Note: Descriptions are shown in the official language in which they were submitted.
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A device in a measuring station
The present invention relates to a device in a measuring
station in a paper or cardboard machine for manufacturing
a paper or cardboard web, said measuring station being
situated between two work stations in said machine and
comprising a scanner with two measuring heads travelling
to and fro across the web and between them defining a
space through which the web passes, driven by drive
members.
The performance of a paper machine, such as a tissue
machine, is often limited by its drying section. Severa~
factors in the drying section. may cause a rupture in the
web and the web produced may then have to be rejected due
to unsatisfactory quality. The main factors affecting the
efficiency of the machine's drying section are loss of
time when there is no paper on the reeling drum and the
quantity of paper that must be rejected after a rupture.
In most machines run at high speed the paper reel is
ejected in the event of a rupture since it is difficult
to rewrap a half-sized reel to continue reeling, and if
it is too small the reel is rejected. One cause of web
rupture is that when a web is running with no support
between two consecutive sections in a paper machine, air
currents will often cause turbulence and web flutter
which sometimes results in the web breaking. Such air
currents may come from the machinery, the layer of air at
the boundary and the rotating drums in the paper machine.
A section of the web draw in which the web runs without
support is termed open draw.
In order to reduce web breakage it is known to stabilize
or support the web by means of flutter suppressers along
its draw. Such flutter~uppressers may be in the form of
plates or wings extending across the width of the
predetermined web draw. When a web is to run along and
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slightly spaced from a flutter suppresser in the form of
a plate, the boundary layer of air between the flutter
suppresser and the web will be reduced an maintained. The
tendency of the web to flutter will therefore decrease.
Web stabilizers or flutter suppressing wings are
described in US-A-4,321,107 (Page) and US-A-3,650,043
(Overly et al.). A section of the web draw where the web
is supported is termed closed draw.
In the drying section of a paper machine the web usually
passes a measuring station with a scanner for measuring
properties such as moisture and basis weight. Scanners
are described for instance in US-A-5,164,048
(Bossen et al.) and US-A-5,233,195 (Hellstrom et al.).
The above-mentioned US-A-4,321,107 states that a
tissue-paper web runs from a Yankee-cylinder past a
suppressing wing, through a scanner and then past a
second suppressing wing. According to this patent
specification the scanner forms an intermediate element
between the suppressing wings. The part of the web draw
where scanning occurs will therefore constitute an open
draw. If a scanner is to be applied on a part of the web
draw, scanning may not of course be impeded by any object
such as a suppressing wing. The part of the web draw
where the web passes a scanner will, however, have a
greater tendency to web flutter and web rupture than the
parts of the web draw where the web can be supported by a
suppressing wing or similar element. The distance the web
travels during passage of the measuring frame of the
measuring station is of course relatively short, but it
is still sufficiently long to be able to cause undesired
flutter. Another drawback with the web flutter caused by
the open draw is that measurement of the web properties
will be less accurate.
-
The object of the present invention is to reduce the
occurrence of web rupture caused by web flutter when the
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web passes a measuring station that includes a scanner
having two movable measuring heads. Another object is to
increase the accuracy of the scanning function.
The device according to the present invention is
characterized in that it comprises a continuous, flexible
support strip extending through said space between the
measuring heads, transverse and parallel to the web from
one edge to the other, and drive members for winding and
unwinding the support strip onto or off rolls arranged at
points outside each edge of the web, for displacement of
the support strip transversely to the web in the same
direction and at the same speed as the measuring heads,
and in that the support strip is provided with at lea s
one through-opening to expose the web to at least one
measuring transducer, the emitter unit and receiver unit
of which are situated in individual measuring heads or
the same measuring head.
The invention will be described in more detail in the
following, with reference to the drawings showing a
presently preferred embodiment.
Figure 1 is a side view of a part of a paper machine
showing a measuring station placed between two work
stations for a continuous paper web.
Figure 2 is a side view showing a part of the web draw
upstream of the measuring station, in greater detail.
Figure 3 shows the measuring station according to
Figure 1 seen in the direction of the machine.
Figure 4 is a top view of the web and a part of the
scanner along the arrows A-A in Figure 3.
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Figure 5 shows the measuring station according to
Figure 1 with the measuring heads of the scanner near an
end position.
Figure 6 is a top view of the web and of a part of the
scanner along the arrows B-B in Figure 5.
Figure 1 shows a measuring station 1 receiving a
continuous paper web 2 with two longitudinal edges
20, 21. The web is delivered from a previous work station
3 such as a pair of calender rolls, and runs in a
predetermined path of movement through the measuring
station 1 where measuring is performed. From the
measuring station 1 the web continues to the next work
station 4, such as a reel-up. As can be seen in Figures 1
and 2 the paper web is delivered to the measuring station
1 via a support member 5 forming a web-supporting or
web-carrying surface. The support member 5 comprises a
plurality of plates 6 arranged one after the other which
extend across the width of the web, and tubes 7, also
extending across the width of web. The plates 6 are
mounted close to the predetermined path of movement of
the web. The tubes 7 are placed as connecting elements
between two consecutive plates 6, and each tube 7 extends
in vertical direction slightly past the plane of the
plate situated downstream. When the paper web 2 passes
one of the tubes 7, therefore, the transition between the
tube 7 and the plate 6 located downstream will create a
zone with partial pressure which will draw the web 2
towards the plate, thus improving, adhesion between the
web and the surface supporting the web. In order to
stabilize the web 2, and particularly for the purpose of
threading paper through the machine, the tubes 7 are
connected to a compressed air source and are provided
with oblong slits or openings (not shown) facing the
direction of movement of the web 2. As is clear from
Figure 2 showing a part of the web draw between the
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calender rolls 3 and measuring station 1, compressed air
is fed out of the slits or openings in the direction of
movement of the web 2. The flow of compressed air will
then follow the web-supporting surface and run between
S the web 2 and the web-supporting surface. The flow of
compressed air will accompany the web 2, helping it to
move forward. Between the web 2 and the supporting
surface the air flow will form a layer of air with
reduced static pressure which will stabilize the web 2
against flutter. The web passes from the support member
5, through the measuring station, to a support member 8
located downstream and forming a web-supporting surface.
The support member 8 resembles the support member 5.
As can be seen best in Figure 3, the measuring station 1
comprises a stand which, in the embodiment shown,
comprises two vertical side beams 9 and 10 joined
together by means of a horizontal top beam 11 and a
horizontal bottom beam 12. These beams thus form a
rectangular open frame and the web 2 passes through the
opening 13 therein.
The measuring station 1 comprises a scanner having two
cooperating measuring heads 14, 15 disposed vertically
one above the other at a predetermined distance from each
other so that a space is formed between their flat
horizontal surfaces facing each other, through which
space the web 2 runs a short distance from said flat
surfaces of the measuring heads. The measuring heads
14, 15 are provided with one or more measuring
- transducers in order to measure various properties of the
paper web 2, such as sheet thickness, moisture content,
. basis weight, etc.
The measuring station 1 also comprises two cantilevers
16, 17, each supporting a measuring head 14, 15,
respectively, and being movably journalled in the top
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beam 11 and bottom beam 12, respectively, along a
horizontal rail (not shown). Each cantilever 16, 17 with
its measuring head 14, 15, respectively, thus forms a
movable, carriage-like unit 18, 19.
The measuring station 1 also comprises drive means for
moving the carriages 18, 19 carrying the measuring heads
14, 15. In the shown preferred embodiment the drive means
comprise a common drive source 22 for the two carriages
18, 19, said drive source 22 consisting of a motor
journalled on one side beam 9. The drive means comprise a
transmission with vertical, rotatably journalled driving
rod 23, driven by the motor 22 via a coupling 24, and
upper and lower endless driving belts 25, 26 running in
the top beam 11 and bottom beam 12, respectively and
round wheels 27 journalled in the side beams 9, 10. The
vertical driving rod 23 is joined at its upper and lower
ends to the wheels 27 so that the rotary movement of the
driving rod 23 is transmitted to the wheels 27 to drive
the belts 25, 26 at exactly the same speed. The
cantilevers 16, 17 are firmly secured to the driving
belts 25, 26 so that the measuring heads 14, 15 will be
moved synchronously with each other, transversely to and
fro across the web 2 as the motor 22 changes driving
direction. In the embodiment shown the measuring heads
have two measuring transducers and each measuring
transducer comprises an emitter unit 42 situated in the
upper measuring head and a receiver unit 43 situated in
the lower measuring head, these two units always being
vertically aligned with each other thanks to the
synchronous movement of the measuring heads 14, 15. The
measured values obtained are processed by means of known
technology not dealt with here.
The invention relates t-o a device in a measuring station
1 of the type described above, for instance, said device
comprising a continuous, flexible support strip 28
,.
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extending through the space between the measuring heads
in a direction transverse to the web 2 and so that the
support strip 28 is situated immediately above and
parallel with the web 2. A roll 29, 30 is rotatably
S arranged in the vicinity of each side beam 9, 10. The
ends of the support strip 28 are attached to the two
rolls 29, 30 for alternate winding onto and unwinding
from the rolls 29, 30 with the aid of drives means 40, 41
so that the support strip 28 acquires the same direction
of movement and exactly the same speed as the measuring
heads 14, 15. The drive means for the support strip 28
are separate from the drive means for the measuring heads
14, 15 but the two systems are synchronized so that the
support strip 28 and the measuring heads 14, 15 are
driven at exactly the same speed, the relative speed
between them thus being zero. The parallel extension of
the support strip 28 with and across the web is assured
by two deflection rollers 31, 32 around which the support
strip 28 passes during its movement up and down from the
winding and unwinding rolls 29, 30. The deflection
rollers 31, 32 are situated in a common plane and are
vertically below the winding and unwinding rolls 29, 30.
It will be understood that the distance between the
deflection rollers 31, 32 is slightly greater than the
width of the web 2 to allow sufficient space for the
carriages 18, 19 in their end positions when the
measuring transducers are situated outside the edges
20, 21 of the web 2.
The device suitably includes a control circuit for
controlling the tension in the support strip 28 to ensure
that the support strip 28 has the desired straight draw,
parallel with the web 2. The control circuit has a
regulator 44 and a measuring transducer, suitably
comprising a load cell 45 which detects the tension in
the support strip 28. The tension (measured value) in the
support strip 28 measured by the load cell is compared in
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the regulator 44 with a set value, i.e. a predetermined
value for the tension which represents said desired
straight draw of the support strip 28. As shown in
Figures 3 and 5, the load cell may be arranged at the
deflection roller 32 close to the side beam 10 on the
right side of the figures. The load cell senses a force F
corresponding to the tension appearing in the support
strip at the time of measuring. When the force F
decreases, for instance, the load cell emits a signal to
the regulator which, after comparison with the set value,
emits a signal to the relevant drive means 40 or 41 to
retard the roller 29 or 30 from which the support strip
is being unwound.
Said drive means may advantageously comprise servomotors
with braking function. Alternatively, one roller may be
driven by a servomotor and the other by a suitable spring
which is placed under tension when the support strip is
driven in one direction of the servomotor, whereupon the
spring force thus built up is responsible for pulling the
support strip back in the other direction.
The support strip 28 is provided with one or more
openings 33 within the section situated in the space
between the measuring heads 14, 15. Each opening 33 is
aligned with at least one measuring transducer 42, 43 0:
the measuring heads. In the embodiment shown the
measuring heads 14, 15 include two measuring transducers,
as mentioned above, and the support strip 28 is provided
with a circular opening 33 for each measuring transducer.
In other words, the openings 33 are arranged to expose
the web 2 to the measuring transducers so that their
function is in no way disturbed. The dimension of each
opening 33 is as small as possible in order to minimize
any collection of dust on the measuring heads, but
sufficiently large to permit free passage of measuring
signals from the measuring transducers.
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The lower measuring head 15 comprises a plate 34 which is
extended in both directions of movement of the support
strip in order to form two downwardly directed parts 35,
ensuring that the web 2 is correctly guided to the upper
side of the plate 34 (said upper side corresponding to
said surface of the lower measuring head 15 which defines
the space between the measuring heads).
The measuring heads 14, 15 are suitably pivotably mounted
on their cantilevers 16, 17 so that their facing surfaces
can be given an inclination in relation to the horizontal
plane which corresponds to the inclined draw of the web
when it passes the measuring station 1. Similarly, the
winding and unwinding rolls 29, 30 and deflection rollers
31, 32 are suitably journalled in a house (not shown)
which is pivotably journalled on the inner side of the
side beams 9 and 10, respectively, about a horizontal
shaft, so that the houses and thus also the support strip
28 can be given the same inclination as the measuring
heads 14, 15, the support strip 28 thus being aligned
parallel to the web.
The distance covered by the measuring heads 14, 15 is
sufficient to allow scanning of the whole width of the
web 2 before the measuring heads stop at the end
positions and then returns in the opposite direction. The
measuring station 1 includes two limit switches 36 which
sense the position of the upper measuring head 14 and
emit a signal to the motor 22 to stop and reverse its
drive direction. Similarly, the measuring station
includes two limit switches 37 which sense suitable marks
~ 38, e.g. metal tabs arranged at predetermined points on
the support strip 28, whereupon a signal is emitted to
the drive means in question to stop the support strip 28
simultaneously with the measuring heads and in the
desired end position so that the openings in the support
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strip are still aligned with the measuring transducers of
the measuring heads.
Some form of control device is advisable in order to
5 ensure that the support strip 28 really does move at the
same speed as the measuring heads 14, 15 while the
scanner is in operation, and that therefore the openings
33 of the support strip 28 are aligned with the measuring
transducers. A first pulse emitter may be arranged, for
10 instance, at the upper wheel 27 near the side beam 9 on
the left side in the drawings, and a second pulse emitter
at the left deflection roller 31, these pulse emitters
comparing pulses with each other and effecting correction
of the speeds of the measuring heads and support strip
should they have been recorded as different through
deviation of the two pulse measurements.
The width of the support strip 28 is suitably somewhat
greater than the distance between the stationary support
members 5, 8 arranged upstream and downstream, so that
the support strip is in sliding contact with the nearest
plates 6 thereof, whereupon the support strip 28 overlaps
these from above (see Figure 2) and from below, at its
edges situated upstream and downstream, respectively.
During passage of the web 2 through the measuring
station, the web 2 will be supported and carried by the
support strip 28 and possibly further stabilized by an
air flow supplied from the tube 7 immediately upstream of
the measuring heads 14, 15, said air flow passing between
the support strip 28 and the web 2. The entire path of
movement of the web 2 through the measuring station 1
thus forms an unbroken closed draw and yet the measuring
transducers on the measuring heads will not be impeded by
any object between them and the moving paper web.
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The invention thus comprises arranging an unbroken closed
draw for a moving fibre web 2 through a measuring station
1 by means of a support strip 28 forming a web-supporting
and web-carrying surface which will support and carry the
fibre web and yet not impede the measuring transducers of
the measuring heads.
The device according to the invention reduces the
occurrence of web ruptures and improves accuracy in
scanning or measuring.
