GB2251736A - Securing pole pieces on permanent magnet rotor - Google Patents

Description

PERFECTED ELECTRIC TORQUE MOTOR c) b, The present invention relates to a

perfected electric torque motor.

Torque motors are devices for exerting considerable torque with very little angular displacement, and comprise a stator consisting of an annular element on which is defined one or more windings; and a rotor consisting of a permanent magnet free to rotate inside the stator and angularly integral with a shaft, which is the only means mechanically connecting the load. The main characteristic of these motors is that, due to the high torque exerted as a function of minimum angular displacement, the load, consisting of an actuator such as a throttle valve, can be operated directly with no intermediate gearing.

one of the main drawbacks of these motors is ' that they only enable electromagnetic interaction between the permanent magnet and the turns of the winding facing the magnet and therefore inside the stator. To overcome this drawback, substantially U-shaped pole pieces are provided, which, in diametrically-opposed directions, 2 - surround a respective portion of the stator, so as to enable electromagnetic interaction involving a larger number of turns of the winding. A portion of the pole piece is connected mechanically to the permanent magnet to which, at present, the pole pieces are secured using either screws or adhesive.

screw fastening creates a number of problems as regards assembly and overall size. Due to its magnetic characteristics, the rotor is made of relatively soft material, while the motors themselves are normally so small as to allow of very little freedom in the definition of an assembly portion with threaded holes and sufficient space in which to torque the screws. In other words, screw fastening requires special machining, and involves assembly difficulties, which inevitably result in higher production cost.

While overcoming the above drawback, the adhesive system poses another, relative to the reliability of the assembly, which, for certain applications of the motor,, such as the control of a vehicle internal combustion engine supply device valve, must present a high degree of dependability. In this case, in fact, a fault on the electric motor automatically results in arrest of the vehicle.

It is an object of the present invention to provide a perfected electric torque motor designed to overcome the aforementioned drawbacks, i.e. featuring a highly reliable system of assembling the pole pieces to the magnet. Further aims and advantages of the present invention will be disclosed in the following description. According to the present invention, there is provided an electric torque motor comprising:

an annular stator made of ferromagnetic material and supporting two diametrically-opposed windings; a rotor consisting of a permanent magnet fitted coaxially inside said stator; and two diametrically-opposed pole pieces made of ferromagnetic material and each comprising a first lateral portion between said magnet and one of said windings; a center portion over said stator; and a second lateral portion outside said stator and parallel to said first lateral portion; characterised. by the fact that it comprises a system of centering and assembling said pole pieces to said magnet, whereby a first projection defined on the inner face of said first lateral portion engages a first recess defined on the lateral surface of said magnet; and whereby a metal retaining ring clicks inside a second recess defined on the outer face of said first lateral portion, so that the inner face of said first lateral portion presses firmly on the lateral surface of said magnet.

4 - A preferred non-limiting embodiment of the present invention will be described by way of examplewith reference to the accompanying drawings, in which:

Fig.1 shows a section of a motor in 4ccordance with the teachings of the present invention; Fig.2 shows a a bottom plan view of the Fig.1 motor; Fig.3 shows an exploded view of a number of components on the Fig.1 motor.

Number 1 in Fig.1 indicates an electric torque motor comprising:

a stator 2 consisting of an annular element 3 made of ferromagnetic material and supporting two diametrically-opposed windings 4; a rotor 5 comprising a central element 6 of a permanent magnet; and two diametrically-opposed pole pieces 7 made of ferromagnetic material; and a system of centering and assembling pole pieces 7 to element 6.

Windings 4 each consist in known manner of a number of turns wound about element 3. Each winding 4 of motor 1 covers an arc of roughly 136, so that two spaces 8, each covering an arc of 44, are defined between the ends of windings 4. Spaces 8 enable the definition inside element 3 of reference points for the assembly of motor 1, which, as described later on, is so designed as to enable it to be assembled automatically. In addition to simplifying assembly, said reference points also consisting provide for perfect timing of the components of motor 1 and, hence, improved operation of the same.

In the embodiment shown, for each space 8, the reference points consist of a diametrical recess. 11 formed on the lower edge of element 3. Recesses 11 may of course act as reference points for mounting motor I on to a fixed supporting structure, or, better still, form part of the system for assembling stator 2 to said supporting structure. For reducing axial size, spaces 8 may be used as a passage for the connectors (preferably blade types) electrically supplying windings 4.

As shown in the accompanying drawings, element 6 is cylindrical in shape, is fitted coaxially inside stator 2, and presents an axial through hole 12 having two opposite flat faces 13 which, in use, are perpendicular to the diametrical axis along which recesses 11 are formed. The above design of hole 12 provides for angularly integral connection of element 6 and the shaft or hub of an actuator, which connection in turn determines the angular position of element 6 in relation to stator 2, and, consequently, the angular position of the actuator in relation to the fixed structure. Halfway down, on the sides corresponding to the curved faces of hole 12, the lateral surface of element 6 presents two milled, diametricallyopposed, flat-bottomed recesses 14 (Fig.3) each facing a center portion of a respective winding 4.

As shown in the accompanying drawings, each pole piece 7 is U-shaped, so as to surround said center portion of respective winding 4, and presents a first lateral portion 15 between element 6 and element 3 at said center portion of winding 4; a center portion 16 over stator 2; and a second lateral portion 17 outside stator 2 and parallel to portion 15.

The bottom end of portion 15 is substantially flush with the bottom end of element 6. Portion 15 is defined in a plane describing a cylindrical surface having as its center the longitudinal axis of element 6, and presents its inner face substantially contacting a lateral surface portion of element 6. Halfway down, said inner face of portion 15 presents a flat-faced projection 18 engaging recess 14. On either side of projection 18 on the inner face of portion 15, there are formed two projections 21 (Fig.3), the respective longitudinal axes of which describe an arc having as its center the axis of element 6. Recess 14, projection 18 and projections 21 are so sized as to enable projections 21 to contact the lateral surface of element 6.

As shown in Fig.1, a first projection 21 contacts the lower lateral edge of element 6; a second projection 21 contacts the lateral surface of element 6 just below recess 14; a third projection 21 contacts said surface just above recess 14; and a fourth projection 21 contacts the upper lateral edge of element 6. Two chambers 22 are thus formed between the two pairs of projections 21 and the corresponding lateral surface of element 6. At projection 18, the outer face of portion 15 presents a recess 23, the longitudinal axis of which describes an arc having as its center the axis of element 6.

Portion 16 is thicker than portions 15 and 17, so that its upper face is higher than the upper edge of element 6. The longitudinal axis of portion 15 describes an arc having as its center the axis of element 6. Portion 17 is geometrically similar to portion 15, except that it is shorter and presents no projections or recesses.

With reference to Fig.1, the system for securing pole pieces 7 to element 6 features a layer of adhesive injected into chambers 22, and a metal retaining ring 24 housed inside recesses 23 on pole pieces 7, for pressing portions 15 on to element 6. Projections 21 thus press on the lateral surface of element 6, and projection 18 on the bottom wall of recess 14.

Motor 1 operates in known manner and will therefore be described only briefly. In particular, the magnetic flux produced by the magnet interacts with the current circulating in the turns of the portion of winding 4 inside pole piece 7, which interaction produces a force for rotating the magnet.

Motor 1 is easy to assemble, and may even be assembled automatically. To assemble rotor 5, pole pieces 7 are first connected to element 6 by engaging projections 18 inside recesses 14. Ring 24 is then clicked inside the semiannular groove defined by recesses 23 on pole pieces 7, and adhesive injected inside chambers 22. At this point, rotor 5 is connected to stator 2 by simply securing stator 2 to the supporting structure and rotor 2 to the shaft or hub of the actuator.

The advantages of the present invention will be clear from the foregoing description.

In particular, the component parts of motor 1 are so designed as to enable fast, troublefree assembly, which may even be performed automatically. The references on motor 1 provide for determining a given angular position of its components, and of the motor itself in relation to the supporting structure and the actuator. In addition, the present invention provides for a highly reliable, straightforward system of securing the pole pieces'to the magnet. The above characteristics all combine to provide for a high degree of reliability of motor I as well as low production cost.

To those skilled in the art it will be clear that changes may be made to the motor as described and illustrated herein without, however, departing from the scope of the present invention.

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Claims (1)

1. An electric torque motor comprising:

   an annular stator (2) made of ferromagnetic material and supporting two diametrically-opposed windings (4); a rotor (5) consisting of a permanent magnet (6) fitted coaxially inside said stator (2); and two diametrically-opposed pole pieces (7) made of ferromagnetic material and each comprising a first lateral portion (15) between said magnet (6) and one of said windings (4); a center portion (16) over said stator (2); and a second lateral portion (17) outside said stator (2) and parallel to said first lateral portion (15); characterised by the fact that it comprises a system of centering and assembling said pole pieces (7) to said magnet (6), whereby a first projection (18) defined on the inner face of said first lateral portion (15) engages a first recess (14) defined on the lateral surface of said magnet (6); and whereby a metal retaining ring (24) clicks inside a second recess (23) defined on the outer face of said first lateral portion (15). so that the inner face of said first lateral portion (15) presses firmly on the lateral surface of said magnet (6). 2) - A motor as claimed in Claim 1. characterised by the fact that said magnet (6) is cylindrical, and that said 1 first lateral portion (15) is defined in a plane describing an arc having as its center the longitudinal axis of said magnet (6).

   3) - A motor as claimed in Claim 2, characterised by the fact that the longitudinal axis of said second recess (23) describes an arc having as its center the longitudinal axis of said magnet (6); said second recess (23) being formed substantially on a level with said first projection (18).

   4) - A motor as claimed in Claim 3, characterised by the fact that the bottom wall of said first recess (14) and the contacting face of said first projection (18) are flat.

   5) - A motor as claimed in at least one of the foregoing Claims, characterised by the fact that, on the inner face ot said first lateral portion (15), and by second projections (21) contacting the lateral surface of said magnet (6), at least one chamber (22) is defined in which a layer of adhesive is injected; each of said second projections (21) being defined on an are having as its center the longitudinal axis of said magnet (6).

   6) - A motor as claimed in Claim 51' characterised by the fact that two said chambers (22) are defined,-one above and one below said first projection (18).

   7) - A motor as claimed in at least one of the foregoing Claims, characterised by the fact that each of said windings (4) as a whole covers such an arc as to define. between the ends of said windings (4), two spaces (8) constituting reference points for the assembly of said motor and for determining a given angular position of the components of said motor, of said stator (2) in relation to a supporting structure, and of said rotor (3) in relation to an actuator angularly integral with said magnet (6).

   8) - A motor as claimed in Claim 7, characterised by the fact that, for each said space (8), said reference points consist of a third diametrical recess (11) formed on the lower edge of said stator (2); said third recesses (11) forming part of a system for securing said stator (2) to said supporting structure.

   9) - An electric torque motor, substantially as described and illustrated herein with reference to the accompanying drawings.