CN116930800B - Fuel cell impedance measuring device and method - Google Patents
Fuel cell impedance measuring device and method Download PDFInfo
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- CN116930800B CN116930800B CN202311197096.0A CN202311197096A CN116930800B CN 116930800 B CN116930800 B CN 116930800B CN 202311197096 A CN202311197096 A CN 202311197096A CN 116930800 B CN116930800 B CN 116930800B
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- 238000000034 method Methods 0.000 title claims abstract description 23
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- 230000005284 excitation Effects 0.000 claims abstract description 33
- 230000008878 coupling Effects 0.000 claims description 151
- 238000010168 coupling process Methods 0.000 claims description 151
- 238000005859 coupling reaction Methods 0.000 claims description 151
- 239000003990 capacitor Substances 0.000 claims description 83
- 238000002847 impedance measurement Methods 0.000 claims description 11
- 230000003993 interaction Effects 0.000 claims description 8
- 238000005259 measurement Methods 0.000 abstract description 28
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
- G01R31/36—Arrangements for testing, measuring or monitoring the electrical condition of accumulators or electric batteries, e.g. capacity or state of charge [SoC]
- G01R31/389—Measuring internal impedance, internal conductance or related variables
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R27/00—Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
- G01R27/02—Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/30—Hydrogen technology
- Y02E60/50—Fuel cells
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Abstract
The invention discloses a fuel cell impedance measuring device and a method, wherein the device comprises: the device comprises a tested power end, a signal excitation circuit, a control unit and a signal acquisition circuit. The invention obtains the low-frequency to high-frequency current analog signal through the noninductive sampling resistor, and can improve the measurement accuracy by utilizing the excellent high-frequency response performance of the noninductive sampling resistor.
Description
Technical Field
The present invention relates to the field of fuel cell measurement, and in particular, to a device and a method for measuring impedance of a fuel cell.
Background
A fuel cell is a chemical device that directly converts chemical energy of fuel into electric energy, also called an electrochemical generator; it is a fourth power generation technology following hydroelectric power generation, thermal power generation, and nuclear power generation. The ac impedance of the fuel cell is the key data of the performance of the fuel cell, and the ac impedance measurement of the fuel cell can identify problematic fuel electromagnetic assemblies and other errors generated in the assembly process; helping to determine the resistance to movement, ohmic resistance, and transport limitations of reactants in a fuel cell system.
In the conventional fuel cell alternating current impedance measurement process, an alternating current injection method is generally adopted, a signal generator is externally connected with a detection device and is injected into a fuel cell, a weak alternating current signal is generated and is connected with a universal low-speed ADC chip, and then the single chip microcomputer is used for carrying out operation to calculate the alternating current impedance of the fuel cell. The device is complex in wiring and cannot measure high-voltage and high-power batteries. Meanwhile, the fuel cell has high energy density, generally operates in measurement, and needs heavy load when fuel such as hydrogen and oxygen is introduced, namely the fuel cell normally works when hundred amperes of current is introduced, alternating current components are superposed on large direct current, interference is formed when alternating current signals are measured, measurement precision is low, alternating current impedance of the fuel cell cannot be accurately obtained, and in addition, the existing measuring device can only measure alternating current impedance with fixed frequency.
Disclosure of Invention
The present invention aims to solve at least one of the technical problems existing in the prior art. Therefore, the invention provides a device and a method for measuring the impedance of a fuel cell, which can accurately obtain the alternating current impedance of the fuel cell and has wide measuring frequency.
According to an embodiment of the first aspect of the present invention, a fuel cell impedance measuring apparatus connected to a fuel cell includes: the power end to be measured comprises an MOS tube, a current feedback resistor and a noninductive sampling resistor, wherein the drain electrode of the MOS tube is connected with the anode of the fuel cell, and the source electrode of the MOS tube is connected with the cathode of the fuel cell through the current feedback resistor and the noninductive sampling resistor which are connected in series; the two ends of the current feedback resistor are respectively connected with the first input end of the signal excitation circuit for feeding back the current of the tested power end, and the output end of the signal excitation circuit is connected with the grid electrode of the MOS tube for forming a constant current control loop according to the current of the tested power end to adjust the conduction degree of the MOS tube; the programming signal output end of the control unit is connected with the second input end of the signal excitation circuit and used for controlling the output of the signal excitation circuit through the programming signal; the signal acquisition circuit comprises a first ADC, a second ADC, a first coupling capacitance switching circuit, a second coupling capacitance switching circuit, a first differential amplifier and a second differential amplifier, wherein the in-phase end and the opposite-phase end of the first differential amplifier are respectively connected with two ends of a noninductive sampling resistor for acquiring a current signal of a fuel cell, the output end of the first differential amplifier is connected with the input end of the first coupling capacitance switching circuit for eliminating a direct current component in the current signal through a coupling capacitance, the output end of the first coupling capacitance switching circuit is connected with the input end of the first ADC, and the output end of the first ADC is connected with the first signal input end of the control unit; the in-phase end and the opposite-phase end of the second differential amplifier are respectively connected with two ends of the fuel cell through a second coupling capacitance switching circuit so as to be used for collecting voltage signals of the fuel cell and eliminating direct current components in the voltage signals through the second coupling capacitance switching circuit, the output end of the second differential amplifier is connected with the input end of the second ADC, the output end of the second ADC is connected with the second signal input end of the control unit, and the control unit is respectively connected with the first coupling capacitance switching circuit and the control end of the second coupling capacitance switching circuit so as to be used for calculating and controlling to switch corresponding coupling capacitances according to signal frequencies and line load resistances.
The fuel cell impedance measuring device according to the embodiment of the first aspect of the present invention has at least the following advantageous effects:
in the embodiment of the invention, the tested fuel cell is connected through the tested power end, the control unit generates a programming signal to the signal excitation circuit, on one hand, a direct current signal is generated to pull the tested fuel cell to work normally and output an alternating current signal at the same time, the signal excitation circuit generates corresponding current according to the direct current signal and the alternating current signal at the tested power end to flow through the fuel cell, the control unit collects current signals of different frequencies of the fuel cell through the noninductive sampling resistor, collects voltage signals of the fuel cell through differential sampling, then calculates and controls to switch coupling capacitors of corresponding capacitance values in the first coupling capacitance switching circuit and the second coupling capacitance switching circuit through signal frequency and line load resistance so as to accurately remove direct current components in the current signals and the voltage signals, then obtains alternating current voltage and current data through the first ADC and the second ADC, and the control unit calculates alternating current impedance of the fuel cell according to the alternating current voltage and current data.
The invention obtains the low-frequency to high-frequency current analog signal through the noninductive sampling resistor, and can improve the measurement accuracy by utilizing the excellent high-frequency response performance of the noninductive sampling resistor.
According to some embodiments of the invention, the signal excitation circuit includes a third differential amplifier, an error amplifier, and a DAC unit, where the control unit is connected to an input end of the DAC unit for generating a current signal, an output end of the DAC unit is connected to an in-phase end of the error amplifier, an in-phase end and an opposite-phase end of the third differential amplifier are respectively connected to two ends of the current feedback resistor, an output end of the third differential amplifier is connected to an opposite-phase end of the error amplifier, and an output end of the error amplifier is connected to a gate of the MOS transistor.
According to some embodiments of the invention, the DAC unit comprises a first DAC, a second DAC, and an adder, the control unit is connected to an input of the first DAC for outputting a dc current signal, the control unit is connected to an input of the second DAC for outputting an ac current signal, both the output of the first DAC and the output of the second DAC are connected to an input of the adder, and an output of the adder is connected to an in-phase end of the error amplifier.
According to some embodiments of the invention, the control unit comprises an MCU and an FPGA, a programming signal output end of the FPGA is connected to a second input end of the signal excitation circuit for controlling an output of the signal excitation circuit by the programming signal, the FPGA is respectively connected to control ends of the first coupling capacitance switching circuit and the second coupling capacitance switching circuit for calculating and controlling switching of the corresponding coupling capacitance according to a signal frequency and a line load resistance, an output end of the first ADC is connected to a first signal input end of the control unit, an output end of the second ADC is connected to a second signal input end of the FPGA, and the MCU is connected to the FPGA for writing configuration parameters and reading a current-voltage signal acquired by the FPGA to calculate an ac impedance of the fuel cell.
According to some embodiments of the invention, the control unit further comprises a man-machine interaction module connected to the MCU for manual parameter setting and for drawing a cole-cole graph from ac impedance values of the fuel cell at respective frequencies.
According to some embodiments of the invention, the first coupling capacitor switching circuit includes a first analog switch and a first coupling capacitor group, the first coupling capacitor group includes a plurality of coupling capacitors connected in parallel and having different capacitance values, an output terminal of the first differential amplifier is connected to an input terminal of the first analog switch, an output terminal of the first analog switch is connected to an input terminal of the first coupling capacitor group, an output terminal of the first coupling capacitor group is connected to an input terminal of the first ADC, and the control unit is connected to a control terminal of the first analog switch for switching different coupling capacitors in the first coupling capacitor group.
According to some embodiments of the invention, the second coupling capacitor switching circuit includes a second analog switch, a second coupling capacitor group, a third analog switch, and a third coupling capacitor group, where the second coupling capacitor group and the third coupling capacitor group each include a plurality of coupling capacitors with different capacitance values connected in parallel, an anode of the fuel cell is connected to an input end of the second analog switch, an output end of the second analog switch is connected to an input end of the second coupling capacitor group, an output end of the second coupling capacitor group is connected to an in-phase end of the second differential amplifier, the control unit is connected to a control end of the second analog switch for switching different coupling capacitors in the second coupling capacitor group, a cathode of the fuel cell is connected to an input end of the third analog switch, an output end of the third analog switch is connected to an input end of the third coupling capacitor group, an output end of the third coupling capacitor group is connected to an inverting end of the second differential amplifier, and the control unit is connected to a control end of the third analog switch for switching different coupling capacitors in the third coupling capacitor group.
A fuel cell impedance measuring method according to an embodiment of the second aspect of the invention includes the steps of:
acquiring configuration parameters and inputting the configuration parameters into a control unit;
the control unit outputs a direct current signal through the signal excitation circuit to pull the fuel cell to work normally and simultaneously outputs an alternating current signal;
the signal excitation circuit generates corresponding current at the tested power end according to the direct current electric signal and the alternating current electric signal and flows through the fuel cell;
the control unit collects current signals of different frequencies of the fuel cell through the noninductive sampling resistor, collects voltage signals of the fuel cell through differential sampling, and then switches a coupling capacitor with a corresponding capacitance value through a coupling capacitor selection algorithm to accurately remove direct current components in the current signals and the voltage signals, so that alternating voltage and current data are obtained;
the control unit performs Fourier operation on the alternating current voltage flow data, performs complex modulus to obtain alternating current voltage and current amplitude, and converts the alternating current voltage and current amplitude to obtain alternating current impedance of the fuel cell.
The fuel cell impedance measuring method according to the embodiment of the second aspect of the invention has at least the following advantageous effects:
in the embodiment of the invention, configuration parameters are firstly obtained and input into a control unit; then the control unit outputs a direct current signal through the signal excitation circuit to pull the fuel cell to work normally and simultaneously outputs an alternating current signal; the signal excitation circuit generates corresponding current at the tested power end according to the direct current electric signal and the alternating current electric signal and flows through the fuel cell; the control unit collects current signals of different frequencies of the fuel cell through the noninductive sampling resistor, collects voltage signals of the fuel cell through differential sampling, and then switches a coupling capacitor with a corresponding capacitance value through a coupling capacitor selection algorithm to accurately remove direct current components in the current signals and the voltage signals, so that alternating voltage and current data are obtained.
The invention obtains the low-frequency to high-frequency current analog signal through the noninductive sampling resistor, and can improve the measurement accuracy by utilizing the excellent high-frequency response performance of the noninductive sampling resistor.
According to some embodiments of the invention, the formula of the coupling capacitance selection algorithm is c=1/(2×pi×r×f), where C is the capacitance value of the coupling capacitance, f is the signal frequency, and R is the resistance value of the line load resistor.
According to some embodiments of the invention, the method further comprises a drawing step, wherein the steps are repeated to obtain alternating current impedance values of the fuel cells at each frequency, and then a Kerr-Kerr graph is drawn according to the alternating current impedance values of the fuel cells at each frequency.
Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
Drawings
The invention is further described with reference to the accompanying drawings and examples, in which:
FIG. 1 is a circuit diagram of a fuel cell impedance measuring apparatus in an embodiment of the invention;
FIG. 2 is a flow chart of a fuel cell impedance measurement method according to an embodiment of the present invention;
FIG. 3 is a flowchart of a coupling capacitance selection algorithm according to an embodiment of the present invention;
FIG. 4 is a schematic diagram illustrating the measurement of AC impedance in an embodiment of the present invention;
fig. 5 is an ideal fuel cell internal resistance Nyquist diagram.
Detailed Description
Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein like or similar reference numerals refer to like or similar elements or elements having like or similar functions throughout. The embodiments described below by referring to the drawings are illustrative only and are not to be construed as limiting the invention.
In the description of the present invention, it should be understood that the direction or positional relationship indicated with respect to the description of the orientation, such as up, down, etc., is based on the direction or positional relationship shown in the drawings, is merely for convenience of describing the present invention and simplifying the description, and does not indicate or imply that the apparatus or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as limiting the present invention.
In the description of the present invention, plural means two or more. The description of the first and second is for the purpose of distinguishing between technical features only and should not be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated or implicitly indicating the precedence of the technical features indicated.
In the description of the present invention, unless explicitly defined otherwise, terms such as arrangement, installation, connection, etc. should be construed broadly and the specific meaning of the terms in the present invention can be reasonably determined by a person skilled in the art in combination with the specific contents of the technical scheme.
First, a fuel cell is described, which is a chemical device that directly converts chemical energy of fuel into electric energy, and is also called electrochemical generator; it is a fourth power generation technology following hydroelectric power generation, thermal power generation, and nuclear power generation. The ac impedance of the fuel cell is the key data of the performance of the fuel cell, and the ac impedance measurement of the fuel cell can identify problematic fuel electromagnetic assemblies and other errors generated in the assembly process; helping to determine the resistance to movement, ohmic resistance, and transport limitations of reactants in a fuel cell system.
Referring to fig. 5, the internal resistance of the fuel cell can be divided into three parts:
an activated polarization internal resistance Ract determined by the nature of the electrochemical reaction system;
the resistance of the electrode, the diaphragm, the electrolyte, the connecting strip, the pole and other components in the battery is the ohmic internal resistance Rohm;
the internal resistance Rcon is polarized by the concentration due to the change in the concentration of the reactive ions. In the conventional fuel cell alternating current impedance measurement process, an alternating current injection method is generally adopted, a signal generator is externally connected with a detection device and is injected into a fuel cell, a weak alternating current signal is generated and is connected with a universal low-speed ADC chip, and then the single chip microcomputer is used for carrying out operation to calculate the alternating current impedance of the fuel cell. The device is complex in wiring and cannot measure high-voltage and high-power batteries. Meanwhile, the fuel cell has high energy density, generally operates in measurement, and needs heavy load when fuel such as hydrogen and oxygen is introduced, namely the fuel cell normally works when hundred amperes of current is introduced, alternating current components are superposed on large direct current, interference is formed when alternating current signals are measured, measurement precision is low, alternating current impedance of the fuel cell cannot be accurately obtained, and in addition, the existing measuring device can only measure alternating current impedance with fixed frequency.
The invention designs a hardware system, which can rapidly measure the alternating current impedance of the fuel cell and has high measurement accuracy; meanwhile, the measuring range can be switched, and the fuel cell with high power and low power can be measured in a compatible manner; the fuel cell impedance analyzer also has the functions of single frequency measurement and multi-frequency automatic measurement, the measuring frequency range can reach 0.01 Hz-50 kHz alternating current signal, and the multi-frequency measurement function result can also draw a Kerr-Kerr curve through upper computer software to analyze the impedance of the fuel cell. Meanwhile, the circuit method realizes integration of the excitation source and the detection function equipment, greatly saves equipment cost and simplifies the measurement structure.
The specific structure and implementation of the present invention will be described in detail below, with reference to fig. 1, a fuel cell impedance measuring apparatus, connected to a fuel cell, comprising: the device comprises a tested power end, a signal excitation circuit, a control unit and a signal acquisition circuit. Specifically, the detected power end comprises a MOS tube Q1, a current feedback resistor R1 and a noninductive sampling resistor R2, the drain electrode of the MOS tube Q1 is connected with the anode of the fuel cell, and the source electrode of the MOS tube Q1 is connected with the cathode of the fuel cell through the current feedback resistor R1 and the noninductive sampling resistor R2 which are connected in series. The two ends of the current feedback resistor R1 are respectively connected with a first input end of the signal excitation circuit for feeding back the current of the tested power end, and an output end of the signal excitation circuit is connected with the grid electrode of the MOS tube Q1 for adjusting the conduction degree of the MOS tube Q1 according to the current of the tested power end to form a constant current control loop; the programming signal output end of the control unit is connected with the second input end of the signal excitation circuit and used for controlling the output of the signal excitation circuit through the programming signal; the signal acquisition circuit comprises a first ADC, a second ADC, a first coupling capacitance switching circuit, a second coupling capacitance switching circuit, a first differential amplifier U1 and a second differential amplifier U2, wherein the in-phase end and the anti-phase end of the first differential amplifier U1 are respectively connected with the two ends of a noninductive sampling resistor R2 for acquiring current signals of the fuel cell, the output end of the first differential amplifier U1 is connected with the input end of the first coupling capacitance switching circuit for eliminating direct current components in the current signals through coupling capacitance, the output end of the first coupling capacitance switching circuit is connected with the input end of the first ADC, and the output end of the first ADC is connected with the first signal input end of the control unit for inputting alternating current signals; the in-phase end and the opposite-phase end of the second differential amplifier U2 are respectively connected with two ends of the fuel cell through a second coupling capacitor switching circuit so as to be used for collecting voltage signals of the fuel cell and eliminating direct current components in the voltage signals through the second coupling capacitor switching circuit, the output end of the second differential amplifier U2 is connected with the input end of the second ADC, the output end of the second ADC is connected with the second signal input end of the control unit so as to be used for inputting alternating current voltage signals, and the control unit is respectively connected with the control ends of the first coupling capacitor switching circuit and the second coupling capacitor switching circuit so as to be used for calculating and controlling switching of corresponding coupling capacitors according to signal frequency and line load resistance.
The non-inductive sampling resistor R2 is used as the current sampling resistor of the fuel cell in the present application to reduce the parasitic inductance of the resistor to form a phase shift. A non-inductive resistor is a resistor that only needs to be on a wire or coil to function, and should not be inductive or voltage generated by induction. The common noninductive resistor is formed by combining two insulated wires, wherein the terminal is short-circuited, the head end is externally connected with a power supply and an application circuit, the current directions of the two insulated wires are opposite, no external mutual inductance exists, and the noninductive resistor can be wound into a coil for use and only plays a role in resistance.
Specifically, the signal exciting circuit in the invention comprises a third differential amplifier U3, an error amplifier U4 and a DAC unit, wherein the DAC unit comprises a first DAC, a second DAC and an adder, the control unit is connected with the input end of the first DAC for outputting a direct current signal, in the embodiment, the control unit controls a load to generate 0-600A direct current through the first DAC so that a fuel cell normally works, the control unit is connected with the input end of the second DAC for outputting an alternating current signal, in the embodiment, the control unit excites a load through the second DAC to generate a programming voltage of 0.01-50 kHz alternating current, the output ends of the first DAC and the second DAC are both connected with the input end of the adder, the output end of the adder is connected with the in-phase end of the error amplifier U4, the in-phase end and the anti-phase end of the third differential amplifier U3 are respectively connected with two ends of a current feedback resistor R1, the output end of the third differential amplifier U3 is connected with the anti-phase end of the error amplifier U4, and the output end of the error amplifier U4 is connected with the grid of a MOS tube Q1.
It should be understood that the current feedback resistor R1, the MOS transistor Q1, the third differential amplifier U3, and the error amplifier U4 form a constant current control loop to supply a set current to the fuel cell.
Specifically, the control unit comprises an MCU and an FPGA, the programming signal output end SPI1 of the FPGA is connected with the first DAC, the programming signal output end SPI2 of the FPGA is connected with the second DAC, the FPGA is respectively connected with the control ends of the first coupling capacitance switching circuit and the second coupling capacitance switching circuit so as to be used for calculating and controlling to switch the corresponding coupling capacitance according to the signal frequency and the line load resistance, the output end of the first ADC is connected with the first signal input end SPI3 of the FPGA, the output end of the second ADC is connected with the second signal input end SPI4 of the FPGA, and the MCU is connected with the FPGA so as to be used for writing in configuration parameters and reading the current voltage signals acquired by the FPGA to calculate the alternating current impedance of the fuel cell.
It should be noted that, in the present application, the control unit structure of mcu+fpga is adopted because the MCU executes instructions one instruction by one instruction, but in the present application, the DAC unit sets the excitation signal to the ADC to collect the excitation signal, and otherwise, the impedance accuracy of measurement may be affected. Therefore, the FPGA is responsible for generating and sending programming signals to the DAC unit, controlling the switching of the coupling capacitors in the first coupling capacitor switching circuit and the second coupling capacitor switching circuit, collecting the alternating current voltage data fed back by the first ADC and the second ADC, and calculating the processing of man-machine interaction data such as alternating current impedance, configuration parameters and the like according to the alternating current voltage data. According to the method and the device, the synchronous and parallel characteristics of the FPGA are utilized, the phase shift of voltage and current signals is reduced, and the measurement accuracy is ensured.
Specifically, the first coupling capacitor switching circuit in the invention comprises a first analog switch K1 and a first coupling capacitor group, wherein the first coupling capacitor group comprises a plurality of coupling capacitors which are connected in parallel and have different capacitance values, the output end of the first differential amplifier U1 is connected with the input end of the first analog switch K1, one ends of the plurality of coupling capacitors are connected in parallel and are connected to the input end of the first ADC, the other ends of the plurality of coupling capacitors are respectively connected with different contacts of the first analog switch K1, and the control unit is connected with the control end of the first analog switch K1 to be used for switching the different coupling capacitors in the first coupling capacitor group, namely switching the capacitance values of the coupling capacitors between the first ADC and the first differential amplifier U1.
Specifically, the second coupling capacitor switching circuit in the invention includes a second analog switch K2, a second coupling capacitor group, a third analog switch K3, and a third coupling capacitor group, where the second coupling capacitor group and the third coupling capacitor group both include a plurality of coupling capacitors connected in parallel and having different capacitance values, the positive electrode of the fuel cell is connected to the input end of the second analog switch K2, one ends of the plurality of coupling capacitors are connected in parallel and connected to the same phase end of the second differential amplifier U2, the other ends of the plurality of coupling capacitors are respectively connected to different contacts of the second analog switch K2, the control unit is connected to the control end of the second analog switch K2 to switch different coupling capacitors in the second coupling capacitor group, that is, to switch the capacitance values of the coupling capacitors between the positive electrode of the fuel cell and the same phase end of the second differential amplifier U2, the negative electrode of the fuel cell is connected to the input end of the third analog switch K3, one ends of the plurality of coupling capacitors are connected in parallel and are connected to the opposite phase end of the second differential amplifier U2, the other ends of the plurality of coupling capacitors are respectively connected to different contacts of the third analog switch K3, and the control unit is connected to the control end of the third analog switch between the second coupling capacitor and the different coupling capacitors in the second coupling capacitor group.
It should be noted that, in the present application, the FPGA removes the dc component on the voltage and current signal by selecting and switching the coupling capacitor with a suitable capacitance value, so as to improve the measurement accuracy, and the principle of removing the dc component by the coupling capacitor is as follows:
when the coupling capacitor is connected to a power supply, the free charge does not pass through the insulating medium between the two electrodes, and the direct current cannot pass through the coupling capacitor. The alternating current causes voltage change between the two polar plates because of the change of the charges accumulated on the two polar plates, and when the voltage rises, the charges are accumulated on the polar plates of the coupling capacitor to form charging current; when the voltage decreases, the charge leaves the plate, forming a discharge current. The coupling capacitor is alternately charged and discharged, and a current is present in the circuit, which is represented by an alternating current "passing" through the coupling capacitor. The dc component on the voltage current signal can be removed by the coupling capacitance.
It should be noted that, in this embodiment, three-stage coupling capacitors are respectively disposed in the first coupling capacitor switching circuit, the second coupling capacitor switching circuit and the third coupling capacitor switching circuit, so long as the time constant is smaller than the rising time of the voltage, the coupling waveforms will not be distorted, and the capacitance value of the coupling capacitor is selected adaptively in consideration of the low-frequency measurement application of 10mHz and the withstand voltage of the integrated capacitor.
Specifically, the capacitance value calculation formula of the coupling capacitor is:
C= 1/(2*π*R*f)
wherein C is the capacitance of the coupling capacitor, f is the signal frequency, and R is the resistance of the line load resistor.
It should be noted that, the line load resistor refers to an input resistor of the next stage, and a load resistor is added to ground after capacitive coupling. The FPGA automatically switches the size of the coupling capacitor according to the frequency of the signal and the resistance value of the line load resistor input in advance under different frequencies, and because the signal frequency is firstly obtained by the FPGA, the signal frequency can be calculated well in advance by the FPGA, and phase shift generated during calculation is avoided.
Specifically, the control unit in the embodiment of the invention further comprises a man-machine interaction module, wherein the man-machine interaction module is connected with the MCU for manually setting parameters and drawing a Kerr-Kerr graph according to the alternating current impedance value of the fuel cell under each frequency, and the man-machine interaction module comprises a display screen and a keyboard, and can also adopt other man-machine interaction equipment such as a touch screen to realize manual parameter setting and display of the Kerr-Kerr graph for performance analysis of the fuel cell.
In the invention, the first ADC and the second ADC both adopt high-speed ADC modules, and the first DAC and the second DAC both adopt high-speed DAC modules so as to improve the measurement speed. The first differential amplifier U1, the second differential amplifier U2 and the third differential amplifier U3 adopt instrument amplifiers, so that multiple switching can be realized, and the sizes of the acquired signals can be switched. The first ADC and the second ADC can adjust gain coefficients, the size of a switching signal is realized, and the acquired alternating voltage and alternating current signal multiple can be switched through the setting, so that the switching of measurement ranges is realized to be compatible with the fuel cells for measuring high power and low power.
The invention adopts the FPGA+MCU mode, the synchronous time sequence of the FPGA can ensure that the set current and the readback acquisition and FFT operation are synchronous, and the data output (communication) is not affected.
Referring to fig. 2, the present invention also relates to a fuel cell impedance measurement method, comprising the steps of:
s100, acquiring configuration parameters and inputting the configuration parameters into a control unit;
specifically, the configuration parameters comprise a current frequency, a measurement mode and the like, the measurement mode comprises a fixed frequency mode, a sweep frequency mode, a single mode, a plurality of modes and the like, and the configuration parameters are input into the MCU through the man-machine interaction module.
S200, the control unit outputs a direct current signal through the signal excitation circuit to pull the fuel cell to work normally and simultaneously outputs an alternating current signal;
specifically, the MCU writes the configuration parameters into the FPGA, and the FPGA controls the first DAC and the second DAC to pull direct current and inject alternating current for the operation of the fuel cell.
S300, the signal excitation circuit generates corresponding current at the tested power end according to the direct current electric signal and the alternating current electric signal and flows through the fuel cell;
specifically, the fuel cell is fully reacted, and the alternating current in this embodiment is at most one tenth of the direct current.
S400, the control unit collects current signals of different frequencies of the fuel cell through a non-inductive sampling resistor R2, collects voltage signals of the fuel cell through differential sampling, and then switches a coupling capacitor with a corresponding capacitance value through a coupling capacitor selection algorithm to accurately remove direct current components in the current signals and the voltage signals, so as to obtain alternating voltage and current data;
specifically, the FPGA acquires analog quantity signals of currents with different frequencies through a non-inductive sampling resistor R2, acquires alternating voltage signals through double-twisted differential sampling, removes direct current components of the voltage current signals through a coupling capacitor, and feeds back the direct current components to the first ADC and the second ADC through a second operational amplifier and a third operational amplifier.
Referring to fig. 3, the calculation formula of the coupling capacitance selection algorithm is c=1/(2×pi×r×f), where C is the capacitance value of the coupling capacitance, f is the signal frequency, and R is the resistance value of the line load resistor.
The following describes the calculation process of the coupling capacitance selection algorithm of the FPGA in detail, specifically as follows:
after the power-on system is initialized, the FPGA firstly acquires measurement parameters including a preset line load resistance value and a measurement frequency of a measurement current, then calculates the capacitance value of the coupling capacitor according to the preset line load resistance value and the measurement frequency of the measurement current, and controls the analog switch to switch the coupling capacitor according to the calculation result.
S500, the control unit performs Fourier operation on the alternating current voltage flow data, performs complex modulus to obtain alternating current voltage and current amplitude, and converts the alternating current voltage and current amplitude to obtain alternating current impedance of the fuel cell.
The specific steps of step S500 are:
s501, firstly, converting the alternating voltage current waveform signal into a complex number with a real part as an acquisition value and an imaginary part as 0 according to the data format requirement of Fourier operation.
S502, performing Fourier operation on the alternating voltage and current data, wherein a calculation formula is as follows
Wherein, X (k) is a frequency domain value, X (N) is a time domain sampling point, N is a sequence index of the time domain sampling point, k is an index of the frequency domain value, N is the number of sampling points for conversion, j is an imaginary unit, and t represents time.
It should be noted that, the voltage and current value obtained after fourier operation is performed on the ac voltage and current data is also a complex number, so that complex number modulo calculation is also required to obtain the ac voltage and current signal amplitude at the measured frequency, and then the ac voltage and current amplitude is converted to obtain the ac impedance value of the fuel cell, which specifically includes the steps of:
after carrying out Fourier operation on the alternating voltage and current data, obtaining the amplitude of the voltage and the current of each frequency point through complex modulo calculation, and obtaining the maximum value of the group of data through calculation, wherein the maximum value is the alternating voltage and current amplitude under the measured frequency, and comprises an alternating voltage amplitude U 'and an alternating current amplitude I'. Because the voltage and current value obtained after the Fourier operation is also a complex number; therefore, the measured complex impedance can be calculated according to the ohm's law r=u/I and the complex division formula, and if U ' =a+bi, I ' =c+di, (a, b, c, d e R), the complex division formula is
And performing modular operation on the complex impedance to obtain an actual impedance value, wherein the calculation formula of the modular operation is as follows:
Z=
wherein Z is impedance, R is resistance, X is reactance, R=，X=/>。
By the calculation method, the alternating current impedance value of the set frequency can be measured rapidly and accurately.
Referring to fig. 4, the device and the method of the invention are used for measuring the alternating current impedance of the fuel cell, the first DAC initially controls the loaded direct current of the MOS transistor Q1 to enable the fuel cell to operate, then the first DAC is excited by the superimposed alternating current through the adder to start measuring the alternating current impedance, the fuel cell generates an alternating current differential pressure on the superimposed alternating current, and the alternating current impedance is calculated by collecting the alternating current and the differential pressure.
Specifically, the embodiment further includes S600, a drawing step, where the steps are repeated to obtain ac impedance values of the fuel cell at each frequency, and then a cole-cole graph is drawn according to the ac impedance values of the fuel cell at each frequency. The kerr-kerr plot is a plot describing the frequency dependence between the real and imaginary parts of the complex dielectric constant (resistivity), from which deviations of the dielectric relaxation behavior from a single relaxation time course can be determined.
In summary, the invention obtains the low-frequency to high-frequency current analog signal through the noninductive sampling resistor, and can improve the measurement accuracy by utilizing the excellent high-frequency response performance of the noninductive sampling resistor.
In addition, the invention adopts an FPGA+MCU mode, the synchronous time sequence of the FPGA can ensure that the set current, the readback acquisition and the FFT operation are synchronous, the data output and the communication are not affected, the invention adopts a high-speed ADC and a high-speed DAC to ensure the actual and accurate establishment of waveforms, the speed of controlling and acquiring the measured signals can be further accelerated, and meanwhile, the invention adopts a direct current superposition communication mode, the alternating current signal is not required to be superimposed after the direct current temperature is waited, and the measuring speed can be further accelerated.
The embodiments of the present invention have been described in detail with reference to the accompanying drawings, but the present invention is not limited to the above embodiments, and various changes can be made within the knowledge of one of ordinary skill in the art without departing from the spirit of the present invention.
Claims (8)
1. A fuel cell impedance measuring apparatus connected to a fuel cell, comprising:
the power end to be measured comprises an MOS tube, a current feedback resistor and a noninductive sampling resistor, wherein the drain electrode of the MOS tube is connected with the anode of the fuel cell, and the source electrode of the MOS tube is connected with the cathode of the fuel cell through the current feedback resistor and the noninductive sampling resistor which are connected in series;
the two ends of the current feedback resistor are respectively connected with the first input end of the signal excitation circuit for feeding back the current of the tested power end, and the output end of the signal excitation circuit is connected with the grid electrode of the MOS tube for forming a constant current control loop according to the current of the tested power end to adjust the conduction degree of the MOS tube;
the programming signal output end of the control unit is connected with the second input end of the signal excitation circuit and used for controlling the output of the signal excitation circuit through the programming signal;
the signal acquisition circuit comprises a first ADC, a second ADC, a first coupling capacitance switching circuit, a second coupling capacitance switching circuit, a first differential amplifier and a second differential amplifier, wherein the in-phase end and the opposite-phase end of the first differential amplifier are respectively connected with the two ends of a noninductive sampling resistor for acquiring a current signal of a fuel cell, the output end of the first differential amplifier is connected with the input end of the first coupling capacitance switching circuit for eliminating a direct current component in the current signal through a coupling capacitance, the output end of the first coupling capacitance switching circuit is connected with the input end of the first ADC, and the output end of the first ADC is connected with the first signal input end of the control unit for inputting an alternating current signal; the in-phase end and the opposite-phase end of the second differential amplifier are respectively connected with two ends of the fuel cell through a second coupling capacitance switching circuit so as to be used for collecting voltage signals of the fuel cell and eliminating direct current components in the voltage signals through the second coupling capacitance switching circuit, the output end of the second differential amplifier is connected with the input end of the second ADC, the output end of the second ADC is connected with the second signal input end of the control unit so as to be used for inputting alternating current voltage signals, and the control unit is respectively connected with the control ends of the first coupling capacitance switching circuit and the second coupling capacitance switching circuit so as to be used for calculating and controlling to switch corresponding coupling capacitances according to signal frequency and line load resistance;
the signal excitation circuit comprises a third differential amplifier, an error amplifier and a DAC unit, wherein the control unit is connected with the input end of the DAC unit for generating a current signal, the output end of the DAC unit is connected with the in-phase end of the error amplifier, the in-phase end and the opposite-phase end of the third differential amplifier are respectively connected with the two ends of the current feedback resistor, the output end of the third differential amplifier is connected with the opposite-phase end of the error amplifier, and the output end of the error amplifier is connected with the grid electrode of the MOS tube;
the DAC unit comprises a first DAC, a second DAC and an adder, the control unit is connected with the input end of the first DAC and used for outputting direct current signals, the control unit is connected with the input end of the second DAC and used for outputting alternating current signals, the output ends of the first DAC and the second DAC are connected with the input end of the adder, and the output end of the adder is connected with the in-phase end of the error amplifier.
2. The fuel cell impedance measurement apparatus according to claim 1, wherein the control unit comprises an MCU and an FPGA, a programming signal output end of the FPGA is connected to a second input end of the signal excitation circuit for controlling an output of the signal excitation circuit by the programming signal, the FPGA is respectively connected to control ends of the first coupling capacitance switching circuit and the second coupling capacitance switching circuit for calculating and controlling switching of the corresponding coupling capacitance according to a signal frequency and a line load resistance, an output end of the first ADC is connected to a first signal input end of the control unit, an output end of the second ADC is connected to a second signal input end of the FPGA, and the MCU is connected to the FPGA for writing configuration parameters and reading a current-voltage signal acquired by the FPGA to calculate an ac impedance of the fuel cell.
3. The fuel cell impedance measurement device of claim 2, wherein the control unit further comprises a human-machine interaction module connected to the MCU for manual parameter setting and for plotting a cole-cole graph from the ac impedance values of the fuel cell at each frequency.
4. The fuel cell impedance measurement apparatus according to claim 1, wherein the first coupling capacitance switching circuit includes a first analog switch and a first coupling capacitance group, the first coupling capacitance group includes a plurality of coupling capacitances connected in parallel and having different capacitance values, an output terminal of the first differential amplifier is connected to an input terminal of the first analog switch, an output terminal of the first analog switch is connected to an input terminal of the first coupling capacitance group, an output terminal of the first coupling capacitance group is connected to an input terminal of the first ADC, and the control unit is connected to a control terminal of the first analog switch for switching the different coupling capacitances in the first coupling capacitance group.
5. The fuel cell impedance measurement apparatus according to claim 1, wherein the second coupling capacitance switching circuit includes a second analog switch, a second coupling capacitance group, a third analog switch, and a third coupling capacitance group, the second coupling capacitance group and the third coupling capacitance group each include a plurality of coupling capacitances connected in parallel and having different capacitance values, an anode of the fuel cell is connected to an input terminal of the second analog switch, an output terminal of the second analog switch is connected to an input terminal of the second coupling capacitance group, an output terminal of the second coupling capacitance group is connected to an in-phase terminal of the second differential amplifier, the control unit is connected to a control terminal of the second analog switch for switching different coupling capacitances in the second coupling capacitance group, a cathode of the fuel cell is connected to an input terminal of the third analog switch, an output terminal of the third analog switch is connected to an input terminal of the third coupling capacitance group, an output terminal of the third coupling capacitance group is connected to an inverting terminal of the second differential amplifier, and the control unit is connected to a control terminal of the third analog switch for switching different coupling capacitances in the third coupling capacitance group.
6. A fuel cell impedance measurement method applied to the apparatus of any one of claims 1 to 5, comprising the steps of:
acquiring configuration parameters and inputting the configuration parameters into a control unit;
the control unit outputs a direct current signal through the signal excitation circuit to pull the fuel cell to work normally and simultaneously outputs an alternating current signal;
the signal excitation circuit generates corresponding current at the tested power end according to the direct current electric signal and the alternating current electric signal and flows through the fuel cell;
the control unit collects current signals of different frequencies of the fuel cell through the noninductive sampling resistor, collects voltage signals of the fuel cell through differential sampling, and then switches a coupling capacitor with a corresponding capacitance value through a coupling capacitor selection algorithm to accurately remove direct current components in the current signals and the voltage signals, so that alternating voltage and current data are obtained;
the control unit performs Fourier operation on the alternating current voltage flow data, performs complex modulus to obtain alternating current voltage and current amplitude, and converts the alternating current voltage and current amplitude to obtain alternating current impedance of the fuel cell.
7. The method of claim 6, wherein the formula of the coupling capacitance selection algorithm is c=1/(2×pi×r×f), where C is a capacitance value of the coupling capacitance, f is a signal frequency, and R is a resistance value of the line load resistor.
8. The method of measuring impedance of fuel cells according to claim 6, further comprising a drawing step of repeating the above steps to obtain ac impedance values of the fuel cells at the respective frequencies, and then drawing a cole-cole graph based on the ac impedance values of the fuel cells at the respective frequencies.
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