GB2125222A - Electro-magnetic actuator - Google Patents

Abstract

An electromagnetic actuator has three, spaced limbs (18, 19, 20) projecting from a common base (17) and a permanent magnet (24) which moves in an aperture in the middle limb (20). An energising winding (30) surrounds the middle limb. The magnet (24) is fixed on a shaft 25 carrying a valve closure member (15) and guided in pole pieces (22, 23). <IMAGE>

Description

SPECIFICATION Electro-magnetic actuator This invention relates to an electro-magnetic actuator for providing an output when the actuator is energised by an electric current. The output may be provided to a further device to control that further device. Alternatively, the output may be used to control a flow path by means of elements incorporated in the actuator.

According to the invention, there is provided an electro-magnetic actuator comprising a permanent magnet, guide means for defining a path along which the magnet can move under the influence of a magnetic field, the magnet having opposite pole faces spaced apart along the path, first and second pole pieces adjacent to opposite ends of said path, a third pole piece adjacent to an intermediate part of the path and an electrically conductive winding have a core which is in a magnetic circuit with the pole pieces, the magnetic circuit being so arranged that, when a direct current flows through the winding, the first and second pole pieces have like polarity which is unlike the polarity of the third pole piece.

Reference herein to a magnetic circuit does not necessarily imply that the circuit is a closed circuit.

There will usually be an air gap in the magnetic circuit.

When the permanent magnet is at one end of its path and no current flows through the winding, magnetic flux is established in said circuit by the permanent magnet and therefore work must be done to move the permanent magnet from this end of its path. If the permanent magnet is coupled mechanically with an external device, that external device can be controlled by the actuator, since it is necessary for a force exceeding a predetermined threshold force to be exerted on the permanent magnet by the associated device in order for the device to move the magnet along its path.

Alternatively, the magnet can be moved along its path by causing a current pulse to flow through the winding in a direction such that the flow of magnetic flux in the circuit is reversed, the flow of flux arising from the permanent magnet being dominated by the flow of flux arising from energisation of the winding. Once the permanent magnet has been moved from a first end of its path to a second end, flow of current in the winding can be terminated and the magnet will induce a flow of flux in the circuit in a direction such that the magnet tends to remain at the second end of its path.

The guide means preferably includes a shaft on which the permanent magnet is fixedly mounted and a pair of bearings arranged to support the shaft for longitudinal sliding, the permanent magnet lying between the bearings. Bearings which will reliably maintain the permanent magnet accurately on a predetermined path throughout a long working life of the actuator can readily be provided and this makes it possible for the third pole piece to be formed with an aperture in which the permanent magnet is a close sliding fit, so that good magnetic coupling can be established between the permanent magnet and the third pole piece without substantial friction arising from contact between the magnet and the third pole piece.

There is also provided in accordance with the invention a combination comprising a valve and a magnetic actuator according to the invention, the permanent magnet being mechanically connected with a control element of the valve.

Alternatively, an actuator in accordance with the invention may incorporate a valve.

An example of a valve in combination with an actuator in accordance with the invention will now be described, with reference to the accompanying drawing, wherein FIGURE 1 shows in side elevation the actuator and valve, with the valve in an open condition, and FIGURE 2 shows a cross-section through the actuator and valve with the valve in a closed condition.

The valve 10 shown in the drawing comprises a housing 11 defining a chamber 12 to which there is an inlet 13. An outlet duct 14 of the valve extends through the wall of the housing 11 and presents at a position within the chamber 12 a seat with which a closure element 1 5 can engage to close the valve.

The actuator 1 6 comprises a body 1 7 having a high magnet permeability. The body is elongated and includes at opposite ends respective limbs 1 8 and 19. The body includes a third limb 20 which lies between the first and second limbs and is spaced at least approximately equally from them.

The limbs 18 to 20 are parallel to one another and are united buy a common base 21 of the body 17.

In each of the limbs 18 to 20, there is formed a cylindrical aperture, these apertures being co-axial with one another and aligned with the path along which the closure element 1 5 moves towards and away from its seat. In the aperture defined by the first limb 18, there is mounted a pole piece 22 and in the aperture defined by the second limb 1 9 there is mounted a further pole piece 23. These pole pieces also are formed of a material having a high magnet permeability and, in the particular example illustrated, are generally cylindrical and are a force fit in the apertures of the limbs.

The actuator 1 6 further comprises a permanent magnet 24 of generally cylindrical form which is a close sliding fit in the aperture defined by the third limb 20. One end face of the magnet is a north pole face and the opposite end face is a south pole face, as indicated in the drawing. The length of the magnet exceeds the corresponding dimension of the third limb, so that a part of the magnet can protrude from the aperture in the third limb whilst that aperture is fully occupied by a further part of the magnet. The magnet 24 is fixedly mounted on a rectilinear shaft 25 which extends longitudinally of the magnet through a central hole therein.

Guide means is provided for guiding the magnet 24 along a rectilinear path, opposite ends of which are defined by the pole pieces 22 and 23.

The pole piece 22 is of annular form and has a central passage 26 into which the shaft 25 can extend with radial clearance. A corresponding passage 27 is provided in the pole piece 23 and the shaft 25 extends through the passage 27 from the magnet 24 into the housing 11 of the valve.

The closure element 1 5 may be mounted directly on an end portion of the shaft 25. Alternatively, the closure element may be provided with its own shaft which is attached to the shaft 25. The guide means comprises a bearing bush 28 disposed in the passage 26 adjacent to that end of the pole piece 22 which is nearer to the third limb 20 and a further bearing bush 29 disposed in the passage 27 at an end thereof nearer to the third limb.

These bushes engage the shaft 25 in sliding contact and are formed of material selected to have a low coefficient of friction with respect to the shaft.

An electrically conductive winding 30 embraces a part of the third limb 20 which lies between the base 21 and an end portion 34 of the limb which surrounds the aperture therein. This winding is connected to terminals 32 mounted on the body 1 7 through the intermediary of an electrically insulating carrier 33. With this arrangement, when a current is passed through the winding, magnetic flux induced in the body 17 such that the pole pieces 22 and 23 have a like polarity which is opposite to the magnetic polarity of the end portion 34.

When the actuator is prepared for use, the terminals 32 are connected via a suitable switch with a source of electric power, for example a primary cell or other low voltage d.c. source.

When there is no current flow through the winding 30 and the magnet 24 is in the position shown in Figure 1, there is a substantially closed path for magnetic flux from the magnet through the pole piece 22, the limb 18, the base 21 and the limb 20 back to the magnet. To move the magnet away from the pole piece 22, work must be done and therefore the magnet resists movement of the closure element 1 5 towards its seat, unless a force exceeding a predetermined threshold force is exerted on the magnet, shaft 25 or the closure element 1 5.

If the winding 30 is energised, whilst the magnet 24 is in the position shown in Figure 1, by a current flowing in a direction such that the flow of flux in the magnetic circuit is reversed, there will be established on the pole piece 22, a magnetic pole face which faces towards the immediately adjacent pole face of the magnet 24 and has a polarity like the polarity of that face of the magnet. Accordingly, there will be repulsion between the magnet and the pole piece 22. There will also be repulsion between the end portion 34 of the third limb and an end portion of the magnet 24 disposed in the aperture defined thereby.It will be noted that, when energisation of the winding 30 commences, the magnet is in contact with the pole piece 22 and is spaced by a negligible distance from the end portion 34 of the third limb so that magnetic coupling between the magnet on the one hand and the pole piece 22 and third limb on the other hand will be good and a large magnetic force will be exerted on the magnet in a direction tending to displace the magnet and the closure element 1 5 towards the seat defined by the outlet 14. As this movement occurs, an air gap appears between the magnet and the pole piece 22, the magnetic coupling decreases and the magnitude of the force exerted on the magnet falls.However, as the magnet approaches the pole piece 23, attraction between one end of the magnet and this pole piece and attraction between an opposite end portion of the magnet and the third limb 20 will increase so that the magnitude of the force exerted on the magnet will rise once more. Furthermore, flow of magnetic flux in a circuit comprising pole piece 23, second limb 19, base 21 and third limb 20 which is established by energisation of the winding 30 is enhanced by flow of magnetic flux induced by the magnet 24 in the same circuit. If the winding 30 is de-energised, the magnet will remain in contact with the pole piece 23 unless there is exerted on the magnet, shaft 25 or closure element 1 5 a force exceeding a threshold value.

To move the magnet 24 from the pole piece 23 to the pole piece 22 and thereby open the valve 10, the winding 30 is energised by a current flowing in the opposite direction to induce on the pole pieces 22 and 23 and on the end portion 34 of the third limb magnetic poles as represented in Figure 2.

The position of the third limb 20 relative to the pole pieces 22 and 23 may be such that the force which must be exerted mechanically to displace the magnet 24 from contact with one of the pole pieces, when the winding 30 is de-energised, exceeds the corresponding force which must be exerted to displace the magnet from the other pole piece. For example, the threshold force exerted on the closure element 1 5 which is just sufficient to open the valve may be smaller than the threshold force exerted on the closure element which is just sufficient to close the valve.

The length of the magnet 24 relative to the corresponding dimension of the third limb 20 and to the spacing of the third limb from the pole pieces 22 and 23 may be such that the magnet always protrudes on opposite sides of the third limb, such that when in one of its extreme positions an end face of the magnet is flush with a corresponding face of the third limb 30 or such that, when in one of its extreme positions, an end face of the magnet lies within the aperture defined by the third limb 20.

As shown in Figure 2, there may be provided in the housing 11 a seal 35 through which the shaft 25 slides and which seals the opening in the housing through which the shaft passes against egress of fluid from the housing. Alternatively, the housing 11 may be sealed to the pole piece 23, the passage 26 closed at its end remote from the third limb and the space within which the magnet 24 moves sealed by an envelope (not shown) so that fluid can pass from the interior of the valve housing 11 along the passage 27 but cannot escape completely from the assembly or contaminate the winding 30. In a case where fluid from the valve is permitted to enter the actuator, bleed passages (not shown) may be provided to facilitate flow of fluid between opposite ends of the magnet 24.

The actuator shown in the drawing may be modified by the provision on the magnet 24 of a radially outwardly protruding flange adjacent to one end of the magnet or by the provision of two such flanges at opposite ends of the magnet. The or each flange would overlap with a pole face of the third limb 20 and enhance the magnetic coupling between the magnet and that limb.

The magnet, pole pieces and the body 17 may be constructed of known materials in a known manner. For example, the body 10 may have a laminated construction.

The combination illustrated in the drawing may be modified by providing the valve outlet duct 14 in the pole piece 23 with a valve seat presented towards the magnet 24 and permitting or adapting the magnet to act as a closure element which can engage the seat to close the outlet.

Since, in this modified arrangement, the shaft 25 would not be required to transmit motion to a device external to the actuator, the shaft may be fixed with respect to the body of the actuator, the magnet being slidable on the shaft and guided thereon by a suitable bearing. If alternative guide means for the magnet are provided, the shaft may be omitted entirely.

Claims (10)

1. An electro-magnetic actuator comprising a permanent magnet, guide means for defining a path along which the magnet can move under the influence of a magnetic field, the magnet having opposite pole faces spaced apart along the path, first and second pole pieces adjacent to opposite ends of said path, a third pole piece adjacent to an intermediate part of the path and an electrically conductive winding having a core in a magnetic circuit with the pole pieces, the magnetic circuit being so arranged that, when a direct current flows through the winding, the first and second pole pieces have like polarity which is unlike the polarity of the third pole piece.

2. An actuator according to claim 1 wherein the guide means includes a shaft on which the magnet is fixedly mounted and a pair of bearings arranged to support the shaft for longitudinal sliding, the permanent magnet lying between the bearings.

3. An actuator according to claim 2 wherein the bearings lie one in each of the first and second pole pieces.

4. An actuator according to any preceding claim wherein the permanent magnet is cylindrical.

5. An actuator according to any preceding claim wherein the third pole piece defines an aperture in which the permanent magnet is a free sliding fit.

6. An actuator according to claim 5 so arranged that, when one end of the permanent magnet is engaged with the first pole piece, an opposite end of the magnet is near to or is aligned with a surface of the third pole piece which faces towards the second pole piece.

7. An actuator according to any preceding claim wherein the force exerted on the permanent magnet by the magnetic field established by flow of a constant current in the winding is greater at the ends of the stroke of the magnet than at the middle of said stroke.

8. The combination comprising a valve for controlling the flow of a fluid and an actuator according to any preceding claim wherein a control element of the valve is mechanically connected with the permanent magnet for movement therewith.

9. An actuator substantially as herein described with reference to and as shown in the accompanying drawing.

10. Any novel feature or novel combination of features disclosed herein or in the accompanying drawing.