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Fuzzy Quality Control of a TMP Plant

Submitted by K. Leiviska, 26.06.2001, IBA A

Problem

The quality control in a TMP plant (maintaining freeness and fibre length at optimum levels) has to cope with two different kinds of disturbances: slowly proceeding wearing of refiner plates and faster variations in raw material quality. Information about the pulp quality is usually obtained from laboratory testing of pulp or finished paper samples, so this information is a few hours old and therefore of no use for real-time control. Also, delays in on-line freeness and fibre length measurements have limited the use of automatic on-line quality control.

Generally, refiner control systems are based on the principle that freeness is directly related to specific energy consumption in the process. Motor load usually controls this. The strategy presented here is based on the fact that the refining consistency is another key variable in the TMP-plant control because it affects on how energy is transferred to fibres. When the consistency and motor load are stabilised there exists a good basis for the actual quality control of the TMP-plant. In this case it is done using conventional adaptive controllers.

Solution

The fuzzy quality control is based on on-line measurements of freeness, fibre length and consistency. The fuzzy quality controller works as a master controller adjusting the set points of the motor loads and primary stage consistency. Inputs are the difference between the freeness set point and measurement, consistency set point and mean fibre length measurement. The consistency set point is used as a control input instead of the measurement, because the actual consistency always includes short-term fluctuations around the set point and therefore may cause some unnecessary output changes.

The fuzzy quality controller has been implemented in the mill's automation system using standard fuzzy controller toolbox available in the system. Minimum-maximum method was used in inference and singletons were used for outputs. The controller has been tuned based on experience and also a test simulator was used. The controller has been tuned so that the main control variable is the fibre length. Therefore, the freeness level is allowed to increase, when the motor load control opens the plate position more than the consistency controller requests.

Status and results

The fuzzy quality controller was implemented as a part of AutoTMP control system at four refiner lines at Holmen Paper, Hallstavik mill in Sweden. The control system features the TCA consistency measurement in the blow line. Based on it, the refining consistency and motor load are controlled using adaptive control methods. The fuzzy control strategy maintains the fibre length at an acceptable level and the freeness is controlled to be on a target level.

Operator confidence in the control system has been very high; 95-100 % uptime for the consistency and motor load controls and over 90 % uptime for the fuzzy quality controls have been experienced. The six months follow-up period proved a 20-50 % decrease in the standard deviations of the key quality and process variables.

Adaptivity and portability

The quality controller is an intelligent supervisory controller without adaptive features. It is a part of a commercial control system and using it in another application requires usual fuzzy controller tuning.

More information

Jaakko Myllyneva, Lennart Karlsson, Ismo Joensuu, Fuzzy Quality Control of a TMP Plant. In: Industrial Applications of Soft Computing (K. Leiviska, Ed.). Studies in Fuzziness and Soft Computing, Vol. 71, Springer Verlag, 2001.

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