GB2312962A

GB2312962A - Commodity meter employing a calibrated bucket counter

Info

Publication number: GB2312962A
Application number: GB9607996A
Authority: GB
Inventors: Ian Graham
Original assignee: Siemens Measurements Ltd
Current assignee: Siemens Metering Ltd
Priority date: 1996-04-18
Filing date: 1996-04-18
Publication date: 1997-11-12
Anticipated expiration: 2016-04-18
Also published as: GB9607996D0; NO971728D0; GB2312962B; ZA973027B

Abstract

A commodity meter comprises a central processing unit (cpu) with means 2, 4 and 6 arranged to calculate instantaneous values indicative of the quantities of a commodity used. These values are fed to a bucket counter 8 until the value within the bucket counter 8 reaches a threshold level 12, at which point an output pulse 14 is generated. This output pulse 14 is calibrated to accurately indicate the use of a certain amount of the said commodity. The threshold level 12 may be set at the capacity of the bucket counter 8. The threshold level 12 may be input and compared with the level within the bucket counter 8 by the cpu to control the bucket counter reset. The counter 8 may include a feed back system to allow for overflow and underflow conditions within the counter. A further bucket counter may be employed to extend the dynamic range of the meter. The meter may measure electrical power in which the output pulse 14 is calibrated to indicate one kilowatt hour.

Description

IMPROVEMENTS IN OR RELATING TO COMMODITY METERS The present invention relates to commodity meters and in particular to electricity meters where a method of calibrating such meters is required so that the Kilowatt hour reading is compensated in respect of inaccuracies in the measuring system.

When a measurement is taken of an applied voltage and current, many influences exist that will cause the measurement as seen by central processing unit, to vary from meter to meter. The variations may be caused by mechanical variation in the sensor used to determine the current, for example a shunt, or a variation of tolerance in the components used as the signal measured is passed to the central processing unit.

The net result of any electricity meter is required to be outputted as a Kilowatt hour reading, which represents the integral of the product of the voltage and the current and must be in a range of +/- 2% for example, as specified by the Standard IEC 1036.

A known calibration method scales the incoming parameters of voltage and current by a suitable multiplication factor.

However, this is very difficult to carry out in a situation where many samples are required in a second, such as found in an electricity meter, or where the computational power of the central processing unit is limited.

An aim of the present invention is to provide means for calibrating an a commodity meter which is simple to operate and which can conveniently be used where the computational power of the central processing unit is limited and where many samples are required in a short period of time.

According to the present invention there is provided a commodity meter comprising a central processing unit arranged to calculate the amount of a commodity used by a consumer, the central processing unit being provided with means arranged to calculate an instantaneous product indicative of the commodity being consumed at predetermined times, said product values being fed to an accumulator which is arranged to accumulate the product values until a threshold value is reached, whereby the accumulator generates an output impulse which is indicative of an amount of the commodity consumed and is calibrated in accordance with the chosen value of said threshold.

According to an aspect of the present invention, the capacity of the accumulator determines the desired threshold value.

According to a further aspect of the present invention the accumulator is provided with a threshold input and a reset input and said central processing unit is arranged to apply the required threshold value to the threshold input and compares the threshold value with the count in the accumulator.

An embodiment of the present invention will now be described with reference to the accompanying drawing, which shows a block diagram of an accumulator controlled by the central processing unit to calibrate an electricity meter.

It will be appreciated that the invention is not necessarily limited to electricity meters.

Referring to the drawing, there is shown an accumulator, e.g.

a bucket counter 2, having a first input connected to an output of an adder 4, and a second input connected to an output of a subtractor 6. An output of the bucket counter 2 is connected to an input of a comparator 8, and is also connected to an input of the adder 4 and the subtractor 6. The adder 4 receives an input signal on line 10. The comparator 8 receives a threshold signal on a line 12 and generates an output signal on line 14. It will be appreciated that the function of the bucket counter may be performed in software and could conveniently be incorporated in the software algorithm that is loaded in the central processessing unit of the electricity meter. This algorithm is used to convert the arbitrarily scaled samples of voltage and current measurements taken by the meter to convert them into accurate Kilowatt hour pulses for integration.

The central processing unit generates uncompensated products of the voltage and current measurements. For each measurement a number is given to that measurement which is applied to the input line 10, and therefore represents a value which is proportional to power. This value is constantly added by the adder 4 with the output from the bucket counter via line 16, so that the bucket counter can count up towards the threshold value. The output value generated on the output line 16 is applied to the input of the comparator 8 where it is compared with a value representing the threshold on the input line 12. If the value of this signal on the output line 16 is equal to or greater than the value of the signal on the input line 12 to the comparator, then the microprocessor causes the value on the output line 16 not to be added in the adder 4, and is subtracted from the bucket counter by the subtractor 6 leaving the overflow in the bucket counter 2. The output signal from the comparator 8, on the output line 14, represents the true power value.

Therefore, depending upon the results obtained by the comparator circuit, the output signal on line 16 is either added or subtracted in the bucket counter 2.

It will be appreciated that the threshold will determine the rate at which the overflow will occur. If the threshold is low, the overflow will be frequent, and if the threshold is high, then the overflow will occur less frequently.

It will be appreciated that the bucket counter may be provided with an input to receive a threshold signal and an input to receive a reset signal, but this will require additional processing power to compare the count value with the threshold value. To avoid the use of extra processing power, the threshold is offset to the capacity of the bucket counter. The central processing unit therefore, can easily determine overflow without time consuming subtractions or comparisons. The threshold therefore, is represented by an initial offset, which is equivalent to the capacity of the bucket counter and indicates the point at which overflow occurs.

Therefore, by judicious choice of the initial offset, matching the overflow rate to exactly the desired calibration value for the product is possible.

It will be appreciated that in the case of an electricity meter, which may be subject to unfavourable phase shifts between voltage and current, the product which is accumulated may also run in the negative direction. In this case, a similar operation can be performed with a negative offset value being applied to the underflow of the bucket counter.

A further bucket counter may be used to increase the dynamic range of the measurements under circumstances where the error values are changed due to the selection of different external circuitry, which have different tolerances. Conveniently, this bucket counter may be a third of the size of the other counter, thus increasing the gain of the system by ratio of 3 to 1 and would be eminently suitable when both large and small readings are required.

Claims

1. A commodity meter comprising a central processing unit arranged to calculate the amount of a commodity used by a consumer, the central processing unit being provided with means arranged to calculate an instantaneous product indicative of the commodity being consumed at predetermined times, said product values being fed to an accumulator which is arranged to accumulate the product values until a threshold value is reached, whereby the accumulator generates an output impulse which is indicative of an amount of the commodity consumed, and is calibrated in accordance with the chosen value of said threshold.

2. A commodity meter as claimed in Claim 1, wherein the capacity of the accumulator determines the desired threshold value.

3. A commodity meter as claimed in Claim 1, wherein the accumulator is provided with a threshold input and a reset input and said central processing unit is arranged to apply the required threshold value to the threshold input and to compare the threshold value with the count in the accumulator.

4. A commodity meter as claimed in any preceding Claim, wherein the output of said accumulator is fed back to the input of the accumulator by way of a subtractor and any overflow is retained in the accumulator and used in the next count sequence.

5. A commodity meter as claimed in any preceding Claim, wherein a further accumulator is provided and is used to increase a dynamic range of the measurements in respect of the commodity.

6. A commodity meter as claimed in any preceding Claim, wherein the meter is an electricity meter, the instantaneous product calculated is power, and the output impulse from the accumulator is indicative of a Kilowatt hour reading.

7. A commodity meter as claimed in Claim 6, wherein the accumulator is provided with a negative threshold value arranged to handle the product which is accumulated in a negative direction, and is used to control the underflow of the accumulator.

8. A commodity meter substantially as hereinbefore described with reference to the accompanying drawing.