GB2453591A

GB2453591A - Magnetic contact

Info

Publication number: GB2453591A
Application number: GB0720003A
Authority: GB
Inventors: John Darby; Kim Lloyd; Graham Franklin
Original assignee: ELMDENE INTERNAT Ltd
Current assignee: ELMDENE INTERNAT Ltd
Priority date: 2007-10-12
Filing date: 2007-10-12
Publication date: 2009-04-15
Anticipated expiration: 2027-10-12
Also published as: GB2453591B; GB0720003D0

Abstract

A magnetic contact for mounting onto a surface comprises an operating face for mounting substantially perpendicular to said surface opposite an operating magnet; an operating reed switch adjacent said operating face; and shield walls of high magnetic permeability material forming an enclosure around the operating reed switch on all sides of the operating reed switch except for: (a) the side adjacent said surface, and (b) the side adjacent said operating face. The operating reed switch is spaced from the shield walls by at least 2mm. In another embodiment, the magnetic contact for mounting onto a surface comprises an operating face arranged to be substantially parallel and opposite to said surface, opposite an operating magnet; an operating reed switch adjacent said operating face; two tamper detection switches, one located on each side of the operating reed switch; and magnetic shielding comprising high permeability material located between each tamper detection switch and a housing of the magnetic contact.

Description

1 2453591

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 to overcome the security protection for the window or door.

The physics of magnetic fields is such that it is not possible to make a detector totally immune to all possible strengths and configurations of a potential sabotage magnetic field. However, a recently developed Technical Specification (draft standard), BSI DD CLCITS 50131-2-6:2004, specifies desirable 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

One embodiment of the invention provides a magnetic contact for mounting onto a surface. The magnetic contact comprises an operating face arranged to be substantially perpendicular to said surface opposite an operating magnet; an operating reed switch adjacent said operating face; and shield walls of high magnetic permeability material forming an enclosure around the operating reed switch on all sides of the operating reed switch except for: (a) the side adjacent said mounting surface, and (b) the side adjacent said operating face. The operating reed switch is spaced from the shield walls by at least 2mm (in one particular embodiment, the operating reed switch is spaced from the shield walls by approximately 3-4mm). This configuration helps to protect the operating reed switch from a sabotage magnetic

field.

In one embodiment, the high magnetic permeability material is mu-metal, although other similar materials could be used. The shield wall further comprises steel, where the steel is thicker than the high magnetic permeability material (to compensate for the lower magnetic permeability). This helps to reduce costs. The steel is in a layer inside the high magnetic permeability material to reduce the risk of the steel becoming permanently magnetised.

In one embodiment, a tamper detection reed switch is located outside the enclosure around the operating reed switch. The operating reed switch is located between the tamper detection reed switch and the operating face. This configuration helps to protect against a sabotage magnetic field that is strong enough to overcome the shielding.

On the other hand, some embodiments may achieve sufficient protection from a sabotage magnet without the use of a tamper detection reed switch. This is particularly (but not exclusively) the case with a relatively large magnetic contact device. In one particular embodiment the enclosure is spaced both from the portion of the housing opposite said surface, and also from the portion of the housing opposite the operating face, for example by approximately 8mm or more. In this embodiment, the increased distance between the operating reed switch and any sabotage magnet provides additional protection, and the tamper detection reed switch may be omitted.

Another embodiment of the invention provides a magnetic contact for mounting onto a surface. The magnetic contact comprises an operating face arranged to be substantially parallel and opposite to said surface, opposite an operating magnet; an operating reed switch adjacent said operating face; two tamper detection switches, one located on each side of the operating reed switch; and magnetic shielding comprising high permeability material located between each tamper detection switch and a housing of the magnetic contact. The magnetic shield provided next to the tamper detection switches helps to prevent accidental triggering of the tamper detection switches by the operating magnet.

In one embodiment, the magnetic contact further comprises shield walls of high magnetic permeability material forming an enclosure around the operating reed switch on all sides of the operating reed switch except for: (a) the side adjacent said mounting surface, and (b) the side adjacent said operating face.

In one embodiment, the operating reed switch and the two tamper detection reed switches are mounted on a PCB, which helps to control the proper positioning of the reed switches (although other forms of mounting may be used in other embodiments).

Brief Description of the Drawings

I

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 plan view depicting a magnetic contact and sabotage magnet; Figure 2 is a schematic sectional view depicting a magnetic contact and sabotage magnet; Figure 3 is a section through a magnetic contact in accordance with one embodiment of the invention; Figure 4 is an isometric view of the magnetic contact of Figure 3; Figure 5 is a section through a magnetic contact in accordance with another embodiment of the invention; Figure 6 is an isometric view of the magnetic contact of Figure 5; Figure 7 is a section through a magnetic contact in accordance with another embodiment of the invention; and Figure 8 is a plan view of the magnetic contact of Figure 7.

Detailed Description

Figures 1 and 2 are schematic drawings of a magnetic contact comprising a reed switch 10 and an operating magnet 12. Figure 1 can be regarded as a plan view while Figure 2 represents a sectional view. (Note however that the plan view may have any orientation, such as vertical or horizontal, or be at some intermediate configuration).

The reed switch 10 is installed onto the surface of member 100, while the operating magnet 12 is installed onto member 200. Member 100 adjoins member 200 along an edge of member 100 and member 200. The reed switch 10 and the operating magnet 12 are surface-mounted onto member 100 and member 200 respectively adjacent the adjoining edge. The reed switch 10 and the operating magnet 12 are positioned so as to closely face one another.

Figures 1 and 2 illustrate a closed position for members 100 and 200. For example, member 100 may comprise a door frame or a window frame, while member

I

comprises a corresponding door or window. In other embodiments, both member and member 200 may both be movable (for example in a pair of shutters that close together). In principle, the movement of one member may occur in any direction, except where the presence of the other member would physically impede such motion. However, the actual range and type of movement permitted in any particular implementation is usually limited to a single direction (as for most doors and windows).

If one of the members is fixed, it is generally more convenient for the reed switch 10 to be mounted to the fixed member, such as a window frame or door frame.

This makes it easier to provide wiring (not shown in Figure I) for the reed switch 10; in contrast the operating magnet 12 generally does not need a power supply.

However, in other embodiments, the reed switch may be mounted on the moving portion while the operating magnet is mounted on a fixed frame portion. In Figures 1 and 2 it is assumed that member 100 is fixed, while member 200 is movable.

Figures 1 and 2 illustrate motion along three different axes, as indicated by arrows A, B and C. Arrows A and B represent motion within the plane of members and 200. This might represent the sliding motion of a sash window or patio door for example. More particularly, arrow A represents a longitudinal motion, where the operating magnet 12 slips past the reed switch 10. This movement would occur if the operating magnet and reed switch were located on the side of a sash window. Arrow B represents a motion where the operating magnet 12 recedes directly from the reed switch 10. This movement would occur if the operating magnet and reed switch were located at the top of a sash window.

In contrast, arrow C represents motion out of the plane of members 100 and 200. This generally represents a hinged or rotational motion, such as the opening of a door. This movement of arrow C would occur if the operating magnet was located on the top or side of a conventional door and the reed switch had a corresponding location on the door frame.

For ease of discussion, we shall henceforth assume (without limitation) that member 100 is a door frame and member 200 is a door. The operating magnet 12 and reed switch 10 therefore act to detect illicit opening of the door.

An adversary may try to subvert the operation of the reed switch 10 via the use of a sabotage magnet 30. The aim of the sabotage magnet 30 is to simulate the presence of operating magnet 12 so as to stop the reed switch 10 triggering even if the door is opened.

It is contemplated in BSI DD CLOTS 50131-2-6:2004 that the sabotage magnet may approach the reed switch from any position or direction on the same side as the mounting of the reed switch itself. In addition, sabotage magnet 30 may be introduced with any polarity -i.e. with N or S directed towards the operating magnet 12.

Figures 1 and 2 illustrate three different axes of potential approach for a sabotage magnet. In Figure 1, sabotage magnet 30A is shown approaching the reed switch along an axis parallel to the line between the door and the door frame, while sabotage magnet 30B is shown approaching the reed switch in the plane parallel to the door, but perpendicular to the edge between the door and the door frame. In Figure 2, sabotage magnet 30C is shown approaching the reed switch along an axis perpendicular to the plane of the door. It will be appreciated that the direction of approach for sabotage magnet 30 does not have to be one of the 3 orthogonal directions shown in Figures 1 and 2, but may instead comprise any intermediate or diagonal direction. (The operating magnet 12 and the reed switch 10 are usually close together when the door is shut, and this can prevent the sabotage magnet from being inserted between them).

In most situations, the sabotage magnet is located on the same side of the door 200 and door frame 100 as the operating magnet 12 and reed switch 10; in other words, in the configuration and orientation of Figure 2, the sabotage magnet approaches from above rather than from below the door and door frame. In general, it is more difficult to attack the reed switch 10 from the opposite side of the door and door frame. Firstly, an adversary may not have access to this far side of the door.

Secondly, if the adversary is located on the far side of the door, it is difficult for them to locate the operating magnet 12 and reed switch 10, which are then normally hidden from view on the opposite side of the door. Thirdly, if the adversary does determine the location of the operating magnet and reed switch, the door 200 and door frame 100 provide at least some shielding for the reed switch against the sabotage magnet, which makes it more difficult for the sabotage magnet to subvert the operation of the reed switch.

Figures 3 and 4 illustrate a magnetic contact 300 in accordance with one embodiment of the invention. Figure 3 illustrates a transverse section through the centre of magnetic contact 300 attached to surface 100. It is assumed that an operating magnet (not shown in Figure 3) is attached to surface 200, opposite the operating face 310 of the magnetic contact 300. Figure 4 is an isometric view of the magnetic contact 300 from underneath the surface that would normally be attached to surface 100.

Magnetic contact 300 is provided with a plastic outer casing 305, which includes punch-holes 30 IA, 301 B and fixing hole 302. Magnetic contact 300 further includes an operating reed switch 320 and a tamper detection reed switch 340. The operating reed switch 320 is located towards the operating face 310 of the magnetic contact, while the tamper detection reed switch 340 is located away from the operating face 310. In other words, operating reed switch 320 is located between the tamper detection reed switch 340 and the operating face 3 10.

In one embodiment, the operating reed switch 320 is held normally closed when the door (or other item for detection) is closed. The tamper detection reed switch 340 is also held normally closed (absent the presence of a sabotage magnet).

This gives a fail-safe situation such that if the cable to the magnetic contact 300 is cut, a circuit interruption can be used to signal an interference condition.

The operating face is protected by shielding in four directions. The shielding comprises two layers, a first layer of steel 335 and a second layer of mu-metal 336.

The mu-metal is on the outside of the steel (as seen from the operating reed switch 320) to protect against the risk of the steel becoming magnetized.

Mu-metal is an alloy of 75% nickel, 15% iron, plus copper and molybdenum, which has undergone heat treatment to increase its magnetic permeability. The relative permeability of mu-metal is approximately 50,000. The thickness of the mu-metal wall 336 is approximatelyOSmm The thickness of the steel waIl 335 is approximately 1.2mm (the relative permeability of steel is approximately 100). In other embodiments, different thicknesses and/or different materials may be used instead of mu-metal and/or steel. In general, if a material with lower magnetic permeability is used, then a greater thickness of wall is required to provide the same level of shielding. For example, if the shield walls 330 were made entirely of mu-metal, the overall thickness of the walls could be reduced (but the shielding would be more expensive). Conversely, if the shield walls were made entirely of steel, the overall thickness of the walls would be increased.

As can be seen in Figure 3, one wall 330A of the shielding is parallel to the operating face 310, and is located between the operating reed switch 320 and the tamper detection reed switch 340. Another wall 330B of the shield is parallel to the surface 100 to which the magnetic contact is attached, and is located so that operating reed switch 320 is located between wall 330B and member 100. The other two walls 330C and 330D are located at the respective ends of the operating reed switch 320 (see Figure 4). The operating reed switch 320 is supported within shield walls 330 by a locating frame 350, which in one embodiment is formed from moulded plastic (non-magnetic). In another embodiment, the operating reed switch 320 may be supported and positioned within the shield walls 330 using a printed circuit board (PCB) or other structure formed of a non-ferrous material.

The four shielding walls 330A, 330B, 330C and 330D, together with operating face 310 and member 100, define in effect a rectangular box that encloses the operating reed switch 320. The shielding walls protect the operating reed switch 320 against the main directions of attack. (It is difficult to attack the magnetic contact 300 from the side of operating face 310, since in this case the operating reed switch 320 is closer, and hence more sensitive, to the proper operating magnet).

In one particular embodiment, operating reed switch 320 has a rating of 10- I5AT (ampere-turns), while the tamper detection reed switch 340 has a rating of 20- 25AT. (It will be appreciated that other embodiments may use different reed switches, with different sensitivities). The tamper detection reed switch 340 is therefore slightly less sensitive than the operating reed switch 320. The tamper detection reed switch 340 is further away from the operating magnet than the operating reed switch 320, and is separated from the operating magnet by shield wall 330A. As a result, the tamper detection reed switch 340 is not triggered by the operating magnet.

The protection of magnetic contact 300 from a sabotage magnet 30 is primarily controlled by the provision and positioning of the shield walls 330 and also by the positioning and sensitivity of the reed switches 320, 340. The protection provided in magnetic contact 300 generally satisfies the requirements of BSI DD CLCiTS 50131-2-6:2004 and is based on: (a) the internal shielding 330 to limit the influence of an external sabotage field; and (b) an additional tamper detection reed switch 340 for providing further robustness and protection against an external sabotage field that overcomes the internal shielding.

If the sabotage magnet 30 is relatively weak, the shield walls 330 prevent the sabotage magnet from affecting the operating reed switch 320. On the other hand, if a stronger sabotage magnet is used, which may perhaps be sufficiently powerful to affect the operating reed switch 320, this will also trigger the tamper detection reed switch 340, thereby generating a detection signal.

In one embodiment, the dimensions of magnetic contact 300 are approximately 16mm deep (from surface 100), 18mm wide, and 70mm long (although it will be appreciated that other embodiments may have different dimensions). As indicated in Figure 3, the operating reed switch 320 is spaced from the operating face 310 by about 6mm and from the shield walls 330A and 330B by approximately 3.2mm. The spacing of the operating reed switch 320 from the shielding 330 avoids the reed switch 320 being influenced by the magnetic field within the shield wall itself. (If the operating reed switch were too close to the shield walls, the operating p reed switch might be triggered by the magnetic field from the shield wall caused by a sabotage magnet).

Figures 5 and 6 illustrate a magnetic contact 400 in accordance with another embodiment of the invention. Figure 5 illustrates a transverse section through the centre of magnetic contact 400 attached to surface 100. It is assumed that an operating magnet (not shown in Figure 5) is attached to surface 200, opposite the operating face 410 of the magnetic contact 400. Figure 6 is an isometric view of the magnetic contact 400 from underneath the surface that would normally be attached to surface 100.

Magnetic contact 400 is generally similar to magnetic contact 300, with mu-metal 436 and steel 435 shielding 430 provided in four directions around an operating reed switch 420. Accordingly, the following description will focus on the differences between these two devices. Magnetic contact 400 is larger than magnetic contact 300, about twice the size. In particular, magnetic contact 400 has approximate dimensions of depth 32mm (from surface 100), width 32mm, and length 130mm (although it will be appreciated that other embodiments may have different dimensions).

The casing 405 for magnetic contact 400 includes an internal wall 408 which supports shielding 430 against the operating face 410. The shielding 430 in turn supports the plastic frame 450 that holds the operating reed switch 420. The operating reed switch 420 is again spaced by approximately 3.2mm from the shield wall 430, for the same reasons as described above in relation to magnetic contact 300. In another embodiment, the operating reed switch 420 may be supported and positioned within the shielding 430 using a printed circuit board (PCB) or other structure formed of a non-ferrous material.

Unlike magnetic contact 300, magnetic contact 400 does not include a tamper detection reed switch. Due to the greater size of magnetic contact 400 (compared to magnetic contact 300), the operating reed switch 420 is spaced significantly further from walls of magnetic contact 400 that are most exposed to potential attack by a sabotage magnet 30 (shown as walls 406A, 406B, 406C and 406D). This increase in spacing reduces the influence of any sabotage magnet on operating reed switch 420 l0 (compared to the situation for magnetic contact 300), since it is no longer possible for the sabotage magnet to get very close to operating reed switch 420. This allows the shielding 430 to provide effective protection against an attack by a sabotage magnet, thereby avoiding the need for a tamper detection reed switch. In some embodiments, the tamper detection reed switch may also be omitted even from relatively small devices (e.g. comparable in size to magnetic contact 300), depending on the effectiveness of the shielding, etc. Figures 7 and 8 illustrate a magnetic contact 500 in accordance with another embodiment of the invention. Magnetic contact 500 is typically mounted on the ground or some other base, and is used as protection for roller shutter doors, etc. Figure 7 illustrates a transverse section through the centre of magnetic contact 500 attached to surface 100. The geometry of the magnetic contact 500 is somewhat different from that of magnetic contacts 300 and 400, in that it is assumed that an operating magnet (not shown in Figure 7) is provided opposite surface 100. In other words, operating face 510 is opposite (rather than perpendicular) to surface 100.

Figure 8 is a plan view of the magnetic contact 500 from underneath the surface that would normally be attached to surface 100.

In use, the cavity of magnetic contact 500 may be filled and covered in potting compound (the cavities of magnetic contacts 300 and 400 may likewise be filled and covered in potting compound). Note that this potting compound does not impact the magnetic properties and operation of the device, rather it provides physical and environmental protection for the potentially fragile components of the magnetic contact, as well as helping to hold these components in place.

Magnetic contact 500 includes a metal (e.g. cast aluminium) casing 505 with two screw holes 502A, 502B for attaching the magnetic contact to the ground. The magnetic contact further includes an operating reed switch 520 (for example, the same operating reed switch as for magnetic contacts 300 and 400). The operating reed switch 520 is mounted on a PCB 560. In another embodiment, the operating reed switch 520 may be supported and positioned within the shield walls 530 using a suitable frame or other structure formed of a non-ferrous material such as plastic.

II *1

The operating reed switch 520 is protected by two shield walls 530A, 530B, one on either side of the operating reed switch, and extending parallel to it. These shield walls 530A and 530B are perpendicular to the surface 100 and to the operating face 510. In addition, shield wall 530C protects one end of the operating reed switch 520, and there is a similar shield wall at the opposite end of the operating reed switch 520. These end shield walls are also perpendicular to the surface 100 and to the operating face 5 10 (there are no shield walls parallel to the surface I O0/operating face 510). As in magnetic contacts 300 and 400, the shield walls 530 comprise layers 536, 535 of mu-metal and steel respectively.

The magnetic contact 500 further includes two tamper detection reed switches 540A, 540B, which run parallel to operating reed switch 520. Tamper detection reed switch 540A is located on the opposite side of shield wall 530A from operating reed switch 520. Tamper detection reed switch 540B is located on the opposite side of shield wall 530B from operating reed switch 520. If magnetic contact 500 is attacked with a sufficiently strong magnet to overcome shielding 530, then this will also trigger at least one of the two tamper detection reed switches 540A, 540B. In one embodiment, the tamper detection reed switches 540A, 540B may be supported and positioned within the shield walls 430 using a printed circuit board (PCB) or other frame formed of a non-ferrous material such as plastic.

The magnetic contact 500 further includes mu-metal shield walls 545A and 545B. These shield walls run parallel to shield walls 530A and 530B, but are tilted at an angle to shield walls 530A and 530B. The shield walls 545A and 545B are positioned between respective tamper detection reed switches 540A, 540B and the casing 505. The tilt of the shield walls 545A and 545B conforms approximately to the profile of the casing 405. Shield walls 545A and 545B reduce the sensitivity of the tamper detection reed switches 540A, 540B to the operating magnet, and so help to prevent spurious activations of the tamper detection reed switches 540A, 540B.

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

I

Claims I. A magnetic contact for mounting Onto a surface, the magnetic contact comprising: an operating face arranged to be substantially perpendicular to said surface and opposite an operating magnet; an operating reed switch adjacent said operating face; and shield walls of high magnetic permeability material forming an enclosure around the operating reed switch on all sides of the operating reed switch except for: (a) the side adjacent said mounting surface, and (b) the side adjacent said operating face; wherein the operating reed switch is spaced from the shield walls by at least 2mm.
2. The magnetic contact of claim I, wherein the high magnetic permeability material is mu-metal.
3. The magnetic contact of claim I or 2, wherein the shield wall further comprises steel, where said steel is thicker than the high magnetic permeability material.
4. The magnetic contact of claim 3, wherein the steel is in a layer inside the high magnetic permeability material.
5. The magnetic contact of any preceding claim, wherein the operating reed switch is spaced from the shield walls by approximately 3-4mm.
6. The magnetic contact of any preceding claim, further comprising a tamper detection reed switch located outside the enclosure around the operating reed switch.
7. The magnetic contact of claim 6, wherein the operating reed switch is located between the tamper detection reed switch and the operating face.
8. The magnetic contact of any of claims I to 6, wherein the contact contains only one reed switch and has a housing, and wherein the enclosure is spaced from both the portion of the housing opposite said surface, and also the portion of the housing opposite the operating face.
9. The magnetic contact of claim 8, wherein said spacing of the portion of the housing opposite said surface and also the spacing of the portion of the housing opposite the operating face is approximately 8mm or more.
10. A magnetic contact for mounting onto a surface, the magnetic contact comprising: an operating face arranged to be substantially parallel and opposite to said surface, and opposite an operating magnet; an operating reed switch adjacent said operating face; two tamper detection switches, one located on each side of the operating reed switch; and magnetic shielding comprising high permeability material located between each tamper detection switch and a housing of the magnetic contact.
II. The magnetic contact of claim 10, further comprising shield walls of high magnetic permeability material forming an enclosure around the operating reed switch on all sides of the operating reed switch except for: (a) the side adjacent said mounting surface, and (b) the side adjacent said operating face.
12. The magnetic contact of claim 11, wherein the operating reed switch is spaced from the shield walls by at least 2mm.
13. The magnetic contact of claim 11 or 12, wherein the shield wall further comprises steel, where said steel is thicker than the high magnetic permeability material.
14. The magnetic contact of claim 13, wherein the steel is in a layer inside the high magnetic permeability material.
15. The magnetic contact of any of claims 10 to 14, wherein the high permeability material is mu-metal.
16. The magnetic contact of any of claims 10 to 15, wherein the operating reed switch and the two tamper detection reed switches are mounted on a PCB.
17. A magnetic contact substantially as described herein with reference to the attached drawings.