Improvement of thickness tolerances for a two-stand aluminium cold rolling mill

The implementation of additional control loops to compensate backup roll eccentricity and harmonic hardness variation allows a significant improvement in strip thickness performance.

Control loops (active controllers) for eccentricity and hardness were successfully implemented as an extension to an exist ing two-stand aluminium cold rolling mill.

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Main facts


Industry Aluminium production
Customer Alunorf
Country Germany
Solutions

  • Control loops (active controllers) for eccentricity and hardness compensation
  • Simulation study

Maximizing productivity in cold rolling

An essential quality characteristic in the cold rolling sector is the minimum thickness deviation that can be achieved at final gauge, i.e. from the finishing pass. However, the need to maximize productivity requires that tight thickness tolerances have to be achieved even under sub-optimal mill and entry strip conditions. To meet this requirement there is a need for improved and advanced control concepts.

Process-related disturbances during rolling

The analysis of a two-stand aluminium cold rolling mill has shown that process-related disturbances such as backup roll eccentricities and harmonic strip hardness variations can make it difficult to meet the target thickness tolerance.

Backup roll eccentricity is not only the result of grinding and roll assembly tolerances, but can also be thermally induced during work roll changes and other stoppages.

Strip hardness variation is less well understood, but an earlier ABB study has shown that for thin strip, cyclic hardness variations as small as 2% in the incoming strip can have a large impact on thickness deviation. Such hardness variations can easily occur in aluminium alloys as a result of uneven cooling.

Typically the hardness variation occurs as a soft (or hard) region on the circumference of the coil. The variation is thus synchronous with the rotational speed of the decoiler and the frequency increases steadily even at constant rolling speed, as the decoiler speed rises with decreasing entry coil diameter.


Cyclic hardness variations as small as 2% in the incoming strip can have a large impact on thickness deviation. Such hardness variations can easily occur in aluminium alloys as a result of uneven cooling

Concept for the compensation of harmonic disturbances

The key feature of the selected approach for compensating harmonic disturbances is the use of active controllers which automatically adapt to the changing conditions of the system.

While the use of active controllers to compensate harmonic backup roll eccentricities is well established, applying this concept to the compensation of harmonic hardness variations was a step in a new direction and presented new challenges.

Therefore it was decided to first analyze, verify and optimize these concepts within the framework of a simulation study before implementation on the plant.


Simulation and implementation without affecting operation

Within the framework of the simulation study the new concepts for active compensation of thickness disturbances due to backup roll eccentricities (Rec) and harmonic hardness variations (Hdc) were to be tested and optimized.

For this purpose the two-stand mill, see Fig. 1, including decoiler, coiler and all relevant control loops was modelled and simulated.

The simulation platform in Matlab/Simulink® as used and developed by ABB enables a scalable, modular and dynamic simulation of single and multi-stand rolling mills for the development and analysis of new control concepts.

After the successful simulation study, the control concepts were implemented on the two-stand cold rolling mill.

In observation mode the functions for eccentricity and hardness compensation could be monitored and preoptimized in parallel to normal operation with no effect on the current operation of the mill.

Additional monitoring functions were provided to safeguard against signal errors, e.g. to switch to a backup mode or deactivate the compensation should important signals fail or the evaluation or suppression of harmonic disturbances give implausible results.

If normal operation is detected again, the compensation loops are automatically and bumplessly reactivated.

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Customer experience

After more than six years of successful operation with eccentricity and hardness compensation the customer summarizes the benefits from production point of view as follows:

"The ABB backup roll eccentricity and cyclic hardness variation has successfully corrected two major sources of strip thickness variation on the Alunorf 2-stand cold mill without any negative impact on mill productivity. The system has been virtually maintenance-free, requiring no additional sensors and no operator action.

One limitation of the sys-tem is that it only calculates a parallel position correction for both hydraulic screwdowns. This limits the size of errors that can be corrected, as gross errors in eccentricity or hardness usually have a significant asymmetric component which then leads to a cyclic tilt error. For the production of high quality strip this is not a serious restriction".
"The system has been virtually maintenance-free, requiring no additional sensors and no operator action"

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