WO2008078108A2

WO2008078108A2 - Magnetic field indicator and utility meter incorporating the same

Info

Publication number: WO2008078108A2
Application number: PCT/GB2007/005006
Authority: WO
Inventors: Andrew James
Original assignee: Polymeters Response International Limited
Priority date: 2006-12-27
Filing date: 2007-12-27
Publication date: 2008-07-03
Also published as: WO2008078108A3; GB0625939D0

Abstract

A device is disclosed for indicating the application of a magnetic field to an apparatus, the device comprising a magnetically conductive member (4) arranged in a first position, the member being movable to a second position (fig. 2) on application of a magnetic field with a magnitude above a predetermined threshold, and means for maintaining the member in the second position on removal of the magnetic field. The device is suited for use in a utility meter where attempts may be made to fraudulently effect the detection of the utility consumption by an external magnetic field.

Description

MAGNETIC FIELD INDICATOR AND UTILITY METER INCORPORATING THE

SAME

The present invention relates to magnetic field indication and particularly to a device for indicating the presence of an abnormally large magnetic field. Further, the invention relates to a utility meter which comprises such an indicating device.

It is known that certain instruments providing measurements which result in payment being demanded are subject to fraud. This applies for example to utility meters such as electricity meters. The present invention is particularly suited for use with such apparatus and the following description will mention electricity meters as an example though it must be understood that the invention described is widely applicable to other apparatus.

Criminals and others bent on defrauding utility meters such as electricity meters devote much effort to finding ways of interfering with their operation. Nowadays many such meters are based on electronic circuitry possibly including microprocessors and it is known that the application of a powerful magnetic field can interfere with some electronic circuits as well as with certain types of transducer used in a meter such as current transformers. For example a typical switched mode power supply has as a principal component an inductor. The presence of a very strong magnetic field can cause saturation of the inductor and result in the failure of the power supply to act as designed, in turning causing non-operation or maloperation of the meter.

Modern magnetic materials are capable of producing very strong magnetic fields at short range. It is difficult to design circuitry capable of resisting the classes of powerful magnets now widely available and their application by a fraudster to the outside of an instrument is a known method of causing maloperation. Attempts can be made to shield sensitive components with materials such as mumetal having a high permeability. Another approach is to place sensitive components far from the outside of the case of the meter so that it is physically impossible to place a magnet very close to them. As the field of the magnet decreases sharply with distance, a degree of protection is thereby obtained. However trends toward miniaturisation lessen the possibility of this.

Electronic detectors sensitive to magnetism can be incorporated in the circuitry of the meter but in general they may not operate if the power supply itself is not working. So for example a method of tamper which they may not detect is to wait for an interruption of the electricity supply to an electricity meter and then, while the electronic circuitry of the meter is not working because of the absence of any power, to apply the magnet in such a way as to prevent the power supply from operating when the electricity supply is restored.

It is not possible to prevent all attempts at tampering with a meter to prevent it from recording energy. To take an extreme example, a meter can be physically destroyed by a fraudster. However the evidence of this is visually apparent on inspection and as a result criminal proceedings or other measures may be taken against the perpetrator.

From a first aspect, the present invention provides a device for indicating the application of a magnetic field to an apparatus, the device comprising a magnetically conductive member arranged in a first position which is mechanically stable, the member being movable to a second position which is mechanically stable on application of a magnetic field with a magnitude above a predetermined threshold, and means for maintaining the member in the second position on a removal of the magnetic field, wherein the member itself is arranged to move under the influence of the magnetic field above the predetermined threshold and indicate the application of the magnetic field above the predetermined threshold when in the second position.

Advantageously, this results in a simple mechanical device which moves on application of a strong magnetic and itself provides indication of the application of a strong magnetic field. The device cannot easily be reset, so providing lasting evidence that a fraud has been attempted. From a second aspect, the present invention provides a utility meter comprising a housing; means for detecting consumption of a utility; and a device as hereinbefore defined arranged within the housing.

The movable member is preferably in a form of a piece of material attracted by a magnet and is placed inside the meter housing. It is formed in such a way that in the absence of a magnetic field it remains in one position which is stable. If a sufficiently powerful magnet is brought near to it then it adopts a second position which is stable. In this manner, it is considered bistable. It cannot be moved from the second position back to the first position by simple external means such as removal of the magnet or applying a magnet of different polarity, or by applying the magnet elsewhere outside the case. Accordingly it is maintained in the second position.

The movable member may be positioned near any circuitry particularly sensitive to magnetic interference so that when a magnet is brought nearby in an attempt to cause malfunctioning, the movable member is attracted from the first position to the second position. As to the construction of the movable member in a first embodiment it is a thin strip of ferrous material such as steel which is held inside the meter housing at its ends in such a way that it is slightly bowed with its centre being further from the inside of the cover than its ends. This position is stable. Sufficient space is provided between it and the inside of the cover to allow it to adopt a second position which is also bowed but with the centre closer to or even touching the inside of the cover- and which is also stable. The springiness- of the steel and its low mass prevent it from changing to the second position in the event of any reasonable mechanical shock being applied. The force exerted by a magnet sufficiently strong to interfere with circuitry is more than enough to overcome the spring force which normally holds the strip in its first position and to pull the strip into the second position. In order that the present invention be more readily understood embodiments thereof will be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a perspective view of a section of a utility meter including a device according to a first embodiment of the present invention in a first position;

Figure 2 is a perspective view of the device shown in Fig. 1 in a second position;

Figure 3 is a perspective view of a section of a utility meter including a device according to a second embodiment of the present invention in a first position;

Figure 4 is a perspective view of the device shown in Fig. 3 in a second position;

Figure 5 is a perspective view of the movable member from above which is used in the device of Fig. 3 and 4;

Figure 6 is a perspective view of alternative forms of movable member which can be used in further embodiments.

The invention is preferably provided in an electronic utility meter containing electronic circuitry which is affected by the application of a powerful magnetic field. The following embodiments will be described with reference to this preferred use but it will be appreciated that the invention can be utilised with other types of similar instruments which require the indication of a magnetic field.

Fig. 1 and 2 shows a section through part of an electricity meter which encompasses a device according to a first embodiment. Such a meter will typically include electronic circuitry such as microprocessors powered by a power supply and a transducer such as one or more current transformer(s) which is used to detect the consumption of electricity. The performance of the power supply and the current transformer(s) is potentially affected by the application of an abnormally large magnetic field. The reference 7 indicates circuitry which is sensitive to a magnetic field. Other components within an electricity meter are known to the skilled man and do not form part of the invention.

The meter has a casing 1 and the circuitry 7 is contained within the casing 1. Preferably at least part of the casing 1 is transparent so that the contents of the meter are visible. Fig 1 and 2 show only part of the casing 1 with the inside of the meter being above the casing 1. It will be appreciated that the casing 1 forms an enclosure (not shown) which contains the components of the electricity meter including the circuitry 7.

Supports 2,6 are spaced apart and fixed to the inside of the meter casing 1. A member 4 which is preferably formed of a steel strip is located by its ends in grooves 3, 5 in the supports 2, 6. It will be appreciated that any type of material which is attracted by a magnet may be used as the member 4.

Fig. 1 shows the member 4 in the first stable position. The length of the member 4 is slightly greater than the distance between the grooves 3, 5 so that it is constrained to a bowed shape as shown. After application of a magnet (not shown) to the outside of the meter casing 1 (that is, below in these figures) the member 4 being attracted by the magnet is flexed and very rapidly passes through an unstable shape which is not a simple arc but may for example be like a flattened "S" though the precise form is immaterial until it adopts a second stable state as shown in Figure 2. The change from the first state in Fig. 1 to the second state in Fig. 2 is irreversible so providing lasting evidence that a fraud or tamper has been attempted. In this case as shown it is in contact with the inside of the casing 1. It will be understood that it is necessary to provide some constraints to prevent the strip being drawn by determined manipulation of the magnet out of its location between the supports. The inside of the cover is one such constraint but other means (not shown) may be required to prevent the strip being drawn by the magnet in a direction at right angles to the plane of the Figure.

The member 4 is magnetically conductive which is not designed to remain magnetised, is not part of an electric switch, and is itself mechanically stable in two positions (bistable) thereby indicating directly the presence at some time of a disturbing magnetic field. Accordingly, electrical power is not required to power the member 4.

Figs 3 and 4 show a second embodiment of the device where a member 4a of similar properties to that in the first embodiment is furnished with holes 8 as shown in Fig. 5. These holes 8 are located on studs or projections 9,10 on supports 11 ,12 which are fixed to the casing 1. It will be appreciated that the supports 11 ,12 can be based on the shape of the supports 2,6 of the first embodiment but in addition provided with suitably shaped studs or projections 9,10. The member 4a is held by fixing washers (not shown) or other means e.g. nuts, glue or welding to the studs or projections 9,10. The holes 8 are slightly further apart than the studs 9,10 resulting in a bowing of the member 4 as shown. In a similar manner to the previous arrangement the member 4a is attracted by the magnet from the first stable position shown in Figure 3 to the second stable position shown in Figure 4 and maintained in the second stable position in a similar manner to the first embodiment.

It must be understood that the curvature shown is for example only and the precise amount and shape of curvature in, both first and second positions may be varied. The elasticity of the member 4,4a and its thickness may be varied to obtain different sensitivity. Forms other than a simple strip can be used for example a strip with parts cut away giving more than two anchor points. The member 4,4a need not be of uniform width. Three further examples of members 4b,4c,4d of a non-uniform width are shown in Figure 6. Members 4b and 4c comprise a strip which is suitable for use in the second embodiment of Fig. 3 and 4 as there are holes 8a, 8b on each end of the strip. Member 4d is suitable for use in the first embodiment as its ends are received in grooves 3,5 in supports 2,6. Many other forms of the device are possible such as a star-like shape having two or more sections anchored at extremities. Such a modification is shown as cruciform member 4e,4f in Figure 7 and there may be similar shapes having a different number of spokes. With this type of shape, the number of supports may be increased. Moreover a degree of curvature may be imposed on the strip or other form in its unconstrained state before assembly and again different properties will be obtained according to the direction of that curvature relative to the meter casing on assembly.

Other methods of construction can be envisaged. For example the member 4 may be arranged in such a way that when attracted by a magnet it moves under a lip inside the casing 1 which mechanically prevents it from returning to the first stable position.

The strip gives detection of magnetic field at a particular part of the casing, which may be chosen as the point most susceptible to magnetic attack. If it is felt necessary to detect fields at other parts then more than one indicating device can be incorporated at very little additional cost.

Although in the above embodiments the casing is described as transparent, it will be appreciated that the change in position of the movable member 4,4a can be detected in a variety of ways. A part of a transparent casing may be roughened, grooved or matted on the inside surface so that items seen through it are not clearly seen unless they -are very close to the inside surface, and the second position of the movable member may be arranged so that part of it is suitably close to the surface or rests against it so that symbols or lettering on it become clearly legible whereas they are not clearly legible in the first position. For example the housing may have an optically diffuse section adjacent the movable member 4,4a only and the warning text "Magnetic Tamper Occurred" could be made to appear legible only when in the second position. Additionally or instead of this it is possible to arrange means (not shown) by which the circuitry 7 of the meter can detect the change in position. A simple way is by use of a microswitch. Alternatively the change in position may be detected by an optical sensor which for example is uncovered in the second position. Still another way is for the change in position to affect the capacitance or inductance of a piece of circuitry which is then detected by any of a number of means and such methods which are known in the art.

Claims

1. A device for indicating the application of a magnetic field to an apparatus, the device comprising a magnetically conductive member arranged in a first position which is mechanically stable, the member being movable to a second position which is mechanically stable on application of a magnetic field with a magnitude above a predetermined threshold, and means for maintaining the member in the second position on removal of the magnetic field, wherein the member itself is arranged to move under the influence of the magnetic field above the predetermined threshold and indicate the application of the magnetic field above the predetermined threshold when in the second position.

2. The device of claim 1 further comprising a plurality of supports arranged to receive the movable member.

3. The device of claim 2 wherein each support comprises a groove to receive the end of the movable member.

4. The device of claim 2 or 3 wherein each support comprises a projection.

5. The device of any preceding claim wherein the movable member comprises a hole at each end thereof.

6. The device of any of claims 2 to 5 wherein the length of movable member is greater than the distance between the supports.

7. The device of any preceding claim wherein the movable member is formed of a strip of magnetically conductive material.

8. The device of any preceding claim wherein the movable member is formed of steel.

9. A utility meter comprising a housing; means for detecting consumption of a utility; and a device of any preceding claim arranged within the housing.

10. The utility meter of claim 9 wherein at least part of the housing is transparent.

11. The utility meter of claim 10 wherein the movable member is arranged in the housing such that the second position of the movable member is located adjacent the transparent part of the housing.

12. The utility meter of claim 9, 10, or 11 comprising means for detecting the change in position of the movable member.

13. The utility meter of claim 12 wherein the position detecting means is a microswitch.

14. The utility meter of any of claims 9 to 13, the meter being an electronic meter and the consumption detecting means comprising a current transformer.

15. The device as hereinbefore described and with reference to the accompanying drawings.

16. The utility meter as hereinbefore described and with reference to the accompanying drawings.