Method for an impedance analysis, and measuring assembly

Abstract

The invention relates to a method for an impedance analysis of an electrical component, having the steps of: - periodically exciting the component by means of an electrical excitation signal with a preselected signal shape and with a first repetition frequency, - measuring an electrical current curve and an electrical voltage curve on the component as a function of time over at least one repetition period, wherein - the excitation signal is interrupted at least once within a period duration, and - evaluating the measured current curve and voltage curve over the plurality of repetition periods in order to determine the impedance behavior of the electrical component.

Description

[0001] Description

[0002] Method for impedance analysis and measurement setup

[0003] The invention relates to a method for the impedance analysis of electrical components, as well as a corresponding measuring arrangement. The method and the measuring arrangement are suitable for the investigation of various electrical components, in particular they are suitable for the analysis of electrochemical elements, such as galvanic primary or secondary cells, batteries, electrolysis or fuel cells or stacks.

[0004] Impedance spectroscopy, among other methods, can be used for in-depth analysis of the condition of electrical components. This method is relevant, for example, for the analysis of electrochemical elements such as lithium-ion batteries, fuel cells, or electrolysis cells. However, it can also be usefully applied to other electrical components. In the impedance analysis method, the component under investigation is periodically excited externally during operation. This excitation can be a cyclic increase and decrease of the output current (e.g., via a frequency-modulated load) or an excitation voltage superimposed on the operating voltage. Typically, a frequency generator is used for this purpose. This generates a fully symmetrical signal, such as a sine wave, which is transmitted to the component via a voltage source (called "potentiostatic") or an electrical source or sink (called "galvanostatic").The resulting response signal (voltage fluctuation in the case of galvanostatic excitation, or current signal fluctuation in the case of potentiostatic excitation) is measured on the component under test. A Fourier analysis of the resulting phase shift from excitation to response signal at different excitation frequencies provides information about the real and imaginary parts of the component's intrinsic impedance. The basic function and process can be found in numerous books and publications.

[0005] Technically, this method is complex and susceptible to interference for several reasons: The generation of the periodic excitation signals typically requires a frequency generator capable of generating a wide spectrum of different frequencies. Typically, many frequencies are scanned per measurement in order to map a complex, frequency-dependent impedance pattern. Furthermore, the signals must be transmitted cleanly and with signal fidelity to the cell using suitable voltage sources / current sinks, etc. Interference from cables, lines, or the measurement setup, such as inductances and capacitances, must be laboriously circumvented or measured out using calibration in order to derive a "clean" diagnosis of the component under investigation from the analysis. Ideally, one setup should be identical for all measurements (e.g., aging measurements over time or after component damage, possibly due to temperature influences, etc.). This means:A location- and / or configuration-accurate, repeatable arrangement is necessary. Likewise, external disturbances on the signal path to be measured / excited must be avoided (e.g., load jumps during a measurement cycle). The signals to be measured must be resolved very precisely (both in terms of signal strength and temporal correlation to the excitation). The procedure—particularly for a broad frequency spectrum (depending on the observed characteristics of the component under investigation)—requires a certain amount of measurement time. This is difficult to implement, particularly in mobile applications (e.g., for online analysis in a vehicle). The described restrictions regarding the required hardware severely limit implementation in mobile or temporally non-stationary applications. In addition, the required devices / configurations are comparatively cost-intensive.

[0006] German patent application DE 102023204236.3, which was not published at the time of filing, discloses a simplified method for impedance analysis that eliminates the need for a frequency generator and instead uses cyclic excitation via a passive component to excite the component under test. Cyclic excitation via a pulse, step, or step signal with a repetition frequency enables the application of a Fourier analysis and thus analysis across a broad frequency spectrum.

[0007] A corresponding evaluation method for impedance analysis based on cyclically applied excitation signals is described in more detail in a parallel application with the same filing date as this application. This enables an impedance calculation that, however, is not related to a specific excitation frequency of the interference signal, but rather only takes into account the sampling rate and the interference repetition rate of the excitation signal.

[0008] The present invention is based on the object of simplifying the known methods for impedance analysis of electrical components and further improving the aforementioned simplified methods for impedance analysis. This object is achieved by a method according to the independent method claim and by a measuring arrangement and a circuit according to the independent device claims. Advantageous developments of the invention are specified in the dependent claims.

[0009] According to the inventive method for impedance analysis of an electrical component, an electrical component to be examined is periodically excited by means of an electrical excitation signal with a preselected signal shape and at a first repetition frequency. The course of the electrical current and the electrical voltage at the component is measured as a function of time (at a preselected sampling rate) over at least one repetition period. Preferably, the measurement is carried out over 2, 3, 4 or more repetition periods. According to the invention, the excitation signal is interrupted at least once within a period. The measured current and voltage course over the at least one repetition period is then evaluated for the impedance behavior of the electrical component.

[0010] For the evaluation, reference is made to the German patent application DE 102023204236.3 and the German patent application filed by the same applicant at the same time as the present application.

[0011] To excite the electrical component, a symmetrical, periodic signal does not have to be used; instead, a non-symmetrical excitation signal is sufficient, preferably with a predetermined or known signal shape. This excitation signal can be represented, for example, as a pulse excitation or a step or step function. The signal structure, i.e. the signal shape of the excitation signal itself, is not to be seen as periodic excitation. Instead, the excitation signal itself, e.g. the pulse, is repeatedly impressed cyclically with a repetition frequency. This, in turn, can be used to subject the excitation-response signals measured at the component in the form of the time-dependent voltage and current curves at the component to a so-called Fourier analysis. (For example, a fast Fourier transformation (FFT), a dynamic Fourier transformation (DFT), but also other mathematical methods such as the Laplace transformation can be used.) This transforms the time-dependent measured current and voltage waveforms into frequency-dependent current and voltage spectra, which, however, are not related to a specific excitation frequency of the excitation signal, but rather to the repetition frequency (and the sampling rate of the current or voltage measurement). This, in turn, allows for an impedance calculation in the usual way.

[0012] The secondary frequency amplitudes (also called image frequencies or spectral frequencies in communications engineering) found during the Fourier evaluation can be subjected to a filter and an impedance value can be evaluated for a large number of these secondary frequencies.

[0013] According to the invention, the excitation signal is interrupted at least once during a period (or repetition period). This can be achieved, for example, by opening and closing an electrical switch, e.g., a transistor. The excitation signal is thus divided into at least two parts during a period and applied with an interruption to the electrical component under test. The excitation signal is thus broken down into several parts, thus generating an excitation disturbance with a higher excitation frequency. At the same time, the repetition frequency of the actual excitation signal remains unchanged.

[0014] This makes it possible according to the invention to analyze complex resistances or impedances at higher frequencies. It is particularly advantageous if the excitation signal is interrupted several times within a period. Preferably, the multiple interruptions also occur cyclically at a second frequency which is higher than the repetition frequency. The frequency spectrum to be analyzed can be further increased if a further fragmentation (cyclical interruption) of the excitation signal is provided with at least one further frequency. The second frequency, at least one further frequency and the repetition frequency should advantageously be different from one another. In particular, at least one further frequency is higher than the second frequency. According to a particularly advantageous variant, the second frequency is an integer, in particular an even-numbered, multiple of the repetition frequency.The at least one further frequency can in turn be an integer (e.g. even-numbered) multiple of the second frequency.

[0015] According to a preferred embodiment of the method according to the invention, the excitation signal is generated by an excitation device connected to the electrical component in a circuit, wherein the excitation device has at least one passive component. This can then be interrupted within a period by opening and closing a switch. The passive component is, for example, a capacitor, a coil (inductance), or a resistor. These can each be "precharged" or brought into a defined initial state and then impress a corresponding charge or discharge signal as an excitation signal into the circuit. The initial state is, for example, a charge with a specific voltage in the case of a capacitor, a specific temperature of a (temperature-dependent) resistor, or a specific magnetization state of a coil. The defined initial state can, however, also be a ground state, i.e.for example, in the case of a capacitor the voltage is 0 or in the case of a coil, no magnetization.

[0016] While the passive component is connected to the circuit, it emits an excitation signal into the circuit (e.g., within a repetition period). The excitation signal of a capacitor is, for example, a charging or discharging current. This charging (discharging) current follows a characteristic temporal characteristic depending on the capacitor's design. Different capacitors can differ, for example, in their capacitance and dielectric strength, as well as their charging and discharging characteristics and their time constant (t).

[0017] Typical capacitances are, for example, in the range from less than 100pF up to 10mF, especially in the range from 200pF to 2mF.

[0018] Accordingly, resistors can differ in terms of their temperature dependence, and inductances can differ, for example, in terms of their magnetic permeability, number of windings, etc. These properties of passive components, along with the excitation state, are crucial for the course of the excitation signal in the circuit and, in the context of this application, are also referred to as the characteristic of the respective passive component. In the simplest case, the passive component can be a simple resistor that consumes power whenever it is connected to the component.

[0019] According to the invention, this excitation signal, whose signal form and duration are specified by the passive component's hardware, is interrupted at least once during a period or repetition period. This is preferably done by cyclically opening and closing a switch, as described above. This advantageously increases the analyzable frequency, while the repetition frequency is selected according to the passive component's hardware design, thus lowering the excitation analysis frequency for the impedance analysis.

[0020] As described above, the excitation signal can be interrupted within a repetition period by opening and closing a switch. Preferably, the switch is opened and closed cyclically at at least a second and / or further frequency, e.g., using a clock.

[0021] According to a preferred development, the method according to the invention can be carried out successively at different frequencies. A combined implementation with different frequency combinations, which in turn can be repeated cyclically, is also conceivable.

[0022] According to a second aspect, the invention relates to a measuring arrangement for diagnosing an electrical component in an electrical circuit, comprising: an excitation device configured to periodically emit an excitation signal having a preselected signal shape into the electrical circuit (11) during a repetition period (period duration); a first clock generator for specifying a repetition frequency corresponding to the period duration; an ammeter for measuring an electrical current in the electrical component as a function of time during the period duration, and a voltmeter for measuring an electrical voltage at the electrical component as a function of time during the period duration; and a switch for preferably briefly disconnecting (and reconnecting) the excitation device from the electrical circuit within the repetition period.

[0023] Preferably, the measuring arrangement further comprises at least one clock generator for specifying a second frequency and / or at least one further frequency with which the switch is periodically opened and closed.

[0024] The excitation device preferably comprises at least one passive component, which is periodically charged and discharged (at the repetition frequency) to deliver the excitation signal to the circuit. In the case of a resistor as a passive component, it can also be periodically connected to and disconnected from the circuit. The excitation device can also comprise an arrangement of passive components. According to a further aspect, the invention relates to a circuit for use in the method according to the invention or in a measuring arrangement according to the invention, which circuit comprises switching elements for controlling and interrupting the excitation signal from the excitation device. This circuit can be designed, for example, as an integrated circuit.

[0025] This circuit can advantageously have at least one clock generator for specifying a repetition frequency and / or a second and / or further frequency. Furthermore, the circuit can contain at least one current measurement and / or voltage measurement circuit. Advantageously, the circuit can also have analog-to-digital converters, inputs / outputs, a time control via an internal clock generator, a microcontroller, and communication interfaces.

[0026] Preferably, the entire electrical circuit required for excitation-Zdecomposition of the excitation signal (with the exception of the passive components, e.g., the required capacitors) can be integrated into an integrated circuit (semiconductor circuit, or ASIC). A measurement system designed in this way represents, according to current knowledge, the maximum miniaturization and reduction of the described invention.

[0027] The invention is particularly advantageous for determining the impedance of electrochemical cells, such as fuel cells or fuel cell stacks. It is particularly advantageous to implement this method by means of excitation by switching on / off a passive component (in particular a capacitor), which at the beginning of the excitation is at a lower voltage level than the electrochemical cell / cell stack to be examined (e.g.

[0028] Fuel cell (Z-n stack or Li-ion battery cell). Connecting the capacitor to the (fuel) cells causes a current with the typical charging current characteristic of the installed capacitor and the underlying voltage difference to act as a disturbance signal on the cells.

[0029] The observable voltage applied to the cells will, depending on the current flow, exhibit a voltage curve which is characteristic of the cell under investigation in terms of both magnitude and phase relationship. An analysis of this current-voltage relationship can be used to determine the frequency-dependent impedance. In order to be able to analyze not only high-frequency components but also low-frequency behavior in the cell(s), the capacitor (particularly electrical capacitance) is designed so that the charging curve (charging current curve) represents a significant, reproducible, and easily measurable curve. In order to represent impedance components of the electrochemical cell, the charging current curve can now be interrupted using the described method by clocked switching of the switch (e.g. transistor). This interruption can occur in several steps and intervals in order to generate a current curve broken down into known segments.The decomposition can be performed in a clocked manner and by superimposing multiple switching frequencies, which allows the analysis of the impedance at multiples of the original charging current repetition rates. The evaluation of the main and secondary frequencies is carried out, for example, analogously to the aforementioned previous or parallel applications.

[0030] Advantageously, the method according to the invention can also be applied in parallel to several cells of the cell stack (e.g., series connection of battery cells or individual cells in fuel cells) or to several cell blocks simultaneously. This allows the evaluation of the impedance at different sections of the overall system.

[0031] Optionally, the energy stored in the capacitor during the charging process can be discharged to a lower voltage level or a low-voltage circuit of the consumer system after the charging process. This is a highly efficient, resource-saving analysis system (e.g., to keep energy consumption low in a battery or fuel cell vehicle).

[0032] The invention will be explained in more detail below with reference to the exemplary embodiments illustrated in the drawings. They show schematically:

[0033] Figure 1: shows an excitation signal periodically generated according to a first embodiment of the method according to the invention, with cyclic interruptions at two additional frequencies; Figure 2: shows a current profile on a component to be examined with an excitation signal from a capacitor according to an advantageous embodiment of the method according to the invention;

[0034] Figure 3: a voltage signal on the component according to the embodiment of Figure 2;

[0035] Figure 4: a frequency-dependent current spectrum measured and transformed on the component according to a further embodiment;

[0036] Figure 5: an impedance calculated according to a further embodiment of the method according to the invention compared to an impedance determined with a sinusoidal excitation; and

[0037] Figure 6: an embodiment of a measuring arrangement according to the invention.

[0038] A first embodiment of an excitation signal generated according to a first embodiment of the method according to the invention is shown schematically in Figure 1. The excitation signal 20 consists of a current pulse with a signal shape 21, which is generated cyclically with a repetition frequency f1 corresponding to a repetition period p1 and impressed on the component 10 to be examined. However, the current pulse is not maintained over the entire repetition period p1, but is interrupted several times with a frequency f2 corresponding to a period p2 and with a frequency f3 corresponding to a period p3. A further interruption occurs here with the repetition period at approximately half the repetition period p1. This can also occur at a different point in the repetition period p1. Sections of the current pulse with a low amplitude can be cut off in this way.The interruption occurs by cyclical opening (and closing) of a switch. The underlying excitation signal 20 can, for example, be generated by a passive component, which determines the signal shape 21. This excitation signal of the repeatedly interrupted current pulse is cyclically impressed on the component under test at the repetition frequency f1. The repetitions and interruptions at the repetition frequency f1, as well as the higher-frequency interruptions, can be implemented, for example, using a switch and a corresponding clock generator, whereby the first repetition frequency f1 is limited by the hardware of the passive component. If, according to an advantageous embodiment, the excitation signal 20 is generated by a capacitor, a corresponding time-dependent current waveform 22 can be measured, for example, at the component 10 (Figure 2).At the same time, for the method according to the invention, a time-dependent voltage curve 23 will be measured on the component 10, shown in Figure 3.

[0039] In order to evaluate the current (22) and voltage curves (23) measured on the component (10) with regard to an impedance behavior of the component (10), the time-dependent current (22) and voltage curves (23) are transformed into frequency-dependent current or voltage spectra by means of a mathematical method (e.g. a Fourier transformation, e.g. FFT).

[0040] A transformed current spectrum is shown as an example in Figure 4. The absolute values ​​of the complex currents are plotted on the Y-axis. Clear maxima can be seen at the frequencies f1, f2, and f3 used for excitation.

[0041] In a corresponding manner, the time-dependent voltage curve 23 can also be transformed into a frequency-dependent voltage spectrum. It should be noted that the measured values ​​and spectra shown belong to various exemplary embodiments and are intended only to illustrate the invention. A valuative relationship between the measurements and curves shown should not be inferred from them.

[0042] Finally, the real and imaginary parts of the impedance can be calculated from the transformed current and voltage spectra in a known manner. Suitable frequency values ​​are preferably considered for this purpose. Figure 5 shows exemplary impedance values ​​determined using the method according to the invention (shown as crosses) with an impedance curve determined using a conventional method for impedance measurement with a sinusoidal excitation (shown as a solid line).

[0043] Finally, Figure 6 shows a measuring arrangement according to the invention with an electrical circuit 11 in which an electrical component 10 is connected to a load 16. An excitation device 12 is connected to the circuit via a switch 40. A voltmeter 46 and an ammeter 47 enable the measurement of the voltage and current at the component 10. The excitation device 12 can, for example, comprise a passive component or an arrangement of passive components. In the simplest case, it is an ohmic resistor that is cyclically connected to the electrical component by the switch 40. Not shown is a clock generator that specifies the cyclical control of the switch. Such a device can, for example, be contained in a microcontroller.

[0044] All features of the invention described above may be relevant to the invention both individually and in combination.

Claims

Patent claims 1 . Method for impedance analysis of an electrical component (10) comprising the steps: - periodically exciting the component (10) by means of an electrical excitation signal (20) with a preselected signal shape and with a first repetition frequency (f1), - measuring an electrical current profile (22) and an electrical voltage profile (23) on the component (10) as a function of time over at least one repetition period (p1), - wherein the excitation signal is interrupted at least once within a repetition period (p1 ), - evaluating the measured current curve (22) and voltage curve (23) over the at least one repetition period (p1) for an impedance behavior of the electrical component (10).

2. Method for impedance analysis of an electrical component (10) according to claim 1, wherein the excitation signal (20) is periodically interrupted more than once within a repetition period (p1) with a second frequency (f2), wherein the second frequency (f2) is greater than the first repetition frequency (f1).

3. Method for impedance analysis of an electrical component (10) according to claim 2, wherein the excitation signal (20) is periodically interrupted more than once within a repetition period (p1) with at least one further frequency (f3), wherein the at least one further frequency (f3), the second frequency (f2) and the first repetition frequency (f1) each differ from one another.

4. Method for impedance analysis of an electrical component (10) according to one of claims 2 or 3, wherein the at least one further frequency (f3) and / or the second frequency (f2) are integer multiples of the repetition frequency (f1).

5. Method for impedance analysis of an electrical component (10) according to one of the preceding claims, wherein the excitation signal (20) is generated by an excitation device (12) connected to the electrical component (10) in a circuit (11), and is interrupted within a repetition period (p1) by opening and closing a switch (40), and wherein the excitation device (12) has at least one passive component.

6. Method for the impedance analysis of an electrical component (10) according to one of the preceding claims, wherein the excitation signal (20) is interrupted within a repetition period (p1) by cyclically opening and closing a switch (40) in a circuit (11) with the electrical component (10) by a clock generator with at least a second and / or a further frequency (f2, f3).

7. Method for impedance analysis of an electrical component (10) according to one of the preceding claims, wherein the method is carried out successively with different frequencies.

8. Measuring arrangement for diagnosing an electrical component (10) in an electrical circuit (11), comprising: - an excitation device (12) which is arranged to periodically emit an excitation signal (20) with a preselected signal shape into the circuit (11) during a repetition period (p1); - a first clock generator for specifying a repetition frequency (f1) corresponding to the repetition period (p1), - a current measuring device (47) for measuring an electric current in the electrical component (10) as a function of time during the repetition period (p1) - a voltage measuring device (46) for measuring an electrical voltage on the electrical component (10) as a function of time during the repetition period (p1), - a switch (40) for disconnecting the excitation device (12) from the circuit (11) during a repetition period (p1).

9. Measuring arrangement for diagnosing an electrical component (10) in an electrical circuit (11) according to claim 8, further comprising at least one clock generator for specifying a second frequency (f2) and / or at least one further frequency (f3) with which the switch (40) is periodically opened and closed.

10. Measuring arrangement for diagnosing an electrical component (10) in an electrical circuit (11) according to one of claims 8 to 9, wherein the excitation device (12) comprises at least one passive component which is periodically charged and discharged, or which is periodically connected to and disconnected from the electrical circuit in order to output the excitation signal (20) into the electrical circuit (11).

11. Circuit (11) for use in a method according to one of claims 1 to 7 or in a measuring arrangement according to one of claims 9 to 10, comprising switching elements for controlling and interrupting the excitation signal (20) from the excitation device (12).

12. Circuit (11) according to claim 11, which is designed as an integrated circuit.

13. Circuit according to one of claims 11 to 12, comprising at least one clock generator for specifying a repetition frequency (f1), and / or a second and / or further frequency (f2, f3).

14. Circuit (11) according to one of claims 11 to 13, comprising at least one current measuring (47) and / or voltage measuring circuit (46).

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