METHOD AND POTATO CONTROL DEVICE ADVANCING A FIBROUS PLATE PERFORATED BY NEEDLES Background of the Invention The invention relates to fiber structures that are punctured by needles. A particular but non-exclusive field of the invention is to make plates, sleeves, or other preforms with needles, for example annular, suitable for constituting the reinforcement of composite parts. In a well-known manner, a fiber structure for piercing with needles is advanced by passing a set of needles carried by a head with needles, and the needles are periodically inserted into the fiber structure and then recovered from it, imparting a return movement to the head of "needles in a direction extending transversely relative to the structure of the forward direction." Reference may be made in particular to U.S. Patent Number 4,790,052 which describes making fiber structures perforated by needles by successively piercing with needles layers formed by flat superimposed layers or by turns wound on one another The intended field of that patent is to make fiber reinforcement for thermostructural composite parts, and in particular parts of material Carbon-carbon composite or composite ceramic matrix parts in which the fiber reinforcement it is densified by means of a carbon or ceramic matrix. Needle-punched fiber structures are made of refractory fibers, typically carbon fibers or ceramic fibers, it being possible to perform needle puncturing on the fibers when the material is in a precursor state for carbon or ceramic, and after drilling with needles the precursor is transformed by heat treatment. The intended applications of the aforementioned patent are brake discs or the divergent portions of rocket engines, these applications require materials having good mechanical properties and the ability to preserve them at high temperatures. Drilling with superposed layers of fibers fiber needles to transfer fibers in the Z direction, that is, transversely in relation to the layers. This produces a structure that exhibits less disuniformity and increased capacity to withstand delamination, ie increased resistance to the layers that separate due to the shear forces to which they can be subjected, particularly in the brake discs. In order to perform needle punctures on the entire surface area of a fiber structure, the structure is advanced past a needle head. When needle piercing is performed on each new superimposed layer, a forward movement is performed each time a new layer is put in place so that the needle head sweeps over the entire surface area of the more recently superimposed layer. If the fiber structure is continuously advanced at a constant speed, then it moves transversely in relation to the needles during the whole duration of the penetration of needles. In particular, when the structure is thick or once it has become thick, the forced advance causes the needles to bend and break. In addition to the fact that broken needles need to be replaced, the presence of broken guides within the fiber structure may be undesirable in the later use of that structure. It could be considered to ensure that the structure proceeds very slowly, thereby minimizing the bending forces applied to the needles while they are present within the fiber structure, or also advancing the structure discontinuously so that it is stationary during the penetration of the needles. . However, these solutions have the clear disadvantage of considerably increasing the time and thus also the cost required for the process of completely penetrating the structure with needles. Another process for controlling the advancement in a needle punching machine is described in the French patent FR 22 729 404, in which the rotational speed of the extraction rollers for the fiber structure perforated by needles is modulated, so that the speed of rotation has different values for different positions of the needles. It is then necessary to provide a system that allows the rollers to be driven at variable speed. Also, the variation in velocity does not take into account the actual instantaneous forces applied to the needles.
OBJECT AND SUMMARY OF THE INVENTION The object of the invention is to propose a method of drilling with needles a fiber structure that makes it possible to solve the problem of needle rupture without significantly penalizing the speed of the method, even with structures that are thick. . According to the invention, the instantaneous speed of advance of the fiber structure decreases in response to the resistance to advance exerted by the needles penetrating the structure, and increases after the needles have been removed, so that the force exerted on the needles by the advance of the structure is limited, but without completely interrupting the advance during the entire duration of the needles that are present in the structure. In a preferred implementation, the decrease in the forward speed is directly caused by the resistance to advance exerted by the needles penetrating the structure. When the structure is moved by means of a controlled member connected to a driving motor by means of a transmission, the decrease in the forward speed can be absorbed by a mechanical play in the transmission. In this case, since the motor is driven at a constant speed, the play which is preferably elastic is automatically picked up as soon as the needles have been removed. In this way, while the needles are penetrating, the speed of advance of the structure decreases below an average speed of advance corresponding to the speed of the engine, and as soon as the needles have been removed, this speed increases above the speed progress average. A measurement of the torque can be carried out at the level of a driving element coupled with the fiber structure, in order to decrease the speed of the engine when the torque becomes less than a given threshold, as a consequence of the decrease in speed of the fiber structure. In another embodiment of the method, a representative value of the force exerted to drive the fiber structure is measured, the impulse velocity of the fiber structure is reduced when the measured value becomes equal to or greater than a first threshold value, and, after the speed has been reduced, the pulse speed increases when the measured value becomes less than a second threshold value. The second threshold value can be equal to or less than the first. The measured value is for example representative of the torque exerted by a conduction element for the fiber structure. Another object of the invention is to provide an installation that enables the method to be implemented. This object is achieved by means of an installation for needle-piercing a fiber structure, the installation comprising a needle-piercing head having a plurality of needles, a device for controlling the head for piercing with needles carrying a plurality of needles. , a device for driving the head for piercing with needles to impart alternating movement to the needles, a support for the fiber structure to be pierced with needles located facing the needle head for piercing, and a device for driving the structure of fibers for imparting a forward movement thereto in said support, in this installation, according to the invention, the structure driving device is designed to enable the advance speed of the fiber structure carried out by the support for diminish momentarily in response to the resistance to the advance exerted by the needles that penetrate the structure of fibers, without completely interrupting the advance over the entire duration that the needles are present in the fiber structure.
BRIEF DESCRIPTION OF THE DRAWINGS The description will be better understood by reading the following description given by way of non-limiting indication and with reference to the accompanying drawings, in which: Figure 1 is a very schematic front view of a drilling facility with needles; Figure 2 is a very schematic fragmentary view on a larger scale of the installation of Figure 1 in lateral elevation and in section; Figure 3 is a very schematic view of a pulse device that makes it possible, during needle drilling, for a fiber structure to advance through an installation of the kind shown in Figures 1 and 2, for one embodiment of the invention. invention; Figure 4 is a graph showing how the forward speed of a fiber structure being pierced by needles varies as a function of time for the mode shown in Figure 3; Figure 5 is a graph illustrating the displacement of a fiber structure that is being perforated with needles in the embodiment of Figure 3; Figure 6 is a very schematic view of a pulse device for a structure being pierced with needles, according to a second implementation of the invention; and Figure 7 is a diagram showing the process of controlling the conduction of the fiber structure in the second implementation of the invention.
DETAILED DESCRIPTION OF THE IMPLEMENTATIONS OF THE INVENTION Figures 1 and 2 show an installation for needle-piercing a fiber structure in the form of a plate 10. The structure 10 is composed of two flat layers stacked flat and perforated with needles with one another for gluing the layers between them and give the plate resistance against delamination.
located below a needle-piercing head 24. An impulse system 26 comprising at least one crane and connecting rod assembly imparts alternating vertical movement to the piercing head with needles 24 under the control of a motor (not shown) . The needle piercing head 24 extends over the entire width of the plate 10 and carries a plurality of needles 28. The holes 22a are formed through the platen 22 in registration with the needles 28. The plate 10 is advanced by means of pairs of press rolls 40, 50 located between the drilling platen with needles 22 and each of tables 12 and 14, respectively. In each pair 40 or 50 of the press rolls, the rollers 42 and 44, 52 and 54 are driven in rotation, with at least one of the rollers in each pair being disengageable, for example the lower roller 44, 54. When the plate 10 moves from the table 12 towards the table 14, impulse is provided by the rollers 52 and 54 pressed against one another while the roller 44 is disengaged, and possibly also the roller 42. Conversely, when the plate 10 is moved from the table 14 to the table 12, then the impulse is made by the rollers 42 and 44 pressed against each other, while the roller 54 is disengaged, and possibly also the roller 52.
When only one roller is disengaged on a pair of non-driven rollers, it is also advantageous to eliminate the pressure exerted by the rollers to avoid any impulse effect from the non-disengaged roller. It will be noted that the lower rollers 44, 54 can be replaced by conveyor belts which can then also form tables 12 and 14. After each needle piercing step, when the plate 10 has reached table 12 or 14, it overlaps a new layer and a new step of piercing with needles is made moving the plate 10 towards the other table 14 or 12. During each step of perforation with needles, the needles 28 penetrate vertically in the plate 10. The depth of penetration of the needles 28 in the plate 10 is a function of the position of the needle piercing head, at one end of its vertical travel, measured in relation to the needle piercing plate 22. The depth of the needle penetration can be extend through several thicknesses of overlapping layers. This depth can be adjusted depending on the desired distribution for needle piercing density through the thickness of the plate. When a substantially uniform depth of penetration is desired, then the distance between the needle piercing platen 22 and the needle piercing head increases incrementally after each overlap of a new layer, imparting a downward step towards the piercing table with needles Reference may be made to the aforementioned U.S. Patent No. 4,790,052. A step down similar to tables 12 and 14 can be imparted with the tables and the platen mounted on a vertically movable common frame. Thus, during the piercing with needles of the first strata constituting the plate 10, the needles 28 pass through all the strata and penetrate the holes 22a. As soon as the plate 10 has accumulated to a certain thickness, the needles 28 no longer reach the plate 22. By means of the invention, the advancing speed of the plate 10 becomes slower when the needles penetrate the plate to limit the bending force applied to the needles due to the advance of the plate and thus eliminate or at least minimize the risk of needle breakage. To this end, in a preferred implementation of the invention, the lowering of speed is produced directly by the penetration of the needles that exert a force that slows the advance of the plate. This can be achieved by including a mechanical play in the transmission between a pulse motor and the pairs of the press rolls. Figure 3 shows a device for driving the press rolls and comprising a motor 60 rotating at constant speed and driving a strip 62 passing over the press rolls 42, 44 and 52, 54. In its path between the motor 60 and the upper press roller 42, the band 62 passes over a tension roller 64 and a deflector roller 65, and on its path between the press roller 52 and the motor 60, the band 62 passes over a deflector roller 67 and a tension roller 66. In order to prevent slippage between the band 62 and the rollers on which it passes, it is preferable to use a corrugated double-sided band with corresponding relief formed on the surfaces of the rollers where they come into contact with the band . The tension rollers 64 and 66 are fixed at the ends of the respective arms 68 and 70 which form articulated levers which are subjected to the elastic return force exerted by the respective devices 72 and 74 so as to keep the belt 62 permanently under tension . The devices 72 and 74 exert elastic return forces which may be in the form of springs, or preferably in the form of pneumatic dampers. The pressure in the pneumatic dampers is advantageously adjustable. The operation is as follows: When a plate 10 moves from the table 14 towards the table 12, the plate is driven by the press rolls 42,44 while at least the lower roller 54 is disengaged in the press 50. The motor 60 rotates in the direction represented by the arrow Fl. When the needles penetrate the plate 10, the plate lowers the speed by the needles, whereby the speed of rotation of the rolls 42, 44 is reduced. Because the band 62 is driven at constant speed by the motor 60.1 the length of the belt between the motor 60 and the roller 42 increases. This increase in length is absorbed by the tension roller 64 under the action of the elastic return force exerted by the damper 72 by pivoting the lever 68 in the direction indicated by the arrow F2. Conversely, the length of the web between the roller 52 and the motor 60 decreases, whereby it causes the lever 70 to pivot in the direction indicated by the arrow F3 against the force exerted by the damper 74. When the needles subsequently exit the plate , the press rolls 42, 44 accelerate and the length of the band which has accumulated between the motor 60 and the roll 42 is collected until the two tension rolls 64 and 66 have returned to an equilibrium situation. By adjusting the pressure in the shock absorbers 72, 74 it is possible to adjust the return force exerted and achieve synchronization and thus the proper operation of the impulse system.
When plate 10 moves from table 12 to table 14, the operation is symmetric to that described above. Figure 4 shows how the forward speed of the plate 10 varies as a function of time as the plate goes from the table 12 to the table 14, or vice versa, the curve Ci representing the speed of the plate entering the station of drilling with needles and curve C2 its speed when leaving it. The speed can be measured by means of a sensor carrying a follower wheel that rests on the plate and rotates through the advancing plate. Curve A represents the displacement of the needles between their high and low positions. The difference between the speeds of advance in the entrance and the exit are due to the perforation with needles. Because the fibers are transferred in the Z direction, the velocity measured in the upper layer not perforated with needles from the needle piercing station is greater than the speed of the plate measured after the needle piercing. Times ti and t2 mark the beginning of the penetration of the needle into the plate and the complete removal of the needles from the plate. The difference in time At between the times ti and t2 depends on the depth of penetration selected for the needles. Due to the elastic play present in the transmission between the motor 60 and the presses 40 and 50, the forward speed of the plate 10 continuously varies between a maximum speed that is higher than the average forward speed corresponding to the motor speed, while that the needles are not on the plate, and a minimum speed that is slower than the average speed of advance, while the needles are on the plate, but the advance is not interrupted during the time interval between ti and t2. In Figure 5, the curves represent the displacement of the plate as a function of time, measured first in the last layer entering the area of needle punching (curve Da), and secondly leaving the area of needles (curve D2) ), and curve A represents the displacement of the needles. The displacement at the entrance and at the exit is measured by the respective sensors that provide signals representative of the distance the plate advances. It can be seen that at the outlet, that is, immediately downstream of the press rollers driving the plate, the advance of the plate decreases briefly after the instant ti into which the needles penetrate, and begins to increase briefly after the instant t2 when the needles have been completely removed from the plate. At the entrance, in the area of needle drilling, that is, immediately upstream of the pressure rollers disengaged, the advance continues to increase after the ti time before reverting. This can be explained by the ability of the blade to deform elastically in the longitudinal direction combined with the fact that, in the embodiment concerned, no disassembly element of the type constituted by a press foot is used which holds the plate towards down during the penetration of the needles. Consequently, when the needles are raised, they tend to lift. Slightly plate before it is free and fall back onto the piercing plate with needles. In a variant of the embodiment described above, the lowering of the speed of the plate can be detected by measuring the torque at one end of the arrow of one or both pressure rollers ensuring the momentum of the plate. The stretching of the portion of the band between the motor and the driving roller as a consequence of the measured torque shows the slowing down of the plate. It is then possible to control a reduction of the speed of 1 pulse motor from an assigned value when the decrease in measured torque reaches a given threshold, which is added to the effect of the elastic play in the transmission to quickly react for resistance to the advance exerted by the needles. After removal of needles from fiber structure, the shortening of the portion of the belt causes the impeller press rolls to accelerate. The motor speed can then increase and return to its assigned value in response to detecting an increase in measured torque. Obviously, the measurement of the torque is carried out alternately on one pair of press rolls and on the other pair, as a function of the direction of plate displacement. In the previous variant, the motor is controlled at a speed that is constant but adjustable. One embodiment of the invention is described above for piercing with needles a plate that moves in straight translation through the needle piercing station. However, the person skilled in the art will immediately see that the invention also applies to needle-puncturing the annular fiber structures formed by helically winding a fiber structure as flat overlapping turns or for needle-punched sleeve-shaped structures formed by laminating a fabric or superimposed turns. These structures are driven in rotation by passing a drill head with needles. In these circumstances, the advance movement of the fabric is one dimension. Causing that the speed of advance of the structure of fibers lowered the speed directly by the resistance to advance caused by the needles with elastic game included in the transmission has several advantages: it is autoadaptive, in particular as the layers accumulate and according to the thickness of the structure increases at the beginning of the accumulation, and makes it possible to maintain an average forward speed which is relatively high since the forward acceleration when the needles are not in the structure compensates for the advance deceleration caused by the penetration of the needles. However, the forward speed of the fiber structure could be controlled as a function of the measurement of a value representing the force exerted to move the fiber structure. This value is for example the torque that must be exerted to displace the fiber structure. The torque can be measured at the level of a pulse element, for example at the level of the drive motor. Figure 6 shows an impulse device that differs from that of Figure 3 because it does not include an elastic set. The motor 60 drives the press rolls 42-44 and 52-54 by means of the band 62 passing over deflecting rollers 65, 67 and 64 ', 66', the latter having a fixed axis opposite the rollers 64, 66 of Figure 3. A sensor (not shown) provides a signal Sc representative of the value of the torque CM exerted by the motor 60, for example by measuring the current drawn by the motor. The motor may consist of a stepper motor controlled by the control circuit 80. As illustrated by FIG. 7, the motor 60 is originally controlled at a predetermined rated speed Vc (step 81). If the measured torque becomes equal to or greater than a maximum threshold value Cmx (test 82), the motor speed decreases by an increase? (step 83) before returning to test 82. If the torque torque CM is less than Cmax (test 82), and if the motor speed is less than the assigned value Vc (test 84), the VM speed increases by an increase ñ'V equal or not to AV (step 85). Otherwise, if the velocity VM is equal to or greater than Vc, it remains unchanged or is brought back to the value Vc, (returns to step 81). As a variant, as shown by the lines interrupted in Figure 7, when CM is equal to or less than Cmax and VM <; Vc, it can be verified if the torque CM has become smaller than a threshold Cmin less than Cmax (test 86). If it is, the VM speed is increased by an increase ??. Otherwise, it remains unchanged and the process returns to test 82. When the needles penetrate the fiber structure, the resistance to advance exerted by the needles causes an increase in the torque required to continue driving the fiber structure. at the assigned speed. The speed is reduced by an AV increase as soon as the torque reaches the CmaX threshold. Several consecutive increases in the decrease in speed may be necessary during the penetration of the needles. After removal of the needles, the speed is increased by one of several successive increments when the torque CM becomes less than Cmax or Cmin, until the value of the assigned speed is reached again. In the embodiment of Figures 6 and 7, it is assumed that there is no slippage between the texture of the fibers and the driven press rolls.