U.S. patent application number 10/491831 was filed with the patent office on 2005-01-27 for method and apparatus for controlling the operation of stock preparation of a paper machine.
Invention is credited to Huhtelin, Taisto.
Application Number | 20050016704 10/491831 |
Document ID | / |
Family ID | 8562092 |
Filed Date | 2005-01-27 |
United States Patent
Application |
20050016704 |
Kind Code |
A1 |
Huhtelin, Taisto |
January 27, 2005 |
Method and apparatus for controlling the operation of stock
preparation of a paper machine
Abstract
A method and an apparatus for controlling the operation of stock
preparation of a paper machine for preparing machine stock from
component stocks. The stock preparation includes a plurality of
successive blending points, where the component stocks are blended
with each other, a second raw material of the machine stock is
added to the stock and/or the stock is diluted by blending dilution
water with the stock. The flow and/or consistency of one or more
stocks arriving at a blending point and/or the flow and consistency
of the stock leaving a blending point and/or the concentration of
the second raw material of the machine stock in the stock is
adjusted in such a manner that the flow and/or consistency of the
stock and/or the concentration of the second raw material of the
machine stock in the stock follow target values determined for
them.
Inventors: |
Huhtelin, Taisto; (Tampere,
FI) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 19928
ALEXANDRIA
VA
22320
US
|
Family ID: |
8562092 |
Appl. No.: |
10/491831 |
Filed: |
May 18, 2004 |
PCT Filed: |
October 18, 2002 |
PCT NO: |
PCT/FI02/00811 |
Current U.S.
Class: |
162/198 ;
162/253; 162/258; 162/259 |
Current CPC
Class: |
G05D 11/136 20130101;
D21F 1/66 20130101; D21H 23/06 20130101; D21G 9/0018 20130101 |
Class at
Publication: |
162/198 ;
162/253; 162/258; 162/259 |
International
Class: |
D21F 007/00; D21F
001/06; D21F 001/08 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 19, 2001 |
FI |
20012037 |
Claims
1. A method of controlling the operation of stock preparation of a
paper machine, the stock preparation being configured to produce
machine stock to be fed to the short circulation of the paper
machine either from one or more component stocks by blending them
with each other, and the stock preparation including a plurality of
successive blending points where the component stocks are blended
with each other, a second raw material of the machine stock is
added to the stock and/or the stock is diluted by blending dilution
water with the stock the method comprising determining the
consistency of one or more stocks arriving at a blending point or
the concentration of the second raw material of the machine stock
in the stock arriving at a blending point and by determining the
consistency of the stock leaving a blending point or the
concentration of the second raw material of the machine stock in
the stock leaving a blending point, by determining the flow of one
or more stocks arriving at the blending point and the flow of the
stock OM3) leaving the blending point determining a consistency
prediction for the consistency of one or more stocks arriving at
the blending point or a concentration prediction of the second raw
material of the machine stock in the stock arriving at the blending
point, determining a flow prediction for the flow of the stock
leaving the blending point, determining a consistency target for
the consistency of one or more stocks arriving at the blending
point or a target concentration of the second raw material of the
machine stock in the stock arriving at the blending point and/or by
determining a consistency target for the consistency of the stock
leaving the blending point or the target concentration of the
second raw material of the machine stock in the stock leaving the
blending point, determining a flow target for the flow of one or
more stocks arriving at the blending point and/or a flow target for
the flow of the stock leaving the blending point and adjusting the
flow and/or consistency of one or more stocks arriving at the
blending point and/or the concentration of the second raw material
of the machine stock in the stock arriving at the blending point
based on the flow prediction of the stock leaving the blending
point and/or the consistency prediction of one or more stocks
arriving at the blending point and/or the concentration prediction
of the second raw material of the machine stock in such a manner
that the flow of one or more stocks arriving at the blending point
follows the determined flow target and/or the consistency follows
the determined consistency target and/or the concentration of the
second raw material of the machine stock in the stock follows the
determined target concentration and/or adjusting the flow and
consistency of the stock leaving the blending point and/or the
concentration of the second raw material of the machine stock in
the stock leaving the blending point based on the flow prediction
of the stock leaving the blending point and/or the consistency
prediction of one or more stocks arriving at the blending point
and/or the concentration prediction of the second raw material of
the machine stock in such a manner that the flow of the stock
leaving the blending point follows the determined flow target and
the consistency follows the determined consistency target and/or
the concentration of the second raw material of the machine stock
in the stock (KM, OM1, OM2, OM3) follows the determined target
concentration.
2. A method as claimed in claim 1, wherein the second raw material
of the machine stock is a filler, additive or chemical.
3. A method as claimed in claim 1, comprising controlling the
machine stock flow from a blending/machine chest of the stock
preparation by determining a machine stock flow control message
based on the machine stock consistency prediction, the machine
stock flow, a machine stock flow control set value and a machine
stock fiber flow target value trajectory, and by controlling the
machine stock flow from the blending/machine chest based on said
machine stock flow control message.
4. A method as claimed in claim 3, comprising determining the
machine stock flow prediction based on the measured machine stock
flow and the machine stock control message and controlling the
dosing of the component stocks and/or the second raw material of
the machine stock based on the machine stock flow prediction.
5. A method as claimed in claim 4, comprising determining the
component stock total flow target based on measured component stock
flows, a blending/machine chest surface level trajectory and the
machine stock flow prediction.
6. A method as claimed in claim 5, comprising determining a
consistency prediction for each component stock based on a
component stock chest, output consistency prediction and the
measured component stock consistency.
7. A method as claimed in claim 6, comprising determining a flow
target for each component stock based on the consistency prediction
of each component stock and the component stock total flow
target.
8. A method as claimed in claim 7, comprising controlling the
dosing of each component stock from the component stock chest by
determining a component stock flow control message based on the
flow of the component stock from the component stock chest, a
component stock flow control set value and the component stock flow
target and controlling the dosing of the component stock from the
component stock chest based on said component stock flow control
message.
9. A method as claimed in claim 8, comprising determining the flow
prediction of the component stock flowing from the component stock
chest based on the component stock flow control message and the
measured component stock flow.
10. A method as claimed in claim 9, comprising controlling the
dosing of the component stock from a stock tower based on the
component stock flow prediction.
11. A method as claimed in claim 9, comprising controlling the
dilution of the component stock flowing from the component stock
chest with dilution water by determining a dilution water flow
control message based on the component stock chest output
consistency prediction, the component stock consistency target
value trajectory, the measured dilution water flow, a dilution
water flow control set value and the component stock flow
prediction and controlling the dilution water flow based on the
determined dilution water flow control message.
12. A method as claimed in claim 11, comprising determining a
dilution water flow prediction based on the dilution water flow
control message and the measured dilution water flow.
13. A method as claimed in claim 12, comprising controlling the
dosing of the component stock from the stock tower of the stock
preparation based on the dilution water flow prediction.
14. A method as claimed in claim 13, comprising controlling the
dosing of the component stock from the stock tower by by
determining a control message for the component stock flowing from
the stock tower to the component stock chest based on the flow of
the component stock flowing from the stock tower, a component stock
flow control set value, a flow prediction for the component stock
leaving the component stock chest and the component stock chest
surface level target value trajectory and by controlling the dosing
of the component stock from the stock tower based on the control
message of the component stock flowing from the stock tower to the
component stock chest.
15. A method as claimed in claim 14, comprising determining the
flow prediction for the component stock flowing from the stock
tower to the component stock chest based on the control message of
the component stock flowing from the stock tower and the measured
flow of the component stock flowing from the stock tower.
16. A method as claimed in claim 15, comprising controlling the
dilution of the component stock flowing from the stock tower to the
component stock chest with dilution water to be mixed with the
component stock by determining the dilution water flow control
message based on a consistency prediction for the component stock
flowing from the stock tower, a consistency target value trajectory
for the component stock flowing from the stock tower, the measured
dilution water flow, a dilution water flow control set value and
the flow prediction for the component stock flowing from the stock
tower and by controlling the dilution water flow based on the
determined dilution water flow control message.
17. A method as claimed in claim 1, wherein the consistency of one
or more stocks arriving at the blending point or leaving the
blending point or the concentration of the second raw material of
the machine stock in the stock is determined by measuring or
indirectly by using a process model descriptive of the operation of
the stock preparation or a part thereof.
18. A method as claimed in claim 1, wherein the flow of one or more
stocks arriving at the blending point or leaving the blending point
is determined by measuring or indirectly by using a process model
descriptive of the operation of the stock preparation or a part
thereof.
19. A method as claimed in claim 1., comprising a control message
including several future control steps for the flow of the machine
stock, a component stock or the second raw material of the machine
stock, a flow target value trajectory for the flow of the machine
stock, a component stock or the second raw material of the machine
stock, a blending/machine chest or component stock chest surface
level target value trajectory and/or a control message including
several future control steps of the dilution water flow by using a
model predictive control method comprising a process model
descriptive of the stock preparation or a part thereof and
optimization, so that the cost function associated with the
optimization is minimized.
20. A method as claimed in claim 17, wherein the process model is a
dynamic process model.
21. An apparatus for controlling the operation of stock preparation
of a paper machine, the stock preparation being configured to
produce machine stock to be fed to the short circulation of the
paper machine either from one or more component stocks by blending
them with each other, and the stock preparation including a
plurality of successive blending points where the component stocks
are blended with each other, a second raw material of the machine
stock is added to the stock and/or the stock is diluted by blending
dilution water with the stock, in that the apparatus being
configured to determine the consistency of one or more stocks
arriving at a blending point or the concentration of the second raw
material of the machine stock in the stock arriving at a blending
point and to determine the consistency of the stock leaving a
blending point (DP4, DP6) or the concentration of the second raw
material of the machine stock in the stock leaving a blending
point, to determine the flow of one or more stocks arriving at the
blending point and the flow of the stock leaving the blending
point, to determine a consistency prediction for the consistency of
one or more stocks arriving at the blending point or a
concentration prediction of the second raw material of the machine
stock in the stock arriving at the blending point, to determine a
flow prediction for the flow of the stock leaving the blending
point, to determine a consistency target for the consistency of one
or more stocks arriving at the blending point or a target
concentration of the second raw material of the machine stock in
the stock arriving at the blending point and/or to determine a
consistency target for the consistency of the stock leaving the
blending point or a target concentration for the second raw
material of the machine stock in the stock leaving the blending
point, to determine a flow target for the flow of one or more
stocks arriving at the blending point and/or a flow target for the
flow of the stock leaving the blending point and to adjust the flow
and/or consistency of one or more stocks arriving at the blending
point and/or the concentration of the second raw material of the
machine stock in the stock arriving at the blending point based on
the flow prediction of the stock leaving the blending point and/or
the consistency prediction of one or more stocks arriving at the
blending point and/or the concentration prediction of the second
raw material of the machine stock in such a manner that the flow of
one or more stocks arriving at the blending point follows the
determined flow target and/or the consistency follows the
determined consistency target and/or the concentration of the
second raw material of the machine stock in the stock follows the
determined target concentration and/or to adjust the flow and
consistency of the stock leaving the blending point and/or the
concentration of the second raw material of the machine stock in
the stock leaving the blending point based on the flow prediction
of the stock leaving the blending point and/or the consistency
prediction of one or more stocks arriving at the blending point
and/or the concentration prediction of the second raw material of
the machine stock in such a manner that the flow of the stock
leaving the blending point follows the determined flow target
(KA4FFTr, and the consistency follows the determined consistency
target and/or the concentration of the second raw material of the
machine stock in the stock follows the determined target
concentration.
22. An apparatus as claimed in claim 21, wherein the second raw
material of the machine stock is a filler, additive or
chemical.
23. An apparatus as claimed in claim 21 or 22, wherein the
apparatus is configured to control the machine stock flow from the
blending/machine chest of the stock preparation in such a manner
that the apparatus is configured to determine a machine stock flow
control message based on the machine stock consistency prediction,
the machine stock flow, a machine stock flow control set value and
a machine stock fiber flow target value trajectory, and to control
the machine stock flow from the blending/machine chest based on
said machine stock flow control message.
24. An apparatus as claimed in claim 23, wherein the apparatus is
configured to determine the machine stock flow prediction based on
the measured machine stock flow and the machine stock control
message and to control the dosing of the component stocks and/or
the second raw material of the machine stock based on the machine
stock flow prediction.
25. An apparatus as claimed in claim 24, wherein the apparatus is
configured to determine the component stock total flow target based
on measured component stock flows, a blending/machine chest surface
level target value trajectory and the machine stock flow
prediction.
26. An apparatus as claimed in claim 25, wherein the apparatus is
configured to determine a consistency prediction for each component
stock based on a component stock chest output consistency
prediction and the measured component stock consistency.
27. An apparatus as claimed in claim 26, wherein the apparatus is
configured to determine a flow target for each component stock
based on the consistency prediction of each component stock and the
component stock total flow target.
28. An apparatus as claimed in claim 27, wherein the apparatus is
configured to control the dosing of each component stock from the
component stock chest by determining a component stock flow control
message based on the flow of the component stock from the component
stock chest, a component stock flow control set value and the
component stock flow target and by controlling the dosing of the
component stock from the component stock chest based on said
component stock flow control message.
29. An apparatus as claimed in claim 28, wherein the apparatus is
configured to determine the flow prediction of the component stock
flowing from the component stock chest based on the component stock
flow control message and the measured component stock flow.
30. An apparatus as claimed in claim 29, wherein the apparatus is
configured to control the dosing of the component stock from a
stock tower based on the component stock flow prediction.
31. An apparatus as claimed in claim 29, wherein the apparatus is
configured to control the dilution of the component stock flowing
from the component stock chest with dilution water by determining a
dilution water flow control message based on the component stock
chest output consistency prediction, the component stock
consistency target value trajectory, the measured dilution water
flow, a dilution water flow control set value and the component
stock flow prediction and by controlling the dilution water flow
based on the determined dilution water flow control message.
32. An apparatus as claimed in claim 31, wherein the apparatus is
configured to determine a dilution water flow prediction based on
the dilution water flow control message and the measured dilution
water flow.
33. An apparatus as claimed in claim 32, wherein the apparatus is
configured to control the dosing of the component stock from the
stock tower of the stock preparation based on the dilution water
flow prediction.
34. An apparatus as claimed in claim 33, wherein the apparatus is
configured to control the dosing of the component stock from the
stock towers by by determining a control message for the component
stock flowing from the stock tower to the component stock chest
based on the flow of the component stock flowing from the stock
tower, a component stock flow control set value, a flow prediction
for the component stock leaving the component stock chest and the
component stock chest surface level target value trajectory and by
controlling the dosing of the component stock from the stock tower
based on the control message of the component stock flowing from
the stock tower to the component stock chest.
35. An apparatus as claimed in claim 34, wherein the apparatus is
configured to determine the flow prediction for the component stock
flowing from the stock tower to the component stock chest based on
the control message of the component stock flowing from the stock
tower and the measured flow of the component stock flowing from the
stock tower.
36. An apparatus as claimed in claim 35, wherein the apparatus is
configured to control the dilution of the component stock flowing
from the stock tower to the component stock chest with dilution
water to be mixed with the component stock by by determining the
dilution water flow control message based on a consistency
prediction for the component stock flowing from the stock tower, a
consistency target value trajectory for the component stock flowing
from the stock tower, the measured dilution water flow, a dilution
water flow control set value and the flow prediction for the
component stock flowing from the stock tower and by controlling the
dilution water flow based on the determined dilution water flow
control message.
37. An apparatus as claimed in claim 21, wherein the apparatus is
configured to determine the consistency of one or more stocks
arriving at the blending point or leaving the blending point or the
concentration of the second raw material of the machine stock in
the stock by measuring or indirectly by using a process model
descriptive of the operation of the stock preparation or a part
thereof.
38. An apparatus as claimed in claim 21, wherein the apparatus is
configured to determine the flow of one or more stocks arriving at
the blending point or leaving the blending point by measuring or
indirectly by using a process model descriptive of the operation of
the stock preparation or a part thereof.
39. An apparatus as claimed in claim 21, wherein the apparatus is
configured to determine a control message including several future
control steps for the flow of the machine stock a component stock
or the second raw material of the machine stock, a flow target
value trajectory for the flow of the machine stock, a component
stock or the second raw material of the machine stock, a
blending/machine chest or component stock chest surface level
target value trajectory and/or a control message including several
future control steps of the dilution water flow by using a model
predictive control method comprising a process model descriptive of
the stock preparation or a part thereof and optimization, so that
the cost function associated with the optimization is
minimized.
40. An apparatus as claimed in claim 37, wherein the process model
is a dynamic process model.
41. A method of controlling the operation of stock preparation of a
paper machine, the stock preparation being configured to produce
machine stock to be fed to the short circulation of the paper
machine either from one or more component stocks by blending them
with each other, and in which stock preparation a second raw
material of the machine stock is added to the stock and/or the
stock is diluted by blending dilution water with the stock by the
method comprising determining flow data and a flow prediction for
the machine stock, determining flow data and a flow prediction for
one or more component stocks, transferring the flow data and flow
prediction of a component stock along the dosing line of the stock
preparation backwards in such a manner that adjustments controlling
the stock flows forward along the dosing line utilize predicted
future flow changes, determining consistency data and a consistency
prediction of one or more component stocks and/or the concentration
of the second raw material of the machine stock in the stock and a
concentration prediction in the stock and transferring the
consistency data and consistency prediction of a component stock
and/or the concentration of the second raw material of the machine
stock in the stock and a concentration prediction in the stock
forward along the dosing line in the stock preparation and
adjusting the flow and consistency of the stock flowing forward in
the dosing line of the stock preparation and/or the concentration
of the second raw material of the machine stock in the stock in
such a manner that the adjustments controlling the consistency of
the stock or the concentration of the second raw material of the
machine stock in the stock utilize the predicted future consistency
changes or the changes in the concentration of the second raw
material of the machine stock in the stock.
42. A method as claimed in claim 41, wherein the second raw
material of the machine stock is a filler, additive or chemical.
Description
[0001] The invention relates to method of controlling the operation
of stock preparation of a paper machine, the stock preparation
being configured to produce machine stock to be fed to the short
circulation of the paper machine either from one or more component
stocks by blending them with each other, and the stock preparation
including a plurality of successive blending points, where the
component stocks are blended with each other, a second raw material
of the machine stock is added to the stock and/or the stock is
diluted by blending dilution water with the stock, and in which
method the flow and/or consistency of one or more stocks arriving
at a blending point and/or the concentration of the second raw
material of the machine stock in the stock arriving at a blending
point is adjusted and/or the flow and consistency of the stock
leaving a blending point and/or the concentration of the second raw
material of the machine stock in the stock leaving a blending point
is adjusted.
[0002] The invention further relates to an apparatus for
controlling the operation of stock preparation of a paper machine,
the stock preparation being configured to produce machine stock to
be fed to the short circulation of the paper machine either from
one or more component stocks by blending them with each other, and
the stock preparation including a plurality of successive blending
points, where the component stocks are blended with each other, a
second raw material of the machine stock is added to the stock
and/or the stock is diluted by blending dilution water with the
stock, the apparatus being configured to adjust the flow and/or
consistency of one or more stocks arriving at a blending point
and/or the concentration of the second raw material of the machine
stock in the stock arriving at a blending point and/or to adjust
the flow and consistency of the stock leaving a blending point
and/or the concentration of the second raw material of the machine
stock in the stock leaving a blending point.
[0003] The invention also relates to a method of controlling the
operation of stock preparation of a paper machine, the stock
preparation being configured to produce machine stock to be fed to
the short circulation of the paper machine either from one or more
component stocks by blending them with each other, and in which
stock preparation a second raw material of the machine stock is
added to the stock and/or the stock is diluted by blending dilution
water with the stock, and in which method the flow and consistency
of the stock flowing forward in the dosing line of the stock
preparation and/or the concentration of the second raw material of
the machine stock in the stock are adjusted.
[0004] In papermaking, the measured basis weight or variables
derived from it, such as the air-dry or completely dry basis
weight, of a paper web being made are adjusted by controlling the
dosing of the paper stock or paper pulp being transferred to the
short circulation of the paper machine from the stock preparation
of the paper machine. The stock fed into the short circulation of
the paper machine is typically called machine stock. Since the
quality and amount of machine stock produced cannot be made so even
that the stock could be led from the manufacturing equipment
directly to the paper machine, the stock preparation comprises a
plurality of different storage and intermediate chests. The various
component stocks included in the machine stock, i.e. stocks
containing different kinds of fibers, constitute the first raw
material of the machine stock, i.e. the fibrous raw material of the
machine stock. Different fillers, additives or chemicals added to
the machine stock or the component stocks constitute the second raw
material of the machine stock. These different fillers, additives
and chemicals are used to improve the quality and printability of
finished paper or the operability of the manufacturing process.
Typically, the component stocks, fillers, additives and chemicals
are stored in large storage chests. The composition of the machine
stock conveyed to the paper machine is adjusted in a dosing system
at the stock preparation, where the different stock components
included in the stock are blended with each other both in a pipe
leading to the blending chest and in the blending chest itself,
from where the stock is conveyed to the machine chest and from
there further to the short circulation of the paper machine. The
consistency of the machine stock conveyed to the short circulation
is typically kept at three percent. Since the consistency of the
component stocks stored in storage chests is usually 10 to 14%, and
the consistency of repulped stock is usually about 5%, the
consistency of the different component stocks and, if need be, that
of the blended stock are diluted by addition of water, which is
typically white water separated from the short circulation of the
paper machine. Accordingly, the consistency of the stock to be fed
into the paper machine is adjusted by changing the amount of
dilution water fed into the stock, i.e. the adjustment of the
consistency of the stock always relates to the addition of dilution
water to the stock in a suitable ratio to the amount and
consistency of the stock.
[0005] The basis weight of the paper web to be made is adjusted by
changing the fiber flow conveyed to the paper machine. In practice,
the basis weight is adjusted by changing the flow of machine stock.
Since the basis weight adjustment is unaware of future variations
in stock consistency, consistency variations can be eliminated for
instance by including, in the machine stock flow request, an
additional specification `at 3% consistency` relating to the
consistency of the stock to be fed, i.e. the desired consistency of
the machine stock to be fed is 3%. If the measured consistency
deviates from the target, the flow target is amended respectively.
The desired fiber flow is thus conveyed to the paper machine. The
basis weight adjustment requests for the necessary amount of fiber
flow or machine stock flow from the machine chest in the stock
preparation, the intent being to keep a constant amount of stock
therein at all times. The change in the machine stock flow caused
by a change in the basis weight, i.e. a flow disturbance, travels
from the paper machine towards the storage towers for the component
stocks, the flow disturbance being strengthened further by the
action of the adjustment of the surface level in each intermediate
chest in the dosing line. Because the flow disturbances are strong
and rapid, the consistency adjustments are unable to keep up,
causing consistency disturbances that proceed along with the stock
flow towards the paper machine. Because of the large volume of the
intermediate chests and the significant length of the dosing line,
the process involves long delays, wherefore the stock preparation
adjustment is extremely sensitive to variations both in the
consistency of the component stocks and in the concentrations of
fillers, additives and chemicals, which, in turn, easily lead to
retention variation in the wire section of the paper machine.
Retention variation in the wire section also causes changes in the
ash content and basis weight of the paper web. Accordingly, the
flow disturbance caused by a change in the basis weight first
proceeds as a flow disturbance from the machine chest through the
dosing chests towards the stock towers and returns as a consistency
disturbance through the dilution steps in the dosing line to the
machine stock and further all the way to the basis weight of the
paper. The basis weight is measured at the dry end of the paper
machine immediately before the web is reeled into a machine roll,
whereby a basis weight error detected in the measurement causes a
new change, i.e. a flow disturbance, by changing the machine stock
flow. As a result, a state of vibration, which is difficult to
manage, is created, during which paper or board having the wrong
basis weight and ash content is produced. This vibration also
causes other disturbances in the operation of the process via
dilution lines, for example.
[0006] The above-described dosing solution based on the blending
capacity of the machine chest and the blending chest is not the
only usable solution. Component stock dosing and blending can also
be solved in other ways. For example, the machine chest and the
blending chest may be two successive blending chests and the
machine chest as a third chest, whereby blending is believed to be
under still better control. If the consistency and other properties
of the component stocks are well controlled, one chest may be
sufficient. In new solutions, the aim is to blend the component
stocks with each other in a separate blending device in the short
circulation, whereby the process does not include any blending
chest or machine chest.
[0007] At present, stock preparation dosing is controlled by
adjusting the surface levels of the different stock chests and the
consistencies and flow rates of the stock flows at different points
of the process with unit controllers based on feed forward
coupling, examples of which are the method for regulating the
surface level and the consistency in a stock chest for a component
stock disclosed in U.S. Pat. No. 6,210,529, and the method for
regulating the basis weight of paper or board by dosing component
stocks disclosed in U.S. Pat. No. 6,203,667, both methods utilizing
feed forward coupling to adjust the process. However, the use of
feed forward coupling in the adjustment is problematic, since when
feed forward coupling is used, the action of adjustment changes on
the process part succeeding the controller cannot be taken into
account.
[0008] The object of the present invention is to provide a new type
of solution for controlling the operation of stock preparation.
[0009] The method of the invention is characterized by determining
the consistency of one or more stocks arriving at a blending point
or the concentration of the second raw material of the machine
stock in the stock arriving at a blending point and by determining
the consistency of the stock leaving a blending point or the
concentration of the second raw material of the machine stock in
the stock leaving a blending point, by determining the flow of one
or more stocks arriving at the blending point and the flow of the
stock leaving the blending point, by determining a consistency
prediction for the consistency of one or more stocks arriving at
the blending point or a concentration prediction of the second raw
material of the machine stock in the stock arriving at a blending
point, by determining a flow prediction for the flow of the stock
leaving the blending point, by determining a consistency target for
the consistency of one or more stocks arriving at the blending
point or a target concentration of the second raw material of the
machine stock in the stock arriving at the blending point and/or by
determining a consistency target of the consistency of the stock
leaving the blending point or the target concentration of the
second raw material of the machine stock in the stock leaving the
blending point, by determining a flow target for the flow of one or
more stocks arriving at the blending point and/or a flow target for
the flow of the stock leaving the blending point, and by adjusting
the flow and/or consistency of one or more stocks arriving at the
blending point and/or the concentration of the second raw material
of the machine stock in the stock arriving at the blending point
based on the flow prediction of the stock leaving the blending
point and/or the consistency prediction of one or more stocks
arriving at the blending point and/or the concentration prediction
of the second raw material of the machine stock in such a manner
that the flow of one or more stocks arriving at the blending point
follows the determined flow target and/or the consistency follows
the determined consistency target and/or the concentration of the
second raw material of the machine stock in the stock follows the
determined target concentration and/or by adjusting the flow and
consistency of the stock leaving the blending point and/or the
concentration of the second raw material of the machine stock in
the stock leaving the blending point based on the flow prediction
of the stock leaving the blending point and/or the consistency
prediction of one or more stocks arriving at the blending point
and/or the concentration prediction of the second raw material of
the machine stock in such a manner that the flow of the stock
leaving the blending point follows the determined flow target and
the consistency follows the determined consistency target and/or
the concentration of the second raw material of the machine stock
in the stock follows the determined target concentration.
[0010] The apparatus of the invention is characterized in that the
apparatus is configured to determine the consistency of one or more
stocks arriving at a blending point or the concentration of the
second raw material of the machine stock in the stock arriving at a
blending point and to determine the consistency of the stock
leaving a blending point or the concentration of the second raw
material of the machine stock in the stock leaving a blending
point, to determine the flow of one or more stocks arriving at the
blending point and the flow of the stock leaving the blending
point, to determine a consistency prediction of the consistency of
one or more stocks arriving at the blending point or a
concentration prediction of the second raw material of the machine
stock in the stock arriving at the blending point, to determine a
flow prediction for the flow of the stock leaving the blending
point, to determine a consistency target for the consistency of one
or more stocks arriving at the blending point or a target
concentration of the second raw material of the machine stock in
the stock arriving at the blending point and/or to determine a
consistency target for the consistency of the stock leaving the
blending point or a target concentration for the second raw
material of the machine stock in the stock leaving the blending
point, to determine a flow target for the flow of one or more
stocks arriving at the blending point and/or a flow target for the
flow of the stock leaving the blending point and to adjust the flow
and/or consistency of one or more stocks arriving at the blending
point and/or the concentration of the second raw material of the
machine stock in the stock arriving at the blending point based on
the flow prediction of the stock leaving the blending point and/or
the consistency prediction of one or more stocks arriving at the
blending point and/or the concentration prediction of the second
raw material of the machine stock in such a manner that the flow of
one or more stocks arriving at the blending point follows the
determined flow target and/or the consistency follows the
determined consistency target and/or the concentration of the
second raw material of the machine stock in the stock follows the
determined target concentration and/or to adjust the flow and
consistency of the stock leaving the blending point and/or the
concentration of the second raw material of the machine stock in
the stock leaving the blending point based on the flow prediction
of the stock leaving the blending point and/or the consistency
prediction of one or more stocks arriving at the blending point
and/or the concentration prediction of the second raw material of
the machine stock in such a manner that the flow of the stock
leaving the blending point follows the determined flow target and
the consistency follows the determined consistency target and/or
the concentration of the second raw material of the machine stock
in the stock follows the determined target concentration.
[0011] The method of the invention for adjusting the flow and
consistency of the stock leaving the blending point and/or the
concentration of the second raw material of the machine stock in
the stock is further characterized by determining flow data and a
flow prediction for the machine stock, determining flow data and a
flow prediction for one or more component stocks, transferring the
flow data and flow prediction of a component stock along the dosing
line of the stock preparation backwards in such a manner that
adjustments controlling the stock flows forward along the dosing
line utilize predicted future flow changes, determining consistency
data and a consistency prediction of one or more component stocks
and/or the concentration of the second raw material of the machine
stock in the stock and a concentration prediction in the stock, and
transferring the consistency data and consistency prediction of a
component stock and/or the concentration of the second raw material
of the machine stock in the stock and a concentration prediction in
the stock forward along the dosing line in the stock preparation in
such a manner that the adjustments controlling the consistency of
the stock or the concentration of the second raw material of the
machine stock in the stock utilize the predicted future consistency
changes or the changes in the concentration of the second raw
material of the machine stock in the stock.
[0012] An essential idea of the invention is to control the
operation of the stock preparation of a paper machine, the stock
preparation being adapted to produce machine stock to be fed into
the short circulation of the paper machine from either one
component stock or several component stocks by blending them with
each other and comprising a plurality of successive blending points
where the component stocks are blended with each other, the second
raw material of the machine stock is added to the stock and/or the
stock is diluted by mixing dilution water to the stock, by
adjusting the flow and/or consistency of one or more stocks
arriving at a blending point and/or the concentration of the second
raw material of the machine stock in the stock arriving at a
blending point and/or by adjusting the flow and consistency of the
stock leaving a blending point and/or the concentration of the
second raw material of the machine stock in the stock leaving a
blending point. The essential idea comprises determining the
consistency of one or more stocks arriving at the blending point or
the concentration of the second raw material of the machine stock
in the stock arriving at the blending point, determining the
consistency of the stock leaving the blending point or the
concentration of the second raw material of the machine stock in
the stock leaving the blending point, determining the flow of one
or more stocks arriving at the blending point and the flow of the
stock leaving the blending point. The essential idea further
comprises determining a consistency prediction for the consistency
of one or more stocks arriving at the blending point or a
prediction for the concentration of the second raw material of the
machine stock in the stock arriving at the blending point,
determining a flow prediction for the flow of the stock leaving the
blending point, determining a consistency target for the
consistency of one or more stocks arriving at the blending point or
a target concentration of the second raw material of the machine
stock in the stock arriving at the blending point and/or
determining a consistency target for the consistency of the stock
leaving the blending point or the target concentration of the
second raw material of the machine stock in the stock leaving the
blending point and determining a flow target for the flow of one or
more stocks arriving at the blending point and/or a flow target for
the flow of the stock leaving the blending point. The essential
idea further comprises adjusting the flow and/or consistency of one
or more stocks arriving at the blending point and/or the
concentration of the second raw material of the machine stock in
the stock arriving at the blending point in such a manner that the
flow of one or more stocks arriving at the blending point follows
the determined flow target and/or the consistency follows the
determined consistency target and/or the concentration of the
second raw material of the machine stock in the stock follows the
determined target concentration and/or adjusting the flow and
consistency of the stock leaving the blending point and/or the
concentration of the second raw material of the machine stock in
the stock leaving the blending point in such a manner that the flow
of the stock leaving the blending point follows the determined flow
target and the consistency follows the determined consistency
target and/or the concentration of the second raw material of the
machine stock in the stock follows the determined target
concentration. According to the essential idea of the invention, a
predicted consistency change can be used instead of a consistency
prediction, and a predicted concentration change can be used
instead of a prediction for the concentration of the second raw
material of the machine stock in the stock. In a preferred
embodiment of the invention, a model predictive control method is
used for controlling the operation of the stock preparation,
comprising a process model descriptive of the process or a part
thereof and optimization in such a manner that the cost function
associated with the optimization is minimized for optimal control
of the operation of the stock preparation. According to a second
preferred embodiment of the invention, dynamic process models are
used as process models.
[0013] An advantage of the invention is that the stock preparation
is rapidly and exactly able to respond in different states of paper
machine production changes, such as paper web breaks, paper machine
start-up, grade changes and speed changes. The solution presented
eliminates the presently very common vibrations in stock
preparation flows, surface levels, consistencies and concentrations
and, consequently, the effect of these disturbances on paper
quality, and enables a much more accurate adjustment in time than
methods being used at present.
[0014] The solution of the invention is quite similar in the
production of board and soft tissue, and, consequently, in the
present description, the term `paper` refers not only to paper, but
also to board and soft tissue.
[0015] The invention will be described in detail in the attached
drawings, in which
[0016] FIG. 1 schematically shows a stock preparation department in
a paper machine,
[0017] FIG. 2 schematically shows the operational principle of
controlling a machine stock flow,
[0018] FIG. 3 schematically shows the principle of determining the
total amount of the flow of component stocks arriving at a
blending/machine chest,
[0019] FIG. 4 schematically shows the principle of controlling
component stock dosing from a component stock chest,
[0020] FIG. 5 schematically shows the principle of diluting a
component stock after the component stock chest,
[0021] FIG. 6 schematically shows the principle of dosing a
component stock from a stock tower,
[0022] FIG. 7 schematically shows the dilution of a component stock
after a stock tower,
[0023] FIG. 8 schematically shows the calculation of a component
stock consistency prediction,
[0024] FIG. 9 schematically shows the determination of a chest
output flow without a dilution step,
[0025] FIG. 10 schematically shows the modelling of a dilution
step,
[0026] FIG. 11 schematically shows dosing into a blending
chest,
[0027] FIGS. 12 and 13 schematically show the modelling of a flow
into a chest, and
[0028] FIG. 14 schematically shows the principle of the adjustment
solution used in the solution of the invention.
[0029] FIG. 1 schematically shows a stock preparation department or
stock production and dosing line in a paper machine. FIG. 1 shows
the paper machine 8 very schematically way by means of a
rectangular box. In the stock preparation of FIG. 1, machine stock
KM to be fed to the paper machine 8 is composed of three component
stocks OM1, OM2 and OM3, which are mixed with each other. For the
sake of clarity, the dosing line of only the first component stock
OM1 is shown in its entirety. The dosing lines of the second
component stock OM2 and the third component stock OM3 are
substantially similar. The dosing line for component stock OM1
includes a stock tower 1 acting as the storage chest for component
stock OM1. From the stock tower 1, component stock OM1 is fed with
a first pump P1 along a feeding pipe 2 to a component stock chest 3
acting as a dosing chest. From the component stock chest 3,
component stock OM1 is fed with a second pump P2 along a dosing
pipe 4 to a main line 6 in the stock preparation, leading to a
blending/machine chest 5, to which main line 6 components stocks
OM2 and OM3 are led in the same way. The component stocks OM1, OM2
and OM3 start to blend with each other in the main line 6, but more
efficient blending of the component stocks OM1, OM2 and OM3 occurs
only in the blending/machine chest 5, where efficient blenders are
used to blend the component stocks OM1, OM2 and OM3 with each
other. From the blending/machine chest 5, the machine stock KM
composed of the component stocks OM1, OM2 and OM3 is fed with a
third pump P3 along a machine stock dosing pipe 7 to the short
circulation of the paper machine 8 and further to the headbox for
feeding the paper stock to the wire section of the paper machine 8.
The stock preparation of FIG. 1 includes three component stocks to
be blended with each other, but it is evident that the number of
component stocks used in the production of a paper web may vary
such that one or more component stocks are used in the production
of the web. Typically, 2 to 6 component stocks are used in the
production. Furthermore, FIG. 1 shows the blending chest and the
machine chest as a combined blending/machine chest 5, but they may
be, and usually are, physically entirely separate chests.
[0030] The consistency of the paper stock fed into the wire section
of a paper machine typically varies between 0.3 and 1.5%. At an
upper section 1a of the stock tower 1, whereto new component stock
OM1 is fed, the consistency of component stock OM1 is typically 10
to 14%. Thus, component stock OM1 has to be diluted before being
pumped to the paper machine 8. The component stocks OM1, OM2 and
OM3 are diluted by addition of dilution water into the stock in
such a manner that the consistency of the machine stock KM to be
fed in due course into the short circulation is about 3%. As
dilution water is typically used white water, which is separated
from the short circulation of the paper machine 8 and from which
fibers and fine matter and ash are usually removed with a disc
filter. The component stocks are diluted in several steps. FIG. 1
shows the dilution of component stock OM1 with dilution water fed
immediately after the stock tower 1 at a blending point DP6 to the
suction side of the first pump P1 via an adjusting valve V6 and a
dilution water duct DW6. At this point, the consistency of the
stock is diluted from a consistency level of 10 to 14% to a level
of 5 to 6%. After the component stock chest 3, component stock OM1
is further diluted with dilution water fed at a blending point DP4
to the suction side of the second pump P2 via an adjusting valve V4
and a dilution water duct DW4, typically to a level of 3.2 to 3.5%.
The component stock dosing line may comprise a plurality of
successive component stock chests and, after them, blending points,
but for the sake of clarity FIG. 1 only shows one component stock
chest 3. One more stock dilution step is usually arranged between
the physically separate blending and machine chests. Component
stock OM1 can also be diluted in a lower section 1b of the stock
tower 1 by recycling the stock and adding dilution water to
component stock OM1 at a blending point DP7 via an adjusting valve
V7 and a dilution water duct DW7.
[0031] The size of the stock tower 1, the component stock chest 3
and the blending/machine chest 5 depends on the production capacity
of the paper machine 8 and the paper qualities produced with it,
wherefore the size of the chests may vary significantly. When the
same paper grade is manufactured at all times, larger chests are
used than when the paper grade changes very often. In newspaper
mills, large stock towers 1 and component stock chests 3 are
typically used. In this case, the volume of the stock tower may be
up to thousands of cubic meters. In a fine paper mill manufacturing
a plurality of paper grades, the stock tower 1 or the component
stock chest 3 may have a volume of some tens of cubic meters only.
The stock tower 1 is usually considerably larger than the component
stock chest 3 and the blending/machine stock chest 5.
[0032] When the basis weight of paper is being adjusted, a basis
weight adjustment unit 9 requests for the necessary fiber flow or
machine stock KM flow. Since the desired grammage is generated from
the fiber flow in the paper machine, the solution of the basis
weight adjustment is based on equation
MS.multidot.L.multidot.BW=F.multidot.Cs.multidot.k, (1)
[0033] wherein MS is machine speed at reeler [m/s],
[0034] L is web width at reeler [m],
[0035] BW is basis weight caused by fibers or dry weight of paper
if no filler is metered [g/m.sup.2],
[0036] F is machine stock flow KM [l/s],
[0037] Cs is machine stock consistency KM [g/l] and
[0038] k is adjustment factor that takes into account the stock
loss in the long circulation and the portion of rejects in the
short circulation.
[0039] Since the basis weight adjustment unit 9 cannot be aware of
future stock consistency variations, the term `at 3% consistency`
is added to the machine stock KM flow request, i.e. in all cases
the desired fiber flow is led to the paper machine. The machine
stock KM is pumped from the blending/machine chest 5 with pump P3
along the machine stock dosing pipe 7 to the short circulation of
the paper machine 8. From the blending chest, new stock is pumped
into the machine chest in such a manner that stock flows at all
times from the machine chests via an overflow bin back into the
blending chest. This ensures a constant state for machine stock KM
pumping and, similarly, the machine chest contains a constant
amount of machine stock KM at all times. The blending chest surface
level varies, and blending chest surface level measurement is used
to adjust the flow of component stocks entering the blending chest
in order to keep the surface of the blending chest at the desired
level. Process-dynamically, the blending chest is an integrating
chest, wherefore the adjustment of the blending chest surface level
is slow and results in overshoots, since as the output flow
increases for instance 0.01 m.sup.3/s, the flow into the blending
chest has to be momentarily changed to level 0.02 m.sup.3/s before
the surface is at the desired level. In this way, a single machine
stock KM flow change increases gradually towards the stock towers 1
as high as up to 0.2 m.sup.3/s, and presently used adjusting
methods are unable to adjust the basis weight BW of the web
sufficiently rapidly in a controlled manner.
[0040] The solution of the invention for controlling the operation
of the stock preparation in a paper machine 8 utilizes the ability
of model predictive control (MPC) to calculate a prediction, i.e.
future control commands, for control messages required for
controlling the operation of the stock preparation. These
calculated control predictions are utilized by shifting the flow
data and prediction for component stocks OM1, OM2 and OM3,
determined based on the machine stock KM flow data and prediction,
which take into account the machine stock KM flow change caused by
the basis weight adjustment unit 9 backwards along the process flow
produced by the dosing line, whereby the adjustments guiding the
process stock flow forward utilize the predicted future flow
changes. This means that the prediction for the feed flow of a
given chest can be used as an indication of the amount of stock to
be pumped from the chest. Besides stock flows, stock dilution steps
and stock consistencies also need to be administered, and therefore
the consistency data and consistency prediction transferred forward
along the dosing line can be used by dynamic process models to
predict and take into account the stock consistency variation
caused by stock flow changes when adjusting the fiber flow and
dilution.
[0041] In the following, the operation of the solution of the
invention will be studied by way of example in the adjustment of
the basis weight of a paper web. The operation of the stock
preparation is dividable into sections and, for the sake of
clarity, the solution of the invention is presented by means of
subprocesses descriptive of the stock preparation portions.
[0042] When the basis weight of paper is being adjusted, the basis
weight adjustment unit 9 requests for the required fiber flow or
machine stock KM flow from the blending/machine chest 5, from where
the dosing of machine stock KM is controlled with a first control
unit CONTROL1. The first control unit CONTROL1 controls the dosing
of machine stock KM by controlling the third pump P3 or by
controlling the set value of flow control. Flow control can also
take place by means of an adjusting valve mounted after pump P3.
The specially structured valve is called a basis weight valve and
it is extremely accurate. Generally, flow control or flow rate
control can be carried out by changing the valve opening, pump
speed or rotational volume or all these manners known per se. The
basis weight BW of a paper web is measured at the dry end of the
paper machine 8, for example immediately before the reeler, whereby
the basis weight adjustment unit 9 requests for the necessary
machine stock KM flow based on the difference between the desired
basis weight BW value and the measured value. Machine stock KM flow
control constitutes a first subprocess 10, which is schematically
shown in FIG. 2, which also shows the inner operation of the first
control unit CONTROL1 as a block diagram. In the first step, a
consistency prediction KMCsPr for the machine stock KM discharged
from the blending/machine chest 5 and determined at the previous
calculation cycle is read. Machine stock KM consistency DT1 is then
measured, and it can be either total consistency or fiber
consistency, and a machine stock consistency prediction DT1 Pr is
calculated based on the machine stock output consistency prediction
KMCsPr and the measured machine stock consistency DT1. A machine
stock flow FT1 is then measured and a machine stock flow control
set value FIC1 and a machine stock fiber flow target value
trajectory KMFFTr, i.e. volume flow, wherein machine stock
consistency is 3%, calculated by the basis weight adjustment unit
9, are read. The calculated machine stock consistency prediction
DT1Pr, the measured machine stock flow FT1, the machine stock flow
control set value FIC1 and the machine stock fiber flow target
value trajectory KMFFTr are used as the basis in model predictive
control, i.e. MPC, to calculate a machine stock control message
KMFmv, which may be a new flow control set value FIC1 or a control
message SIC1 for the speed of a corresponding actuator, in this
case pump P3. The measured machine stock flow FT1 and machine stock
control message KMFmv are used to calculate a new machine stock
flow prediction KMFPr, i.e. a prediction stating how much machine
stock KM is pumped from the blending/machine chest 5 to the paper
machine 8. The MPC model includes a response from the PID control's
set value to the flow. This is a preferred alternative, since known
methods of control engineering enable the determination of a
response regarding how a control circuit has to behave in set value
changes. In accordance with known tuning methods of control
circuits, a control circuit can be tuned to give said response. MPC
evens out the flow target within the control performance limits
optimizing the cost function generated by the output error and the
control change. The machine stock flow prediction KMFPr is relayed
further to a second control unit CONTROL2 controlling the dosing of
component stocks OM1, OM2 and OM3.
[0043] FIG. 2 does not show the dilution step between the blending
chest and the machine chest. If said step is in use, the solutions
presented for component stock OM1 dosing and dilution in FIGS. 4
and 5 can be used.
[0044] Modern paper machine basis weight control calculates several
future flow changes for the fiber flow, which constitute the future
target value trajectory. Based on this information and by
calculating the future consistency trajectory of the preceding
chest, an optimal flow trajectory can be adjusted and it is
implemented by flow control. Since this method provides, at the
dilution step, information about the consistency trajectory of the
stock arriving at a blending point, i.e. future consistency, and
the flow trajectory and consistency target of the stock leaving the
blending point are also known, an optimal dilution water or
component stock flow trajectory can be set, which is implemented by
flow control. Instead of stock consistency, the concentration of
the second raw material of the machine stock, such as various
fillers, additives or chemicals in the stock, can also be
controlled, i.e. in addition to or instead of stock consistency,
the solution of the invention can be used to control the
concentration of fillers, additives or chemicals in the stock.
Dilution water and various fillers, additives and chemicals can
also be added at the same blending point, which is usually before
the pump. Fillers, additives or chemicals can also be fed into the
inlet of a chest not containing dilution. For the sake of clarity,
the figures do not show the addition of fillers, additives or
chemicals to machine stock or component stocks, or the measurement
of their concentration in the stock.
[0045] FIG. 14 schematically shows the principle of model
predictive control. Model predictive control (MPC) is a method
known per se in control engineering. FIG. 14 schematically shows a
control message 12 or a control variable 12 and a variable 13 to be
measured or controlled. The point in time t0 is set to correspond
to the present moment, at which time history data on control
variable 12 and the variable 13 to be controlled are available. The
solution of the invention utilizes the capability of MPC to
calculate a prediction for the process output, i.e. the variable 13
to be controlled, based on the capability of MPC to calculate a
so-called free or unrealised response of the process by means of
previous control variables, measurements, predicted disturbance
variables, i.e. the difference between the current and desired
states of the process, and the process model, whereby MPC can be
used to solve the process control problem by means of the available
control variables, i.e. manipulable variables, in such a manner
that the process output variables, i.e. adjustable or controllable
variables are as close to the target value as possible at each
particular point in time. The several control changes 16 calculated
for the control variable 12 by means of control and optimization
and to be implemented in future are presented as step-like changes
after the present moment to. As target value for variable 13 to be
controlled by means of MPC can also be used a time-dependent
variable value, i.e. target value trajectory 14, which can be
arranged to start from the last measured value of variable 13 or
which may be preset, as in the example of FIG. 14. FIG. 14 also
shows predictions 15a and 15b calculated for the controllable
variable 13. Prediction 15b corresponds to a situation that arises
if no new control measures are taken. This corresponds to the
initial state in optimization. When optimization calculates control
changes 16, the result is a prediction 15a that is descriptive of
process output and is the result of the control measures. The
control implements the first measure and, after control interval
dt, at point in time t.sub.-+dt, new control changes are calculated
in the same way. In addition to process models, an essential part
of model predictive control is thus optimization, wherein future
process controls for the desired operation of the process are
determined based on the predicted disturbance variables, the cost
function descriptive of the quality or target of the control, and
the limitations set on the optimization. Accordingly, the invention
utilizes the capability of the optimization cost function, included
in MPC technology, to give a penalty for both a process output
error and a control change calculated by a controller. This enables
the operation of the process to be stabilized and so-called soft,
i.e. slowly acting, changes to be achieved, the changes yet being
well timed. Literature dealing with model predictive control is
abundantly available, an example being D. Clarke: Advances in
Model-Based Predictive Control, Oxford Science Publications, 1994
and R. Soeterboek: Predictive Control a Unified Approach, Prentice
Hall, 1992.
[0046] The dynamic stock making process models, used in the
solution of the invention, are fully known per se. For instance
Donald P. Campbell: Process Dynamics, John Wiley & Sons, Inc.,
1958 describes a basic theory of creating dynamic models for
physical processes. The invention presents a solution for coupling
together general models descriptive of process dynamics and model
predictive controls. The present invention utilizes the capability
of dynamic models to calculate a prediction for process flows,
surfaces and consistencies and the capability of model predictive
controls to bind the prediction to the last measurement result of
the process and to utilize it in control calculation. In addition,
model predictive control couples successive control cycles together
in an intelligent manner both by giving control changes a penalty
and by utilizing historical data on the control changes on previous
control cycles. The present invention utilizes the ability of MPC
to calculate a prediction, i.e. future control commands, for a
control message. This control prediction is utilized by conveying a
flow prediction caused by basis weight control or a corresponding
measure, such as grade change, backwards in the process flow,
whereby the controls pumping process flow forward utilize the
predicted future flow changes, whereby the feed flow prediction of
the chest know how much is going to be pumped from the chest. Since
it is desirable to control not only flows but also dilution
processes and consistencies, the consistency variation proceeding
with the process can be predicted and taken in account by means of
dynamic process models in both fiber flow controls and dilution
controls. In this case, the calculation proceeds stepwise.
[0047] FIG. 3 schematically shows the determination of the total
amount of the flow of compound stocks OM1 to OM3 into the
blending/machine chest 5, forming a second subprocess 20. FIG. 3
also shows a block diagram of the inner operation of a second
control unit CONTROL2 controlling the second subprocess 20.
Component stock flows FT3.sub.1-n, where n is the number of
component stocks, and the surface level LT2 in the blending chest
are measured and a blending/machine chest 5 surface level
trajectory LT2Tr is calculated. In addition, the machine stock flow
prediction KMFPr, calculated in the first subprocess, is read. A
common component stock flow target OMFTr is then calculated using
MPC. When the target value trajectory OMFTr for the total flow to
be fed into the blending/machine chest 5 is calculated, component
stock consistency measurements DT3.sub.1-n and component stock
chest output consistency predictions OMCsPr.sub.1-n from the
previous calculation cycle are read. These are used to calculate a
consistency prediction DT3Pr.sub.1-n for each component stock.
Since a given amount of each component stock is desired and the
intention is to maintain the desired ratio of component stocks, a
flow target OMFTr.sub.1-n is calculated for each composite stock
based on the previous data by means of formulas (7), (8a), (8b) and
(8c), presented later.
[0048] FIG. 4 shows the dosing of component stock OM1 from the
component stock chest 3, forming a subprocess 30, which is
controlled by a third control unit CONTROL3. FIG. 4 also shows a
block diagram of the inner operation of the third control unit
CONTROL3, which controls the third subprocess 30. FIG. 4 only shows
the dosing of composite stock OM1 from the dosing chest 3, but the
principle shown in FIG. 4 concerns similarly all composite stocks
OM1, OM2 and OM3, wherefore the notations of the variables lack the
subscript 1 denoting component stock OM1. A component stock flow
FT3, a component stock flow control set value FIC3 and the
component stock flow target OMFTr calculated by the second control
unit CONTROL2 are first read. A component stock control message
OMFmv, which may be a new flow control set value FIC3 or a control
message SIC3 for a corresponding actuator, in this case for the
speed of pump P2, is then calculated by MPC on the basis of the
measured composite stock flow FT3, the component stock flow control
set value FIC3 and the component stock flow target OMFTr. A
component stock flow prediction FT3Pr is then calculated based on
the composite stock control message OMFmv and the measured
component stock flow FT3. The component stock OM1 flow prediction
FT3Pr is transferred to a fourth control unit CONTROL4 controlling
the dilution of component stock OM1 and to a fifth control unit
CONTROL5 controlling the dosing of component stock OM1 from the
stock tower 1.
[0049] FIG. 5 schematically shows the dilution of component stock
OM1 after the component stock chest 3, forming a fourth subprocess
40, which is controlled by the fourth control unit CONTROL4. FIG. 5
also shows a block diagram of the inner operation of the fourth
control unit CONTROL4 that controls the fourth subprocess 40. If
several dilution steps are in use, then in each dilution step the
process is the same. FIG. 5 relates similarly also to component
stocks OM2 and OM3. A component stock chest 3 output consistency
prediction OMCsPr calculated in the previous cycle is first read.
The component stock consistency is then measured, either total
consistency or fiber consistency, DT3 and a component stock
consistency trajectory DT3Tr is calculated, by means of which the
consistency is directed to the desired target value. The DT3Tr may
also be a preset consistency target that does not change as a
function of time. A dilution water flow FT4 is then measured, and
both a dilution water flow control set value FIC4 and the component
stock flow prediction FT3Pr calculated by the third control unit
CONTROL3 are read. A dilution water control message DFmv,
descriptive of the position of a dilution water duct DW4 adjusting
valve V4, or as in FIG. 5, the flow control set value, is then
calculated based on the component stock chest output consistency
prediction OMCsPr, the component stock consistency trajectory
DT3Tr, the measured dilution water flow FT4, the dilution water
flow control set value FIC4 and the component stock flow prediction
FT3Pr by using MPC. A dilution water flow prediction FT4Pr is
calculated based on the dilution water control message DFmv, the
dynamic process model and the measured dilution water flow FT4, and
transferred further to the fifth control unit CONTROL5 controlling
the dosing of component stock OM1 from the stock tower 1. A
consistency prediction DT3Pr is calculated in the same way and
transferred to the third control unit CONTROL3 and to the process
model predicting the consistency of the blending/machine chest 5
output.
[0050] FIG. 6 schematically shows the dosing of component stock OM1
from the stock tower 1, which constitutes a fifth subprocess 50
that is controlled by the fifth control unit CONTROL5. FIG. 6 also
shows a block diagram of the inner operation of the fifth control
unit CONTROL5 that controls 10 the fifth subprocess 50. FIG. 6
relates similarly also to component stocks OM2 and OM3. The surface
level LT1 in the component chest 3 is first read and a surface
level target trajectory LT1Tr is computed, by means of which the
surface of the chest is directed to the desired level. A component
stock flow FT5 is then measured and a component stock flow control
set value FIC5 is read. In addition, a component stock chest 3
output flow prediction FT3Pr-FT4Pr is calculated. The component
stock flow control message OMFmv, which may be a new flow control
set value FIC5 or a speed control message SIC5 of a corresponding
actuator, in this case pump P1, is then calculated using MPC and
based on the measured component stock flow FT5, the component stock
flow control set value FIC5, the component stock chest 3 output
flow prediction FT3Pr-FT4Pr and the component stock 3 surface level
target value trajectory LT1TR. A component stock flow prediction
FT5Pr is then calculated based on the component stock control
message OMFmv and the measured component stock flow FT5 and
transferred to a sixth control unit CONTROL6 controlling the
dilution of component stock OM1 to be metered from the stock tower
1.
[0051] FIG. 7 shows the dilution of component stock OM1 after the
stock tower 1, constituting a sixth subprocess 60, which is
controlled by the sixth control unit CONTROL6. FIG. 7 also shows a
block diagram of the inner operation of the sixth control unit
CONTROL6 that controls the sixth subprocess 60. FIG. 7 relates
similarly also to component stocks OM2 and OM3. A stock tower 1
output consistency prediction MTCsPr, calculated in the previous
cycle, is first read. The component stock consistency is then
measured, either total consistency or fiber consistency, DT5 and a
component stock consistency trajectory DT5Tr is calculated, by
means of which the consistency is directed to the desired target
value. Further, a dilution water flow FT6 is measured, and a
dilution water flow control set value FIC6 and the component stock
flow prediction FT5Pr determined by the fifth control unit CONTROL5
are read. MPC is then used to calculate the dilution water control
message DFmv, which is based on the calculated stock tower output
consistency prediction MTCsPr, the component stock target
consistency trajectory DT5Tr, the measured dilution water flow FT6,
the dilution water flow control set value FIC6 and the component
stock flow prediction FT5Pr and in this case is illustrative of the
new position of a dilution water duct DW6 adjusting valve V6 or the
flow control FIC6 set value. A dilution water flow prediction FT6Pr
is calculated based on the dilution water control message DFmv and
the measured dilution water flow FT6. A consistency prediction
DT5Pr for the consistency after the dilution step is also
calculated and transferred to process models predicting the output
consistency after the blending point. The dilution water flow
prediction FT6Pr and the measured dilution water flow FT6 are
transferred further to a seventh control unit CONTROL7 controlling
the dilution of the stock at the lower part 1b of the stock tower
1, if such dilution is in use at the lower part 1b of the stock
tower 1. The function of the seventh control unit CONTROL7
corresponds to that of the sixth control unit CONTROL6. The stock
tower 1 output consistency prediction MTCsPr can be determined by
taking into account the action of the flow of dilution water at the
lower part 1b of the stock tower 1 by measuring the component stock
consistency in the stock tower 1 by an indirect method or by
statistical methods.
[0052] FIG. 8 is a block diagram of the calculation of the
component stock OM1 consistency prediction. FIG. 8 relates
similarly also to component stocks OM2 and OM3. In a first step,
the flow prediction FT5Pr for stock flowing to the component stock
chest 3 and the consistency prediction DT5Pr are read. At the same
time, the component stock flow FT5 and the component stock
consistency DT5 are measured. These four variables are used to
calculate a component stock chest 3 feed consistency prediction or
fiber flow prediction F5CsinPr. The surface level LT1 in the
component stock chest 3 is measured and a component stock chest 3
surface level prediction LT1 Pr, and the component stock flow
prediction FT3Pr and the flow prediction FT4Pr for the dilution
water to be added to the stock after the component stock chest 3
are read. These four variables and the component stock chest feed
consistency prediction F5CsinPr are used to calculate a volume
prediction VOMsPr for the component stock chest 3 and a component
stock chest 3 consistency prediction OMCsPr, and these two
variables and the component stock flow prediction FT3Pr, the
dilution water flow prediction FT4Pr and the measured component
stock consistency DT3 are used to calculate a component stock
consistency prediction DT3CsPr at the output of the component stock
chest 3.
[0053] FIG. 9 schematically shows the determination of the output
flow of a chest, particularly the machine chest or the last
component stock chest in the dosing line without the dilution step.
The consistency of the flow discharged from the chest at a
measuring point can be calculated by the following formula
Cs(t)=Csto(t-td1)-Csto(t0-td1)+Cs(t0), (2)
[0054] wherein Cs(t) is chest output consistency [g/l],
[0055] Csto(t) is stock consistency in chest [g/l],
[0056] t0 is calculation point in time,
[0057] td1 is delay caused by flow from chest to consistency
measurement, and
[0058] t is future point in time after calculation point in time
t0.
[0059] Formula (2) can be used to correct the effect of errors in
the process model. The desired future flow at the blending point
can be solved by the formula
F(t)=FF(t)/Cs(t-td2), t.sub.max>t>t0, (3)
[0060] wherein td2 is delay caused by flow from consistency
measurement to blending point SP and
[0061] F(t) is output flow [l/s] and
[0062] FF(t) is the desired fiber flow.
[0063] The dilution step modelling can be schematically expressed
by means of FIG. 10. Chest output consistency is obtained from the
following formulas 1 Cstou ( t0 - td3 - td4 ) = Cs ( t0 ) F ( t0 -
td4 ) F ( t0 - td4 ) - F2 ( t0 - td4 ) , ( 4 )
Cstou(t)=Csto(t)-Csto(t0-td3-td4)+Cstou(t0-td3-td4) and (5) 2 F2 (
t ) = F ( t ) ( 1 - Cs ( t + td2 ) Cstou ( t - td1 ) ) , ( 6 )
[0064] wherein Cstou(t) is chest output consistency [g/l],
[0065] Cs(t) is output consistency [g/l],
[0066] Csto(t) is chest consistency [g/l],
[0067] F(t) is output flow [l/s],
[0068] F2(t) is dilution water flow in dilution step [l/s],
[0069] t0 is calculation point in time,
[0070] td3 is delay caused by flow from chest to blending
point,
[0071] td4 is delay caused by flow from blending point to
consistency measurement, and
[0072] t is future point in time after calculation point in time
t.sub.0.
[0073] Formulas (4) to (6) serve to determine the level of chest
output consistency at the start of the calculation. This level
calibrates the measurements by maintaining mass balance. Output
consistency is derived from the chest consistency prediction taking
into account the predicted changes in the chest consistency.
[0074] FIG. 11 and the following formulas (7) and (8a) to (8c)
schematically show dosing into a blending chest: 3 X ( t ) = F ( t
) K1 Cs1 ( t - td1 ) + K2 Cs2 ( t - td2 ) + K3 Cs3 ( t - td3 ) , (
7 ) F1 ( t ) = K1 Cs1 ( t - td1 ) X ( t ) , ( 8 a ) F2 ( t ) = K2
Cs1 ( t - td1 ) X ( t ) , ( 8 b ) F3 ( t ) = K3 Cs1 ( t - td1 ) X (
t ) , ( 8 c )
[0075] wherein X(t) is total fiber flow determined based on feed
consistencies,
[0076] F(t) is the desired total flow [l/s],
[0077] K1 is the desired fiber fraction of component stock OM1,
[0078] F1(t) is the component stock OM1 flow [l/s] corresponding to
fiber fraction K1 of component stock OM1,
[0079] K2 is the desired fiber fraction of component stock OM2
and
[0080] F2(t) is the component stock OM2 flow [l/s] corresponding to
fiber fraction K2 of component stock OM2,
[0081] K3 is the desired fiber fraction of component stock OM3,
and
[0082] F1(t) is the component stock OM3 flow [l/s] corresponding to
fiber fraction K3 of component stock OM3.
[0083] During dosing into the blending chest, the desired component
stock flow can be determined at the same time in such a manner that
both the total flow target and the desired fiber fraction target
for each component stock are simultaneously fulfilled. The total
consistency target cannot, however, be fulfilled. The formulas
presented do not take into account the changes between the disc
filter 11 input and the return flow, for example, but they are
eliminated in chest surface level management, allowing one to
assume that the fiber flows to and from the disc filter 11 are the
same at all times.
[0084] The modelling of the flow to a chest is schematically shown
in FIGS. 12 and 13 by means of the component stock chest 3. The
flow can be modelled by formulas 4 Fi ( t ) = Fo ( t ) + A ( LTr (
t ) ) LTr ( t ) t , ( 9 ) LTr(t)=f(Lsp,L(t0)),
t.sub.max>t>t0, (10)
[0085] wherein Fi(t) is input flow to chest [l/s],
[0086] Fo(t) is output flow from chest [l/s],
[0087] L(t) is surface level in chest [m],
[0088] LTr(t) is the desired variation curve for surface level,
[0089] Lsp is the desired surface level and
[0090] A is chest area at level L.
[0091] Depending on the difference between the measurement and the
set value, different target functions LTr(t) can be used. If, in
addition, an unknown disturbance flow is associated with the
process, its effect can be eliminated by control engineering
methods known per se.
[0092] The solution of the invention thus utilizes normal process
operation, and all stock preparations can be adjusted in the manner
of the solution presented. The solution is also well suitable for
managing water cycles, whereby the water amounts and flows in
chests can be managed by management of chest inputs and outputs.
The solution utilizes the capability of the optimization cost
function belonging to the MPC technology to give a penalty for both
a process output error and a control change calculated by the
controller. This allows process operation to be stabilized and
enables the achievement of so-called soft, i.e. slowly acting, but
timely control measures. The solution presented also enables exact
monitoring of the operation of measurements, actuators and
adjustments, and calling the operator if the process does not
operate in the way predicted by the models.
[0093] Accordingly, the disc filter 11 shown in FIG. 1 or another
fiber recovery apparatus is associated with nearly every blending
chest. A disc filter 11 requires long-fibered stock (sweetener), to
which fines and filler are bonded. The assumption in the solution
presented is that the stocks to and from the disc filter 11 are at
equilibrium as regards both consistency and flow, i.e. sum flow and
sum fiber flow are zero. If this is not the case, both flows and
their consistencies can be taken into account in the calculation of
the second control unit CONTROL2.
[0094] The drawings and the related specification are only intended
to illustrate the inventive idea. The details of the invention may
vary within the scope of the claims. It is thus clear that the
solution of the invention is in continuous use in the control of
stock preparation, i.e. it is not intended only for use in states
of change, such as for basis weight control or another grade change
relating to the product being produced. Furthermore, the solution
presented is usable not only in the manufacture of paper and board
stock but also in other chained processes, where changing and
adjusting consistencies and concentrations constitute a significant
factor in the process. The control units used to control stock
preparation are preferably microprocessor or signal processor-based
data processor units, in which at least part of the required
functions can be implemented by software. It would also be possible
to use only one single control unit for controlling the operation
of stock preparation and it would implement all necessary
functions, but the functions are preferably distributed to several
separate control units. Flows, consistencies and concentrations can
be measured using any sensors and other measuring devices known per
se.
* * * * *