Electromagnetic actuator

Abstract

 An electromagnetic actuator comprises a yoke (18) of a material having a low magnetic remanence, and a rare earth permanent magnet (24) in series with said yoke (18), providing a magnetic circuit having no flux gap. A coil (28) can induce in the yoke (18) a flux which is opposed to that from the permanent magnet (24), and an armature (26) which is responsive to the combined flux from said permanent magnet (24) and said coil (28).

Description

[0001] UK Patent 1195629 discloses an electro-magnetic switching device in which a first permanent magnet forms part of a magnetic circuit which has no flux gap and which is provided with a coil for inducing an additional flux into the magnetic circuit. The coil may be used to induce a flux which is either in the same sense or in opposition to the flux from the first permanent magnet. The magnetic circuit also includes a further permanent magnet whose polarity may be reversed by energisation of the coil.

[0002] When the fluxes from the coil and the further permanent magnet are opposed to the flux from the fist permanent magnet a resulting combined flux path passes externally of the gapless magnetic circuit and attracts an armature. When the fluxes from the coil and further permanent magnet are in the same sense as that from the first magnet, the total flux is retained in the gapless circuit and the armature is returned to an unoperated position.

[0003] The device shown in the prior art thus has three sources of magnetism, the first and second permanent magnets, and the coil. The inclusion of the further permanent magnet provides magnetic latching, since the polarity of the further magnet is set by the coil at each operation thereof and coacts with the flux from the first magnet even when the coil is de-energised.

[0004] It is a disadvantage that at each operation of the device the remanent magnetism of the further magnet must be overcome before the device can change state, and this will require an additional coercive force from the coil. Additionally, the further magnet, by reason of its lower relative permeability, requires a much larger magneto-motive force to attain a flux level comparable with a soft iron core of the same dimensions.

[0005] It is an object of the present invention to provide an electro-magnetic actuator in which the gapless magnetic circuit includes only one permanent magnet and which the operating mmf to achieve a given attractive force is reduced, or alternatively that an increased attractive force is obtained with the same mmf in the coil.

[0006] Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:-

Figure 1 is a section through an electrical switch including an actuator according to the invention;

Figure 2 is a section on line 2-2 in Figure 1;

Figure 3 is a section, corresponding generally to Figure 1, through an alternative form of electrical switch, and

Figure 4 is a section, also corresponding generally to Figure 1, of a further alternative form of electrical switch.

[0007] The switch shown in Figure 1 has fixed contacts 10, 11 which are engageable by respective movable contacts 12, 13. The contacts 12, 13 are carried on a lever arm 14 which is mounted on a pivot 15. The pivot 15 is carried between brackets 16 mounted on a base 17. Mounted between the brackets 16 is a yoke 18 of a soft iron having a high magnetic permeability and low remanence.

[0008] The yoke 18 comprises a core 19 which is secured to an end plate 20 and is a close sliding fit in an angled plate 21 which also forms part of the yoke 18. The yoke 18 also includes a plate 22 which is secured to the end plate 20 and is aligned with a horizontally-extending portion 23 of the plate 21. A samarium cobalt permanent magnet 24 extends through aligned openings 25 in the brackets 16, the upper edge of the openings 25 being aligned with the upper edges of the plate 22 and the portion 23 of the plate 21. The axial position of the core 19 and its attached plates 20, 22 is adjustable, so that the plate 22 and the portion 23 abut opposite sides of the permanent magnet 24.

[0009] The yoke 18 thus provides a magnetic circuit which has substantially no flux gap, and in the absence of any additional flux, the flux from the permanent magnet 24 remains within the yoke 18. In these circumstances an armature 26 which is secured to the lever arm 14 is not attracted by the magnet 24, and the arm 14 remains biassed by a compression spring 27 to the position shown in Figure 1.

[0010] A coil 28 is wound about the core 19 and is energisable so as to induce in the yoke 18 a flux which is opposed to the flux from the permanent magnet 24.

[0011] The permanent magnet 24 has an energy product of 159 kilojoules/m³, It is arranged that the energy level of the electro-magnet shall be higher than that of the permanent magnet 24 whereby, when the coil 28 is energised, flux from the permanent magnet 24 no longer passes through the circuit of the yoke 18, but the combined flux of the permanent magnet 24 and the coil 28 passes externally of the yoke 18 through the armature 26. The lever 14 is thereby pivoted against the spring 27 to operate the switch. The attractive force applied to the armature 26 is very high, being effectively derived from the sum of the fluxes from the permanent magnet 24 and the coil 28. It has been found that the response time of the heaviest type of switch assembly expected to be encountered in aircraft is of the order of 30 milliseconds when operated by an actuator according to the invention. If the switch is actuated by energisation of the coil 28, sufficient flux from the permanent magnet 24 may continue to pass through the armature 26 to maintain it in its attracted state even after subsequent de-energisation of the coil 28. Accordingly, therefore, a non-magnetic shim '29 is secured to the yoke 18 to maintain a small flux gap which enables the spring 27 to overcome residual attraction from the magnet 24.

[0012] In the embodiment shown in Figure 3 a high . permeability, low remanence yoke has a core 40 about which is wound a coil 41. The yoke also includes arms 42, 43, 44, a samarium cobalt permanent magnet 45 being included in the arm 44. The magnet 45 has the polarity shown in Figure 3 and combines with the core 40 and arms 42, 43, 44 to provide a first magnetic circuit 46 having no flux gap. The yoke also includes arms 47, 48 which combine with the core 40 to provide a second magnetic circuit 49 which includes a flux gap 50. An armature 51 is pivotally mounted on a pin 52 and includes a samarium cobalt permanent magnet 53 which has the polarity shown and which is positioned adjacent the flux gap 50.

[0013] With the coil 41 de-energised the flux from the magnet 45 remains within the magnetic circuit 46, and does not attract the armature 51. The flux from the magnet 53 passes through the circuit 49 and causes the armature 51 to pivot clockwise to place the actuator in a first operating condition. The coil 41 is energisable to provide a flux in the yoke which is opposed to the fluxes from the magnets 45, 53. Energisation of the coil 41 thus causes a combined flux from this coil and from the magnet 45 to pass externally of the yoke and attract the armature 51 anti-clockwise. The top end of the core 40 becomes a north pole, and the top end of the plate 48 a south pole, repelling the magnet 53 and assisting the anti-clockwise movement of the armature 51. It has been found that the foregoing arrangement can substantially improve the response times of the actuator.

[0014] Figure 4 shows the actuator arrangement of Figure 3 as applied to a change-over switch, and parts corresponding to those of Figure 3 have been given identical reference numerals. In this case, however, a contact arm 60 is mounted for movement about a fixed pivot 70 and carries two contact bars 61, 62. Bar 61 has terminals 63, 64 which engage fixed terminals 65, 66 in one operating condition of the switch and bar 62 has terminals 67, 68 which engage terminals 66, 69 in the other operating condition of the switch.

[0015] It will be apparent that electro-mechanical actuators as described above may be used to operate any desired configuration of switch contact, or other apparatus, for example fluid control valves. It will also be apparent that a plurality of actuators according to the invention may be ganged together to provide additional motive force or may be stacked in a desired configuration to provide a number of separately- operable devices.

Claims

1. An electromagnetic actuator comprising a yoke of magnetisable material a permanent magnet in series with a part of said yoke, said yoke part and said permanent magnet combining to provide a magnetic circuit which has substantially no flux gap, an electro-magnet mounted on said yoke for generating therein a flux of opposite polarity to the flux from said permanent magnet, and an armature movable by a magnetic flux which is external to said yoke and which originates in the combined fluxes of said electro-magnet and said permanent magnet, characterised in that said yoke part (20, 21, 22) is wholly comprised of a material of low magnetic remanence, and that said permanent magnet (24) and said electromagnet (28) are the only flux sources in said magnetic circuit.

2. An electromagnetic actuator as claimed in claim 1 in which said permanent magnet (24) is a rare-earth magnet.

3. An electromagnetic actuator as claimed in claim 1 or claim 2 in which said electromagnet (28) can be energised to a level higher than that of said permanent magnet (24).

4. An electromagnetic actuator as claimed in any of claims 1 to 3 in which a further part (47, 48) of said yoke (18) provides a second magnetic circuit having a flux gap (50), and there is a second permanent magnet (53) mounted on said armature (51) for movement towards and away from said flux gap (50), said second permanent magnet (53) generating within said yoke (49) a magnetic flux of opposite polarity to that provided by said electromagnet (41).

Drawing