MEASURING METHOD FOR DETERMINING THE CURRENT THROUGH A SHUNT RESISTOR

Abstract

What is proposed is a method for accurately determining an electric current (I.sub.in, I.sub.in,0) with reduced technical outlay, comprising: connecting a circuit branch (2, 12) in parallel with an inaccurate but current-loadable shunt resistor (R.sub.sh), wherein a reference resistor (R.sub.ref) that is more accurate in comparison with the shunt resistor (R.sub.sh) but less current-loadable is connected into the circuit branch (2, 12), such that the circuit branch (2, 12) branches off in each case at a node point (K) upstream and downstream of the shunt resistor, generating a temporally changeable reference current (I.sub.ref, I′.sub.ref, I″.sub.ref) through the circuit branch (2, 12), measuring the voltages (V′.sub.sh, V″.sub.sh, V′.sub.ref, V″.sub.ref) across the shunt resistor (R.sub.sh) and across the reference resistor (R.sub.ref), determining the current strength (I.sub.in, I.sub.in,0) upstream and downstream of the node point (K).

Claims

1. A method for accurately determining an electric current, the method comprising: connecting a circuit branch in parallel with an inaccurate but current-loadable shunt resistor, wherein a reference resistor that is more accurate in comparison with the shunt resistor but less current-loadable is connected into the circuit branch, such that the circuit branch branches off in each case at a node point upstream and downstream of the shunt resistor, generating a temporally changeable reference current through the circuit branch, measuring the voltages, across the shunt resistor and across the reference resistor, and determining the current strength upstream and downstream of the node point.

2. The method of claim 1, wherein the reference current is modified by connecting in the circuit branch in parallel with the shunt resistor and disconnecting it again, and/or the voltages are measured with and without the circuit branch connected in.

3. The method of claim 1, wherein a reference current source is connected into the circuit branch and the flow of current in the circuit branch is thereby increased and the flow of current through the shunt resistor is reduced.

4. The method of claim 1, wherein the current strength upstream and downstream of the node point is kept constant even in the event of a temporal modification in the reference current through the circuit branch.

5. The method of claim 1, wherein a series of values for the resistance of the shunt resistor and/or the current strength is determined on the basis of the measured voltages that are dropped across the shunt resistor and the reference resistor and then filtered.

6. The method of claim 5, wherein the series of values are filtered using one or more of: an average filter, median filter, or a low-pass filter.

7. A measuring device for accurately determining an electric current, comprising a circuit that has an inaccurate but current-loadable shunt resistor and a circuit branch that is able to be attached and/or connected in parallel with the shunt resistor, wherein a reference resistor that is more accurate in comparison with the shunt resistor but less current-loadable and at least one switch and/or a current source for generating a reference current is connected into the circuit branch such that the flow of current at the node point at which the circuit branch is connected in parallel with the shunt resistor branches into the two paths through the shunt resistor and the reference resistor and a reference current is able to flow through the circuit branch, wherein the circuit branch is designed to bring about a temporally changeable reference current, wherein the measuring device is designed to measure the voltages that are each dropped across the shunt resistor and across the reference resistor in order therefrom to determine the current strength that flows towards and/or away from the respective node point at which the circuit branch branches off.

8. The measuring device of claim 7, wherein the circuit branch has a switch in order to interrupt and/or to activate the flow of current through the circuit branch in order thereby to bring about the temporally changeable reference current.

9. The measuring device of claim 7, wherein the circuit branch comprises a reference current source in order thereby to bring about the temporally changeable reference current.

10. The measuring device of claim 7, wherein the reference current source is connected in series with the reference resistor in the circuit branch in order thereby to bring about the temporally changeable reference current.

11. The measuring device of claim 7, wherein the switch is formed by at least one transistor, in particular a field-effect transistor.

12. The measuring device of claim 11, wherein the at least one transistor comprises a field-effect transistor.

13. The measuring device of claim 7, wherein the measuring device is designed such that the current strength upstream and downstream of the node point is kept constant even in the event of a temporal modification in the reference current through the circuit branch.

14. The measuring device of claim 7, wherein the reference current source is galvanically isolated.

15. The measuring device of claim 7, wherein the reference current source in the circuit branch contains at least one solar cell.

16. The measuring device of claim 15, wherein the at least one solar cell comprises a solar cell exposed to infrared radiation generated by a light-emitting diode (LED).

17. The measuring device of claim 7, wherein the temporally changeable reference current is generated by way of at least one switch.

18. The measuring device of claim 17, wherein the temporally changeable reference current is generated by multiple field-effect transistors in a bridge circuit comprising a current source that changes little over time.

19. The measuring device of claim 18, wherein the current source comprises a solar cell.

20. The measuring device of claim 7, wherein the reference current source is connected such that the flow of current in the circuit branch is increased and the flow of current through the shunt resistor is reduced.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1 shows a schematic circuit diagram of a measuring device according to one example with a circuit branch that is able to be connected in,

[0030] FIG. 2 shows a schematic circuit diagram of a measuring device according to one example with a changeable reference current source,

[0031] FIG. 3 shows a schematic circuit diagram for implementing the switch for a measuring device according to FIG. 1, and

[0032] FIG. 4-5 show schematic circuit diagrams for implementing the reference current source for measuring devices according to FIG. 2, here with solar cells for galvanic isolation.

DETAILED DESCRIPTION

[0033] FIG. 1 shows a schematic circuit diagram of a measuring device 1 according to one example having a switch SW.sub.1 for connecting in or disconnecting a circuit branch 2.

[0034] The situation in which the circuit branch 2 is connected in, that is to say the switch SW.sub.1 is closed, is indicated by a “dash(′)”, and the situation with an open switch SW.sub.1 is indicated by “two dashes(″)” in the variables. With an open switch SW.sub.1:

I.sub.ref″=0

[0035] With a closed switch SW.sub.1:

I.sub.ref′=I.sub.ref,0

[0036] The resistances of the resistors, that is to say shunt resistor and reference resistor, are assumed to be constant for the short time between the switching alternations:

R.sub.sh′=R.sub.sh″=R.sub.sh,0

and

R.sub.ref′=R.sub.ref″=R.sub.ref,0

[0037] The circuit is also operated (the switching alternations are performed so quickly) that the current strengths I.sub.in may be assumed to be constant upstream and downstream of the node point at which the path into the branch through the shunt resistor and through the circuit branch containing the reference resistor branch, that is to say:

I.sub.in′=I.sub.in″=I.sub.in,0

[0038] For closed switches SW.sub.1, this gives:

[00001]$V_{\mathrm{sh}}^{\prime }=\frac{R_{\mathrm{sh}}^{\prime }.Math.R_{\mathrm{ref}}^{\prime }}{R_{\mathrm{sh}}^{\prime }+R_{\mathrm{ref}}^{\prime }}{I}_{\mathrm{in}}^{\prime }=\frac{R_{\mathrm{sh},0}.Math.R_{\mathrm{ref},0}}{R_{\mathrm{sh},0}+R_{\mathrm{ref},0}}{I}_{\mathrm{in},0}$

while with an open switch SW.sub.1:

V.sub.sh″=R.sub.sh″.Math.I.sub.in″=R.sub.sh,0.Math.I.sub.in,0

[0039] This ultimately gives overall for I.sub.in:

[00002]$V_{\mathrm{sh}}^{\prime }=\frac{\frac{V_{\mathrm{sh}}^{″}}{I_{\mathrm{in},0}}.Math.R_{\mathrm{ref},0}}{\frac{V_{\mathrm{sh}}^{″}}{I_{\mathrm{in},0}}+R_{\mathrm{ref},0}}{I}_{\mathrm{in},0}.fwdarw.I_{\mathrm{in},0}=\frac{1}{R_{\mathrm{ref},0}}.Math.\frac{V_{\mathrm{sh}}^{\prime }.Math.V_{\mathrm{sh}}^{″}}{\left(V_{\mathrm{sh}}^{″}-V_{\mathrm{sh}}^{\prime }\right)}$

[0040] The calculation is performed here assuming that the switch SW.sub.1 behaves like a mechanical switch and has a practically infinitely large resistance in the open state and has no ohmic resistance in the closed state.

[0041] For the rest, high current strengths I.sub.in should generally be expected.

[0042] FIG. 2 shows an embodiment similar to FIG. 1 (measuring device 11), but in which the flow of current through the circuit branch 12 is not completely interrupted, but rather in which the polarity of the reference current I.sub.ref alternates (phase 1, identified by a “dash(′)”, in reversal to phase 2, identified by a “dash(″)”, that is to say

I.sub.ref′=+I.sub.ref,0

and

I.sub.ref″=−I.sub.ref,0

[0043] For this purpose, use is made of a current source 13 that is connected into the circuit branch 12 in series with the reference resistor R.sub.ref and whose polarity is able to be alternated. The flow of current is furthermore set such that I.sub.in remains constant, that is to say:

I.sub.in′=I.sub.in″=I.sub.in,0

[0044] Regardless of the circuit situation, the resistance of the shunt resistor and reference resistor is assumed to be constant for the short time between the switching alternations, that is to say

R.sub.sh′=R.sub.sh″=R.sub.sh,0

and

R.sub.ref′=R.sub.ref″=R.sub.ref,0

[0045] This ultimately gives, for the two phases with different polarity of the reference current strength:

V.sub.sh′=(I.sub.in′−I.sub.ref′).Math.R.sub.sh′=(I.sub.in,0−I.sub.ref,0).Math.R.sub.sh,0

V.sub.ref′=I.sub.ref′.Math.R.sub.ref′=I.sub.ref,0.Math.R.sub.ref,0

and

V.sub.sh″=(I.sub.in″−I.sub.ref″).Math.R.sub.sh″=(I.sub.in,0−I.sub.ref,0).Math.R.sub.sh,0

V.sub.ref″=I.sub.ref″.Math.R.sub.ref″=I.sub.ref,0.Math.R.sub.ref,0

[0046] Rearrangements ultimately give:

[00003]$\frac{V_{\mathrm{ref}}^{\prime }-V_{\mathrm{ref}}^{″}}{2R_{\mathrm{ref},0}}=\frac{I_{\mathrm{ref},0}.Math.R_{\mathrm{ref},0}+I_{\mathrm{ref},0}.Math.R_{\mathrm{ref},0}}{2R_{\mathrm{ref},0}}=I_{\mathrm{ref},0}$

and also

[00004]$\frac{V_{\mathrm{sh}}^{″}+V_{\mathrm{sh}}^{\prime }}{V_{\mathrm{sh}}^{″}-V_{\mathrm{sh}}^{\prime }}=\frac{\left(I_{\mathrm{in},0}+I_{\mathrm{ref},0}\right).Math.R_{\mathrm{sh},0}+\left(I_{\mathrm{in},0}-I_{\mathrm{ref},0}\right).Math.R_{\mathrm{sh},0}}{\left(I_{\mathrm{in},0}+I_{\mathrm{ref},0}\right).Math.R_{\mathrm{sh},0}-\left(I_{\mathrm{in},0}-I_{\mathrm{ref},0}\right).Math.R_{\mathrm{sh},0}}=\frac{I_{\mathrm{in},0}}{I_{\mathrm{ref},0}}$

from which the following is concluded for the current strength

[00005]$\frac{V_{\mathrm{ref}}^{\prime }-V_{\mathrm{ref}}^{″}}{2R_{\mathrm{ref},0}}.Math.\frac{V_{\mathrm{sh}}^{″}+V_{\mathrm{sh}}^{\prime }}{V_{\mathrm{sh}}^{″}-V_{\mathrm{sh}}^{\prime }}=I_{\mathrm{in},0}$

[0047] This thus gives, to within a factor of ½, a formula similar to the exemplary embodiment according to FIG. 1.

[0048] The exemplary embodiments according to FIGS. 1 and 2 have the common feature that only voltages, which are also able to be measured very accurately, are required and have to be measured. The resistance of the reference resistor is likewise very accurately known.

[0049] It is also possible to accurately determine the measurement current I.sub.in,0 and the resistance R.sub.sh,0 of the shunt resistor when the absolute values of the reference current during the two phases are not identical, that is to say:

|I.sub.ref′|≠|I.sub.ref″|

[0050] The following relationships apply here:

[00006]$I_{\mathrm{ref}}^{\prime }=\frac{V_{\mathrm{ref}}^{\prime }}{R_{\mathrm{ref}}^{\prime }}=\frac{V_{\mathrm{ref}}^{\prime }}{R_{\mathrm{ref},0}}$$I_{\mathrm{ref}}^{″}=\frac{V_{\mathrm{ref}}^{″}}{R_{\mathrm{ref}}^{″}}=\frac{V_{\mathrm{ref}}^{″}}{R_{\mathrm{ref},0}}$$R_{\mathrm{sh},0}=\frac{V_{\mathrm{sh}}^{″}-V_{\mathrm{sh}}^{\prime }}{\left(I_{\mathrm{in},0}-I_{\mathrm{ref}}^{″}\right)-\left(I_{\mathrm{in},0}-I_{\mathrm{ref}}^{\prime }\right)}=\frac{V_{\mathrm{sh}}^{″}-V_{\mathrm{sh}}^{\prime }}{I_{\mathrm{ref}}^{\prime }-I_{\mathrm{ref}}^{″}}$

this gives:

[00007]$R_{\mathrm{sh},0}=R_{\mathrm{ref},0}\frac{V_{\mathrm{sh}}^{″}-V_{\mathrm{sh}}^{\prime }}{V_{\mathrm{ref}}^{\prime }-V_{\mathrm{ref}}^{″}}$

[0051] Therefore, for each switching cycle, the current resistance of the shunt resistor may be determined purely from the measurable voltages and the known resistance of the reference resistor.

[0052] If the ascertained values of the resistance of the shunt resistor over multiple switching cycles, which are ascertained at times t=t.sub.1, t=t.sub.2, etc., are joined together, then it is possible to form a shunt resistor signal R.sub.sh,0(t):

R.sub.sh,0(t)={R.sub.sh,0|t=t.sub.1,R.sub.sh,0|t=t.sub.2 . . . }

[0053] It should be expected that this shunt resistor signal, due to noise in the voltage measurements for determining V′.sub.sh, V″.sub.sh, V′.sub.ref, and V″.sub.ref, will in turn contain noise, that is to say fast and small random changes. Since it should be expected that the resistance change, to be expected due to the heating of the shunt resistor caused by the current loading, will however take place relatively slowly, for example over a time interval of a few seconds, the shunt resistor signal may also be filtered in order to improve accuracy. Applying a filter function f to the shunt resistor signal R.sub.sh,0(t) gives the filtered shunt resistor signal R*.sub.sh,0(t):

R.sub.sh,0*(t)=ƒ(R.sub.sh,0(t))

[0054] An average filter, median filter, low-pass filter or other filter function common in signal processing may be used as suitable filter function f, for example.

[0055] The measurement current I.sub.in,0 may then be ascertained using the following equation:

[00008]$I_{\mathrm{in},0}=\frac{1}{2}\left(\left(\frac{V_{\mathrm{sh}}^{″}}{R_{\mathrm{sh},0}^{*}\left(t\right)}+I_{\mathrm{ref}}^{″}\right)+\left(\frac{V_{\mathrm{sh}}^{\prime }}{R_{\mathrm{sh},0}^{*}\left(t\right)}+I_{\mathrm{ref}}^{\prime }\right)\right)$$\mathrm{Or}:$$I_{\mathrm{in},0}=\frac{1}{2}\left(\left(\frac{V_{\mathrm{sh}}^{″}}{R_{\mathrm{sh},0}^{*}\left(t\right)}+\frac{V_{\mathrm{ref}}^{″}}{R_{\mathrm{ref},0}}\right)+\left(\frac{V_{\mathrm{sh}}^{\prime }}{R_{\mathrm{sh},0}^{*}\left(t\right)}+\frac{V_{\mathrm{ref}}^{\prime }}{R_{\mathrm{ref},0}}\right)\right)$

[0056] The measuring device according to one example may thus be used: [0057] to determine a current strength very accurately, [0058] even when a very high current strength is involved, [0059] and also to precisely measure the unknown resistance of the shunt resistor.

[0060] FIG. 3 shows a schematic illustration of how it is possible to implement the switch SW.sub.1 that is required for the embodiment according to FIG. 1: The light-emitting diode 31 (emission in the infrared region) is supplied by a voltage source 32; the circuit is switched via the field-effect transistor A.

[0061] In order to perform complete galvanic isolation, a solar cell may for example be used as current source. In a manner similar to an optocoupler circuit, the light-emitting diode 31 illuminates a solar cell 34, which in turn switches a field-effect transistor B, such that this causes either the off state or the on state.

[0062] Finally, FIGS. 4 and 5 each show exemplary embodiments in which the reference current source 13 is able to be modified in terms of its current strength by alternating the polarity of the current. The variant embodiment according to FIG. 4 uses a bridge circuit (also called H circuit) to alternate the polarity. Depending on whether the field-effect transistor pairs A-A′ or B-B′ are respectively in the on state or off state, the current of the solar cell contributes to increasing or to reducing the reference current strength I.sub.ref. In this case too, the solar cell 34 is illuminated by an infrared light-emitting diode. The switching in order to modify the reference current strength I.sub.ref takes place solely through the transistors A-A′ or B-B′ in the circuit branch that also comprises the solar cell 34.

[0063] However, two solar cells 54, 55 could also be connected in antiparallel instead. From the point of view of the reference current I.sub.ref, the polarity depends on which of the solar cells 54, 55 is illuminated.

[0064] Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

LIST OF REFERENCE SIGNS

[0065] 1 Measuring device [0066] 2 Circuit branch [0067] 11 Measuring device [0068] 12 Circuit branch [0069] 13 Reference current source [0070] 31 Light-emitting diode [0071] 32 Voltage source [0072] 34 Solar cell [0073] 51 Infrared light-emitting diode [0074] 52 Infrared light-emitting diode [0075] 54 Solar cell [0076] 55 Solar cell [0077] A Field-effect transistor [0078] A′ Field-effect transistor [0079] B Field-effect transistor [0080] B′ Field-effect transistor [0081] I.sub.ref,0 Reference current strength [0082] I.sub.ref′ Reference current strength (phase 1) [0083] I.sub.ref″ Reference current strength (phase 2) [0084] I.sub.in′ Current strength (phase 1) [0085] I.sub.in″ Current strength (phase 2) [0086] I.sub.in Current strength [0087] I.sub.in,0 Current strength [0088] K Node point [0089] R.sub.sh′ Resistance of shunt resistor (phase 1) [0090] R.sub.sh″ Resistance of shunt resistor (phase 2) [0091] R.sub.sh,0 Resistance of shunt resistor [0092] R.sub.sh Shunt resistor [0093] R.sub.sh,0(t) Shunt resistor signal [0094] R.sub.sh,0*(t) Filtered shunt resistor signal [0095] ƒ Filter function [0096] R.sub.ref′ Resistance of reference resistor (phase 1) [0097] R.sub.ref″ Resistance of reference resistor (phase 2) [0098] R.sub.ref,0 Resistance of reference resistor [0099] R.sub.ref Reference resistor [0100] V.sub.sh′ Voltage across shunt resistor (phase 1) [0101] V.sub.sh″ Voltage across shunt resistor (phase 2) [0102] SW.sub.1 Switch