CD control of fourdrinier paper machines – how it all began ...

CD control of fourdrinier paper machines – how it all began
Hansson, Å. (2010)
Control Systems 2010, SPCI, Stockholm.
INNVENTIA AB
Drottning Kristinas väg 61, Box 5604
SE-114 86 Stockholm, Sweden
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www.innventia.com

CD CONTROL OF FOURDRINIER
PAPER MACHINES- HOW IT ALL
BEGAN
Åke Hansson
Innventia AB, P.O. Box 5604,
SE-114 86 Stockholm, SWEDEN
[Ake.Hansson@innventia.com]
ABSTRACT
In the early/mid 1970’ies work at STFI - the Swedish
Pulp and Paper Research Institute (now with the new
name “Innventia”) led to the then revolutionary concept
of considering the basis weight response to an actuator
change as a transfer function which could be modelled,
simulated and used for profile optimisation in a CD
control system.
The original work paved the way for all CD control
systems to follow by establishing several basic concepts
and understandings.
Several systems were placed on the Nordic market by
STFI until the concept was transferred to
Sentrol/Valmet (now Metso) in the mid 80’ies.
INTRODUCTION
Figure 1: The profile optimisation challenge
This paper is given in recognition of the visions of three
persons that I want to specifically name in this context.
Namely, from STFI there were the researchers Inge
Lundqvist and Thomas Östman and from the Skoghall
PM5, the production manager Idon Gladh.
At the time IPR issues prevented STFI from presenting
much detail about their work although, over the years,
some presentations were made, mostly in Swedish. I
have tried to make up for that on some later occasions,
e.g. a Tappi meeting in Vancouver and a PIRA meeting
in Edinburgh in the late 90’ies.
In the mid 80’ies the author was appointed the
instrumentation manager at the Stora Enso Skoghall
mill. At the time when the CD Control work was
conducted at Skoghall I was STFI’s systems
Control Systems 2010 1
engineering counterpart at the Sackpaper Divisions
Development department on the PM5 at the Skoghall
Mill. As such I was working on the profile optimisation
task closely with STFI and with the machine tenders on
the PM5.
The following is based on my recollection of the work
and experiences we made over a period of many years
in the late 70’ies and the first half of the 80’ies.
HISTORY
Of course many people have been pondering over the
cross machine profiles on fourdrinier machines for
decades already. Lennart Haglund at STFI had
measured the effects of slice adjustments on the STFI
Pilot Machine in the mid 70’ies and several others
(Jasper Mardon, for one) have done similar work even
earlier. (See also references [1] and [2])
However, at that time, controlling the cross machine
profile was in reality considered an art, or even “black
magic”, that only a few very gifted machine tenders
mastered. Of the machine tenders on the PM5 there was
one person in particular that could repeatedly improve
the profiles within an hour or two, and another one that,
more often than not, left the profile in a worse state than
when he went up to the headbox to adjust the slice. The
other machine tenders were somewhere in-between..
On the machine, the sideways extension of the basis
weight response was not fully considered. The “bizarre”
side effects that could result from an adjustment was
often referred to as a result of “cross currents” on the
wire or as a specific result coupled to the history of the
adjustments made previously.
In the 70’ies the conditions were changing rapidly.
Computer based scanning measurement systems had
been around since the late 60’ies (albeit with limited CD
resolution), “portable” computers, like the PDP 11/36
used by STFI in this project, were introduced to the
market, and Idon Gladh at the Skoghall Mill had
installed and patented a high resolution system for
hysteresis free measurements of the slice lip adjustment
screw positions.
Idon’s idea was that if you could reset the slice lip to a
previously acceptable position, you should be able to
restore the CD profile to an acceptable state after a
grade change or a maintenance shut down. Much to his
dismay, it did not seem to work that way. Therefore,
when STFI approached him with the suggested CD
control project, he immediately saw its potential.
THE SKOGHALL PM5
The PM5 (which was dismantled and sold to Asia in
1995) was a fourdrinier sack paper machine built by
KMW in 1968. Some important machine characteristics
of the PM5 are summarised here.
• Production: 130 tpa sack kraft (700 mpm @ 70
gsm)
• Headbox: Conventional air pad (but wet
surfaces “electro-polished” to reduce scaling),

7.1 metres, 48 slice screws (150 mm slice
screw spacing)
• Slice opening: 50-55 mm (Headbox
consistency around 0.18-0.24 %)
• Scanning measurement system: AccuRay 800
with Krypton based Basis Weight
measurement, RF Moisture measurement and
semi non-contacting Caliper.
As it turned out, this is a type of machine which really
exhibits an extremely troublesome slice screw actuator
response.
The scanning measurement system, although employing
a computer, was still pretty much built on analogue
technology, allowing STFI to extract the raw profile
measurement signals at a resolution much higher than
the native of about “one average per slice screw” that
was thought to be sufficient at the time.
Another interesting feature of the machine was that Idon
Gladh had decided to let almost the full width of the
sheet through the machine (in order to save on expanda
wear) and trim the sheet just before the reel using hipressure
water jets.
And, as mentioned before, it had a very high resolution
slice position measurement system based on LVDTs
connected directly between the slice lip and the front
wall of the headbox (bypassing the adjustment screws).
Figure 2: Principle for the back-lash free measurement
of the slice screw positions on the PM5.
Although the resolution was better than 1/100 of a
millimetre, no one at that time really thought that
anything beyond tenths of millimetres was significant.
THE BASIS WEIGHT RESPONSE
Definition: The basis weight response in this context
refers to the effect on the Dry Weight CD profile caused
by a hypothetical 1 mm adjustment of a slice screw.
With accurate measurements of the sheet quality
profiles and of the slice position profile, work begun to
Control Systems 2010 2
measure the effect of a slice screw change on the quality
profiles of the sheet.
The Digital Equipment PDP 11/36 computer easily
collected measured data and “Idpac” (a Matlab like
precursor from the University of Lund) was used to
perform process identification. Results were then
converted into mathematical models implemented in
Fortran 77. The Idpac tool was also used for diverse
profile calculations in the on-line system.
Some of the early findings were
• that the effect of a slice screw adjustment was
in fact not “arbitrary” but very repeatable, also
between slice screws on the machine
• that the effect of many simultaneous
adjustments could be approximated by the
super positioned effects of the individual
adjustments
• that the effect of the sheet edges on the PM5
were, for practical purposes, neglectable
Initially the development team also had an alternate
hypothesis; namely that the sheet edges acted more or
less like a mirror to the waves on the wire. However, the
machine tenders did what they could to prevent
reflections by using stepped deckle boards and wire
wedges. Further, the outermost parts of the sheet never
reached the reel to be measured by the scanning system.
Therefore, the measurable effects of any edge effects
could be disregarded on this machine.
Numerous experiments were performed to model the
sheet profile response under varying machine
conditions. Finally the development team arrived at a
model, and a theory, that could predict the basis weight
(and moisture, and caliper) responses from known
machine operating conditions, most notably machine
speed, basis weight and slice opening (and of course
several constants like “wire length to the dry line”).
Response generation
The basis weight response effect is similar to that made
by a fast boat on shallow waters. When depressed, the
slice lip (boat) creates a void in the water surface by
pushing water up to the sides. Behind the boat the void
is being filled by waves travelling in from the sides,
into, and past, the void, creating the characteristic waves
that you see after the boat. As waves do when the paths
cross, the individual waves are unaffected by each other,
but their respective amplitude effects are superimposed
onto each other. It turned out that the dispersive effects
did not have to be accounted for, within the operating
range of the PM5. On the wire, the waves travel slower
as the suspension depth decreases. Obviously the wave
sideways movement stops when the dry line is reached.
The fibre distribution is thus “frozen” in this MD
position.
The response model created by STFI accounted for the
major effects. Increased slice opening (i.e. increased
average suspension depth on the wire) and increased
time-to-dry-line both increase the width of the resulting

asis weight response, whereas higher machine speed
decreases it.
For a fast twin wire machine, where the sheet is
consolidated very quickly after the slice, the response is
quite narrow, whereas slower fourdrinier machines
exhibit gradually broader and more dispersed responses.
Figure 3: Some typical basis weight responses.
The PM5 Response
Figure 4: The nominal Skoghall PM5 Basis Weight
response. The tick marks on the horizontal axis
represent individual slice screw positions (150 mm
spacing at the slice). The full machine width is shown.
Figure 5: The individual components of the Skoghall
PM5 Basis Weight response model
Control Systems 2010 3
As the illustration shows, the effect from one single
slice screw extends over a width of about 3.3 metres in
this case.
The two separating waves are each characterized by a
peak (about 60 centimetres away from the actuated slice
screw) and an equally large valley some 90 cm away.
At the centre of this response is seen what the
developers named “the mountain” which they attributed
to an effect of the actual increase in slice opening at the
slice position affected.
It is not hard to understand why any simple control
scheme will run into problems with such an actuator
response. Nor why the operators, more often than not,
had a hard time to figure out what was actually
happening on the wire.
Figure 6: Example: The effect of adjusting two adjacent
slice screws in opposite directions. Horizontal tick
marks per slice screw position. Dotted lines mark the
two adjusted screws. The solid line indicates the
resulting basis weight response.
Humans do have a problem evaluating complex profiles.
When you are presented with two similar profiles, you
can easily be led into characterizing one as “having a
light streak problem” at a certain position, and the other
as “having a heavy streak problem” at a neighbouring
position. It all comes down to how you interpret minute
differences in the local environment in each profile.
It is my experience that effects like these, account for
the seemingly arbitrary “cross currents” that the wet end
operators often referred to.
Assessing the true profile
The discussion above is further complicated by the fact
that there are several problems associated with assessing
the true CD profile (and thus the true Basis Weight
response) on a typical paper machine.
The scanning speed of the traversing beta-based basis
weight measurement system is a compromise which is
fundamentally limited by the characteristics of
radioactive sources as such. The smaller the
measurement integration area and the faster the
scanning speed, the more uncertain is the measurement
result. At the PM5 the BW measurement area was about
an inch across (2.5 cm) and the scanning speed was
about one minute side to side.
As a result the “CD profile” actually contained one
hundred times the length of information in the MD
direction, as compared to in the CD direction. And
when one edge has been measured, it could take two full
minutes before another sample could be made of the

profile in the same position again. Obviously individual
profiles had to be accumulated for a considerable time
to arrive at anything that could be considered to be “the
true measured CD profile” of the machine.
Add to that the transport delay from slice to reel and
secondary dynamic effects for moisture and caliper and
you can appreciate why the control system’s typical
“cycle time” was more like 15 minutes to half an hour.
Thus, the speed of the slice actuators was never really
considered an issue.
Figure 7: Some typical CD weight profiles during a 90
minute period
At this stage, let me point out that the typical
measurement system’s “average profiles” (which were
created by exponentially adding subsequent profiles
together) is not very efficient in suppressing periodical
MD variations. In fact, with a typical averaging factor
of 0.5 one could hope to suppress, at best, some 50
percent of the disturbance, often much less.
To counter the scanning systems sensitivity for
periodical MD variations it was considered to randomly
change the scanning speed or the starting time for each
scan, but it was never implemented at that time.
However, as the STFI CD control system could generate
its own straight-average profiles from its saved raw
measurement data, it was more efficient in suppressing
MD variations than the “host” system was.
Note also, that in this system there was one disturbance
that was perfectly in sync with the profile build-up – the
MD basis weight control output, which occurred at the
same CD position(s) at each scan.
A word on profile resolution and alignment
The “two big ones” in the seventies were AccuRay and
Measurex (with Sentrol and Lippke following).
AccuRay’s approach was to let the screen resolution
determine the profile resolution which resulted in
profiles being built from less than 64 data boxes.
Measurex, on the other hand, averaged the data per slice
screw (“since there is no way you can adjust a partial
slice screw anyway”). In either case, the basic resolution
was inadequate for efficient model based profile control
of the STFI kind.
The development team therefore opted, as mentioned
before, to sample the scanning sensors directly into their
own computer with a resolution of approximately three
measurement points per slice screw position.
The critical issue is not as much positioning the
response as such in the right place, but to know where
the steep transitions on either side of the peak falls.
Control Systems 2010 4
There the response goes practically from minimum to
maximum within a distance of one slice screw width.
If the systems counts on that an adjustment will
increase the weight at a certain position, but the actual
response width is more narrow than anticipated, it
would lead to a substantial decrease instead. As a result
the control system could easily enter into an unstable
condition.
When a slice screw is adjusted, the two waves start to
travel away from the slice screw. The model is used to
predict how far they have travelled before they reach the
dry line (accounting for current machine parameters).
On its way down to the reel, two more physical effects
come into play, the shrinkage profile and the sideways
movement of the sheet through the dryers.
STFI made experiments to model the cross machine
shrinkage profile on the PM5 and found that it could be
modelled by a second order function, a parabola, across
the sheet (other machines have displayed a shrinkage
profile with a more “flattened” top.).
On-line measurements of the pissers on the wire, and
camera based sheet edge detectors at the reel made it
possible to accurately assess the sheet position and the
average shrinkage of the sheet.
Thus, the slice screw position at the headbox could be
accurately mapped onto the measured sheet CD profiles
for basis weight, moisture and caliper.
An error in the mapping function, i.e. in the placement
of the individual response features relative to its
originating slice screw, often resulted in the
characteristic “trumpet” oscillation across the sheet.
As the optimiser was working its way across the profile,
the effects at the very edges of the response was badly
estimated, even the direction of the effect could be
wrong, especially near the steep flanks. This situation
leads to that gradually more drastic control efforts are
(thought to be) required trying to keep the profile
variations under control (which of course fails
eventually).
The situation was often liked with trying to flatten a
wrinkled table cloth by striking your hand over it. You
are initially successful but at some point you loose
control and you end up with some major wrinkles in the
end. Similarly the CD profiles, and the slice lip profile,
seem quite good at one side, but the situation gradually
worsens across the sheet creating the characteristic
“trumpet” like envelopes of the profiles.
The situation was prevented by evaluating and
comparing the slice lip profile and the target profile
development during the optimisation.
Response amplitudes
The development team found that they did not need to
assess the response amplitudes to a very high degree of
precision. Simply put, given the times involved for a
full optimisation cycle, even if the amplitudes were off

y a considerable amount, you would quickly enough
arrive at acceptable profiles anyhow.
However, it was noted that the effect of a slice screw
adjustment generally was much greater than anticipated,
based on the relative change of the slice opening at the
position.
In fact, on the PM5, the effect was about ten times
stronger, which explains why, instead of adjusting the
slice by 1/10 of millimetres (as was the assumption
before the project) you had to make changes to the slice
screw positions on the 1/100 part of a millimetre scale.
The optimisation algorithm treated “long wave”
optimisation (i.e. smiling or frowning profiles)
separately from the ones handled by the response
model.
The Idon Hypothesis
One attempt to explain the excessive effect of a small
slice opening change has been made by considering the
possible effect of what was termed the “Idon-distance”
on the flow pattern when the suspension passes the
slice.
Figure 8: How, according to the “Idon Hypothesis”, the
apparent slice opening (ASO) changes when the “Idon
distance changes
According to the hypothesis, a small “Idon distance”
change results in a proportional change in the flow (A
above). And for a larger Idon distance, the flow along
the front wall is completely diverted to an initial
downward flow, and again, the flow changes
proportionally (C above). But in-between (B) there is a
region where a small change in the Idon distance not
only changes the physical slice opening, but also
changes the deflection angle of the flow along the front
wall, thus changing the “efficient slice opening” by the
downward effect of the suspension flow nearest the slice
lip.
OPERATOR INVOLVEMENT (AND CLEVER
SOFTWARE) – THE KEY TO SUCCESS.
The system was developed by STFI on the PM5 system,
in close cooperation with the machine operators and the
development and production personnel at the machine.
Initially the system was designed as an “operator
guidance” system only, (but was later expanded with
stepper motors on the slice screws.)
An advisory system
The result of a profile optimisation run was presented to
the operators as a list of suggested slice screw set
points. On the wall, clearly seen from any position on
the headbox, the operator could call up the data for a
particular slice screw. The control error was displayed
Control Systems 2010 5
in a thermometer like fashion. Thus, all the operator had
to do, was to adjust the screw in question until the error
bar showed zero and then move on to the next screw.
The system worked quite well, the operators quickly
learnt how to handle the screw backlash to get to the
setpoint efficiently. Nevertheless, performing a new
slice screw adjustment took considerable time.
(Although the software always minimized the number
of slice screws to adjust, by consolidating many slice
screws into one, or entirely removing a slice screw from
the calculations if its effect could be considered
negligible).
When the system development stabilised, it was
converted to a fully automatic system with stepper
motor actuators added to the manual adjustment shafts
of the screws.
HUMAN INTERFACE DESIGN
The system had lots of software features to allow the
operators to take part in the development and to make
them feel that they were in control of the system at all
times.
Flexible target profiles
Previous investigations had indicated that a good
moisture profile was more important for satisfactory
conversion of the PM5 paper than an optimized dry
weight profile (and, incidentally, that a good web
tension profile might be even more important).
The STFI control system was highly programmable in
the sense that any profiles could be weighted into the
optimization. Under typical conditions the target
optimising efforts were distributed between weight and
moisture in a 30/70 fashion.
Later developments could also cope with multiple
headboxes and weighing in the effects of a fibre
orientation profile.
All sorts of limitations were of course built into the slice
optimization, including max and min values, maximum
bending, screw to screw limitations etc. At times the
optimization algorithm could be made favour a more
“simple” slice profile, even at a small cost in the
moisture and weight profile variations. The reason being
that, although several possible slice profiles might yield
approximately the same quality profiles, the more
complex ones depended on an intricate balance between
different slice screw responses; a balance that would
quickly be upset when the response width changed, such
as when the machine speed was changed.
What-if simulations
A simulation feature allowed the operators to actually
ask the system “if I actually follow these
recommendations, what would be the expected resulting
profiles”. Alternatively, the operator could simulate
implementing his own strategy, screw by screw, and
study the expected profile results from each change.

Operator involvement
A “Save” function allowed the operators to compare the
two results. A host of other tools were available to the
operators, like excluding one or more slice screws from
the calculations (because the screw was deemed
unreliable, or to get out of a suspected “local
minimum”)
Another important feature was that the operators could
not only save results, but also give comments to the
developers in the process.
It was a great help to receive comments like “In this
example I have, according to the simulator, manually
managed to outperform the automatic optimisation
algorithm quite considerably. I believe this is
because…”. Such a comment could result in improved
optimisation algorithms, improved simulation
algorithms, or at least lead to stimulating discussions
between operators and developers.
The PDP 11’s operating system (RSX11/M, later
inspiring to Windows NT and its successors) had a
command line interpreter called CCL which the
development team at STFI further developed into TCL.
TCL was a parser that could translate natural language
operator given commands into computer parlance.
In all its simplicity it was very efficient in actually
giving the user the perception of an intelligent
computer. As an example, the operator command “Plot
Basis Weight and Moisture versus Slice” was easily
converted to a call to the plotting program with the
proper parameters.
The one-year average profile
The command “Plot average dry weight profile from
DATE to DATE” readily presented us with a year average
profile
One one-year-average profile showed three distinct
features, one low streak related to the “centre of the
headbox”, two high streaks related to the head box front
wall supporting beams and a fourth feature near the
drive side end that later proved to be caused by the slice
lip production process. Namely: For shop space reasons
the manufacturer made the slice in two pieces that were
then welded together to create the 7 metre slice lip
required by the PM5. Although every effort was made
to polish the joint to perfection, obviously it made its
mark on the sheet anyhow.
This finding led to a modified slice production process
at the suppliers shop and another source of bad profiles
being removed from the PM5.
CONCLUSIONS
The STFI Profile Optimisation system was very
advanced for its time, and at least for the decade to
follow. The flexible and user friendly software, along
with clever algorithms was very sophisticated and
paralleled modern software in many aspects.
The work put the focus on the CD variations which led
to many new ideas regarding how to assess the “truest
Control Systems 2010 6
possible CD Profiles” in the most efficient way. It
changed the way people thought about CD resolution
and quickly led to systems with much improved CD
profile measurements. It also inspired the development
of many new actuating systems to control various CD
profiles more efficiently (the dilution headbox was a
very successful innovation in this respect).
The STFI research project can indeed be called a
commercial success, fulfilling the emerging needs of the
paper industry for more efficient production (improved
profiles allow for higher speeds and reduced energy
consumption) and more competitive products with less
property variations across the sheet.
Figure 9: Ideas about “near instantaneous CD basis
weight profiles” are not new, as can be seen from this
old illustration from the TAPPI Journal of June 1957
ACKNOWLEDGEMENTS
Apart from the persons already mentioned, the author
acknowledges all the other dedicated researchers and
project leaders at STFI/Innventia, operating personnel at
the PM5 and at the other mills involved and with the
innovation partner Metso. They have all worked hard to
develop the “STFI OPTI Profile” and its descendants
over the years.
REFERENCES
[1] Cuffey, William H., ”Some factors involved in basis
weight uniformity”, TAPPI Journal, 40 (6): 1957
[2] Beecher, A.E., Bareiss, R.A., ”Theory and practice
of automatic control of basis weight profiles”, TAPPI
Journal 58 (5): 1970
[3] Haglund, Lennart, ”Control of basis weight profiles
by the slice opening” ,
STFI B-310, Stockholm, 1975.
[4] Eriksson, L., Hill, J., Lundqvist, I., ”Control of the
papermaking process and of the CD basis weight profile
on a paper machine” ,
Das Papier, 32 (10A), 1978.
[5] Karlsson, H., Lundqvist, I., Östman, T., ”Principles
and potentials of CD basis weight control”, Proceedings
from Control Systems 1982,, Stockholm, 1982.

[6] Karlsson, H., Hansson, Å., ”Optimal cross-direction
basis weight and moisture profile control on paper
machines”, Pulp and Paper Canada, 86 (8): 1985.
[7] Gladh, Idon, ”The IDON-distance, the story of a
hypothesis by Idon Gladh” , Svensk papperstidning, 90 (6): 92 1987.
Control Systems 2010 7