An optical scanning eddy current magneto-optical imaging nondestructive testing device

Abstract

The present invention discloses an optical scanning eddy current magneto-optical imaging nondestructive testing device. In the magneto-optical imaging system, a polarization beam splitter prism is used for polarization analysis. The analysis angle is set to 45 degrees, which is the most sensitive to the magnetic field response. An anti-reflection film is used to eliminate the influence of reflected light. The change of the magnetic field on the surface of the test piece will affect the polarization angle of the linearly polarized light incident on the polarization beam splitter prism, thereby affecting the light intensity difference of the two output linear polarized lights of the polarization beam splitter prism. Photoelectric conversion is performed by a self-balancing photoelectric detector, and amplification is performed by a lock-in amplifier. Then, the amplitude and phase of the signal are scanned and imaged through an imaging and signal processing system. Finally, the brightness change of the image is reflected.

Description

Technical Field

[0001] The present invention belongs to the technical field of non-destructive testing, and more specifically, relates to an optical scanning eddy current magneto-optical imaging non-destructive testing device. Background Art

[0002] In non-destructive testing applications, magneto-optical imaging based on the Faraday rotation effect has been successfully used in weld defect detection, corrosion and crack defect detection of aircraft components by imaging constant or alternating magnetic fields.

[0003] Currently, eddy current-stimulated magneto-optical imaging (MET) can achieve non-contact detection of surface and sub-surface defects in conductive materials. Compared with traditional eddy current detection methods, MET can significantly improve detection efficiency and spatial resolution. However, conventional camera-based MET detection methods have a low responsivity to magnetic fields, at the millitesl or sub-mitesl level. Furthermore, they require a relatively large excitation current, typically ranging from several amperes to hundreds of amperes, depending on the excitation method. This low responsivity limits the promotion and application of MET methods. However, MET detection based on the Faraday rotation effect can achieve nanotesla or even petascale magnetic field responsivity. This is mainly because in magneto-optical magnetic field detection with high magnetic field responsiveness, a transmission point detection method is usually used, which does not require the elimination of the influence of reflected light. When using a photodetector for detection, the circuit can be used to perform differential reduction of the influence of constant light and suppress common-mode interference, and the analyzer angle is set at the most sensitive 45 degrees. However, in eddy current magneto-optical imaging non-destructive testing, in order to eliminate the influence of reflected light, the analyzer angle is usually set to the least sensitive 90 degrees. If it is set at the most sensitive 45 degrees, the reflected light can easily saturate the camera. Only very weak light intensity can make the camera unsaturated, but reducing the light intensity will also reduce the responsiveness of the magnetic field. Summary of the Invention

[0004] The purpose of the present invention is to overcome the shortcomings of the existing technology and provide an optical scanning eddy current magneto-optical imaging non-destructive testing device, which adopts eddy current magneto-optical imaging to improve the magnetic field responsiveness of the magneto-optical imaging non-destructive testing system and quickly judge defects through the brightness value of the imaging image.

[0005] To achieve the above-mentioned object of the invention, the present invention provides an optical scanning eddy current magneto-optical imaging nondestructive testing device, characterized in that it includes, in order of optical transmission direction: a visible light laser, a polarizer, a micro-vibration mirror, a first focusing mirror, a depolarizing beam splitter prism, a sensor unit, an eddy current excitation coil, a polarizing beam splitter prism, a second focusing mirror, a third focusing mirror, a reflector, a self-balancing photodetector, a lock-in amplifier, and an imaging and processing system;

[0006] The visible light laser generates a laser signal which is input to a polarizer to obtain linearly polarized light with a high extinction ratio. The angle of the polarizer is adjusted so that the polarization direction of the detection light passing through the polarizer forms a 45-degree angle with the directions of the two polarized lights of the polarization beam splitter prism.

[0007] The micro-vibration mirror is placed at the focal position of the first focusing mirror, and the detection light is reflected to the first focusing mirror at different angles by rotating the micro-vibration mirror;

[0008] The first focusing mirror causes the detection lights from different directions emitted from the focus to be incident on the depolarization beam splitter prism in different positions in a vertical direction;

[0009] The depolarization beam splitter prism does not change the polarization state of the detection light, but reflects part of the detection light, while the other part of the detection light passes through the depolarization beam splitter prism and is vertically incident on the sensor unit;

[0010] The sensor unit is divided into three layers: the first layer is an anti-reflection film, the second layer is a Faraday rotator material, and the third layer is a reflective layer. The anti-reflection film is plated on the front of the Faraday rotator material to increase the transmittance of the detection light, and the back of the Faraday rotator material is plated with germanium as a reflective layer to reflect the detection light back to the depolarization splitter prism.

[0011] The sensor unit is placed directly above the test object and at a certain distance from it. It is used to detect the magnetic field B distributed vertically on the test object's surface. When the probe light passes through the anti-reflection coating and enters the Faraday rotator material, the polarization direction of the probe light is changed according to the magnetic field B distributed on the test object's surface, and the polarization angle is rotated by θ. After being reflected by the reflective layer, the probe light passes through the Faraday rotator material again. Due to the non-reciprocity of the Faraday rotation effect, the polarization direction of the probe light is rotated again by θ in the same direction. Finally, after passing through the anti-reflection coating, it returns to the depolarizing beam splitter prism. At this time, the depolarizing beam splitter prism redirects the reflected portion of the probe light to enter the polarizing beam splitter prism in a horizontal direction.

[0012] The eddy current excitation coil is placed directly above the test piece, and the sensor unit and the eddy current excitation coil are on the same horizontal plane. The imaging and signal processing system inputs an excitation signal of a reference frequency to the eddy current excitation coil, thereby generating an excitation magnetic field of a corresponding frequency. Under the action of the excitation magnetic field, eddy currents are generated in the test piece. Defects in the test piece affect the distribution of the eddy currents and thus generate a secondary magnetic field. The magnetic field distribution B in the direction perpendicular to the surface of the test piece is finally detected by the sensor unit and is the superposition of the excitation magnetic field and the secondary magnetic field.

[0013] The polarization beam splitter prism decomposes the horizontal detection light into two polarization directions, polarized light 1 and polarized light 2, with polarization directions perpendicular to each other. The light intensity of the two polarized light beams is related to the polarization direction of the incident light. Polarized light 1 is irradiated onto one photoelectric sensor of the self-balancing photodetector through the second focusing mirror. Polarized light 2 is irradiated onto the other photoelectric sensor of the self-balancing photodetector through the reflecting mirror and the third focusing mirror.

[0014] The self-balancing photodetector includes two photoelectric sensors, one of which is located at the focal position of the second focusing lens, and the other is located at the focal position of the third focusing lens. The two photoelectric sensors respectively perform photoelectric conversion on the input polarized light, perform differential and amplification processing based on the signal strength, and finally output the differential voltage signal in the form of voltage.

[0015] The lock-in amplifier performs phase-locked amplification on the voltage signal output by the self-balancing photodetector according to the reference frequency provided by the imaging and signal processing system, and then transmits the amplitude and phase to the imaging and signal processing system;

[0016] The imaging and signal processing system uses the same reference frequency as the eddy current excitation coil and the phase-locked amplifier input frequency, and simultaneously performs imaging based on the amplitude and phase feedback from the phase-locked amplifier, and performs defect detection through changes in image brightness.

[0017] The object of the invention of the present invention is achieved like this:

[0018] The present invention discloses an optical scanning eddy current magneto-optical imaging nondestructive testing device. In a magneto-optical imaging system, a polarization beam splitter prism is used for polarization analysis. The analysis angle is set to 45 degrees, which is the most sensitive to magnetic field response. An anti-reflection film is used to eliminate the influence of reflected light. Changes in the magnetic field on the surface of a test piece affect the polarization angle of linearly polarized light incident on the polarization beam splitter prism, thereby affecting the intensity difference of the two linearly polarized lights output by the polarization beam splitter prism. Photoelectric conversion is performed by a self-balancing photodetector, amplified by a lock-in amplifier, and then the amplitude and phase of the signal are scanned and imaged through an imaging and signal processing system. Finally, changes in the brightness of the image are used to reflect the magnetic field distribution in the vertical direction on the surface of the test piece, and defects are detected by abnormalities in the magnetic field distribution.

[0019] At the same time, the optical scanning eddy current magneto-optical imaging nondestructive testing device of the present invention also has the following beneficial effects:

[0020] (1) Improve the magnetic field response of eddy current magneto-optical imaging.

[0021] (2) In eddy current magneto-optical imaging, a smaller excitation current can be used for excitation, and a higher excitation frequency can be used.

[0022] (3) Compared with traditional eddy current detection, the detection efficiency can be improved through high-sensitivity magneto-optical imaging.

[0023] (4) The use of anti-reflection film improves the transmittance of light and reduces the impact of reflected light on imaging.

[0024] (5) By using germanium as the reflective layer, the attenuation of high-frequency electromagnetic fields when using conductive materials such as silver or aluminum as the reflective layer can be reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a diagram illustrating an embodiment of the optical scanning eddy current magneto-optical imaging nondestructive testing device of the present invention; DETAILED DESCRIPTION

[0026] The following describes the specific embodiments of the present invention in conjunction with the accompanying drawings so that those skilled in the art can better understand the present invention. It should be noted that in the following description, when detailed descriptions of known functions and designs may dilute the main content of the present invention, such descriptions will be omitted here.

[0027] Example

[0028] Figure 1 This is a diagram of the architecture of a specific embodiment of the optical scanning eddy current magneto-optical imaging nondestructive testing device of the present invention.

[0029] In this embodiment, if Figure 1 As shown, the present invention is a magneto-optical imaging nondestructive testing device based on interference extinction, which includes, in order of light transmission direction: a visible light laser 1, a polarizer 2, a micro-vibration mirror 3, a first focusing mirror 4, a depolarizing beam splitter prism 5, a sensor unit 6, an eddy current excitation coil 7, a test piece 8, a polarizing beam splitter prism 9, a reflector 10, a second focusing mirror 11, a third focusing mirror 12, a self-balancing photodetector 13, a lock-in amplifier 14, and an imaging and processing system 15;

[0030] The visible light laser 1 generates a laser signal which is input to the polarizer 2. The polarizer 2 obtains linearly polarized light with a high extinction ratio. The angle of the polarizer 2 is adjusted so that the polarization direction of the detection beam passing through the polarizer 2 and the directions of the two polarized lights output by the polarization beam splitter 9 form a 45-degree angle, because 45 degrees is the most sensitive analyzer angle.

[0031] The micro-vibration mirror 3 is placed at the focal position of the first converging lens 4. The rotation of the micro-vibration mirror 3 makes the detection beam reflected to the first converging lens 4 at different angles;

[0032] The first converging lens 4 directs the detection light from the focal point in different directions to the depolarization beam splitter prism 5 at different positions in a perpendicular direction;

[0033] The depolarizing beam splitter prism 5 does not change the polarization state of the detection light, but can reflect 50% of the detection light and transmit 50% of the detection light, while the portion of the detection light that passes through the depolarizing beam splitter prism 5 is vertically incident on the sensor unit 6;

[0034] The sensor unit 6 is divided into three layers, such as Figure 1 As shown, the first layer is an antireflection film 6.1, the second layer is a Faraday rotator material 6.2, and the third layer is a reflective layer 6.3. The antireflection film 6.1 is applied to the front surface of the Faraday rotator material 6.2 to increase the transmittance of the detection light and reduce the impact and interference caused by reflection. The back surface of the Faraday rotator material 6.2 is coated with germanium as a reflective layer 6.3 to reflect the detection light back to the depolarizing beam splitter prism 5. In this embodiment, germanium reflects visible light but is transparent to electromagnetic fields with frequencies below the infrared band. This does not affect the transmission of high-frequency magnetic field signals, thereby extending the detectable frequency range of magneto-optical imaging.

[0035] The sensor unit 6 is placed directly above the test object 8 at a certain distance. It is used to detect the magnetic field B distributed perpendicular to the surface of the test object 8. When the detection light passes through the anti-reflection film 6.1 and enters the Faraday rotator material 6.2, the polarization direction of the detection light is changed according to the magnetic field B distributed on the surface of the test object 8. The polarization angle is θ, θ = VBd, where V is the Verdet constant of the Faraday rotator material, B is the magnetic field strength distributed perpendicular to the surface of the test object, and d is the thickness of the Faraday rotator material.

[0036] After being reflected by reflective layer 6.3, the probe light passes through Faraday rotator material 6.2 again. Due to the non-reciprocal nature of the Faraday rotation effect, the polarization direction of the probe light rotates again by θ in the same direction. Finally, it passes through anti-reflection film 6.1 and returns to depolarizing beam splitter prism 5. At this time, depolarizing beam splitter prism 5 transmits the reflected portion of the probe light in a horizontal direction to polarizing beam splitter prism 9.

[0037] In this embodiment, the test piece is made of conductive material. When a conductive material is selected, the material needs to be eddy current excited. When there is a defect in the test piece, its electromagnetic properties change. The defect will cause a change in the magnetic field distribution perpendicular to the surface of the material under the excitation of the magnetic field or eddy current, thereby bringing a different magnetic field distribution in the detection light irradiation area, thereby forming a different brightness distribution during subsequent imaging.

[0038] The eddy current excitation coil 7 is placed directly above the test piece 8, and the sensor unit 6 and the eddy current excitation coil 7 are on the same horizontal plane. The imaging and signal processing system 15 inputs an excitation signal of a reference frequency to the eddy current excitation coil 7, thereby generating an excitation magnetic field of the corresponding frequency. Under the action of the excitation magnetic field, eddy currents are generated in the test piece. Defects in the test piece affect the distribution of the eddy currents and thus generate a secondary magnetic field. The magnetic field distribution B in the direction perpendicular to the surface of the test piece detected by the sensor unit 6 is the superposition of the excitation magnetic field and the secondary magnetic field.

[0039] Polarization beam splitter prism 9 acts opposite to the analyzer and is used to decompose horizontal detection light into two polarization beams, polarized light 1 and polarized light 2, with polarization directions perpendicular to each other. The light intensity of the two polarized light beams is related to the polarization direction of the incident light. Polarized light 1 passes through second focusing mirror 11 and is incident on one photoelectric sensor of self-balancing photodetector 13. Polarized light 2 passes through reflector 10 and third focusing mirror 12 and is incident on the other photoelectric sensor of self-balancing photodetector 13.

[0040] In this embodiment, the depolarization beam splitter prism is parallel to the beam splitting plane of the polarization beam splitter prism and parallel to the reflector, so that the polarized light 2 is incident and reflected at an angle of 45 degrees. Assuming that the intensity of the detection light incident on the polarization beam splitter prism 9 is I0, according to Malus's law, the intensity of the polarized light 1 is I1 = I0 cos 2 (45+2θ), the intensity of polarized light 2 I2=I0 cos 2 (45-2θ), after substituting into θ, we can get and The intensity of these two beams is related to the magnetic field being measured. The probe light passes through the Farad rotator twice, and the polarization direction is rotated by 2θ degrees.

[0041] The self-balancing photodetector 13 includes two photosensors, one located at the focal point of the second focusing lens 11 and the other located at the focal point of the third focusing lens 12. The two photosensors perform photoelectric conversion on the input polarized light, perform differential processing and amplification based on the signal strength, and finally output the differential voltage signal in the form of a voltage. In this embodiment, detection using the self-balancing photodetector is equivalent to performing photoelectric conversion and differential processing on the light intensities of polarized light 1 and polarized light 2. The output signal reflects the magnitude and direction of I2-I1=I0sin4VBd. Therefore, when V and d are constant, the larger I0 is, the greater the change in light intensity caused by the same magnetic field B, which is equivalent to a higher responsiveness to the magnetic field.

[0042] The lock-in amplifier 14 performs phase-locked amplification on the voltage signal output by the self-balancing photodetector 13 according to the reference frequency provided by the imaging and signal processing system 15, further improving the signal-to-noise ratio, and then extracts the amplitude and phase and transmits them to the imaging and signal processing system;

[0043] The imaging and signal processing system 15 inputs the same reference frequency to the eddy current excitation coil 7 and the lock-in amplifier 14. It also generates images based on the amplitude and phase feedback from the lock-in amplifier 14, and detects defects based on changes in image brightness. In this embodiment, when a defect exists in the test piece, due to changes in electromagnetic properties, the defect will cause changes in the magnetic field distribution perpendicular to the material surface under the excitation of the magnetic field or eddy currents. This results in a different magnetic field distribution in the area illuminated by the probe light, and thus a different brightness distribution in subsequent imaging. The area with the highest brightness distribution indicates the defect location.

[0044] Although the above describes the illustrative specific embodiments of the present invention to facilitate understanding of the present invention by those skilled in the art, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the appended claims, these changes are obvious, and all inventions and creations using the concepts of the present invention are protected.

Claims

1. An optical scanning eddy current magneto-optical imaging nondestructive testing device, characterized in that: The system includes, in order of optical transmission direction: a visible light laser, a polarizer, a micro-vibration mirror, a first focusing mirror, a depolarizing beam splitter prism, a sensor unit, an eddy current excitation coil, a polarizing beam splitter prism, a second focusing mirror, a third focusing mirror, a reflector, a self-balancing photodetector, a lock-in amplifier, and an imaging and signal processing system; The visible light laser generates a laser signal which is input to a polarizer to obtain linearly polarized light with a high extinction ratio. The angle of the polarizer is adjusted so that the polarization direction of the detection light passing through the polarizer forms a 45-degree angle with the directions of the two polarized lights of the polarization beam splitter prism. The micro-vibration mirror is placed at the focal position of the first focusing mirror, and the detection light is reflected to the first focusing mirror at different angles by rotating the micro-vibration mirror; The first focusing mirror causes the detection lights from different directions emitted from the focus to be incident on the depolarization beam splitter prism in different positions in a vertical direction; The depolarization beam splitter prism does not change the polarization state of the detection light, but reflects part of the detection light, while the other part of the detection light passes through the depolarization beam splitter prism and is vertically incident on the sensor unit; The sensor unit is divided into three layers: the first layer is an anti-reflection film, the second layer is a Faraday rotator material, and the third layer is a reflective layer. The anti-reflection film is plated on the front of the Faraday rotator material to increase the transmittance of the detection light, and the back of the Faraday rotator material is plated with germanium as a reflective layer to reflect the detection light back to the depolarization splitter prism. The sensor unit is placed directly above the test object and at a certain distance from it. It is used to detect the magnetic field B distributed vertically on the test object's surface. When the probe light passes through the anti-reflection coating and enters the Faraday rotator material, the polarization direction of the probe light is changed according to the magnetic field B distributed on the test object's surface, and the polarization angle is rotated by θ. After being reflected by the reflective layer, the probe light passes through the Faraday rotator material again. Due to the non-reciprocity of the Faraday rotation effect, the polarization direction of the probe light is rotated again by θ in the same direction. Finally, after passing through the anti-reflection coating, it returns to the depolarizing beam splitter prism. At this time, the depolarizing beam splitter prism redirects the reflected portion of the probe light to enter the polarizing beam splitter prism in a horizontal direction. The eddy current excitation coil is placed directly above the test piece, and the sensor unit and the eddy current excitation coil are on the same horizontal plane. The imaging and signal processing system inputs an excitation signal of a reference frequency to the eddy current excitation coil, thereby generating an excitation magnetic field of a corresponding frequency. Under the action of the excitation magnetic field, eddy currents are generated in the test piece. Defects in the test piece affect the distribution of the eddy currents and thus generate a secondary magnetic field. The magnetic field distribution B in the direction perpendicular to the surface of the test piece is finally detected by the sensor unit and is the superposition of the excitation magnetic field and the secondary magnetic field. The polarization beam splitter prism decomposes the horizontal detection light into two polarization directions, polarized light 1 and polarized light 2, with polarization directions perpendicular to each other. The light intensity of the two polarized light beams is related to the polarization direction of the incident light. Polarized light 1 is irradiated onto one photoelectric sensor of the self-balancing photodetector through the second focusing mirror. Polarized light 2 is irradiated onto the other photoelectric sensor of the self-balancing photodetector through the reflecting mirror and the third focusing mirror. The self-balancing photodetector includes two photoelectric sensors, one of which is located at the focal position of the second focusing lens, and the other is located at the focal position of the third focusing lens. The two photoelectric sensors respectively perform photoelectric conversion on the input polarized light, perform differential and amplification processing based on the signal strength, and finally output the differential voltage signal in the form of voltage. The lock-in amplifier performs phase-locked amplification on the voltage signal output by the self-balancing photodetector according to the reference frequency provided by the imaging and signal processing system, and then transmits the amplitude and phase to the imaging and signal processing system; The imaging and signal processing system uses the same reference frequency as the eddy current excitation coil and the phase-locked amplifier input frequency, and simultaneously performs imaging based on the amplitude and phase feedback from the phase-locked amplifier, and performs defect detection through changes in image brightness.

2. The optical scanning eddy current magneto-optical imaging nondestructive testing device according to claim 1, characterized in that: The polarization rotation angle 2θ satisfies: 2θ=2VBd Wherein, V is the Verdet constant of the Faraday rotator material, B is the vertical magnetic field strength of the tested object, and d is the thickness of the Faraday rotator material.

3. The optical scanning eddy current magneto-optical imaging nondestructive testing device according to claim 1, characterized in that: The depolarization beam splitter prism is parallel to the beam splitting plane of the polarization beam splitter prism and parallel to the reflector, so that the polarized light 2 is incident and reflected at an angle of 45 degrees.

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