Note: Descriptions are shown in the official language in which they were submitted.
CA 02439238 2008-03-05
Titl:e of .the invention
Method and control device for advancing a needle
punch'ed fibrous sheet
Background of the invention
The invention relates to needling fiber structures.
A particular but non-exclusive field of the invention is
making plates, sleeves, or other peedled prefort~s, e.g.
of annular shape, suitable for constituting the
reinforcement of composite material parts.
In well-known manner, a fiber structure for needling
is advanced past a set of needles carried by a needling
head, and the needles are;periodically inserted into the
fiber structure and then withdrawn therefrom, by
imparting go-and-return motion to the needling head in a
direction that extends transversely relative to.the
structure advance direction:
Reference can be made in particular to US patent
No. 4 790 052 which describes making needled fiber
structures by successively needling layers formed by
plies superposed flat or.by turns wound one on another.
The intended field of that patent is making fiber
reinforcement for thermostructural composite material
parts, and in particular carbon-carbon composite material
parts,or ceramic matrix composite material parts in which
the fiber reinforcement is densified by means of a carbon
or a ceramic.matrix. Needled fiber structures are made
of refractory fibers, typically carbon fibers or ceramic
fibers, with it being possible to perform needling on the
fibers while the fiber material is in a precursor state
for carbon or ceramic, and after needling the precursor
is transformed by heat treatment. The.'intended'.
applications of the above-cited patent are brake disks or
the diverging portions of rocrket engines, which
applications require materials that have good mechanical
properties and the ability to conserve them at high
temperatures.
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Needling superposed layers of fibers serves to
transfer fibers in the Z direction, i.e. transversely
relative to the layers. This produces a structure which
presents less non-uniformity and increased ability to
withstand delamination, i.e. increased resistance to the
layers separating due to shear forces to which they can
be subjected, particularly in brake disks.
In order to perform needling over the entire surface
area of a fiber structure, the structure is advanced past
a needling head. When needling is performed on each new
superposed layer, an advance movement is performed each
time a new layer is put into place so that the needling
head sweeps over the entire surface area of the most
recently superposed layer.
If the fiber structure is caused to advance
continuously at constant speed, then it moves
transversely relative to the needles throughout the
duration of needle penetration. In particular, when the
structure is thick or once it has become thick, the
forced advance causes the needles to bend and they can
break. In addition to the fact that broken needles need
to be replaced, the presence of broken needles within the
fiber structure can be undesirable in subsequent use of
that structure.
It might be envisaged to ensure that the structure
advances very slowly, thereby minimizing the bending
forces applied to the needles while they are present
within the fiber structure, or else to advance the
structure discontinuously so that it is stationary during
needle penetration.
However those solutions present the clear drawback
of considerably increasing the time and thus also the
cost required for the process of fully needling the
structure.
Another process for controlling advance in a
needling machine is disclosed in FR 2 729 404, in which
the rotational speed of calling rollers for the needled
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fiber structure is modulated, so that the rotational
speed has different values for different positions of the
needles. It is then necessary to provide a system
allowing the rollers to be driven at a variable speed.
Also, the variation in speed does not take into account
the actual instantaneous forces applied to the needles.
Summary of the invention
The present invention is directed towards the provision
of a method of needling a fiber structure which makes it
possible to resolve the problem of needles breaking without
significantly penalizing the speed of the method, even with
structures that are thick.
According to one aspect of the present invention, there
is provided a method of needling a fiber structure in which
a fiber structure for needling is driven so as to impart an
advance movement thereto past a needling head carrying a
plurality of needles driven with reciprocating motion during
which they penetrate into the fiber structure and are
extracted therefrom, wherein the instantaneous speed of
advance of the fiber structure decreases in response to the
resistance to advance exerted by the needles penetrating
into the structure, and increases after the needles have
been withdrawn, so that the force exerted on the needles by
the advance of the structure is limited, but without
completely interrupting advance throughout the entire
duration of needles being present in the structure.
In a preferred implementation, the decrease in the
speed of advance is caused directly by the resistance to
advance exerted by the needles on penetrating into the
structure.
When the structure is moved by means of a controlled
member connected to a drive motor by a transmission, the
decrease in the speed of advance can be absorbed by
mechanical slack in the transmission. In which case, since
the motor is driven at constant speed, the slack which is
preferably resilient is automatically taken up once the
needles have been withdrawn.
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Thus, while the needles are penetrating, the speed of
advance of the structure decreases to below a mean speed of
advance corresponding to the speed of the motor,and once the
needles have been withdrawn, this speed increases to above
the mean speed of advance.
A measure of the torque at the level of a driving
element engaged with the fiber structure may be carried out,
in order to decrease the speed of the motor when the torque
becomes smaller than a given threshold, as a consequence of
the slowing down of the fiber structure.
In another implementation of the method, a value
representative of the force exerted to drive the fiber
structure is measured, the driving speed of the fiber
structure is reduced when the measured value becomes equal
to or larger than a first threshold value, and, after the
speed has been reduced, the driving speed is increased when
the measured value becomes lower than a second threshold
value.
The second threshold value may be equal to or lower
than the first one.
The measured value is for example representative of the
torque exerted by a driving element for the fiber structure.
The present invention is further directed towards the
provision of an installation enabling the method to be
implemented.
In accordance with a further aspect of the invention,
there is provided an installation for needling a fiber
structure, the installation comprising a needling head
carrying a plurality of needles, a device for driving the
needling head to impart reciprocating motion to the needles,
a support for the fiber structure to be needled situated
facing the needling head, and a device for driving the fiber
structure so as to impart an advance movement thereto on
said support, in which installation, according to the
invention, the fiber structure drive device is designed to
enable the speed of advance of the fiber structure carried
by the support to decrease momentarily in response to the
resistance to advance exerted by the needles penetrating
into the fiber structure, without
CA 02439238 2003-08-22
completely interrupting advance throughout the duration
of the needles being present in the fiber structure.
Brief description of the drawings
5 The invention will be better understood on reading
the following description given by way of non-limiting
indication and with reference to the accompanying
drawings, in which:
- Figure 1 is a highly diagrammatic front view of a
needling installation;
- Figure 2 is a highly diagrammatic fragmentary view
on a larger scale of the Figure 1 installation in lateral
elevation and in section;
- Figure 3 is a highly diagrammatic view of a drive
device making it possible, during needling, for a fiber
structure to be advanced through an installation of the
kind shown in Figures 1 and 2, for one embodiment of the
invention;
- Figure 4 is a graph showing how the advance speed
of a fiber structure being needled varies as a function
of time for the embodiment shown in Figure 3;
- Figure 5 is a graph illustrating the displacement
of a fiber structure being needled in the Figure 3
embodiment;
- Figure 6 is a highly diagrammatic view of a drive
device for a fiber structure being needled, according to
a second implementation of the invention; and
- Figure 7 is a chart showing the process of
controlling the driving of the fiber structure in the
second implementation of the invention.
Detailed description of implementations of the invention
Figures 1 and 2 show an installation for needling a
plate-shaped fiber structure 10.
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The structure 10 is built up from two-dimensional
plies stacked up flat and needled to one another so as to
bind the plies together and give the plate resistance to
delamination.
By way of example, the individual plies are strips
of woven cloth or other two-dimensional fabric, for
example a fabric made up of one-dimensional sheets
superposed in different directions and bonded together by
light needling.
By way of example, the plies are needled
individually, with a needling pass being performed over
the entire surface area of the fiber plate after each new
ply has been superposed thereon. Nevertheless, the ambit
of the present invention extends to performing a needling
pass after superposing two or more plies thereon, and
even to performing two or more needling passes after each
ply making up the plate has been superposed thereon.
The plate 10 being needled is moved horizontally
through a needling station 20 between a first support
table 12 and a second support table 14 situated on either
side of the needling station. The plate is moved
alternately in one direction and then in the opposite
direction from the table 12 to the table 14, and back
again.
In the needling station 20, the plate passes over a
needling platen 22 situated beneath a needling head 24.
A drive system 26 comprising at least one crank-and-
connecting-rod assembly imparts vertical reciprocating
motion to the needling head 24 under the control of a
motor (not shown). The needling head 24 extends over the
entire width of the plate 10 and carries a plurality of
needles 28. Holes 22a are formed through the platen 22
in register with the needles 28.
The plate 10 is caused to advance by means of pairs
of presser rollers 40, 50 situated between the needling
platen 22 and each of the tables 12 and 14, respectively.
In each pair 40 or 50 of presser rollers, the rollers 42
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& 44 and 52 & 54 are driven in rotation, with at least
one of the rollers in each pair being declutchable, e.g.
the bottom roller 44, 54. When the plate 10 is moved
from the table 12 towards the table 14, drive is provided
by the rollers 52 and 54 pressed towards each other while
the roller 44 is declutched, and possibly also the roller
42. Conversely, when the plate 10 is moved from the
table 14 towards the table 12, then drive is performed by
the rollers 42 and 44 pressed towards each other, while
the roller 54 is declutched, and possibly also the roller
52.
When only one roller is declutched in a pair of non-
driving rollers, it is also advantageous to eliminate the
pressure exerted by the rollers so as to avoid any effect
of drive from the non-declutched roller.
It will be noted that the lower rollers 44,54 may be
replaced by conveyor belts which may then constitute also
tables 12 and 14.
After each needling pass, when the plate 10 has
reached the table 12 or 14, a new ply is superposed and a
new needling pass is performed by moving the plate 10
towards the other table 14 or 12. During each needling
pass, the needles 28 penetrate vertically into the plate
10. The penetration depth of the needles 28 in the plate
10 is a function of the position of the needling head, at
one of the ends of its vertical stroke, as measured
relative to the needling platen 22.
The needle penetration depth can extend through
several thicknesses of superposed plies. This depth can
be adjusted depending on the distribution desired for
needling density through the thickness of the plate.
When a substantially uniform penetration depth is
desired, then the distance between the needling platen 22
and the needling head is increased incrementally after
each superposition of a new ply, by imparting a down step
to the needling table. Reference can be made to above-
cited document US 4 790 052. A similar down step can be
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imparted to the tables 12 and 14 with the tables and the
platen 22 all being mounted on a common vertically-
movable frame. Thus, during needling of the first plies
constituting the plate 10, the needles 28 pass through
all of the plies and penetrate into the holes 22a. Once
the plate 10 has been built up to a certain thickness,
the needles 28 no longer reach the platen 22.
By means of the invention, the speed of advance of
the plate 10 is slowed when the needles penetrate into
the plate so as to limit the bending force applied to the
needles due to plate advance and thus eliminate or at
least minimize the risk of needles breaking.
For this purpose, in a preferred implementation of
the invention, the slowdown is produced directly by
penetration of the needles exerting a force that brakes
the advance of the plate. This can be achieved by
including mechanical slack in the transmission between a
drive motor and the pairs of presser rollers.
Figure 3 shows a device for driving the presser
rollers and comprising a motor 60 turning at constant
speed and driving a belt 62 passing over the presser
rollers 42, 44 and 52, 54. On its path between the motor
60 and the top presser roller 42, the belt 62 passes over
a tensioning roller 64 and a deflector roller 65, and on
its path between the top presser roller 52 and the motor
60, the belt 62 passes over a deflector roller 67 and a
tensioning roller 66.
In order to avoid relative slip between the belt 62
and the rollers over which it passes, it is preferable to
use a double-sided cog belt meshing with corresponding
relief formed on the surfaces of the rollers where they
come into contact with the belt.
The tensioning rollers 64 and 66 are fixed at the
ends of respective arms 68 and 70 forming hinged levers
that are subjected to resilient return force exerted by
respective devices 72 and 74 so as to keep the belt 62
permanently under tension. The devices 72 and 74
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exerting resilient return force can be in the form of
springs, or preferably in the form of pneumatic dampers.
The pressure in the pneumatic dampers is advantageously
adjustable.
Operation is as follows:
When the plate 10 is moved from the table 14 towards
the table 12, the plate is driven by the presser rollers
42, 44 while at least the bottom roller 54 is declutched
at the press 50. The motor 60 turns in the direction
represented by arrow F1. When the needles penetrate into
the plate 10, the plate is slowed down by the needles,
thereby reducing the speed of rotation of the rollers 42,
44. Because the belt 62 is driven at constant speed by
the motor 60, the length of belt between the motor 60 and
the roller 42 increases. This increase in length is
absorbed by the tensioning roller 64 under the action of
the resilient return force exerted by the damper 72 by
pivoting the lever 68 in the direction indicated by arrow
F2. Conversely, the length of belt between the roller 52
and the motor 60 decreases, thereby causing the lever 70
to pivot in the direction indicated by arrow F3 against
the force exerted by the damper 74. When the needles
subsequently come back out of the plate, the presser
rollers 42, 44 accelerate and the length of belt that has
accumulated between the motor 60 and the roller 42 is
taken up until the two tensioning rollers 64 and 66 have
returned to an equilibrium situation.
By adjusting the pressure in the dampers 72, 74 it
is possible to adjust the return force they exert and to
achieve synchronization and thus proper operation of the
drive system.
When the plate 10 is moving from the table 12
towards the table 14, operation is symmetrical to that
described above.
Figure 4 shows how the speed of advance of the plate
10 varies as a function of time while the plate is going
from the table 12 towards the table 14, or vice versa,
CA 02439238 2003-08-22
with curve C. representing the speed of the plate on
entering the needling station and curve C2 its speed on
leaving it. Speed can be measured by means of a sensor
carrying a follower wheel resting on the plate and
5 rotated by the plate advancing. Curve A represents
displacement of the needles between their high and low
positions. The difference between the speeds of advance
at the inlet and at the outlet are due to needling.
Because fibers are transferred in the Z direction, the
10 speed measured on the non-needled top layer upstream from
the needling station is greater than the speed of the
plate as measured after needling. Times tl and t2 mark
the beginning of needle penetration into the plate and
full extraction of the needles from the plate. The time
difference At between times tl and t2 depends on the
penetration depth selected for the needles.
Because of the resilient slack present in the
transmission between the motor 60 and the presses 40 and
50, the speed of advance of the plate 10 varies
continuously between a maximum speed which is greater
than the mean speed of advance corresponding to the speed
of the motor, while the needles are not in the plate, and
a minimum speed that is slower than said mean speed of
advance, while the needles are in the plate, but advance
is not interrupted during the time interval between tl and
t2.
In Figure 5, the curves represent displacement of
the plate as a function of time, as measured firstly on
the last layer on entering the needling zone (curve D1),
and secondly on leaving the needling zone (curve D2), and
curve A represents the displacement of the needles. The
displacement at the inlet and at the outlet is measured
by respective sensors providing signals representative of
the distance the plate advances.
It can be seen that at the outlet, i.e. immediately
downstream from the presser rollers driving the plate,
plate advance diminishes shortly after instant tl at which
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the needles penetrate, and it begins to increase shortly
after instant t2 when the needles have been extracted
completely from the plate.
At the inlet, in the needling zone, i.e. immediately
upstream from the unclutched presser rollers, advance
continues to increase after time tl, before reversing.
This can be explained by the ability of the sheet to
deform elastically in the longitudinal direction combined
with the fact that, in the embodiment concerned, no use
is made of a stripper element of the type constituted by
a presser foot holding the plate down during needle
penetration. Consequently, when the needles rise, they
tend to raise the plate slightly before it comes free and
drops back onto the needling platen.
In a variant of the above described embodiment, the
slowdown of the plate may be detected by measuring the
torque at an end of the shaft of one or both presser
roller(s) ensuring the driving of the plate. The
lengthening of the belt portion between the motor and the
driving roller as a consequence of the penetration of the
needles results in a decrease of the measured torque
evidencing the slowdown of the plate. It is then possible
to control a reduction of the speed of the driving motor
from an assigned value when the decrease of the measured
torque reaches a given threshold, which adds to the
effect of the resilient slack in the transmission to
quickly react to the resistance to advance exerted by the
needles. Upon withdrawal of the needles from the fiber
structure, the shortening of the belt portion causes the
driving presser rollers to accelerate. The speed of the
motor may then be increased and returned to its assigned
value as a response to the detection of an increase of
the measured torque.
Obviously, the measure of torque is carried out
alternatively on one pair of presser rollers and on the
other pair, as a function of the direction of
displacement of the plate.
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In the above variant, the motor is controlled at a
speed which is constant but adjustable.
An embodiment of the invention is described above
for needling a plate that is moved in rectilinear
translation through a needling station. Nevertheless,
the person skilled in the art will see immediately that
the invention is equally applicable to needling annular
fiber structures formed by helically winding a fiber
structure as flat superposed turns or for needling
sleeve-shaped structures formed by rolling up a fabric of
superposed turns, which structures are driven in rotation
past a needling head. Under such circumstances, the
advance motion of the fabric is one-dimensional.
Causing the speed of advance of the fiber structure
to slow down directly by the resistance to advance caused
by the needles with resilient slack being included in the
transmission presents several advantages: it is self-
adapting, in particular as the plies build up and as the
thickness of the structure increases at the beginning of
buildup, and it makes it possible to maintain a mean
speed of advance that is relatively high since advance
acceleration when the needles are not in the structure
compensates for advance deceleration caused by the
needles penetrating.
Nevertheless, the speed of advance of the fiber
structure could be controlled as a function of the
measure of a value representing the force exerted for
moving the fiber structure.
Such a value is for example the torque which must be
exerted for displacing the fiber structure. The torque
may be measured at the level or a driving element, for
example at the level of the driving motor.
Figure 6 shows a drive device which differs from the
one of Figure 3 in that it does not include a resilient
slack. The motor 60 drives the presser rollers 42-44 and
52-54 by means of belt 62 passing over deflector rollers
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65, 67 and 64', 66', the latter having a fixed axis
contrary to the rollers 64, 66 of Figure 3.
A (non-represented) sensor provides a signal Sc
representative of the value of the torque CM exerted by
the motor 60, for example by measuring current drawn by
the motor. The motor may consist in a step-motor
controlled by control circuit 80.
As illustrated by Figure 7, motor 60 is originally
controlled at a predetermined assigned speed Vc (step 81).
If the measured torque becomes equal to or larger than a
maximum threshold value Cmax (test 82), the speed of the
motor is decreased by an increment AV (step 83) before
returning to test 82.
If the measured torque CM is lower than Cmax (test
82), and if the motor speed VM is lower than the assigned
value Vc (test 84), the speed VM is increased by an
increment 0'V equal or not to OV (step 85). Otherwise, if
the speed VM is equal to or larger than Vc, it is
maintained unchanged or brought back to the value Vc
(return to step 81).
As a variant, as shown by interrupted lines on
Figure 7, when CM is equal to or lower than Cmax and VM<Vc,
it can be checked whether the torque CM has become lower
than a threshold Cmin lower than Cmax (test 86) . If yes, the
speed VM is increased by an increment A'V. Otherwise, it
remains unchanged and the process returns to test 82.
When the needles penetrate into the fiber structure,
the resistance to advance exerted by the needles causes
an increase in the torque required to continue to drive
the fiber structure at the assigned speed. The speed is
reduced by an increment AV as soon as the torque reaches
the threshold Cmax. Several consecutive speed diminution
increments may be necessary during penetration of the
needles. Upon withdrawal of the needles, the speed is
increased by one of several successive increments when
the torque CM becomes lower than Cmax or Cmin, until the
assigned speed value is again reached.
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In the embodiment of Figures 6 and 7, it is supposed
that there is no sliding between the fiber texture and
the driving presser rollers.
