Method and device for measuring an electrical current

Abstract

The invention relates to a method for measuring an electric current. In the method, current-dependent measurement signals are repeatedly acquired and, for each measurement signal, the measurement signal is filtered by means of a band-pass filter (5) in order to form a filtered measurement signal. Furthermore, for each measurement signal, a first derivative of the measurement signal with respect to time and a first derivative of a power density value with respect to time are determined, the power density value being formed from the spectral power density of the measurement signal. A weighted average of the measurement signal with the filtered measurement signal is formed as a measured value of the current intensity of the current, the weight of the measurement signal monotonically increasing with an increase in the first derivative of the measurement signal with respect to time and with an increase in the first derivative of the power density value with respect to time.

Description

[0001] The present invention relates to a method and a measuring device for measuring current.

[0002] The measurement signal for current measurement is usually superimposed with broadband noise. To suppress this noise, a filter is used. A filter is a system that relates an input signal to a transfer function and provides the modified signal at its output. In measurement technology, filters are mainly used to partially separate the broadband noise that is usually superimposed on the measurement signal from the signal. Thus, the narrowband region of the measurement signal can be isolated, which contains less noise. In the case of a current sensor for measuring current, the main frequency of the measurement signal is usually 50 Hz or 60 Hz. In this case, low-pass filtering of the measurement signal is advantageous. The filtered curve of the obtained measurement signal depends on its frequency, which is smooth, contains little noise, and is easy to process using further algorithms. However, the disadvantage of low-pass filtering is that the filtered curve responds slowly to changes in the measurement signal. In other words, improved signal quality is obtained at the cost of time lag. In the case of a measurement signal using low-pass filtering, it is impossible to detect rapid changes in the measurement signal.

[0003] The object of the present invention is to provide an improved method and an improved measuring device for measuring current.

[0004] According to the present invention, this object is achieved by a method having the features described in claim 1 and a measuring device having the features described in claim 10.

[0005] The dependent claims relate to advantageous embodiments of the present invention.

[0006] In the method for measuring current according to the present invention, a measurement signal dependent on current is repeatedly acquired, and for each measurement signal, a filtered measurement signal is formed by filtering the measurement signal using a band-pass filter. In addition, for each measurement signal, the first derivative of the measurement signal with respect to time and the first derivative of the power density value with respect to time are determined, where the power density value is formed from the spectral power density of the measurement signal. A weighted average of the measurement signal and the filtered measurement signal is formed as a measured value of the number of amperes of the current, where the weight of the measurement signal increases monotonically as the first derivative of the measurement signal with respect to time increases and as the first derivative of the power density value with respect to time increases.

[0007] Accordingly, the present invention provides for forming a weighted average of an actual measurement signal and a measurement signal filtered using a band-pass filter as a measured value of the amperage of the measured current. In this case, the weight of the measurement signal increases monotonically as the first derivative of the measurement signal with respect to time and as the first derivative of the power density value with respect to time increase. Correspondingly, the weight of the filtered measurement signal decreases monotonically as the first derivative of the measurement signal with respect to time and as the first derivative of the power density value with respect to time increase. In this case, the power density value is formed from the spectral power density of the measurement signal. The first derivative of the power density value with respect to time is a measure of the change in the frequency distribution of the measurement signal over time. According to the present invention, forming the measured value of the amperage of the measured current from the actual measurement signal and the filtered measurement signal takes into account the above statement, namely that filtering of the measurement signal suppresses noise and contributes to the evaluability of the measurement signal, but is not suitable for evaluating rapidly changing measurement signals. Thus, the measurement signal provided by the method according to the present invention has little noise in the case of a slowly changing measurement signal, but has a high bandwidth in the case of a rapidly changing measurement signal.

[0008] In one embodiment of the present invention, a stability factor is defined that decreases monotonically as the first derivative of the measurement signal with respect to time and as the first derivative of the power density value with respect to time increase, and the weighted average of the measurement signal and the filtered measurement signal is formed using the stability factor such that the weight of the measurement signal decreases as the stability factor increases, while the weight of the filtered measurement signal increases as the stability factor increases.

[0009] For example, the stability factor is assigned a value within the interval [0, 1], and the weighted average is formed according to A·(1 - S)+B·S, where A represents the measurement signal, B represents the measurement signal filtered by the band-pass filter, and S represents the value of the stability factor. In the case of forming the weighted average, the phase shift generated by the band-pass filter is also adjusted stably and continuously by the weighted average.

[0010] In another embodiment of the present invention, a first threshold is specified for the first derivative of the measurement signal with respect to time, and the stability factor is defined in such a way that for all values of the first derivative of the measurement signal with respect to time that exceed the first threshold, the stability factor assumes the minimum value of its value range.

[0011] In another embodiment of the present invention, a second threshold is specified for the first derivative of the power density value with respect to time, and the stability factor is defined in such a way that for all values of the first derivative of the power density value with respect to time that exceed the second threshold, the stability factor assumes the minimum value of its value range.

[0012] In another embodiment of the present invention, a third threshold is specified for the first derivative of the measurement signal with respect to time, and the qualification factor is defined in such a way that for all values of the first derivative of the measurement signal with respect to time below the third threshold, the stability factor assumes the maximum value of its value range.

[0013] In another embodiment of the present invention, a fourth threshold is specified for the first derivative of the power density value with respect to time, and the stability factor is defined such that for all values of the first derivative of the power density value with respect to time below the fourth threshold, the stability factor assumes the maximum value of its value range.

[0014] In this case, the first threshold and the second threshold each specify the first derivative value of the measurement signal with respect to time and the first derivative value of the power density value with respect to time. When the first derivative value exceeds the threshold, the weight of the measurement signal reaches the maximum. Correspondingly, the third threshold and the fourth threshold each specify the first derivative value of the measurement signal with respect to time and the first derivative value of the power density value with respect to time. When the first derivative value is below the threshold, the weight of the filtered measurement signal reaches the maximum.

[0015] In another embodiment of the present invention, the band-pass filter is a low-pass filter. This embodiment of the present invention is particularly advantageous when the main frequency of the measurement signal is relatively small, such as 50 Hz or 60 Hz.

[0016] In another embodiment of the present invention, the power density value is formed by integrating the spectral power density value of the measurement signal within a frequency range.

[0017] According to the present invention, a measuring device for measuring current includes:

[0018] - a current sensor configured to repeatedly acquire a measurement signal depending on the current,

[0019] - a band-pass filter configured to form a filtered measurement signal for each measurement signal, and

[0020] - an evaluation unit configured to:

[0021] - determine the first derivative of the measurement signal with respect to time and the first derivative of the power density value of the measurement signal with respect to time for each measurement signal, wherein the power density value is formed from the spectral power density of the measurement signal, and

[0022] - form a weighted average of the measurement signal and the filtered measurement signal as a measured value of the current in amperes, wherein the weight of the measurement signal increases monotonically with an increase in the first derivative of the measurement signal with respect to time and with an increase in the first derivative of the power density value with respect to time.

[0023] For example, a band-pass filter is a low-pass filter. For example, a current sensor is an optical current sensor.

[0024] For example, the evaluation unit is configured to form a power density value by integrating the spectral power density of the measurement signal over a frequency range.

[0025] The measuring device according to the invention enables the method according to the invention to be carried out. The advantages of such a measuring device correspond to the above-mentioned advantages of the method according to the invention.

[0026] The above-mentioned features, characteristics and advantages of the invention, as well as the manner of achieving these features, characteristics and advantages, will become more clearly understandable in the description in connection with the following embodiments, which will be explained in more detail in conjunction with the accompanying drawings, wherein:

[0027] Figure 1 is a block diagram of an embodiment of a measuring device for measuring current according to the invention;

[0028] Figure 2 is a flowchart of an embodiment of a method for measuring current according to the invention.

[0029] Figure 1 is a block diagram of an embodiment of a measuring device 1 for measuring current according to the invention. The measuring device 1 includes a current sensor 3, a band-pass filter 5 and an evaluation unit 7.

[0030] The current sensor 3 is configured to repeatedly acquire a measurement signal that depends on the current. For example, the current sensor 3 is an optical current sensor.

[0031] The band-pass filter 5 is configured to form a filtered measurement signal for each measurement signal. For example, the band-pass filter 5 is an analog low-pass filter or a digital low-pass filter.

[0032] The evaluation unit 7 is configured to determine, for each measurement signal, the first derivative of the measurement signal with respect to time and the first derivative of the power density value of the measurement signal with respect to time, wherein the power density value is formed from the spectral power density of the measurement signal. For example, the evaluation unit 7 is configured to form a power density value by integrating the spectral power density of the measurement signal over a frequency range.

[0033] Furthermore, the evaluation unit 7 is configured to form an average value obtained by weighting the measurement signal and the filtered measurement signal by a stability factor, which decreases monotonically as the first derivative of the measurement signal increases and as the first derivative of the power density value increases, wherein the weight of the measurement signal decreases as the stability factor increases, while the weight of the filtered measurement signal increases as the stability factor increases.

[0034] Figure 2is a flowchart of an embodiment of a method according to the present invention, including method steps 11 to 15 for measuring current. The method is performed by using the measurement device 1 described with reference Figure 1 thereof.

[0035] In step 11 of the first method, a measurement signal dependent on the current is acquired by the current sensor 3 of the measurement device 1.

[0036] In step 12 of the second method, the measurement signal acquired in step 11 of the first method is filtered by the band-pass filter 5 of the measurement device 1.

[0037] In step 13 of the third method, for the measurement signal acquired in step 11 of the first method, the evaluation unit 7 of the measurement device 1 determines the first derivative of the measurement signal with respect to time and the first derivative of the power density value of the measurement signal with respect to time. In this case, a plurality of measurement signals acquired by the current sensor 3 at different times are used to determine the first derivative of the measurement signal with respect to time and the first derivative of the power density value of the measurement signal with respect to time. The power density value is formed by the spectral power density of the measurement signal, for example, by integrating the spectral power density of the measurement signal over a frequency range.

[0038] In step 14 of the fourth method, the evaluation unit 7 forms a stability factor that depends on the first derivative of the measurement signal with respect to time and the first derivative of the power density value with respect to time. The stability factor is defined such that it decreases monotonically as the first derivative of the measurement signal with respect to time increases and as the first derivative of the power density value with respect to time increases.

[0039] For example, a first threshold is specified for the first derivative of the measurement signal with respect to time, and the stability factor is defined such that for all values of the first derivative of the measurement signal with respect to time that exceed the first threshold, the stability factor assumes the minimum value of its value range.

[0040] In addition, for example, a second threshold is specified for the first derivative of the power density value with respect to time, and the stability factor is defined such that for all values of the first derivative of the power density value with respect to time that exceed the second threshold, the stability factor assumes the minimum value of its value range.

[0041] Alternatively or additionally, a third threshold is specified for the first derivative of the measurement signal with respect to time, and the stability factor is defined such that for all values of the first derivative of the measurement signal with respect to time that are below the third threshold, the stability factor assumes the maximum value of its value range.

[0042] In addition, for example, a fourth threshold value is specified for the first derivative of the power density value with respect to time, and the stability factor is defined such that for all values of the first derivative of the power density value with respect to time that are below the fourth threshold value, the stability factor assumes the maximum value of its value range.

[0043] For example, the stability factor assumes a value in the interval [0, 1] such that the minimum value of its value range is 0 and the maximum value of its value range is 1.

[0044] In step 15 of the fifth method, the evaluation unit 7 forms a weighted average of the measurement signal acquired by the current sensor 3 in the first method step 11 and the measurement signal filtered by the band-pass filter 5 in the second method step 12 as the measured value of the current in amperes, where the weight of the measurement signal decreases as the stability factor increases, and the weight of the filtered measurement signal increases as the stability factor increases.

[0045] For example, according to C = A·(1 - S)+B·S, the weighted average C is formed as the measured value of the current in amperes, where A represents the measurement signal acquired by the current sensor 3 in the first method step 11, B represents the measurement signal filtered by the band-pass filter 5 in the second method step 12, and S represents the stability factor formed by the evaluation unit 7 in the fourth method step 14.

[0046] Although the present invention has been shown and described in more detail based on preferred embodiments, the present invention is not limited to the disclosed examples, and those skilled in the art can derive other variations therefrom without departing from the scope of protection of the present invention.

Claims

1. A method for measuring current, wherein - repeatedly obtaining a measurement signal depending on the current, - for each measurement signal, filtering the measurement signal using a band-pass filter (5) to form a filtered measurement signal, - for each measurement signal, determining a first derivative of the measurement signal with respect to time and a first derivative of the power density value with respect to time, wherein, the power density value is formed from the spectral power density of the measurement signal, and - forming a weighted average of the measurement signal and the filtered measurement signal as a measured value of the amperage of the current, wherein the weight of the measurement signal increases monotonically as the first derivative of the measurement signal with respect to time increases and as the first derivative of the power density value with respect to time increases.

2. The method according to claim 1, wherein, a stability factor is defined, the stability factor decreases monotonically as the first derivative of the measurement signal with respect to time increases and as the first derivative of the power density value with respect to time increases, and the weighted average of the measurement signal and the filtered measurement signal is formed using the stability factor such that the weight of the measurement signal decreases as the stability factor increases, and the weight of the filtered measurement signal increases as the stability factor increases.

3. The method according to claim 2, wherein, the stability factor is assigned a value within the interval [0, 1], and the weighted average is formed according to A·(1 - S)+B·S, where A represents the measurement signal, B represents the filtered measurement signal, and S represents the value of the stability factor.

4. The method according to claim 2 or 3, wherein, a first threshold is specified for the first derivative of the measurement signal with respect to time, and the stability factor is defined such that for all values of the first derivative of the measurement signal with respect to time exceeding the first threshold, the stability factor takes the minimum value of its value range.

5. The method according to any one of claims 2 to 4, wherein, a second threshold is specified for the first derivative of the power density value with respect to time, and the stability factor is defined such that for all values of the first derivative of the power density value with respect to time exceeding the second threshold, the stability factor takes the minimum value of its value range.

6. The method according to any one of claims 2 to 5, wherein, a third threshold is specified for the first derivative of the measurement signal with respect to time, and the stability factor is defined such that for all values of the first derivative of the measurement signal with respect to time below the third threshold, the stability factor takes the maximum value of its value range.

7. The method according to any one of claims 2 to 6, wherein, a fourth threshold is specified for the first derivative of the power density value with respect to time, and the stability factor is defined such that for all values of the first derivative of the power density value with respect to time below the fourth threshold, the stability factor takes the maximum value of its value range.

8. The method according to any one of the preceding claims, wherein, The band-pass filter (5) is a low-pass filter.

9. The method according to any one of the preceding claims, wherein, the power density value is formed by integrating the spectral power density of the measurement signal over a frequency range.

10. A measuring device (1) for measuring current, comprising: - a current sensor (3) configured to repeatedly acquire a measurement signal depending on the current, - a band-pass filter (5) configured to form a filtered measurement signal for each measurement signal, and - an evaluation unit (7) configured to: - determine, for each measurement signal, a first derivative of the measurement signal with respect to time and a first derivative of the power density value of the measurement signal with respect to time, wherein the power density value is formed from the spectral power density of the measurement signal, and - form a weighted average of the measurement signal and the filtered measurement signal as a measured value of the amperage of the current, wherein the weight of the measurement signal increases monotonically with an increase in the first derivative of the measurement signal with respect to time and with an increase in the first derivative of the power density value with respect to time.

11. The measuring device (1) according to claim 10, wherein, the band-pass filter (5) is a low-pass filter.

12. The measuring device (1) according to claim 10 or 11, wherein, the current sensor (3) is an optical current sensor.

13. The measuring device (1) according to any one of claims 10 to 12, wherein, the evaluation unit (7) is configured to form the power density value by integrating the spectral power density of the measurement signal over a frequency range.

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