A method and device for detecting surface defects of a steel plate based on magnetic flux leakage principle

Abstract

The application provides a kind of steel plate surface defect detection method and device based on magnetic flux leakage principle, belong to steel plate surface defect detection field, including successively connected excitation unit, magnetic sensor, signal conditioning unit, data acquisition unit and host computer;Excitation unit is used to contact steel plate surface to form closed magnetic field, and the partial magnetization of steel plate is saturated state, and the leakage magnetic field is detected;Magnetic sensor is used to induct leakage magnetic field, and the differential voltage signal of leakage magnetic field is output;Signal conditioning unit is used to amplify and filter differential voltage signal, and obtains defect characteristic signal;Data acquisition unit is used to carry out digital signal conversion to defect characteristic signal, and obtains leakage detection data;Host computer is used to carry out waveform display, feature extraction and defect discrimination to leakage detection data, and obtains steel plate surface defect detection result.The application solves the problem that the sensitivity of existing steel plate surface defect detection device is low, the volume is large, and the data automatic acquisition function is lacked.

Description

Technical Field

[0001] This invention belongs to the field of steel plate surface defect detection, and particularly relates to a method and device for steel plate surface defect detection based on the principle of magnetic flux leakage. Background Technology

[0002] Steel plates, as an important ferromagnetic material, are widely used in construction, metallurgy, petroleum, coal mining, ports, and other fields. However, during the production and use of steel plates, defects such as inclusions, cracks, and pits are prone to appear on the surface. These defects can significantly reduce the load-bearing capacity and service life of the steel plates, and if they are not detected and dealt with in time, they may lead to serious safety accidents.

[0003] Currently, the main methods for detecting surface defects in steel plates include: Visual inspection: This method relies on the experience and visual observation of the inspector, and can only detect obvious surface defects. It has low reliability and is prone to missed defects. Eddy current testing: This method suffers from the skin effect, making it unable to identify deeper internal defects, and it requires a high degree of cleanliness on the steel plate surface. Ultrasonic testing: This method requires a coupling agent, has a slow testing speed, and a low detection rate for small near-surface defects. Magnetic particle testing: Although it has high detection accuracy, it requires demagnetization of the steel plate after testing, resulting in high cost and low efficiency.

[0004] Magnetic flux leakage (MFL) detection technology boasts advantages such as simple structure, non-contact operation, high sensitivity, and the ability to detect surface and near-surface defects. It is widely used for defect detection in ferromagnetic materials such as rails and pipes. However, most existing MFL detection devices are bulky, expensive, and often employ low-sensitivity Hall sensors, limiting their ability to detect minute defects. Furthermore, most existing devices lack automatic data acquisition and transmission capabilities, resulting in inaccurate data acquisition and hindering real-time recording and offline analysis of detection data. Summary of the Invention

[0005] To address the aforementioned shortcomings in existing technologies, this invention provides a method and apparatus for detecting surface defects in steel plates based on the principle of magnetic flux leakage, which solves the problems of low sensitivity, large size, and inaccurate data acquisition in existing steel plate surface defect detection devices, resulting in low defect detection accuracy.

[0006] To achieve the above objectives, the technical solution adopted by this invention is as follows: a method for detecting surface defects in steel plates based on the principle of magnetic flux leakage, comprising the following steps: S1: Place the excitation unit on the surface of the steel plate to be tested, so that the excitation unit and the steel plate to be tested form a closed magnetic circuit, and magnetize the local part of the steel plate to be tested to saturation. S2: The differential voltage signal is obtained by sensing the leakage magnetic field generated at the defect on the surface of the steel plate under test by a magnetic sensor; S3: The differential voltage signal is amplified and filtered by the signal conditioning unit to obtain the defect characteristic signal; S4: The incremental encoder detects the movement distance of the probe on the steel plate surface. Data acquisition is triggered every fixed distance movement. The data acquisition unit collects the defect feature signal and converts it into a digital signal, which is then temporarily stored in the external SRAM. S5: The digital signal temporarily stored in the external SRAM is transmitted to the host computer via USB to serial port. The host computer receives and displays the waveform corresponding to the digital signal, and judges the existence, location and relative severity of defects on the steel plate surface based on the waveform changes.

[0007] The present invention also provides a steel plate surface defect detection device based on the principle of magnetic leakage, comprising an excitation unit, a magnetic sensor, a signal conditioning unit, a data acquisition unit and a host computer connected in sequence; The excitation unit is used to form a closed magnetic field by contacting the surface of the steel plate, locally magnetizing the steel plate to a saturated state, and detecting the leakage magnetic field. The magnetic sensor is used to sense the leakage magnetic field and output a differential voltage signal of the leakage magnetic field. The signal conditioning unit is used to amplify and filter the differential voltage signal to obtain the defect feature signal; The data acquisition unit is used to perform digital signal conversion on the defect feature signals to obtain magnetic flux leakage detection data; The host computer is used to display waveforms, extract features, and identify defects in the magnetic flux leakage detection data to obtain the surface defect detection results of the steel plate.

[0008] To ensure the accuracy of steel plate surface defect detection, most current devices are based on magnetic flux leakage detection technology. However, these devices primarily use electromagnet excitation, requiring a high-current external power supply, resulting in bulky and heavy equipment suitable only for laboratory testing, making them unsuitable for portable on-site testing. Furthermore, existing magnetic flux leakage detection devices often use Hall effect sensors due to circuit limitations, which are not very effective at detecting the weak magnetic flux leakage generated by inclusions, cracks, and other minute defects, limiting their detection capabilities. Most also lack signal conditioning and data acquisition functions, resulting in low signal-to-noise ratios and the inability to achieve real-time recording and offline analysis of defect signals. This makes it difficult to meet the needs of quantitative analysis and quality management of test results in steel plate production sites. To address this issue, this invention provides a steel plate surface defect detection device based on the principle of magnetic leakage. It forms a closed magnetic field through permanent magnet excitation, eliminating the need for an external high-current power supply, thus reducing the device's size and weight and enabling portable on-site detection. Furthermore, based on permanent magnet excitation, to ensure detection accuracy, a more sensitive tunnel magnetoresistive sensor is selected instead of a traditional Hall sensor. A matching signal conditioning unit, consisting of a differential amplifier, a high-pass filter, and a low-pass filter cascaded together, is designed to suppress baseline drift and high-frequency noise from the tunnel magnetoresistive sensor, improving the detection sensitivity for weak magnetic leakage field signals. Combined with a data acquisition unit and a host computer, it enables real-time acquisition, transmission, display, and quantitative analysis of steel plate surface defects, meeting the needs of practical applications for high-sensitivity, portable, and visualized detection of steel plate surface defects.

[0009] Furthermore: the excitation unit includes an industrial magnetic yoke, a first permanent magnet, and a second permanent magnet; The first permanent magnet and the second permanent magnet are respectively located at both ends of the industrial yoke, and are placed parallel to each other along the length of the industrial yoke; The first and second permanent magnets placed therein have opposite polarities; When the excitation unit comes into contact with the surface of the steel plate, a closed magnetic field is formed through the industrial yoke, the first permanent magnet, the steel plate, and the second permanent magnet.

[0010] The further beneficial effects mentioned above are as follows: This invention uses two permanent magnets with opposite polarities in conjunction with an industrial magnetic yoke, and utilizes the static magnetic field of the permanent magnets themselves to form a closed magnetic circuit with the steel plate, instead of using an electromagnet structure in the conventional way. It can locally magnetize the steel plate without external power supply, reducing the weight and power consumption of the device, realizing portable detection of steel plate surface defects, and meeting the needs of steel plate defect detection in steel plate production sites or outdoor environments.

[0011] Furthermore, the magnetic sensor is a tunnel magnetoresistive sensor, fixed between two permanent magnets with opposite polarities, maintaining a constant lift-off distance from the steel plate surface.

[0012] The further beneficial effects mentioned above are as follows: the present invention uses a tunnel magnetoresistive sensor to replace the traditional Hall sensor, which has higher sensitivity and can capture weak leakage magnetic field signals; at the same time, the present invention fixes the magnetic sensor between the two permanent magnets of the excitation unit and maintains a constant lift-off distance, which improves the detection efficiency of steel plate defects.

[0013] Furthermore, the signal conditioning unit includes a differential-to-single-ended amplifier circuit, an RC high-pass filter circuit, and a second-order low-pass active filter circuit; The differential-to-single-ended amplifier circuit includes an operational amplifier U1. The output terminal 1 of the operational amplifier U1 is connected to one end of a resistor R1 and an RC high-pass filter circuit. The other end of the resistor R1 is connected to the inverting input terminal 2 of the operational amplifier U1 and one end of a resistor R4. The other end of the resistor R4 is connected to one end of a capacitor C1 and one end of a resistor R7. The other end of the resistor R7 is grounded. The positive power supply terminal of the operational amplifier U1 is connected to a +5V power supply, and the negative power supply terminal is grounded. The non-inverting input terminal of the operational amplifier U1 is connected to a magnetic sensor and one end of a resistor R8. The other end of the resistor R8 is grounded. The RC high-pass filter circuit includes resistors R5 and R6 and capacitor C3. One end of resistor R5 is connected to the output terminal 1 of operational amplifier U1, and the other end is connected to one end of capacitor C3 and one end of resistor R6. The other end of capacitor C3 is connected to the second-order low-pass active filter circuit. The other end of resistor R6 is connected to one end of capacitor C4 and the second-order low-pass active filter circuit. The other end of capacitor C4 is grounded. The second-order low-pass active filter circuit includes an operational amplifier U2. The output terminal of the operational amplifier U2 is connected to the other end of resistor R5, one end of resistor R3, and the data acquisition unit, respectively. The other end of resistor R3 is connected to the inverting input terminal 6 of the operational amplifier U1 and one end of resistor R2, respectively. The other end of resistor R2 is grounded. The non-inverting input terminal of the operational amplifier U2 is connected to the other end of resistor R6.

[0014] The further beneficial effects mentioned above are as follows: The present invention, through a signal conditioning unit composed of a differential-to-single-ended amplifier circuit, an RC high-pass filter circuit, and a second-order low-pass active filter circuit, sequentially completes signal conversion, drift elimination, and noise suppression, and can extract defect feature signals with high signal-to-noise ratio, providing clear data for subsequent data acquisition and defect identification, and improving the reliability of the detection results of the steel plate surface defect detection device.

[0015] Furthermore: the data acquisition unit includes a microcontroller chip U3, a USB to serial port chip U4, external SRAM, and an incremental encoder; The microcontroller chip U3 includes an analog-to-digital converter module, whose input terminal is connected to the output terminal of the signal conditioning unit, and is used to acquire defect feature signals and convert them into digital signals. The serial transmit and receive pins of the microcontroller chip U3 are connected to the receive and transmit pins of the USB to serial port chip U4, respectively. The USB to serial port chip U4 is connected to the host computer through a USB interface and is used to transmit digital signals to the host computer. The external SRAM is connected to the FSMC bus interface of the microcontroller chip U3 and is used to temporarily store the collected magnetic flux leakage detection data. The signal output terminal of the incremental encoder is connected to the external interrupt input pin of the microcontroller chip U3. The incremental encoder is coaxially connected to the guide wheel. When the guide wheel rolls on the steel plate surface, it converts the moving distance into a pulse signal. Every fixed distance moved triggers an analog-to-digital conversion acquisition, which is used for equal spatial sampling and locating the defect position on the steel plate surface.

[0016] The further beneficial effects mentioned above are as follows: Existing magnetic flux leakage detection devices mostly adopt an equal-time sampling method. Uneven manual probe movement speed leads to waveform distortion of the defect signal, and the amount of data collected is limited by the internal RAM capacity of the microcontroller, which cannot meet the data storage requirements of long-distance continuous detection, making it difficult to accurately locate the defect position. The incremental encoder of this invention is coaxially connected to the guide wheel, converting the probe movement distance into a pulse signal. It triggers an analog-to-digital conversion acquisition once every fixed movement distance, realizing equal-space sampling, solving the influence of uneven detection speed on the signal waveform, and enabling accurate location of the defect.

[0017] The beneficial effects of this invention are as follows: This invention uses two permanent magnets with opposite polarities combined with an industrial yoke to form an excitation unit, which can locally magnetize the steel plate to saturation without the need for an external high-current power supply, reducing the size and weight of the steel plate surface defect detection device and meeting the portable detection needs of steel plate production sites or outdoor environments; it uses a tunnel magnetoresistive sensor instead of a traditional Hall sensor, which has higher sensitivity than Hall elements and can capture leakage magnetic field signals generated by tiny defects, improving the detection accuracy of steel plate surface defects; through differential to single-ended amplification, RC high-pass filtering and second-order low-pass active filtering, signal conversion and amplification, DC baseline drift elimination and high-frequency noise suppression are performed in sequence, improving the quality of defect signals when using tunnel magnetoresistive sensors; through the data acquisition unit, the defect signals are automatically acquired and digitized, and can be transmitted to the host computer in real time for waveform display and discrimination, making the steel plate surface defect detection results easy to display intuitively, meeting the needs of high-sensitivity, portable and intelligent detection at the steel plate defect detection site. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of a steel plate surface defect detection device based on the principle of magnetic flux leakage. Figure 2 This is a schematic diagram of the excitation unit. Figure 3 This is a schematic diagram of the circuit principle of a steel plate surface defect detection device based on the principle of magnetic flux leakage. Detailed Implementation

[0019] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the present invention as defined and determined by the appended claims. All inventions utilizing the concept of the present invention are protected.

[0020] Example 1 like Figure 1 The diagram shows a structural schematic of a steel plate surface defect detection device based on the principle of magnetic leakage. The present invention provides a steel plate surface defect detection device based on the principle of magnetic leakage, including an excitation unit, a magnetic sensor, a signal conditioning unit, a data acquisition unit, and a host computer connected in sequence. The excitation unit is used to form a closed magnetic field by contacting the surface of the steel plate, locally magnetizing the steel plate to a saturated state, and detecting the leakage magnetic field. The magnetic sensor is used to sense the leakage magnetic field and output a differential voltage signal of the leakage magnetic field. The signal conditioning unit is used to amplify and filter the differential voltage signal to obtain the defect feature signal; The data acquisition unit is used to perform digital signal conversion on the defect feature signals to obtain magnetic flux leakage detection data; The host computer is used to display waveforms, extract features, and identify defects in the magnetic flux leakage detection data to obtain the surface defect detection results of the steel plate.

[0021] In a specific embodiment of the present invention, the excitation unit includes an industrial magnetic yoke, a first permanent magnet, and a second permanent magnet; The first permanent magnet and the second permanent magnet are respectively located at both ends of the industrial yoke, and are placed parallel to each other along the length of the industrial yoke; the polarities of the first permanent magnet and the second permanent magnet are opposite; when the excitation unit contacts the surface of the steel plate, a closed magnetic field is formed through the industrial yoke, the first permanent magnet, the steel plate and the second permanent magnet.

[0022] In a specific embodiment, such as Figure 2The diagram shows the structure of the excitation unit, which includes an industrial pure iron yoke and two neodymium iron boron permanent magnets with opposite polarities. The industrial pure iron yoke measures 20mm × 30mm × 80mm. The two neodymium iron boron permanent magnets are located at both ends of the industrial pure iron yoke. The two neodymium iron boron permanent magnets each measure 20mm × 30mm × 10mm and are placed parallel to each other along the length of the industrial pure iron yoke with opposite polarities, i.e., one with the N pole facing down and the other with the S pole facing down. After placing the excitation unit on the surface of the steel plate to be tested, a closed magnetic circuit is formed by an industrial pure iron yoke and two neodymium iron boron permanent magnets with opposite polarities, locally magnetizing the steel plate to saturation or near-saturation. When the steel plate surface is intact and without defects, no leakage magnetic field can be formed, the magnetic sensor cannot detect the leakage magnetic field, and no differential voltage signal can be generated, resulting in a test result indicating that the steel plate surface is intact. When defects such as inclusions, cracks, and pits exist on the steel plate surface, the magnetic permeability of the defect area changes abruptly. At this time, the magnetic lines of force bend at the defect and partially leak to the surface of the steel plate, forming a leakage magnetic field. This invention uses permanent magnets for excitation in the excitation unit, which can form a closed magnetic field without the need for an external high-current power supply. The device has a compact overall structure, low cost, and is easy to use handheld or portable in the field.

[0023] After the excitation unit generates a leakage magnetic field, this invention uses a magnetic sensor to sense and detect the leakage magnetic field. The magnetic sensor is a tunnel magnetoresistive sensor, specifically a TMR2102 tunnel magnetoresistive linear sensor in an SOP8 package. Its operating voltage is 5V, and its sensitivity is 4.9mV / V / Oe. It is fixed between two permanent magnets with opposite polarities and maintains a constant lift-off distance from the steel plate surface, which can be set to 2-4mm. The sensitive direction of the magnetic sensor is parallel to the steel plate surface and perpendicular to the direction of the magnetic field lines. It is used to sense the radial magnetic field component leaking at the defects on the steel plate surface and outputs a differential voltage signal.

[0024] In specific embodiments of the present invention, such as Figure 3 The diagram shows a circuit diagram of a steel plate surface defect detection device based on the principle of magnetic leakage. TMR2102 is a TMR2102 tunnel magnetoresistive linear sensor. Its 4th and 5th ports are connected to the differential-to-single-ended amplifier circuit in the signal conditioning unit, which can transmit the detected differential voltage signal to the conditioning unit.

[0025] The signal conditioning unit includes a differential-to-single-ended amplifier circuit, an RC high-pass filter circuit, and a second-order low-pass active filter circuit; The differential-to-single-ended amplifier circuit includes an operational amplifier U1. The output terminal 1 of the operational amplifier U1 is connected to one end of a resistor R1 and an RC high-pass filter circuit. The other end of the resistor R1 is connected to the inverting input terminal 2 of the operational amplifier U1 and one end of a resistor R4. The other end of the resistor R4 is connected to one end of a capacitor C1 and one end of a resistor R7. The other end of the resistor R7 is grounded. The positive power supply terminal of the operational amplifier U1 is connected to a +5V power supply, and the negative power supply terminal is grounded. The non-inverting input terminal of the operational amplifier U1 is connected to a magnetic sensor and one end of a resistor R8. The other end of the resistor R8 is grounded. The differential-to-single-ended amplifier circuit of this invention can be constructed using a TLC2254 operational amplifier, that is, a TLC2254 operational amplifier is selected as the operational amplifier U1, and the gain is set to 10.

[0026] The RC high-pass filter circuit includes resistors R5 and R6 and capacitor C3. One end of resistor R5 is connected to output terminal 1 of operational amplifier U1, and the other end is connected to one end of capacitor C3 and one end of resistor R6. The other end of capacitor C3 is connected to a second-order low-pass active filter circuit. The other end of resistor R6 is connected to one end of capacitor C4 and a second-order low-pass active filter circuit. The other end of capacitor C4 is grounded. The RC high-pass filter circuit of this invention is composed of capacitors and resistors, and its cutoff frequency is 1.59Hz.

[0027] The second-order low-pass active filter circuit includes an operational amplifier U2. The output terminal of operational amplifier U2 is connected to the other end of resistor R5, one end of resistor R3, and a data acquisition unit. The other end of resistor R3 is connected to the inverting input terminal 6 of operational amplifier U1 and one end of resistor R2. The other end of resistor R2 is grounded. The non-inverting input terminal of operational amplifier U2 is connected to the other end of resistor R6. The second-order low-pass active filter circuit of this invention also uses a TLC2254 operational amplifier. Its cutoff frequency is 160Hz.

[0028] The signal conditioning unit of this invention converts differential signals to single-ended signals and performs primary amplification through a differential-to-single-ended amplifier circuit, filters out DC baseline drift through an RC high-pass filter circuit, and filters out high-frequency noise through a second-order low-pass active filter circuit, finally obtaining a high-quality defect feature signal.

[0029] In specific embodiments of the present invention, existing magnetic flux leakage detection devices mostly employ an equal-time sampling method for data acquisition, i.e., signals are collected at fixed time intervals. When the operator moves the probe across the steel plate surface, it is difficult to maintain a completely uniform speed. At higher speeds, the number of sampling points per unit distance decreases, and the defect waveform is compressed; at lower speeds, the number of sampling points increases, and the defect waveform is stretched. The waveform of the same defect varies at different detection speeds, leading to reduced accuracy in subsequent feature extraction and analysis. Furthermore, existing magnetic flux leakage detection devices struggle to correlate detection signals with spatial location, mostly relying on manual marking of defect positions by the operator, resulting in large positioning errors and poor repeatability. To address these issues, the present invention designs a data acquisition unit; the data acquisition unit includes a microcontroller chip U3, a USB-to-serial chip U4, external SRAM, and an incremental encoder. The microcontroller chip U3 includes an analog-to-digital converter module, whose input terminal is connected to the output terminal of the signal conditioning unit, and is used to acquire defect feature signals and convert them into digital signals. The serial transmit and receive pins of the microcontroller chip U3 are connected to the receive and transmit pins of the USB to serial port chip U4, respectively. The USB to serial port chip U4 is connected to the host computer through a USB interface and is used to transmit digital signals to the host computer. The external SRAM is connected to the FSMC bus interface of the microcontroller chip U3 and is used to temporarily store the collected magnetic flux leakage detection data. The signal output terminal of the incremental encoder is connected to the external interrupt input pin of the microcontroller chip U3. The incremental encoder is coaxially connected to the guide wheel. When the guide wheel rolls on the steel plate surface, it converts the moving distance into a pulse signal. Every fixed distance moved triggers an analog-to-digital conversion acquisition, which is used for equal spatial sampling and locating the defect position on the steel plate surface.

[0030] In this invention, the guide wheel converts the linear movement distance of the probe into encoder pulse signals as it rolls on the steel plate surface. Each time the microcontroller receives an encoder pulse, it triggers an analog-to-digital conversion acquisition, ensuring a one-to-one correspondence between the sampling point and the spatial position on the steel plate surface. This solves the waveform distortion problem caused by uneven manual movement speed under the equal-time sampling method, guaranteeing the consistency and repeatability of the defect signal waveform. Furthermore, this invention adds an external SRAM interface to the microcontroller's FSMC bus, expanding the microcontroller's original data storage capacity to meet the data storage requirements of long-distance continuous detection in practical engineering. Additionally, since the encoder pulse count corresponds one-to-one with the sampling point sequence number, this invention can also accurately locate the defect, significantly improving the defect location accuracy and practicality of the steel plate surface detection.

[0031] like Figure 3As shown, the microcontroller chip U3 is an STM32F103 with analog-to-digital conversion capabilities. Its serial transmit pin PA9 and receive pin PA10 are connected to the receive pin RXD and transmit pin TXD of the USB-to-serial chip U4, respectively. The external interrupt input pin PA1 of the microcontroller chip U3 is connected to the incremental encoder. The general-purpose input / output pin PB0 of the microcontroller chip U3 is connected to the buzzer U6 for alerting and alarming. The FSMC bus interface of the microcontroller chip U3, i.e., pins D0 to D15, is connected to the LCD display U7. Through the LCD display, the received defect detection data can be displayed in a visual manner as a waveform, making it easy for inspectors to judge the presence, location, and relative severity of defects based on waveform changes. Most existing magnetic flux leakage detection devices only have signal acquisition and storage functions in their data acquisition section, lacking real-time feedback and location capabilities on-site. This results in inspectors not being able to obtain the test results immediately during the inspection process, reducing on-site inspection efficiency. This invention designs a data acquisition unit that can automatically alarm via a buzzer, enabling inspectors to immediately know the location of steel plate defects and mark them to avoid missed detections. The real-time display of the detection waveform via an LCD screen allows inspectors to intuitively observe the trend of defect signal changes, complete preliminary detection and judgment, and improve the flexibility and efficiency of on-site inspection.

[0032] Example 2 This invention provides a method for detecting surface defects in steel plates based on the principle of magnetic flux leakage. The method is implemented using a steel plate surface defect detection device based on the principle of magnetic flux leakage provided in Example 1, and includes the following steps: S1: The excitation unit is placed on the surface of the steel plate to be tested, so that the excitation unit and the steel plate to be tested form a closed magnetic circuit, and the steel plate to be tested is locally magnetized to a saturated state; wherein, the excitation unit is placed on the surface of the steel plate to be tested, so that the first permanent magnet and the second permanent magnet at both ends of the industrial pure iron yoke are simultaneously in close contact with the surface of the steel plate. The magnetic lines of force start from the N pole of the first permanent magnet, are conducted through the industrial pure iron yoke to the second permanent magnet, pass through the second permanent magnet and enter the interior of the steel plate and flow back to the first permanent magnet, forming a complete closed magnetic circuit; under the action of this closed magnetic circuit, the local area of ​​the steel plate in contact with the two permanent magnets is magnetized to a saturated or near-saturated state.

[0033] S2: The leakage magnetic field generated at the defect on the surface of the steel plate under test is sensed by a magnetic sensor, and a differential voltage signal is obtained. When there are defects such as inclusions, cracks or pits on the surface of the steel plate, the magnetic permeability of the defect area changes abruptly, and the magnetic lines of force bend at the defect and partially leak to the surface of the steel plate to form a leakage magnetic field. The magnetic sensor is fixed in the center between two permanent magnets, maintaining a constant lift-off distance of 2-4 mm from the surface of the steel plate. Its sensitive direction is parallel to the surface of the steel plate and perpendicular to the direction of the magnetic lines of force. It senses the radial component of the leakage magnetic field in the direction of maximum response and outputs a differential voltage signal.

[0034] S3: The differential voltage signal is amplified and filtered by the signal conditioning unit to obtain the defect feature signal. The signal conditioning unit converts the differential voltage signal into a single-ended signal through a differential-to-single-ended amplifier circuit and performs a 10-fold primary amplification. Then, it filters out the DC bias and temperature drift components of the sensor's static voltage through an RC high-pass filter circuit. Finally, it filters out high-frequency electromagnetic interference in the industrial environment through a second-order low-pass active filter circuit, outputting a defect feature signal with a high signal-to-noise ratio.

[0035] S4: The incremental encoder detects the movement distance of the probe on the steel plate surface. A data acquisition is triggered every fixed distance movement. The data acquisition unit collects defect characteristic signals and converts them into digital signals, which are then temporarily stored in external SRAM. The incremental encoder is coaxially connected to the guide wheel. As the guide wheel rolls on the steel plate surface, it converts the linear movement distance of the probe into pulse signals. A pulse is output every fixed distance movement to trigger an analog-to-digital conversion acquisition, ensuring a one-to-one correspondence between the sampling points and the spatial positions on the steel plate surface, achieving equisional sampling. The acquired data is written to the external SRAM in real time via the FSMC bus for temporary storage, preventing data loss due to insufficient internal RAM capacity.

[0036] S5: The digital signal temporarily stored in the external SRAM is transmitted to the host computer via USB to serial port. The host computer receives and displays the waveform corresponding to the digital signal, and judges the existence, location and relative severity of defects on the steel plate surface based on the waveform changes. After the detection is completed, the data upload is triggered by pressing a button. The host computer judges the presence or absence of defects based on the peak-trough abrupt change in the waveform, calculates the physical location of the defect based on the encoder pulse count value corresponding to the waveform abrupt change, and evaluates the relative severity of the defect based on the amplitude of the waveform. All detection data is automatically stored in the host computer.

[0037] The beneficial effects of this invention are as follows: Compared with existing leakage magnetic field detection devices that use electromagnet excitation and Hall sensors, this invention uses two permanent magnets with opposite polarities in conjunction with an industrial pure iron yoke to form an excitation unit. This eliminates the need for an external high-current power supply to locally magnetize the steel plate to saturation, significantly reducing the size and weight of the device and enabling handheld portable operation of the detection probe. The use of a tunnel magnetoresistive sensor instead of a traditional Hall sensor provides a much higher sensitivity than Hall elements, clearly capturing the weak leakage magnetic field signal generated by minute defects and improving the detection rate of minute defects. Furthermore, the invention utilizes a three-stage process: differential-to-single-ended amplification, RC high-pass filtering, and second-order low-pass active filtering. The cascaded conditioning circuit sequentially performs signal conversion, DC baseline drift elimination, and high-frequency noise suppression, solving the problems of low signal-to-noise ratio and easy noise submersion of defect features in existing defect detection devices. The introduction of an incremental encoder coaxially connected to the guide wheel enables equisional sampling, ensuring that the sampling points correspond one-to-one with the spatial positions on the steel plate surface. This eliminates waveform distortion caused by uneven manual detection speed. Combined with the large-capacity data temporary storage of external SRAM and real-time upload via USB serial port, it achieves precise defect positioning, complete recording and automatic storage of detection data. This enables highly sensitive, portable, and intelligent detection and quality assessment of steel plate surface defects.

Claims

1. A method for detecting surface defects in steel plates based on the principle of magnetic flux leakage, characterized in that, Includes the following steps: S1: Place the excitation unit on the surface of the steel plate to be tested, so that the excitation unit and the steel plate to be tested form a closed magnetic circuit, and magnetize the local part of the steel plate to be tested to saturation. S2: The differential voltage signal is obtained by sensing the leakage magnetic field generated at the defect on the surface of the steel plate under test by a magnetic sensor; S3: The differential voltage signal is amplified and filtered by the signal conditioning unit to obtain the defect characteristic signal; S4: The incremental encoder detects the movement distance of the probe on the steel plate surface. Data acquisition is triggered every fixed distance movement. The data acquisition unit collects the defect feature signal and converts it into a digital signal, which is then temporarily stored in the external SRAM. S5: The digital signal temporarily stored in the external SRAM is transmitted to the host computer via USB to serial port. The host computer receives and displays the waveform corresponding to the digital signal, and judges the existence, location and relative severity of defects on the steel plate surface based on the waveform changes.

2. A steel plate surface defect detection device based on the principle of magnetic flux leakage, used in the steel plate surface defect detection method based on the principle of magnetic flux leakage as described in claim 1, characterized in that, It includes an excitation unit, a magnetic sensor, a signal conditioning unit, a data acquisition unit, and a host computer connected in sequence; The excitation unit is used to form a closed magnetic field by contacting the surface of the steel plate, locally magnetizing the steel plate to a saturated state, and detecting the leakage magnetic field. The magnetic sensor is used to sense the leakage magnetic field and output a differential voltage signal of the leakage magnetic field. The signal conditioning unit is used to amplify and filter the differential voltage signal to obtain the defect feature signal; The data acquisition unit is used to perform digital signal conversion on the defect feature signals to obtain magnetic flux leakage detection data; The host computer is used to display waveforms, extract features, and identify defects in the magnetic flux leakage detection data to obtain the surface defect detection results of the steel plate.

3. The steel plate surface defect detection device based on the principle of magnetic flux leakage as described in claim 1, characterized in that, The excitation unit includes an industrial magnetic yoke, a first permanent magnet, and a second permanent magnet; The first permanent magnet and the second permanent magnet are respectively located at both ends of the industrial yoke, and are placed parallel to each other along the length of the industrial yoke; The first and second permanent magnets placed therein have opposite polarities; When the excitation unit comes into contact with the surface of the steel plate, a closed magnetic field is formed through the industrial yoke, the first permanent magnet, the steel plate, and the second permanent magnet.

4. The steel plate surface defect detection device based on the principle of magnetic flux leakage according to claim 3, characterized in that, The magnetic sensor is a tunnel magnetoresistive sensor, fixed between two permanent magnets with opposite polarities, maintaining a constant lift-off distance from the steel plate surface.

5. The steel plate surface defect detection device based on the principle of magnetic flux leakage according to claim 1, characterized in that, The signal conditioning unit includes a differential-to-single-ended amplifier circuit, an RC high-pass filter circuit, and a second-order low-pass active filter circuit. The differential-to-single-ended amplifier circuit includes an operational amplifier U1. The output terminal 1 of the operational amplifier U1 is connected to one end of a resistor R1 and an RC high-pass filter circuit. The other end of the resistor R1 is connected to the inverting input terminal 2 of the operational amplifier U1 and one end of a resistor R4. The other end of the resistor R4 is connected to one end of a capacitor C1 and one end of a resistor R7. The other end of the resistor R7 is grounded. The positive power supply terminal of the operational amplifier U1 is connected to a +5V power supply, and the negative power supply terminal is grounded. The non-inverting input terminal of the operational amplifier U1 is connected to a magnetic sensor and one end of a resistor R8. The other end of the resistor R8 is grounded. The RC high-pass filter circuit includes resistors R5 and R6 and capacitor C3. One end of resistor R5 is connected to the output terminal 1 of operational amplifier U1, and the other end is connected to one end of capacitor C3 and one end of resistor R6. The other end of capacitor C3 is connected to the second-order low-pass active filter circuit. The other end of resistor R6 is connected to one end of capacitor C4 and the second-order low-pass active filter circuit. The other end of capacitor C4 is grounded. The second-order low-pass active filter circuit includes an operational amplifier U2. The output terminal of the operational amplifier U2 is connected to the other end of resistor R5, one end of resistor R3, and the data acquisition unit, respectively. The other end of resistor R3 is connected to the inverting input terminal 6 of the operational amplifier U1 and one end of resistor R2, respectively. The other end of resistor R2 is grounded. The non-inverting input terminal of the operational amplifier U2 is connected to the other end of resistor R6.

6. The steel plate surface defect detection device based on the principle of magnetic flux leakage according to claim 1, characterized in that, The data acquisition unit includes a microcontroller chip U3, a USB to serial port chip U4, an external SRAM, and an incremental encoder; The microcontroller chip U3 includes an analog-to-digital converter module, whose input terminal is connected to the output terminal of the signal conditioning unit, and is used to acquire defect feature signals and convert them into digital signals. The serial transmit and receive pins of the microcontroller chip U3 are connected to the receive and transmit pins of the USB to serial port chip U4, respectively. The USB to serial port chip U4 is connected to the host computer through a USB interface and is used to transmit digital signals to the host computer. The external SRAM is connected to the FSMC bus interface of the microcontroller chip U3 and is used to temporarily store the collected magnetic flux leakage detection data. The signal output terminal of the incremental encoder is connected to the external interrupt input pin of the microcontroller chip U3. The incremental encoder is coaxially connected to the guide wheel. When the guide wheel rolls on the steel plate surface, it converts the moving distance into a pulse signal. Every fixed distance moved triggers an analog-to-digital conversion acquisition, which is used for equal spatial sampling and locating the defect position on the steel plate surface.

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