Method and device for manufacturing a helically magnetised rod

Abstract

The invention relates to a method for manufacturing a helically magnetised rod (12), comprising the following steps: - injecting into a cylindrical tube (T12) a mixture (M12) comprising magnetic particles and at least one polymer forming a binder; - heating the mixture to a heating temperature of between the glass-transition temperature of the binder and the Curie temperature of the magnetic particles; - aligning the magnetic particles so as to assign them a magnetic orientation having a direction which extends in a plane orthogonal to the longitudinal axis (X12) of the tube and which varies continuously as a function of the distance separating the plane and one end of the tube; and - cooling the mixture to a cooling temperature lower than the glass-transition temperature of the binder.

Description

[0001] METHOD AND DEVICE FOR MANUFACTURING A HELICALLY MAGNETIZED ROD

[0002] The present invention relates to the field of linear position sensors, and in particular to a method and a device for manufacturing a helical magnetized rod for such a sensor.

[0003] BACKGROUND OF THE INVENTION

[0004] The brake of an aircraft wheel generally comprises a stack of friction discs, a thrust plate, and controllable actuators to selectively exert a braking force on the thrust plate and therefore on the stack of discs. Such a brake conventionally comprises a wear indicator formed by a rod secured to the thrust plate and whose linear position is representative of the wear of the stack of discs. During maintenance, an operator then manually checks the position of the rod to determine the state of wear of the discs. While such a check is reliable, it does not allow for precise estimation of the wear of the discs and makes any automated monitoring of this wear difficult.

[0005] It was therefore considered to automatically monitor the wear of the disc stack by measuring, using a magnetic sensor, the linear position of the rod.

[0006] To do this, it was proposed that the rod comprise an assembly of several permanent magnets each having, in a plane perpendicular to the longitudinal axis of the rod, a South pole and a North pole extending on either side of said longitudinal axis. The poles of the same polarity of two adjacent magnets have a non-zero angular offset around the longitudinal axis of the rod, so that the rod emits a magnetic field whose orientation changes depending on the position of the rod, which makes it possible to determine wear on the stack of discs.

[0007] The major drawback of this solution is that the manufacture of such a rod, due to the number of magnets and the complexity of their assembly, turns out to be particularly long and expensive.

[0008] Moreover, the orientation of the magnetic field emitted by the rod does not change continuously, which makes the determination of disc wear imprecise.

[0009] SUBJECT OF THE INVENTION

[0010] The invention therefore aims to propose a simple and inexpensive method of manufacturing a rod emitting a magnetic field varying, in a plane perpendicular to a longitudinal axis of the rod, continuously as a function of the distance separating the plane from one end of said rod.

[0011] SUMMARY OF THE INVENTION

[0012] To this end, the invention proposes a method for manufacturing a helical magnetized rod, comprising the following steps:

[0013] - injection into a cylindrical tube of a mixture comprising magnetic particles and at least one polymer forming a binder;

[0014] - heating the mixture to a heating temperature between the glass transition temperature of the binder and the Curie temperature of the magnetic particles;

[0015] - alignment of the magnetic particles so as to assign them a magnetic orientation having a direction which extends in a plane orthogonal to the longitudinal axis of the tube and which varies continuously as a function of the distance separating the plane and one end of the tube; and

[0016] - cooling the mixture to a cooling temperature below the glass transition temperature of the binder.

[0017] From then on, the orientation of the magnetic field emitted by the rod evolves continuously, which allows the linear position of the rod to be precisely measured using a magnetic sensor.

[0018] In particular, the method further comprises debinding and sintering of the mixture.

[0019] In particular, the method further comprises a strengthening of the magnetic field emitted by the magnetic particles.

[0020] The invention also relates to a magnetic alignment device for implementing a method. The device comprises:

[0021] - a motorized platform arranged to simultaneously ensure a translation of the tube along its longitudinal axis and a rotation of said tube around said longitudinal axis;

[0022] - a heating element fixed relative to the motorized platform and arranged to locally increase the temperature of the mixture contained in the tube up to the heating temperature;

[0023] - a cooling element fixed relative to the motorized platform and arranged to locally reduce the temperature of the mixture contained in the tube to the cooling temperature; and

[0024] - a unidirectional flux generator arranged between the heating element and the cooling element, and arranged to locally subject the magnetic particles to a unidirectional magnetic flux orthogonal to the longitudinal axis of the tube.

[0025] According to a particular feature, the heating element comprises a heating collar arranged to extend around a section of the tube.

[0026] In particular, the heating collar is of the resistive type.

[0027] According to another particular feature, the cooling element comprises a cooling collar arranged to extend around a section of the tube.

[0028] In particular, the cooling collar is a copper ring cooled by circulating water or by spraying a decompressed gas.

[0029] According to another particular characteristic, the unidirectional magnetic flux generator comprises an electromagnet selectively powered by an electric current to generate a magnetic field, and a channeling element arranged to obtain the unidirectional magnetic flux from the magnetic field generated by the electromagnet.

[0030] BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The invention will be better understood in light of the following description, which is purely illustrative and not limiting, and must be read in conjunction with the appended drawings, among which:

[0032] [Fig. 1] Figure 1 is a schematic perspective view of an aircraft wheel brake;

[0033] [Fig. 2] Figure 2 is a schematic perspective view of a measuring device for determining wear of the brake disc stack shown in Figure 1;

[0034] [Fig. 3] Figure 3 illustrates a method of manufacturing a rod of the measuring device illustrated in Figure 2;

[0035] [Fig. 4] Figure 4 is a schematic perspective view of a magnetic alignment device for implementing the manufacturing method illustrated in Figure 3.

[0036] DETAILED DESCRIPTION OF THE INVENTION

[0037] With reference to Figure 1, the invention is described here in application to a brake, designated 1, of a wheel of an aircraft A.

[0038] The brake 1 comprises a torsion tube onto which is threaded, along an axis X, a stack of discs 2 (also called heat sink) comprising alternately stator discs 2a and rotor discs 2b. The stator discs 2a and the rotor discs 2b are provided with brake linings and are respectively rotationally connected to the torsion tube and to a rim of the wheel.

[0039] The brake 1 further comprises a hydraulic crown 3 fixed to one end of the torque tube 2. The hydraulic crown 3 carries pistons 4 arranged to exert a braking force on a thrust plate 5 which is axially supported on the stack of discs 2 and which uniformly distributes the braking force thus exerted on a front face of the stack of discs 2.

[0040] This well-known arrangement of the brake 1 will not be described in further detail. It will be noted that the invention is also applicable to an electromechanical braking device.

[0041] The brake 1 is further equipped with a linear measuring device 10 for determining wear of the discs 2a, 2b of the stack of discs 2. With reference to FIG. 2, the measuring device 10 comprises a substantially cylindrical body 11 which extends along an axis Xn parallel to the axis X and which is fixed on an external face 3.1 of the hydraulic crown 3 substantially orthogonal to the axis X.

[0042] The measuring device 10 also comprises a magnetic rod 12 which extends along an axis X12 parallel to the axis X and which is slidably mounted in a bore of the body 11, so that the axis X12 is substantially coincident with the axis Xn. The rod 12 comprises a first end 12.1 which is articulated to an external face 5.1 of the thrust plate 5 via a pivot 13 with an axis substantially orthogonal to the axis X, and a second end 12.2, opposite the first end 12.1, which is free. The rod 12 is thus integral in translation, along the axis X, with the external face 5.1 of the thrust plate 5.

[0043] The rod 12 is arranged to emit a radial magnetic field C12 comprising a north pole and a south pole which are arranged on either side of the axis X12 and which each extend along a helix 12N, 12S around the axis X12, here at an angle of 360 degrees between the first end 12.1 and the second end 12.2. Thus, the orientation of the magnetic field varies, in a plane perpendicular to the axis X12, continuously as a function of a distance separating this plane from the first end 12.1 of the rod 12.

[0044] The measuring device 10 also comprises a first magnetic sensor 14.1 and a second magnetic sensor 14.2, both of the Hall effect type. The first sensor 14.1 and the second sensor 14.2 are arranged on a free face of the body 11, in the same measuring plane substantially orthogonal to the axis Xn, so that the second sensor 14.2 corresponds to an image of the first sensor 14.1 according to a rotation of the axis Xn and of amplitude equal to 90 degrees. The first sensor 14.1 is arranged to deliver a first signal representative of the intensity of the magnetic field emitted by the rod 12 in a first direction D14.1 which is included in the measuring plane Pm and which is substantially orthogonal to the axis X12. The second sensor 14.2 is arranged to deliver a second signal representative of the intensity of the magnetic field emitted by the rod 12 in a second direction D14.2 which is included in the measurement plane and which is substantially perpendicular to the axis X12 and to the first direction D14.1.

[0045] The first sensor 14.1 and the second sensor 14.2 are connected to a control unit UC of the aircraft A. The control unit UC comprises a memory module, a processor forming a processing module, and a display. The memory module contains computer programs whose execution by the processing module allows the operation of the control unit UC. The memory module further contains a database which associates measurement pairs of intensity of the magnetic field emitted by said rod 12 in the first direction D14.1 and the second direction D14.2 with an angular orientation of said magnetic field around the axis X12 and therefore with a position of said rod 12 along said axis X12, relative to the body 11.

[0046] When it is desired to determine the state of wear of the linings of the stack of discs 2, an output of the pistons 4 of the hydraulic crown 3 is commanded so that the thrust plate 5 moves along the X axis and compresses the stack of discs 2. By virtue of its connection with the thrust plate 5, the rod 12 then moves along the X12 axis. The first signal and the second signal delivered respectively by the first sensor 14.1 and the second sensor 14.2 constitute a pair of intensity measurements representative of the angular orientation of the magnetic field emitted by the rod 12 around the X12 axis. Using the database, the processing module of the control unit UC determines the angular orientation of the rod 12 associated with the pair of intensity measurements delivered by the first sensor 14.1 and the second sensor 14.2.The processing module then deduces a position of the rod 12 relative to the body 11 and therefore a wear value of the brake linings of the stack of discs 2.

[0047] The measuring device 10 thus makes it possible to obtain a reliable, precise and rapid measurement of a state of wear of the brake linings of the stack of discs 2 without it being necessary to carry out manual intervention on the brake 1.

[0048] With reference to figures 3 and 4, a method of manufacturing the rod 12 will now be detailed.

[0049] A cylindrical T12 tube with an X12 axis is first filled with an M12 mixture, also called "feedstock", comprising magnetic particles and a polymer mixture composed of thermoplastics, plasticizers and wetting agents. The polymer mixture forms a viscous binder. The T12 tube is made of stainless steel (for example 316L, 304L or equivalent steel) and forms a conformer having here an external diameter substantially between 7 and 8 millimeters and a length substantially between 5 and 8 centimeters. The magnetic particles are in the form of a permanent magnet powder such as for example: a powder of particles made of an alloy of samarium and cobalt (SmCo); a powder of particles made of an alloy of neodymium, iron and boron (NdFeB); a powder of ferrite particles... The volumetric loading rate of the magnetic particles in the M12 mixture is here substantially equal to 40%.The M12 mixture is inserted under pressure into the T12 tube via an injection press (not shown) which forces the introduction of said M12 mixture into a mold in which the T12 tube is previously received.

[0050] Once filled with the M12 mixture, the T12 tube is ready to be inserted into a magnetic alignment device 20 making it possible to orient the magnetic particles of the M12 mixture in a direction D12 which extends in a magnetization plane PM perpendicular to the X12 axis of the T12 tube and which varies continuously depending on the distance separating the magnetization plane PM from a free end of the T12 tube.

[0051] Referring to Figure 4, the magnetic alignment device 20 comprises a motorized platform 21, a heating element 22, a cooling element 23 and a unidirectional magnetic flux generator 24.

[0052] The motorized platform 21 is mounted movably on a frame and arranged to simultaneously ensure a translation of the tube T12 along its longitudinal axis X12 and a rotation of said tube T12 around said axis X12. The translation speed and the rotation speed of the tube T12 are controllable independently of each other. The motorized platform 21 comprises, for example, a carriage driven in translation along a rail by a first motor provided with a barrel which is driven in rotation around a horizontal axis by a second motor and which is provided with means for clamping one end of the tube Ti2. It is also possible to provide a single motor and a device for synchronizing the movements, for example a set of gears.

[0053] The heating element 22 is fixed relative to the frame and is arranged to locally increase the temperature of the mixture M12 contained in the tube T12 up to a predetermined heating temperature. The heating temperature is between the glass transition temperature (Tg) of the binder below which said binder is said to be glassy and exhibits the behavior of an elastic solid body, and the Curie temperature of the magnetic particles of the powder above which said magnetic particles lose their ferromagnetic properties. The heating temperature is chosen so that at this temperature, the binder used is both sufficiently liquid to allow orientation of the magnetic particles via the unidirectional magnetic flux generator 24, and sufficiently solid to prevent any movement of the magnetic particles due to the displacement and rotation of the tube T12 via the motorized platform.

[0054] 21. The heating element 22 here comprises a heating collar arranged to extend around a section of the tube T12 and to travel the entire length of the tube T12 as said tube T12 moves along its axis X12 via the motorized platform 21. The heating collar is for example of the resistive type.

[0055] The cooling element 23 is fixed relative to the frame and is arranged to locally reduce the temperature of the mixture M12 heated by the heating element

[0056] 22, up to a predetermined cooling temperature. The cooling temperature is lower than the glass transition temperature (Tg) of the binder and is chosen so that at said cooling temperature, the binder used is sufficiently solid or viscous to prevent any movement of the magnetic particles. The cooling element 23 here comprises a cooling collar arranged to extend around a section of the tube Ti2 and to travel the entire tube T12 as said tube T12 moves along its axis X12 via the motorized platform 21. The cooling collar is for example a copper ring cooled by circulating water or by spraying a decompressed gas.

[0057] The unidirectional magnetic flux generator 24 is arranged between the heating element 22 and the cooling element 23, and is arranged to locally subject the mixture M12 heated by the heating element 22 to a unidirectional magnetic flux. This unidirectional magnetic flux is orthogonal to the axis X12 of the tube T12 and has sufficient intensity to orient, in the direction of the magnetic flux, the magnetic particles of the mixture M12 heated by the heating element 22. Here, it comprises an electromagnet 24.1 selectively powered by an electric current, and a channeling element 24.2 arranged to obtain the unidirectional magnetic flux from the magnetic field generated by the electromagnet. The channeling element 24.2 is for example produced by an air gap.

[0058] The tube T12 containing the mixture M12 is thus inserted into the magnetic alignment device 20. While the motorized platform 21 causes a translation and a rotation of the tube T12 around its axis X12, each section of said tube T12 is successively subjected to:

[0059] - to the heating element 22 in order to locally heat the mixture M12 until it reaches the heating temperature and thus allow a modification of the orientation of the magnetic particles contained in said mixture M12;

[0060] - to the magnetic alignment device 24 in order to orient the magnetic particles in the direction D12 which extends in a plane orthogonal to the axis X12 of the tube T12 and which varies continuously as a function of the distance separating this plane from one end of the tube T12;

[0061] - to the cooling element in order to locally cool the heated M12 mixture until it reaches the cooling temperature and thus freeze the orientation of the magnetic particles bound by the binder.

[0062] Thus, at the output of the magnetic alignment device 20, a tube T12 is obtained containing a powder of linked magnets emitting a magnetic field of helical shape. This magnetic field comprises a north pole and a south pole which are arranged on either side of the axis X12 and which each extend around the axis X12, along the helix 12N, 12S. The pitch of the helix 12N, 12S depends on the translation speed and the rotation speed to which the tube T12 is subjected via the motorized platform 12. In the case where the translation speed and the rotation speed are constant, the pitch of the helix 12 N , 12s is constant. The linked magnets, alone or surrounded by the tube T12, thus form the rod 12.

[0063] In order to strengthen the magnetic field emitted by the permanent magnets, it is possible to insert the tube T12 again into the magnetic alignment device 20 in order to once again subject said permanent magnets to the unidirectional magnetic flux emitted by the generator 24, but by deactivating the heating element 22 and the cooling element 23.

[0064] For the same purpose, it is also possible to increase the relative density of permanent magnets inside the T12 tube by debinding and then densifying the M12 mixture by sintering. Debinding, in other words the removal of the binder present inside the T12 tube, can for example be carried out by placing the T12 tube vertically in a furnace under partial argon pressure, having previously screwed a cap onto the lower end of the tube or pinched said lower end. Such debinding makes it possible to obtain a relative density of permanent magnets greater than 90%. If a relative density equal to 100% is required, the end of the T12 tube that remained open will in turn be closed and then the T12 tube will be inserted into a HIP (Hot Isostatic Pressing) chamber to undergo hot isostatic compaction.Once sintering is complete, we obtain a T12 tube containing a solid magnet emitting a helical-shaped magnetic field.

[0065] It should be noted that the binder must have properties compatible with the implementation of the manufacturing process of the rod 12, but also the application in which said rod 12 is used. It must thus have a rheology suitable for injection operations, but also, if necessary, for debinding and sintering operations (low residual impurity level of the permanent magnet powder, in particular the residual carbon level): polymer mixtures composed of polyethylene (PE), stearic acid (SA), polyethylene glycol (PEG), paraffin (PW), polymethyl methacrylate (PMMA)...

[0066] It will also be noted that such a manufacturing method and such a magnetic alignment device 20 allow the manufacturing of a rod 12 having a length ranging from ten millimeters to several meters, and this without modification of said magnetic alignment device.

[0067] It will further be noted that the T12 tube can be cut into sections. Of course, the invention is not limited to the embodiment described but encompasses any variant falling within the scope of the invention as defined by the claims.

[0068] Although the propeller pitch 12 N, 12s is here constant, it can also be variable by for example varying the translation speed and / or the rotation speed of the Ti2 tube containing the M12 mixture. The magnetic alignment device 20 thus makes it possible to manufacture a rod 12 having a helical magnetization with variable pitch, but also to manufacture rods having helical magnetizations having different constant pitches.

[0069] To carry out the debinding and sintering operations, it may be considered to remove the T12 tube and / or to install a new one.

[0070] Although the rod 12 is used here in a measuring device for determining wear on the discs of an aircraft braked wheel, it can also be used in other types of linear position sensors, such as for example passive electrical sensors of the LVDT (Linear Variable Differential Transformer) type or active electrical sensors of the RVDT (Rotary Variable Differential Transformer) type.

[0071] The dimensions and shape of the T12 tube may vary from those described.

[0072] Although the binder here comprises a polymer mixture composed of thermoplastics, plasticizers and wetting agents, it can also comprise a mixture composed of a single polymer.

[0073] The final viscosity of the M12 mixture will be adapted according to the binder, the size and geometry of the magnetic particles, the geometry of the conformer, etc.

Claims

CLAIMS 1. Method for manufacturing a rod (12) with helical magnetization, comprising the following steps: - injection into a cylindrical tube (T12) of a mixture (M12) comprising magnetic particles and at least one polymer forming a binder; - heating the mixture to a heating temperature between the glass transition temperature of the binder and the Curie temperature of the magnetic particles; - alignment of the magnetic particles so as to assign them a magnetic orientation having a direction which extends in a plane orthogonal to the longitudinal axis (X12) of the tube and which varies continuously as a function of the distance separating the plane and one end of the tube; and - cooling the mixture to a cooling temperature below the glass transition temperature of the binder.

2. Method according to claim 1, further comprising debinding and sintering the mixture (M12).

3. A method according to any preceding claim, further comprising strengthening the magnetic field emitted by the magnetic particles.

4. Magnetic alignment device for implementing the method according to any one of claims 1 to 3, comprising: - a motorized platform (21) arranged to simultaneously ensure a translation of the tube (T12) along its longitudinal axis (X12) and a rotation of said tube around of said longitudinal axis; - a heating element (22) fixed relative to the motorized platform and arranged to locally increase the temperature of the mixture (M12) contained in the tube up to the heating temperature; - a cooling element (23) fixed relative to the motorized platform and arranged to locally reduce the temperature of the mixture (M12) contained in the tube to the cooling temperature; and - a unidirectional flux generator (24) arranged between the heating element (22) and the cooling element (23) and arranged to locally subject the magnetic particles to a unidirectional magnetic flux orthogonal to the longitudinal axis of the tube.

5. Device according to claim 4, wherein the heating element (22) comprises a heating collar arranged to extend around a section of the tube (T12).

6. Device according to claim 5, in which the heating collar is of the resistive type.

7. Device according to claim 4, wherein the cooling element (23) comprises a cooling collar arranged to extend around a section of the tube (T12).

8. Device according to claim 7, in which the cooling collar is a copper ring cooled by circulation of water or by spraying of a decompressed gas.

9. Device according to claim 4, in which the unidirectional magnetic flux generator (24) comprises an electromagnet (24.1) selectively powered by an electric current to generate a magnetic field, and a channeling element (24.2) arranged to obtain the unidirectional magnetic flux from the magnetic field generated by the electromagnet.

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