Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Plural inputs
Reexamination Certificate
1999-03-25
2001-05-29
Nguyen, Vinh P. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Plural inputs
C324S10300R
Reexamination Certificate
active
06239589
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a measuring device for electrical power and to a method for measuring the electrical power. The measuring device has a multiplexer connected to an analog-to-digital (A/D) converter. The multiplexer receives current and voltage signals that are digitized by the A/D converter. A digital processing device controlled by a control unit further processes the digitized signals for formulating power values.
In order to determine or measure electrical power of an electrical load, it is known to feed the current and voltage signals acquired at the load in each case to an analog to digital converter (A/D converter) and to multiply them by one another in a downstream digital processing device.
Contemporaneous values of current and voltage are combined with one another in the process.
In order to compensate for angle errors originating, by way of example, from a shunt or a current transformer, a digital filter may be connected downstream of the A/D converter. As an alternative, external compensation using RC circuitry (analog filter) is also possible. A measuring device of this type requires a high outlay on components, so it is appropriate essentially only for high-quality meters having a high level of accuracy.
German Patent DE 26 30 359 C2 discloses a measuring device in which an analog time delay element is used to compensate for a time delay between two signals, which time delay element is connected upstream of a multiplexer.
Published, European Patent Applications EP 0 681 235 A1 and EP 0 634 662 A1 each disclose an electricity meter in which an A/D converter, as in the case of Published, European Patent Application EP 0 377 282 A1, corresponding to U.S. Pat. No. 5,017,860, is used in each case for current and voltage measurement. The A/D converter is in each case configured as a sigma-delta converter.
Published, Non-Prosecuted German Patent Applications DE 195 21 609 A1 and DE 195 21 610 A1 disclose, in connection with electrical energy measurement, using a decimation filter which has a selectable decimation ratio. A reference to the problems in the present case is not provided therein.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a measuring device for electrical power and a method for measuring the electrical power that overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, in which, despite a low outlay on components, the highest possible measuring accuracy is achieved during measurement of the electrical power with regard to an angle error.
With the foregoing and other objects in view there is provided, in accordance with the invention, an electrical power measuring device, including: a multiplexer for receiving input signals including a current signal and a voltage signal to be measured; an analog-to-digital converter connected to and disposed downstream of the multiplexer, the analog-to-digital converter is a sigma-delta modulator receiving the current signal and the voltage signal and outputs a digital serial signal formed of digitized values of the current signal and the voltage signal; a digital processor connected to and disposed downstream of the sigma-delta modulator and receives the digital serial signal from the sigma-delta modulator; a decimation filter connected between the sigma-delta modulator and the digital processor; and a control logic unit connected to the decimation filter and generating predeterminable waiting cycles in each case between the digitized values of the current signal and the voltage signal forming the digital serial signal for forming respective power values, the control logic unit driving the decimation filter such that a predetermined phase angle is generated between the digitized values of the current signal and the voltage signal in the digital serial signal.
The invention is based on the fundamental concept that, taking the prior art as a departure point, it is possible to eliminate a processing path with an A/D converter, a multiplexer being connected upstream of the remaining processing path as a replacement. However, this has the disadvantage that the respective samples of current and voltage which are provided in the sense of value pairs for a power value to be determined no longer appear simultaneously but rather succeed one another in time.
The present invention uses the disadvantage of the temporal offset in order to compensate for angle errors, error compensation being achieved in an astonishingly simple manner.
Compensation of external angle errors is afforded in this case without additional measures or further components being required. In addition, the angle error compensation has the capability of being adjusted.
The decimation filter used may be part of the processing device or of the A/D converter. This allows the formation of multi-bit data words, as a result of which simple digital measured-value processing by a computer is possible. The error rate is particularly low in this case.
It is expedient if the processing device includes an averaging unit to which the digital values of one of the input signals can be fed. The output average values formed are then used with the digitized values of the respective other input signal in order to form the respective power values. An internal angle error resulting from a temporal offset of the current and voltage signals is thus compensated for.
This can be done, by way of example, as follows: in all instances respectively successive current and voltage values are multiplied directly by one another, thereby producing a doubling of power values which, in total, achieve an average value effect. As an alternative, the average value of, for example, two voltage values adjacent to a current value can also be formed directly and then be multiplied by the current value.
It is expedient if the A/D converter includes a sigma-delta modulator. This makes it possible for a digital signal to be formed in a simple manner. This component makes it possible to achieve good linearity and a high degree of stability in the measured-value processing.
The power values are preferably accumulated in the processing device for the purpose of forming at least one energy value. By way of example, the energy value can then be stored for accounting or else be summed with further subsequent energy values.
The measuring device is preferably used in meters for electrical energy and/or power, integration in a control device for energy measurement also being conceivable, of course.
In the method of the invention, the serial signal is advantageously an output signal of a multiplexer. The use of a particularly simple component is possible as a result of this.
It is expedient if the output signal is subjected to analog-to-digital conversion and a digital signal is generated in the process. Simple downstream digital signal processing is possible in this way. The digital signal is preferably configured as a 1-bit stream. This affords fast signal processing with few errors. 1-bit A/D converters have a simple analog circuit, the formation of high-resolution digital words in the digital section being afforded.
The 1-bit stream can subsequently be converted into multi-bit words, thereby affording conventional digital signal processing in which conventional components, in particular digital signal processors, are used.
During the conversion of the 1-bit stream into multi-bit words, waiting cycles are provided between the current and voltage values, and have a predeterminable length. Adjustable compensation is afforded in this way, it being possible to make use of waiting cycles which are possibly present in any case in decimation filters. The jargon also speaks of waiting times in this context.
The power values generated are preferably accumulated for the purpose of forming an energy value. This affords simple measurement of energy consumption.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invent
Greenberg Laurence A.
Lerner Hebert L.
Nguyen Vinh P.
Siemens Aktiengesellschaft
Stemer Werner H.
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