GB2391948A

GB2391948A - Utility meter with magnetic field detection

Info

Publication number: GB2391948A
Application number: GB0219035A
Authority: GB
Inventors: Kaushik Ghosh
Original assignee: Polymeters Response International Ltd; PRI Ltd
Current assignee: Secure Meters UK Ltd
Priority date: 2002-08-15
Filing date: 2002-08-15
Publication date: 2004-02-18
Anticipated expiration: 2022-08-15
Also published as: ZA200501303B; IL166819A; IL166819A0; NZ538200A; AU2003252966B2; EP1532457A1; MY135680A; CN1688887A; WO2004017077A1; GB2391948B; AU2003252966A1; EG23466A; GB0219035D0

Abstract

A utility meter is provided with a magnetic field sensor 20 to detect the presence of abnormally high magnetic fields in the vicinity of the meter, such as those caused by a magnet placed to tamper with the meter readings. The sensor may be a coil wound on a ferrite core which saturates when a magnetic field above a threshold is present. The saturation is detected by monitoring a signal applied to the coil by signal generator 23. The presence or duration of a high magnetic field may be stored in memory 12 or displayed on means 22.

Description

2391 948

APPARATUS FOR DETECTING TAMPERING WITH A UTILITY METER

The present invention relates to utility meters and more particularly to apparatus for detecting tampering with such meters.

5 Meters for measuring consumption of gas and electricity are well known and many rely on electromagnetic or electronic principles. Such meters can be tampered with by applying magnets to the meter in order to reduce or actually stop the registration of consumption. This has resulted in proposals to provide the meters or at least the active part of such meters with magnetic shields.

10 However, in response to this, persons wishing to tamper with meters have resorted to using stronger and stronger magnets.

It is an object of the present invention to monitor magnetic fields in

utility meters and to indicate the presence of abnormally large magnetic fields.

When an abnormally high magnetic field is detected, this fact is

15 preferably recorded by the meter in some convenient fashion, e.g. by noting the date and time of the occurrence. Likewise, the removal of the abnormally high magnetic field can also be detected and recorded. This enables the utility

supplier to take appropriate steps which may include increasing the cost of the supply of the utility during such periods.

20 Preferably, the apparatus utilises a ferrite core provided with a coil which is used to provide a sense signal which can be evaluated.

In order that the present invention be more readily understood, an embodiment thereof will now be described with reference to the accompanying drawings, in which: 25 Fig 1. shows a block diagram of a utility meter provided with apparatus according to the present invention; and Fig 2. shows a diagrammatic representation of apparatus according to the present invention.

The preferred embodiment of the present invention is an electricity 30 meter which may be either a conventional electromagnetic meter or a more

modern electronic meter which utilises a current transformer in order to detect consumption of electricity. It will be appreciated that this is simply an example of a utility which could be metered and that the present invention is capable of being utilised with different types of meter for recording the consumption of 5 different flowing substances.

Referring to Fig 1., this shows a block diagram of an electricity meter where it is assumed it is an electronic meter. Electricity consumption is metered by detecting current utilising a current detector 10 in the form of a current transformer whose output is fed to a computation section which is preferably 10 constituted by a microprocessor 11. The result of the computation is then recorded and periodically stored in a memory 12. Thus far the meter is standard. However, it is known that the current detection can be affected by the presence of a high external magnetic field. The present invention is designed to

1 S detect the presence of such a field and to cause the presence of such a field to

be recorded by the meter and preferably also to be indicated to the user in the hope of deterring tampering with the meter. The indication can be any suitable visible and/or audible indication such as flashing a display or flashing a red light. It is preferred to record the commencement and cessation of the existence 20 of the abnormally high magnetic field so that the utility provider can take

appropriate steps.

The presently preferred arrangement for detecting the magnetic field is

represented in Fig 1. by the magnetic field detector 20 whose output is fed to a

computation circuit 21. The circuit 21 includes a comparator for comparing the 25 output from the magnetic field detector with a threshold level which is chosen

to indicate the presence of the abnormally high magnetic field. The circuit 21

outputs a signal to the memory 12 and records the date and time of the commencement and cessation of the existence of the abnormally high magnetic field. Additionally, if desired, the circuit 21 also triggers an indicator 22.

If one now refers to Fig 2., this shows the preferred arrangement for detecting magnetic fields. It consists of an inductance L constituted by a ferrite

core provided with a coil. One end of the coil is earthed via a resistor and this end constitutes the output from the detector. Lee other end of the coil 5 constitutes the input to the detector and is fed with a wavefonn from a signal generator 23 which is shown in Figl.. In this example, the signal generator outputs a square wave which will normally result in the output of the coil being a somewhat saw toothed waveform due to the inductance L. The slopes of the saw tooth are determined by the inductance value of the coil L and the ferrite i 10 core and coil are arranged to saturate at a predetermined level of external magnetic field. Saturation reduces the value of the inductance quite l

substantially and this in turn increases the rate of change of the slope of the current output wavefonn so that during saturation the square wave is more accurately transferred from the input to the output. The output from the coil is 15 fed to an average value determining circuit in the computation circuit 21 and it will be appreciated that the average value under normal conditions will be much less than the average value of the output in the presence of an abnormally high magnetic field which saturates the coil. The output of the average value

circuit is then evaluated. This can be readily achieved by a comparator circuit 20 for a simple comparison with a preset threshold level.

It is preferred that when a meter is constructed incorporating a magnetic field detector circuit, on initial power up of the meter following manufacture,

the ambient magnetic field can be registered and this will provide a datum level

to which an offset indicative of the abnormally high magnetic field will be

25 added.

It will be appreciated that the above detector could be utilised with a magnetic shield for the meter. If the magnetic field detector were located i

outside the shield, then this would result in someone wishing to tamper with the meter would have to utilise a sufficiently high magnetic field which would

30 inevitably be detected by detector circuit.

Additionally, it will be appreciated that the detector coil provided with the ferrite core will be able to detect both alternating and direct magnetic fields

and that because ferrite materials are easy to form, the ferrite core can be made to detect omni-directional magnetic fields.

S Various modifications to the circuitry are envisaged and various components of the meter may be shared with the magnetic field detector. For

example, rather than using a comparator at threshold level, the microprocessor may be utilised to evaluate the output of the detector. This may require a separate A/D channel being available. Also, rather than having a separate 10 detector core, it is possible to modify the current transformer or power supply transformer for an electronic meter by adding an extra winding which will be supplied with the detection waveform. This, however, may require some additional computation for the actual meter reading so as to avoid false readings due to the presence of the detection waveform.

Claims

CLAIMS:

1. A utility meter comprising means for detecting consumption of a utility, means for indicating the result of the detection and means for detecting the 5 presence of a magnetic field originating outside the meter.

2. A utility meter according to claim 1 and comprising memory means for storing the occurrence of the detection of the magnetic field.

10

3. A utility meter according to claim 2, and wherein the magnetic field

detection means is used to indicate the removal to the magnetic field.

4. A utility meter according to claim 1, and comprising a real time clock and memory means for storing the date and/or time during which the detection 15 of the magnetic field has occurred.

5. A utility meter according to any one of the preceding claims, wherein the magnetic field detection means includes a detection circuit and a waveform

generator applying a detection waveform to the detection circuit.

6. A utility meter according to claim 5, wherein the detection circuit includes an inductance in the form of a coil wound around a ferrite core.

7. A utility meter substantially as hereinbefore described with reference to 25 the accompanying drawings.