Device and method for measuring the quiescent current of a vehicle sensor

Abstract

The application relates to a device (10) for measuring the quiescent current of a vehicle sensor (12), in particular by way of the peripheral sensor interface 5, PSI5, comprising: - a voltage source (14) and a shunt resistor (16), which is arranged between the voltage source (14) and a reference node, wherein the shunt resistor (16) has a first and a second terminal; - a comparator (22), - an evaluation unit (26) for evaluating the comparator output signal and for outputting an evaluation unit output signal; - a control unit (30) designed to record the evaluation unit output signal and to output a control unit output signal based on the recorded evaluation unit output signal; and - a digital-to-analogue conversion unit (28) designed to convert the control unit output signal into an offset signal and to feed the offset signal into an input path of the comparator (22).

Description

[0001] Device and method for measuring the quiescent current of a vehicle sensor

[0002] The present invention relates to a device for measuring the quiescent current of a vehicle sensor, in particular via the peripheral sensor interface 5 (PSI5), and a corresponding measuring method.

[0003] Knowing the quiescent current of a vehicle sensor is essential for its reliable operation. Knowing the quiescent current is particularly important for communication between a control unit and a vehicle sensor via a communication interface, such as PSI5. Only with knowledge of the quiescent current can the signal received by the vehicle sensor be correctly evaluated by distinguishing between a quiescent current component and a signal pulse superimposed on the quiescent current signal. For example, the quiescent current may be 35 mA, while the current pulse used for communication with the control unit has an amplitude of 25 mA.

[0004] In addition, the quiescent current is an important indicator for the proper functioning of a vehicle sensor. For example, if the quiescent current for a vehicle sensor deviates significantly from the typical quiescent current of that sensor, this can be interpreted as an indication of a faulty function of the vehicle sensor.

[0005] Various devices for measuring quiescent current are known from the state of the art. However, the methods used to date show a strong, sometimes systematic, susceptibility to interference (e.g., EMC).

[0006] DE 10 2007 003 542 A1 describes a method for controlling a personal protection system, in which a sensor signal is provided, and the personal protection system is controlled depending on the sensor signal. To provide the sensor signal, the sensor signal is processed using first and second signal processing to generate a first and a second comparison signal. The first and second comparison signals are compared to generate an input signal for a decoder, wherein the decoder provides the decoded input signal for controlling the personal protection system.

[0007] DE 10 2016 116 059 A1 describes a method for detecting the presence and correct functioning of a data bus capacitance of a data bus interface. In this method, a first voltage level with a predetermined or predefinable voltage value is provided against the reference potential. A second voltage level, whose voltage value against the reference potential depends on the voltage value of the data bus connection against the reference potential, is determined. A voltage supply line is connected to the data bus connection. In addition, a data bus current value of the data bus current flowing out of the PSI5 data bus connection is determined. The determined data bus current value of the data bus current is compared with a first threshold value, whereby a comparison result is generated. Finally, the correct functioning or incorrect functioning of the data bus capacitance and / or the presence orthe absence of data bus capacity is concluded depending on the comparison result.

[0008] Based on the problem described above, the object of the present invention is to provide a device for measuring the quiescent current of a vehicle sensor, which provides reliable measurement results even in the presence of interference.

[0009] To achieve the above-mentioned object, the present invention proposes a device for measuring the quiescent current of a vehicle sensor, in particular via the peripheral sensor interface 5, PSI5, comprising: - a voltage source and a shunt resistor arranged between the voltage source and a reference node, wherein the shunt resistor has a first and a second terminal;

[0010] - a comparator designed to

[0011] - to receive at its first input a voltage value which depends on the voltage at the first terminal of the shunt resistor;

[0012] - to receive at its second input a voltage value which depends on the voltage at the second terminal of the shunt resistor; and

[0013] - to provide at its output a comparator output signal which depends on the two input values; the comparator output signal has a bandwidth of fl;

[0014] - an evaluation unit for evaluating the comparator output signal and for outputting an evaluation unit output signal;

[0015] - a control unit designed to receive the evaluation unit output signal and to output a control unit output signal depending on the received evaluation unit output signal; and

[0016] - an offset generator designed to convert the control unit output signal into an offset signal and to feed the offset signal into the comparator or into an input path of the comparator; wherein

[0017] - the evaluation unit is designed to sample the comparator output signal at a frequency f2, where f2 > 2 • fi.

[0018] The device according to the invention allows a robust measurement of the quiescent current, which delivers reliable measurement results even in the presence of interference. In particular, the oversampling of the comparator output signal with a frequency f2 > 2 • fi allows a reliable measurement of the quiescent current, which is not significantly impaired by interference signals at a frequency of fi. In previously known measuring devices, which are used in particular in conjunction with PSI5, the first frequency at which the measurement data is evaluated is fi = 1 MHz. In practice, problems are often observed when interference signals are present which also have a frequency of f « 1 MHz. The device according to the invention, on the other hand, allows reliable measurement results to be achieved even in the presence of interference signals with a frequency of approximately 1 MHz, by deliberately oversampling the comparator output signal.On the other hand, the device according to the invention allows the components and evaluation frequency used in previously used measuring devices to be retained.

[0019] To measure the quiescent current, the vehicle sensor can be connected to the reference node.

[0020] The shunt resistor (often referred to as a shunt or measuring resistor) has a low resistance.

[0021] The comparator inputs are connected (at least indirectly) to the terminals of the shunt resistor. This allows the comparator to compare the voltage values ​​present at the two terminals of the shunt resistor. The comparator can be connected directly to the terminals of the shunt resistor. Alternatively, individual components can be placed between the terminals of the shunt resistor and the comparator inputs, for example, to adjust the voltage values ​​to a range that can be evaluated by the comparator.

[0022] The comparator is designed to output a "high" signal when the voltage at the positive input is greater than the voltage at the negative input. If the voltage at the positive input is not greater than the voltage at the negative input, a "low" signal is output at the comparator's output.

[0023] The comparator can be designed to receive and evaluate an offset signal at one of its inputs. Furthermore, the adjustable offset can be generated in the comparator. The present invention is therefore not limited to a specific method of providing the offset signal to the comparator.

[0024] In the device according to the invention, it can be provided that the evaluation unit is designed to determine whether the sampled comparator output signal is set to "high" at least n times within a measurement period T = 1 / fi, where n = 1 / 2 x fz / fi is preferably used. For example, fi = 1 MHz and f2 = 12 MHz, where the evaluation unit is designed to determine whether the sampled comparator output signal is set to "high" at least six times within a measurement period T = 1 ps. In this respect, the evaluation unit can be designed to make a majority decision regarding the received values ​​and provide an output signal that depends on whether the received values ​​are set to "high" at least n = 1 / 2 x fz / fi. Alternatively, n can be any value between 1 and fz / fi.

[0025] In addition, it can be provided that the evaluation unit is designed to generate an evaluation unit output signal depending on whether at least a first predetermined proportion of the values ​​received within a measuring period T = 1 / fi is set to "High" or to generate an evaluation unit output signal depending on whether at least a first predetermined proportion of the values ​​received within a measuring period T = 1 / fi is set to "Low".

[0026] According to a preferred embodiment of the device according to the invention, the evaluation unit can be designed to output a "1" if at least a first predetermined portion (for example, half) of the values ​​of the comparator output signal received within a measurement period T = 1 / fi is set to "High"; and to output a "0" if at least a second predetermined portion (for example, half) of the values ​​of the comparator output signal received within a measurement period T = 1 / fi is set to "Low." This results in a majority decision in the evaluation unit, according to which it is determined whether, on average, a limit value is exceeded or undershot. For example, the majority decision can be used to determine whether the quiescent current measured in the previous measurement period was too high or too low compared to the actual quiescent current.Depending on the majority decision, a counter reading of the control unit can then be increased or decreased. For example, the first frequency fi can be set to 1 MHz and the second frequency fz to 12 MHz, with the output signal of the evaluation unit being determined as follows: Furthermore, the device according to the invention can be designed to increase a current counter reading by one value if the value received from the evaluation unit is set to "1"; and to reduce the current counter reading by one value if the value received from the evaluation unit is set to "0". In this way, a digital control unit is provided that allows iterative determination of the quiescent current, wherein the control unit exhibits integral behavior. Furthermore, the control unit can operate at a frequency fi, but preferably decides on the adjustment of the offset signal using a different temporal, linear, or non-linear evaluation method.

[0027] Furthermore, according to the present invention, it can preferably be provided that an additional low-pass filter circuit is arranged at the input of the comparator. The low-pass circuit allows the suppression of high-frequency interference signals that may occur during the measurement. This increases the robustness of the measuring device against high-frequency interference signals. The combination of oversampling and the low-pass circuit further offers the advantage that a low-pass filter with a higher cutoff frequency can be provided without the occurrence of aliasing effects (Nyquist-Shannon sampling theorem). The desired low-pass function can also be provided via the intrinsic properties of the comparator.

[0028] Furthermore, the device according to the invention can be provided with a low-pass filter circuit comprising a capacitor arranged between the two inputs of the comparator, as well as two resistors, each arranged in series with the two inputs of the comparator. This arrangement allows for efficient filtering of differential, high-frequency interference signals that could undesirably affect the measurement result without low-pass filtering. This further increases the accuracy and robustness of the device according to the invention against interference. Furthermore, this arrangement is simple and cost-effective to implement.

[0029] Furthermore, the device according to the invention can be provided with the low-pass filter circuit having a cut-off frequency fi < f c < f2 / 2, preferably fi < f c < 1.4 x fi, particularly preferably fi < f c< 1.3 x fi or fi < fc < 1.2 x fi, and in particular fi < f c < 1.1 x fi. This allows interference signals, which in practice strongly influence the measurement results, to be suppressed, further increasing the robustness of the device according to the invention. The first frequency can preferably be fi = 1 MHz.

[0030] The device according to the invention can also be provided with a second frequency of f2 > 6 • fi, preferably f2 > 12 • fi, and particularly preferably f2 > 24 • fi. In particular, the first frequency can be 1 MHz, while the second frequency can be 6 MHz, 12 MHz, or 24 MHz. Initial studies have shown that oversampling with the aforementioned sampling frequencies (f2) enables a particularly robust and interference-independent measurement of the quiescent current.

[0031] In addition, to solve the problem described above, a method for measuring the quiescent current of a vehicle sensor, in particular via the peripheral sensor interface 5, PSI5, is proposed, the method comprising the following steps:

[0032] Receiving a first voltage value and a second voltage value at the inputs of a comparator and outputting a comparator output signal that depends on the two input values, wherein the first voltage value depends on a voltage at a first terminal of a shunt resistor; the second voltage value depends on a voltage at a second terminal of a shunt resistor; and the comparator output signal has a bandwidth of fl; - sampling the comparator output signal by an evaluation unit;

[0033] - Evaluating the sampled comparator output signal by the evaluation unit and outputting an evaluation unit output signal;

[0034] - Generation of a control unit output signal by a control unit depending on the evaluation unit output signal and outputting the control unit output signal;

[0035] - converting the control unit output signal into an offset signal using an offset generator and feeding the offset signal into the comparator or into an input path of the comparator; wherein

[0036] - the evaluation unit is designed to sample the comparator output signal at a frequency f2, where f2 > 2 • fi.

[0037] The method according to the invention allows a more robust measurement of the quiescent current of a vehicle sensor compared to the methods known from the prior art, wherein the measurement results remain unadulterated even in the presence of interference signals, in particular interference signals with a frequency of fi.

[0038] In addition, the method according to the invention can provide that the evaluation unit outputs an evaluation unit output signal which depends on whether at least a first predetermined proportion of the values ​​received within a measuring period T = 1 / fl is set to "High" or outputs an evaluation unit output signal which depends on whether at least a second predetermined proportion of the values ​​received within a measuring period T = 1 / fl is set to "Low".

[0039] In the method according to the invention, the following method steps can preferably be provided:

[0040] - Outputting a "1" at the output of the evaluation unit if at least half of the values ​​received within a measuring period T = 1 / fi are set to "High"; and - Outputting a "0" at the output of the evaluation unit if less than half of the values ​​received within a measuring period T = 1 / fi are set to "Low".

[0041] According to the method according to the invention, the following method steps can also be provided:

[0042] - Increasing a current counter reading of the control unit by one value if the value received from the evaluation unit is set to "1"; and

[0043] - Reduce the current counter reading of the control unit by one value if the value received from the evaluation unit is set to "0".

[0044] It can also preferably be provided that the method according to the invention comprises the following method step:

[0045] - Performing low-pass filtering of the input signals of the comparator by using a low-pass filter circuit, which is preferably arranged at the input of the comparator.

[0046] Alternatively, it may also be provided that the low-pass filter function is provided by the intrinsic properties of the comparator and that the comparator is selected such that its manufacturing-related processing or delay properties provide the desired low-pass filter properties.

[0047] Furthermore, the method according to the invention can provide that the low-pass filtering of the input signals of the comparator is carried out by using a low-pass filter circuit which has a capacitor arranged between the two inputs of the comparator and two resistors which are each arranged in series with the two inputs of the comparator.

[0048] Furthermore, it can be provided that the low-pass filter circuit has a cutoff frequency fi < f c < f2 / 2, preferably fi < f c < 1.4 x fi, particularly preferably fi < f c< 1.3 x fi, fi < f c < 1.2 x fi, and in particular fi < fc < 1.1 x fi. According to a preferred embodiment, fi = 1 MHz. As already explained above, the desired low-pass function can be provided by a separate circuit at the input of the comparator or by intrinsic properties of the comparator.

[0049] Finally, in the method according to the invention, it can be provided that the second frequency is f2 > 6 • fi, preferably f2 > 12 • fi, and particularly preferably f2 > 24 • fi.

[0050] The present invention will be explained in more detail below with reference to the figures.

[0051] Fig. 1 Measuring device according to the prior art,

[0052] Fig. 2 shows an embodiment of the measuring device according to the invention, and Fig. 3 shows an embodiment of the measuring method according to the invention.

[0053] Fig. 1 schematically shows a device 10 for measuring the quiescent current of a vehicle sensor 12. The device 10 has a voltage source 14 and a shunt resistor 16. The shunt resistor 16 is arranged between the voltage source 14 and a reference node to which the vehicle sensor 12 can be connected. The device 10 also has a comparator 22, wherein the inputs of the comparator 22 are connected to the two terminals of the shunt resistor 16 via a second resistor 18 and a third resistor 20. The first and second resistors 18, 20 can be used to convert the voltage values ​​present at the terminals of the shunt resistor 16 into a range that can be processed by the comparator 22. The comparator 22 is designed to output a comparator output signal at its output which depends on the values ​​received at its inputs.The output signal of comparator 22 may additionally depend on an adjustable internal offset of comparator 22. The comparator output signal has a bandwidth of fi.

[0054] The comparator output signal is received and processed by a control unit 30. The control unit 30 samples the comparator output signal at a frequency fi, evaluates the sampled comparator output signal, and determines a controlled variable dependent on the evaluated signal. For example, it can be provided that an average sensor current is estimated, with the estimate being made based on the past m values. Here, m can be, for example, 4, 6, or 8. Thus, an integral control unit can be provided. However, the present invention is not limited to integral control units. Instead, the control unit described above serves as an illustrative example of the measuring principle used according to the prior art.

[0055] The estimated controlled variable (for example, an estimated average quiescent current) can therefore be adjusted depending on the values ​​received from the control unit 30. The controlled variable can be updated, for example, once per microsecond. The control unit 30 then generates a control unit output signal and transmits this signal to an offset generator 28. The offset generator generates an offset signal that can, for example, be fed into an input path of the comparator 28. Alternatively, it is also possible to feed the offset signal directly into the comparator 22. The comparator 22, the control unit 30, and the offset generator 28 together represent a control loop designed to adapt the controlled variable of the control loop until the controlled variable (for example, the estimated average current of the vehicle sensor) adjusts to the actual value.

[0056] As already mentioned above, the system frequency of the measuring device shown can be 1 MHz. In this case, significant distortions of the measurement results are observed in practice when interference signals with a frequency of f < 1 MHz occur, which are eliminated by the present invention.

[0057] Fig. 2 schematically illustrates an embodiment of the device 10 according to the invention. Just like the device shown in Fig. 1, the device according to the invention also comprises a voltage source 14 and a shunt resistor 16 arranged between the voltage source 14 and a reference node to which a vehicle sensor 12 can be connected. Furthermore, the device 10 comprises a comparator 22 configured to receive a voltage value at its first input that depends on the voltage value at the first terminal of the shunt resistor 16, and a voltage value at its second input that depends on the voltage value at the second terminal of the shunt resistor.A first resistor 18 and a second resistor 20 are provided between the shunt resistor 16 and the inputs of the comparator 22. These resistors are designed to convert the voltage values ​​at the terminals of the shunt resistor 16 into a range of values ​​that can be evaluated by the comparator 22. Furthermore, the device 10 has a third resistor 32, a fourth resistor 34, and a capacitor 36, which together form a low-pass filter circuit 38. The low-pass filter circuit 38 allows the suppression of high-frequency interference signals that could negatively affect the measurement result. As a result, the device 10 is highly robust against high-frequency interference.

[0058] In addition, the device 10 has an evaluation unit 26 designed to sample the comparator output signal at a frequency f2, where f2 > 2 • fi, and then evaluate it. For example, it can be provided that the first frequency fi = 1 MHz and the second frequency f2 = 12 MHz. The evaluation unit can record and evaluate 12 values ​​within a measurement period, which is, for example, 1 ps. It can be provided that the evaluation unit is designed to make a majority decision and generate an evaluation unit output signal that depends on whether a certain proportion (for example, at least 50%) of the received values ​​is set to "High". The evaluation unit output signal is then output to the control unit 30.The control unit 30 can then adjust a controlled variable (for example, an estimated average current of the vehicle sensor) depending on the received values ​​and generate a control unit output signal. The control unit output signal is then transmitted to an offset generator 28, which is designed to generate an offset signal. As already explained above, there are different ways to use the offset signal. For example, the offset signal can be fed into an input path of the comparator 22. Alternatively, the offset signal can also be fed directly into the comparator itself.

[0059] The evaluation unit 26 and the control unit 30 can be designed to generate an output signal with a first frequency fi, where fi can preferably be 1 MHz. The comparator output signal has a bandwidth of fi. In contrast, the comparator output signal is sampled with a second frequency f2, where the second frequency f2 is at least twice as high as the first frequency fi and preferably six times as high, twelve times as high, or 24 times as high as the first frequency fi. In particular, it can be provided that the evaluation unit 26 is designed to generate an evaluation unit output signal that indicates whether the current currently flowing through the shunt resistor 16 is on average above or below the current value determined during the previous measuring period. This can preferably be done via a majority decision in which the sampled values ​​are evaluated within a measuring period.For example, if the first frequency is 1 MHz and the second frequency is 12 MHz, a total of twelve comparator output values ​​are evaluated within a measurement period with a period of 1 microsecond. In this case, the evaluation unit 26 can be configured to determine whether at least six of the twelve output values ​​are greater than the current value determined during the previous measurement period. A majority decision can thus be made, and an evaluation unit output signal updated at a frequency of 1 MHz can be generated based on the majority decision.

[0060] The device according to the invention allows for a robust measurement of the quiescent current of a vehicle sensor, with oversampling in particular allowing greater robustness against interference signals with a frequency of f « fi. Furthermore, the evaluation unit, which is designed to perform a majority decision and generates an output signal with a frequency of f = fi, allows the use of the same components used in previously available measurement systems, thus ensuring high compatibility with previously available measurement systems.

[0061] Fig. 3 schematically shows an embodiment of the method 100 according to the invention. In a first method step 110, a first voltage value and a second voltage value are recorded at the inputs of a comparator, and a comparator output signal is generated depending on the recorded voltage values. The first voltage value depends on the voltage at the first terminal of the shunt resistor, while the second voltage value depends on the voltage at the second terminal of the shunt resistor. In a second method step 120, the generated comparator output signal is sampled by an evaluation unit designed to sample and evaluate the comparator output signal. In a third method step 130, the sampled comparator output signal is evaluated by the evaluation unit, and a corresponding evaluation unit output signal is generated.Depending on the evaluation unit output signal, the counter reading of a control unit is adapted in a fourth method step 140, whereby a control unit output signal is generated. In a fifth method step 150, the control unit output signal is converted into an offset signal using an offset generator, whereby the analog offset signal is fed into the comparator or into an input path of the comparator. In the method 100 according to the invention, the control unit output signal and the evaluation unit output signal can each have a first frequency fi, while the evaluation unit is designed to perform the sampling of the comparator output signal at a second frequency f2, whereby the second frequency f2 is at least twice as high as the first frequency fi. The order of the embodiment of the method according to the invention described above is to be regarded as purely exemplary.The method according to the invention is not limited to an implementation of the method steps in the above-mentioned order.

[0062] LIST OF REFERENCE SYMBOLS Device Vehicle sensor Voltage source Shunt resistor First resistor Second resistor Comparator Evaluation unit Offset generator Control unit Third resistor Fourth resistor Capacitor Low-pass filter circuit Method First method step Second method step Third method step Fourth method step Fifth method step

Claims

CLAIMS 1. Device (10) for measuring the quiescent current of a vehicle sensor (12), in particular via the peripheral sensor interface 5, PSI5, comprising: - a voltage source (14) and a shunt resistor (16) arranged between the voltage source (14) and a reference node, the shunt resistor (16) having a first and a second terminal; - a comparator (22) designed to - to receive at its first input a voltage value which depends on the voltage at the first terminal of the shunt resistor (16); - to receive at its second input a voltage value which depends on the voltage at the second terminal of the shunt resistor (16); and - to provide at its output a comparator output signal which depends on the two input values; the comparator output signal has a bandwidth of fl; - an evaluation unit (26) for evaluating the comparator output signal and for outputting an evaluation unit output signal; - a control unit (30) which is designed to receive the evaluation unit output signal and to output a control unit output signal as a function of the received evaluation unit output signal; and - an offset generator (28) designed to convert the control unit output signal into an offset signal and to feed the offset signal into the comparator or into an input path of the comparator (22); wherein - the evaluation unit (26) is designed to perform the sampling of the comparator output signal at a frequency f2, where f2 > 2 • fi.

2. Device (10) according to claim 1, characterized in that the evaluation unit (26) is designed to output a "1" if at least a first predetermined proportion of the values ​​received within a measurement period T = 1 / fi is set to "High"; and to output a "0" if at least a second predetermined proportion of the values ​​received within a measurement period T = 1 / fi is set to "Low".

3. Device (10) according to one of claims 1 or 2, characterized in that the control unit (30) is designed to increase a current counter reading by one value if the value received from the evaluation unit (26) is set to "1"; and to reduce the current counter reading by one value if the value received from the evaluation unit (26) is set to "0".

4. Device (10) according to one of claims 1 to 3, characterized by an additional low-pass filter circuit (38) arranged at the input of the comparator (22).

5. Device (10) according to claim 4, characterized in that the low-pass filter circuit (38) has a capacitor (36) arranged between the two inputs of the comparator (22), and two resistors (32, 34) each arranged in series with the two inputs of the comparator (22).

6. Device (10) according to one of claims 4 or 5, characterized in that the low-pass filter circuit (38) has a cut-off frequency fi < f c < fz / 2, preferably fi < f c < 1.4 x fi, particularly preferably fi < fc < 1.3 x fi or fi < f c < 1.2 x fi, and in particular fi < f c < 1.1 x fi.

7. Device (10) according to one of claims 1 to 6, characterized in that the second frequency is f2 > 6 • fi, preferably f2 > 12 • fi, and particularly preferably f2 > 24 • fi.

8. Method for measuring the quiescent current of a vehicle sensor (12), in particular via the peripheral sensor interface 5, PSI5, the method comprising the following steps: - receiving (110) a first voltage value and a second voltage value at the inputs of a comparator (22) and outputting a comparator output signal which depends on the two input values, wherein - the first voltage value depends on a voltage at a first terminal of a shunt resistor (16); - the second voltage value depends on a voltage at a second terminal of a shunt resistor (16); and - the comparator output signal has a bandwidth of fi; - sampling (120) of the comparator output signal by an evaluation unit (26); - evaluating (130) the sampled comparator output signal by the evaluation unit (26) and outputting an evaluation unit output signal; - generation (140) of a control unit output signal by a control unit (30) as a function of the evaluation unit output signal and outputting the control unit output signal; - converting (150) the control unit output signal into an offset signal using an offset generator (28) and feeding the offset signal into the comparator or into an input path of the comparator (22); wherein - the evaluation unit (26) is designed to perform the sampling of the comparator output signal at a frequency f2, where f2 > 2 • fi.

9. Method according to claim 8, characterized by the following method steps: - outputting a "1" at the output of the evaluation unit (26), provided that at least a first predeterminable portion of the values ​​received within a measuring period T = 1 / fi is set to "High"; and - Outputting a "0" at the output of the evaluation unit (26), provided that at least a second predeterminable portion of the values ​​received within a measuring period T = 1 / fi is set to "Low".

10. Method according to claim 8 or 9, characterized by the following method steps: - increasing a current counter reading of the control unit (30) by one value if the value received from the evaluation unit (26) is set to "1"; and - Reducing the current counter reading of the control unit (30) by one value if the value received from the evaluation unit (26) is set to "0".

11. Method according to one of claims 8 to 10, characterized by the following method step: - performing low-pass filtering of the input signals of the comparator (22) by using a low-pass filter circuit (38) arranged at the input of the comparator (22).

12. The method according to claim 11, characterized in that the low-pass filtering of the input signals of the comparator (22) is carried out by using a low-pass circuit which has a capacitor (36) arranged between the two inputs of the comparator (22) and two resistors (32, 34) which are each arranged in series with the two inputs of the comparator (22).

13. Method according to one of claims 11 or 12, characterized in that the low-pass filter circuit (38) has a cut-off frequency fi < f c < fz / 2, preferably fi < f c < 1.4 x fi, particularly preferably fi < f c < 1.3 x fi, fi < f c < 1.2 x fi, and in particular fi < f c < 1.1 x fi.

14. Method according to one of claims 8 to 13, characterized in that the second frequency is f2 > 6 • fi, preferably f2 > 12 • fi, and particularly preferably f2 > 24 • fi.

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