Reinforcing steel bar nondestructive testing equipment and method based on double electromagnetic induction

Abstract

The invention relates to the field of reinforcing steel bar detection, and discloses a reinforcing steel bar nondestructive testing device and method based on double electromagnetic induction, and the device comprises a main control chip, a peripheral circuit, a displacement encoder module, an LCD serial port screen, a double electromagnetic induction coil and a power supply module; the main control chip is electrically connected with the peripheral circuit, the displacement encoder module and the LCD serial port screen; and the power supply module is electrically connected with the main control chip, the peripheral circuit, the displacement encoder module, the LCD serial port screen and the double electromagnetic induction coils and is used for supplying power. The beneficial effect of the invention is that the technical problem that an existing steel bar detector cannot accurately evaluate the diameter of a steel bar and the thickness of a protective layer at the same time because of only a single induction coil is solved.

Description

Technical Field

[0001] The present invention relates to the field of steel bar detection, and in particular to a steel bar nondestructive detection device and method based on double electromagnetic induction. Background Art

[0002] Reinforced concrete structure is the largest and most widely used structure in the world. It has been widely used in buildings, bridges and tunnels. This structure improves the efficiency of engineering construction while effectively enhancing the strength of the building. For the maintenance of key locations of old buildings, sampling and testing, and infrastructure renovation, it is necessary to accurately understand the distribution of steel bars inside the building. However, due to the age of many buildings, it is difficult to obtain construction drawings, which brings great difficulties to the detection of key locations and internal structures of the building and the resulting infrastructure renovation.

[0003] According to the relevant regulations of the Ministry of Housing and Urban-Rural Development, the mandatory acceptance inspection report of the project quality provided by the construction party must include the important indicator parameter of the concrete steel bar protective layer thickness. The concrete structures of beams and slabs should be inspected separately, and it is prohibited to use steel bars with diameters lower than the design requirements. In current construction, due to the limitations of construction technology, the position and burial depth of steel bars in concrete (often referred to as the protective layer thickness) usually cannot fully meet the design specifications. When the protective layer thickness is seriously inconsistent, the compressive and impact resistance of reinforced concrete structures will be greatly reduced.

[0004] Common methods for reinforced concrete structure detection can be divided into three categories:

[0005] The first is completely non-destructive testing methods, such as electromagnetic induction / eddy current testing, ultrasonic testing and ground penetrating radar, which do not cause any damage to the concrete structure;

[0006] The second is semi-destructive testing methods, such as pull-out and core testing methods. Semi-destructive testing does not endanger the overall structure of the concrete and is easy to repair;

[0007] The third is destructive testing methods, such as tensile testing and macro-sectioning. In contrast to non-destructive testing and semi-destructive testing, destructive testing methods will completely destroy and change the concrete structure. Even if the concrete structure passes the test, it is no longer suitable for continued use.

[0008] Existing non-destructive steel bar testing instruments based on the principle of electromagnetic induction require the steel bar diameter to be input in advance to obtain the predicted value of the protective layer thickness. The accuracy of the preset diameter determines the accuracy of the predicted steel bar protective layer thickness. In actual engineering applications, the steel bar meter, as a portable testing tool, needs to give accurate results quickly. However, in the face of some old buildings or buildings with missing drawings, the existing testing instruments cannot provide accurate prior information, so they cannot accurately evaluate the two important parameter information of protective layer thickness and steel bar diameter. Summary of the invention

[0009] The purpose of the present invention is to propose a steel bar nondestructive testing device and method based on dual electromagnetic induction, so as to solve the technical problem that the existing steel bar detector cannot accurately evaluate the diameter and protective layer thickness of the steel bar at the same time due to only a single induction coil.

[0010] A steel bar nondestructive testing device based on double electromagnetic induction, comprising:

[0011] Main control chip, peripheral circuit, displacement encoder module, LCD serial port screen, dual electromagnetic induction coil and power supply module;

[0012] The main control chip is electrically connected to the peripheral circuit, displacement encoder module, and LCD serial port screen;

[0013] The dual electromagnetic induction coil is electrically connected to the peripheral circuit and is used to receive an excitation signal;

[0014] The peripheral circuit is used to process the excitation signal and obtain the steel bar induction signal detection data;

[0015] The displacement encoder module is used to trigger the main control chip to read the steel bar sensing signal detection data output by the peripheral circuit;

[0016] LCD serial port screen is used to display test data and human-computer interaction control;

[0017] The power supply module is electrically connected to the main control chip, the peripheral circuit, the displacement encoder module, the LCD serial port screen, and the dual electromagnetic induction coils for power supply.

[0018] A steel bar nondestructive testing method based on dual electromagnetic induction is applied to a steel bar nondestructive testing device based on dual electromagnetic induction, comprising the following steps:

[0019] S1, the main control chip sends two pulse width modulation signals with different frequencies and duty cycles to the peripheral circuit;

[0020] S2, after the peripheral circuit passes through the signal power amplifier circuit, the amplified pulse width modulation signal is input into the dual electromagnetic induction coil to obtain a mixed electrical signal including the original excitation signal and the steel bar induction signal;

[0021] S3, the signal limiting circuit in the peripheral circuit filters out the original excitation signal in the mixed electrical signal to obtain a steel bar sensing signal;

[0022] S4, the steel bar sensing signal is extracted and amplified by the signal gating circuit and the amplifying circuit to obtain an amplified detection signal;

[0023] S5, the amplified detection signal is transmitted to the main control chip through the signal integration circuit and the A / D conversion circuit;

[0024] S6. The main control chip stores a pre-calibrated relationship table of the amplified detection signal U, the steel bar diameter D, and the protective layer thickness h:

[0025] S7. Find the common solution from the calibration relationship table through the double table lookup method to obtain the actual diameter and protective layer thickness of the steel bar to be measured.

[0026] The beneficial effects provided by the present invention are:

[0027] (1) Dual electromagnetic induction coils with different excitation signals are used to detect the same steel bar to simultaneously evaluate its diameter and protective layer thickness, avoiding the non-uniqueness of the detection results of a single coil.

[0028] (2) The hardware circuit part mainly includes the design and customization of the coil, the design and implementation of the main control chip and peripheral circuits, the excitation signal generating circuit, the signal extraction circuit, the integration circuit and the A / D conversion circuit. The hardware circuit automatically extracts and processes the sensing signal, which improves the efficiency of obtaining results.

[0029] (3) The table lookup algorithm proposed for dual-coil detection superimposes the difference between the voltage peaks detected by the two coils to avoid over-reliance on the detection results of one coil. The voltage peaks of the two coils are combined to reduce the diameter error of the device and the thickness error of the protective layer, thereby improving the accuracy of the detection results.

[0030] Based on the principle of electromagnetic induction, by improving the induction coil and excitation signal, the steel bar diameter and protective layer thickness can be accurately predicted without prior knowledge of the steel bar. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic diagram of the device of the present invention;

[0032] Figure 2 It is the schematic diagram of the peripheral circuit;

[0033] Figure 3 It is a schematic diagram of the coil signal waveform;

[0034] Figure 4 It is the steel bar parameter calibration table;

[0035] Figure 5 It is a flowchart of the double lookup table method. DETAILED DESCRIPTION

[0036] To make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0037] Please refer to Figure 1 It is a schematic diagram of the structure of the device of the present invention.

[0038] The present invention provides a steel bar nondestructive testing device based on dual electromagnetic induction, comprising: a main control chip, a peripheral circuit, a displacement encoder module, an LCD serial port screen, a dual electromagnetic induction coil and a power supply module;

[0039] The main control chip is electrically connected to the peripheral circuit, displacement encoder module, and LCD serial port screen;

[0040] The dual electromagnetic induction coil is electrically connected to the peripheral circuit and is used to receive an excitation signal;

[0041] The peripheral circuit is used to process the excitation signal and obtain the steel bar induction signal detection data;

[0042] The displacement encoder module is used to trigger the main control chip to read the steel bar sensing signal detection data output by the peripheral circuit;

[0043] LCD serial port screen is used to display test data and human-computer interaction control;

[0044] The power supply module is electrically connected to the main control chip, the peripheral circuit, the displacement encoder module, the LCD serial port screen, and the dual electromagnetic induction coils for power supply.

[0045] It should be noted that the peripheral circuit includes: a signal power amplification circuit, a signal limiting circuit, a signal gating circuit, a voltage amplification circuit, a signal integration circuit and an A / D conversion circuit.

[0046] It should be noted that the main control chip is electrically connected to the signal power amplification circuit; the signal power amplification circuit is electrically connected to the dual electromagnetic induction coil; the dual electromagnetic induction coil is electrically connected to the signal limiting circuit; the signal limiting circuit is electrically connected to the signal selection circuit; the signal selection circuit is electrically connected to the voltage amplification circuit; the voltage amplification circuit is electrically connected to the signal integration circuit; the signal integration circuit is electrically connected to the A / D conversion circuit; and the A / D conversion circuit is electrically connected to the main control chip.

[0047] It should be noted that the device also includes a WIFI module, which is electrically connected to the main control chip and is used to transmit the steel bar sensing signal detection data to the host computer.

[0048] It should be noted that the dual electromagnetic induction coils are both transceiver-integrated coils.

[0049] As an embodiment, the main control chip of the present invention adopts STM32 / F103ZET6 chip. Of course, as other embodiments, other replaceable chips can also be used.

[0050] The main control chip in the present invention mainly controls the operation of the entire device.

[0051] Please refer to Figure 2 , Figure 2 It is the schematic diagram of the peripheral circuit.

[0052] The peripheral circuit is controlled by the STM32 chip and is directly connected to the coil. It mainly includes a signal power amplifier circuit, a signal limiter circuit, a signal gating circuit, a voltage amplifier circuit, a signal integration circuit and an A / D conversion circuit. Their main functions are to amplify the excitation signal, process the steel bar induction signal and transmit it to the STM32 chip;

[0053] The displacement encoder module is mainly triggered by the movement of the roller. After moving a fixed distance, it emits a pulse signal, triggering the STM32 chip to read the detection voltage of the peripheral circuit.

[0054] The LCD serial port screen is directly connected to the STM32 chip to perform human-computer interaction control during detection; the WIFI module is connected to the STM32 chip to transmit the detection data to the host computer; the power supply module uses a lithium battery, which powers the entire device; the two electromagnetic induction coils are connected to the peripheral circuit to detect the steel bars at the same time.

[0055] The detection principle of the present invention is briefly described as follows:

[0056] The main purpose of the present invention is to accurately predict the diameter of the steel bar and the thickness of the protective layer at the same time without prior knowledge of the steel bar based on the principle of electromagnetic induction by improving the induction coil and the excitation signal.

[0057] The present invention takes the eddy current effect of electromagnetic induction as its theoretical basis and designs two identical transceiver coils. When alternating current is passed through the coils, a changing primary magnetic field will be generated in space. When the steel bars are in the changing magnetic field, eddy currents will be generated on the surface, and the changing eddy currents will excite a secondary magnetic field in space.

[0058] Therefore, the induced voltage received by the coil is the superposition of the primary magnetic field excited by the coil and the secondary magnetic field generated by the eddy current of the steel bar.

[0059] The hardware processing circuit can extract the induction signal of the steel bar itself from the received mixed electrical signal. When the coil moves to the top of the steel bar, the steel bar induction voltage signal U extracted by the coil is the largest. At this time, the relationship between the coil extraction voltage and the steel bar diameter and the protective layer thickness is:

[0060] U=f(D,h), (1)

[0061] Where: D is the diameter of the steel bar, h is the thickness of the steel bar protective layer.

[0062] It can be seen from the above formula that when there is only one detection coil, only one voltage value can be obtained, and the two parameters of the diameter of the steel bar and the thickness of the protective layer cannot be deduced;

[0063] Only when the steel bar diameter is known in advance can the thickness of the steel bar protective layer be accurately calculated. However, when two coils with different excitation signals detect the steel bar at the same time, two sets of voltage signals of the steel bar to be tested can be obtained. The corresponding relationship between the coil voltage and the steel bar diameter and the protective layer thickness is:

[0064]

[0065] Where: U1 is the induced voltage value of coil 1, and U2 is the induced voltage value of coil 2. The diameter of the steel bar and the thickness of the protective layer can be uniquely determined by the two voltage values ​​corresponding to the two sets of relationship equations. In two identical physical coils, namely coil 1 and coil 2, square wave pulse signals of different frequencies and duty cycles are passed through, and two different sets of induced voltage values ​​U1 and U2 under the same steel bar diameter and protective layer thickness can be obtained. From the perspective of solving the equation, there is a unique solution.

[0066] Therefore, the present invention provides a method for nondestructive detection of steel bars based on dual electromagnetic induction, comprising the following steps:

[0067] S1, the main control chip sends two pulse width modulation signals with different frequencies and duty cycles to the peripheral circuit;

[0068] S2, after the peripheral circuit passes through the signal power amplifier circuit, the amplified pulse width modulation signal is input into the dual electromagnetic induction coil to obtain a mixed electrical signal including the original excitation signal and the steel bar induction signal;

[0069] It should be noted that in the two groups of coils, the excitation signals used are both square wave pulse signals, and the signal received by each coil is the superposition of the excitation signal and the steel bar induction signal.

[0070] S3, the signal limiting circuit in the peripheral circuit filters out the original excitation signal in the mixed electrical signal to obtain a steel bar sensing signal;

[0071] It should be noted that since the secondary magnetic field excited by the steel bar lags behind the primary magnetic field in time, the steel bar induction signal received by the receiving coil is mainly located after the falling edge of the main pulse of the excitation signal passes through zero. Figure 3 As shown, Figure 3 It is a schematic diagram of coil signal waveform.

[0072] S4, the steel bar sensing signal is extracted and amplified by the signal gating circuit and the amplifying circuit to obtain an amplified detection signal;

[0073] In the present invention, the signal in the coil is intercepted, amplified and integrated, and the secondary induction signal from the steel bar is extracted from the aliased signal.

[0074] S5, the amplified detection signal is transmitted to the main control chip through the signal integration circuit and the A / D conversion circuit;

[0075] S6. The main control chip stores a pre-calibrated relationship table of the amplified detection signal U, the steel bar diameter D, and the protective layer thickness h:

[0076] As an example, in actual testing and evaluation, it is necessary to use these two coils to perform calibration measurements on various steel bar parameters in the laboratory in advance, thereby establishing two sets of calibration data tables, referring to Figure 3 As shown, Figure 4 It is the steel bar parameter calibration table;

[0077] S7. Find the common solution from the calibration relationship table through the double table lookup method to obtain the actual diameter and protective layer thickness of the steel bar to be measured.

[0078] As an embodiment, a two-table lookup method is used in actual evaluation, that is, a set of steel bar diameter and protective layer thickness values ​​corresponding to the two coil measurement values ​​in the corresponding two sets of calibration data tables are found respectively.

[0079] Specifically, coil 1 and coil 2 are first used to measure steel bars of various diameters and different protective layer thicknesses, and the two sets of electromagnetic induction data obtained form calibration data table 1 and calibration data table 2. Each set of calibration data tables may contain an induced voltage value corresponding to multiple sets of parameter values ​​to be measured.

[0080] During actual detection and evaluation, the two coils are located directly above the steel bar to be tested to measure the voltage values ​​U1 and U2. In the calibration data table 1, several possible groups of diameter and protective layer thickness values ​​corresponding to U1 are found, such as U(D1, h3), U(D2, h2), U(D2, h3), and U(D3, h2). In the calibration data table 2, several possible groups of diameter and protective layer thickness values ​​corresponding to U2 are found, such as U'(D1, h1), U'(D1, h1), U'(D2, h3), and U'(D3h3). The group of diameter and protective layer parameter values ​​with the same values ​​are the actual diameter and protective layer thickness of the steel bar to be tested, that is, (D2, h3).

[0081] Please refer to Figure 5 , Figure 5 It is a flowchart of the double lookup table method.

[0082] First, according to the steel bar induced voltage peak value detected by coil 1, multiple groups are found in the calibration data table corresponding to coil 1. Take 8 groups as an example below: 8 groups of calibration values ​​are closest to the measured peak value of the steel bar diameter and protective layer thickness, and the absolute value of the difference between the measured value and the calibration peak value in the table is saved; after narrowing the range to 8 groups, according to the peak value of the induced voltage detected by coil 2, 8 groups of diameter and protective layer thickness results corresponding to the voltage measurement peak value are found in the calibration data table of coil 2, and the absolute value of the difference between the measurement result and these 8 groups of voltages is returned, and superimposed in the difference array of coil 1; finally, the steel bar diameter and protective layer thickness corresponding to the group of voltage data with the smallest difference result are regarded as the actual diameter and protective layer thickness of the measured steel bar.

[0083] It should also be noted that when the device of the present invention is used, in order to subtract the background voltage generated by the coil mutual inductance and circuit noise, the coil voltage is first calibrated, the coil voltage value is read multiple times and the average is taken as the reference voltage, and then the coil voltage is read as the calibration value. If the reference voltage differs greatly from the calibration value, it indicates that the error is caused by the hardware circuit, and the reference voltage is corrected and updated to complete the calibration;

[0084] When the steel bar detection device moves on the concrete surface, every time the roller moves 1 mm, the displacement encoder will trigger a pulse to control the STM32 chip to read the voltage value of the A / D conversion circuit of coil 1 and coil 2, that is, the steel bar induced voltage is detected at the current position.

[0085] When the two coils are moved to the top of the steel bar, the secondary induced voltage of the two coils is the largest. The horizontal position of the steel bar is located by the trend change of the steel bar induced voltage collected during the scanning process from rising to falling.

[0086] After the scan is completed, the two sets of voltage data detected by the two coils are saved in the electrically erasable programmable read-only memory (EEPROM) of the STM32 chip. The data is in the form of two one-dimensional arrays, corresponding to the induced voltage values ​​obtained by the two coils in the measured displacement distance, and transmitted to the host computer through the WIFI module, so that the data can be processed on the host computer or on site. When the STM32 chip processes the data, it extracts the maximum value of the voltage in the two sets of "displacement-voltage" data measured by the two coils, and searches for the corresponding steel bar diameter and protective layer thickness values ​​in the calibration data table corresponding to coil 1 and coil 2. The last set of overlapping steel bar diameter and protective layer thickness parameter values ​​is the diameter and protective layer thickness of the steel bar to be measured.

[0087] In order to verify the effectiveness of the device and method of the present invention, the present invention provides a reinforced concrete block verification experiment.

[0088] Since it is difficult to find reinforced concrete buildings and walls that can be destructively verified in real life, the method of pouring reinforced concrete was chosen instead of actual building testing to further verify the effect of the proposed steel bar detection device in actual building detection.

[0089] This time, concrete blocks containing 9 different steel bars were poured for testing, with diameters of 8mm, 10mm, 12mm, 14mm, 16mm, 18mm, 20mm, 22mm, and 25mm. The developed steel bar detector was used to conduct positioning tests on these 9 steel bars with different diameters and protective layer thicknesses. The test results are shown in Table 1. It can be seen that the maximum positioning error is 3mm and the average error is 1.8mm. The concrete surface is rough and not flat, which will cause errors when using the device for mobile measurement, affecting the measurement accuracy of the device.

[0090] Table 1 Results of steel bar positioning in concrete

[0091]

[0092] After confirming that the device can accurately locate the steel bars, use the calibrated device to accurately measure the diameter and protective layer thickness of the steel bars in the concrete. After scanning the steel bars, click "End" on the screen. The device analyzes the test results, finds the measured voltage peaks of the two sets of coils, displays them on the serial port screen, and outputs the results of the table lookup method, that is, the diameter and protective layer thickness of the steel bars. At this time, you can click "Save" to save the data in the TF card, or click "Return" for the next test. The results of the device table lookup method are in Table 2.

[0093] Table 2 Detection results of steel bars in concrete

[0094]

[0095]

[0096] As can be seen from Table 2, the actual measurement effect of the device is relatively accurate, but the accuracy is slightly lost due to the uneven concrete surface. The results of the steel bar meter table lookup method show that in this concrete pouring test, the error of the steel bar diameter is within 1 specification, and the maximum error of the protective layer thickness is 2mm, which proves the accuracy of the steel bar equipment developed by the present invention.

[0097] The beneficial effects of the present invention are:

[0098] (1) Dual electromagnetic induction coils with different excitation signals are used to detect the same steel bar to simultaneously evaluate its diameter and protective layer thickness, avoiding the non-uniqueness of the detection results of a single coil.

[0099] (2) The hardware circuit part mainly includes the design and customization of the coil, the design and implementation of the main control chip and peripheral circuits, the excitation signal generating circuit, the signal extraction circuit, the integration circuit and the A / D conversion circuit. The hardware circuit automatically extracts and processes the sensing signal, which improves the efficiency of obtaining results.

[0100] (3) The table lookup algorithm proposed for dual-coil detection superimposes the difference between the voltage peaks detected by the two coils to avoid over-reliance on the detection results of one coil. The voltage peaks of the two coils are combined to reduce the diameter error of the device and the thickness error of the protective layer, thereby improving the accuracy of the detection results.

[0101] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A steel bar nondestructive testing device based on dual electromagnetic induction, characterized in that: include: Main control chip, peripheral circuit, displacement encoder module, LCD serial port screen, dual electromagnetic induction coil and power supply module; The main control chip is electrically connected to the peripheral circuit, displacement encoder module, and LCD serial port screen; The dual electromagnetic induction coil is electrically connected to the peripheral circuit and is used to receive an excitation signal; The peripheral circuit is used to process the excitation signal and obtain the steel bar induction signal detection data; The displacement encoder module is used to trigger the main control chip to read the steel bar sensing signal detection data output by the peripheral circuit; LCD serial port screen is used to display test data and human-computer interaction control; The power supply module is electrically connected to the main control chip, the peripheral circuit, the displacement encoder module, the LCD serial port screen, and the dual electromagnetic induction coils for power supply.

2. A steel bar nondestructive testing device based on dual electromagnetic induction as claimed in claim 1, characterized in that: The peripheral circuits include: signal power amplifier circuit, signal limiter circuit, signal selection circuit, voltage amplifier circuit, signal integration circuit and A / D conversion circuit.

3. A steel bar nondestructive testing device based on dual electromagnetic induction as claimed in claim 2, characterized in that: The main control chip is electrically connected to the signal power amplification circuit; the signal power amplification circuit is electrically connected to the dual electromagnetic induction coil; the dual electromagnetic induction coil is electrically connected to the signal limiting circuit; the signal limiting circuit is electrically connected to the signal selection circuit; the signal selection circuit is electrically connected to the voltage amplification circuit; the voltage amplification circuit is electrically connected to the signal integration circuit; the signal integration circuit is electrically connected to the A / D conversion circuit; and the A / D conversion circuit is electrically connected to the main control chip.

4. The steel bar nondestructive testing equipment based on dual electromagnetic induction according to claim 1, characterized in that: It also includes a WIFI module, which is electrically connected to the main control chip and is used to transmit the steel bar sensing signal detection data to the host computer.

5. The steel bar nondestructive testing equipment based on dual electromagnetic induction as claimed in claim 4, characterized in that: The dual electromagnetic induction coils are both transceiver-integrated coils.

6. A method for nondestructive testing of steel bars based on dual electromagnetic induction, applied to a nondestructive testing device for steel bars based on dual electromagnetic induction as claimed in any one of claims 1 to 5, characterized in that: The method comprises the following steps: S1, the main control chip sends two pulse width modulation signals with different frequencies and duty cycles to the peripheral circuit; S2, after the peripheral circuit passes through the signal power amplifier circuit, the amplified pulse width modulation signal is input into the dual electromagnetic induction coil to obtain a mixed electrical signal including the original excitation signal and the steel bar induction signal; S3, the signal limiting circuit in the peripheral circuit filters out the original excitation signal in the mixed electrical signal to obtain a steel bar sensing signal; S4, the steel bar sensing signal is extracted and amplified by the signal gating circuit and the amplifying circuit to obtain an amplified detection signal; S5, the amplified detection signal is transmitted to the main control chip through the signal integration circuit and the A / D conversion circuit; S6. The main control chip stores a pre-calibrated relationship table of the amplified detection signal U, the steel bar diameter D, and the protective layer thickness h: S7. Find the common solution from the calibration relationship table through the double table lookup method to obtain the actual diameter and protective layer thickness of the steel bar to be measured.

7. A method for nondestructive testing of steel bars based on dual electromagnetic induction as claimed in claim 6, characterized in that: Before conducting steel bar inspection, the coil voltage is first calibrated. The coil voltage values ​​are read multiple times and the average is taken as the reference voltage. The coil voltage is then read as the calibration value. If the reference voltage differs from the calibration value by more than the preset value, the reference voltage is corrected and updated to complete the calibration.

8. A method for nondestructive testing of steel bars based on dual electromagnetic induction as claimed in claim 7, characterized in that: When performing steel bar detection, every time the roller of the displacement encoder module rolls a preset distance, it will trigger the pulse control main control chip to read the detection signal of the A / D conversion circuit.

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