GB2409528A

GB2409528A - Utility meter with magnetic shielding and field detector

Info

Publication number: GB2409528A
Application number: GB0330057A
Authority: GB
Inventors: Kaushik Ghosh
Original assignee: Polymeters Response International Ltd
Current assignee: Secure Meters UK Ltd
Priority date: 2003-12-24
Filing date: 2003-12-24
Publication date: 2005-06-29
Anticipated expiration: 2023-12-24
Also published as: GB2409528B; CN1637390B; CN1637390A; GB0330057D0; MY138974A

Abstract

A utility meter, such as an electricity meter, has a coil 27, the core of which is magnetically linked to a shield 26 surrounding the meter. The coil can detect a magnetic field applied to the meter in order to detect tampering. The meter may have a memory in which to record the detected tampering and may record the time and date the field was applied and removed. A signal may be passed through the coil to detect the field.

Description

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.

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 susceptible part of such meters with magnetic shields.

The susceptible parts of a meter may be, for example, current transformers, transformers of meter power supplies, relays of pulsing outputs for remote reading, and/or latching relays of prepayment meters. However, in response to the shielding, persons wishing to tamper with meters have resorted to using stronger and stronger magnets.

The use of reed switches for detecting external magnetic fields is already known. However such detectors are more expensive, have highly polarized sensitivity and can also be triggered by inadvertently created stray magnetic fields. There are several patents suggesting use of careless transformers for making them immune to external DC magnetic fields. Such current sensors are however influenced by alternating magnetic fields. US5343143 and FR2800167 disclose such current sensing devices that employ a magnetic shield to prevent external influence and improve flux linkage. US4894610 discloses a set of two coils in series to cancel out the effect of external influencing fields. GB2353598 reveals use of series connected coils on a PCB to cancel out effects of external fields. However, all these arrangements fail to protect the measurement circuit against strong or highly polarized influencing fields that are deployed by fraudstcrs intending to make the meters run slower.

It is an object of the present invention to overcome problems related to the prior art and provide an alternative way of monitoring 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 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.

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

Since the shielding means must first be saturated in order for an external magnetic field to influence susceptible parts of the utility meter, the shielding means acts as a detector of abnormally large magnetic fields.

In order that the present invention be more readily understood, embodiments thereof will now be described with reference to the accompanying drawings, in which: 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.

Fig. 3 shows a susceptible element which may be used in a utility meter according to a first embodiment of the present invention.

Fig. 4 shows a second embodiment of the present invention.

The preferred embodiment of the present invention is an electricity 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 itlty which could be metered and that the present invention is capable of being tilised with different types of meter for recording the consumption of 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. Electronic meters differ from traditional electromechanical meters in that electronic meters do not use a meter system with a disc which rotates and which is driven at a rate dependent upon the current drawn. Instead, electronic meters measure electricity consumption by detecting current utilising a current detector 10 in the form of a current transponder whose output is fed to a computation section which is preferably constituted by a microprocessor 11. The result of the computation is then recorded and periodically stored in a memory 12. In addition, the electronic meter may be provided with communication means well known in the art to allow for- the meter to be read remotely through a communication link. 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 shield the current sensors or any similar influenced part from the effect of normal external fields and detect the presence of abnormally strong fields which tend to make the shielding itself ineffective. This causes the presence of such a field to be recorded by the meter and preferably also to be indicated to the riser 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 tile existence of the abnormally high magnetic field so that the utility provider can take appropriate steps.

al he 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 opt from the magnetic field detector with a threshold level which is chosen to ndrcatc the presence of the abnormally high magnetic field. The circuit 21 outposts 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 core provided with a coil.

The core according to the present invention is constituted by at least part 5of a nag'etic shield which is provided to shield some, if not all, of the elements of tile meter whose operation may be affected by high external magnetic fields.

Tic coil Is preferably wound directly on part of the shield and a typical cmbodinent of the arrangement is shown in Fig 3.

F'g 3 shows a susceptible switch element 25 which may be used in a 10utility meter and a shield 26 which protects the switch 25 from high external magnetic fields. Only part of the shield 26 is required to be provided with a coil 27 and the shield 26 and coil 27 combination form the inductance L shown in Fig 2.

According to this embodiment, the shield 26 which protects the ISsusceptible switch element 25 is formed of a main body 26a and an extension 26b. Me extension 26b is substantially in the shape of an 'L' and firstly projects away from the main body 26a before being arranged in the L shape.

Preferably, only one key of the L shaped extension 26b is provided with the magnetic detection arrangement which includes the coil 27.

20One end of the coil is grounded via a resistor and this end constitutes the outpost Tom the detector. The other end of the coil constitutes the input to the detector and is fed with a waveform from a signal generator 23 which is shown in F'gl.. In this example, the signal generator outputs a rapidly varying signal, for example a square wave which will normally result in the output of the coil belly a somewhat asymptotic (or saw toothed) waveform due to the inductance L. The r use and fall times of the saw tooth are determined by the inductance value of the coil L and the shield and coil are arranged to saturate at a predetermined level of external magnetic field. In fact, saturation can be dctcctcd relatively accurately as the shield 26 itself must first be saturated in order to influence the susceptible elements of the meter. Further, the presence of a high external magnetic field can be detected by the arrangement as an indication may be when the shielding capability of the shield is being compromised. Saturation reduces the value of the inductance quite substantially and this in turn increases the rate of change of the slope of the cuncnt output waveform so that during saturation the square wave is more accurately transferred from the input to the output. It may be appreciated that a magnetic detection circuit made using a different saturable material has to be specifically chosen so that it does not detect inadvertently caused stray fields.

The output from the coil is fed to an average value determining circuit in the lO co,ptation 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 opt of the average value circuit is then evaluated. This can be readily achieved by a comparator circuit for a simple comparison with a preset threshold level.

Fig. 4 is another embodiment of the present invention and shows a susceptible current transformer 35 which may be used in a utility meter. The sled 26 protects the current transformer 35 from high external magnetic field.

As in the previous embodiment, only part of the shield 26 is required to be provided with the coil 27 and the shield 26 and coil 27 combination form the inductance L shown in Fig 2. The details of the current transformer itself are not explained herein but in this example it is linked with a primary conductor and pr ovides a secondary output as shown in the Figure.

According to this embodiment, the shield 26 is formed in a similar Nancy to the previous embodiment such that there is a main body 26a and an cxtenson portion 26b. In this particular embodiment, the extension 26b is in talc shape of an 'L' and is formed such that one leg of the extension extends away from the main body 26a and the other leg of the extension is provided with Else magnetic detection arrangement which includes the coil 27.

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 S adUcd Additionally, it will be appreciated that the detector coil provided with the shielding means will be able to detect both alternating and direct magnetic fields and that the shielding means can be made to detect omni-directional

magnetic fields.

lO It will be appreciated that although the two embodiments described hecubefore relate to the shield being applied to either the switch element 25 or the current transformer 35, the utility meter may comprise one or more of either element. Further, the utility meter may include both these elements and/or any other element susceptible to external magnetic fields. Moreover, the shield may be applied to the entire utility meter or parts of the utility meter. Further, it is possible that only some susceptible areas of the meter are provided with the shielcl.

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 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 adcltonal computation for the actual meter reading so as to avoid false rcadigs due to the presence of the detection waveform.

Claims

CLAIMS: 1. An utility meter comprising means for detecting consumption of

a utility and means for indicating the results of the detection wherein at least part of the utility meter is provided with a means for shielding from a magnetic field and wherein at least part of the shielding means is provided with a coil, wherein the part of the shielding means and the coil form a magnetic field detection means to detect the presence of a magnetic field originating from outside the meter.
2. An utility meter according to claim 1 and comprising memory means for storing the occurrence of the detection of the magnetic field.
3. An utility meter according to claim 2, and wherein the magnetic field detection means is used to indicate the removal to the magnetic field.
4. An 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

of the magnetic field has occurred.
5. An utility meter according to any one of the preceding claims, wherein the magnetic field detection means includes a waveform generator applying a detection waveform to the coil.
6. An utility meter substantially as hereinbefore described with reference to the accompanying drawings.