Eddy current magnetic braking device, braked vehicle wheel and aircraft landing gear and aircraft provided with such a wheel

Abstract

The invention relates to a magnetic braking device for a vehicle wheel, the device comprising at least one stator element (2) arranged so as to be rotatably connected to a portion of the vehicle supporting the wheel, a rotor element (3) arranged so as to be rotationally connected to the wheel, magnets for producing, between these elements, a magnetic flux capable of generating eddy currents in one of the elements when the wheel rotates and a braking control unit (11) that receives, as input, a braking command to apply a braking force according to the braking command. The magnets comprise at least one electromagnet (9) and the device comprises at least one anti-locking control unit (16) that receives, as input, a vehicle speed signal and a wheel rotational speed signal and provides, as output, a control signal of the electromagnet according to a comparison of speeds from the signals received as input. The invention also relates to an aircraft wheel and a landing gear comprising such a device.

Description

[0001]EDDY CURRENT MAGNETIC BRAKING DEVICE, BRAKED VEHICLE WHEEL AND AIRCRAFT LANDING GEAR, AIRCRAFT EQUIPPED WITH SUCH A WHEEL The present invention relates to the field of braking vehicle wheels such as aircraft wheels. BACKGROUND OF THE INVENTION An aircraft wheel generally comprises a rim surrounded by a tire and connected by a disc to a hub mounted to rotate on a wheel support shaft (axle or spindle). Friction braking devices are known comprising a stack of brake discs which is housed in an annular space extending between the rim and the hub and which comprises an alternation of rotor discs linked in rotation with the wheel and stator discs fixed relative to the wheel support shaft.The braking device also includes hydraulic or electromechanical actuators mounted on an actuator holder and arranged to apply a pressing force to the stack of discs so as to generate a braking torque to brake the rotation of the wheel. Many vehicles equipped with a friction braking device include a wheel anti-lock (or anti-skid) system associated with the friction braking device. Indeed, if the braking torque is too high in relation to the speed of the braked vehicle and the coefficient of friction of the wheel tires relative to the ground, the braking device locks the wheels which then skid on the ground: such skidding reduces the vehicle's maneuvering capacity and causes wear and rapid heating of the tires which can burst.To prevent the wheel from locking, the anti-lock system compares the rotational speed of the wheels with the speed of the vehicle and controls the actuator to release the pressing force when the gear ratio is greater than a threshold (corresponding substantially to the circumference of the wheel). To obtain effective anti-lock, the release of the pressing force must be obtained in a response time of between 50 and 100 ms, a response time compatible with most friction braking devices. Eddy current magnetic braking devices are also known, used for braking vehicle wheels and more particularly aircraft wheels.Document FR-A-3122405 describes such a device comprising a rotor rotatably connected to the wheel, two stators which frame the rotor and which are rotatably connected to the wheel support shaft and free in translation relative to said shaft, magnets for producing an axial magnetic flux between the stators and the rotor, and linear actuators for axially moving the stators between a maximum braking position in which the stators are close to the rotor and a free rotation position in which the stators are distant from the rotor. In an eddy current magnetic braking device, the braking torque produced is all the greater as the relative rotational speed of the rotor relative to the stators is high and the braking torque decreases with the relative rotational speed. For this reason, an anti-lock system for the wheels was not envisaged.However, it appeared that the risk of wheel locking also existed with an eddy current braking device. It was therefore considered to control the actuators to separate the stators from the rotor exactly as in an anti-lock system of a friction braking device. However, it was determined that the stroke of the actuators necessary for a sufficient reduction in the braking torque to avoid wheel locking is five times greater with a magnetic braking device than with a friction braking device. As a result, conventional magnetic braking devices have too long a response time to allow effective anti-locking to be obtained. OBJECT OF THE INVENTION The invention aims in particular to propose a braking device that at least partially overcomes the aforementioned drawbacks.SUMMARY OF THE INVENTION To this end, according to the invention, there is provided an eddy current magnetic braking device for a vehicle wheel, comprising at least one stator element arranged to be rotationally connected to a part of the vehicle supporting the wheel, a rotor element arranged to be rotationally connected to the wheel, magnets for producing between said elements a magnetic flux capable of generating eddy currents in one of the elements when the wheel rotates, and a braking control unit receiving as input a braking command for applying a braking force as a function of the braking command.The magnets comprise at least one electromagnet and the device comprises at least one anti-lock control unit receiving as input a vehicle speed signal and a wheel rotation speed signal and providing as output a control signal for the electromagnet based on a comparison of speeds from the signals received as input. The comparison of speeds from the signals received as input allows at least the anti-lock control unit to detect wheel lock and to provide as output a control signal for the electromagnet to remedy this lock. By controlling the electromagnet, it is possible to very quickly cancel the magnetic flux produced and therefore cancel the braking torque.It has thus been observed that anti-lock using an electromagnet is several times faster than anti-lock using modification of the stator / rotor air gap in order to increase or decrease the braking torque in an eddy current braking device. Indeed, when the air gap is increased, the braking torque takes longer to decrease than when the electromagnet current is decreased. The same phenomenon can be observed when the air gap is decreased: the braking torque takes longer to increase than when the electromagnet current is increased. The response time of anti-lock by modification of the electromagnet current is therefore relatively low: the modification of the electromagnet supply current allows an amplitude of variation of the braking torque equivalent to a rapid release of the press force for a friction braking device.Furthermore, the anti-lock control circuit can have a rapid dynamic response. Preferably, the anti-lock control unit is arranged to control the electromagnet to generate a reduction in the braking torque in the event of detection of wheel lock and then an increase in the braking torque when the lock disappears, the anti-lock control unit controlling the electromagnet in such a way that the increase in the braking torque is slower than the decrease in the braking torque. This makes it possible to limit the risk of rapid wheel lock upon application of the braking force. The invention also relates to a braked wheel equipped with such a device; a landing gear comprising a leg having one end carrying a shaft on which the hub of such a wheel is mounted, the stator element of the braking device being rotationally connected to the leg via the shaft; and an aircraft equipped with such a landing gear.Other characteristics and advantages of the invention will emerge from reading the following description of particular and non-limiting embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Reference will be made to the appended drawings, among which: [Fig. 1] Figure 1 is a partial schematic front view of an aircraft equipped with landing gear according to the invention; [Fig. 2] Figure 2 is a partial schematic view of a wheel according to the invention, in axial half-section; [Fig. 3] Figure 3 is a partial schematic perspective view of the stators and rotor of the braking device of this wheel; [Fig. 4] Figure 4 is a curve relating a skid ratio (slip ratio, equal to 1 minus the ratio between the product of the angular speed of the wheel and the radius of the wheel on the speed of the vehicle) and a coefficient of friction (Mu) for a given state of the landing runway used by the aircraft (here, dry runway); [Fig.5] Figure 5 illustrates the modification of the supply current of the electromagnets and shows the superposition of the target current and the supply current of the electromagnets; [Fig. 6] Figure 6 superimposes a curve representing the response time of the braking torque after modification of the air gap and a curve representing the response time of the braking torque after modification of the supply current of the electromagnets; [Fig. 7] Figure 7 represents the braking torque obtained by modification of the supply current of the electromagnets as a function of time; [Fig. 8] Figure 8 is a schematic view of the braking control logic circuit according to the invention. DETAILED DESCRIPTION OF THE INVENTION With reference to Figures 1 to 3, the braking system according to the invention is carried by an aircraft 100 comprising landing gear 101.Each landing gear 101 comprises a leg having one end provided with two coaxial shafts 102 on each of which a wheel 103 is mounted to pivot. Each wheel 103 comprises, in a manner known per se, a hub 104 mounted to pivot on the shaft 102 and a rim 105 connected to the hub 104 by a web 106. The rim 105 and the hub 104 define between them an annular space having one end closed by the web 106 and one end open towards the outside of the wheel 103. The rim 105 is surrounded by a tire not shown. According to the invention, the wheels 103 are equipped with a magnetic braking device generally designated 1, which extends here in the annular space defined by the rim 105 and the hub 104. The magnetic braking device 1 comprises fixed elements, or stators 2, and mobile elements, or rotors 3.More precisely here, the stators 2 and the rotor 3 are in the form of discs, coaxial with the wheel 103, therefore having collinear central axes. The rotor 3 is arranged between the two stators 2 and has two opposite main faces 3.1 each facing a main face 2.1 of one of the stators 2. The main faces 2.1, 3.1 are parallel to each other. The stators 2 are rotationally connected to the shaft 102 or to the leg of the landing gear 101, here by means of a ribbed torque tube 4 which is fixed to an external collar of the shaft 102: the stators 2 are engaged on the torque tube 4. The rotor 3 is rotationally connected to the wheel 103, here to the rim 105 of the wheel 103 by means of bars extending in projection towards the inside of the rim 105. Thus, the rotor 3 rotates on itself around its central axis relative to the stators 2 which frame it: during this movement of the rotor 3 in a circumferential direction, the main faces 3.1 remain opposite the main faces 2.1 and parallel to them. Each of the stators 2 is mounted to slide (without rotation) on the torque tube 4 to be movable in an axial direction of the torque tube 4 between a first position, or maximum braking position, in which the rotor 3 and the stator 2 are close to each other and have their main faces 3.1, 2.1 separated by a first predetermined air gap and a second position, or free rotation position, in which the rotor 3 and the stator 2 are spaced apart from each other and have their main faces 3.1, 2.1 separated by a second predetermined air gap greater than the first predetermined air gap. Each stator 2 is connected by a mechanism symbolized at 5 to at least one electromechanical actuator symbolized at 6, controllable by the pilot of the aircraft in a manner known per se, to actuate the mechanism 5 which moves the stator 2 between the two aforementioned positions.For example: the actuator may be a linear actuator and the mechanism may be a direct connection of the movable element of the actuator to the stator or comprise one or more intermediate levers; the actuator may be rotary and the mechanism a screw / nut system. An axial stop, of the rolling bearing or needle bearing type, is preferably provided interposed between the rotor 3 and the stators 2 (or between parts connected to them) to ensure that the stators 2 cannot be brought closer to the rotor 3 beyond the first air gap. Each stator 2 comprises a plurality of permanent magnets 7 having magnetic fields capable of generating eddy currents in the rotor 3 when the stator 2 is in the first position and the rotor 3 pivots opposite the stator 2. The permanent magnets 7, here based on rare earths, are preferably fixed on a support 8 for example made of magnetic steel.Each stator 2 also comprises a plurality of electromagnets 9 fixed on the support 8 alternating with the permanent magnets 7. The electromagnets 9 comprise a coil connected to a current source and are dimensioned to be able to generate a magnetic field canceling the magnetic field produced by the permanent magnets 7 or even reversing the magnetic field emitted by the stator 2. The rotor 3 is made of copper or any other electrically conductive material. The rotor 3 has a thickness such that a skin effect (otherwise called the film effect or Kelvin effect) is generated from each face 3.1 of the rotor 3 over more than half of the thickness of the rotor 3 at least over a range of possible relative speeds of the rotor 3 with respect to the stators 2. The eddy currents generated from the two faces 3.1 will then circulate in the central part of the rotor 3, which will increase the braking torque.This results in a "superposition of skin effects", the thickness of the rotor 3 being sufficiently low to obtain this effect while satisfying the thermal and mechanical constraints. In one example, this effect gives approximately 60% more performance. We will now look in more detail at the braking control. It is understood that to cause braking, the electromechanical actuators 6 are controlled to bring the stators 2 into the first position and that, to interrupt braking, the electromechanical actuators 6 are controlled to bring the stators 2 into the second position, a position in which the permanent magnets 7 do not allow sufficient eddy currents to be generated in the rotor 3 to cause braking of the rotor 3. It will be noted that below a certain rotational speed of the rotor 3, the braking torque is negligible regardless of the position of the stators 2.It will then be necessary to consider an additional brake. It is also understood that braking can be interrupted by powering the electromagnets 9 so as to cancel the magnetic field produced by the permanent magnets 7. The control circuit 10 of the electromechanical actuators 6 and the electromagnets 9 will now be described in relation to FIG. 8. The braking control circuit 10 defines a braking servo loop and comprises a braking control unit 11 (or BCU) having: - a first input connected to the braking control instrument actuated by the pilot of the aircraft 100, - a second input connected to the return channel of the braking loop, - a first output connected to a first input of a first adder 12, - a second output connected to a first input of a second adder 13.The first adder 12 has an output connected to an input of an actuator control unit 14 (or ACU) having an output connected to the actuators 6. The second adder 13 has an output connected to an input of an electromagnet control unit 15 (or CCU) having an output connected to the electromagnets 9. The braking control circuit 10 further comprises an anti-lock control unit 16 having: - a first input connected to a communication bus of the aircraft 100 to receive in particular the speed of the aircraft 100 provided by sensors of the aircraft 100, such as the speed relative to the ground (inertial, GNSS or hybrid) provided by the navigation unit of the aircraft 100; - a second input connected to an angular speed sensor 17 arranged to determine the rotation speed of the wheels 103; - a first output connected to a second input of the first adder 12; - a second output connected to a second input of the second adder 13.The units 11, 14, 15 and 16 are electronic units whose structure is known in itself. They may comprise one or more integrated circuits, microcontrollers, FPGAs or others. The braking control unit 11 comprises for example a computer in the form here of an integrated circuit programmed to process the signals coming from its inputs so as to develop: - an actuator braking control signal sent to the first output; - an electromagnet braking control signal sent to the second output. The anti-lock control unit 16 comprises for example a computer in the form here of an integrated circuit programmed to process the signals coming from its inputs so as to develop: - an actuator anti-lock control signal sent to the first output; - an electromagnet anti-lock control signal sent to the second output.The first adder 12 provides an actuator control signal as output (corresponding either to the actuator braking control signal or to the actuator anti-lock control signal as will be seen later). The second adder 13 provides an electromagnet control signal as output (corresponding either to the electromagnet braking control signal or to the electromagnet anti-lock control signal as will be seen later). The actuator control unit 14 comprises, for example, a computer in the form here of an integrated circuit programmed to process the actuator control signal arriving at its input so as to develop, via a power circuit associated with the control circuit, an actuator power signal to power the actuators 6.The electromagnet control unit 15 comprises, for example, a control circuit in the form here of an integrated circuit programmed to process the electromagnet control signal arriving at its input so as to develop, via a power circuit associated with the control circuit, an electromagnet power signal to power the electromagnets 9. As visible in FIG. 5, the electromagnet power signal powering the electromagnets 9 can here vary between positive and negative values, as required. In operation, the anti-lock control unit 16 has in memory the radius of the tires used on the aircraft 100 and can, from said radius, the angular speed of rotation provided by the tachometer 17 and the speed of the aircraft 100 provided by the communication bus, calculate a skid rate and compare it to a threshold here set at 0.12 in the case of a dry runway (see figure 4).The threshold here corresponds to the optimal slip rate: as long as the slip rate is less than or equal to this threshold, the tire is in a stable zone and it is considered that braking is carried out without locking the wheel. If the slip rate exceeds the threshold, skidding is detected. As long as no skidding is detected, the electromagnets 9 and the actuators 6 are controlled by the braking control unit 11 only, without intervention from the anti-lock control unit 16, to deliver a braking torque proportional to the driver's braking command. The braking control unit 11 sends control signals to the actuator control unit 14 to modify the value of the air gap between the stators 2 and the rotor 3 and / or to the electromagnet control unit 15 to modify the magnetic field produced by the electromagnets 9 according to the braking force required by the pilot.The modification of the air gap by the actuators 6 and the modification of the magnetic field of the electromagnets 9 will increase or decrease the braking torque and therefore the speed of the aircraft 100. The modification of the air gap by the actuators 6 affects the magnetic flux transmitted to the rotor 3 by the permanent magnets 7 and by the electromagnets 9 (at constant supply current). The modification of the supply current of the electromagnets 9 affects the magnetic flux transmitted to the rotor 3 by the electromagnets 9 and also the magnetic flux transmitted by the permanent magnets 7 since the electromagnets 9 can cancel the magnetic field transmitted by the permanent magnets 7.For example, the braking control unit 11 can be configured to: - only act on the actuators 6 as long as the speed of variation of the braking force requested by the pilot (determined for example as a function of the amplitude of movement of the control instrument manipulated by the latter) is less than a threshold; and - act jointly on the actuators 6 and the electromagnets 9 when the speed of variation of the braking force requested by the pilot (determined for example as a function of the amplitude of movement of the control instrument manipulated by the latter) is greater than the threshold. In the second case, it is understood that the production of the braking torque is distributed between the permanent magnets 7 and the electromagnets 9.This distribution can change over time during braking: - at the start of braking, the actuators 6 and the electromagnets 9 are acted upon by a modification, for example of the same amplitude, but the increase in the magnetic flux of the electromagnets 9 being much faster than that of the magnetic flux of the permanent magnets 7, the share of the electromagnets 9 in the increase in the braking torque is greater, - as the braking continues and the air gap between the rotor 3 and the stator 2 decreases, the braking control unit 11 can reduce the supply current of the electromagnets 9 so as to balance the contributions of the permanent magnets 7 and the electromagnets 9 to the braking torque, or even ensure that the contribution of the permanent magnets 7 gradually replaces that of the electromagnets 9.The brake control unit 11 can be configured in the same way to release the braking force. Other strategies for controlling the actuators 6 and the electromagnets 9 are of course conceivable for applying the braking force or for releasing it. As soon as a skid is detected, the anti-lock control unit 16 sends an interrupt to the brake control unit 11. The outputs of the brake control unit 11 are blocked and the anti-lock control unit 16 takes control of the braking loop.The anti-lock control unit 16 then sends control signals to the electromagnet control unit 15 to modify the magnetic field produced by the electromagnets 9, and possibly to the actuator control unit 14 to modify the value of the air gap between the stators 2 and the rotor 3, depending on the difference between the speed of the aircraft calculated from the rotational speed of the wheels and the speed of the aircraft from the communication bus. The actuator control unit 14 will send a voltage to the actuators 6 to modify the value of the air gap. The electromagnet control unit 15 will send a voltage to the electromagnets 9 to modify the current of the electromagnets 9. These actions aim to reduce the braking torque and to release / unlock the wheel 103 in order to stop the skidding.Once the skidding has been detected by the anti-lock control unit 16, the torque is gradually increased (using the electromagnets and / or the air gap adjustment) up to a value corresponding substantially to the theoretical optimum slip rate of the tire-ground contact (the anti-lock control unit 16 has in memory a curve such as that of FIG. 4 or a table corresponding to it for different runway conditions), depending on the actual conditions (dry, wet, icy runways). The information on the actual conditions is for example provided to the aircraft by the airport control tower and is transmitted to the anti-lock control unit 16 by the communication bus.Since anti-lock using electromagnets 9 is several times faster than anti-lock using actuators 6 to modify the air gap in order to increase or decrease the torque, modifying the supply current of the electromagnets 9 is preferred to achieve the anti-lock function and reduce the braking torque. Indeed, the braking torque must decrease abruptly so that the wheel lock lasts a minimum amount of time. Figure 6 illustrates the difference between the response times of anti-lock by modifying the air gap and anti-lock by modifying the current of the electromagnets 9 (it is illustrative because it does not represent the actual behavior of the brake and the actual changes in torque amplitude for the electromagnets depend on the arrangement of the magnets 7, 9).If necessary, if a greater torque variation is required, the air gap can also be modified (by the actuators 6) at the same time as the current of the electromagnets 9 is modulated. This combination improves the effectiveness of the anti-lock of the eddy current brake. As much as the braking torque decrease must be as rapid as possible, it is also necessary to ensure a gradual increase in the braking torque after the rapid decrease in the braking torque, as opposed to the abrupt decrease in the braking torque, as illustrated in Figure 7.Thus, the anti-lock control unit 16 is arranged to control the electromagnets 9 to generate a reduction in the braking torque in the event of detection of wheel lock and then an increase in the braking torque when the lock disappears, the anti-lock control unit 16 controlling the electromagnets 9 in such a way that the increase in the braking torque is slower than the decrease in the braking torque. It can be seen in Figure 7 that the increase in the braking torque follows a curve as a function of time having a substantially straight starting section having a substantially vertical slope (around 80°) and a curved terminal section having a slope gradually decreasing to zero at the time of application of the maximum braking force.The curve in Figure 7 shows that the response of the anti-lock system is very rapid when acting on the electromagnets 9 and in any case faster than if acting on the air gap only. The electromagnets allow this progressive increase in the braking torque while avoiding instability in the anti-lock loop. To this end, according to a first control mode, the electromagnets are controlled between three states from a first positive intensity value, a second zero intensity value, and a third intensity value inverse to the first value. A second control mode consists of modifying the time constant of the control of the electromagnets 9 directly in the anti-lock control unit 16 as a function of the required braking torque.Thus, by way of example, the supply current of the electromagnets is controlled between two extreme values ​​corresponding to +100% and -100% of a maximum current value in steps of 1%. The braking torque is therefore actively modulated. A third control mode comprises the combination of air gap control and control of the current of the electromagnets 9, for example according to one of the first two control modes. Of course, the invention is not limited to the embodiments described but encompasses any variant falling within the scope of the invention as defined by the claims. In particular, the device may have a structure different from that described. The magnets may be carried by the rotor instead of the stator, two rotors framing a stator. The shape, arrangement and dimensions of the magnets may be different from those described or shown.For example, the magnets may have different dimensions and / or be arranged in a HALBACH pattern. The number of rotors and / or the number of stators may be different from those mentioned. Although the rotor and the stator have been described in the form of parallel discs facing each other, the stator and the rotor may have other shapes. The device described is axial flux but the invention is applicable to radial flux operation or hybrid operation combining axial flux and radial flux. Thus, the stators may for example be arranged in the form of an outer drum and an inner drum between which extends a central drum forming the rotor in such a way that the central drum has an outer surface facing an inner surface of the outer drum and an inner surface facing an outer surface of the inner drum.The magnets are carried by the outer surface of the inner drum and by the inner surface of the outer drum. The magnetic braking device according to the invention can be associated with a conventional friction braking device which comprises friction members, for example a stack of carbon discs, and a plurality of electromechanical actuators carried by an actuator holder. Each electromechanical actuator comprises an electric motor and a pusher capable of being moved by the electric motor to press the stack of discs. The electromechanical actuator is thus intended to produce a controlled braking force on the stack of discs. A method of controlling the braking devices is for example known from document FR-A-2953196.Alternatively, the magnets may be directly attached to the rotor discs or stator discs of the friction brake, or the magnets may be covered with a friction lining, so that the braking device provides magnetic braking to slow the wheel when the discs are spaced apart from each other by an adequate air gap and friction braking when the discs are pressed against each other. There is therefore no longer an axial stop between the discs in this embodiment. It is possible to use, for the mechanical actuation of the magnetic braking device, an actuator acting on several stators sliding in the same direction to be brought closer to the adjacent rotor, rather than an actuator for each stator. A single actuator may also act on two stators to move them in opposite directions. The electromagnets 9 are here controlled to produce the braking torque with the permanent magnets.It is conceivable that they are only used in the event of a blockage to cancel the magnetic field of the permanent magnets. Preferably, however, the electromagnets are driven to work with the permanent magnets so as to complete the magnet array, and therefore the field, so that it functions optimally. This is particularly the case with a Halbach array where the interaction between adjacent magnets in the magnet array is important. The device can only include electromagnets. The example logic shown in Figure 8 is only one possible solution and other ways of arranging the logic can be imagined to incorporate one or the other or both means of anti-skid control for eddy current braking: for example, using both air gap control and electromagnet control, or conversely, electromagnet control only.The control modes can be combined, for example, based on external information such as the state of the runway or atmospheric conditions to determine first external conditions in which coarse control of the electromagnets is sufficient (first control mode), second conditions requiring fine control (second control mode) and third conditions requiring fine control of greater amplitude (third control mode). Any useful parameter (aircraft speed, wheel speed, torque, etc.) can be used to create a servo or feedback loop in the control of the braking system. Other modes of actuation of the magnetic brake are possible: for example, solely by means of coils generating a magnetic field canceling that of the permanent magnets, rotors and stators being axially fixed.Alternatively, the first adder 12 can provide as output an actuator control current resulting from the subtraction of the actuator anti-lock control signal from the actuator braking control signal. In the same way, the second adder 13 can provide as output an electromagnet control current resulting from the subtraction of the electromagnet anti-lock control signal from the electromagnet braking control signal. The support 8 can be made of a non-magnetic material. The invention can be used on any type of vehicle, for example aerial, land or amphibious vehicles, or for applications other than a vehicle, for example for any industrial or transport mobile equipment requiring braking.

Claims

CLAIMS 1. Eddy current magnetic braking device for a vehicle wheel, comprising at least one stator element (2) arranged to be rotationally connected to a part of the vehicle supporting the wheel, a rotor element (3) arranged to be rotationally connected to the wheel, magnets for producing between said elements a magnetic flux capable of generating eddy currents in one of the elements when the wheel rotates, and a braking control unit (11) receiving as input a braking command to apply a braking force as a function of the braking command,characterized in that the magnets comprise at least one electromagnet (9) and the device comprises at least one anti-lock control unit (16) receiving as input a vehicle speed signal and a wheel rotation speed signal and providing as output a control signal for the electromagnet as a function of a comparison of speeds from the signals received as input.

2. Device according to claim 1, in which the anti-lock control unit (16) is arranged to control the electromagnet (9) to generate a reduction in the braking torque in the event of detection of wheel lock and then an increase in the braking torque when the lock disappears, the anti-lock control unit (16) controlling the electromagnet (9) in such a way that the increase in the braking torque is slower than the decrease in the braking torque.

3. Device according to claim 2,wherein the increase in braking torque follows a curve as a function of time having a straight starting section having a substantially vertical slope and a curved terminal section having a slope gradually decreasing to zero., 4. Device according to any one of the preceding claims, in which the comparison is based on the calculation of a slip rate of the wheel relative to the ground and the anti-lock control unit (16) comes into action when the calculated slip rate is greater than an optimum slip rate.

5. Device according to any one of the preceding claims, in which the magnets comprise permanent magnets (7) carried by one of the elements and the device comprises at least one actuator (6) for modifying the air gap between the elements, the anti-lock control unit (16) being connected to said actuator to modify the air gap as a function of the comparison of the speeds from the signals received at the input.

6. Device according to claim 5, comprising several electromagnets (9) each interposed between two of the permanent magnets (7). 7.Device according to any one of claims 5 and 6, in which the braking control unit (11) and the anti-lock control unit (16) each have a first output connected to a first adder (12) connected to a control unit (14) of the actuator (6) and a second output connected to a second adder (13) connected to a control unit (15) of the electromagnet (9).

8. Braked vehicle wheel (103), comprising a rim (105) and a hub (104) connected by a disc (106) defining an annular space receiving a braking device according to any one of the preceding claims.

9. Landing gear (101) comprising a leg having one end carrying a shaft (102) on which is mounted the hub of a wheel (103) according to claim 8, the stator element (2) of the braking device being rotationally linked to the leg via the shaft (102).

10. Aircraft comprising at least one landing gear according to the. claim 9.

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