Impedance sensing measurement system and method for multidirectional monitoring of coating degradation

Abstract

The invention belongs to the field of structural coating corrosion degradation degree monitoring, and relates to an impedance sensing measurement system and method for multi-azimuth monitoring of coating degradation, and the system comprises a multi-probe sensing device and an AC impedance measurement circuit. The AC impedance measurement circuit comprises a single-chip microcomputer module, an excitation signal module, a measurement output signal amplification module and an impedance switching and impedance double-excitation measurement module, and the single-chip microcomputer module is connected with the excitation signal module, the measurement output signal amplification module and the impedance switching and impedance double-excitation measurement module. The excitation signal module and the measurement output signal amplification module are both connected with the impedance switching and impedance double-excitation measurement module, and the impedance switching and impedance double-excitation measurement module is connected with the multi-probe sensing device; according to the invention, the impedance of the coating in multiple directions can be measured, and the degradation condition of the coating in each direction can be monitored.

Description

Technical Field

[0001] The present invention belongs to the field of monitoring the corrosion degradation degree of structural coatings, and particularly relates to an impedance sensing measurement system and method for multi-directional monitoring of coating degradation. Background Art

[0002] Corrosion monitoring technology is of great significance both in terms of corrosion prediction and anti-corrosion work, and can greatly reduce economic losses. To prevent and reduce the impact of various corrosions, many engineering structures or systems adopt anti-corrosion coating technologies, such as anti-corrosion compounds, anti-corrosion paint films, anti-corrosion tapes, etc. Affected by factors such as moisture absorption, climate change, external force damage, and construction defects, these anti-corrosion coatings will age over time, resulting in a decline in anti-corrosion performance and ultimately the failure of the overall structure. The maintenance cycle of anti-corrosion coatings is usually determined based on experience and the appearance of the coatings, often resulting in premature maintenance or delayed maintenance, causing unnecessary waste. By measuring the impedance and phase angle of the coating, changes in the coating performance can be detected before substrate corrosion occurs. By using a sensor device and a coating aging probe to monitor the performance and status of the coating in real time online, the maintenance department can accurately grasp the process of coating aging and the current latest aging degree information, and accordingly formulate the best maintenance time and maintenance plan, thereby improving the effectiveness of the anti-corrosion coating, minimizing the possibility of substrate corrosion caused by aging, reducing the maintenance workload at the same time, and saving maintenance costs.

[0003] Currently, the sensors used in the method of monitoring coating degradation by impedance only include two probes, which can only achieve single-impedance measurement, and the monitoring area for the surface coating degradation situation is limited. In order to more comprehensively monitor the coating degradation situation in each area of the surface, more probes need to be added to achieve multi-directional monitoring, and the designed circuit should meet the requirements of multi-impedance measurement. Summary of the Invention

[0004] The present invention proposes an impedance sensing measurement system and method for multi-directional monitoring of coating degradation to solve the problem that the sensors used in the current method of monitoring coating degradation by impedance only include two probes, which can only achieve single-impedance measurement, and the monitoring area for the surface coating degradation situation is limited.

[0005] Therefore, the purpose of the present invention is to provide an impedance sensing measurement system and method for multi-directional monitoring of coating degradation, which can measure the impedance of the coating in multiple directions and realize the monitoring of the degradation situation of the coating in each direction.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] An impedance sensing measurement system for multi-directional monitoring of coating degradation, comprising a multi-probe sensing device and an alternating current impedance measurement circuit. The alternating current impedance measurement circuit includes a single-chip microcomputer module, an excitation signal module, a measurement output signal amplification module, and an impedance switching and impedance dual-excitation measurement module. The single-chip microcomputer module is respectively connected to the excitation signal module, the measurement output signal amplification module, and the impedance switching and impedance dual-excitation measurement module. The excitation signal module and the measurement output signal amplification module are both connected to the impedance switching and impedance dual-excitation measurement module, and the impedance switching and impedance dual-excitation measurement module is connected to the multi-probe sensing device.

[0008] Furthermore, the multi-probe sensing device includes a metal substrate with a certain thickness. A central hole is provided on the metal substrate, and a variable number of multi-directional distribution holes are arranged equiangularly along the circumference of the central hole. Metal probes with insulated surfaces are provided in the central hole and the plurality of multi-directional distribution holes.

[0009] Furthermore, the single-chip microcomputer module includes an ADC module and 4 ADC channels connected thereto. The 4 ADC channels respectively collect 2 groups of excitation signals generated by the excitation signal module, and the measurement output signals generated by the impedance switching and impedance dual-excitation measurement module and 2 groups of output signals after the measurement output signals are processed by the amplification module.

[0010] The single-chip microcomputer module includes a dual-channel DAC module. The dual-channel DAC module generates 2-channel sinusoidal analog signals with a certain frequency and provides them to the excitation signal module.

[0011] The single-chip microcomputer module includes 8 output channels of the first GPIO. The 8 output channels of the first GPIO are used to control the amplitude and phase difference of the 2-channel sinusoidal analog signals generated by the dual-channel DAC module.

[0012] The single-chip microcomputer module includes n output channels of the second GPIO, which are used to control the switching of the impedance switching and impedance dual-excitation measurement module to realize the measurement of impedances in different directions.

[0013] Furthermore, the excitation signal module includes a first excitation signal generation circuit and a second excitation signal generation circuit. The first excitation signal generation circuit includes an operational amplifier U1, resistors R1 to R6, and a capacitor C1. One end of the resistor R1 is connected to the output of the dual-channel DAC module to receive a sinusoidal analog signal. The other end of the resistor R1 is connected to the positive input terminal of the operational amplifier U1. One end of the resistor R2 and one end of the capacitor C1 are both connected to the positive input terminal of the operational amplifier U1. The other ends of the resistor R2 and the capacitor C1 are respectively connected to one end of the resistors R3 to R6. The other ends of the resistors R3 to R6 are respectively connected to 4 output channels of the first GPIO of the single-chip microcomputer module. The negative input terminal of the operational amplifier U1 is connected to the output terminal, and the output terminal of the operational amplifier U1 outputs the first excitation signal. The vector representation of the AC excitation signal is used to simplify the synthesis calculation of the sine signal in the later operation;

[0014] The second excitation signal generation circuit includes an operational amplifier U2, resistors R7 to R12, and a capacitor C2. One end of the resistor R7 is connected to the output of the dual-channel DAC module to receive the other sine analog signal. The other end of the resistor R7 is connected to the positive input terminal of the operational amplifier U2. One end of the resistor R8 and one end of the capacitor C2 are both connected to the positive input terminal of the operational amplifier U2. The other ends of the resistor R8 and the capacitor C2 are respectively connected to one end of the resistors R9 to R12. The other ends of the resistors R9 to R12 are respectively connected to the output channels of the other 4 first GPIO of the single-chip microcomputer module. The negative input terminal of the operational amplifier U2 is connected to the output terminal, and the output terminal of the operational amplifier U2 outputs the second excitation signal

[0015] Furthermore, the impedance switching and impedance dual-excitation measurement module includes a reference resistor R0, an electronic switching switch, and an operational amplifier U3. One end of the reference resistor R0 is connected to the output terminal of the operational amplifier U1 to receive the first excitation signal The other end is connected to the central hole probe. The probes in several multi-directionally distributed holes are respectively connected to the output channels of the electronic switching switch. The central hole probe serves as a common electrode, and together with the electrodes formed by the probes in the circumferential multi-directionally distributed holes, they form a multi-directional dual electrode for measuring the coating impedance Z in each direction x ; The analog signal common terminal of the electronic switching switch is used as the input to receive the second excitation signal The control ports of the electronic switching switch are respectively connected to the output channels of n second GPIO of the single-chip microcomputer module for selecting the output channels of the second excitation signal The other end of the reference resistor R0 is also connected to the positive input terminal of the operational amplifier U3. The negative input terminal of the operational amplifier U3 is connected to the output terminal, and the output signal of the operational amplifier U3 is

[0016] Furthermore, the measurement output signal amplification module includes an operational amplifier U4, resistors R13 to R14, and capacitors C3 to C4. One end of the resistor R13 is connected to the output terminal of the operational amplifier U3, and the other end is connected to one end of the capacitor C3. The other end of the capacitor C3 is connected to the negative input terminal of the operational amplifier U4, and the positive input terminal of the operational amplifier U4 is grounded; both ends of the resistor R14 are respectively connected to the negative input terminal and the output terminal of the operational amplifier U4, and both ends of the capacitor C4 are respectively connected to the negative input terminal and the output terminal of the operational amplifier U4. The output terminal of the operational amplifier U4 is connected to one of the 4 ADC channels of the single-chip microcomputer module.

[0017] A method for measuring the multi-directional impedance of a coating using the impedance sensing measurement system includes the following steps:

[0018] Step 1: Adopt a two-step excitation method to roughly measure the impedance of a certain azimuth of the coating and simultaneously calculate the input resistance of operational amplifier U3

[0019] Step 2: Adopt an incremental iteration method to accurately measure the impedance of a certain azimuth of the coating Precisely measure;

[0020] Step 3: Switch the output channel of the electronic switch and repeat Steps 1 - 2 to complete the rough measurement and accurate measurement of the impedance of other azimuths of the coating.

[0021] Furthermore, Step 1 specifically includes the following steps:

[0022] Step 1.1: The theoretical derivation of the two-step excitation method using the vector formula is as follows, that is, given:

[0023]

[0024] Step 1.2: Set two excitation conditions to calculate the rough measurement of the impedance of a certain azimuth and the input impedance of operational amplifier U3 That is:

[0025] Condition 1): Set the first excitation signal The second excitation signal Then the output signal at the output terminal of operational amplifier U3 is According to formula (4), there is:

[0026]

[0027] Among them, A is the amplitude of the first excitation signal under Condition 1) and A1 is the amplitude of the output signal under Condition 1) is the phase angle of the output signal under Condition 1), and t is the time independent variable;

[0028] Condition 2): Set the first excitation signal The second excitation signal Then the output signal at the output terminal of operational amplifier U3 is According to formula (4), there is:

[0029]

[0030] Among them, A' is the amplitude of the second excitation signal under Condition 2) and A2 is the amplitude of the output signal under Condition 2) ​​The phase angle, where t is the time independent variable;

[0031] By combining equations (5) and (6), we get:

[0032]

[0033] The input impedance of operational amplifier U3 and the impedance of a certain azimuth of the coating can be calculated simultaneously from equations (5), (6), and (7). And measure the impedance of a certain azimuth of the coating

[0034] Furthermore, step 2 specifically includes the following steps:

[0035] Step 2.1: Calculate the amplitude ratio and phase difference of the excitation signal through equation (8):

[0036]

[0037] After the initial adjustment of the measurement results of the two-step excitation method, the bridge formed by the reference resistor R0 and the impedance of a certain azimuth is in a roughly balanced state;

[0038] Step 2.2: Conduct the theoretical derivation of the incremental iteration method. That is, after transforming equation (4), we have:

[0039]

[0040] Step 2.3: Perform fine adjustment on the first excitation signal That is:

[0041] When the first excitation signal occurs a change, will also correspondingly occur a change, that is:

[0042] To further balance the bridge, it is required that Then from equation (10), we can obtain:

[0043]

[0044] By combining equations (9) and (11), we have:

[0045]

[0046] Substituting equation (8) into equation (12), we get:

[0047]

[0048] Combining equation (10) with the incremental iteration method, perform fine adjustment on the first excitation signal ;

[0049] During the adjustment process of the incremental iteration method, each adjustment needs to determine the output voltage of the middle node of the bridge. Since the operational amplifier U3 only functions as a voltage follower, the output voltage of the middle node of the bridge is equal to the output voltage of the output terminal of the operational amplifier U3. Therefore, when is satisfied, the iteration stops, and no further fine adjustment is made to the first excitation signal . When

[0050] is not satisfied, based on the previous adjustment , the change amount of is continuously adjusted until is satisfied. Then, cyclic iterative adjustment is performed. After the iteration is completed, the bridge is in a precise balance state, and the impedance of a certain azimuth of the coating is accurately measured through formula (12); During the adjustment process of the incremental iteration method, since the bridge is basically balanced, at this time is very small. Affected by the quantization error of the actual sampled ADC, directly collecting signal will result in a large error in the collected signal. Therefore, in the actual measurement process, after collecting the amplified signal

[0051] , and then dividing it by the amplification factor to obtain , thus obtaining a more accurate signal and improving the measurement accuracy; When the second excitation signal changes during the adjustment of the incremental iteration method, will also correspondingly change. Therefore, the second excitation signal can be adjusted, that is:

[0052] In order to further balance the bridge, it is required that Then, from formula (14), we can obtain:

[0053]

[0054]

[0055]

[0056]

[0057]

[0058] By combining formulas (9) and (15), we have:

[0057]

[0058] Substituting formula (8) into formula (16), we can obtain:

[0059]

[0060] Combining formula (14) with the incremental iteration method, the second excitation signal Make fine adjustments;

[0061] In the incremental iteration method, each adjustment The output voltage of the middle node of the bridge needs to be The output voltage of the middle node of the bridge is The output voltage of the output terminal of the operational amplifier U3 Equal, therefore, when satisfied Stop iteration and stop the second stimulus signal Make fine adjustments, if not satisfied After the last adjustment Based on this, continue to adjust until the change in After the iteration is completed, the bridge is in a fine balance state, and the impedance of a certain position of the coating is calculated by formula (16). Accurate calculation.

[0062] Because iterative adjustment belongs to fine-tuning, in actual iterative calculation, in order to improve measurement accuracy, iterative adjustment is always performed on the excitation signal with a larger amplitude.

[0063] Furthermore, after completing the impedance measurement of a certain direction of the coating in step 3, the output channels of the electronic switch are switched in sequence, and steps 1 to 2 are repeated to complete the rough and precise measurement of the impedance of other directions of the coating.

[0064] Beneficial Effects of the Invention

[0065] The impedance sensing measurement system and method for multi-directional monitoring of coating degradation proposed by the present invention includes a multi-probe sensing device, a dual-excitation AC impedance measurement circuit, and a method for obtaining measurement results. A multi-impedance coating sensor is designed by designing a central probe as a common electrode and multiple circumferential probes designed at equal angles as distributed electrodes; under the control of a single-chip microcomputer module, the coating impedance between each circumferential probe and the central probe is measured in a time-sharing manner by electronic switching, so as to monitor the degradation of the coating in all directions; the present invention has the following specific advantages:

[0066] (1) Comprehensive and multi-directional monitoring of coating degradation in all areas of the surface;

[0067] (2) The measurement of multi-directional impedance can be used as a mutual reference to eliminate the influence of environmental factors on the impedance measurement results. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Figure 1 This is the structural schematic diagram of the impedance sensing measurement system for multi-directional monitoring of coating degradation of the present invention;

[0069] Figure 2(a) 、 2(b) This is the schematic diagram of the metal substrate of the present invention;

[0070] Figure 3(a) 、 3(b) This is the circuit diagram of the excitation signal module of the present invention;

[0071] Figure 4 This is the circuit diagram of the impedance switching and impedance dual-excitation measurement module of the present invention;

[0072] Figure 5 This is the circuit diagram of the measurement output signal amplification module of the present invention;

[0073] Figure 6 This is the flowchart for roughly calculating the impedance in a certain direction of the coating by using the two-step excitation method of the present invention and the input resistance of operational amplifier U3 of the flowchart;

[0074] Figure 7 This is the flowchart for accurately calculating the impedance in a certain direction of the coating by using the incremental iteration method of the present invention of the flowchart;

[0075] Figure 8 This is the method flowchart for multi-directional impedance measurement of the coating by using the impedance sensing measurement system of the present invention. Detailed implementation manners

[0076] In order to make the content of the present invention be more clearly understood, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0077] As Figure 1 shown, an embodiment of the present invention provides an impedance sensing measurement system for multi-directional monitoring of coating degradation, including a multi-probe sensing device and an alternating current impedance measurement circuit. The alternating current impedance measurement circuit includes a single-chip microcomputer module, an excitation signal module, a measurement output signal amplification module, and an impedance switching and impedance dual-excitation measurement module. The single-chip microcomputer module is respectively connected to the excitation signal module, the measurement output signal amplification module, and the impedance switching and impedance dual-excitation measurement module. The excitation signal module and the measurement output signal amplification module are both connected to the impedance switching and impedance dual-excitation measurement module. The impedance switching and impedance dual-excitation measurement module is connected to the multi-probe sensing device.

[0078] In this embodiment, the multi-probe sensing device includes a metal substrate with a certain thickness. A central hole is provided on the metal substrate, and a variable number of multi-directional distribution holes are arranged at equal angles along the circumference of the central hole. Metal probes with insulated surfaces are provided in the central hole and the plurality of multi-directional distribution holes.

[0079] In this embodiment, the single-chip microcomputer module includes an ADC module and 4 ADC channels connected thereto. The 4 ADC channels respectively collect 2 groups of excitation signals generated by the excitation signal module, and 2 groups of output signals generated by the impedance switching and impedance dual-excitation measurement module and processed by the output signal amplification module.

[0080] The single-chip microcomputer module includes a dual-channel DAC module. The dual-channel DAC module generates 2 sinusoidal analog signals with a certain frequency and provides them to the excitation signal module.

[0081] The single-chip microcomputer module includes 8 output channels of the first GPIO. The 8 output channels of the first GPIO are used to control the amplitude and phase difference of the 2 sinusoidal analog signals generated by the dual-channel DAC module.

[0082] The single-chip microcomputer module includes n output channels of the second GPIO, which are used to control the switching of the impedance switching and impedance dual-excitation measurement module to realize the measurement of impedances in different directions.

[0083] In this embodiment, the excitation signal module includes a first excitation signal generation circuit and a second excitation signal generation circuit. The first excitation signal generation circuit includes an operational amplifier U1, resistors R1 to R6, and a capacitor C1. One end of the resistor R1 is connected to the output of the dual-channel DAC module to receive a sinusoidal analog signal. The other end of the resistor R1 is connected to the positive input terminal of the operational amplifier U1. One end of the resistor R2 and one end of the capacitor C1 are both connected to the positive input terminal of the operational amplifier U1. The other ends of the resistor R2 and the capacitor C1 are respectively connected to one ends of the resistors R3 to R6. The other ends of the resistors R3 to R6 are respectively connected to the 4 output channels of the first GPIO of the single-chip microcomputer module in a one-to-one correspondence. The negative input terminal of the operational amplifier U1 is connected to the output terminal, and the output terminal of the operational amplifier U1 outputs the first excitation signal

[0084] The described second excitation signal generation circuit includes operational amplifier U2, resistors R7 to R12, and capacitor C2. One end of resistor R7 is connected to the output of the dual-channel DAC module to receive the other sinusoidal analog signal. The other end of resistor R7 is connected to the positive input terminal of operational amplifier U2. One end of resistor R8 and one end of capacitor C2 are both connected to the positive input terminal of operational amplifier U2. The other ends of resistor R8 and capacitor C2 are respectively connected to one end of resistors R9 to R12. The other ends of resistors R9 to R12 are respectively connected to the output channels of the other 4 first GPIO of the single-chip microcomputer module in one-to-one correspondence. The negative input terminal of operational amplifier U2 is connected to the output terminal, and the output terminal of operational amplifier U2 outputs the second excitation signal

[0085] In this embodiment, the impedance switching and impedance dual-excitation measurement module includes reference resistor R0, electronic switching switch, and operational amplifier U3. One end of reference resistor R0 is connected to the output terminal of operational amplifier U1 to receive the first excitation signal The other end is connected to the central hole probe. The probes in several multi-directionally distributed holes are respectively connected to the output channels of the electronic switching switch in one-to-one correspondence. The central hole probe serves as a common electrode, and together with the electrodes formed by the probes in the circumferential multi-directionally distributed holes, forms a multi-directional dual electrode for measuring the coating impedance Z in each direction x ; The analog signal common terminal of the electronic switching switch is used as the input to receive the second excitation signal The control ports of the electronic switching switch are respectively connected to the output channels of n second GPIO of the single-chip microcomputer module for selecting the output channels of the second excitation signal The other end of reference resistor R0 is also connected to the positive input terminal of operational amplifier U3. The negative input terminal of operational amplifier U3 is connected to the output terminal, and the output signal of the output terminal of operational amplifier U3 is

[0086] In this embodiment, the described measurement output signal amplification module includes operational amplifier U4, resistors R13 to R14, and capacitors C3 to C4. One end of resistor R13 is connected to the output terminal of operational amplifier U3, and the other end is connected to one end of capacitor C3. The other end of capacitor C3 is connected to the negative input terminal of operational amplifier U4, and the positive input terminal of operational amplifier U4 is grounded; both ends of resistor R14 are respectively connected to the negative input terminal and the output terminal of operational amplifier U4, and both ends of capacitor C4 are respectively connected to the negative input terminal and the output terminal of operational amplifier U4. The output terminal of operational amplifier U4 is connected to one of the 4 ADC channels of the single-chip microcomputer module

[0087] The second embodiment of the present invention also provides an impedance sensing measurement system for multi-directional monitoring of coating degradation, including a multi-probe sensing device and a dual-excitation AC impedance measurement circuit; a multi-impedance coating sensor is designed by designing a central probe as a common electrode and multiple circular probes designed at equal angles as distributed electrodes; under the control of a single-chip microcomputer, the AC impedance between each circular probe and the central probe is measured in a time-sharing manner by electronic switching, so as to monitor the degradation of the coating in all directions.

[0088] The overall structure of the measurement system is as follows: Figure 1 As shown, in addition to the multi-probe sensor device, the measurement system is mainly divided into four modules in terms of circuit:

[0089] (1) A microcontroller module based on an STM32 microcontroller, including four ADC conversion channels, two DAC output channels, a group of 8 GPIO output channels and a group of n GPIO output channels;

[0090] (2) an excitation signal module for generating two follower drive excitation signals;

[0091] (3) Impedance switching and impedance dual excitation measurement module;

[0092] (4) Measurement output signal amplification module.

[0093] In this embodiment, in order to eliminate the influence of the connecting wire on the measurement result as much as possible and to minimize the structure of the sensor as much as possible, the single-chip microcomputer module, the excitation signal module and the measurement output signal amplification module are integrated on a circuit board as a signal processing circuit board. The multi-probe sensing device and the impedance switching and impedance dual excitation measurement module are integrated on a circuit board as a sensing circuit board. In this way, the error caused by the impedance of the connecting wire itself is eliminated, the number of connecting wires is reduced, and at the same time, the measurement system is guaranteed to have greater anti-interference ability against spatial electromagnetic noise.

[0094] Multi-probe sensing device:

[0095] The structural design of the multi-probe sensor device is as follows Figure 2(a) , 2(b) As shown, for a metal substrate with a certain thickness and variable shape, first process the small holes shown in the figure, including a central hole and a variable number of multi-directional holes arranged at equal angles along the circumference. Then insert the surface-insulated metal probe or conductive metal wire into each small hole. At the same time, ensure that the exposed part of the probe is flush with the metal surface and smooth and flat. Finally, apply an anti-corrosion coating on the metal surface.

[0096] The center hole probe is used as a common electrode, and the electrodes formed by the probes in the circumferential multi-directional holes form a multi-directional dual electrode to measure the coating impedance in all directions. The number of directions is 2n pieces, which is convenient for selecting a conventional analog electronic switch chip, and generally n < 5.

[0097] Single-chip microcomputer module:

[0098] The single-chip microcomputer module is mainly based on the STM32 single-chip microcomputer and includes:

[0099] (1) Externally, 4 ADC channels are provided through the pins of the STM32 single-chip microcomputer to collect 2 groups of excitation signals and 2 groups of output signals respectively. The three ADC modules inside the STM32 single-chip microcomputer can synchronously collect the signals of 3 channels. By switching channels, the channels to be collected can be selected.

[0100] (2) A dual-channel DAC module, through DDS technology, generates 2-channel sine analog signals with a certain frequency. The amplitude and phase difference of the 2-channel analog sine signals can be arbitrarily changed within a certain range in a program-controlled manner. The output of 8 GPIOs can further reduce the output amplitude of the analog signal.

[0101] (3) The output of n GPIOs is used to switch the electronic switch to realize the time-sharing measurement of impedances in different directions. For the multi-select-one electronic switch, the output of n GPIOs can select 2 n measurements of impedances in different directions.

[0102] Excitation signal module that can generate 2-channel follow-up drive signals:

[0103] This module can make the amplitude of the output sine excitation signal change arbitrarily within a larger range in a direct or voltage-dividing follow-up manner for the analog sine signal input by the dual-channel DAC module. At the same time, the output excitation signal has a greater driving ability. Among them, the direct or voltage-dividing method is determined by the output states of 8 GPIOs. Every 2 GPIOs determine a voltage-dividing state, and each analog signal has 2 voltage-dividing states; the two follow-ups are respectively completed by two operational amplifiers in a common-mode follow-up output manner. The circuit diagram is as shown in Figure 3(a) 、 3(b) .

[0104] When designing each component in the circuit of the excitation signal module, for the component design of the first excitation signal generation circuit in Fig. 3(a): If the outputs of GPIO1 to GPI4 are 1, the corresponding level is VCC, and the reference voltage of the DAC1 module is Vref, then when the gain value of the operational amplifier U1 is set to 1 / k1, the values of R3 and R4, R5 and R6 can be designed and calculated according to the following formulas (1) and (2):

[0105]

[0106] Among them, the value of resistor R1 is the designated value in the design, which is a known quantity, and it satisfies R2 >> R1. Therefore, a resistance value much larger than that of R1 can be selected as the resistance value of R2. At the same time, the reduction effect of the selected C1 on the AC signal can be ignored and can be arbitrarily selected. k1 is a constant greater than 1.

[0107] The component design of the second excitation signal generation circuit in Fig. 3(b) is the same as that of the first excitation signal generation circuit in Fig. 3(a). That is, the designs of R7, R8, R9, R10, R11, R12 and C2 corresponding to them are exactly the same. And it satisfies R8 >> R7. The reduction effect of the selected C2 on the AC signal can be ignored.

[0108] Using the designed circuit diagrams, namely Fig. 3(a) and Fig. 3(b), by outputting different states of GPIO, three different divided voltage excitation output signals, direct following, reduced to 1 / k1 and 1 / k2, can be obtained as shown in Table 1 below. OD in the status output column represents open-drain output, and PP represents push-pull output.

[0109] Table 1 Pin Status Table of GPIO

[0110]

[0111] Impedance Switching and Impedance Dual-Excitation Measurement Module:

[0112] This module serves as the main processing circuit of the sensing circuit board and consists of a multiple-choice electronic switch, a reference resistor R0, and a single operational amplifier U3. The analog signal common terminal of the electronic switch is used as the input, connected to the excitation signal Ex2. The control ports ABC are connected to n GPIO output channels. Normally, n < 5, which is used to select the output channel of the excitation signal Ex2. Therefore, the state of the electronic switch determines the currently selected impedance of the measured position. Different states select different azimuth probes as the measured impedance. One end of the reference resistor is connected to the excitation signal Ex1, the other end is connected to the center hole probe, and at the same time, it is connected to the positive input terminal of the operational amplifier. The operational amplifier is designed in a non-inverting follower mode, and its output is used as out1 to output the first-stage measurement signal. Zxi shown in the figure represents the coating impedance between each azimuth.

[0113] Measurement Output Signal Amplification Module:

[0114] This module adopts the method of band-pass filtering and amplification. According to the frequency of the excitation signal, a second-order band-pass filtering and amplification circuit with the center frequency consistent with the excitation frequency is designed to ensure that the second-stage amplification has the same phase as the first-stage output. At the same time, the output has a higher signal-to-noise ratio, which is beneficial to further improving the measurement accuracy. The circuit schematic diagram is as Figure 5 .

[0115] The main relationship satisfied by the selection of resistors and capacitors is:

[0116]

[0117] Where: ω is the angular frequency of the excitation signal, and k is the amplification factor of the frequency signal.

[0118] In this embodiment, the component parameters in each module are set as follows:

[0119] (1) The single-chip microcomputer module selects STM32F407, and the GPIO of the reduction module selects the PA0 - PA3 and PE10 - PE13 ports of STM32F407; the GPIO of the electronic switch selects the PD0 - PD2 ports of STM32F407

[0120] (2) The operational amplifiers select the dual operational amplifier GS358 and the single operational amplifier GS321. The dual operational amplifier GS358 is responsible for generating 2 channels of follower drive excitation signals, such as U1 and U2, and the single operational amplifier GS321 is responsible for following the signal and amplifying the output signal, such as U3 and U4;

[0121] (3) The electronic switching switch selects the analog switch RS2251XS16, and 3 I / O ports can be used for free switching of 8 channels;

[0122] (4) The reference resistor R0 selects a 1MΩ resistor, and the excitation frequency selects 1kHz;

[0123] (5) Select the reduction multiples 1 / k1 = 1 / 6 and 1 / k2 = 1 / 31. Therefore, R1 = R7 = 100kΩ, R2 = R8 = 10MΩ, C1 = C2 = 10μF, R3 = R9 = 52.9kΩ, R4 = R10 = 32.3kΩ, R5 = R11 = 5.4kΩ, R6 = R12 = 8.8kΩ,

[0124] (6) Select the amplification factor k = 20 times; according to formula (3), set R1 = 3.5k, R2 = 100kΩ, C1 = 45nF, C2 = 1.59nF in the measurement output signal amplification module.

[0125] The third embodiment of the present invention is to provide a method for multi-directional impedance measurement of a coating using an impedance sensing measurement system. In order to eliminate Figure 4 the influence of the capacitive leakage impedance of the operational amplifier U3 during the measurement process, first, a two-step excitation method is used for rough measurement, and the input impedance of the operational amplifier is calculated simultaneously Then, using the measurement results of the two-step method, an incremental balance method is used for precise measurement; as Figure 8 shown, it includes the following steps:

[0126] Step 1: Adopt the two-step excitation method to measure the impedance of a certain azimuth of the coating Rough measurement and calculation of the input resistance of operational amplifier U3

[0127] Step 2: Adopt the incremental iteration method to accurately measure the impedance in a certain azimuth of the coating Accurate measurement;

[0128] Step 3: Switch the output channels of the electronic switch and repeat Steps 1-2 to complete the rough measurement and accurate measurement of the impedance in other azimuths of the coating.

[0129] In this embodiment, Step 1 specifically includes the following steps:

[0130] Step 1.1: The theoretical derivation of the two-step excitation method using the vector formula is as follows, that is, given:

[0131]

[0132] Step 1.2: Set two excitation conditions and calculate the rough measurement of the impedance in a certain azimuth and the input impedance of operational amplifier U3 That is:

[0133] Condition 1): Set the first excitation signal The second excitation signal Then the output signal at the output terminal of operational amplifier U3 is According to formula (4), we have:

[0134]

[0135] Among them, A is the amplitude of the first excitation signal under Condition 1 The amplitude of the output signal under Condition 1 is A1 The amplitude of the output signal under Condition 1 is The phase angle of the output signal under Condition 1 is t is the time independent variable;

[0136] Condition 2): Set the first excitation signal The second excitation signal Then the output signal at the output terminal of operational amplifier U3 is According to formula (4), we have:

[0137]

[0138] Among them, A' is the amplitude of the second excitation signal under Condition 2 The amplitude of the output signal under Condition 2 is A2 The amplitude of the output signal under Condition 2 is The phase angle of the output signal under Condition 2 is t is the time independent variable;

[0139] Combining equations (5) and (6) gives:

[0140]

[0141] The input impedance of operational amplifier U3 and the impedance of a certain azimuth of the coating can be calculated simultaneously from equations (5), (6), and (7). And measure the impedance of a certain azimuth of the coating

[0142] In this embodiment, in step 2, after the bridge is initially adjusted to balance using the two-step excitation method, it is in a roughly balanced state. At this time, according to the output signal of the potential at the middle node of the bridge to the pair or for incremental adjustment. This way of continuously adjusting according to the incremental value of this balance point is called the incremental iteration method. The incremental iteration method adjusts to gradually balance the bridge, reducing the influence on the measurement result and improving the measurement accuracy. Since the operational amplifier U3 acts as a voltage follower, the potential at the middle node of the bridge is equal to the output voltage .

[0143] Step 2 specifically includes the following steps:

[0144] Step 2.1: Calculate the amplitude ratio and phase difference of the excitation signal through equation (8):

[0145]

[0146] After the initial adjustment of the measurement result of the two-step excitation method, the bridge formed by the reference resistor R0 and the impedance of a certain azimuth is in a roughly balanced state;

[0147] Step 2.2: Conduct the theoretical derivation of the incremental iteration method, that is, after transforming equation (4), we have:

[0148]

[0149] Step 2.3: Fine-tune the first excitation signal , that is:

[0150] When the first excitation signal changes , will also correspondingly change , that is:

[0151] To further balance the bridge, it is required that Then, from equation (10), we can obtain:

[0152]

[0153] Combining equations (9) and (11), we get:

[0154]

[0155] Substituting equation (8) into equation (12), we obtain:

[0156]

[0157] Combining equation (10) with incremental iteration to finely adjust the first excitation signal for fine adjustment;

[0158] During the adjustment process of the incremental iteration method, each adjustment needs to determine the output voltage of the middle node of the bridge while the operational amplifier U3 only functions as a voltage follower. Therefore, the output voltage of the middle node of the bridge is equal to the output voltage of the output terminal of the operational amplifier U3 . Thus, when

[0159] the iteration stops, and no further fine adjustment is made to the first excitation signal . When is not satisfied, based on the after the previous adjustment, continue to adjust the variation until is satisfied for cyclic iterative adjustment. After the iteration is completed, the bridge is in a fine balance state, and the impedance of a certain azimuth of the coating is accurately measured through equation (12) ;

[0160] During the adjustment process of the incremental iteration method, since the bridge is basically balanced, at this time is very small. Affected by the quantization error of the actual sampled ADC, directly collecting the signal will result in a large error in the collected signal. Therefore, in the actual measurement process, after collecting the amplified signal , and then dividing by the amplification factor to obtain so as to obtain a more accurate signal and improve the measurement accuracy;

[0161] In the adjustment of the incremental iteration method, when the second excitation signal undergoes a change, will also correspondingly undergo a change. Therefore, the second excitation signal can be adjusted, that is:

[0162]

[0163] To further balance the bridge, it is required that Then, from formula (14), we can obtain:

[0164]

[0165] Combining formulas (9) and (15), we have:

[0166]

[0167] Substituting formula (8) into formula (16), we can get:

[0168]

[0169] Combining formula (14) with incremental iteration, for the second excitation signal Perform fine adjustment;

[0170] During the adjustment process of the incremental iteration method, each adjustment Needs to determine the output voltage of the middle node of the bridge And the operational amplifier U3 only plays the role of voltage following. Therefore, the output voltage of the middle node of the bridge Is equal to the output voltage at the output terminal of the operational amplifier U3 When satisfied Stop the iteration and no longer perform fine adjustment on the second excitation signal When not satisfied Based on the previous adjustment Continue to adjust The change amount of until satisfied Perform cyclic iterative adjustment. After the iteration is completed, the bridge is in a fine balance state, and the impedance of a certain azimuth of the coating is accurately measured through formula (16) Accurately measured.

[0171] Since the iterative adjustment belongs to fine tuning, in actual iterative calculations, in order to improve the measurement accuracy, the excitation signal with a larger amplitude is always used for iterative adjustment.

[0172] In this embodiment, after step 3 completes the impedance measurement of a certain azimuth of the coating, the output channels of the electronic switch are sequentially switched, and steps 1-2 are repeated to complete the rough measurement and accurate measurement of the impedance of other azimuths of the coating; under normal circumstances, iterating 2-3 steps can reach the fine balance state, and the subsequent output error is mainly affected by random errors.

[0173] The above are only the preferred embodiments of the present invention patent and are not intended to limit the present invention patent. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention patent shall be included within the protection scope of the present invention patent.

Claims

1. An impedance sensing measurement system for multi-directional monitoring of coating degradation, characterized in that It includes a multi-probe sensing device and an AC impedance measurement circuit. The AC impedance measurement circuit includes a single-chip microcomputer module, an excitation signal module, a measurement output signal amplification module, and an impedance switching and impedance dual excitation measurement module. The single-chip microcomputer module is respectively connected to the excitation signal module, the measurement output signal amplification module, and the impedance switching and impedance dual excitation measurement module. The excitation signal module and the measurement output signal amplification module are both connected to the impedance switching and impedance dual excitation measurement module. The impedance switching and impedance dual excitation measurement module is connected to the multi-probe sensing device.

2. The impedance sensing measurement system for multi-directional monitoring of coating degradation according to claim 1, wherein The multi-probe sensing device comprises a metal substrate with a certain thickness, a central hole and a variable number of multi-directional holes arranged at equal angles along the circumference of the central hole. The central hole and the multi-directional holes are provided with surface-insulated metal probes.

3. The impedance sensing measurement system for multi-directional monitoring of coating degradation according to claim 1, wherein The single-chip microcomputer module includes an ADC module and four ADC channels connected thereto, and the four ADC channels respectively collect two groups of excitation signals generated by the excitation signal module, as well as the measurement output signals generated by the impedance switching and impedance dual excitation measurement module and two groups of output signals after the measurement output signals are processed by the amplification module; The single-chip microcomputer module includes a dual-channel DAC module, which generates two sinusoidal analog signals of a certain frequency and provides them to the excitation signal module; The single-chip microcomputer module includes 8 first GPIO output channels, and the 8 first GPIO output channels are used to control the amplitude and phase difference of the two sinusoidal analog signals generated by the dual-channel DAC module; The single-chip microcomputer module comprises n second GPIO output channels, which are used to control the switching of impedance switching and impedance dual excitation measurement modules, so as to realize the measurement of impedances in different directions.

4. The impedance sensing measurement system for multi-directional monitoring of coating degradation according to claim 3, characterized in that, The described excitation signal module includes a first excitation signal generation circuit and a second excitation signal generation circuit. The first excitation signal generation circuit includes an operational amplifier U1, resistors R1 to R6, and a capacitor C1. One end of resistor R1 is connected to the output of the dual-channel DAC module to receive a sinusoidal analog signal. The other end of resistor R1 is connected to the positive input terminal of operational amplifier U1. One end of resistor R2 and one end of capacitor C1 are both connected to the positive input terminal of operational amplifier U1. The other ends of resistor R2 and capacitor C1 are respectively connected to one ends of resistors R3 to R6. The other ends of resistors R3 to R6 are respectively and correspondingly connected to the output channels of 4 first GPIO of the single-chip microcomputer module. The negative input terminal of operational amplifier U1 is connected to the output terminal, and the output terminal of operational amplifier U1 outputs the first excitation signal The described second excitation signal generation circuit includes an operational amplifier U2, resistors R7 to R12, and a capacitor C2. One end of resistor R7 is connected to the output of the dual-channel DAC module to receive the other sinusoidal analog signal. The other end of resistor R7 is connected to the positive input terminal of operational amplifier U2. One end of resistor R8 and one end of capacitor C2 are both connected to the positive input terminal of operational amplifier U2. The other ends of resistor R8 and capacitor C2 are respectively connected to one end of resistors R9 to R12. The other ends of resistors R9 to R12 are respectively and correspondingly connected to the output channels of the other 4 first GPIO of the single-chip microcomputer module. The negative input terminal of operational amplifier U2 is connected to the output terminal, and the output terminal of operational amplifier U2 outputs the second excitation signal 5. The impedance sensing measurement system for multi-directional monitoring of coating degradation according to claim 4, wherein The impedance switching and impedance dual-excitation measurement module includes a reference resistor R0, an electronic switching switch, and an operational amplifier U3. One end of the reference resistor R0 is connected to the output end of the operational amplifier U1 to receive the first excitation signal The other end is connected to the central hole probe. The probes in several multi-directionally distributed holes are respectively connected to the output channels of the electronic switching switch. The central hole probe serves as a common electrode, and the electrodes formed by the probes in the circumferential multi-directionally distributed holes constitute multi-directional dual electrodes for measuring the coating impedance Z in each direction x ; The analog signal common terminal of the electronic switching switch is used as the input to receive the second excitation signal The control ports of the electronic switching switch are respectively connected to the output channels of n second GPIO of the single-chip microcomputer module for selecting the output channels of the second excitation signal The other end of the reference resistor R0 is also connected to the positive input terminal of the operational amplifier U3. The negative input terminal of the operational amplifier U3 is connected to the output terminal, and the output signal of the output terminal of the operational amplifier U3 is 6. The impedance sensing measurement system for multi-directional monitoring of coating degradation according to claim 5, wherein The measurement output signal amplification module includes an operational amplifier U4, resistors R13~R14, and capacitors C3~C4. One end of the resistor R13 is connected to the output end of the operational amplifier U3, and the other end is connected to one end of the capacitor C3. The other end of the capacitor C3 is connected to the negative input end of the operational amplifier U4, and the positive input end of the operational amplifier U4 is grounded; the two ends of the resistor R14 are respectively connected to the negative input end and the output end of the operational amplifier U4, and the two ends of the capacitor C4 are respectively connected to the negative input end and the output end of the operational amplifier U4, and the output end of the operational amplifier U4 is connected to one of the four ADC channels of the single-chip microcomputer module.

7. A method for multi-directional impedance measurement of a coating using the impedance sensing measurement system as described in claim 6, characterized in that, The following steps are involved: Step 1: Adopt a two-step excitation method to roughly measure the impedance of a certain azimuth of the coating, and simultaneously calculate the input resistance of operational amplifier U3 ​ Step 2: Use the incremental iteration method to accurately measure the impedance of a certain azimuth of the coating accurately measure; Step 3: Switch the output channel of the electronic switch and repeat steps 1 to 2 to complete the rough and precise measurement of the impedance in other directions of the coating.

8. The method according to claim 7, characterized in that, Step 1 specifically includes the following steps: Step 1.1: Using the vector formula, the theoretical derivation of the two-step excitation method is as follows, that is, it is known that: Step 1.2: Set two excitation conditions and calculate the rough measurement of the impedance in a certain azimuth and the input impedance of operational amplifier U3 That is: Condition 1): Set the first excitation signal The second excitation signal Then the output terminal of the operational amplifier U3 outputs a signal According to Equation (4), we have: Among them, A is the amplitude of the first excitation signal under condition 1) , A1 is the amplitude of the output signal under condition 1) , is the phase angle of the output signal under condition 1) , and t is the time independent variable; Condition 2): Set the first excitation signal The second excitation signal Then the output terminal of the operational amplifier U3 outputs a signal According to Equation (4), we have: Among them, A' is the amplitude of the second excitation signal under condition 2) and A2 is the amplitude of the output signal under condition 2) . is the phase angle of the output signal under condition 2) , and t is the time independent variable; Combining formulas (5) and (6) we can get: The input impedance of operational amplifier U3 can be calculated simultaneously from formulas (5), (6), and (7). and the impedance in a certain azimuth of the coating can be measured 9. The method according to claim 8, wherein Step 2 specifically includes the following steps: Step 2.1: Calculate the amplitude ratio and phase difference of the excitation signal using formula (8): After the initial adjustment of the measurement results of the two-step excitation method, the bridge formed by the reference resistor R0 and a certain azimuth impedance is in a roughly balanced state; Step 2.2: Perform theoretical derivation of the incremental iteration method, that is, after transforming formula (4), we have: Step 2.3: Perform fine adjustment on the first excitation signal That is: When the first excitation signal occurs a change will also occur correspondingly i.e.: To further balance the bridge, it is required that Then, from Equation (10), we can obtain: Combining formulas (9) and (11), we have: Substituting formula (8) into formula (12), we get: Combine formula (10) with incremental iteration to finely adjust the first excitation signal for fine adjustment; During the adjustment process of the incremental iteration method, each adjustment needs to determine the output voltage of the middle node of the bridge . Since the operational amplifier U3 only plays the role of voltage following, the output voltage of the middle node of the bridge is equal to the output voltage of the output terminal of the operational amplifier U3 . Therefore, when is satisfied, the iteration stops, and no further fine adjustment is made to the first excitation signal . When is not satisfied, based on the after the previous adjustment, continue to adjust the change amount of until is satisfied for cyclic iterative adjustment. After the iteration is completed, the bridge is in a fine balance state, and the impedance of a certain azimuth of the coating is accurately measured through formula (12) ; During the adjustment process of the incremental iteration method, since the bridge is basically balanced, at this time is very small. Affected by the quantization error of the ADC in actual sampling, directly collecting signals will result in a large error in the collected signals. Therefore, during the actual measurement process, after collecting the amplified signal and then dividing it by the amplification factor, we get to obtain a more accurate signal; In the adjustment of the incremental iteration method, when the second excitation signal occurs a change, a corresponding change will also occur. Therefore, the second excitation signal can be adjusted, i.e.: To further balance the bridge, it is required that Then, from Equation (14), we can obtain: Combining formulas (9) and (15), we have: Substituting formula (8) into formula (16), we get: Combine formula (14) with incremental iteration to finely adjust the second excitation signal for fine adjustment; During the adjustment process of the incremental iteration method, each adjustment needs to determine the output voltage of the middle node of the bridge . Since the operational amplifier U3 only plays the role of voltage following, the output voltage of the middle node of the bridge is equal to the output voltage of the output terminal of the operational amplifier U3 . Therefore, when the condition is satisfied, the iteration stops and no further fine adjustment is made to the second excitation signal . When the condition is not satisfied , based on the result after the previous adjustment , the change amount of is continuously adjusted until the condition is satisfied, and cyclic iterative adjustment is performed. After the iteration is completed, the bridge is in a precise balance state, and the impedance of a certain azimuth of the coating is accurately measured through formula (16). To improve the measurement accuracy, the excitation signal with a larger amplitude is always used for iterative adjustment.

10. The method according to claim 9, characterized in that, In step 3, after measuring the impedance in a certain azimuth of the coating, sequentially switch the output channels of the electronic switch, and repeat steps 1-2 to complete the rough measurement and accurate measurement of the impedance in other azimuths of the coating.

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