EP1529220A2

EP1529220A2 - Method and circuit arrangement for current measurement

Info

Publication number: EP1529220A2
Application number: EP03790695A
Authority: EP
Inventors: Reinhard Maier; Jürgen RUPP
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2002-08-14
Filing date: 2003-08-07
Publication date: 2005-05-11
Also published as: DE10237920B3; WO2004021021A3; US7242177B2; WO2004021021A2; US20060164069A1

Abstract

A method with corresponding circuit arrangement is disclosed, whereby for evaluation of a measured signal arising as an analogue value with a potential greater than zero potential in a measuring device requiring a supply current, the measured information therefrom is transmitted as a digital signal to an analytical unit lying at earth potential after an A/D conversion. The generated digital signal thus provides the clock signal for a modulation of the supply current and thus also carries out the function of the carrier for the information content of the measured signal. According to the invention, curved characteristic lines for compression/decompression of the signals are used. Suitable means for temperature compensation are similarly provided.

Description

description

Method and circuit arrangement for current measurement

The invention relates to a method for current measurement according to the preamble of claim 1. In addition, the invention also relates to the associated circuit arrangement.

A known method for current measurement is to measure the voltage drop using a shunt resistor. To measure di- gital-to-analog converter (ADC's) are used in modern measuring and switching devices frequently, which convert the analog signal into a digital measurement ^¬ measured value. The following problems can occur: a) The measurement signal has a high dynamic range, since small currents (<10% nominal current) still have to be measured with an acceptable resolution and large currents (> = 10 times the nominal current) also have to be recorded. b) The accuracy must not be adversely affected by the influence of the ambient temperature. c) The electronic components (ADC, processor, etc.) must be supplied with power. Often two (galvanically) separate power supplies are necessary, since components are available both at low potential (N potential or artificial star point) and at high outer conductor potential. d) In addition to the outer conductor current, the outer conductor voltage must often also be measured against N potential. e) In the event of an overload or short circuit, the main circuit must be switched off.

From the applicant's older, non-prepublished DE 101 05 982 AI, a method of the type mentioned at the outset is known in which a measurement signal is at a potential which is higher than zero potential and a measuring device is available which requires a supply current, the measurement signal nal arises as an analog value and its measurement information is transmitted after an A / D conversion as a digital signal to an evaluation unit which is at ground potential and, after the A / D conversion of the measurement signal, the digital signal generated provides the clock for a modulation of the supply current, which therefore likewise fulfills the function of the carrier for the information content of the measurement signal. The following measures are taken to master the aforementioned problem points: a) High dynamics: analog-digital converters with a linear characteristic and a correspondingly high resolution (number of bits) are used. b) Influence of the ambient temperature on the accuracy: Special (expensive) alloys are chosen for the shunt that have a small temperature coefficient. c) The voltage is supplied either directly to an ADC at low potential via voltage converters, ie using transformers, or via voltage dividers.

Based on the latter, it is the object of the invention to propose technical improvements in a method of the type mentioned at the outset and to create a suitable circuit arrangement for this.

The object is achieved by the measures of the method claims. Associated circuit arrangements are specified in the claims.

The invention is essentially characterized by the following measures or features: a) Dynamics: The ADC is not realized with a linear, but with a curved characteristic. This compresses the measurement signal and a lower number of bits is sufficient for the same dynamics. This method, which is known under the term compression, is intended to be used advantageously for current measurement. Possible characteristics are logarithmic characteristics (with special those zero treatment), the constants kon the advantage of a ^¬, relative accuracy, ie errors based on the measured value, having, or a root function, in a microcontroller, provides the advantage of simple expansion by squaring, for example. b) Influence of the ambient temperature on the accuracy: No special resistance alloy is chosen for the shunt, but a piece of conductor material that is already present is used. To increase the local resistance (for the shunt), the cross-section is reduced by constrictions. The high temperature coefficient of copper is compensated by the fact that the ADC has a reference voltage that has a temperature coefficient that is as large and as constant as possible. The measuring unit, which is at a high potential anyway, can be mounted in good thermal contact with the copper bar used, so that both are at the same temperature level. c) Power supply: The subject of the already mentioned DE 101 05 982 AI with the designation 'method for evaluating a measured value and associated circuit arrangement ^λ is, inter alia, a method for measuring current at potential, in which the supply current of the components at potential is obtained directly from the Relative to the network and transmit the measured value to the low potential by modulating the supply current. This basic idea can now advantageously be expanded such that, in addition to the supply current of the components at high potential, the supply current of the components at low potential is also obtained directly from the network. In order to prevent the loss line from becoming unnecessarily high, the voltage dividers can also be implemented partly with ohmic resistors and partly with capacitors. d) Line voltage measurement: The line voltage is measured with the help of an existing voltage divider. To do this, either the voltage drop across the upper resistor R _{Lx is added to} an additional channel of the measuring device. tung supplied to potential or the voltage drop at the lower resistance R _x to an ADC of Auswerteeinrich ^¬ processing at a low potential. For this purpose, reference is made in detail to the figure β described below.

Compared to the prior art, the following advantages are achieved in particular with the invention: a) High dynamics: The proposed method has the advantage that cheaper ADCs can be used which do not have to provide a high resolution over the entire measuring range. b) Influence of the ambient temperature on the accuracy: The proposed method has the advantage that no expensive special alloy has to be used, but a piece of conductor material that is already present is used. In addition, there is no need to attach (screw, rivet) the resistance element to the copper. Nevertheless, the temperature compensation achieves high measuring accuracy. c) Power supply: The proposed method has the advantage that costs, weight and space requirements are reduced, d) The voltage drop across the resistors occurs in any case with the current measuring device described and can therefore be used with simple means and inexpensively for the additional voltage measurement.

Further details and advantages of the invention result from the following description of figures of exemplary embodiments with reference to the drawing in conjunction with the patent claims. Show it

FIG. 1 shows the arrangement of a measuring device with a shunt at high potential and the shunt resistance,

FIG. 2 shows a logarithmically compressed code for use in the measuring device according to FIG. 1, 3 shows a logarithmically expanded measured value USAGE ^¬ dung at the measuring device according to Figure 1, Figure 4 a corresponding to a square root curve Kompri ^{¬-programmed} code as an alternative to Figure 2, Figure 5 is a corresponding expanded a square function measured value as an alternative to Figure 3. Figure 6 shows an arrangement for current and voltage measurement in a three-phase system and FIG. 7 shows an alternative arrangement with means for short-circuit and / or overload shutdown.

The older DE 101 05 982 AI, the disclosure content of which is part of the subject matter of the present application, is essentially assumed below. Irrespective of this, the subject of the application can be applied in a generalized form to a current and voltage measurement.

An advantageous arrangement of shunt resistance and measuring device is shown in FIG. According to FIG. 1 of DE 101 05 982 AI, a shunt 1 is present in a phase L1 which is at a high potential, from which the measuring voltage Udiff 1 is passed via an amplifier 2 to an analog-digital converter 3 as part of a measuring device 5. The voltage difference Uiff _/, which is a measure of the current through the shunt 1, is converted into a binary measured value by the analog / digital converter (ADC) 3 and transmitted as a binary signal to ground potential. A unit 4 for signal compression is connected upstream of the ADC 3, with which, in particular, great signal dynamics can be taken into account in a cost-effective manner, the binary signal being subjected to a reverse expansion after it has been transmitted. The latter can be carried out by software in the relevant evaluation unit, in particular in the microcontroller that is usually present, at ground potential.

When measuring current via a shunt, the influence of the ambient temperature on the accuracy is known and not too neglect: The shunt resistance is temperature dependent according to the function:

R _s = R ₀ (l + a * T) (1)

With the aid of FIG. 1, it is further clarified that the temperature response of the shunt resistor is compensated for by thermal coupling of the shunt 1 and measuring device 5 by an equally large temperature response of a reference voltage source 6 with the resistance corresponding function U _ref = U ₀ (l + a * T) (2) ,

The signals for a logarithmic compression / expansion are shown in FIGS. 2 and 3, in FIGS. 4 and 5 for a root / square compression / expansion. The relevant characteristic curves are designated with 21, 31, 41 and 51.

FIG. 6 shows a measuring device for the current and voltage measurement in a 3-phase system with phases L1, L2 and L3. The evaluation channel for Ll is complete, the identical channels for L2 and L3 only partially. It is essential here that three identical shunt resistors 61, 61, 61 ^{Λ λ are present} in the individual channels, each of which is connected to a measuring unit 60 corresponding to the measuring device 5 according to FIG. 1.

Both the voltage supply at high and the voltage supply at low potential are obtained from the network via voltage dividers 66, 66 and 67, 67 ^Λ . Furthermore, resistors 69, 69 at high potential and further switching elements, such as capacitors and blocking diodes, are present, with the means for signal rectification being generally referred to as 64, 64 ^λ .

The switching elements with which the desired signal compression or the reverse signal expansion in this regard corresponds to that shown by way of example in FIGS. 2 to 5 Characteristic curves is effected is integrated in the measuring unit 60 in FIG.

In Figure 6, the neutral conductor N is connected to the consumers 70, 70 ^λ , 70 ^V and a star point is formed. Al ternatively ^¬ the three voltage dividers form an artificial neutral point in the switching device when the neutral conductor is not connected, which corresponds to cost reasons, the normal case.

The voltage drops at the lower or upper resistors can alternatively be used for the voltage measurement. If the upper voltage is measured, both measurements, i.e. Current and voltage transferred to low potential via the modulated supply current.

A circuit arrangement with a measuring unit 60 corresponding to FIG. 6 is constructed in FIG. Voltage dividers are controlled by the shunt 71 with circuit elements 72 via a control unit 73 and a rectifier 74: in detail, voltage dividers are constructed here from resistors 76, 76 ^λ and 77, 77 ^λ and alternatively 79, 79 ^λ , the power supply alternatively being high or low potential.

A differential amplifier 78 is connected downstream of the amplifier 2 present in FIG. 1 and the AD converter 3, which is capacitively coupled to the voltage dividers by means of capacitors which are not further designated.

In addition, means for short-circuit or overload shutdown are available: for this purpose, the circuit contains, via the elements already described, a device comprising two comparators 85 and 95 with two mutually independent, respectively adjustable threshold values. The first comparator 85 compares the current value with the threshold I and gives a signal for short-circuit detection when exceeded with which the main circuit can be switched off. The second comparator 95 compares the current temperature ^¬ turwert the load which is obtained by means of a thermal model 94, with the threshold II and are at exceeded, a signal for overload detection, with which the main circuit can be switched off. Only one of the two comparators can also be provided.

From the parallel voltage dividers with resistors 76, 76 and 77, 77 ^Λ and 79, 79, a signal is sent to evaluation unit 75 at a low potential via a differential amplifier 78. The evaluation unit 75 can be a microcontroller and corresponds to the unit 65 from FIG. 6.

The described method with associated circuit arrangement is particularly suitable for current measurement for potential and evaluation of the measurement signals which are obtained as an analog value at a potential which is higher than zero potential and whose measurement information is transmitted after an A / D conversion as a digital signal to an evaluation unit which is at ground potential. The generated digital signal provides the clock for a modulation of the supply current and thus also fulfills the function of the carrier for the information content of the measurement signal. Curved characteristic curves are advantageously used for the compression / decompression of the signals. Suitable means for temperature compensation are also available.

Claims

claims

1. A method for measuring current to a potential which lies on a opposite zero potential higher value, the current value is measured as an analog signal and its Informa ^¬ tion after A / D conversion is transmitted as a digital signal to an on-ground evaluation unit, characterized in that the analog signal is subjected to compression before the A / D conversion and transmission and that the digital signal is subjected to an expansion after transmission to earth potential.

2. The method of claim 1, d a d u r c h g e k e n n z e i c h n e t that the compression and expansion are logarithmic.

3. The method of claim 1, d a d u r c h g e k e n n z e i c h n e t that the compression and expansion take place according to the specification of root functions.

4. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that a temperature compensation takes place.

5. The method according to claim 4, so that a thermal coupling of the measuring device and the shunt is carried out for temperature compensation.

6. The method according to any one of the preceding claims, in which for the evaluation of a measurement signal that is at a potential higher than zero potential as an analog value in a measuring device that requires a supply current, with the following measures: - The compressed information content of the measurement signal is determined according to the A / Transmit D-conversion as digital signal to the evaluation unit which is at ground potential, - After the A / D conversion of the measurement signal, the generated digital signal provides the clock for a modulation of the supply current, so that the modulated supply current of the measuring device likewise fulfills the function of the carrier for the information content of the measurement signal.

7. Circuit arrangement for carrying out the method according to claim 1 or one of claims 2 to 6, for use in current measurement on a shunt, in which the voltage drop is evaluated as a measure of the current after amplification, with a shunt (1; 61 , 61 ^λ , 61; 71), an amplifier (2) for the voltage signal tapped at the shunt (1; 61, 61 ^λ , 61 ^λX ; 71), an analog / digital converter (3) and an evaluation unit (5, 65 , 7) and with means for supplying power to the measuring components (2, 3), characterized in that further means (4, 6) for signal compression and signal expansion are present.

8. Circuit arrangement according to claim 7, d a d u r c h g e k e n n z e i c h n e t that additional means for temperature compensation are available.

9. Circuit arrangement according to claim 7, d a d u r c h g e k e n n z e i c h n e t that a unit for signal compression is connected in front of the A / D converter (3).

10. Circuit arrangement according to claim 7, so that the means for signal expansion are integrated in the evaluation unit (5, 65, 75), preferably software-wise in the existing microcontroller.

11. Circuit arrangement according to claim 8, characterized in that means for temperature compensation have a temperature-dependent reference voltage source (6).

12. Circuit arrangement according to one of claims 7 to 11, d a d u r c h g e k e n n z e i c h n e t that means

(85, 95) for short-circuit shutdown and / or overload shutdown are available.

13. Circuit arrangement according to claim 12, so that a first comparator (85) is present which compares the instantaneous value of the current with a first threshold value and generates a signal for short-circuit shutdown when exceeded.

14. Circuit arrangement according to claim 12, so that a second comparator (95) is present which compares the instantaneous temperature of the load with a second threshold value and outputs a signal for overload shutdown when exceeded.

15. The circuit arrangement as claimed in claim 14, so that a thermal model (94) of the load is present, with which the current temperature of the load (80) is determined from the measured current.