EP0272164B1 - Bistable-polarised electromagnet - Google Patents

Abstract

A bistable polarized electromagnet is provided having a mobile armature with a permanent magnet and pole pieces which cooperate with a fixed yoke through two pairs of air gaps for each of which one is variable whereas the other is substantially constant. This electromagnet may further have asymmetric or symmetric forms of revolution for easy replacement of the excitation coil.

Description

The invention relates to a bi-stable polarized electromagnet comprising a fixed magnetizable circuit which is excited by an associated coil in which an electric current flows according to one of two possible polarities, and a movable armature having a permanent magnet on both sides. opposite poles from which two respective pole pieces are connected, this armature being able to move longitudinally between two positions for each of which these pole pieces cooperate with distinct portions of the fixed circuit by means of two air gaps placed in series, one of which at least is variable.

Such electromagnets which are widely used in industrial installations and automation systems, for example to reduce energy consumption or to ensure that circuits maintain their state in the event of failure of the power sources, can be illustrated for example by French Patent No. 2,358,006 in which the two variable air gaps are placed in series and evolve simultaneously, while in each of the two stable states, the flux of the permanent magnet closes on a magnetic reluctance circuit negligible. In such an electromagnet the replacement of a coil is not easy due to the presence of protruding pole pieces.

Furthermore, the presence of two air gaps placed in series requires that the coil develops a sufficiently high number of ampere-turns to oppose the flux developed by the magnet, while allowing its circulation through two reluctances placed in series; in this known device, there is also a certain technical difficulty for a simultaneous application of the pole pieces to the magnetic circuit, to be properly performed, due to the distance which separates them.

DE-A-3 508 768 describes a polarized electromagnet comprising a moving polarized part devoid of polar extensions, and having two sliding gaps of constant thickness. No magnetically stable position is conferred on this moving part, which moreover requires for its movements the presence of two symmetrical magnetizable systems, therefore of two coils. The moving part, which has no variable air gap, is only subjected to weak tangential components of forces of attraction or repulsion developing perpendicular to the direction of movement; a limit to movement is only given by the meeting of feet associated with the ends of openings.

The electromagnet according to EP-A-179 911, in the embodiment of Figure 5, has a single coil (11) to cause the excitation of a fixed magnetic circuit (17) having a gap of variable thickness (13) and a flow closing gap (15) of constant thickness, to cause the displacement of a movable armature constituted by a permanent magnet (46) and by two opposite pole pieces (45) .

In the structure described, the longitudinal arrangement of the polarization of the magnet requires the use of transverse pole pieces of which only the edges cooperate with the fixed circuit, so that the corresponding reluctances are high.

On the other hand, the fluxes developed by the permanent magnet in each of the two extreme positions of the armature (Figures 5a and 5b) are largely magnetically short-circuited by the presence of pairs of air gaps which, in these positions , do not contribute to maintaining stable positions. No guidance of the movable armature is described.

In the embodiment of Figure 3, the orientation of the permanent magnets is transverse, but no pole piece is associated with the permanent magnet in order to reduce the reluctance of the air gap of constant thickness. In each of the two stable positions, the presence of a residual air gap prevents the development of a holding flow, while no information is given on the location of the guide means.

The invention therefore aims to provide an electromagnet having the general constitution mentioned above and in which measures will be taken, on the one hand to reduce the volume of the coil and, on the other hand, to remove the technical difficulties which arise when it is necessary to obtain the simultaneous closure of two air gaps which are not in the same plane; moreover, the invention also aims to conserve the benefit which results in a known manner from the use of the same magnets to stabilize the armature in its two extreme positions.

It offers a bi-stable electromagnet, comprising a fixed magnetic circuit excited by a winding so as to give magnetic polarizations opposite to two fixed parts of this circuit placed opposite one another and a movable armature mounted to slide between these two parts along a longitudinal axis, this armature comprising a transverse polarized permanent magnet, the internal flux of which flows parallel to the two fixed parts and forms with the fixed magnetic circuit two pairs of air gaps, namely, parallel to the longitudinal axis a pair of working air gaps, of variable thickness and perpendicular to the longitudinal axis a pair of sliding closing air gaps, of substantially constant thickness in series with the work gaps.

• une première pièce polaire munie d'au moins un prolongement ou epanouissement longitudinal formant au moins un entrefer de fermeture, d'épaisseur constante avec le circuit magnétique fixe et
• une deuxième pièce polaire disposée en regard des deux susdites pièces fixes de manière à former avec lesdites pièces lesdits entrefers de travail et en ce que les deux pièces polaires sont disposés de manière à venir sensiblement en contact avec l'une ou l'autre des pièces fixes dans chacune des deux positions stables de l'électro-aimant en établissant un flux de maintien dans chacune de ces positions.

According to the invention, this electromagnet is characterized in that the two poles of the permanent magnet are radially extended by two respective magnetic pole pieces, namely

• a first pole piece provided with at least one longitudinal extension or expansion forming at least one closing air gap, of constant thickness with the fixed magnetic circuit and
• a second pole piece arranged opposite the two aforementioned fixed parts so as to form with said pieces said working air gaps and in that the two pole pieces are arranged so as to come substantially into contact with one or the other of the pieces fixed in each of the two stable positions of the electromagnet by establishing a holding flux in each of these positions.

In an annex, the invention relates to embodiments which make it possible either to reduce the reluctance of the air gaps, the value of which is by construction substantially constant, or to present forms of magnetizable circuits requiring only the application of a number of reduced ampere-turns, or else of producing the electromagnet in forms capable of reducing the parasitic attraction forces which develop between the two neighboring areas of thin air gaps.

Are already known, for example from FR patent ^No. 2,568,402 electromagnets in which measures are taken to reduce the ampere turns developed by the coil through the installation of two air gaps of which, having a variable reluctance to generate attractive forces, is placed in series with a second sliding air gap, the reluctance of which is low and substantially constant; in such an electromagnet where the magnetizable circuits have only one circulation path for the flux of the coil, a permanent magnet cannot be incorporated directly into this circuit without establishing one or other of its states an opposition of the two fluxes developed by the coil and respectively by the magnet.

• Aux figures 1, 2 et 3, un premier mode de réalisation asymétrique d'un électro-aimant selon l'invention représenté dans ses deux états stables et pour une position intermédiaire de l'armature ;
• A la figure 4, une vue de l'électro-aimant qui correspond à la position de l'armature définie à la figure 3, et où est indiqué le partage de flux d'origines différentes ;
• A la figure 5, un second mode de réalisation asymétrique de l'électro-aimant, dans lequel le circuit magnétisable associé à la bobine est modifié ;
• A la figure 6, un électro-aimant dont la constitution répond à celle des figures 1 à 4 dans lequel des mesures sont prises pour réduire des forces d'attraction parasites ;
• A la figure 7, un électro-aimant dont la constitution répond à celle de la figure 5 et dans lequel des mesures sont prises pour réduire des forces d'attraction parasites ;
• Aux figures 8 et 9, deux troisièmes modes de réalisation qui dérivent de ceux représentés aux figures 1 et 5 lorsque l'on donne à leurs éléments des formes de révolution autour d'axes voisins des entrefers coulissants ;
• Aux figures 10 et 11, deux quatrièmes modes de réalisation qui dérivent de ceux représentés aux figures 1 et 5 lorsque l'on donne à leurs éléments des formes de révolution autour d'axes passant par les entrefers de travail ;
• Aux figures 12 et 13, deux cinquièmes modes de réalisation qui, dans le cadre d'organes d'électro-aimants ayant des formes de révolution, font appel à deux entrefers coulissants situés l'un au voisinage de l'axe de symétrie, et l'autre à une distance voisine de la périphérie ;
• Aux figures 14 et 15, deux vues de côté d'armatures ayant des formes de révolution et mettant en oeuvre des aimants permanents de différentes formes ; et
• Aux figures 16 et 17, deux vues en coupes d'armatures ayant des formes de révolution et une orientation axiale de leurs pôles magnétiques.

The invention, as well as various embodiments to which it lends itself, will be better understood on reading the description below and on examining the appended figures which illustrate:

• In Figures 1, 2 and 3, a first asymmetrical embodiment of an electromagnet according to the invention shown in its two stable states and for an intermediate position of the armature;
• In FIG. 4, a view of the electromagnet which corresponds to the position of the armature defined in FIG. 3, and where the sharing of fluxes of different origins is indicated;
• In FIG. 5, a second asymmetrical embodiment of the electromagnet, in which the magnetizable circuit associated with the coil is modified;
• In FIG. 6, an electromagnet whose constitution corresponds to that of FIGS. 1 to 4 in which measures are taken to reduce parasitic attraction forces;
• In FIG. 7, an electromagnet whose constitution corresponds to that of FIG. 5 and in which measures are taken to reduce parasitic attraction forces;
• In FIGS. 8 and 9, two third embodiments which derive from those shown in FIGS. 1 and 5 when their elements are given forms of revolution around adjacent axes of the sliding gaps;
• In FIGS. 10 and 11, two fourth embodiments which derive from those shown in FIGS. 1 and 5 when their elements are given forms of revolution around axes passing through the working air gaps;
• In Figures 12 and 13, two fifth embodiments which, in the context of electromagnets having forms of revolution, use two sliding air gaps located one in the vicinity of the axis of symmetry, and the other at a distance close to the periphery;
• In FIGS. 14 and 15, two side views of armatures having forms of revolution and using permanent magnets of different shapes; and
• In FIGS. 16 and 17, two sectional views of reinforcements having forms of revolution and an axial orientation of their magnetic poles.

According to a first embodiment 1 of an electromagnet according to the invention, visible in particular in Figure 3, a movable frame 2 is guided longitudinally along a slide 18 belonging to a housing 19 along longitudinal directions F and G.

This armature, shown in this figure in an unstable intermediate position - III -, comprises a permanent magnet 3 having transversely opposite pole faces 4 respectively 5, which are respectively connected to magnetizable pole pieces 7 and 6; the pole piece 6 itself comprises two opposite extensions 6 _a , 6 _b , one of which 6 _a cooperates thanks to a constant air gap 10 whose reluctance with a first branch 28 of a fixed magnetizable circuit 13 having a second branch 26 is weak and substantially constant when the armature moves.

With this magnetizable circuit 13 is associated an excitation coil 14 which is wound in a known manner on a carcass 15 placed around a branch 29, and has two supply terminals 16, 17.

Between one end 12 of the branch 29 and the pole piece 7 is a variable air gap 9 whose reluctance varies according to the position of the frame.

The second extension _6b cooperates with an air gap 24 whose reluctance is low and substantially constant, with one end 26 of a second fixed magnetizable part 25; this second magnetizable part has, moreover, another end 27 which is placed opposite the pole piece 7, and is separated from the latter by a gap 23 whose reluctance varies according to the longitudinal position of the frame.

This figure therefore makes it possible to grasp the existence of two pairs 8 and 22 of air gaps, where each pair comprises, on the one hand, an air gap of variable reluctance 9 respectively 23 and, on the other hand, an air gap 10 respectively 24 of low and substantially constant reluctance. The air gaps 10 and 24 are established by surfaces close to and parallel to the direction - F, G -.

Although, in particular for balancing reasons, one or more elastic members acting in direction F or G can be associated with the frame, these members will not be involved in the balance of forces exerted on it. , when opposite excitations are communicated to the coil and to the circuits thanks to the circulation in one direction or the other of a current in the latter.

The operation of the electromagnet 1 will be explained with the aid of FIGS. 1, 2 and 3 which include circuits in solid lines to represent the flux φ _B developed by the coil, and circuits in dashed lines to represent those φ _a , φ _a1 , φ _a2 which are developed by the permanent magnet.

In one of the stable states - I - represented in FIG. 1, the flux φ _a of the magnet closes through the pole piece 7, the branch 29, the core 30, the branch 28, the air gap 10 of low reluctance, the extension _6b and the pole piece 6, because the pieces 7 and 12 are practically in contact and a force of attraction applies the armature against the circuit fixed 13; a small leakage flow φ _f1 also closes through the fixed part 25.

When a current of appropriate direction flows in the coil 14, the flow φ _B flows through the core 30, the branch 29, the air gap 23, the part 25, the air gap 24, the extensions 6 _b , 6 _a and the air gap 10; such circulation implies that, on the one hand, the air gaps 24 and 10 have a very low reluctance and that, on the other hand, the flux φ _B , which cannot pass through the magnet, is developed by ampere-turns of sufficient coil to overcome the reluctance of the air gap 23, which is relatively greater.

When these ampere-turns are sufficiently high, a magnetic polarity appearing at the end 27 attracts the pole piece 7; this phenomenon occurs as soon as φ _B becomes greater than φ _a , and the reinforcement then moves in direction G.

It will be noted that when this displacement takes place, the reluctance of the air gap 32 decreases, while a reluctance appears in the air gap 9.

So that the movement can be prolonged, it is necessary that the overall reluctance R _g , authorizing the circulation of the flow φ _B and then making two partial reluctances intervene in series, does not increase appreciably.

As the armature moves away from the fixed circuit 13, the flow φ _a decreases due to the appearance of the air gap 9 and the attraction undergone in direction F by this armature also decreases.

For a particular position - III - of the armature, visible in FIGS. 3 and 4, the flux of the magnet is divided into two fluxes φ _a1 , φ _a2 , flowing respectively in the fixed circuit 13 and in the fixed circuit 25, so that the motor actions in direction F and in direction G developed by this magnet, are equal and of opposite direction.

It suffices, therefore, that the excitation of the coil is maintained until the movement in direction G of the armature brings it slightly beyond this particular position - III - for the flux φ _a2 becomes preponderant and then attracts the armature to a position close to the fixed part 25; in the stable state - II - corresponding and visible in Figure 2, the armature is therefore still maintained by the permanent magnet. Residual air gaps, 23 _r respectively 9 _r , having a small thickness, can advantageously be established between the pole pieces 7 and the ends 12, respectively 27 to attenuate the effects of adhesion and facilitate the take-off of the reinforcement in one either way.

The return of the armature from the - II - position to the - I - original position is also effected by means of a short-term excitation of the coil which is then traversed by a current in the opposite direction to the previous one.

During this operation, the circulation of the flux φ _B which changes direction, see FIG. 2, and closes in particular through the fixed part 25 in the opposite direction to the flux φ _a of the permanent magnet, so that the latter is forced to close through the pole piece 4, the air gap 9, the magnetizable fixed piece 13, the air gap 10 and the extension 6 _a of the pole piece 6.

The two flows which are then additive in the portions 4, 13, 10 and 6 _a cause the appearance of attractive forces which are applied in direction F on the armature 2 and establish its initial displacement in this direction.

In a manner comparable to that described above, but in opposite directions, the flow φ _a2 decreases, and the flow φ _a1 increases, while the flow φ _B remains substantially constant; here again, it is therefore not necessary to maintain a flow of current in the coil beyond a particular position of the armature, which is substantially close to the previous one, see FIG. 4, so that the displacement of the latter in direction F continues on its own and only thanks to the presence of the permanent magnet.

The U shape given here to the cylinder head 13 makes it possible to replace the coil 14 by relative displacements of direction F and G thereof.

According to an alternative embodiment 30 of the invention, which is visible in FIG. 5, where the armature 31 of the electromagnet is in its equilibrium position, the sliding air gaps of low reluctance previously referenced 10 and 24 are no longer present and the pole piece or expansion 39 of the armature 31 is associated by a single air gap E of low reluctance with a magnetizable cross-member 33 which longitudinally connects the branch 34 of the fixed magnetizable circuit 35 associated with the coil 36, with the second fixed part 37. In this embodiment, the movable frame 32 can be guided longitudinally in directions F and G in the housing, by virtue of a slide of the housing 39 shown diagrammatically at 38 of the housing.

This embodiment is interesting insofar as it allows a reduction in the mass of the armature by requiring only one air gap of low reluctance E; however, the latter combines the functions of the two preceding air gaps, as shown by the dotted line dividing the flux of the magnet into two flows φ _a1 and φ _a2 flowing through two neighboring air gaps e ₁ and e ₂ .

The mode of operation of this variant embodiment is entirely comparable to that of the previous one: however, it will be appreciated that, for the same flux φ _B developed in circuits 34, 37, it will not be necessary to apply a number of the disappearance of the air gaps previously placed in 9 and 24.

In embodiments 1 and 30 of the invention, the means used to operate the longitudinal guide of the armature must take into account the existence of transverse forces of attraction which develop between the magnetizable parts separated by the sliding air gaps. 10, 24 on the one hand and E on the other; these parasitic lateral forces generate additional friction. Furthermore, disassembly of the coil 36 is not directly possible, unless the cylinder head 35 has a pole piece 35 _{a which} can be separated from the branch 34.

One of the means that can be used to very substantially reduce these parasitic lateral forces in an embodiment 1 _a , see FIG. 6, consists in passing through the extensions 6 _a , 6 _b two openings 41, 42 of sections analogs which will be arranged in a leg 28 _a of the fixed circuit 13 _a and respectively in the end 26 _a of the fixed part 25 _a .

In another embodiment 30 _a derived from that of FIG. 5 and visible in FIG. 7, the pole piece 32 _a of the armature 31 _a is provided with an opening 43 surrounding with a low reluctance air gap E ₁ a crosspiece 33 _a of similar section.

If it is further proposed to play these new sliding gaps a longitudinal and transverse mechanical guiding function, it will be possible for example to have there rings of antifriction material, the openings and through parts then advantageously having adjusted circular sections.

According to two other embodiments 50 and 60, which derive from those illustrated in FIGS. 5 and 1 by giving the fixed and mobile parts shapes of revolution, the electromagnets advantageously take the form of magnetizable pots where the balancing of the attraction forces is operated simultaneously as well as an efficient and economical guidance.

In these embodiments, shown in FIGS. 8 and 9, the axis of symmetry XX ′, respectively YY ′ passes substantially through the pole piece 6 or respectively crosspiece 33, described above, and the coils 51 respectively 61 are housed in cavities annulars 52 respectively 62 concentric to XX ', respectively YY'.

The reinforcements 53 respectively 63 use here permanent magnets 54 respectively 64, having, for example, annular shapes visible in FIG. 14.

According to two other embodiments 70 and 80 which are visible in FIGS. 10 and 11, which also derive from those illustrated in FIGS. 1 and 5 by giving the fixed and mobile parts forms of revolution around axes WW 'respectively ZZ' passing longitudinally by the guide leg of the movable armature, advantages comparable to those obtained by embodiments 50 and 60 are obtained.

A common advantage of embodiments 50, 60, 70, 80 is that they make it possible to easily change the coils 51, 61, 71, 81 thanks to the presence of removable bottoms 55, 65, 72, 82 and respectively covers 56, 66 placed in accessible regions of housings 57, 67, 73, 83 shaped to keep the other fixed and non-removable parts.

Variants can be presented by operating combinations of structures, see FIGS. 12 and 13, in which the two sliding air gaps of low reluctance are arranged, one 91 respectively 92 at the periphery of a first fixed and magnetizable part 93 respectively 94, the other 95 respectively 96 in a central region of a second magnetizable fixed part 97 respectively 98.

According to the embodiment of FIG. 12, one of the attraction air gaps which has been moved away from the permanent magnet 100 has also been moved towards the bottom 99 of the fixed part 97.

When the movable armatures, such as 101, respectively 103 take the form of discs as is the case in FIGS. 8 to 13, it is possible to use either a single permanent magnet 102 having an annular shape, see FIG. 14, or again a multiplicity of permanent magnets which are fitted in cells such as 104, respectively 105 and which can take circular shapes 106 or respectively rectangular 107; in all cases shown here, the magnetization axes are radial.

It is also possible to give the magnets mobile armatures having forms of revolution, other non-radial magnetization directions.

In FIGS. 16 and 17, the reinforcements 110 respectively 111 use the same type of annular magnet 112 in which the proportion of the height relative to the average diameter has been increased and where the direction of the magnetization NS is parallel to the axis of revolution KKʹ.

Massive pole pieces 113, 114 which can be associated with each of the magnetic poles along the external and internal surfaces of the magnet, see FIG. 16, lead here to operate an axial offset - d - of the sliding air gaps.

In FIG. 17, turned or stamped pole pieces 115, respectively 116 are associated with the permanent magnet 112 by means of narrowing respectively widening of their average diameters so that the sliding gaps are here arranged by a transverse mean plane MMʹ which is substantially the same as that of the magnet.

Comparable arrangements can naturally be chosen when the movable armatures do not assume a form of revolution, as is done in FIGS. 1 to 7.

Claims (6)

1. A bistable electromagnet having a fixed magnetizable circuit (13) excited by a coil (14, 36, 51, 61, 71, 81) so as to give opposite magnetic polarizations to two fixed pieces (25, 28 ; 34, 37 ; 25a, 28a ; 58, 59 ; 68, 69 ; 75, 76 ; 85, 86 ; 93, 97) of this circuit placed facing each other, and a mobile armature (2, 31, 32, 53, 63, 74, 84, 101, 103) mounted for sliding between said two pieces (25, 28 ; 34, 37 ; 25a, 28a ; 58, 59 ; 68, 69 ; 75, 76 ; 85, 86 ; 93, 97) along a longitudinal axis (F, G), said armature (2, 31, 32, 53, 63, 74, 84, 99, 101, 103) comprising a transversely polarized permanent magnet (3, 54, 64, 100, 102, 104, 105, 106, 107) whose internal flux flows parallel to said two fixed pieces (25, 28 ; 34, 37 ; 25a, 28a ; 58, 59 ; 68, 69 ; 75, 76 ; 85, 86 ; 93, 97) and which forms with said fixed magnetic circuit (13) two pairs of air gaps, namely, parallel to the longitudinal axis (F, G), one pair of working air gaps (9, 23) of variable thickness and perpendicular to the longitudinal axis (F, G) and a pair of sliding closure air gaps (10, 24 ; e₁_, e₂_, E₁_, E₂, E₃) of substantially constant thickness, placed in series with the working air gaps (9, 23),
   characterized in that the two poles of the permanent magnet (3, 54, 64, 100, 102, 104, 105, 106, 107) are radially extended by two respective magnetic polar pieces (6, 7), namely :

   - a first polar piece (6) provided with at least one longitudinal extension or enlarged portion (6a, 6b ; 6a', 6b' ; 32c, 32c', 69a, 69b ; 79, 87a, 87b) forming with the fixed magnetic circuit (13) at least one closure air gap (10, 24 ; e₁, e₂, E₁, E₂, E₃) of constant thickness, and

   - a second polar piece (7) facing the above-mentioned fixed pieces (25, 28 ; 34, 37 ; 28a, 25a ; 58, 59 ; 68, 69 ; 75, 76 ; 85, 86 ; 93, 97) so as to form with said pieces (25, 28 ; 34, 37 ; 25a, 28a ; 58, 59 ; 68, 69 ; 75, 76 ; 85, 86 ; 93, 97) said working air gaps (9, 23) and in that the two polar pieces (6, 7) are disposed so as to substantially enter into contact with one or the other of the fixed pieces (25, 28 ; 34, 37 ; 28a, 25a ; 58, 59 ; 68, 69 ; 75, 76 ; 85, 86 ; 93, 97) for each of the stable positions of the electromagnet and establish a holding flux φ_a for each of said positions.
2. The electromagnet according to claim 1,
   characterized in that the first polar piece comprises two longitudinal extensions (6a, 6b, 32c, 6a', 6b', 69a, 69b, 87a, 87b) forming with the above-mentioned fixed pieces (25, 28 ; 25', 28' ; 28a, 25a ; 68, 69 ; 85, 86) two respective closure air gaps (10, 24) of constant thickness.
3. The electromagnet as claimed in claim 1,
   characterized in that one of the pole pieces (32, 32a, 53b) has an enlarged portion (32c') or extensions (32a, 53b') cooperating with a cross piece (33, 33a, 33d) which magnetically connects together a branch (34, 34', 52) of a first fixed magnetizable circuit (35, 35', 58) and one end or the center of a second magnetizable circuit (37, 37', 59), this enlarged portion or these extensions carrying one of the faces of an air gap E formed by juxtaposition of two contiguous substantially constant air gaps (e₁, e₂) placed in series with the flux (φ_a) developed by the permanent magnet (3).
4. The electromagnet as claimed in one of claims 1 to 3,
   characterized in that the fixed magnetizable circuits ((58, 59 ; 68, 69), the coil (51, 61) and the mobile armature (53, 63) have the form of solids of revolution whose axis (XX'), (YY') which is parallel to the longitudinal axis (F, G) passes through said extensions (69a, 69b) of the first polar piece or through said cross piece ((33d).
5. The electromagnet as claimed in one of claims 1 to 4,
   characterized in that said extension (6a, 6b, 32c) cooperates with the fixed circuit portion concerned (13a, 25a, 33a) through annular air gaps (E₁, E₂, E₃) so as to cancel out the magnetic attractions perpendicular to the direction of movement (-F, G-) of the armature (2a, 31a).
6. The electromagnet as claimed in claim 1,
   characterized in that said fixed magnetizable circuit (75, 76), (85, 86), the coil and the mobile armature (77, 87) have the form of solids of revolution whose axis (WW'), (ZZ') which is parallel to the horizontal axis (F, G) passes through the leg for guiding the mobile armature, the second pole piece being disposed around the guiding leg.

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