WO1998054583A1

WO1998054583A1 - Commodity consumption meter

Info

Publication number: WO1998054583A1
Application number: PCT/GB1998/001506
Authority: WO
Inventors: Alan John Jones; Jonathan David Mills; Timothy Robert Joyce
Original assignee: Abb Metering Systems Limited
Priority date: 1997-05-27
Filing date: 1998-05-22
Publication date: 1998-12-03
Also published as: GB9710912D0; NZ501263A; NO995781D0; NO995781L; GB2327274A; EP0985152A1; GB2327274B; GB9810924D0

Abstract

A commodity consumption meter (2), of a type to which add-on circuitry (54) can be attached to provide at least one function ancillary to metering of the consumption of the commodity, comprises an electrical circuit (6) having metering means (20) for measuring consumption of the commodity in response to at least one electrical signal (vi and vv) applied to an input of said circuit and an interface circuit (22) for controlling data supplied to a display (8) which is connectable to the interface (22). The interface circuit (22) is operable in a first mode of operation to relay consumption data from the metering means (20) to the display (8) and in a second mode of operation to provide data to the display (8) from the add-on circuitry (54) connected to an additional connection (18) to the circuit.

Description

COMMODITY CONSUMPTION METER

This invention relates to commodity consumption meters, such as for example water, gas

or electricity consumption meters and more particularly to the construction of such

meters.

Commodity consumption meters have traditionally measured, and kept a record of, the number of relevant units of commodity consumed since the installation of the meter. Settlement of the consumer's account often took place quarterly, requiring regular inspections of the meter by the commodity supplier to determine the number of units of commodity consumed.

Electricity consumption meters have been developed which can communicate with the commodity supplier by way of radio transmission, a consumer's telephone link, by mains-borne signalling or by other means thereby reducing the need for regular inspection of the meter. In situations where collection of payment proves difficult it is known to use prepayment meters into which the consumer places money, tokens or other forms of credit and the meter continues to provide the commodity whilst credit is present. Such meters can be particularly useful in rented premises where frequent

changes of occupier occur such as, for example, in rented holiday accommodation.

In the UK, with the privatisation of many of the utility companies supplying metered commodities such as electricity, water or gas, new market opportunities have arisen for the supply to both industrial and domestic consumers alike. For example, in the UK, the regional electricity companies (RECs) supply consumers with electricity which the RECs have purchased from a pool system. The various electricity generating companies

submit tenders to supply electricity based on thirty minute periods throughout the day.

The settlement of an RECs purchase from the generating companies occurs monthly. It is proposed to extend this system so that industrial and domestic consumers will be

able to select which REC they purchase electricity from, irrespective of their geographic location and irrespective of which REC owns the cabling connecting the consumer to the electricity distribution network. To enable the REC to settle payment for supplying energy to a customer it is predicted that it will be necessary to log, at half hourly intervals, the number of units of electricity supplied for each customer over a one month period, wherever the customer is geographically located with respect to his/her REC. As a result, new electricity consumption meters will be required.

To meet the above diverse metering needs it is known to design and build a specific meter for each metering application. Designing meters is costly since they must meet stringent regulations laid down by the appropriate authorities and must be approved by those authorities before they can be installed onto the electrical system. Although it is

theoretically possible to build a single multipurpose meter having all the necessary metering functions, this would be costly to manufacture and, in most applications, a

large proportion of the functions would be redundant making the meter cost ineffective.

In an attempt to avoid this redundancy, it has been proposed to produce a modular electricity meter which comprises a basic meter to which add-on modules can be attached. The basic meter module can measure consumption of electricity and keep a running total of the number of units of electricity used which is displayed on a liquid crystal display (LCD). The basic meter also includes an optical communications port

which allows the attachment of add-on modules which are capable of providing an ancillary function to the basic metering function such as, for example, a prepayment

function or remote meter reading function. Whilst such an arrangement provides a flexible metering system, the inventors have appreciated that it has certain limitations.

For example, in the known modular meter arrangements the add-on modules include a further display for displaying the information appropriate to the ancillary function provided by the add-on module. The addition of this display thus renders the display on the basic meter redundant and it is known to arrange the casing of the add-on module to physically obscure the display of the basic meter.

The present invention has arisen in an endeavour to provide a commodity consumption meter which is flexible and can accommodate future metering needs and which at least in part overcomes the limitations of the known arrangements.

According to the present invention there is provided a commodity consumption meter of a type to which add-on circuitry can be attached to provide at least one function

ancillary to metering of the consumption of the commodity, said commodity consumption meter comprising: an electrical circuit having metering means for

measuring consumption of the commodity in response to at least one electrical signal

applied to an input of said circuit and characterised by an interface circuit for controlling data supplied to a display which is connectable to said interface circuit wherein the interface circuit is operable in a first mode of operation to relay consumption data from the metering means to the display and is operable in a second mode of operation to provide data to the display from the add-on circuitry which is connectable to an

additional connection of the circuit.

Constructing a meter in accordance with the invention enables a whole family of meters to be constructed all of which are based on an electrical circuit which is common to

each. The provision of the interface circuit enables a meter to be constructed which uses a single display and consequently the only cost in terms of redundant components is in the inclusion of some additional icons on the display.

Advantageously the interface circuit is further operable to relay consumption data from the measuring means to the add-on circuitry via the additional connection. This is particularly useful where additional processing of the consumption data is required such as for example for multi-tariff meters, that is, a meter in which the cost for a given unit of the commodity is different at different times of the day, or on different days, or where the consumption data recorded in the electrical circuit is to be transmitted by the add-on circuitry to a remote point such as, for example, in remote meter reading applications.

In a preferred embodiment the electrical circuit further comprises circuit means which

is operable to wait for a selected period for a recognised stimulus from the add-on

circuitry and in the event of not receiving a recognised stimulus is operable to generate

an error signal. The absence of a recognised stimulus indicates either that no add-on circuitry is connected or that a fault has arisen in the add-on circuitry. Advantageously this error signal is used to reset the add-on circuitry into a valid mode of operation. Advantageously the interface circuit is operable in a calibration mode to pass

information between the electrical circuit and test circuitry which is connectable thereto

enabling the meter to be readily tested. In such a calibration mode the interface circuit

is further operable to write a calibration constant into the meter, under control of the test

circuitry, ensuring the meter can be accurately calibrated within the tolerance of the

sensor or for different types of sensors which may be used to generate the, or each,

electrical signal.

Preferably the commodity consumption meter further comprises a sealed housing in

which the electrical circuit is housed, preventing unauthorised access and tampering

with the meter's operation. According to a preferred embodiment, the additional connection is only accessible from within the sealed housing and the add-on circuitry

is locatable within said sealed housing. Alternatively, the additional connection is

accessible externally of the sealed housing and said add-on circuitry is locatable

externally of said sealed housing, allowing the add-on circuitry to be fitted without

requiring access to the interior of the meter.

Preferably the commodity consumption meter comprises an electricity consumption

meter and the electrical circuit includes means for detecting a fault condition in the

electricity supply. Advantageously the commodity consumption meter further comprises

non- volatile memory and the electrical circuit is operable to store the consumption data

from the metering means in the memory when a fault condition in the electricity supply

is detected. Furthermore, the electrical circuit advantageously generates an error signal when a fault condition in the electricity supply is detected, and said signal is passed to the additional connection, thereby activating the add-on circuitry to take any necessary

steps before the failure of the electricity.

Advantageously, the electricity meter's input comprises respective inputs for electrical

signals representative of the instantaneous voltage and current and the measuring means comprises means for converting said signals to digital signals and processing means for processing the digital signals to produce a value representative of the electrical power consumed.

In alternative embodiments the commodity consumption meter comprises, for example, a water or gas consumption meter. In any of the embodiments of the invention the electrical circuit preferably comprises an integrated circuit. In the context of the present invention the term 'integrated circuit' is to be construed broadly and thus includes not only circuits which are formed on a single substrate, such as silicon, but also those which comprise a number of separate semi-conductor chips mounted on a common substrate such as for example a ceramic substrate. It will be appreciated therefore that the term 'integrated circuit' refers to a circuit which is formed as a single package as opposed to a circuit constructed from a number of discrete components.

An electricity consumption meter in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic representation of an electricity meter in accordance with the invention;

Figure 2 is a schematic of the metering circuit of the meter shown in Figure 1;

Figure 3 is a schematic representation of the meter of Figure 1 with add-on circuitry

connected thereto and

Figure 4 is a schematic representation of an electricity meter according to a preferred embodiment of the invention.

Referring to Figure 1, a single phase electricity consumption meter 2 in accordance with the present invention is shown. The meter 2 comprises a sealed housing 4 for preventing unauthorised access to the interior workings of the meter. Within the sealed housing there is contained an application specific integrated circuit (ASIC) 6, a liquid crystal display (LCD) 8, a power supply unit 10, a shunt resistor 12, a potential divider arrangement comprising resistors 14 and 16 and a module connector generally denoted 18.

The ASIC 6 includes a metering circuit 20, control logic (or interface logic) 22, a non-

volatile memory 24; a display driver circuit 26 and a serial to parallel (S/P) interface 28.

The metering circuit 20, control logic 22 and memory 24 are each connected to a data

bus 30. The control logic 22 is connected to the driver circuit 26 and S P interface 28 by a respective data bus 32, 34 as shown in Figure 1. The module connector 18 has a number of connections which are denoted "data in", "data out", "data ck", "pulse out", "Vcc" and "OV" in the Figure. The function of these connections is described later.

As is known the meter 2 measures the electrical energy being consumed by measuring the voltages v_; and v_v occurring across the shunt resistor 12 and the resistor 14

respectively in which the voltage v_v is related to the instantaneous voltage and the

voltage V; the instantaneous current. The shunt resistor 12 conveniently comprises a manganin resistor such that its resistance is substantially invariant of its temperature and provides an accurate measurement over the full operating current range of the meter. Typically the shunt resistor 12 has a resistance of approximately 100 μΩ such that it produces a voltage v_; of 10 mV relative to the live supply rail for a load current of 100 amps rms. In the case of the voltage signal v_v the resistors 14 and 16 are 10 kΩ and 2.4 MΩ respectively giving a voltage of 1 volt rms relative to the live rail (denoted ) when operating with a 240 volts rms supply.

These electrical voltage signals v_;and v_v are converted by the metering circuit 20 into values of energy consumed. The metering circuit uses a known measurement technique as described in, for example, our UK patent GB 2239097 which is hereby incorporated

by way of reference thereto.

A schematic representation of the metering circuit 20 is shown in Figure 2. Referring

to this figure the signal v_v corresponding to the instantaneous voltage is applied to an

analogue to digital (A/D) converter 36 within which it is sampled at regular intervals under the control of a clock signal "ck" derived from a clock generator 38. The A/D converter produces two outputs; the first (indicated as "magnitude" in Figure 2) is a quantised digital representation corresponding to the instantaneous ratio of the

magnitude of the voltage v_v to a reference voltage V_re& and the second (indicated as

"polarity" in the Figure) is a single bit representative of the polarity of the instantaneous

voltage v_v . The reference voltage V_ref is generated within the ASIC 6 by a reference

voltage generator 49 which is connected to the PSU 10.

The current representative voltage signal v_; is applied to a sigma-delta (σΔ) modulator

40 which, under control of the pulses "ck" from the clock generator 38 and with

reference to the voltage V_ree produces an output which during each clock pulse period

has either a first or second value, i.e. "1" or "0" dependent on the magnitude of the

voltage V; . As is described in the above mentioned patent the ratio of the number of

"1 's" to "0's" in the output train of the σΔ modulator is determined by the average value of the ratio of the voltage V; to the voltage V_ref . The voltage "polarity" representative

output bit from the A D convertor 36 and the output from the σΔ modulator 40 are

combined together using an exclusive-or gate 42 and the resultant signal, denoted

"add/subtract" in Figure 2, is applied to an accumulator/divider 46. The voltage

"magnitude" representative output from the A/D converter 36 is applied to the input of

the accumulator/divider 46. The accumulator/divider 46 is arranged to either add the

voltage "magnitude" representative value to, or to subtract it from, the count in the

accumulator/divider 46 in dependence on the output of the gate 42, once during each

clock period. As a result, the count in the accumulator/divider 46 changes, in a given

interval, by an amount corresponding to the magnitude of voltage multiplied by the

number of times more in that interval the output of the modulator 40 had one value rather than the other. The count in the accumulator/divider 46 thus changes in proportion to the product of the instantaneous voltage v_v and current v_; supplied, i.e. in proportion to the power supplied to the load.

The accumulator/divider 46 produces a pulse at its output each time its count exceeds a predetermined count and is calibrated such that it produces a known number of pulses

for each kilowatt hour (kWh) of electrical energy passing through the meter 2. The pulsed signal from the accumulator/divider 46 is used to increment either a first counter 48a or a second counter 48b depending on the status of a rate select switch 44. The two counters 48a and 48b are provided to enable the meter to operate at two different tariffs.

In operation each counter 48 stores the number of units of electricity consumed for a respective tariff. The operation of the rate select switch 44 is determined by a control line denoted "tariff select" which is taken to a connection which is accessible from outside the meter's sealed housing to which an external time switch can be connected. To ensure that the metering circuit 20 provides an accurate measure of energy consumption, the accumulator/divider 46 uses a calibration constant to define the predetermined count which is stored in the non- volatile memory 24 and can be loaded

into the accumulator/divider 46 via the data bus 30.

A light emitting diode (LED) 50 (which is shown in Figure 1) which is visible from

outside the sealed housing 4 is connected to the output of the accumulator/divider and "flashes" a known number of times (typically five hundred) for each kWh of energy consumed. The LED 50 gives a visual indication to the consumer that the meter is functioning and allows him/her to check the meter accuracy. The pulsed output from the accumulator/divider 46 is also connected to a respective connection, denoted "pulse out", of the module connector 18. The flashing LED 50 and/or output "pulse out" of the

connector 18 can be utilised by the manufacturer to accurately set the calibration of the

meter. To check the meter accuracy a known load is connected to the meter and the

pulse rate is measured and compared with the expected pulse rate for the given load. If there is any discrepancy between these values the meter calibration can be adjusted by loading a new calibration constant into the non-volatile memory 24 via the module connector 18 and control logic 22.

Referring again to Figure 1 the clock signal "ck" is also used in other parts of the circuit 6 of the meter. For example the clock is used to derive the multiplexing signal for the display driver 26, which in the embodiment shown is a 120 Hz square wave signal. Though a multiplexed connection between display driver 26 and display 8 is preferred, it will be appreciated that this is not essential to the present invention. A multiplexed connection (sixteen electrical connections in the example shown for a forty-eight segment four backplane display) has the advantage of a reduced number of output pins on ASIC 6, thus making the ASIC 6 cheaper to manufacture, but this saving in cost is achieved at the expense of increased complexity in both the display driver 26 and the LCD 8.

The PSU 10 is connected between the live L_h and neutral N^ supply lines and provides

a 5V (Vcc, 0V) supply to the ASIC 6 and to the module connector 18. This arrangement allows power to be supplied to add-on circuitry (not shown) when it is connected to the module connector 18 thereby avoiding the need for the add-on circuitry to be provided with an internal power supply or to be connected to the mains electricity supply.

As described above, in normal operation of the meter the metering circuit 20 measures

the number of units of electrical energy consumed. This information under control of the control logic 22 is periodically copied from the counter 48 to the non-volatile

memory 24 via the data bus 30 and is copied when a failure of the mains electricity supply is detected. Upon restoration of the electricity supply the value of the counter 48 is set using the information in the non- volatile memory 24.

In a first mode of operation of the meter, that is when no add-on circuitry is connected to the meter, the count from counter 48 is transferred via the control logic 22 and data buses 30 and 32 to the display driver 26. The LCD 8 thus displays the total number of units consumed since the installation of the meter. It will be appreciated that the meter 2 thus functions as a basic electricity consumption meter in this first mode of operation. Furthermore, by connecting an external time switch to the "tariff select" line, the meter 2 can operate as a two tariff meter in this first mode of operation. In this case, the display driver 26 can be arranged to display the contents of each counter 48a and 48b

simultaneously or sequentially.

Referring to Figure 3 there is shown the electricity meter 2 with add-on circuitry or

module 54 connected to the module connector 18. In the example illustrated the add-on

module 54 is a four tariff remote meter reading module and enables the meter to log electricity consumption for four tariffs and to be read and programmed remotely using radio transmission. The add-on module 54 comprises a serial to parallel interface 56, a microprocessor 58, a memory 60 (which comprises both volatile and non-volatile parts) and a transceiver arrangement 62 which are connected as shown in the figure. The transceiver 62 enables the commodity provider to communicate with the add-on

module by radio transmission. For the purposes of this example let us assume that the information received defines the tariffs that will be payable for electricity and the times

at which the four tariff periods run. This information is passed by the microprocessor 58 to the memory 60 for storage. When a module is connected to the meter 2 the microprocessor 58 instructs the meter to operate in a second mode of operation. The way in which the module instructs the meter to operate in the second mode is described below. In this second mode of operation the add-on circuitry controls the data being sent to the display 8 and the display 8 no longer simply displays the total number of units consumed.

In the second mode of operation the microprocessor 58 uses the "pulse out" connection of the connector 18 or directly reads the contents of the meter counters 48 using the serial interface "data in", "data out" and "data ck" connection of the connector 18 and, in conjunction with the tariff information stored in memory 60, calculates the requisite information such as, for example, the total value of electricity consumed in terms of

cost, the cost of electricity consumed in the current charging period, the present

consumption rate and cost per unit time and the remaining credit limit to name but a few. The microprocessor 58 passes elements of this information via the "data in", "data ck"

connection of the connector 18 to the display 8 via the control logic 22 and display driver 26. In this second mode of operation the control logic 22 thus acts as an interface and passes information from the module connector 18 to the display 8. In this example the microprocessor 58 can be used to cycle the information appearing on the display 8.

For example the display 8 can cycle through displaying the total number of units used

since the installation of the meter, the current tariff rate, electricity consumption rate or

the value of electricity used in the present charging period. It will be appreciated that

through the provision of the interface/control logic 22 the meter 2 can be operated in any

desired way by connecting suitable circuitry 54 to the connector 18. The add-on

circuitry is preferably located within the sealed housing 4 of the meter to prevent

tampering therewith.

The meter 2 determines when a module 54 has been connected as follows. By default on powering up the meter the interface logic 22 is automatically configured to operate

in the first mode of operation as a stand alone meter. Although the interface

logic/control logic 22 controls the flow of data to the display 8 either from the counter

48 or via the connector 18, it does not have any significant processing power in the

sense of a microprocessor. In the embodiment described above the control logic 22 is

a logic gate array. The provision of such processing power would result in a significant

increase in the cost of the ASIC 6 making the meter prohibitively expensive for many

applications. When a module is connected to the meter the microprocessor 58 in the

module writes data to the meter at a specified address which has the effect of re-

configuring the logic in the interface/control logic 22 such that the meter operates in the

second mode of operation. The mode of operation of the meter is thus determined

externally of the meter via the module connector 18.

As described above the module connector 18 and control/interface logic 22 have an important part to play in the initial calibration and testing of the meter 2 after manufacture. The scaling of the meter 2 is initially set by writing a default calibration

constant into part of the non-volatile memory 24. The meter can then be tested with a

known load and the constant changed accordingly. To reduce the time taken to calibrate and test the meter a smaller calibration constant is initially written into the non-volatile

memory 24 via the module connector 18. This results in an increased pulse rate, by for example a factor of ten, from the metering circuit 20 and thus allows testing to be completed in a shorter time period. The interface logic circuit 22 is also operative to provide the test equipment (not shown) with the values stored in the counters 48 of the metering circuit 20, via the "data out" connection of connector 18, in order to confirm that correct increments of that register are occurring for a predetermined amount of energy consumed.

It will be appreciated that as technology advances or as technology currently too expensive (such as transmitting and receiving information by way of cellular telephone networks) becomes more commercially viable, the meter can be upgraded by opening the sealed housing of the meter and replacing the add-on circuitry 54 without the need to discard the ASIC 6 or display 8 which constitute a substantial proportion of the

meter's cost. The present invention provides support for highly complex add-on circuitry whilst providing basic metering functions in the form of a single integrated

circuit of low cost. In particular, by integrating the metering function 20, the interface

logic circuit 22 and the display driver 26 into the form of an ASIC 6, a basic meter may be produced which can be adapted to meet future needs. Referring to Figure 4 there is shown a preferred form of meter in accordance with the invention in which the ASIC 6 further includes a power supply monitor 64 and a

microprocessor watchdog circuit 66.

The power supply monitor 64, sometimes referred to as a "Brown out" indicator, is operable to detect a likely collapse of the DC power supply to the electrical circuit, such

as would happen if the mains supply voltage either dropped substantially, or failed completely. As is known the PSU 10 includes storage/smoothing capacitors which conveniently comprise polarised electrolytic type capacitors of a few hundred microfarads. The instantaneous voltage on the storage capacitors rises as they are charged by current flowing forward through the rectifying means of the PSU 10 and will decay when the rectifier is not conducting but whilst the electrical circuit continues to draw current. Between the storage capacitors and the output of PSU 10 regulating means are provided which ensure the PSU 10 provides a constant DC voltage output (Vcc, 0V) irrespective of the varying voltage on the reservoir capacitors. In normal operation of the PSU 10 the instantaneous voltage on the reservoir capacitors will typically be in the range 7 volts to 10 volts. In the event that the mains supply fails the

voltage on the reservoir capacitors will fall below this range and this occurrence is detected by the power supply monitor 64 by comparing a fixed proportion of this voltage, conveniently derived by a resistive potential divider, with a threshold reference

voltage.

In the event that a failure of the electrical supply is detected a train of events will be set in motion which includes saving the value of the counters 48a and b (number of units of electricity consumed) into the non-volatile part (E²PROM) of the memory 24 and generating a signal denoted "power failure" which constitutes a further connection on

the module connector 18. On receipt of this signal the add-on circuitry 54 likewise saves its data to the non-volatile part of the memory 60. The electrical energy needed

to accomplish these operations is available from the remaining charge stored in the reservoir capacitors of the PSU 10. In order to conserve energy and enable the meter

2 and module 54 to carry out these tasks the power supply monitor 64 is operable to interrupt the current flow to be LED 50, display driver circuit 26 and parts of the metering circuit 20. When the electrical power to the meter subsequently resumes the counters 48 are set to their previous value from the data saved to the memory 24.

The "watch-dog" circuit 66 is operable to monitor for the continuing operation of the microprocessor 58 of the add-on circuitry 54. The "watch-dog" produces a reset pulse, denoted "reset", at a set time interval if it does not receive a recognised stimulus/data signal from the microprocessor 58 indicating that the microprocessor is operating correctly.

The microprocessor 58 of the add-on circuitry is programmed to generate the recognised

stimulus/data signal, typically once every second, which is passed via the "data in" connection of module connector 18 to reset a timer in the watch-dog circuit 66, causing

the timer to be reinitiated before it reaches the end of its predefined timing period,

typically a period of around two seconds. In the embodiment shown this data signal is recognised by the interface logic 22 as representing the address of the watch-dog circuit 66. In the event that the microprocessor malfunctions it will cease to generate this repetitive data signal and the timer in the watch-dog circuit will then run to its end-state

whereupon it will cause a reset signal to be generated. This reset signal is passed via a respective connection "reset" of the module connector 18 to the microprocessor forcing

the microprocessor into a known starting condition. The microprocessor will thereby

recommence its correct sequence of operations. Whenever the meter 2 is "powered-up" the "watch-dog" circuit 66 produces reset pulses until the electrical supply stabilises which is determined by the power supply monitor 64 which produces a "power on reset"

(POR) signal which is applied to the supply monitor 64.

When no module is connected to the meter the watch-dog circuit 66 will continue to generate a reset signal every two seconds.

It will be appreciated by those skilled in the art that modifications may be made to the meter described which still fall within the scope of the present invention. For example, whilst a single phase electricity consumption meter has been described, it is envisaged that the present invention could be applied to other forms of commodity consumption meters such as, for example, polyphase electricity meters, those meters installed in the commodity supplier's premises for logging the amount of commodity supplied to the

network, water, gas or other commodity meters. In such meters a sensor will be needed

to produce an electrical signal related to the consumption of the commodity, which

signal can be applied to the ASIC 6. For gas and water consumption meters it is also desirable that power is provided from a battery supply since a mains supply of electricity

is not usually feasible. Although the basic meter has been described as being in the form of an ASIC 6, it is also

within the scope of the present invention to construct the electrical circuit using discrete

devices, though this is likely to be more costly. In the context of the present invention

the term 'integrated circuit' is to be construed broadly and thus includes not only circuits

which are formed on a single substrate, such as silicon, but also those which comprise

a number of separate semi-conductor chips mounted on a common substrate such as for

example a ceramic substrate. Thus, in the present context, the term 'integrated circuit'

refers to a circuit which is formed as a single package as opposed to a circuit constructed

from a number of discrete components.

In the embodiment described the module connector 18 is not accessible from outside the meter's sealed housing 4 and the add-on circuitry/module 54 is located within the

housing 4. Such an arrangement is beneficial in that it prevents unauthorised access to,

and tampering with, the meter or add-on circuitry. It does however require access to the

interior of the meter housing when fitting the add-on circuitry, which will limit the

number of persons able to fit the modules 54. It is also envisaged that the module

connector 18 be accessible from outside the sealed housing, enabling modules to be

connected externally of the meter without requiring access to the interior of the meter.

With such an arrangement, however, a sealed connector cover plate or other form of

physical barrier is required to prevent unauthorised access to the module connector 18

when no module is connected to the meter. Furthermore, the add-on circuitry/module

would need to be housed in a suitable tamper-proof enclosure.

It will be appreciated that an important aspect of the invention is the interface logic or control logic 22 which enables the meter to operate as both a basic meter and to co¬

operate with the add-on circuitry 54 to perform ancillary metering functions. In

particular the interface circuit 22 is operable in a first mode of operation to pass

consumption data from the metering circuit 20 to the display driver 26, thereby enabling

the circuit to operate as a basic meter and is further operable in a second mode of

operation under the control of the add-on circuitry 54 to pass data to the display 8 from

the add-on circuitry 54 thereby enabling the meter to use a single display for a wide

range of functions. In the second mode of operation the add-on circuitry 54 controls the

information passed to the display 8 and enables the meter to display information which

could not otherwise have been generated by the meter circuit when operating as a stand¬

alone meter. Furthermore, it is preferable that the interface circuit 22 relays

consumption data to the add-on circuitry 54.

The interface circuit 22 also enables the meter to be tested and calibrated by loading

calibration data into the memory 24. This enables the ASIC 6 to operate with a range

of current and voltage sensors for different types of applications.

Claims

1. A commodity consumption meter (2) of a type to which add-on circuitry (54) can

be attached to provide at least one function ancillary to metering of the consumption of the commodity, said commodity consumption meter comprising:

an electrical circuit (6) having metering means (20) for measuring consumption of the commodity in response to at least one electrical signal (v_;, v_v) applied to an input of said circuit and characterised by an interface circuit (22) for controlling data supplied to a display (8) which is connectable to said interface circuit (22) wherein the interface circuit (22) is operable in a first mode of operation to relay consumption data from the metering means (20) to the display (8) and is operable in a second mode of operation to provide data to the display (8) from the add-on circuitry (54) which is connectable to an additional connection (18) of the circuit (6).

2. A commodity consumption meter according to Claim 1 further characterised in that the interface circuit (22) is further operable to relay consumption data from the metering means (20) to the add-on circuitry (54) via the additional connection (18).

3. A commodity consumption meter according to Claim 1 or Claim 2 characterised

in that the electrical circuit (6) further comprises circuit means (66) which is operable

to wait for a selected period for a recognised stimulus from the add-on circuitry (54) and in the event of not receiving a recognised stimulus is operable to generate an error signal (reset)

4. A commodity consumption meter according to any preceding claim characterised in that the interface circuit (22) is operable in a calibration mode to pass information

between the electrical circuit (6) and test circuitry which is connectable thereto.

5. A commodity consumption meter according to Claim 4 characterised in that the interface circuit (22) is further operable to write a calibration constant into the meter (2),

under control of the test circuitry.

6. A commodity consumption meter according to any preceding claim and characterised by further comprising a sealed housing (4) in which the electrical circuit (6) is housed.

7. A commodity consumption meter according to Claim 6 characterised in that the additional connection (18) is only accessible from within the sealed housing (4) and the add-on circuitry (54) is locatable within said sealed housing (4).

8. A commodity consumption meter according to Claim 6 characterised in that the additional connection (18) is accessible externally of the sealed housing (4) and said

add-on circuitry (54) is locatable externally of said sealed housing (4).

9. A commodity consumption meter according to any preceding claim characterised in that the meter (2) comprises an electricity consumption meter.

10. A commodity consumption meter according to Claim 9 characterised in that the electrical circuit (6) further comprises means (64) for detecting for a fault condition in the electricity supply (L^, N_in).

11. A commodity consumption meter according to Claim 10 characterised by further comprising non-volatile memory (24) and wherein the electrical circuit (6) is operable

to store the consumption data from the metering means (20) in the memory (24) when a fault condition in the electricity supply (L^ N ) is detected.

12. A commodity consumption meter according to Claim 10 or Claim 11 characterised in that the means for detecting a fault condition generates an error signal (Power Failure) when a fault condition in the electricity supply (L^, N_k) is detected, and said signal is passed to the additional connection (18).

13. A commodity consumption meter according to any one of Claims 9 to 12 and characterised in that the input of said circuit (6) comprises respective inputs for electrical signals representative of the instantaneous voltage (v) and instantaneous current (v;) and wherein the measuring means (20) comprises means (36,40) for converting said signals (v,, V;) to digital signals and processing means (46) for processing the digital signals to produce a value representative of the electrical power

consumed.

14. A commodity consumption meter according to any one of Claims 1 to 8 characterised in that the meter comprises a water consumption meter.

15. A commodity consumption meter according to any one of Claims 1 to 8 characterised in that the meter comprises a gas consumption meter.

16. A commodity consumption meter according to any preceding claim characterised in that the electrical circuit (6) comprises an integrated circuit.

17. A commodity consumption meter substantially as described and as illustrated by way of reference to Figure 1 or Figure 4 of the accompanying drawings.

18. Add-on circuitry (54) for use with a commodity consumption meter (2) according to any preceding claim.