WO2008071722A1

WO2008071722A1 - Measuring apparatus for measuring an electrical current

Info

Publication number: WO2008071722A1
Application number: PCT/EP2007/063752
Authority: WO
Inventors: Reinhard Maier; Heinz Mitlehner; Jürgen RUPP
Original assignee: Siemens Aktiengesellschaft
Priority date: 2006-12-13
Filing date: 2007-12-12
Publication date: 2008-06-19
Also published as: DE102006058879A1

Abstract

A measuring apparatus for measuring an electrical current (I) comprises: two connecting nodes (7, 8) for feeding in and discharging the current (I), a measuring network (9) which is arranged between the connecting nodes (7, 8) and contains: an element (10), through which at least one known partial current (I<SUB>1</SUB>) of the current (I) can flow and which has a total resistance (R) which is dependent on the partial current (I<SUB>1</SUB>), and two measuring nodes (13, 14) for tapping off a measuring voltage (U), wherein the latter has a dependence on the current (I) that can be predefined by configuring the measuring network (9).

Description

description

Measuring device for measuring an electric current

The invention relates to a measuring device for measuring an electric current.

A current flowing in an electrical conductor of electric current I can be determined for example by measuring the voltage U via a so-called shunt resistor R _S h _u n _t, where the shunt resistor is connected to the conductor in series and is thus flowed through by the current to be measured I. The current Iv which is a shunt resistor R _S hunt parallel geschal ^¬ tetes voltmeter flows through, as is negligibly small (I _v << I) and the shunt resistor R _S in the hunt Messbe ^¬ rich (I _MES s: I _min ≤ i ≤I _m aχ) is assumed to be constant, so that the measured voltage U is proportional to the current I to be measured according to Ohm's law: U = R _S hunt I shunt resistors are produced specifically for the current measurement so that this proportionality according to the ohmic Law is very well met. Thus, for example, the influence of contact voltages on the terminals of the shunt resistor leading to the current I to be measured in the so-called four-point measurement is avoided by tapping the voltage U at two further, almost no-current terminals. Also, thermal influences are reduced by suitable dimensioning of the shunt resistor Rshunt specifically for the measuring range I _meS s. A Shuntwi ^¬ resistor R _shunt for current measurement can therefore be assumed hereinafter for the operation principle of the power measurement to be constant and thus as a linear component.

Alternatively, the current I can also be measured with current clamps via a magnetic coupling of the magnetic flux generated by the current to be measured and conducted and detected by means of a permeable core to form a sensor circuit. Alternating currents can be measured by means of a coil wound around the core, eg a ferrite core. For measuring direct currents, a current clamp with a Hall be equipped sensor. The current measuring range of such current clamps depends inter alia on the technical data (in particular the saturation induction) of the permeable core.

In the current measurement with shunt increases for high currents I, the electrical power loss P _v in the measuring device, in particular in the shunt resistor R _S hunt and P _v = Rstumt I ² . In order to keep the heat development in the shunt shunt, the power loss P _v should not exceed an upper limit P _V m _a χ. Apart from this excessive heating of the shunt resistor R _S h _u n _t performs the power loss, if appropriate, to the destruction of the shunt resistor. Furthermore, the measurable voltage U is superimposed on a noise as noise voltage U _N and can reduce the measuring accuracy for low currents I.

In addition to further influences by signal amplifiers and sensors used for current measurement, the lower limit I _{min of} the current measuring range is therefore predetermined at least by a minimum required measuring accuracy and the upper limit I _ma χ approximately by a maximum tolerable electrical power loss Pv.

The object of the invention is to specify an improved measuring device for measuring an electric current I.

The object is achieved according to claim 1 by a measuring device for measuring an electric current (I), which is flowed through by the current to be measured via two connection nodes. Between the connection nodes a measuring network is interconnected. This is at least passed through by a partial flow (Ii) of the stream (I). The proportion (Ii) of (I) is known here, so that the unknown current is known when (Ii) is determined. In the measurement network, an element (10), which has a total resistance (R) dependent on the partial flow (Ii), flows through it. Furthermore, the measuring network comprises two measuring nodes (13, 14) for tapping a measuring voltage (U) dependent on the current to be measured (I). This dependence is at least in a measuring range, for example, the above-mentioned I _me ss, unique and known.

The measuring voltage is thus dependent on the current I in a different way than the known current measuring method, namely, virtually any desired current-measuring voltage characteristic can be realized by the nonlinear element. As a result, for example, signal compression, power loss reduction, etc. possible.

The measuring device according to the invention is thus versatile. It is particularly advantageous, for example, for use in contactors, which can carry high peak currents I _pea k, but sometimes have to be switched even with comparatively small currents I _min , wherein these limit values can exceed, for example, a ^peak ratio of 1000: 1 NEN. The measuring device then has a measuring range including the limit values with consistently high accuracy, and works non-destructively and reliably in the measuring range and possibly consumes only a small power loss P _v .

The measurement network can have several non-linear elements. By suitable interconnection, these may be considered in their entirety as a single non-linear element.

The partial flow (Ii) may be the total current to be measured (I), so that the nonlinear element is traversed by the total current to be measured (I).

One or two of the measuring network measuring terminals may be short-circuited to one terminal each of the non-linear element. Thus, for example, the measuring voltage (U) in a context corresponding to Ohm's law U = R Ii with the partial current (Ii) and the total resistance (R) of the non-linear element. The non-linear element may have a decreasing total resistance (R) as the partial current (Ii) increases. In this way, an extension of the measuring range can be achieved compared to an ordinary measuring device with constant shunt resistor (R _S h _u n _t). This is based, in particular, on the fact that the power loss of the measuring device according to the invention for increasing the amount of current | I | due to the decreasing total resistance (R) weaker than the square I ^{2 of} the current increases P _V = RI ² , such as the power loss

a measuring device with constant Shuntwider ^¬ was Rshunt • particularly advantageous a measurement network is laid out ^¬, the electrical power loss in sections, has at least no dependence on the current to be measured. In this way, thermal influences on the measurement result or the measurement voltage (U) can be reduced by keeping the temperature in the area of the measurement network very constant. This can be done, for example, by interconnecting a nonlinear element with a total resistance (R), which at least partially reciprocally to the square of the partial flow (Ii) behaves according to R -.

Alternatively, the non-linear element may have an increasing total resistance (R) with increasing partial current (I ₁ ). Although the power loss in the measuring device can not be reduced in this way, an improvement in the measuring accuracy can be achieved for low currents (I).

A nonlinear element may contain a diode or, in the simplest case, consist of only a single diode. In one embodiment, the non-linear element consists of a parallel connection of a diode and an ohmic resistor. The measuring range can be extended by anti-parallel connection of an additional diode by, for example, both the anode of the first diode with the cathode of the second diode is short-circuited at a first terminal of the non-linear element as well as at the second terminal of the non- linear element, the anode of the second diode to the cathode of the first diode. The two terminals of the element can also be connected to each other via an ohmic resistance, which is thus connected in parallel to the two diodes.

In a further embodiment, the non-linear element includes a transistor, wherein also an anti-parallel connection of a plurality of transistors can be used. For example, at a first terminal of the element the source terminal of a first MOS-FET is short-circuited to the drain terminal of a second MOS-FET and at a second terminal of the element the drain terminal of the first MOS-FET is connected to the source terminal Connection of the second MOS-FET short-circuited. The gate terminal of the MOS FETs may be fed back to one of the terminals of the nonlinear element via a suitable element.

The measuring network can be an active electrical network to which electrical energy is supplied via a further connection.

The measurement network may include an amplifier or a regulator. The controller can be connected in particular for controlling the total resistance (R) of the non-linear element in the measurement network. The non-linear characteristic of the total resistance (R) can thus be dimensioned accordingly for complex applications.

The measuring device can be equipped with a computing device which detects and processes the measuring voltage (U). This can be a digital computer with a microcontroller.

In a further embodiment, the measuring device contains a sensor which detects the temperature in the region of the measuring device. In order to reduce the temperature dependence of the measuring voltage (U), a suitable compensation means may be provided. be present, for example in the form of interconnected in the measurement network components or a suitable, including the temperature detecting equipment of the computing device.

The computing device for determining the value of the current to be measured (I) from the measurement voltage (U) may be equipped in such a way that the current (I) from the measurement voltage (U) is actually calculated for each measurement or a table for this calculation is used. Here, e.g. Temperature dependence can be considered or compensated

The invention will now be described with reference to the appended drawings

Drawings explained in more detail. Show it

Fig. 1 is an electrical network, with current to be measured

2 shows a measuring network with a single, non-linear electrical component,

3 shows a measuring network with a non-linear electrical see component,

4 shows an element with diode,

5 shows an element with diodes,

6 shows an element with MOS-FET,

7 shows an element with MOS FETs, FIG. 8 shows an alternative element with MOS FETs, FIG.

9 shows a measuring network with a controller.

FIG. 1 shows an arbitrary electrical network (1), which may be part of a device or a machine, not shown here. A single path (2) of the electrical network (1) carries an electrical current (I), which is to be detected metrologically. The path (2) can in this case be any electrical conductor, for example a conductor track on a circuit board or a cable, and extends through two nodes (3) and (4), which in FIG. 1 have a direct connection (5) and both flows through the current to be measured (I). For measuring the current (I), the direct connection (5) between the nodes (3) and (4) of the path (2) in the network (1) of Figure 1 is separated and by a measuring device (6) according to Figure 2 in series connection replaced. In this case, the measuring device (6) is connected to a connection node (7) to the node (3) of the path (2) and to a further connection node (8) to the node (4) of the path (2).

Between the nodes (7) and (8) of the measuring device is electrically conductive an electrical measuring network (9) with its at least two terminals (11) and (12) connected, which in Figure 2 from only a single nonlinear element (10) according to the invention. The element (10) may contain, for example, a hot or a cold conductor (negative temperature coefficient resistor, NTC resistor, positive temperature coefficient resistor, PTC resistor). The element (10) can also be a passive or active electrical network from the interconnection of individual components. As is usual in electrical block diagrams, supply lines of a possibly configured as an active network element (10) are not shown for the functioning principle of the current measurement irrelevant, only required for power supply.

The measuring network (9) has two measuring nodes (13) and (14), which are electrically conductively connected to the terminals (7) and (8) of the measuring device (6). In the present case, the measuring node (13) is connected via an electrical short circuit (15) to the terminal (7) and the measuring node (14) in the same way to the terminal (8). Between the measuring nodes (13) and (14) drops a measuring voltage (U), which can be measured with a detection device, not shown, for example, a voltmeter, oscilloscope or similar. The measuring voltage (U) is at least in a measuring range, for example (I _mess ) _/ so, in a clear relationship with the current to be measured in the path (2) current (I). Figure 3 shows an alternative measuring device (6), wherein between a measuring node (13) and a terminal (7) of the measuring device (6) instead of a short-circuit connection (15), the element (10) is connected. Furthermore, in the measuring device (6) shown, although the entire current (I) to be measured flows through the measuring network (9), but not entirely through the element (10). The element (10) is traversed only by a partial flow (Ii), whose connection to the current (I) to be measured is impressed by the current divider circuit formed by the components (Ri, R ₂ ), so that the

Voltage (U) and the current to be measured (I) as described above, at least in a measuring range (I _mess ) are in a clear relationship.

FIGS. 4 to 8 show embodiments of electrical networks for implementing a respective non-linear element (10). The element (10) in FIG. 4 consists of the parallel connection of a diode (Di) and an ohmic resistor (Ri) and can be used primarily for positive currents flowing in the direction from the node (11) to the node (12).

By antiparallel connection of another diode (D ₂ ), as shown in Figure 5, the element can also be used for negative currents or alternating currents.

FIG. 6 shows an element (10) which comprises a self-blocking n-channel metal oxide layer field-effect transistor (MOS-FET) (Ti). Its drain terminal (16) is connected to the terminal (11) and the source terminal (17) to the terminal (12). The voltage (U _G s) between the gate (18) and the source terminal (17) of the MOS-FET (T _x ) is set via an element (20) which here on the one hand to gate terminal (18) and on the other hand connected to the terminal (12). The element (20) may be a passive or an active element or a regulator and possibly further, not shown connections for supplying electrical energy, but which have no indirect, functional influence on the current measuring principle of the device according to the invention. The element shown in Figure 6 (10) is usable for positive currents flowing in the direction from the node (11) to the node (12). The antiparallel circuit shown in FIG. 7 of two elements designed according to FIG. 6 with two MOS FETs (Ti) and (T ₂ ) produces an alternative element (10) which can also be used for negative or alternating currents.

Correspondingly, a series circuit of self-conducting n-channel MOS-FETs (T ₃ ) and (T ₄ ) oriented in the opposite direction to FIG. 8 can also be integrated into the measuring device 6 according to the invention to form an alternative element (10).

Figure 9 shows an embodiment of a measuring device (6) with an alternative element (10) between the terminals (11, 12). Between the current flowing through the measuring node (7) and (8) of the measuring network to the electrical network (1) are antiparallel to each other two MOS-FETs (Ti) and (T ₂ ) connected so that the source terminal (17 ) of (Ti) and drain terminal (16) of (T ₂ ) to terminal node (7) and drain terminal (16) of (Ti) and source terminal (17) of (T ₂ ) to terminal node (8) connected to the measurement network. Between the terminals (7) and (8) drops a voltage (U _A ), which is coupled to two inputs (21) and (22) of a controller (23). The gate terminals (18) of both MOS-FETs (Ti) and (T ₂ ) are also connected to the regulator (23), for example to its terminal (24). A measuring node (13) is connected to the connection (24) of the controller (23) and the other measuring node (14) is short-circuited directly to the connection node (8) of the measuring device.

The voltage (U _A ) between the connection nodes (7) and (8) is determined by the appropriately designed regulator (23) setting an electrical potential at its terminal (24) or the gate terminals (18) of the MOS FETs ( Ti), (T ₂ ) regulated in the manner so that between the measuring node (13) and (14) the voltage to be measured (U) drops, which in the measuring range (I _meSs ) clearly assignable to a current to be measured (I) , This also corresponds to a regulation of the total Stand (R) of the illustrated element (10), which is connected with its terminals (11, 12) to the terminals (7, 8) of the measuring network (9). A particularly advantageously dimensioned controller (23) adjusts a reciprocal ratio of the voltage (U _A ) to the current (I) in the measuring range: U _A ~ 1 / I. This corresponds to the regulation of a total resistance (R) of the element (10), which is reciprocal to the square of the current (I) according to Rl / I ² .

Alternatively, it can also be regulated so that the voltage between the nodes 7 and 8 always remains constant.

Claims

claims

A measuring device for measuring an electric current (I), comprising: - two connection nodes (7, 8) for feeding and discharging the current (I),

a measuring network (9) arranged between the connection nodes (7, 8)

- One of at least a known partial flow (Ii) of the flow (I) through-flow element (10) with one of the partial flow

(Ii) dependent total resistance (R), and

- Two measuring nodes (13, 14) for tapping a measuring voltage (U), this one by design of the measuring network (9) predeterminable dependence on the current (I) contains.

2. Measuring device according to claim 1, wherein the partial flow (Ii) is the current to be measured (I).

3. Measuring device according to claim 1 or 2, wherein the measuring nodes (13,14) are short-circuited to each node of a non-linear component of the element.

4. Measuring device according to one of claims 1 to 3, wherein the electrical power loss (P _v ) of the measuring network (9) is at least partially independent of the current (I).

5. Measuring device according to one of claims 1 to 4, wherein the non-linear element (10) has a decreasing with increasing partial current (Ii) total resistance (R).

6. Measuring device according to claim 5, in which the non-linear element (10) has an at least sectionally in accordance with R - ab-

having total resistance (R)

7. Measuring device according to one of claims 1 to 4, wherein the non-linear element (10) having an increasing partial flow (Ii) increasing total resistance (R).

8. Measuring device according to one of claims 1 to 7, wherein the element (10) includes a diode and / or a transistor.

9. Measuring device according to claim 8, wherein the element (10) includes a parallel connection of a diode and an ohmic resistance.

10. Measuring device according to one of claims 1 to 9, wherein the element (10) includes a feedback MOS-FET.

11. Measuring device according to one of claims 1 to 10, wherein the measuring network includes an amplifier and / or a regulator.

12. Measuring device according to claim 11, with a total resistance (R) of the element (10) regulating controller.

13. Measuring device according to one of claims 1 to 12, with a computing device for further processing of the measuring voltage (U).

14. Measuring device according to one of claims 1 to 13, with a temperature of the measuring device detecting sensor.

15. Measuring device according to one of claims 1 to 14, with a compensation means for reducing the temperature dependence of the measuring voltage (U).