GB2450890A

GB2450890A - Magnetic contact

Info

Publication number: GB2450890A
Application number: GB0713372A
Authority: GB
Inventors: John Darby; Kim Lloyd; Graham Franklin
Original assignee: ELMDENE INTERNAT Ltd
Current assignee: ELMDENE INTERNAT Ltd
Priority date: 2007-07-10
Filing date: 2007-07-10
Publication date: 2009-01-14
Anticipated expiration: 2027-07-10
Also published as: GB0713372D0; GB2450890B

Abstract

A magnetic contact 10 for flush mounting to a surface includes an operating face 155 for mounting flush with the surface opposite an operating magnet; a shield wall 165 of high magnetic permeability material perpendicular to the operating surface; and a pair of reed switches 170A, 170B connected in series and disposed on either side of, and separated from, the shield wall 165.

Description

* 2450890

MAGNETIC CONTACT

Field of the Invention

The present invention relates to magnetic contact switches, especially for use in security applications.

Background of the Invention

A reed switch is an electrical switch that is operated by an applied magnetic field. The switch is held in a first state (either open or closed, depending on the design) when a magnetic field of sufficient strength is present, and transitions to the opposite state when the magnetic field is removed. A reed switch is also known as a magnetic contact (switch).

Reed switches are frequently used as sensors in buildings. For example, the reed switch may be mounted on a fixed window or door frame, with a corresponding magnet on the moving door or window. When the window or door is shut, the magnet is typically adjacent the reed switch. The magnetic field from the magnet holds the reed switch in a particular state (say closed). If the window or door is opened, the magnet moves away from the reed switch. This reduces the magnetic field experienced by the reed switch, which therefore transitions to the open state. This change in state of the reed switch can be used as a trigger mechanism, for example, in relation to an alarm circuit, room lighting, etc. If a magnetic contact is used as a security device, such as to detect opening of a door or window, then an intruder may try to subvert the operation of the reed switch.

This might be done by the intruder introducing a separate ("sabotage") magnet adjacent the reed switch. The magnetic field of this sabotage magnet may hold the reed switch in its existing state even if the actuating magnet in the door or window is moved away from the reed switch. In other words, the sabotage magnet may be used to over-ride the normal operation of the reed switch, thereby allowing the adversary to open the window or door without triggering the reed switch, and hence overcoming the security protection for the window or door.

A recently developed Technical Specification (draft standard), BSI DD CLC!TS 50131-2-6:2004, specifies levels of resistance for magnetic contacts to such attacks involving sabotage magnets. Many (if not all) magnetic contacts that are commercially available at present do not satisfy this standard.

Summary of the Invention

The invention is defined in the appended claims.

One embodiment of the invention provides a magnetic contact for flush mounting to a surfhce. The magnetic contact comprises an operating fce for mounting flush with said surface opposite an operating magnet; a shield wall of high magnetic permeability material perpendicular to the operating surface; anda pair of reed switches connected in series and disposed on either side of, and separated from, said shield wall. Having a shield wall in combination with (and separated from) a pair of reed switches helps to provide protection against a sabotage magnet.

In one embodiment, the shield wall is approximately 1mm thick, and the high magnetic permeability material is mu-metal, although it will be appreciated that other materials and/or dimensions may be used instead. In general, if a material with lower magnetic permeability than mu-metal is used, then the shield wall should be thicker to provide the same level of effect.

In one embodiment, the shield wall is spaced from the operating face by approximately 2-3mm. This spacing helps the operating magnet for the contact to activate the device. In other embodiments, a different spacing (or no spacing) may be employed.

In one embodiment, the reed switches are equally spaced on either side of the shield wall, and each reed switch is spaced from the shield wall by at least 3mm. This spacing makes it more difficult for a sabotage magnet to activate both reed switches at

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once (and so prevent correct operation of the magnetic contact). It will be appreciated that the exact spacing of the reed switches depends on various factors, including the size and strength of the operating magnet, the properties of the shield wall, the strength of the reed switches, etc. In some embodiments, the reed switches may not be equally spaced on either side of the shield wall (this may then require a corresponding offset in the positioning of the operating magnet).

In one embodiment, the shield wall and the pair of reed switches are contained within a cylindrical body. The shield wall is located along one diameter of the cylindrical body, and the reed switches are located along a diameter of the cylindrical body perpendicular to the shield wall. Other shapes may also be used for the body of the magnetic contact -e.g. rectangular. In this case, the shield wall may be disposed along one axis of the rectangle, and the reed switches along a perpendicular axis, although many other configurations are possible.

In one embodiment, the reed switches are spaced from the operating face by approximately 5mm. This separation of the reed switches from the operating face generally lowers the sensitivity of the reed switches to the sabotage magnet. It will be appreciated that a different separation (or no separation) may be used in other embodiments, depending on the strength and position of the reed switches, the strength of the operating magnet, etc.

Brief Description of the Drawings

Various embodiments of the invention will now be described in detail by way of example only with reference to the following drawings: Figure 1 is a schematic, sectional, drawing of a magnetic contact and a sabotage magnet.

Figure 2 is a schematic drawing showing the exposed portion of the magnetic contact of Figure 1.

Figure 3 is a schematic drawing showing a section through the magnetic contact of Figure 1 in accordance with one embodiment of the invention.

Figure 4 is a schematic drawing showing a plan view of the magnetic contact of Figure 1 in accordance with one embodiment of the invention.

Detailed Description

Figure I is a schematic drawing of a magnetic contact comprising a reed switch 10 and an operating magnet 12. The reed switch is installed into member 100, while the operating magnet 12 is installed into member 200. In one embodiment, member 100 comprises a door or window frame 100, while member 200 comprises a corresponding door or window 200. In other embodiments, the reed switch may be located in the moving portion (door or window), and the operating magnet Located in the fixed frame portion. In other embodiments, both member 100 and member 200 may both be movable (for example in a pair of shutters that close together). Member is separated from member 200 by gap 150.

It is generally more convenient for the reed switch 10 to have a fixed location, such as in a window or door frame, since wiring (not shown in Figure 1) for the magnetic contact is received by the reed switch, rather than by the operating magnet.

For ease of discussion, we shall assume below (without limitation) that member 100 is a door frame and member 200 is a door.

Note that opening of the door of Figure 1 may separate the operating magnet 12 from the reed switch in any appropriate direction. For example, one possibility is that the operating magnet 12 lifts up out of the plane of Figure 1 away from reed switch 10. This opening configuration corresponds to a normal hinged door opening.

Another possibility is that the operating magnet moves horizontally (in terms of Figure 1) away from the reed switch, i.e. by motion parallel to the boundary between the door 200 and the door frame 100. This type of motion is especially appropriate for a sliding door, where the magnetic contact is mounted above or below the door.

Another possibility is that the operating magnet moves directly away from reed switch 10, but remaining within the plane of Figure 1. This type of motion is especially appropriate for a sliding door, where the magnetic contact is mounted on the side of the door.

Both the reed switch 10 and the operating magnet 12 are embedded into respective members 100 and 200. In particular, both the reed switch 10 and the

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operating magnet 12 are flush-mounted, so that the reed switch 10 does not protrude beyond the surface of frame 100, while operating magnet 12 does not protrude beyond the surface of door 200.

Figure 2 is a view of the reed switch 10, in effect as seen from the magnetic contact 12. This confirms that reed switch 10 is embedded into (and hence surrounded by) door frame 100. Door frame 100 therefore helps to protect the reed switch 10 from attack by a sabotage magnet, since it generally prevents any sabotage magnet from getting very close to the reed switch.

The flush-mounting of reed switch 10 does however leave one exposed or accessible surface, namely the operating surface which faces operating magnet 12 across gap 150 (this operating surface of the reed switch 100 is flush with the surface of the door frame 100 into which the reed switch is embedded). Accordingly, standard BSI DD CLCITS 5013 1-2-6:2004 specifies that the magnetic contact should be resistant to a sabotage magnet 300 introduced into this gap 150.

Figure 1 illustrates a sabotage magnet 300 being introduced into the magnetic contact. The sabotage magnet is assumed to have a disk shape, with its magnetic axis aligned (and coincident) with the rotational axis of the disk. The sabotage magnetic may be introduced with either polarity -i.e. with its north pole facing the reed switch 10, and its south pole facing the operating magnet 12, or vice versa. Since gap 150 is generally relatively narrow, this limits the potential thickness, and hence strength, of sabotage magnet 300.

Figure 1 shows the sabotage magnet 300 being introduced along the line parallel to the boundary between the door 200 and the door frame 100. However, the sabotage magnet may be introduced along any direction in the plane that separates the door 200 from the door frame 100. For example, in Figure 2, sabotage magnet 300A is introduced along a line perpendicular to the boundary between the door 200 and the door frame 100, sabotage magnet 300C is introduced along a line parallel to the boundary between the door 200 and the door frame 100 (as for Figure 1), and sabotage magnet 300B is introduced along a line intermediate the directions for magnets 300A, 300C.

Figures 3 and 4 illustrate a magnetic contact switch or detector 10 which includes protection against a sabotage magnet 300. One complication in protecting a reed switch against a possible sabotage attack is that a reed switch is sensitive to magnetic fields in multiple directions, not just in the direction of the magnetic field from the operating magnet. In addition, different magnetic fields can bend & distort one other. For example, two magnets may either reinforce one another or cancel each other out, depending on the relative orientation of their poles.

The protection provided in detector 10 includes: (a) internal shielding to limit the influence of an external sabotage field; (b) an additional reed switch for providing Ilirther robustness and protection against an external sabotage field that overcomes the internal shielding.

The overall protection is intended to satis1' the requirements of BSI DD CLCITS 50131-2-6:2004.

Referring to Figures 3 and 4, the magnetic contact 10 includes a cylindrical body 185 and an operating face 155 located at one end of the cylindrical body. The cylindrical body 185 is embedded into a door frame 100 or other block of material, while the operating face 155 sits flush on the surface of door frame 100 (as shown in Figures 1 and 2). The portions of the operating face 155 that extend beyond the cylindrical body 185 include holes 160A, 160B for receiving screws or other fastening means for retaining the magnetic contact 10 within the door frame 100.

The cylindrical body includes an inner cavity which holds a pair of reed switches I 70A, I 70B that are spaced apart on a diameter of the cylindrical body 185.

The reed switches 1 70A, I 70B are both arranged in the housing parallel to the axis to of the cylindrical body 185. The operating magnet 12 therefore generally offers a north or south face to the operating face 155 of the magnetic contact 10 in order to maximise the contact with the reed switches I 70A, I 70B.

In one embodiment, the reed switches 1 70A, 1 70B are approximately 10mm in length (excluding external contacts), and have a sensitivity of approximately 10-15

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Ampere-turns. (It will be appreciated that other embodiments may use different reed switches, with different dimensions and/or sensitivities).

In one embodiment, the reed switches I 70A, I 70B are mounted on a small printed circuit board (PCB) ISO and are also received in corresponding mounting forks I 75A, I 75B, which are formed as part of the moulded structure of magnetic contact 10. The PCB 180 and mounting forks I 75A, I 75B act as a production aid to ensure that the reed switches 1 70A, I 70B are (and remain) properly positioned within magnetic contact 10. (In other embodiments, some different mechanism for positioning the reed switches may be employed).

The magnetic contact further includes a wall 165 of magnetic shielding. Wall lies in the plane parallel to (and including) the axis of cylindrical body 185 and perpendicular to the line joining the two reed switches I 70A, I 70B. Wall 165 therefore provides a (magnetic field) barrier between the two reed switches I 70A, l7OB. In one embodiment, the wall is 1mm thick and is formed of mu-metal, an alloy of 75% nickel, 15% iron, plus copper and molybdenum that has undergone heat treatment to increase its magnetic permeability (the relative permeability of mu-metal is approximately 50,000). In other embodiments, different thicknesses and/or different materials of high magnetic permeability may be used instead of mu-metal.

In general, if a material with lower magnetic permeability than mu-metal is used for wall 165, then wall 165 may be thicker to provide the same level of shielding.

In one embodiment, the cylindrical body is 20mm in diameter, and each of the two reed switches l7OA, 170B is spaced 3.2mm from the shield wall 165. The reed switches are also spaced 5.3mm from the base of the magnetic contact that provides operating face 155. The shield wall is spaced 2.5mm from the flush operating face of the magnetic contact 10. It will be appreciated that other embodiments may use different dimensions and spacings for magnetic contact 10, although the separation of the two reed switches I 70A, I 70B from one another (and from the shield wall 165) helps to protect the device against a sabotage magnet (as described in more detail below).

The shield wall 165 in effect draws or sucks a magnetic field towards itself; this applies to the magnetic field both from the operating magnet 12 and also from any sabotage magnet 300. The spacing between the shield wall 165 and operating face helps to extend the operating distance of the operating magnet 12 across gap 150.

Thus if the shield wall 165 extended all the way to the operating face 155, this would tend to draw more magnetic field from the operating magnet 12 towards the shield wall 165 and away from reed switches I 70A, 1 70B (thereby reducing the operating distance of the magnetic contact).

In one embodiment, reed switches I 70A, I 70B are normally held closed when the door (or other item for detection) is closed. The two reed switches I 70A, 1 lOB are wired in series, so that they must both be under the influence of the field from the operating magnet 12 to make (keep) the circuit closed. In other embodiments, reed switches 1 70A, 1 70B may be normally held open when the door (or other item for detection) is closed. We will assume the former situation herein, since this gives a fail-safe situation such that if the cable to the magnetic contact 10 is cut, then the circuit is interrupted to signal an interference condition.

The protection of magnetic contact 10 from a sabotage magnet 300 is primarily controlled by the provision and positioning of both the shield wall 165 and also the multiple reed switches I 70A, I lOB. It will be appreciated that the physics of magnetic fields is such that it is not possible to make a detector totally immune to any strength or configuration of sabotage magnetic field; nevertheless BSI DD CLC/TS 50131-2-6:2004 defines a plausible physical size and strength for a sabotage magnet.

In general terms, if the sabotage magnet is positioned over one of the reed switches, the shield wall 165 helps to prevent the magnetic field of the sabotage magnetic interfering with the operation of the other reed switch on the far side of the shield wall 165, which will therefore continue to provide a detection facility for the door (bearing in mind the series configuration of the two reed switches). Moreover, if the sabotage magnet 300 is located directly over the shield wall, i.e. midway between the two reed switches, the high magnetic permeability of the shield wall draws the magnetic field into the centre of the cylindrical body 185, and away from the two reed switches I 70A, I 70B, which are spaced out from the shield wall 165 and the centre of the cylindrical body 185. Consequently, the magnetic field from the sabotage magnet 300 placed above shield wall 165 is too weak at the locations of the two reed switches I 70A, I 70B to interfere with their proper detection function.

In conclusion, although a variety of embodiments have been described herein, these are provided by way of example only, and many variations and modifications on such embodiments will be apparent to the skilled person and fall within the scope of the present invention, which is defined by the appended claims and their equivalents.

Claims

Claims I. A magnetic contact for flush mounting to a surface, the

magnetic contact comprising: an operating face for mounting flush with said surface opposite an operating magnet; a shield wall of high magnetic permeability material perpendicular to the operating surface; and a pair of reed switches connected in series and disposed on either side of, and separated from, said shield wall.
2. The magnetic contact of claim 1, wherein the high magnetic permeability material is mu-metal.
3. The magnetic contact of claim I or 2, wherein the shield wall is approximately 1mm thick.
4. The magnetic contact of any preceding claim, wherein the shield wall is spaced from the operating face.
5. The magnetic contact of claim 4, wherein the shield wall is spaced from the operating face by approximately 2-3mm.
6. The magnetic contact of any preceding claim, wherein the reed switches are equally spaced on either side of the shield wall.
7. The magnetic contact of any preceding claim, wherein each of said pair of reed switches is spaced from the shield wall by at least 3mm.
8. The magnetic contact of any preceding claim, wherein the shield wall and the pair of reed switches are contained within a cylindrical body.
9. The magnetic contact of claim 8, wherein the shield wall is located along one diameter of the cylindrical body, and the reed switches are located along a diameter of the cylindrical body perpendicular to the shield wall.
10. The magnetic contact of any preceding claim, wherein the reed switches are spaced from the operating face.
11. The magnetic contact of claim 10, wherein the reed switches are spaced from the operating face by approximately 5mm.
12. The magnetic contact of any preceding claim, wherein the reed switches have a sensitivity of approximately 10-15 AT.
13. The magnetic contact of any preceding claim, wherein the reed switches are mounted on a PCB.
14. The magnetic contact of any preceding claim, further comprising positioning means attached to the operating face for holding the reed switches and shield wall in location relative to the operating face.
15. A magnetic contact substantially as described herein with reference to the attached drawings.