calender.

To effectively control the thickness cross-profile, the pistons of the Nipcorect roll are pressed individually from the inside against the rotating roll shell, thereby exerting different pressures on the paper web in the nip of the calender at each control zone. The adjustment is made above and below the mean linear force. This permits an optimum pressure to be exerted on the paper web.

The mill used deliberate test adjustments (bump tests) to determine the local distribution, amplitude and width of the effects on the paper web. The current process model for cross-profile control was determined from this information. The changes in the control variables for each control zone calculated by the Profilmatic system are co-ordinated with a linear force model of the Nipcorect roll. This occurs before the support points in the Nipcorect roll are actually adjusted.

After the control system was switched on, other control parameters were optimized. Very good profiles were achieved just half an hour after activation. At this point, a 2 Sigma value for the thickness of 0.34 µm in relation to the mean value of the reel was achieved without using the ModuleTherm.

This represents an improvement of 32% in comparison with the previously achieved values. The IR edge dryer was still switched on during this time. At this time, the guaranteed 2 Sigma value of 0.45 µm was already substantially improved.

Fig. 4: patented multi-jet, high-velocity design
with direct air impingement

In the second stage, the engineers also put the ModuleTherm control system into operation. The ModuleTherm directs an air jet with a different temperature for each control zone on the surface of the calender roll. The different surface temperature and the resulting, different radial expansion of the roll leads to a differing pressure in the nip of the calender and thereby affects the thickness cross-profile.

Again, the mill conducted test adjustments (bump tests) at the individual control zones to determine the local distribution, amplitude and the width of the effects on the paper web for the process model. After the control system was activated, other control parameters were optimized step by step in dependency on the control behaviour. The IR edge dryer was then switched off. Due to the combination of Nipcorect with ModuleTherm, the ModuleTherm could be operated at a very low mean power of 15 % (70 kW) of the installed power of 66 kW/m or 462 kW total.

Click to view larger image. The ModuleTherm again substantially reduced the 2 Sigma values for the thickness in relation to the mean value of the reel. At the end, the Voith engineers achieved values between 0.28 µm and 0.18 µm. These values represent an excellent result which could not have been achieved until now by any other control strategy.

The significant improvement of the 2 Sigma value for the thickness and the reduction of the energy consumption are achieved by the interaction of the Nipcorect roll, Profilmatic and the ModuleTherm. The combination of these control components can be regarded as the optimum system for thickness cross-profile control.

Because an anti-deflection roll is necessary for a calender under any circumstances, this technique using the Nipcorect roll as an actuator element is immediately appropriate. The employment of up to 60 control zones does not consume additional energy and requires no more installation space. When an additional ModuleTherm is installed in a calender, it is possible that the full complement of 60 control zones in the Nipcorrect roll are not needed.

Fig. 5: Narrow control zones for precise caliper profile control

This successful installation of a new control concept is the result of the combination and interaction of process knowledge and competence in automation in full compliance with the Voith corporate principle of “engineered reliability.”

The successful control concept is supplemented by a service concept directed at the customer’s requirements. Either by on-site assistance or by remote diagnosis and assistance by modem access, Voith service guarantees constant monitoring of the machine and the process and promotes close co-operation with the customer.

For more information, contact Stephanie Weber at Voith Paper, Heidenheim, Germany, via email: s.weber@voith.com; via telephone: +49 7321 37 3858; or via fax: +49 7321 37 7008; or visit www.voith.com.