GB2445773A - Electromagnetic actuator using magnetic shape memory material - Google Patents

Abstract

An actuator comprises an electromagnet 4, a frame 2 and an element 8 made from a material which changes in shape under the influence of a magnetic field. The said element 8 is mounted in the frame 2 proximate to the electromagnet 4 such that on excitation of the electromagnet 4 at least one dimension of the element 8 changes and this change is used to provide actuation of something. The element 8 may be a rod made from a magnetic shape memory (MSM) material such as a magnetostrictive material or possibly a nickel-manganese-gallium alloy. The actuator may be used to operate a valve in a pneumatic ejector mechanism used in an item sorting apparatus. The actuator may include an electrical winding, a magnetic core with pole pieces 6 and resilient means 24.

Description

ELECTROMAGNETIC ACTUATORS

This invention relates to electromagnetic actuators having particular, but not exclusive application in combination with ejectors in sorting apparatus.

More generally, they will have use in situations in which controllable movements with substantial force are required within a controllable response time, with high reliability and low maintenance requirements.

Arrays of ejectors are used in sorting apparatus in which material to be sorted is directed in a product stream following an aerial path, and certain material is removed from the stream by air pulses from the ejectors. Valve devices are used in the ejectors to control the delivery of air from a pressurised source to an ejector nozzle directed at a particular section of the product stream. An operating mechanism, normally comprising an electrical drive circuit, acts to open valve devices in the array selectively in response to a control signal to deliver a pulse of pressurised air to, and thus from the respective nozzle.

Many current sorting apparatus using pneumatic ejectors grade particulate material according to its ability to reflect light. Typical such apparatus are described in US Patent No: 4 203 522; 4 513 868, and 4 699 273, the disclosures whereof are incorporated herein by reference. In apparatus disclosed in the 522 Patent detectors are responsive to light reflected from the particles of product and generate signals indicative of different qualities of the product. These signals are compared and analysed, to generate a drive signal, generally a current and/or voltage pulse which can activate an ejector to remove the relevant particle from the product stream.

Each ejector in these apparatus will normally use an electrically actuated valve device such as a moving iron disc or plate valve to discharge pulses of pressurised air selectively to remove particles from its respective target area.

I

Ejectors and sorting apparatus are subject to very heavy usage, and are required to operate effectively and accurately over long periods with minimal maintenance. The present invention is directed at an actuator suitable for such heavy usage requirements. The invention exploits the ability of certain materials known as magnetic shape memory (MSM) materials that change their shape when subject to an external magnetic field. The materials return to their original shape when the magnetic field is removed and a suitable external force is applied. According to the invention, an actuator comprises a framework with at least one electromagnetic mounted therein.

An element formed in a magnetic shape memory (MSM) material is mounted in the frame proximate the electromagnet such that excitation of the electromagnet creates a magnetic field to cause a change in dimension of the element. Means are provided for using such a change in dimension. This can be used to generate an output signal, or more directly to operate a device. In the context of ejectors in sorting apparatus, this change in dimension can be sufficient in itself to operate an ejector valve in sorting apparatus of the kind referred to above.

The magnetic shape memory (MSM) effect provokes a change in the shape of an MSM alloy due to a change in its magnetization induced by an external magnetic field. The resultant movement of an element of MSM material is a movement at one extremity relative to another obtained not by a bulk motion of the material, but by deformation from within. The change results from the interactive coupling between the applied magnetic field and the materials individual magnetic domains. Although the effect, known as martensitic transformation, was first identified in quenched steels, a number of materials other than steel are known to exhibit the characteristic. These materials include shape memory alloys, transformation-toughened ceramics, some non-ferrous alloys, pure metals, ceramics, minerals, inorganic compounds, solidified gases and polymers.

A typical actuator according to the invention will include an elongate MSM element fixed in the framework with the electromagnet located to generate a magnetic field transverse to the elongate axis of the element when excited. The consequent dimensional change of the element moves the other end of the element relative to the framework to provide an output. This may be passed oil to the distal end of a rod extending from the framework, and having its proximal end coupled to the other end of the MSM element.

As noted above, the dimensional change of an MSM element in response to an applied magnetic field can be reversed by either reversing the magnetic field or by applying an external force. In actuators of the invention, the latter can conveniently be accomplished by the interposition of a spring effectively between the MSM element and the framework. When the element is elongate or coupled to a separate actuator rod, then the external force can take the form of a coil spring surrounding one or both of the rod and element and directly or indirectly engaging the MSM element. The external force can though, be provided by any suitable device or resilient mechanism such as a gas or leaf spring.

The requisite electromagnetic field can be made available from a single electromagnet with a core extending around the element to define magnetic poles on either side of the element, Alternatively, two separate electromagnets may be used. Most easily these can be aligned such that cores of the electromagnets form magnetic poles on opposite sides of the element, but the cores can be laterally spaced and overlapped to define the magnetic poles at the core tips on either side of the element. Any suitable means can be used for creating the requisite magnetic field.

Whatever arrangement is used to provide for the creation of the magnetic field, it is preferred that the framework in actuators according to the invention forms a magnetic yoke around the element. The framework and element will normally lie on a substantially common plane, but in some embodiments they could lie in perpendicular planes.

The invention will now be described by way of example and with reference to the accompanying schematic drawings wherein: Figure 1 shows a perspective view of a first embodiment of the invention; Figure 2 shows a perspective view of a second embodiment of the invention; Figure 3 is a cross-sectional view taken on line 3-3 of Figure 2 illustrating the arrangement of the electromagnetic cores around the MSM element; and Figure 4 shows a perspective view of a third embodiment of the invention.

The actuator shown in Figure 1 has a unitary generally rectangular framework 2 with two electromagnets 4 mounted therein. Each electromagnet 4 has a core extending towards the other to form poles 6 in juxtaposition on opposite faces of an MSM element 8. One end 10 of the element 8 is held in a bracket 12 fixed in leg 14 of the framework 2. The other end 16 of the element 8 is received in a bushing 18 at the end of a rod 20 that extends through the leg 22 of the framework. A spring 24 surrounds the rod 20 and is compressed between the bushing 18 and the leg 16 of the framework.

The MSM element 8 is a simple bar of rectangular cross-section with parallel flat sides. The framework 2 is a substantially rigid construction which holds the flat juxtaposed faces of the magnetic poles 6 closely adjacent but not in contact with the faces of the MSM element 8. A typical air gap is 0.1mm. The bracket 12, the bushing 24 and the rod 20 hold the MSM element 8 in alignment with the space in between the magnetic poles 6.

To operate the actuator of Figure 1 the electromagnets 4 are connected to a source of direct electrical current to create an electromagnetic field between the juxtaposed faces of the magnetic poles 6, and thereby transverse to the elongate axis of the MSM element 8. The martensitic transformation generated thereby extends the MSM element 8 such that the end 16 thereof moves the bushing 18 and rod 20 against the spring 24. The result is tangible movement at the distal end of the rod 20 which can be used to operate a device coupled thereto, such as an ejector valve in a sorting machine.

Figure 2 shows an actuator similar to Figure 1, but the framework is a composite construction with two E" shaped components 34, on the central legs of which the electromagnets 4 are mounted to be relatively displaced from one another such that their cores extend adjacent opposite sides of the MSM element to form poles 26. This arrangement is illustrated in Figure 3.

Particularly, it facilitates establishment of the spacing between the MSM element 10 and the juxtaposed magnetic poles 26. This is set by spacers 36 which form part of the magnetic circuit. Normally, they would keep the components 34 spaced from one another such that there is a small air gap between the poles 26 and the MSM element 10. However, by using thinner spacers the spacing between the components 34 can be reduced relative to the thickness of the MSM element 10, such that there is engagement between the poles 26 and the element 10. Such contact does not substantially interfere with the martensitic transformation extending the MSM element as described above. The provision of spacers can also provide for better access to the MSM element 8, if this is required. Spacers can also be disposed between the electromagnets 4. Such spacers also serve to avoid discontinuity in the magnetic circuit, thereby maximising the magnetic flux for a given excitation current.

In the embodiments of Figures 1 and 2 the MSM element and the framework lie in a substantially common plane. In the embodiment of Figure 3, they lie in substantially perpendicular planes. The re-orientation of the framework or magnetic yoke further reduces discontinuities in the magnetic circuit, but of course the framework can no longer provide a fixed base for an end of the MSM element 8. In the embodiment of Figure 3 the respective end of the MSM element 8 is fixed relative to the magnetic poles 6 in a boss 30 of magnetically neutral material. The other end of the MSM element 8 is received in a bushing 18 coupled to rod 20 within a spring 24, as in the embodiments of Figures 1 and 2, but the rod passes through a sub-frame 32 which has also to be fixed relative to the boss 30. It will be appreciated that the lateral displacement of the electromagnets and their cores, as illustrated in Figure 3, can also be a feature of this embodiment of the invention.

In the illustrated embodiments of the invention the extent of movement or elongation of the MSM element will depend upon the nature of the element itself, and the strength of the magnetic field applied. A suitable material is a nickel-manganese-gallium alloy. A bar of this alloy having a length of 20mm will typically extend by 1 mm upon application of a magnetic field intensity of 300-500kAIm. This is quite adequate to operate for example an ejector valve in sorting machines of the kind described in the patent publications referred to above.

Claims (17)

1. An actuator comprising a framework with at least one electromagnet mounted therein; an element formed in a magnetic shape memory material, and mounted in the frame to be proximate the electromagnet such that excitation of the electromagnet causes a change in dimension of the element; and means for using such a change in dimension.

2. An actuator according to Claim 1 wherein the element is elongate with an end thereof fixed in the framework, wherein excitation of the electromagnet generates a magnetic field transverse to the elongate axis of the element, movement of the other end of the element relative to the framework providing an output.

3. An actuator according to Claim 2 including a rod extending from the framework and having, a proximal end coupled to said other end of the element.

4. An actuator according to any preceding Claim wherein said change in dimension of the element is against the force of a spring.

5. An actuator according to Claim 3 and Claim 4 wherein the rod passes through a section of the framework, and the spring is disposed between the element in the said framework section.

6. An actuator according to Claim 5 wherein the spring is a coil spring disposed around the rod.

7. An actuator according to any preceding Claim comprising a single electromagnet with a core extending around the element to create magnetic poles on opposite sides thereof.

8. An actuator according to any of Claims I to 6 wherein the electromagnet comprises a pair of excitation coils disposed on opposite sides of the element.

9. An actuator according to Claim 8 wherein each coil is wound round a core forming a magnetic pole in juxtaposition with a side of the element.

10. An actuator according to Claim 9 wherein the cores extend along substantially parallel paths to form magnetic poles adjacent opposite sides of the element.

11. An actuator according to any preceding Claim wherein the framework forms a magnetic yoke around the element.

12. An actuator according to Claim 11 wherein the framework and the element lie in a substantially common plane.

13. An actuator according to Claim 11 wherein the framework and the element lie in substantially perpendicular planes.

14. An actuator according to any preceding Claim including a mechanism for exciting said at least one electromagnet.

15. An actuator according to any preceding Claim 14 comprising a plurality of said elements of magnetic shape memory material.

16. An ejector for use in sorting apparatus including an actuator according to any preceding Claim.

17. A pneumatic ejector according to Claim 16.