DE102019132455B4 - Method for determining the current and arrangement for implementing the method - Google Patents

Abstract

The invention, which relates to a method for current determination, is based on the object of specifying a solution with which a measurement of currents by means of a conductor track shunt resistor is improved in its accuracy and with which the accuracy of the measurement is maintained even with temperature fluctuations. To solve this problem, it is provided that the shunt resistor is provided as a circuit board shunt resistor (6), that a manufacturing error correction is carried out, in which an actual value RShuntist of the circuit board shunt resistor (6) is determined, that a temperature compensation is carried out , in which a temperature-corrected value RShuntkorr of the conductor track shunt resistor 6 is determined on the basis of the actual value RShuntist of the circuit board shunt resistor (6) and that the calculation of the current IShunt is carried out using the value RShuntkorr. To achieve the object, provision is also made for a logic arrangement (11) which carries out the manufacturing error correction and the temperature compensation to be arranged.

Description

The invention relates to a method for current determination in which at least one shunt resistor R _{Shunt is} provided in an inverter, a voltage U _{Shunt being} measured across the shunt resistor and a current I _Shunt being subsequently calculated.

The invention also relates to an arrangement for implementing the method for current determination.

In particular, the method for determining the current is intended to enable a current to be determined precisely, which is robust to temperature fluctuations and manufacturing tolerances. A current determination of this type should take place, for example, in several branches or half bridges of an inverter.

Inverters, also known as inverters, are required, for example, to convert a direct voltage into an alternating voltage. Such inverters are used, for example, to drive electric motors, in particular in vehicles with electrical system voltages of 48 volts or more than 60 volts.

For example, to control a three-phase electric motor controlled by an inverter, such as a motor used in an electric refrigerant compressor (e-compressor), the current must be measured in at least two motor phases, since a third phase can be determined by calculation, for example. A known method for such a current measurement is to measure the currents of the three motor phases via what is known as a shunt resistor.

For example, certain currents in the so-called half-bridges of an inverter, using one shunt resistor for each half-bridge, can be used to calculate the currents in the motor phases (U, V, W).

The term shunt resistor refers to a low-resistance electrical measuring resistor or current measuring resistor, which is inserted directly into the current-carrying line.

The _shunt current flowing through a shunt resistor R _shunt current I generates a voltage drop U _shunt at the shunt resistor, which is measured. Both analog and digital measuring technology can be used to record the voltage drop U _shunt. It is also possible to convert an analog measured value into a digital measured value at a later date. Such a digital measured value, for example, can be recorded via a central control unit, which among other things also controls the inverter, and the current I _Shunt 1, I _Shunt 2 and I _Shunt 3 per motor phase can be determined in this way. These currents I _shunt 1, I _{shunt 2} and I _shunt 3 are used to control the control of the motor by the inverter.

Such shunt resistors are usually applied as a through-hole assembly component or an SMD component on a circuit board of an inverter. In the case of SMD shunt resistors, for example, the costs per shunt resistor are in the order of about 50 euro cents, which is a not inconsiderable cost factor in the production of inverters.

An alternative possibility for such a current detection is to use a current compensation converter. The disadvantages here are that this variant is cost-intensive and has a clearly limited measurement bandwidth.

When measuring the current of the motor phases, sufficient accuracy of the measurement result is essential, which requires a corresponding precision of the shunt resistor used. Correspondingly precisely manufactured shunt resistors, however, lead to a further increase in costs.

A known alternative is that the shunt resistor is formed on a conductor track of a printed circuit board. In this case, in the case of a printed circuit board design, a conductor track forming the conductor track shunt resistor is provided with a corresponding dimensioning. Due to its dimensioning and the known specific electrical resistance of the conductor track material, this forms a conductor track shunt resistor with a corresponding resistance value. Such conductor track material consists, for example, of copper Cu and has a specific electrical resistance of approximately 0.0171 Ohm * mm ² / m.

A major disadvantage of conductor track shunt resistors provided in this way is that, due to the manufacturing tolerances in the manufacture of circuit boards, undesirably large deviations from a required resistance value of the desired shunt resistor occur.

Another disadvantage is that, in contrast to classically manufactured push-through assembly components or SMD components, the resistance value of copper Cu is highly temperature-dependent. The resistance value of a conductor track shunt resistor changes undesirably strongly when there is a change in the ambient temperature, which leads to changed measured values when measuring the current I _Shunt , which thus becomes too imprecise.

The use of shunt resistors, for example to expand the measuring range, is known from the book "Electrical engineering for building technology and mechanical engineering" by Andreas Böker, Ekkehard Boggasch and Hartmuth Paerschke, page 355 ff.

Furthermore, the use of shunt resistors for current measurement in inverters is described in the ATZ offhighway magazine, special issue April 2011, pages 44 to 50.

However, these known solutions are subject to both tolerances during production and temperature fluctuations during operation.

From the DE 10 2006 007 741 A1 a method for current measurement in at least one line of an electrical circuit by means of a shunt resistor is known. The object to be solved is to create a current measuring method and a current measuring device in which shunt resistors can be dispensed with and heat losses and space requirements are minimized.

As a solution, a method is specified in which a current-carrying conductor track of the circuit is used as a shunt resistor for current measurement in at least one line of an electrical circuit. At the factory, the absolute resistance of the conductor track between two voltage pick-up points is determined from a calibration measurement using a test current. To measure the current, the voltage drop between the voltage pick-up points of this conductor track is measured. The temperature of the conductor track is determined via the voltage drop in the case of an impressed test current on a thermally coupled conductor track with known resistance and temperature response.

After the temperature has been determined, the absolute resistance of the conductive track of the circuit is calculated. The current is finally calculated from the voltage drop between the voltage take-off points of the current-carrying conductor track and its calculated absolute resistance.

From the DE 10 2006 006 328 A1 a device for detecting electrical currents with a measuring resistor is known, which is arranged on a printed circuit board and is formed by a conductor track section. The object is to improve a device for detecting electrical currents of the type mentioned at the beginning in such a way that the space requirement on the circuit board is reduced and the detection can take place with increased accuracy.

The object is achieved in that at least one temperature sensor is assigned to the conductor track section. It is provided that the conductor track section is designed in a meandering shape and at least partially encloses the temperature sensor. In addition, the conductor track section is made of copper. It is further disclosed that, in an advantageous development, a measured value amplifier is provided which is formed from three active semiconductor components, the semiconductor components preferably being designed as transistors. It is provided that two transistors are arranged as a current mirror, the voltage drop at the conductor track section being fed to one transistor, which influences the symmetry of the current mirror. The third transistor is arranged in such a way that feedback takes place to cancel the influence on the current mirror symmetry.

There is thus a need for improved accuracy of such conductor track shunt resistors, in particular when used for current measurement.

The object of the invention is to provide a method for current determination, with which a measurement of currents by means of a conductor track shunt resistor is improved in its accuracy and with which the accuracy of the measurement is maintained even with temperature fluctuations.

The object is achieved by a method with the features according to patent claim 1. Further developments are given in the dependent claims.

To solve this problem, it is provided that the use of inexpensive and easy to manufacture conductor track shunt resistors is made possible in that both a manufacturing error correction and a temperature error correction are carried out.

In this case, the manufacturing error correction is carried out once, for example, within a test or final test of an assembly containing the conductor track shunt resistor. In a first variant, an actual resistance value of the conductor track shunt resistor R shunt is determined and stored _{in this manufacturing error correction.}

In a second variant, a first correction _{value R shunt korr} for the deviation between the actual resistance R _{shunt ist of} the conductor track shunt resistance and the nominal value R _{shunt nom} can also be stored.

For subsequent process steps, for example the determination of the current I _Shunt for a Motor phase at which the voltage U across a _shunt formed interconnect shunt resistor is measured, can be determined in the in the fabrication error correction and stored actual resistance value of the conductor path shunt resistor R _{shunt is} to resort. The current I _Shunt to be determined for the motor phase is formed from the quotient of the measured voltage U _shunt and the actual value for the conductor track shunt resistance R _shunt is.

Alternatively, the first correction _{value R shunt korr} determined in the second variant for the conductor track shunt resistance can also be used. The interconnect shunt resistor R _{shunt is} derived from the nominal value R _nom plus _shunt of the first correction value R _{corr shunt,} which is provided with a sign.

In this case, R _{shunt ist ber} is calculated, where: $${\text{R.}}_{\text{shunt is about}}={\text{R.}}_{\text{shunt nom}}+{\text{R.}}_{\text{shunt corr}}$$

Here, R _{shunt korr has} a positive sign (+) if the actual value for the conductor track shunt resistance R _shunt is greater than the specified nominal value R _{shunt nom} . In the event that the actual value for the conductor rail shunt resistor R _shunt is less than the predetermined nominal value _shunt R _nom fails, the first correction value R _{shunt corr} a negative sign (-) on.

In this case, the current I _Shunt to be determined for a motor phase is formed from the quotient of the measured voltage U _shunt and R _{shunt is over} or the sum of R _{shunt nom} and Rshunt _corr .

In the production error correction, it is provided that the conductor track shunt resistor is _{energized with a reference current I reference and the voltage U reference} occurring across this conductor track shunt resistor is measured with a suitable means. The conductor track shunt resistance R shunt _ist can then be determined according to Ohm's law. $${\mathrm{R.}}_{\mathrm{S.}Huntist}=\frac{U_{\mathrm{R.}eferenz}}{{\mathrm{I.}}_{\mathrm{R.}eferenz}}$$

A temperature error correction is also provided, in which a temperature-dependent correction of the resistance value is carried out on the basis of a known temperature coefficient of the material of the conductor track shunt resistor. In the case of a conductor track shunt resistor consisting, for example, of copper Cu, the temperature coefficient α is known for the material copper Cu, which is, for example, 3.93 · 10 ^-3 K ^-1 .

This temperature coefficient α, which is also referred to as the temperature coefficient, describes the relative change in the physical size of the resistance of the copper material in the change in temperature ϑ in relation to a specified reference temperature, such as a so-called room temperature of 20 ° C.

It is also intended to measure a current temperature ϑ of the circuit board on which the conductor path shunt resistor is arranged. A measuring arrangement equipped with a corresponding sensor can be used for this. Alternatively, an existing sensor on the circuit board or, for example, on a refrigerant compressor on which the circuit board is arranged, can be used to determine the temperature ϑ.

genutzt. Hierbei ist R_{Shunt ist @20° C} der Widerstandswert des Leiterbahn-Shuntwiderstands bei 20°C also bei Raumtemperatur, ϑ die aktuell gemessene Temperatur und α_Cu der Temperaturkoeffizient von Kupfer Cu.A temperature error correction is carried out by means of this specific temperature ϑ. For this purpose, an equation according to $${\mathrm{R.}}_{\mathrm{S.}HuntkOrr}={\mathrm{R.}}_{\mathrm{S.}Huntist@\mathrm{20th}°\mathrm{C.}}\ast \left(1+\left(\vartheta -\mathrm{20th}°\mathrm{C.}\right)\ast {\alpha }_{\mathrm{C.}u}\right)$$

used. Here, R _{Shunt is @ 20 ° C} the resistance value of the conductor track shunt resistance at 20 ° C, i.e. at room temperature, ϑ the currently measured temperature and α _Cu the temperature coefficient of copper Cu.

In this way, a temperature measurement and a temperature error correction can be carried out at specified times. This temperature error correction can take place at regular time intervals. Alternatively, such a temperature error correction can take place at times which are predetermined, for example, by the central control unit.

• 1: einen Wechselrichter aus dem Stand der Technik,
• 2: ein Dimensionierungsbeispiel für einen Leiterbahn-Shuntwiderstand aus dem Stand der Technik,
• 3: eine Prinzipskizze der digitalen Fertigungsfehlerkorrektur des Leiterbahn-Shuntwiderstands gemäß der Erfindung,
• 4: einen Ablaufplan der digitalen Fertigungsfehlerkorrektur des Leiterbahn-Shuntwiderstands mit einer Anordnung nach 3,
• 5: eine Prinzipskizze der erfindungsgemäßen Temperaturfehlerkompensation und
• 6: einen Ablaufplan der digitalen Temperaturkompensation mit einer Anordnung nach 5.

Further details, features and advantages of embodiments of the invention emerge from the following description of exemplary embodiments with reference to the associated drawings. Show it:

• 1 : a state-of-the-art inverter,
• 2 : a dimensioning example for a conductor track shunt resistor from the prior art,
• 3 : a schematic diagram of the digital manufacturing error correction of the conductor track shunt resistor according to the invention,
• 4th : a flow chart of the digital manufacturing error correction of the conductor track shunt resistor with an arrangement according to 3 ,
• 5 : a schematic diagram of the temperature error compensation according to the invention and
• 6th : a flow chart of the digital temperature compensation with an arrangement according to 5 .

In the 1 is partially an inverter 1 or inverters from the prior art. The circuit breakers are shown 2a , 2 B of the inverter 1 , which in pairs in three so-called half bridges 3 of the inverter 1 are arranged between the terminals U + and U-. As usual, is in each of the three half bridges 3 one highside circuit breaker each 2a and one lowside circuit breaker each 2 B arranged. Such circuit breakers 2 are mostly semiconductor power switches, such as transistors or thyristors.

Each between a high-side circuit breaker 2a and a low-side breaker 2 B is a branch for the phases U, V and W of a motor to be controlled 4th arranged. As usual, the circuit breakers 2 controlled via their respective control electrode by the central control unit, not shown, and thus generate the motor 4th driving voltages on phases U, V and W.

For realizing an effective control of the motor 4th there is provision for the respective currents through the half bridges 3 , advantageously connected to the negative operating voltage potential at a position, and to measure the measured values of the central control unit for processing or for regulating the operating mode of the motor 4th to provide.

For this purpose, the shunt resistors 5 R _{Shunt 1} , R _{Shunt 2} and R _{Shunt 3} , as in the 1 shown, arranged. Thus it becomes possible on the basis of the known value for the shunt resistances 5 R _{Shunt 1} , R _{Shunt 2} and R _{Shunt 3} and by measuring the voltages U _{Shunt 1} , U _{Shunt 2} and U _{Shunt 3 to determine} the required currents I _{Shunt 1} , I _{Shunt 2} and I _{Shunt 3} .

In the 1 are on the circuit breakers 2a and 2 B , as usual, shown arranged freewheeling diodes according to the prior art.

In the 2 is a dimensioning example for a conductor track shunt resistor 6th shown from the prior art. About one in the 2 illustrated section of a circuit board 7th is in extracts a conductor track 8th , for example made of copper Cu. This conductor run is provided in such a way that it extends over a length 9 I a width 10 w has.

This results in the resulting resistance value of this conductor track shunt resistor 6th R _shunt according to: $${\mathrm{R.}}_{\mathrm{S.}Hunt}=\sigma \ast \frac{l}{H\ast w}$$

Here, h is a height of the conductor track 8th and σ is the electrical conductivity of the conductor track material, in the example of copper Cu this value is 58 * 10 ⁶ S · m-1. A conductor track shunt resistor can be created in this way 6th Dimension accordingly for a given resistance value and on the circuit board 7th manufacture. For this purpose, customary processes for the production of conductor tracks 8th on circuit boards 7th come into use. One over such a conductor shunt resistor 6th measurable voltage U _Shunt is in the 2 also shown.

Advantageous in conductor track shunt resistors manufactured in this way 6th it is that this is when creating the conductor tracks 8th , a process step that is already necessary in the production of a printed circuit board 7th for an inverter, can be generated with. This also leads to a reduction in costs, since no through-hole assembly components or no SMD components are required.

In the 3 is a schematic diagram of the digital manufacturing error correction of the conductor track shunt resistor 6th shown according to the invention. In the left part of the 3 the process of manufacturing error correction is shown, while the right-hand part of the 3 shows the normal operation taking place after the completed manufacturing error correction, which takes place taking into account the result of the manufacturing error correction.

For example by means of a logic arrangement 11 , which has corresponding interfaces for outputting a current I _reference or for measuring a voltage U _reference or U _shunt , the process of manufacturing error correction is controlled.

This manufacturing error correction is carried out, for example, as part of a test or final test of an assembly, such as a central control unit for an inverter. Such a logic arrangement 11 can be, for example, a microprocessor, which via the appropriate interfaces for connection to the circuit board shunt resistor 6th and a temperature sensor 23 or by means of a separate interface arrangement with the circuit board Shunt resistance ( 6th ) and the temperature sensor ( 23 ) connected is.

verwendet. Der Wert des Leiterbahn-Shuntwiderstands 6 R_Shunt ist wird beispielsweise in einer Speichereinheit 14 abgespeichert und steht somit für den nachfolgenden Normalbetrieb der zentralen Steuereinheit zur Verfügung.A calibration mode of operation is used to correct manufacturing errors 12th a defined current I _reference provided and in the conductor track shunt resistor 6th imprinted. This current I _reference leads to a voltage U _reference across the conductor track shunt resistor 6th . This voltage U _reference is measured and, for example, converted into a digital measured value using an analog-digital converter (not shown). This determined digital measured value for the voltage U _reference is used for the exact determination of the value R _Shunt ist for the conductor track shunt resistance 6th in one measuring process 13th according to the relationship $${\mathrm{R.}}_{\mathrm{S.}Huntist}=\frac{U_{\mathrm{R.}eferenz}}{{\mathrm{I.}}_{\mathrm{R.}eferenz}}$$

used. The value of the trace shunt resistance 6th R _Shunt is, for example, in a storage unit 14th and is thus available for the subsequent normal operation of the central control unit.

As in the right part of the 3 is shown, after the completed manufacturing error correction in the normal operating mode 15th switched. In this normal operating mode 15th becomes the conductor shunt resistance 6th traversed by the current to be determined I _Shunt. This current I _Shunt leads to a voltage U _Shunt across the conductor track shunt resistor 6th . This voltage U _Shunt is measured and converted into a digital measured value, for example using an analog-digital converter (not shown). This determined digital measured value for the voltage U _Shunt is used for the exact determination of the value I _Shunt , the one in the memory unit 14th stored actual value for the conductor track shunt resistance 6th R _shunt is used according to the relationship: $${\mathrm{I.}}_{\mathrm{S.}Hunt}=\frac{U_{\mathrm{S.}Hunt}}{{\mathrm{R.}}_{\mathrm{S.}Huntist}}$$

The result of the exact determination of the current I _Shunt is passed on, for example, to the central control unit, as it is in the 3 with the exit 16 is shown for a current value.

the 4th shows a flow chart of the digital manufacturing error correction of the conductor track shunt resistor 6th with an arrangement according to 3 . After the start 17th the manufacturing error correction is done in one step 18th the current I _{reference is} generated and in the conductor track shunt resistor 6th imprinted.

In the following measuring step 19th becomes the voltage U _reference across the conductor track shunt resistor 6th measured and converted into a digital value. In the calculation step 20th the calculation of the value R _Shunt is for the conductor track shunt resistance 6th according to the relationship: $${\mathrm{R.}}_{\mathrm{S.}Huntist}=\frac{U_{\mathrm{R.}eferenz}}{{\mathrm{I.}}_{\mathrm{R.}eferenz}}$$

The one in the crotch 20th The determined exact value R _Shunt is the conductor track shunt resistance 6th will be in crotch 21 and the flow chart of the digital manufacturing error correction ends in step 22nd .

the 5 shows a schematic diagram of the temperature error compensation according to the invention. The representation in the 5 shows at least partially those from the right part of the 3 known arrangement for normal mode 15th in which the current I _Shunt for the conductor track shunt resistor 6th using the in the storage unit 14th stored actual value of the conductor shunt resistance 6th R _Shunt is being determined.

A temperature sensor is also provided 23 , which at the conductor shunt resistor 6th or at least in the vicinity of the conductor track shunt resistor 6th is arranged and such the temperature of the conductor track shunt resistor 6th can measure. As such a temperature sensor 23 A sensor already arranged on the central control unit or on an electric refrigerant compressor can also be used if it is guaranteed that this sensor can measure the temperature of the conductor track shunt resistor 6th can capture with sufficient accuracy.

The measured value of the by means of the temperature sensor 23 determined temperature is for example in digital form to a calculation unit 24 to calculate a temperature-corrected value R _{Shunt corr of} the conductor track shunt resistance 6th forwarded. This calculation in the calculation unit 24 takes place with knowledge of the relationship: $${\mathrm{R.}}_{\mathrm{S.}HuntkOrr}={\mathrm{R.}}_{\mathrm{S.}Huntist@\mathrm{20th}°\mathrm{C.}}\ast \left(1+\left(\vartheta -\mathrm{20th}°\mathrm{C.}\right)\ast {\alpha }_{\mathrm{C.}u}\right)$$

On the basis of this known relationship, the temperature-correct value R _{Shunt korr of} the conductor track shunt resistance 6th can be determined on the basis of the value R _{Shunt that} has already been determined.

Alternatively, it is possible that a correction value used for temperature error compensation for the conductor track shunt resistance 6th or is determined. This correction value describes the deviation in the resistance value of the conductor track shunt resistor caused by the current temperature 6th .

Like in the 5 is shown, the one in the storage unit 14th _Stored actual resistance value R shunt is the conductor shunt resistance 6th used when calculating the current I _Shunt to be determined.

In addition, when determining the current I _Shunt , that of the calculation unit is also used 24 to a correction unit 25th Transferred temperature-correct value R _{Shunt corr of} the conductor track shunt resistance 6th used when calculating the current I _Shunt to be determined.

In the 6th is a flow chart of digital temperature compensation with an arrangement according to 5 shown. After the start 26th the temperature compensation is done in one step 27 by means of a suitable temperature sensor 23 a temperature measurement is carried out at which a value of the temperature ϑ at the conductor track shunt resistor 6th is determined.

This measured value for the current temperature ϑ of the conductor track shunt resistor 6th is in one calculation step 28 for a calculation of the temperature-correct value R _{Shunt corr of} the conductor track shunt resistance 6th or a temperature correction value is used.

In the subsequent correction step 29 a temperature-dependent or temperature-correct correction of the in the storage unit 14th The stored resistance value R _{shunt is} made. This further correction, in which the temperature-correct value R _{Shunt corr of} the conductor track shunt resistance 6th is determined, forms the basis for the exact calculation of the current I Shunt, which is carried out taking into account both the manufacturing _{error correction and the temperature compensation.} The temperature compensation ends in step 30th .

Claims (8)

A method for current determination in which at least one shunt resistor (5, 6) R _{shunt is} provided in an inverter (1), a voltage U _{Shunt being} measured across the shunt resistor (5, 6) and a current I _Shunt being subsequently calculated, characterized in that the shunt resistor is provided as a circuit board shunt resistor (6), that a manufacturing _{error correction is carried out, in which an actual value R Shunt} ist of the circuit board shunt resistor (6) is determined, that a temperature compensation is carried out, in which On the basis of the actual value R _Shunt , the printed circuit board shunt resistor (6) is a temperature-corrected value R _{Shunt corr of} the conductor track shunt resistor 6, it is determined that the current I _{Shunt is} calculated using the value R _{Shunt corr} that occurs during temperature compensation a temperature ϑ is measured on the circuit board shunt resistor (6) and that using a known temp erature coefficient α of a material of the circuit board shunt resistor (6) a calculation of the temperature-corrected value R _{Shunt korr of} the conductor shunt resistor 6 is carried out and that the calculation of the temperature-corrected value R _{Shunt korr of} the conductor shunt resistor 6 according to the relationship $${\mathrm{R.}}_{\mathrm{S.}HuntkOrr}={\mathrm{R.}}_{\mathrm{S.}Huntist@\mathrm{20th}°\mathrm{C.}}\ast \left(1+\left(\vartheta -\mathrm{20th}°\mathrm{C.}\right)\ast {\alpha }_{\mathrm{C.}u}\right)$$

he follows. Procedure according to Claim 1 , characterized in that a fixed current I reference is generated during the manufacturing _{error correction and a voltage U reference is} measured at the circuit board shunt resistor (6) and that the actual value R _shunt ist of the circuit board shunt resistor (6) is calculated. Method according to one of the Claims 1 or 2 , characterized in that the actual value R _Shunt is the circuit board shunt resistor (6) according to the relationship $${\mathrm{R.}}_{\mathrm{S.}Huntist}=\frac{U_{\mathrm{R.}eferenz}}{{\mathrm{I.}}_{\mathrm{R.}eferenz}}$$

is determined. Method according to one of the Claims 1 until 3 , characterized in that the calculation of the current I _Shunt by means of the value R _{Shunt korr takes place} in several half bridges (3) of the inverter (1). Arrangement for the implementation of the procedure according to the Claims 1 until 4th , characterized in that a logic arrangement (11) performing the manufacturing error correction and the temperature compensation is arranged, the logic arrangement (11) having a calculation unit (24) and a correction unit (25) connected to the calculation unit (24), and wherein the calculation unit (24 ) is connected to a temperature sensor (23). Arrangement according to Claim 5 , characterized in that the logic arrangement (11) is a microprocessor. Arrangement according to Claim 6 , characterized in that the microprocessor interfaces to Has connection to the circuit board shunt resistor (6) and the temperature sensor (23) or is connected to the circuit board shunt resistor (6) and the temperature sensor (23) via a separate interface arrangement. Arrangement according to one of the Claims 5 until 7th , characterized in that the logic arrangement (11) has a unit for a calibration mode (12), a unit for a normal mode (15), a unit for carrying out a measuring process (13) and a memory unit (14).

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