Abstract:
A bistable linear electromagnet comprising a first housing ( 10 ) and a second housing ( 11 ) in alignment, a movable armature ( 18 ) comprising a rod ( 19 ) and a shuttle ( 20 ) that is slidably mounted, and a first coil ( 13 ) positioned in the first housing and a second coil ( 15 ) positioned in the second housing. A cavity ( 25 ) is made in a measurement wall ( 4 ) of one of the housings, and the electromagnet comprises a magnetic field sensor ( 26 ) positioned in the cavity and designed to measure a magnetic flux existing in a magnetic path formed by the walls of said housing and by the shuttle, in order to detect whether the shuttle has moved towards or away from the abutment wall of said first or second housing.

Description:
[0001]    The invention relates to the field of monitoring the position of the rod of a bistable linear electromagnet. 
       BACKGROUND OF THE INVENTION 
       [0002]    A braking architecture for the wheel of an aircraft is known that comprises a brake provided with at least one hydraulic actuator for braking the wheel, a source of pressure suitable for delivering a hydraulic fluid at high pressure, a hydraulic braking circuit for normal braking, and a hydraulic circuit for parking. 
         [0003]    The parking hydraulic circuit conventionally comprises a parking valve having an outlet port adapted to be selectively connected either to the source of pressure, or to a return circuit at low pressure relative to said high pressure. 
         [0004]    The parking valve is conventionally operated by a linear electromagnet including a rod sliding between an extended position and a retracted position. The position of the rod is generally monitored by means of a pressure sensor that measures the pressure in the parking hydraulic circuit. The pressure sensor is bulky, heavy, and expensive. It has been envisaged to monitor the position of the rod by incorporating a position sensor directly on the rod. However, the presence of hydraulic fluid in the environment of the rod prevents such incorporation. In addition, the short stroke of the rod would probably not allow the position of the rod to be detected accurately in the most unfavorable configurations (thermal drift, dimensional chains, expansion, etc.). 
       OBJECT OF THE INVENTION 
       [0005]    The object of the invention is to provide monitoring of the position of the rod of an electromagnet that does not present the above-mentioned drawbacks. 
       SUMMARY OF THE INVENTION 
       [0006]    With a view to satisfying this object, there is provided a bistable linear electromagnet comprising a hollow body having walls defining a first housing and a second housing aligned along an axis X, a movable armature comprising a rod linked to a shuttle mounted to slide in the hollow body along the axis X between a first end position in which it comes into abutment against an abutment wall of the first housing and a second end position in which it comes into abutment against an abutment wall of the second housing, and a first coil positioned in the first housing and a second coil positioned in the second housing, in such a manner that the shuttle slides towards the first end position when a first current flows in the first coil and in the second coil, and in such a manner that the shuttle slides towards the second end position when a second current flows in the first coil and in the second coil. A cavity is made in a measurement wall of one of the first and second housings, and the electromagnet includes a magnetic field sensor positioned in the cavity and designed for measuring a magnetic flux existing in a magnetic path formed by the walls of said first or second housing and by the shuttle, in order to detect whether the shuttle has moved towards or away from the abutment wall of said first or second housing. 
         [0007]    The position of the rod of the electromagnet of the invention is thus monitored by the magnetic field sensor that is incorporated in the measurement wall of the hollow body of the electromagnet. This monitoring is therefore performed by means that are compact, lightweight, and inexpensive. The magnetic field sensor, positioned in the cavity made in the measurement wall is positioned in simple manner in an environment that is free from hydraulic fluid. Finally, monitoring of the position of the rod by measuring the magnetic flux makes this monitoring robust, even in the event of unfavorable mechanical and thermal configurations of the rod. 
         [0008]    The invention can be better understood on reading the following description of a particular non-limiting embodiment of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Reference is made to the accompanying drawings, in which: 
           [0010]      FIG. 1  is a section view of the electromagnet of the invention, in a plane passing through a longitudinal axis of the electromagnet, a rod of the electromagnet being in a retracted position; and 
           [0011]      FIG. 2  is a view analogous to that of  FIG. 1 , wherein the rod of the electromagnet is in an extended position. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    With reference to  FIGS. 1 and 2 , the electromagnet  1  of the invention is a linear electromagnet that comprises a hollow body  2  presenting a cylindrical outer shape of circular section and having as its longitudinal axis an axis X. 
         [0013]    The hollow body  2  comprises a plurality of walls, among which a main wall  3  presenting a cylindrical surface having as its longitudinal axis the axis X, a first end wall  4  forming a first face of the hollow body  2 , a second end wall  5  forming a second face of the hollow body  2 , a central wall  6  parallel to the first and second end walls  4 ,  5  and equidistant from the first and second end walls  4 ,  5 , and a first abutment wall  7  and a second abutment wall  8 . The first abutment wall  7 , that presents an annular shape having as its longitudinal axis the axis X, extends from the first end wall  4  towards the inside of the hollow body  2 . The second abutment wall  8 , that presents an annular shape having as its axis the axis X, extends from the second end wall  5  towards the inside of the hollow body  2 . 
         [0014]    The main wall  3 , the first end wall  4 , the first abutment wall  7  and the central wall  6  define a first housing  10  in the hollow body  2 . 
         [0015]    The main wall  3 , the second end wall  5 , the second abutment wall  8  and the central wall  6  define a second housing  11  in the hollow body  2 . 
         [0016]    In this embodiment, the hollow body  2  (and therefore the set of above-mentioned walls) is made from martensitic stainless steel. Specifically, the martensitic stainless steel used is a stainless steel of the X30Cr13 type. 
         [0017]    A permanent magnet  12 , of annular shape, extends from the central wall  6  towards the axis X. The central wall  6  and the permanent magnet  12  separate the first and second housings  10 ,  11 . In this embodiment, the permanent magnet  12  is a neodymium magnet made of SmCo 5 . 
         [0018]    The first housing  10  includes a first main coil  13  and a first auxiliary coil  14 . The second housing  11  includes a second main coil  15  and a second auxiliary coil  16 . The first and second main coils  13 ,  15  are connected in series. The first auxiliary coil  14  and the second auxiliary coil  16  are connected in series. 
         [0019]    The electromagnet  1  further comprises a movable armature  18  comprising a rod  19  and a shuttle  20 . The longitudinal axis of the rod  19  is an axis X′. The shuttle  20  comprises a main portion  21  and a connection portion  22 . The main portion  21  of the shuttle  20  presents a cylindrical outer shape of circular section and has as its longitudinal axis the axis X′. The connection portion  22  is a wall perpendicular to the axis X′ and situated in the center of the main portion  21 . When the movable armature  18  is mounted in the hollow body  2 , the axis X′ coincides with the axis X. 
         [0020]    The rod  19  of the movable armature  18  is made of aluminum. In this embodiment, the shuttle  20  is made of martensitic stainless steel. Specifically, the martensitic stainless steel used is a stainless steel of the X30Cr13 type. The rod  19  is fastened to the shuttle  20  in such a manner as to extend coaxially therein. 
         [0021]    The electromagnet  1  operates as follows. When a first control voltage is applied, a first current flows in a first direction in the first main coil  13  and in the second main coil  15 , a first magnetic field is generated, under the effect of which the shuttle  20  slides in the hollow body  2  along the axis X and moves towards the first abutment wall  7  of the first housing  10  until it reaches a first end position in which the shuttle  20  comes into abutment against the first abutment wall  7  of the first housing  10 . The rod  19  is thus in a retracted position. When the first control current is no longer applied and the first current no longer flows, the shuttle  20  is held in the first end position by the effect of a magnetic field generated by the permanent magnet  12 . The rod  19  is therefore held in the retracted position (situation shown in  FIG. 1 ). 
         [0022]    When a second control voltage is applied, a second current flows in a second direction opposite the first direction in the first main coil  13  and in the second main coil  15 , a second magnetic field is generated, under the effect of which the shuttle  20  slides in the hollow body  2  along the axis X and moves towards the second abutment wall  8  of the second housing  11  until it reaches a second end position in which the shuttle  20  comes into abutment against the second abutment wall  8  of the second housing  11 . The rod  19  is then in an extended position. When the second control voltage is no longer applied and the second current no longer flows, the shuttle  20  is held in the second end position by the effect of a magnetic field generated by the permanent magnet. The rod  19  is therefore held in the extended position (situation shown in  FIG. 2 ). 
         [0023]    The electromagnet  1  is therefore a bistable linear electromagnet. 
         [0024]    The first and second auxiliary coils  14 ,  16  are arranged in the same manner and play exactly the same role as the first and second main coils  13 ,  15 . The first and second auxiliary coils  14 ,  16  are used only when a fault (e.g. a short-circuit or an open circuit) appears on the first main coil  13  and/or on the second main coil  15 . The magnetic circuit of the electromagnet  1  is therefore redundant. 
         [0025]    It should be observed that the hollow body  2  of the electromagnet  1  includes a measurement wall, which is specifically the first end wall  4 . The first end wall  4  includes a cavity  25  in the form of a groove opening out to the outside of the hollow body  2 . The cavity does not open out to the inside of the cavity of the hollow body  2 . The thickness e of the first end wall  4 , at the cavity  25 , lies in the range 0.4 millimeters (mm) to 1 mm. In this embodiment, this thickness e is approximately equal to 0.7 mm. 
         [0026]    A magnetic field sensor, specifically a Hall-effect sensor  26 , is positioned inside the cavity  25 . The sensitive portion of the Hall-effect sensor  26  is positioned against the bottom of the cavity  25 . 
         [0027]    The Hall-effect sensor  26  makes it possible to detect whether the shuttle  20  has moved towards the first abutment wall  7  of the first housing  10  or else towards the second abutment wall  8  of the second housing  11 . The Hall-effect sensor  26  therefore makes it possible to detect whether the shuttle  20  is located in the first end position or in the second end position, and therefore whether the rod  19  is located in the retracted position or in the extended position. 
         [0028]    When the shuttle has moved towards the first abutment wall  7  of the first housing  10 , and, in particular, when the shuttle  20  comes into abutment against the first abutment wall  7  of the first housing  10  (as shown in  FIG. 1 ), a magnetic flux resulting from the magnetic field generated by the first main coil  13  (or by the first auxiliary coil  14  if used) is primarily concentrated in a first magnetic path  30  formed by the walls of the first housing  10 . The magnetic flux is symbolized by magnetic field lines  31 . It can be seen that the magnetic field lines  31  are particularly concentrated in the first end wall  4  at the cavity  25 , and therefore that the magnetic flux there is particularly substantial. The Hall-effect sensor  26  thus detects magnetic field leaks at the bottom of the cavity  25 , and measures a relatively substantial magnetic field. 
         [0029]    Processor means, connected to the Hall-effect sensor  26  and not shown in the figures, acquire the measurements taken by the Hall-effect sensor  26  and, as a function of the amplitude of the magnetic field measured, they detect that the shuttle  20  has moved towards the first abutment wall  7  of the first housing  10 , and, possibly, that the shuttle  20  is in abutment against the first abutment wall  7  of the first housing  10 . The processor means thus detect that the rod  19  is therefore located in the retracted position. 
         [0030]    On the contrary, when the shuttle  20  has moved away from the first abutment wall  7  of the first housing  10 , and, in particular, when the shuttle  20  comes into abutment against the second abutment wall  8  of the second housing  11  (as shown in  FIG. 2 ), the magnetic flux resulting from the magnetic field generated by the second main coil  15  (or by the second auxiliary coil  16  if used) is primarily concentrated in a second magnetic path  32  formed by the walls of the second housing  11 . The Hall-effect sensor  26  therefore measures a relatively weak magnetic field. 
         [0031]    The processor means acquire the measurements taken by the Hall-effect sensor  26  and, as a function of the amplitude of the magnetic field measured, they detect that the shuttle  20  has moved away from the first abutment wall  7  of the first housing  10 , and, possibly, that the shuttle  20  is in abutment against the second abutment wall  8  of the second housing  11 . The rod  19  is therefore located in the extended position. 
         [0032]    It should be noted that it is possible either to perform a “linear” measurement of the position of the shuttle  20  between the first end position and the second end position, and therefore of the position of the rod  19  between the retracted position and the extended position, or to perform a “binary” measurement, which indicates that the shuttle  20  is located either in the first end position, or in the second end position. The binary measurement is simpler to perform and seems more appropriate since the first end position and the second end position are the only stable positions of the shuttle  20 . 
         [0033]    The Hall-effect sensor  26  may be a latch type Hall-effect sensor providing binary information, but it could also be a linear Hall probe. For a linear probe, it is possible to perform a binary measurement by defining a threshold above which the magnetic field measured corresponds to the first end position of the shuttle  20 , and above which the magnetic field measured corresponds to the second end position of the shuttle  20 . The threshold may possibly be adjusted during manufacture of the electromagnet  1 , or also during system tests before delivery. 
         [0034]    Advantageously, the processor means are positioned on a circuit board or in a computer used for powering the electromagnet  1  (and therefore for generating the control voltages applied at the terminals of the coils). Thus, the cost and bulkiness of monitoring the position of the rod  19  are reduced. 
         [0035]    Naturally, the invention is not limited to the embodiment described but covers any variant coming within the ambit of the invention as defined by the claims. 
         [0036]    Although it is stated that the magnetic field sensor used is a Hall-effect sensor, it is entirely possible for a different sensor (e.g., a magnetoresistive sensor) to be used. 
         [0037]    It is also possible to use a plurality of magnetic field sensors, optionally positioned in a common cavity. 
         [0038]    Naturally, the sensor may be incorporated in a measurement wall of the second housing. In addition, the measurement wall, which includes the cavity(ies), may be a wall other than an end wall, e.g. an abutment wall.