Method and device for calibrating a current sensor

Abstract

Method for calibrating a current sensor (10), wherein the current sensor (10) comprises: - a resistor (20) capable of carrying a current (I) to be measured, - an amplifier (30) for amplifying a voltage drop (V) across the resistor (20) as a result of current flowing through the resistor (20) with a gain factor (K), and for outputting a corresponding analog amplifier output signal (Vout*), - an A / D converter (40) for converting the analog amplifier output signal (Vout*) into a corresponding digital converter output signal (Vout), - a digital signal processing device (50) for processing the converter output signal (Vout) using calibration parameters stored in a memory device (54) of the signal processing device (50) to produce an output signal (OUT) of the current sensor (10) representative of the electrical current (I), characterized in that the method comprises the following steps: - detecting (S1) a first value (Vout1) of the converter output signal (Vout) when the resistor (20) is not energized, - detecting (S2) a second value (Vout2) of the converter output signal (Vout) immediately after switching on the current supply to the resistor (20) with a predetermined test current (Itest), when the resistor (20) is supplied with the test current (Itest), - detecting (S3) several third values ​​(Vout3, Vout4, ...) of the converter output signal (Vout) over time after switching on the current supply to the resistor (20) with the test current (Itest), when the resistor (20) is supplied with the test current (Itest) and the temperature of the resistor (20) increases due to the test current and the converter output signal (Vout) changes as a result, - Determining (S4) the calibration parameters by evaluating the recorded first, second and third values ​​(Vout1, Vout2, Vout3, Vout4, ...) of the converter output signal (Vout), - Storing (S5) the determined calibration parameters in the storage device (54).

Description

The present invention relates to a method according to the preamble of claim 1 and to an apparatus according to the preamble of claim 6 for calibrating a current sensor.In known current sensors, the determination of an electric current is usually carried out by measuring the voltage drop across a resistor through which the current flows ("shunt"). The voltage drop across a constant resistance is proportional to the current and thus a suitable measure of the current.However, it is problematic in practice with such current sensors, for example, that the measurement, and in particular, for example, a previous amplification of the voltage dropping across the resistor, which is generally required, can be error-prone due to manufacturing tolerances of the relevant measurement amplifier.Another problem opposing accurate absolute value measurement of the current is some variation in resistance of the resistor (shunt) due to manufacturing tolerances.Furthermore, it has proven problematic in the case of such current sensors, in particular in the case of current sensors for measuring relatively large current intensities, that a not negligible electrical power loss arises when the resistor is energized, which leads to heating of the resistor and consequently to a change in the resistance value and thus to corruption of the current measurement.Various concepts for a calibration of current sensors are known from the prior art, such as, for example, an individual measurement, on the one hand, of the wind resistance value of the shunt and, on the other hand, of a characteristic (e.g. amplification factor) of the measurement amplifier from which a current sensor is built. From the results of this measurement, (at least) one calibration parameter can be formed and stored in a storage device of the current sensor. However, such a calibration method is very complicated and often nevertheless not sufficiently accurate depending on the specific field of application of the current sensor.The publication CN 111 077 489 A describes a calibration of a current sensor by means of a system comprising a current source, a calibrated current sensor and a processor, wherein the current sensor to be calibrated is arranged in series connection with a current source and the calibrated current sensor. A current intensity of the current source is detected by means of the calibrated current sensor in order to obtain a standard value, and a corresponding value to be calibrated is obtained by means of the current sensor to be calibrated. The processor is configured to determine from the default value and the value to be calibrated a calibration parameter that can be stored in the current sensor to be calibrated. Further methods for calibrating current sensors are known, for example, from the publications CN 104 297 710 A and U.S. Pat. No. 8 718 964 B2.A method for calibrating a current sensor according to the preamble of claim 1 is known from DE 10 2015 217 862 A1. In this method, during the current measurement, a first, a second and a third value of a voltage dropping across a measuring resistor ("shunt") are recorded in short succession. When detecting the first value and the third value, there is in each case provision for the resistor to be energized with an "operating current" (to be measured by means of the current sensor), and when detecting the second value, there is provision for the resistor to be energized with a "total current" which is composed of the operating current and a (predefined) additionally impressed "reference current".EP 3 022 737 B1 discloses a method for calibrating a current sensor, in which it is specifically the determination of an offset error of a measurement amplifier. In a "current measurement mode", the measurement amplifier is used in a conventional manner for amplifying the voltage dropping across a measurement resistor ("shunt") as a result of the current being supplied, wherein the two inputs of the amplifier are connected to one another via a further resistor (which is relatively high-ohmic compared to the shunt). During an additionally provided "calibration mode", it is provided for the purpose of determining the offset error that the voltage dropping at the shunt is no longer applied at the two amplifier inputs (by opening a corresponding switch), wherein the amplifier inputs are however still connected to one another via the (relatively high-resistance) further resistor, so that in this mode the offset error of the amplifier can be determined by measuring the output voltage of the amplifier.DE 10 2018 217 625 A1 discloses a method for operating a battery sensor having a current detection device in a vehicle. The current detection device comprises a measuring resistor to which a load current to be measured can be applied, an amplifier for amplifying the voltage dropping across the resistor, and an A / D converter for converting the analog amplifier output signal to a value representative of the current. In addition, a reference current circuit can be provided, with which a reference current of known magnitude can be applied to the resistor in order to thus determine a correction value for correcting the current detection.It is an object of the present invention to provide a novel way of easily and yet accurately calibrating a current sensor.According to the invention, this object is achieved by means of a method for calibrating a current sensor according to claim 1 and by means of a device for calibrating a current sensor according to claim 6.Advantageously, the invention calibrates an already manufactured current sensor, so that the effort is reduced and the accuracy is increased compared to a calibration based on a preceding individual measurement of resistor and amplifier.A further advantage is that the mentioned first, second and third values of the converter output signal can be detected, for example, in three corresponding, directly consecutive time phases (first, second, third phase) and the detection and calibration based thereon can be carried out relatively quickly in this case.A particular further advantage results from the combination provided according to the invention of the detection of these first, second and third values. This enables the determination of calibration parameters, by means of which, during the later current measurement using the calibrated current sensor, thermal or dynamic effects, for example, which are based on variations in the temperature of the resistor as a result of variations in the current and / or an ambient temperature, can advantageously be taken into account and can thus be corrected (compensated) by the calibration.According to one embodiment, when evaluating the acquired first, second and third values of the converter output signal, a model for the value of the converter output signal (as a function of the current I and the resistance value R and at least one further parameter) is used as a basis, for example according to the following equation: wherein:Vout represents the value of the converter output signal,I is the current intensity of the current through the resistor (shunt),R is the resistance (resistance value) of the resistor,k is a gain factor of the amplifier, andVoffset is an offset voltage of the amplifier.In the following, using the example of this model, a possible sequence of the method according to the invention is explained in somewhat more detail by way of example. In the method, it can be provided, for example, that the steps already mentioned above are carried out successively, in the stated order, and one time each:S 1: detecting a (e.g. single) first value "Vout 1" of the converter output signal Vout when the resistor is not energized,S 2: detecting a (e.g. single) second value "Vout 2" of the converter output signal Vout immediately after switching on the energization of the resistor with a predetermined test current intensity "Itest", upon energization of the resistor with the test current intensity Itest,S 3: Acquisition of a plurality of third values "Vout3", "Vout4",.. of the converter output signal Vout in a temporal profile after the switching on of the energization of the resistor with the test current intensity Itest, upon energization of the resistor with the test current intensity Itest and a temperature increase of the resistor caused by the test current and consequently a change of the converter output signal Vout,S4: Determining the calibration parameters by evaluating the detected first, second and third values Vout1, Vout2, Vout3, Vout4,... of the converter output signal Vout, andS5: Storing the determined calibration parameters in the storage device.In a preferred use of the calibration method, this is carried out, for example, "at the strip end" of a series production of current sensors at the individual current sensors, i.e. even before the current sensor is installed in its later electrical installation environment.In step S 1, no special measures are then generally required to ensure the "non-energization" of the resistor. Otherwise, if the current sensor is already installed in an electrical installation environment, the non-energization can be effected, for example, by temporarily (for the duration of the calibration method) separating the resistor from this electrical environment or, for example, by electrically short-circuiting the terminals of the resistor. Then, the first value Vout 1 of the converter output signal Vout may be detected, for example, once. Alternatively, a plurality of (e.g. in the range from 2 to 5) redundant first values Vout 1 can be detected and a single value Vout 1 (e.g. average value) can be formed therefrom, which is used further for the following method steps.In step S 2, the current can be supplied by an e.g. sudden increase in the current intensity I to an optionally settable test current intensity "Itest", which is e.g. fixedly predefined (rise time e.g. at most 100 ms, in particular at most 50 ms), wherein the second value Vout 2 is detected immediately thereafter. Alternatively, a plurality of second values Vout2 could also be detected at time intervals from one another (e.g. equidistant in time) (number e.g. in the range from 2 to 5), e.g. in order to form a further average value therefrom. However, this only occurs if all these detections take place within a (very short) time period immediately after the test current has been switched on, within which a test current-induced temperature increase of the resistor is at most still negligibly small (e.g. less than 5% of a maximum temperature increase achievable as a result of the test current). In practice, for example, a time period of at most 20 ms, in particular at most 10 ms, may be suitable, within which the one or more acquisitions of values for Vout 2 are completed. In one embodiment of the invention, the test current is switched on by means of a (e.g. constant) current source in conjunction with a controllable switching element. Alternatively, a current source with controllably adjustable current intensity can be used, for example.In step S 3, the test current intensity Itestcan be kept constant over time in the temporal profile after the switching on of the current supply, for example, and a plurality of third values Vout3, Vout4,... are detected (number e.g. in the range from 2 to 10) at temporal intervals from one another (e.g. equidistant over time). This is preferably such that these detections are carried out distributed over a time period after the switching on of the test current, within which a test current-induced temperature increase of the resistor already reaches an appreciable extent (e.g. at least 20%, in particular at least 50%, of a maximum temperature increase achievable as a result of the test current). In practice, for example, a time period of at least 1 s, in particular at least 2 s, over which the plurality of detections of values for Vouttakes place, may be suitable in most cases.In step S 4, an evaluation of the acquired first, second and third values Vout 1, Vout 2, Vout 3,... of the converter output signal Vout can be carried out using the relevant mathematical-physical model of the dependence of the converter output signal Vout on the current intensity I (in order to determine one or more of the parameters of the relevant model (e.g. including equation 1).For example, in the example according to Equation 1, it can be assumed that the A / D conversion of the voltage Vout* into the converter output signal Vout occurs without appreciable error (Vout corresponds to Vout*), and thus the values of the model parameters "Voffset" and "K" relate only to the amplifier. Alternatively, however, these values and thus the model could also be obtained for characterizing the series connection of amplifier and A / D converter. However, in step S 4, it would also be possible, for example, to use a different (more complicated) model for the evaluation, which model additionally contains corresponding parameters of the A / D converter (such as "offset" and / or "conversion factor").In the example, Voffset is determined from the first value Vout1 (cf. equation 1): Vout1=Voffset.Then (see equation 1), if the resistance value R is known, the value K can be determined from the second value Vout 2: K=(Vout2- Voffset) / (Itest x R).In this case, the value R can be known, for example, from a manufacturer specification and / or the result of a measurement of this value carried out beforehand.If a determination of a temperature T of the resistor takes place in the method, for example before or in step S 2, then a value R that can be determined more accurately for the specifically determined temperature T can advantageously also be used for the above-mentioned determination of K.In an embodiment in this regard, it is provided that the model takes into account a temperature dependence of R, for example according to the following equation: wherein:R(T) is the resistance R of the resistor as a function of a temperature T of the resistor,a temperature coefficient of resistance,R0 and T0 represent a base resistance (R0) of the resistor and the associated normal temperature (T0).In this case, the values R0and T0and also the temperature coefficient alfamay be known, for example, from a manufacturer specification and / or the result of a previously carried out measurement of these values.If the temperature dependence of R is taken into account in the execution of the calibration method, whether according to equation 2 above or according to another model for this temperature dependence, then for example a measurement of the temperature T of the resistor can be carried out for this purpose (preferably before step S 2, e.g. during step S 1, and e.g. during step S 3) and / or, as explained below, by evaluating the acquired first, second and third values Vout 1, Vout 2, Vout 3,... this temperature T can be estimated on the basis of a "thermal model" (cf. e.g. equation 5 further below).In the modeling of the resistor R(T), as an alternative to equation 2, a different (e.g. more complicated) dependence of the resistor R(T) on the temperature T can also be taken into account in the context of the invention, wherein associated model parameters (or at least a part of such parameters) can again be known from a manufacturer specification and / or prior measurement (and, if appropriate, another part of the parameters can be estimated, e.g. by the calibration method itself).In order to determine a temperature T of the resistor during the calibration method, e.g. before or at step S 2 and e.g. at step S 3, T can be measured e.g. directly at the resistor.Alternatively or additionally, it is possible to measure a temperature "Tsens" in a vicinity of the resistor for this purpose. It should be noted that the temperature value Tsens provided by an "ambient temperature sensor" (in the vicinity of the resistor) corresponds in any case relatively exactly to the temperature value T of the resistor if there was no energization of the resistor for a certain period of time before the temperature measurement. In practice, a time period (without current application) of, for example, at least 1 min, in particular at least 2 min, may often be sufficient for this purpose.If the current sensor to be calibrated (at least) has a temperature sensor, for example for temperature measurement directly at the resistor (T) and / or for ambient temperature measurement (Tsens), this can be used advantageously or jointly used for the calibration method according to the invention. For measuring the ambient temperature Tsens, such a sensor can be arranged, for example, on a circuit carrier (e.g., circuit carrier plate), on which the remaining components of the current sensor mentioned at the beginning (resistor, amplifier, A / D converter, signal processing device) are also arranged.By detecting a third value Vout3, it is advantageously possible to (at least) determine a parameter which is representative of the temporal profile of the converter output signal Vout after the energization with the test current intensity Itest has been switched on. The value of Vout will change (e.g. increase) in practice in this time phase because the resistor (shunt) will heat up as a result of the energization with the test current and thus as a rule its resistance value R changes (temperature-dependent) (cf., for example, model for this according to equation 2).According to the invention, as already explained above, a plurality of third values Vout3, Vout4,... of the converter output signal Vout are detected over time after the switching on of the energization of the resistor with the test current intensity Itest. Advantageously, the (time-dependent) curve of Vout can thus be characterized even more accurately by means of corresponding parameters.In particular, the plurality of third values Vout 3, Vout 4,... can be detected, for example, at least until a point in time at which the converter output signal Vout changes only insignificantly at most. For a specific predefined test current intensity Itest, this time is given when a power loss "Pin" generated as a result of the current application in question is exactly the same as a heat dissipation power "Pout" resulting from a temperature difference between the temperature T of the resistor and the temperature Tsens of the environment of the resistor.The following applies here to the electrical power loss Pin:- general:or upon energization with test current intensity: Pin=Test 2×RIf, for example, heat conduction predominates during heat emission, the heat emission output Pout applies to a good approximation: wherein:W is the thermal conductance of the resistor (towards the environment),T is the temperature of the resistor,Tsens the ambient temperature.In particular, if, within the scope of the evaluation using the detected third values Vout3, Vout4,..., a stationary final value "Voutend" of the converter output signal Vout is estimated or determined, then using this final value Voutend, it is once again advantageously possible, for example, to determine the aforementioned thermal conductance W (or another parameter characterizing the heat emission power Pout of the resistor, such as a time constant "PT", for example, which is described for the exemplary embodiment of FIG. 2 ).This is of great advantage above all because at least one calibration parameter can then be determined by the evaluation and stored in the current sensor (e.g. the parameter W as such, or a parameter derived therefrom), by means of which, during the later operation of the (calibrated) current sensor, independently of a measurement of the resistance temperature T which is then likewise possible (for the purpose of taking into account according to equation 1 or the model in question), a determination by estimating the temperature T on the basis of the temporal profile (history) of the current measurement, i.e. on the basis of the signal Vout or the signal OUT, for example, is also made possible. Advantageously, the (time-resolved) detected ambient temperature Tsens can and should also be used for such a determination of the temperature T in sensor operation, if available (cf. equation 5).On the basis of the equation 4 given by way of example for the heat emission, a model of the thermal or dynamic effects can be provided, for example according to the following equation: wherein:Cp is the heat capacity of the resistor,T is the temperature of the resistor,t is the timeI is the current intensity of the current through the resistor,R is the resistance (resistance value) of the resistor,W is the thermal conductance of the resistor,Tsens the ambient temperature.In this case, the value Cp can be known, for example, from a manufacturer specification and / or the result of a measurement of this value carried out beforehand.As already explained above, a model can be used for modelling the temperature T of the resistor as a function of the current I and as a function of the currently measured temperature Tsens. Thermal or dynamic effects, which are based on variations in the temperature of the resistor, can thus be taken into account, namely in the calibration method and / or in the later operation of the current sensor.In order to take into account a temperature Tsens measured in the current sensor in the calibration method (in step S 4) or by the calibration device, it can be provided that in one or more of steps S 1 to S 3 the measured value of Tsens is recorded and corresponding temperature data (until step S 4) is carried out are temporarily stored. In the calibration device, it can be provided for this purpose that its interface device that can be connected to the current sensor is additionally used for transmitting these measured values or temperature data from the current sensor to the calibration device.In step S 4, the calibration parameters are determined based on the result of the evaluation of the first, second and third values Vout 1, Vout 2, Vout 3,... of the converter output signal Vout acquired in steps S 1 to S 3.As already explained above, in the evaluation in step S 4, in particular, e.g., the values of "Voffset" and / or "K" occurring in the exemplary model according to equation 1 can be determined. These parameters Voffsetand K as such (and optionally also one or more of the parameters R0, T0, alfa, W, PT) or parameters derived therefrom can then be provided as the calibration parameters determined in step S 4, so that during subsequent operation of the current sensor, the sensor output signal OUT is, for example, as exactly as possible proportional to the current intensity I.Alternatively or additionally, in step S 4, for example, the value of "R" occurring in equation 1 can also be determined, for example, if the amplification factor K is already known, for example, from a manufacturer specification and / or the result of a measurement carried out previously. In this case, the amplification factor K can be determined (see equation 1) from the detected values Vout 1, Vout 2.In addition, the value of "R" can be determined in the evaluation in step S 4 if, for example, from the manufacturer specification and / or result of a measurement, (at least) a value pair "R 0, T 0" (base resistance value R 0 and associated normal temperature value) and the temperature coefficient "alfa" of the resistor are known, and a measurement of the resistor temperature T is carried out in the calibration method. Instead of a measurement of T during the calibration method (in particular e.g. during step S 1 and / or during step S 3), an estimation of T by means of a model and e.g. a measurement (e.g. during step S 1 and / or during step S 3) of the ambient temperature Tsens is also considered.For more accurate determination and / or characterization of the resistance, it is preferred that in step S 4 (at least) a value pair "R 0, T 0" (base resistance value R 0 and associated normal temperature value) is used or determined, for example in order to take into account the temperature dependence of the resistance value R when using the "thermal model" (cf. equation 5).The parameters R 0, T 0 (together with the associated parameter alfa) as such, or parameters derived therefrom, can then be provided as calibration parameters determined in step S 4, so that the temperature dependence of the value "R" can advantageously be taken into account during the later operation of the current sensor (in order to provide the sensor output signal OUT, for example, as exactly as possible in proportion to the current intensity I).Utilizing the thermal model based on equation 5 may include, for example, numerically solving the differential equation system defined by equations 1, 2, and 5 with appropriate boundary and initial conditions (e.g., starting values of T and Tsens at the beginning of a modeling period). The same applies analogously to other (for example more complicated) models which can be provided within the scope of the invention (instead of equations 1, 2 and 5).This modeling during the calibration method can be used to initially determine unknown parameters in the model, for example by means of an adaptation calculation (for example with "least square fit"), in order to provide the (adapted) parameters (for example "W" and / or "Cp") thus determined, or parameters derived therefrom, as calibration parameters determined in step S 4.The same modeling can be provided during the later operation of the current sensor and can be used, for example, in particular to determine the current (time-dependent) temperature T of the resistor from the time-dependent values of Vout and / or OUT and Tsens by the adaptation calculation (for example with "least square fit"). This temperature T can then be taken into account during operation of the current sensor according to equations 1 and 2, so that during sensor operation the output signal OUT shows the desired dependence on I, i.e. is proportional as exactly as possible to the current intensity I.Step S 5 of the calibration method is used to store or update (in the case of a "recalibration") the calibration parameters determined in step S 4 in the storage device of the digital signal processing device of the current sensor, and the calibration method is finished.During operation of the current sensor thus calibrated, the conversion of the converter output signal Vout into the output signal OUT carried out by the signal processing device can advantageously be controlled, for example, by a control program which is stored, for example, in the same memory device as the calibration parameters. Alternatively, however, a separate storage device can also be provided for this purpose. The signal processing device can have a digital computing device and (at least) a memory device and can be implemented, for example, as a microcontroller.The apparatus provided according to a further aspect of the invention for calibrating a current sensor of the type described here comprises:an interface device connectable to the current sensor,a controllable current source for supplying current to the resistor of the current sensor via the interface device,an actuatable (e.g. at least one switchable and switchable) detection device for detecting the digital converter output signal of the current sensor via the interface device, anda control device for controlling the current source and the detection device and for determining calibration parameters by evaluating the converter output signal detected by means of the detection device,wherein the control device is configured to store the determined calibration parameters via the interface device in the memory device of the current sensor and thus to execute a method for calibrating the current sensor according to one of claims 1 to 5.The embodiments and special configurations described for the method according to the invention can, individually or in any combination, also be provided in an analogous manner as embodiments or special configurations of the device according to the invention, and vice versa.The interface device of the device has an electrical interface part, e.g. a two-pole electrical connection (e.g. plug connector or the like), at least for supplying current to the resistor of the current sensor.For the acquisition of the digital converter output signal "Vout" of the current sensor and for the storage (transmission) of the determined calibration parameters into the storage device of the current sensor, the interface device can also have an electrical interface part (e.g. plug connectors or the like, e.g. for digital data transmission). Alternatively, however, the interface device could also have an interface part designed for wireless data transmission for this purpose.The drivable current source of the calibration device should be able to bring about the energization of the resistor with at least one specific test current intensity "Itest" defined as precisely as possible. In one embodiment, this test current intensity is adjustable (e.g., by a user input), e.g., in steps or continuously.The controllable detection device for detecting the digital converter output signal Vout of the current sensor can be implemented, for example, as a functional component of the control device, which also serves for controlling the current source. Alternatively, however, a detection device (e.g. comprising a digital data interface according to a data bus standard) implemented separately from the control device but communicatively connected to the control device can also be provided, for example.The control device for driving the current source and said detection device (which may represent a functional component of the control device) and for determining calibration parameters may be implemented as a program-controlled digital data processing device (e.g. microcontroller or the like).The control device can have, for example, a computing device and an associated storage device for storing data and a program controlling the operation of the computing device.The control device can evaluate the converter output signal Vout (in particular its values Vout 1, Vout 2 and Vout 3,... ) detected by the detection device on the basis of an evaluation algorithm predefined by the control program in order to determine the calibration parameters on the basis of the result of the evaluation and to bring about their transmission via the interface device into the storage device of the current sensor.The control device or the relevant control program is designed to execute a calibration method of the type already described above in order to calibrate a current sensor connected to the calibration device.In one embodiment variant of the invention, the calibration device is designed as a portable device or comprises at least one portable device which has at least the interface device of the calibration device, so that a user can connect the device to the latter for the purpose of calibrating a current sensor.In another variant embodiment, the calibration device is designed as a "tool insert" or comprises at least one such tool insert which has at least the interface device of the calibration device, so that an automated tool, such as a robot or the like, can connect the tool insert to the latter for the purpose of calibrating a current sensor.For both variant embodiments, according to a preferred use of the calibration method or the calibration device, it is provided that the calibration takes place at the end of a production line (e.g. for series production) of current sensors of the type described here, or at least before installation of such current sensors in their use environment (e.g. in a vehicle).According to a further preferred use of the calibration method or the calibration device, it is provided that a current sensor is calibrated therewith, which is provided for measuring a current, the current intensity of which can assume values of more than 100 A during operation.In one embodiment of the invention, the test current intensity provided during calibration is at least 50%, in particular at least 80%, of a maximum current intensity operationally provided for the current to be measured. On the other hand, it is usually sufficient and expedient if the test current intensity is at most 100% of the maximum current intensity provided during operation.According to a further preferred use of the calibration method or the calibration device, it is provided that a current sensor is calibrated therewith, which is provided for measuring a current of an electrical energy storage device (e.g. battery or accumulator) on board a vehicle (whether during charging or during discharging of the energy storage device). In particular, this can be an energy storage device from which an electric drive device of the relevant vehicle is supplied (e.g. electrically operated road motor vehicle).The invention will be described further below with reference to exemplary embodiments with reference to the attached drawings. They represent: FIG. 1 is a block diagram of a current sensor together with a calibration device connected thereto for the purpose of calibrating the current sensor, FIG. 2 shows exemplary time curves of a current intensity and of a converter output signal of the current sensor during a calibration method carried out by means of the calibration device, and FIG. 3 is a flow chart of the calibration method.FIG. 1 shows a current sensor 10 and a device 100 for calibrating the current sensor 10 according to an exemplary embodiment. The current sensor 10 has a resistor 20 ("shunt") with a resistance value "R", which can be supplied with a current I to be measured, so that a voltage "V" dropping thereon as a result of an energization of the resistor 20 results in V=R.times.I.Current sensor 10 further includes an amplifier 30 for amplifying the voltage V dropped across resistor 20, between its terminals 21, 22, and outputting a corresponding analog amplifier output signal Vout*. In the example shown, a voltage amplification takes place with a fixed predetermined amplification factor "K". For this purpose, the amplifier 30 is implemented, for example (not shown), as an operational amplifier which is connected in a corresponding manner for voltage amplification.The current sensor 10 further comprises an A / D converter 40 for converting the analog amplifier output signal Vout* into a corresponding digital converter output signal Vout, i.e. the "digital value" of Vout corresponds to the value of the voltage Vout*: Vout ≙ Vout*.The current sensor 10 further comprises a signal processing device 50 (e.g. microcontroller) which in the example shown comprises a digital computing device 52 and a digital storage device 54. The storage device 54 stores a processing program that controls the operation of the signal processing device 50, by means of which the converter output signal Vout is processed into a (e.g. proportional) output signal OUT of the current sensor 10 that is representative of the electric current I. In the example shown, the "digital value" of OUT shall correspond to the value of I: OUT ≙ I.Given the hypothetical assumption ("ideal case") that the resistance value R is invariable and known and that the amplifier 30 operates offset-free and with invariable and known amplification factor K, the converter output signal Vout would already be representative of the electric current I:However, this is not the case in practice. In the example shown, it is rather assumed, for example, in particular that the resistance value R depends on a temperature T of the resistor 20, and that the amplifier 30 has an output offset "Voffset" and a gain factor K, wherein the exact values of these latter characteristic values Voffset and K are not known exactly on account of manufacturing tolerances. In this case, the following applies:Vout=I x R(T) x K+Voffset, with a resistance value R dependent on the temperature T and an offset voltage Voffset.The calibration method provided according to the invention, which is carried out in the example by means of the calibration apparatus 100, serves to determine "calibration parameters" and to store them in the storage device 54 of the current sensor 10 (or update them by means of recalibration).Advantageously, this enables that during the later operation of the current sensor 10, the signal processing device 50 of the current sensor 10 can "convert" the (defective) digital converter output signal Vout into a "correct" sensor output signal OUT using the stored calibration parameters.It is understood that values toe determined as calibration parameters within the scope of the invention, in particular by the calibration method, which quantitatively specify the properties of the resistor 20 (e.g. resistance value R and / or base resistance and associated normal temperature and / or temperature coefficient, etc.) and / or the properties of the amplifier 30, optionally with consideration of the A / D converter 40 (e.g. offset voltage Voffsetand / or amplification factor K, etc.). Alternatively or additionally, however, mathematically derived parameters (which from mathematical point of view are equivalent for calculating the correct output signal OUT of the current sensor teyen) can also be provided as calibration parameters within the scope of the invention.In the example shown, the current sensor 10 furthermore has an ambient temperature sensor 60, by means of which a temperature "Tsens" in a vicinity of the resistor 20 can be measured. The sensor 60 is arranged, for example, on a circuit carrier plate which also carries the resistor 20, the amplifier 30, the A / D converter 40 and the signal processing device 50. In particular, the components 40 and 50 (and / or 30) can be implemented here, for example, in a combined manner in a common integrated circuit, for example an ASIC module.In one embodiment of the invention, during operation of the (calibrated) current sensor 10, the ambient temperature Tsensdetected by means of the temperature sensor 60 is used by the signal processing device 50 to take account of (compensate for) a temperature dependence of the resistance value R when generating the sensor output signal OUT, for example by estimating (modelling) T based on the measurement of Tsens. In a development of this embodiment, the sensor 60 is additionally used during the calibration method (by the device 100).The device 100 for calibrating the current sensor 10 has an interface device which can be connected to the current sensor 10 and which, in the example shown, is formed by interface parts 110, 120 and 130, each in the form of electrical connections. The interface device can be implemented, for example, by one or more plug connectors or the like.The device 100 further comprises an actuatable current source 140, by means of which the resistor 20 of the current sensor 10 can be energized with a precisely predefinable current intensity via the interface part 110.The apparatus 100 furthermore has a controllable detection device 150, by means of which the digital converter output signal Vout of the current sensor 10 can be detected via the interface part 120.The apparatus 100 furthermore has a control device 160 designed as a program-controlled device (e.g. microcontroller), by means of which, when performing the calibration method, on the one hand the current source 140 and the detection device 150 are driven and, on the other hand, the calibration parameters are determined by evaluating the converter output signal Vout detected by means of the detection device 150. At the end of the calibration process, the controller 160 effects a transfer of the determined calibration parameters via the interface portion 130 to store the calibration parameters in the storage device 54 of the current sensor 10. The calibration method executed by the apparatus 100 is predefined by a calibration program stored in a storage device of the control device 160.In one embodiment, the interface part 130 is designed for bidirectional data transfer, so that, in particular, e.g., the value Tsens measured by the temperature sensor 60 can also be transmitted during the calibration method in order to be able to take corresponding temperature data into account during the evaluation in step S 4 of the calibration method.FIG. 3 shows a flow chart of the calibration method, which has the following stepsStep S 1: detecting (at least) a first value Vout 1 of the converter output signal Vout when the resistor 20 is not energized.Step S 2: detecting (at least) a second value Vout 2 of the converter output signal Vout immediately after the energization of the resistor 20 is switched on with a predetermined test current Itest, when the resistor 20 is energized with a predetermined test current Itest.Step S 3: Acquisition of a plurality of third values Vout 3,... of the converter output signal Vout in a temporal profile after the switching on of the energization of the resistor 20 with the test current intensity Itest, upon energization of the resistor 20 with the predefined test current intensity Itest and a temperature increase of the resistor 20 due to the test current and consequently a change of the converter output signal Vout.Step S 4: Determining the calibration parameters by evaluating the acquired first, second and third values Vout 1, Vout 2, Vout 3,... of the converter output signal Vout.Step S 5: Storing the determined calibration parameters in the storage device 54.FIG. 2 shows in the upper part, by way of example, a temporal profile of a current intensity I produced by means of the current source 140 in a calibration method, controlled by means of the control device 160 of the calibration apparatus 100. In the lower part of FIG. 2, the temporal profile of the converter output signal Vout resulting as a result of this energization is shown by way of example.In the example, the calibration method begins with step S 1 at a time t 1, at which the controller 160 controls the current source 140 such that no current flows through the resistor 20 (I=0). Then, the controller 160 controls the detector 150 to detect the first value Vout 1 of the converter output signal Vout and stores it in the storage 162 for later processing (step S 4). In FIG. 2, this detection is symbolized by a point.Step S 2 is performed at a time t 2. The control device 160 controls the current source 140 such that the current I suddenly increases to a value of the test current intensity Itest, which can be predetermined, for example, by a user. In the example of FIG. 2, Itest=500 A. Then, the controller 160 controls the detector 150 to detect the second value Vout 2 of the converter output signal Vout and also stores it for later processing. In FIG. 2, this detection is symbolized by a point.Step S 3 begins, for example, immediately after time t 2 and can last, for example, until the indicated time t 3 (or be provided shorter, for example). In the example shown, the control device 160 continues to drive the current source 140 in step S 3 such that I=Itest applies. In addition, the control device 160 controls the detection device 150 in order to detect the third value Vout 3 of the signal Vout beginning shortly after t 2 and then in the example to detect "further third values" Vout 4,... at points in time predefined by the calibration program. The plurality of third values Vout3, Vout4,... are also temporarily stored in the control device 160 for the later processing steps. In FIG. 2, five of these individual detections (taking place after the time t2) are symbolized by way of example in each case by a point. In the example, the individual detections take place in a time-equidistant manner.The detection in step S 3 serves for (subsequent) determination of one or more parameters for characterizing the time profile of the converter output signal Vout in the time period immediately following the time t 2 (in step S 4).As can be seen from FIG. 2, in the example illustrated, the value of Vout increases in this phase (after t 2) because the resistor 20 heats up as a result of the energization (here: I=Test) and therefore the resistance value R (temperature-dependent) increases, which increases the voltage drop V and accordingly the value of Vout with constant energization. With permanent current flow with I=Itest, the converter output signal Vout would increase asymptotically up to a steady-state final value designated "Voutend" in FIG. 2.In one embodiment, a time constant "PT" drawn in FIG. 2 is determined as a parameter characterizing the temporal profile of Vout starting from the time t 2, on the basis of the second value Vout 2 and the plurality of third values Vout 3,... and specifies the time interval between, on the one hand, the time t 2 and, on the other hand, the time at which a linear continuation of the profile of Vout would reach the final value Voutend immediately after t 2.Alternatively or additionally to the determination of the aforementioned time constant PT, (in step S 4), for example, the slope of the time-dependent curve Vout(t) in the region immediately after t 2 may be determined as a parameter characterizing the time curve of Vout, for example as (Vout3- Vout2) divided by the time interval of the detections of Vout2and Vout3.A time-dependent curve R(t) can be calculated in step S 4 from the time-dependent curve Vout(t) determined with the aid of Itest(t) (e.g. constant). On the basis of a model of the temperature dependence of R (cf. e.g. equation 2), the time-dependent curve R(t) can optionally be further converted into a time-dependent curve T(t).It is understood that (in step S 4) the time profile of Voutfrom the time t 2 can be characterized more accurately and reliably the more individual data points (acquisitions of third values Vout 3,... and associated acquisition times, and optionally associated temperature values T and / or Tsens) are carried out in step S 3.Step S 3 may be ended upon completion of the last single detection of the plurality of third values Vout 3,... If step S 3 is carried out in the example shown, for example, until the time t 3, the converter output signal Vout changes only insignificantly at this time (t 3), i.e., has already almost reached the stationary end value Voutend. On the basis of the third values Vout 3,... determined in step S 3, a very accurate estimation of the value Voutendcan be carried out (in step S 4), for example.For example, in order to shorten the calibration time, however, step S 3 can also be ended already earlier, for example after completion of the five individual detections of third values Vout 3,... symbolized by the five points in FIG. 2. In this case, within the scope of the evaluation, for example, the further profile can be estimated using a model, for example in order to ascertain parameters such as PTand / or Voutend. Upon termination of the detection of third values Vout 3,..., energization can also be terminated.In step S 4, which can begin e.g. immediately after the end of step S 3, the control device 160 in the example causes the current source 140 to be switched off, so that the current falls back to I=0. In any case, the control device 160 evaluates all the first, second and third values Vout 1, Vout 2, Vout 3,... of the converter output signal Vout (and temperature data T and / or Tsens possibly detected in the phase of steps S 1 to S 3) (in step S 4) according to an evaluation algorithm which is defined by the calibration program stored in the control device 160 in order to determine the calibration parameters according to the result of this evaluation and to buffer them, for example (until step S 5) starts.In step S 5, which follows step S 4 directly, for example, the control device 160 causes the calibration parameters determined in step S 4 to be stored in the storage device ( 54) of the current sensor 10 by carrying out a corresponding data transmission via the interface part 130. The method for calibrating the current sensor 10 is thus ended.

Claims

Method for calibrating a current sensor (10), wherein the current sensor (10) comprises: - a resistor (20) to which a current (I) to be measured can be supplied, - an amplifier (30) for amplifying a voltage (V) dropping across the resistor (20) as a result of the resistor (20) being supplied with current by a gain factor (K), and for outputting a corresponding analog amplifier output signal (Vout*), - an A / D converter (40) for converting the analog amplifier output signal (Vout*) into a corresponding digital converter output signal (Vout), a digital signal processing device (50) for processing the converter output signal (Vout) using calibration parameters stored in a storage device (54) of the signal processing device (50) to form an output signal (OUT) of the current sensor (10) representative of the electric current (I), characterized in that the method has the following steps: - detecting (S1) a first value (Vout1) of the converter output signal (Vout) when the resistor (20) is not energized, - detecting (S2) a second value (Vout2) of the converter output signal (Vout) immediately after the energization of the resistor (20) with a predetermined test current intensity (Itest) is turned on, when the resistor (20) is energized with the test current intensity (Itest), - detecting (S3) a plurality of third values (Vout3, Vout4,... ) of the converter output signal (Vout) in a temporal profile after the switching on of the energization of the resistor (20) with the test current intensity (Itest), upon energization of the resistor (20) with the test current intensity (Itest) and test current-dependent temperature increase of the resistor (20) and consequently a change of the converter output signal (Vout), - determining (S4) the calibration parameters by an evaluation of the detected first, second and third values (Vout1, Vout2, Vout3, Vout4,... ) of the converter output signal (Vout), - storing (S5) the determined calibration parameters in the storage device (54).Method according to Claim 1, wherein the plurality of third values (Vout3, Vout4,...) are detected at least until a time (t3) at which the converter output signal (Vout) changes only insignificantly at most.Method according to one of the preceding claims, wherein the evaluation of the detected first, second and third values (Vout1, Vout2, Vout3, Vout4,...) of the converter output signal (Vout) is based on a model for the value of the converter output signal (Vout) according to the following equation: Vout = I × R × K + Voffset wherein: Vout denotes the value of the converter output signal (Vout), I denotes the current intensity of the current (I), R denotes the resistance of the resistor (20), K denotes the amplification factor (K) of the amplifier (30), Voffset denotes an offset voltage of the amplifier (30).Method according to claim 3, wherein the model takes into account a temperature dependence of the resistance R, for example according to the equation R ( T ) = R 0 × ( 1 + alfa × ( T - T 0 ) ) wherein: R(T) denotes the resistance R of the resistor (20) as a function of a temperature T of the resistor (20), alfa denotes a temperature coefficient of the resistor (20), R0 and T0 denotes a base resistance (R0) of the resistor (20) and the associated normal temperature (T0), and wherein for this consideration - a measurement of the temperature T of the resistor (20) and / or an ambient temperature (Tsens) is carried out, and / or - by evaluating the acquired first, second and third values (Vout1, Vout2, Vout3, Vout4,...) an estimation of the temperature T of the resistor (20) is performed.Method according to claim 3 or 4, wherein in the determination (S4) of the calibration parameters by evaluating the acquired first, second and third values (Vout1, Vout2, Vout3, Vout4,...) of the converter output signal (Vout), at least the values of Voffset and K or at least the values of Voffset and R are determined.Device (100) for calibrating a current sensor (10), comprising - a resistor (20) to which a current (I) to be measured can be supplied, - an amplifier (30) for amplifying a voltage (V) dropping across the resistor (20) as a result of the resistor (20) being supplied with current by a gain factor (K), and for outputting a corresponding analog amplifier output signal (Vout*), - an A / D converter (40) for converting the analog amplifier output signal (Vout*) into a corresponding digital converter output signal (Vout), - a digital signal processing device (50) for processing the converter output signal (Vout) using calibration parameters stored in a storage device (54) of the signal processing device (50) to form an output signal (OUT) of the current sensor (10), which output signal is representative of the electric current (I), characterized in that, the device (100) comprising: an interface device (110, 120, 130) connectable to the current sensor (10), a controllable current source (140) for supplying current to the resistor (20) of the current sensor (10) via the interface device (110, 120, 130), a controllable detection device (150) for detecting the digital converter output signal (Vout) of the current sensor (10) via the interface device (110, 120, 130), a control device (160) for controlling the current source (140) and the detection device (150) and for determining calibration parameters by evaluating the converter output signal (Vout) detected by the detection device (150), and wherein the control device (160) is configured to determine the determined calibration parameters via the interface device (110, 120, 130) to be stored in the storage device (54) of the current sensor (10) and thus to carry out a method (S1 to S5) for calibrating the current sensor (10) according to one of the preceding claims.Use of a method according to any one of claims 1 to 5 and / or of a device (100) according to claim 6, for calibrating a current sensor (10) provided for measuring a current (I) whose current intensity can assume values of more than 100 A in operation.Use of a method according to one of Claims 1 to 5 and / or of a device (100) according to Claim 6 or 7, for calibrating a current sensor (10) which is provided for measuring a current (I) of an electrical energy storage device on board a vehicle, wherein the energy storage device is provided for supplying a power to an electrical drive device of the vehicle.

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