Method for determining the resistance value of a measurement resistor and assembly for carrying out such a method

Abstract

The invention relates to a method for determining the resistance value of a measurement resistor, comprising the following steps: a) providing an assembly for measuring the strength of an electric current, wherein the assembly comprises a measurement resistor and a reference resistor, b) defining a resistance ratio, c) measuring the resistance value of the measurement resistor by means of a first measurement device, d) measuring the resistance value of the reference resistor, e) adjusting the resistance value of the reference resistor such that the ratio of the resistance value of the reference resistor to the resistance value of the measurement resistor equals the defined resistance ratio, f) integrating the assembly in an application environment, g) measuring the resistance value of the reference resistor in the application environment by means of a second measurement device, h) determining the resistance value of the measurement resistor from the resistance value of the reference resistor measured in step g) and the resistance ratio known from step b). The invention also relates to an assembly for carrying out such a method.

Description

[0001] Description

[0002] Method for determining the resistance value of a measuring resistor and assembly for carrying out such a method

[0003] The invention relates to a method for determining the resistance value of a measuring resistor and an assembly for carrying out such a method.

[0004] Current measurements in electronic circuits are carried out using measuring resistors connected in series with the component to be monitored. The current is determined according to Ohm's law from the voltage drop across the measuring resistor, known as the shunt resistor. The resistance value is assumed to be known – within certain accuracy limits. Accurate current measurement is particularly important, for example, in the battery management system of an electric or hybrid vehicle. Due to the high currents involved, measuring resistors of 10 pOhm to 200 pOhm are typically used here.

[0005] Due to the high accuracy requirements for current measurement, it is necessary to reduce the uncertainty in the resistance value of the measuring resistor. The requirement is to reduce the uncertainties to less than ± 0.5%. For technical reasons, even within a batch of measuring resistors, the resistance values ​​of individual measuring resistors can vary by significantly more than ± 0.5%. To maintain tolerance, resistors with resistance values ​​outside the tolerance range can be rejected. However, this would increase scrap and ultimately increase manufacturing costs.

[0006] As an alternative to sorting, the individual resistance value of each measuring resistor can be measured with the required accuracy, i.e., calibrated, and this information can be made available to the user of the measuring resistor or the application environment in which the measuring resistor is used in a suitable manner. The application environment could, for example, be a battery management system. Measuring resistances in the range of 100 pOhms requires a precision measuring device. With such small resistance values, the resistances of the measuring leads must also be taken into account, making resistance measurement using four-wire technology mandatory.Such measurement technology is typically not available in the application environment in which the measuring resistor is to be used, so calibrating each individual measuring resistor in its application environment, i.e., on-site, is not possible or would at least require significant additional effort. However, the calibration of an individual measuring resistor can be performed without great effort by the manufacturer of the measuring resistor. The measured individual, i.e., product-specific resistance value of each individual measuring resistor must then be added to each individual measuring resistor as product-specific information in such a way that this measured value is available when the measuring resistor is integrated into its application environment.

[0007] From the publication DE 11 2016 002 799 T5 it is known that the individual

[0008] The resistance value of a shunt resistor is stored as information in a code display section on the shunt. The code can, in particular, be a Data Matrix code (DMC). Document DE 20 2022 104228 U1 further proposes attaching such a code display section in a recess of the shunt surface to protect it from damage. However, reading the code requires the shunt user to have a suitable readout device and a secure process through which the read information is then correctly assigned, for example, by programming it into the exact battery management system in which the shunt resistor is used. The process is complex and fraught with risks.

[0009] The invention is therefore based on the object of providing an alternative method for determining the resistance value of a measuring resistor. Furthermore, the invention is based on the object of providing an assembly that enables the implementation of such a method.

[0010] The invention is represented with respect to a method by the features of claim 1 and with respect to an assembly by the features of claim 7. The further dependent claims relate to advantageous embodiments and developments of the invention.

[0011] The invention relates to a method for determining the resistance value of a measuring resistor, the method comprising the following steps: a) providing an assembly for measuring the strength of an electrical current, the assembly comprising a measuring resistor and a reference resistor, b) defining a resistance ratio, c) measuring the resistance value of the measuring resistor with a first measuring device, d) measuring the resistance value of the reference resistor, e) adjusting the resistance value of the reference resistor so that the ratio of the resistance value of the reference resistor to the resistance value of the measuring resistor is equal to the defined resistance ratio, f) integrating the assembly into an application environment, g) measuring the resistance value of the reference resistor in the application environment by means of a second measuring device,h) Determining the resistance value of the measuring resistor from the resistance value of the reference resistor measured in step g) and the resistance ratio known from step b).

[0012] The invention is based on the idea that resistances of at least 1000 ohms can be measured precisely much more easily than resistances of the order of 100 pOhms. Due to the high resistance value, the resistance of the test leads is insignificant at 1000 ohms, so a two-wire technique provides sufficiently accurate results. To measure the resistance value, it is sufficient to apply a current of a few milliamperes to the resistor to obtain a voltage drop of a few volts as the measurement signal.

[0013] Against this background, the invention proposes providing an assembly comprising a measuring resistor and a reference resistor. Furthermore, a specific resistance ratio is defined between the resistance value of the reference resistor and the resistance value of the measuring resistor. The resistance ratio is preferably at least 100,000, more preferably at least 1,000,000, and at most 100,000,000,000, more preferably at most 1,000,000,000. Thus, if the resistance value of the measuring resistor is, for example, 100 pOhm, the resistance value of the reference resistor can be, for example, 1000 ohms, so that the resistance ratio is 10,000,000. The resistance value of the measuring resistor is measured with sufficient accuracy using a first measuring device. The resistance value of the reference resistor is also measured. This is preferably done using the same measuring device in order to minimize systematic measurement errors.If the ratio of the resistance value of the reference resistor to the resistance value of the measuring resistor does not correspond to the predefined resistance ratio, the resistance value of the reference resistor is adjusted by machining the reference resistor (step e). This can be done, for example, by laser trimming. Here, the reference resistor is adjusted (usually an increase) of up to 10% of the original resistance value by ablation and / or an incision in the resistor layer or in the resistor material. Preferably, the resistance value is trimmed by ablation and / or incisions perpendicular to the direction of current flow through the resistor. The vertical trimming can be performed multiple times next to each other or in a meandering pattern.Furthermore, the vertical trimming process can be combined with removal and / or incision in another direction, for example, to create an L-shaped incision, in order to achieve the most precise adjustment of the reference resistor. The resistance value is adjusted in an active process in which the reference resistor is continuously measured during the adjustment process and the adjustment is terminated when the target value is reached. The reference resistor is therefore adjusted until the ratio of the measured resistance values ​​is sufficiently identical to the predefined resistance ratio. The resistance value of the reference resistor is then a defined multiple of the resistance value of the measuring resistor. In this way, the reference resistor is provided with information based on an individual measurement regarding the actual resistance value of the measuring resistor.The reference resistor is thus calibrated to a precisely known multiple of the measuring resistance. These steps can be performed with a variety of assemblies, for example, with each assembly in a batch or with each assembly of a specific type. The resistance value of the reference resistor is always adjusted so that the resistance ratio is identical for all assemblies with sufficient accuracy, even if the resistance value of the individual measuring resistors varies from assembly to assembly. The resistance ratio is thus a property that applies equally to all assemblies.

[0014] An assembly containing the measuring resistor and the reference resistor matched to the measuring resistor is integrated into an application environment. The user or operator of the application environment knows the resistance ratio as a property that is identical for all assemblies, for example from a data sheet or another information source. In the application environment, the resistance value of the reference resistor is measured using a second measuring device. Because the resistance value of the reference resistor is several orders of magnitude greater than the resistance value of the measuring resistor, a simple measuring technique is sufficient to achieve sufficient accuracy. The resistance value of the measuring resistor of the assembly is determined by division from the measured resistance value of the reference resistor and the known resistance ratio, which applies equally to every assembly.

[0015] The particular advantage is that a simple measurement technique, which is available in many applications, can be used to measure the resistance of the reference resistor in the application environment. Due to the high resistance value, the resistance of the test leads can be neglected, eliminating the need for a four-wire technique. The reference resistor only needs to be supplied with a current of a few milliamperes to obtain a voltage signal of a few volts. Voltages of a few volts can be measured with sufficient accuracy without great effort. Furthermore, parasitic voltages, such as contact voltages or thermoelectric voltages, no longer play a role in this measurement range.

[0016] Furthermore, steps g) and h) can be automated and software-controlled in the application environment. For example, after the module has been integrated into the application environment, a software-controlled measuring device can perform the resistance measurement on the reference resistor. The measured value is read by a program, divided by the previously known resistance ratio, and the resulting resistance value of the measuring resistor is stored in the program for further use in current measurement. This allows a high degree of automation to be achieved. This reduces costs and problems caused by human error.

[0017] Within the scope of one embodiment, the assembly may comprise a carrier element, in particular a printed circuit board, and the reference resistor may be mounted on the carrier element. The reference resistor is thus arranged on a component that is, in many cases, already present.

[0018] Within the scope of a further embodiment, the reference resistor can be an SMD resistor, in particular a thin-film or thick-film SMD resistor. SMD resistors are particularly well suited for mounting on a printed circuit board. Furthermore, the resistance value of an SMD resistor can be adjusted particularly well and precisely. Types 1206, 0612, 0805, or 0603 are particularly preferred. Within the scope of a further embodiment, the first measuring device can be a precision measuring device suitable for measuring a resistance value in the order of 100 pOhm with an uncertainty of at most 0.5%, preferably at most 0.2%.

[0019] In another embodiment, the application environment can be a battery management system. A battery management system typically includes a measuring device capable of measuring the reference resistance with sufficient accuracy.

[0020] Within the scope of a further embodiment, the second measuring device may be suitable for measuring a resistance value in the order of 1000 ohms with an uncertainty of at most 0.5%, preferably at most 0.2%. In particular, the second measuring device may contain an analog-to-digital converter.

[0021] With regard to further technical features and advantages of the method according to the invention, reference is hereby explicitly made to the explanations in connection with the assembly according to the invention and to the exemplary embodiment.

[0022] A further aspect of the invention relates to an assembly for carrying out the method described above. The assembly comprises a measuring resistor and taps for measuring the voltage drop across the measuring resistor when current flows. Furthermore, the assembly comprises a reference resistor and at least one further tap for measuring the voltage drop across the reference resistor when current flows. The further tap can be a pin, for example. The voltage drop across the reference resistor can be measured against the zero potential, for which a tap is usually already present on the assembly. In one embodiment, the reference resistor can be machined such that its resistance value is changed compared to its original resistance value as a result of the machining. The machining can be or include a removal of resistor material, in particular a reduction in the thickness of the resistor material.It is also possible for at least one incision to be formed in the resistor material during machining. The resistance value is preferably trimmed by removal and / or incisions perpendicular to the current flow direction of the resistor. The vertical trimming can be performed multiple times next to each other or in a meandering pattern. Furthermore, the vertical trimming process can be combined with removal and / or incision in another direction, for example to form an L-shaped incision. In this way, a highly precise adjustment of the reference resistor is achieved. The described machining typically increases the resistance value of the reference resistor. Consequently, the reference resistor must be selected such that its resistance value in the unmachined state is so low that it can be adjusted to the smallest possible resistance value of the measuring resistor.By machining, the resistance value of the reference resistor can typically be increased by up to 10% of the original resistance value.

[0023] With regard to further technical features and advantages of the assembly according to the invention, reference is hereby explicitly made to the explanations in connection with the method according to the invention and to the exemplary embodiment.

[0024] The invention is explained in more detail using an embodiment.

[0025] A variety of assemblies are manufactured, each containing a measuring resistor with a nominal resistance of 100 pOhm and a reference resistor. For manufacturing reasons, the variation in the resistance value of the measuring resistor is ± 5%, so the actual resistance value of a measuring resistor can be between 95 and 105 pOhm. The resistance ratio of the reference resistor to the measuring resistor is specified as 10,000,000. The resistance value of the reference resistor is adjusted accordingly by laser trimming. Consequently, depending on the actual resistance value of the measuring resistor, the resistance value of the reference resistor after adjustment can be between 950 and 1050 ohms. Each assembly is then installed in a battery management system.The battery management system has a 3.3 V voltage source and a voltage divider with a fixed resistance of 1000 ohms, to which the module's reference resistor is connected in series. If the reference resistor is 950 ohms, a voltage drop of 1.608 V occurs across it. If the reference resistor is 1050 ohms, a voltage drop of 1.690 V occurs across it. The tolerance range of the measuring resistors, totaling 10 pOhm, is thus mapped to a voltage signal with a variation range of 82 mV. If a simple 10-bit analog-to-digital converter at 3.3 V is used for the voltage measurement, the resolution is 3.2 mV per digit. The variation range of the voltage signal is thus divided into 25 digits. Each digit then corresponds to a resistance change of the measuring resistor of 0.4 pOhm, or 0.4% based on the nominal value of 100 pOhm.With the proposed method, it is thus possible to improve the accuracy of knowing the resistance value of the measuring resistor by approximately a factor of 10 using a simple analog-to-digital converter. A higher accuracy can be achieved with a higher-resolution analog-to-digital converter.

Claims

Patent claims 1. A method for determining the resistance value of a measuring resistor, comprising the following steps: a) providing an assembly for measuring the strength of an electrical current, the assembly comprising a measuring resistor and a reference resistor, b) defining a resistance ratio, c) measuring the resistance value of the measuring resistor with a first measuring device, d) measuring the resistance value of the reference resistor, e) adjusting the resistance value of the reference resistor such that the ratio of the resistance value of the reference resistor to the resistance value of the measuring resistor is equal to the defined resistance ratio, f) integrating the assembly into an application environment, g) measuring the resistance value of the reference resistor in the application environment by means of a second measuring device,h) Determining the resistance value of the measuring resistor from the resistance value of the reference resistor measured in step g) and the resistance ratio known from step b).

2. Method according to claim 1, characterized in that the assembly comprises a carrier element and that the reference resistor is mounted on the carrier element.

3. Method according to claim 1 or 2, characterized in that the reference resistor is an SMD resistor.

4. Method according to one of claims 1 to 3, characterized in that the first measuring device is suitable for measuring a resistance value of 100 pOhm with an uncertainty of at most 0.5%.

5. Method according to one of claims 1 to 4, characterized in that the application environment is a battery management system.

6. Method according to one of claims 1 to 5, characterized in that the second measuring device is suitable for measuring a resistance value of 1000 ohms with an uncertainty of at most 0.5%.

7. An assembly for carrying out a method according to any one of claims 1 to 6, wherein the assembly comprises a measuring resistor and taps for measuring the voltage drop across the measuring resistor, characterized in that the assembly comprises a reference resistor and at least one further tap for measuring the voltage drop across the reference resistor.

8. An assembly according to claim 7, characterized in that the reference resistor is processed in such a way that its resistance value is changed by the processing compared to its original resistance value.