Monostable electromagnetic actuator and method of manufacturing such an actuator

Abstract

The invention relates to a monostable electromagnetic actuator (1), comprising: a magnetic casing (11) extending around a translation axis (X) of the actuator; an annular excitation coil (12) arranged inside the casing to generate a control magnetic flux, the coil being mounted on a support (13) comprising a cavity whose longitudinal axis coincides with the translation axis; a magnetic core (21) mounted to move in the cavity along the translation axis between a retracted position and an advanced position towards which the core is returned by elastic return means (40); and at least one elastically deformable member (15) arranged to be compressed under the action of the elastic return means only when the core is in the advanced position and in the vicinity thereof. FIGURE OF THE ABSTRACT: Fig.1

Description

Title of the invention: Monostable electromagnetic actuator and method of manufacturing such an actuator

[0001] The invention relates to the field of electromagnetic actuators and, more particularly, to a monostable electromagnetic actuator.

[0002] BACKGROUND OF THE INVENTION

[0003] A monostable electromagnetic actuator generally comprises a magnetic yoke extending around a translation axis of the actuator, an annular excitation coil arranged coaxially inside the casing to generate a control magnetic flux, and a movable armature engaged in the coil to move between a retracted position and an advanced position to which the armature is returned by a spring.

[0004] To bring the movable armature from the advanced position to the retracted position, the coil is electrically powered so as to generate a magnetic field exerting on the movable armature an attraction force greater than the return force exerted by the spring.

[0005] When dimensioning such an actuator, it was found that for a large stroke of the armature and / or a restricted volume of the yoke, the tensile force generated by the coil was, in the advanced position, only very slightly greater than the nominal return force generated by the spring. Thus, depending on the manufacturing tolerance of the spring stiffness, the tensile force generated by the coil may be less than the effective return force generated by the spring, and therefore be insufficient to bring the armature into the retracted position. There is therefore a risk that the actuator will not be operational.

[0006] One solution consists of precisely controlling the tolerance of the spring stiffness, which generates a cost, or even an industrial impossibility, the control of each spring in series production being hardly feasible.

[0007] SUBJECT OF THE INVENTION

[0008] The object of the invention is therefore to propose an electromagnetic actuator making it possible to overcome the aforementioned problem, as well as a method of manufacturing such an actuator. Summary of the invention

[0009] For this purpose, according to the invention, a monostable electromagnetic actuator is provided comprising: • a magnetic frame extending around a translation axis of the actuator; • an annular excitation coil arranged inside the carcass to generate a control magnetic flux, the coil being mounted on a support comprising a cavity of which a longitudinal axis coincides with the translation axis; • a magnetic core mounted movably in the cavity along the translation axis between a retracted position and an advanced position towards which the core is returned by elastic return means; and • at least one elastically deformable member arranged to be compressed only when the core is in the advanced position and in the vicinity of this position.

[0010] To bring the movable armature from the advanced position to the retracted position, the coil is electrically powered so as to generate a magnetic field exerting on the movable armature a tensile force greater than the return force exerted by the elastic return means.

[0011] Thus, when the core is in the advanced position, the elastically deformable member exerts on said core a thrust force which slightly helps the coil to bring the core into the retracted position against the elastic return means.

[0012] According to a particular characteristic of the invention, the elastically deformable member is made of elastomer.

[0013] In particular, the elastically deformable member is an O-ring received in an annular groove formed in an end face of the carcass, the core being coupled to a stop element comprising a bearing surface arranged to cooperate with the end face and compress the seal when the core is in the advanced position.

[0014] In particular, the end face extends substantially in a plane orthogonal to the translation axis.

[0015] According to another particular characteristic, the cavity is cylindrical in shape.

[0016] According to another particular characteristic, the elastic return means com take a compression coil spring.

[0017] According to another particular characteristic, the elastic return means and the elastically deformable member form an elastic assembly having, along the translation axis, a stiffness whose manufacturing tolerance is less than that of the stiffness of the elastic return means along said translation axis.

[0018] The invention also relates to a method for manufacturing such an actuator in which the elastic return means exert on the core in the advanced position a return force substantially greater than the tensile force exerted by the coil on said core. The method comprises adjusting the stiffness of the elastically deformable member so that said member exerts on the core in the advanced position a pushing force and that the pulling force and the pushing force are together greater than the restoring force.

[0019] In particular, if it is found that the coil cannot initiate the movement of the core from the advanced position to the retracted position, the elastically deformable member is replaced by another elastically deformable member of greater stiffness. Brief description of the drawings

[0020] The invention will be better understood in light of the following description, which is purely illustrative and non-limiting, and must be read in conjunction with the appended drawings, among which:

[0021] [Fig-1] [Fig.l] is an axial sectional view of an electromagnetic actuator according to a particular embodiment of the invention, in which the core is in an advanced position;

[0022] [Fig.2] [Fig.2] is a view identical to [Fig.l], in which the core is at vicinity of the advanced position;

[0023] [Fig.3] [Fig.3] is a view identical to [Fig.l], in which the core is in retracted position;

[0024] [Fig.4] [Fig.4] is a graph representing the evolution of the restoring force exerted by the spring, and the sum of the tensile force exerted by the coil and the thrust force exerted by the elastically deformable member. DETAILED DESCRIPTION OF THE INVENTION

[0025] With reference to Figures 1 to 3, a monostable electromagnetic actuator 1 comprises, according to a first embodiment of the invention, a fixed assembly generally designated by the reference 10, as well as an assembly mobile in translation along an axis X and generally designated by the reference 20.

[0026] The fixed assembly 10 comprises a carcass 11 made of ferromagnetic material. The carcass 11 comprises a first part 11.1 and a second part 11.2 which extend around the axis X and which together define an annular housing receiving an electromagnetic coil 12. The coil 12 is mounted on an annular support 13 made of non-magnetic material. The support 13 comprises an internal surface 13a defining a cylindrical cavity 13b whose longitudinal axis coincides with the axis of the coil 12 and with the axis X of movement of the mobile assembly 20.

[0027] The carcass 11 comprises a first end comprising a first boss 11a, and a second end, opposite the first end, comprising a second boss 11b. The first boss 11a and the second boss 11b are respectively provided with a first orifice 11c of generally cylindrical shape and a second orifice 11d of generally conical shape. The first orifice 11c and the second orifice 1 Id extend along the X axis and each open onto one end of the cylindrical cavity 13b. The first orifice 1 le comprises an internal annular groove 1 le receiving a first ring 14 for guiding the mobile assembly 20. The second orifice 1 Id comprises an internal surface 1 If of truncated cone shape.

[0028] The second boss 11b comprises a radial end face 11g having an annular groove 11h in which a toric seal 15 is housed. The groove 11h has a depth p and a width l respectively less than and greater than the diameter d of the seal 15. The seal 15, here made of elastomer, is in contact with a bottom of the groove 11h and is elastically deformable between a rest state in which the seal 15 extends projecting from the end face 11g (Figures 2 and 3), and a compressed state in which the seal 15 extends upstream of the end face 11g ([Fig. 1]).

[0029] The fixed assembly 10 is fixed inside a receiving volume delimited by internal walls of a casing 30 made of non-magnetic material.

[0030] The movable assembly 20 comprises a core 21 of generally cylindrical shape whose central axis coincides with the axis X. The core 21 is made of ferromagnetic material and extends into the cylindrical cavity 13b of the coil support 13. The core 21 comprises a first end coupled to an actuating interface 22 which projects from the first boss 11a of the carcass 11, and a second end, opposite the first end, coupled to a stop element 23 which projects from the second boss 11b of the carcass 11. The second end has an external surface 21a of frustoconical shape which has, with the internal surface 11f of the second boss 11b, a variable air gap E during the movement of the movable assembly 20. The external surface 21a is arranged to cooperate with the internal surface 11f.

[0031] The stop element 23, of generally tubular shape, comprises a shoulder forming a radial bearing surface 23a arranged to compress the seal 15 and cooperate with the end face 11g of the second boss 11b, and a free end comprising an external annular groove 23b receiving a second guide ring 24 of the mobile assembly 20. The second guide ring 24 is received to fit in a bore 30a of the casing 30.

[0032] The movable assembly 20 comprising the core 21, the actuation interface 22, the stop element 23 and the second guide ring 24 forms an actuation member movable in translation along the axis X between a retracted position in which an end face 23c of the stop element 23 cooperates with a bottom 30b of the bore 30a of the casing 30, the air gap E then being almost zero ([Fig. 3]), and an advanced position in which the end face 1 If of the carcass 11 cooperates with the bearing surface 23a of the stop element 23, the air gap E then being maximum and the seal 15 in the compressed state ([Fig. 1]). It will be noted that in the compressed state, the seal 15 exerts on the mobile assembly 20 an axial thrust force Pi5 tending to move the bearing surface 23a of the stop element 23 away from the end face 1 If of the carcass 11, and therefore to reduce the air gap E.

[0033] The movable assembly 20 is returned to the advanced position by a helical compression spring 40 extending along the axis X between a bearing surface 23d of the stop element 23 and a bottom 30c of the casing 30. Thus, the spring 40 exerts on the movable assembly 20 an axial return force R40 so that in the absence of supply current in the coil 12, the advanced position of the movable assembly 20 is a stable position. The spring 40 here has a constant stiffness along the axis X.

[0034] It will be noted that the actuator 1 is autonomous and does not require any external resetting phase.

[0035] The operation of the actuator 1 will now be described.

[0036] To move the movable assembly 20 from the advanced position ([Fig. 1]) to the retracted position ([Fig. 3]), the coil 12 is electrically powered so as to generate a magnetic field of attraction of the core 21. The magnetic field generated by the coil 12 is of generally toroidal shape and comprises field lines which pass through the ferromagnetic parts of the actuator 1. Each of the field lines forms a loop and successively passes through the first part 11.1 of the carcass 11, the core 21 and the second part of the carcass 11.2, so that the coil 12 exerts on the core 21 an axial tensile force T[2 tending, with the thrust force P15 exerted by the compressed seal 15, to move the movable assembly 20 towards the retracted position while opposing the return force R40 exerted by the spring 40.

[0037] As the moving assembly 20 moves away from the advanced position ([Fig.l]), the thrust force Pi5 exerted by the joint 15 decreases until it becomes zero as soon as said joint 15 returns to its rest state ([Fig.2]). The movement of the moving assembly 20 to the retracted position is then ensured only by the axial tensile force Tn exerted by the coil 12 which, like the return force R40 exerted by spring 40, tends to increase as the moving assembly 20 approaches the retracted position and the air gap E decreases, while remaining greater than said return force Rio.

[0038] Once in the retracted position ([Fig.3]) and as long as the coil 12 is electrically powered, the mobile assembly 20 is held in said retracted position by the axial tensile force Tn exerted by said coil 12. The air gap E is then almost zero.

[0039] When it is no longer electrically powered, the coil 12 no longer exerts a tensile force T[2 on the movable assembly 20 so that said movable assembly 20 tends, under the action of the spring 40, to move from the retracted position to the advanced position. As the movable assembly 20 approaches the advanced position and the air gap E increases, the return force R40 exerted by the spring 40 decreases.

[0040] When the mobile assembly 20 is close to its advanced position, the seal 15 comes into contact with the bearing surface 23a of the stop element 23 before being compressed by said bearing surface 23a under the action of the spring 40. The seal 15 then again exerts on the mobile assembly 20 a thrust force Pi5 opposing the return force R40 exerted by the spring 40. The thrust force Pi5 exerted by the seal 15 is maximum when the bearing surface 23a cooperates with the end face 11g of the carcass 11, said seal 15 then being in the compressed state.

[0041] [Fig. 4] represents a graph illustrating the evolution of the nominal return force R40 exerted by the spring 40 as a function of the air gap E, in other words as a function of the position of the mobile assembly 20. It will be noted that the evolution of said return force R40 is linear and corresponds to the fact that the stiffness of the spring 40 is constant here.

[0042] Also illustrated on the same graph are the changes in the maximum and minimum effective return force R40 corresponding to the manufacturing tolerance of the stiffness of the spring 40.

[0043] The same graph also illustrates the evolution of the tensile force T[2 exerted by the coil 12, said tensile force T[2 being coupled to the thrust force P15 exerted by the joint 15 when the mobile assembly 20 is in the advanced position and close to it. It is understood that in the vicinity of the advanced position, the joint 15 helps the coil 12 to move the mobile assembly 20 towards the retracted position and to slow down the movement of the mobile assembly 20 as it approaches the advanced position depending on the direction of movement of said mobile assembly 20.

[0044] It is also understood that the spring 40 and the seal 15 form an elastic assembly exerting a force on the mobile assembly 20 over a length pd at the start of the travel of the mobile assembly 20, from its advanced position to its retracted position. It is at this start of travel that the air gap E is the largest and therefore that the tensile force Tn exerted by the coil 12 is the weakest. Therefore, it is possible to facilitate the start of the movement of the mobile assembly 20 to its retracted position by acting solely on the stiffness of the seal 15 along the axis X.

[0045] Thus, when the spring 40 exerts on the mobile assembly 20 in the advanced position a return force R40 substantially greater than the tensile force Tn exerted by the coil 12 on the core 21, the stiffness of the joint 15 is adjusted so that said joint 15 exerts on the core 21 in the advanced position a thrust force P15 and that the tensile force T[2 and the thrust force P15 are together greater than the return force R40

[0046] When manufacturing the actuator 1, springs 40 are provided having stiffnesses identical to the manufacturing tolerances and it is possible to compensate for the differences stiffness differences resulting from the tolerances by using joints 15 of adapted stiffness. It is therefore easy to obtain, by pairing the springs 40 and the joints 15, elastic assemblies having stiffnesses with narrower tolerances, which is more economical than using only springs 40 with tight tolerances.

[0047] Thus, during a test operation of an actuator 1, if it is found that the coil 12 cannot initiate the movement of the mobile assembly 20 from its advanced position to its retracted position for the expected supply current, it is sufficient to replace the seal 15 with a seal 15 of greater stiffness.

[0048] 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.

[0049] The seal 15 can be replaced by one or more elastically deformable members, for example made of elastomer or any material having physical properties compatible with the intended application.

[0050] Although the seal 15 is here carried by the carcass 11, it can be carried by the stop element 23. Generally, the seal 15 can be carried by the fixed assembly 10 as well as by the mobile assembly 20.

[0051] Although the cavity 13b of the support 13 is cylindrical here, it can be of different shape (parallelpiped, hexagonal, etc.).

[0052] The spring 40 can be replaced by any elastic return means.

Claims

Claims

1. Monostable electromagnetic actuator (1), comprising: • a magnetic casing (11) extending around a translation axis (X) of the actuator; • an annular excitation coil (12) arranged inside the casing to generate a control magnetic flux, the coil being mounted on a support (13) comprising a cavity (13b) whose longitudinal axis coincides with the translation axis; • a magnetic core (21) mounted movably in the cavity along the translation axis between a retracted position and an advanced position towards which the core is returned by elastic return means (40); and • at least one elastically deformable member (15) arranged to be compressed under the action of the elastic return means only when the core is in the advanced position and in the vicinity thereof.

2. Actuator (1) according to claim 1, in which the elastically deformable member (15) is made of elastomer.

3. Actuator (1) according to claim 2, in which the elastically deformable member is an O-ring (15) received in an annular groove (11h) formed in an end face (11g) of the carcass (11), the core being coupled to a stop element (23) comprising a bearing surface (23a) arranged to cooperate with the end face and compress the seal when the core is in the advanced position.

4. Actuator (1) according to claim 3, wherein the end face (11g) extends substantially in a plane orthogonal to the translation axis (X).

5. Actuator (1) according to any one of the preceding claims, wherein the cavity (13b) is cylindrical in shape.

6. Actuator (1) according to any one of the preceding claims, in which the elastic return means comprise a helical compression spring (40).

7. Actuator (1) according to any one of the preceding claims, in which the elastic return means (40) and the elastic member (15) tically deformable form an elastic assembly having, along the translation axis (X), a stiffness whose manufacturing tolerance is less than that of the stiffness of the elastic return means along said translation axis.

8. A method of manufacturing an actuator (1) according to any one of the preceding claims and in which the elastic return means (40) exert on the core (21) in the advanced position a return force (R40) substantially greater than the tensile force (Ti2) exerted by the coil (12) on said core, the method comprising an adjustment of the stiffness of the elastically deformable member (15) along the translation axis (X) so that said member exerts on the core (21) in the advanced position a thrust force (Pi5) and that the tensile force and the thrust force are together greater than the return force.

9. Manufacturing method according to claim 8, in which if it is found that the coil (12) cannot initiate the movement of the core (21) from the advanced position to the retracted position, the elastically deformable member (15) is replaced by another elastically deformable member of greater stiffness.

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