CN108195928B - Metal magnetic material defect detection device based on image fusion - Google Patents
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Abstract
The invention discloses a metal magnetic material defect detection device based on image fusion, which is characterized in that sine and cosine signals are respectively applied to two pairs of mutually vertical U-shaped magnetic yokes, a rotating magnetic field with the synthesis direction uniformly changed along with time but the amplitude unchanged is used as an excitation magnetic field for metal magnetic material test piece detection. If the metal magnetic material test piece has defects, a leakage magnetic field is generated above the defects, then the leakage magnetic field information is converted into a magneto-optical image through a magneto-optical imaging system, finally, frequency domain images of the magneto-optical image sequence are normalized, then are subjected to linear weighting fusion, and then are converted into a gray value range, so that the defect image of the metal magnetic material test piece is obtained. The invention adopts a low-frequency rotating magnetic field below 50Hz to excite the test piece, realizes effective excitation of the defects in all directions, simultaneously adopts a magneto-optical imaging technology to collect magneto-optical images in a single excitation period, and then linearly weights and fuses the defects in all directions in a frequency domain, thereby realizing rapid visual detection of the defects in all directions.
Description
Technical Field
The invention belongs to the technical field of defect detection, and particularly relates to a metal magnetic material defect detection device based on image fusion.
Background
In the defect detection of metal magnetic materials, a Magneto-optical imaging (MOI) technology combined with magnetic flux leakage detection is rapidly developed due to visualization, rapidity and accuracy. The magneto-optical imaging detection technology is based on the Faraday effect, converts a leakage magnetic field (leakage magnetic field information generated by defects) reflecting defect information into light intensity information, namely a leakage magnetic field image, and realizes defect detection by analyzing the leakage magnetic field image, so that the health state of the metal magnetic material is evaluated.
The magnetic flux leakage detection is a traditional nondestructive detection technology and is divided into forms of permanent magnet, direct current, alternating current, pulse and the like according to different magnetization modes. Wherein the permanent magnet and the direct current are excited by a static magnetic field, and the alternating current and the pulse are excited by a dynamic magnetic field. The conventional magnetic flux leakage detection uses static excitation or dynamic high-frequency rotating magnetic field excitation above 50Hz to obtain stable magnetic field information.
Static magnetic leakage detection generally adopts direct current excitation magnetization or permanent magnetism magnetization, can effectively detect both inside and surface layer defect, though this technique has obtained extensive application, nevertheless because direct current excitation not only detection speed is slow, the magnetizer is bulky, the energy consumption is high, need demagnetize the test piece after the detection is accomplished, and under invariable magnetization field, the magnetic leakage field is invariable moreover, and information content is limited, is unfavorable for the extraction of defect characteristic.
Dynamic leakage flux detection is usually excited by a high-frequency rotating magnetic field of 50Hz or higher. In foreign countries, Yasuhiro Kataoka in Japan uses 100Hz rotating magnetic field to excite and simultaneously uses a giant magnetoresistance sensor to detect leakage magnetic signals under the excitation of the rotating magnetic field to obtain images of the leakage magnetic field; B.B.Lahiri et al in India researches and applies magnetic flux leakage detection to thermal imaging detection, obtains an infrared thermal image of a test piece by using a unidirectional 50Hz excitation magnetic field and 3mm groove-shaped defects, and realizes defect imaging; shigeru Ando and takaakkiana of the university of tokyo excited with a 150Hz rotating magnetic field and obtained an eddy current alternating magnetic field image caused by defects using a three-phase correlation type image sensor. In China, Liwei uses a double U-shaped orthogonal excitation array to apply a 6kHz rotating electromagnetic field to a test piece, and a semi-elliptical defect profile of 16mm x 40mm is inverted according to information obtained by a magnetic field detection array. In industrial production, in order to reduce the cost of circuit design, the commercial power of 50Hz is often used as the excitation rotating magnetic field.
Low frequency leakage detection has not been paid sufficient attention at present. Keiji Tsukada uses a single U-shaped yoke excitation, applies 1-50Hz excitation, and obtains the leakage magnetic signal of a single defect by using a magnetic resistance gradiometer. The influence of cracks at the depth and position (upper surface or lower surface) of the cracks on the spatial distribution of characteristic parameters of a leakage magnetic field when the defects in a single direction are detected by using a single-direction excitation magnetic field under the low-frequency excitation of 5-50Hz is researched by numerical simulation and detection experiments of the worship products and the like.
In the above several modes, the high-frequency excitation has weak detection capability for internal defects due to the skin effect of eddy current, and the generated leakage magnetic field signal is weak. Static excitation can detect defects only in a single direction, although the stray field signal is strong. And the unidirectional low-frequency magnetic flux leakage detection is equivalent to static excitation of a variable amplitude value, has long detection time and is only sensitive to unidirectional defects.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a metal magnetic material defect detection device based on image fusion, which can obtain clear defect outline images while improving the internal defect detection capability and realize the rapid visual detection of defects in all directions.
In order to achieve the above object, the device for detecting defects of a metal magnetic material based on image fusion is characterized by comprising
An excitation signal circuit for generating an excitation frequency f below 50HzexThe low-frequency sine and cosine power signals are loaded on two U-shaped magnetic yokes vertically arranged in a magneto-optical imaging system;
the magneto-optical imaging system is used for applying low-frequency rotating magnetic field excitation to the metal magnetic material test piece by using two vertically arranged U-shaped magnetic yokes; the magneto-optical imaging system takes red natural light as a light source, converts leakage magnetic field information of defects in all directions into optical signals, and uses a CCD (charge coupled device) sensor to collect optical signals in an integral period of an excitation signal to form a magneto-optical image sequence;
a fusion algorithm module for performing Fourier transform on the amplitude response of each position pixel point in the magneto-optical image sequence, namely converting the amplitude response into a frequency domain to obtain frequency response, and then converting the excitation frequency f in the frequency response of all the position pixel pointsexThe n frequency multiplication responses form a frequency domain image y (nf)ex) Then, a frequency domain image corresponding to the low frequency of the front 1/10 frequency domain, namely y (nf), is takenex) N is 1,2, L/10 is normalized to obtain a normalized frequency domain image y (nf)ex1) Finally, the fused frequency domain image y is calculated by using the following formulafuseUpper pixel point amplitude:
wherein L denotes the length of the sequence of magneto-optical images,
representing the fused frequency domain image yfuseAmplitude of the middle pixel point (i, j), y (nf)ex1)i,jFor normalizing the frequency domain image y (nf)ex1) The amplitude value of the pixel point (i, j), wherein (i, j) is the horizontal and vertical coordinates of the image;
the fused frequency domain image yfuseAnd linearly converting the amplitude value of the upper pixel point into a gray value range to obtain a defect image of the metal magnetic material test piece.
The object of the invention is thus achieved.
The invention discloses a metal magnetic material defect detection device based on image fusion.A sine signal and a cosine signal are respectively applied to two pairs of mutually vertical U-shaped magnetic yokes, a rotating magnetic field with the synthesis direction uniformly changed along with time but the amplitude unchanged is synthesized, and the rotating magnetic field is used as an excitation magnetic field for magnetic flux leakage detection of a metal magnetic material test piece. If the metal magnetic material test piece has defects, a leakage magnetic field is generated above the defects, then the leakage magnetic field information is converted into a magneto-optical image through a magneto-optical imaging system to obtain defect information, finally, frequency domain images of the magneto-optical image sequence are normalized, then are subjected to linear weighting fusion, and then are converted into a gray value range to obtain a defect image of the metal magnetic material test piece. The invention adopts a low-frequency rotating magnetic field below 50Hz to excite the test piece, realizes effective excitation of the defects in all directions, simultaneously adopts a magneto-optical imaging technology to collect magneto-optical images in a single excitation period, and then linearly weights and fuses the defects in all directions in a frequency domain, thereby realizing rapid visual detection of the defects in all directions.
Meanwhile, the metal magnetic material defect detection device based on image fusion also has the following beneficial effects:
the invention can effectively enhance the defect information in each direction and display the defect information on a single image, does not apply a filtering algorithm so as not to cause the distortion of the defect information, and uses red natural light to inhibit interference fringes and light intensity transmission loss in a light path. Meanwhile, the frequency domain fusion algorithm can fuse dynamic magneto-optical images to realize omnidirectional rapid detection, greatly eliminate static interference, enhance periodic magnetic leakage signals, and remarkably increase the penetration depth of excitation signals by ultra-low frequency magnetic field excitation, thereby greatly improving the detection capability of the magnetic leakage and magneto-optical imaging detection method on open hole and buried hole defects.
Drawings
FIG. 1 is a schematic diagram of the low frequency rotating field excitation generation of the present invention;
FIG. 2 is a schematic diagram of the single bridge amplifier of FIG. 1;
FIG. 3 is a schematic diagram of a magneto-optical imaging system according to an embodiment of the present invention;
FIG. 4 is a flowchart of an embodiment of frequency domain image linear weighted fusion according to the present invention;
FIG. 5 is a defect diagram of an embodiment of a metallic magnetic material test piece;
FIG. 6 is a fused frequency domain image and a binarization result of the fused frequency domain image of the metallic magnetic material specimen shown in FIG. 5, wherein (a) is the fused frequency domain image, and (b) is the binarization result;
fig. 7 is a magneto-optical image of the metallic magnetic material test piece shown in fig. 5 obtained by a conventional dynamic magnetic flux leakage test.
Detailed Description
The following description of the embodiments of the present invention is provided in order to better understand the present invention for those skilled in the art with reference to the accompanying drawings. It is to be expressly noted that in the following description, a detailed description of known functions and designs will be omitted when it may obscure the subject matter of the present invention.
The metal magnetic material defect detection device based on image fusion is divided into a hardware module and a software module, wherein the hardware module comprises an excitation signal circuit and a magneto-optical imaging system, and the software module comprises a fusion algorithm module. The following detailed description of the various modules refers to the accompanying drawings
1. Excitation signal circuit
In this embodiment, as shown in FIG. 1, the excitation signal circuit includes a function generator 101 and twoBridge amplifiers 102, 103. The function generator 101 provides two paths of sine signals with a phase difference of 90 degrees, namely sine signals and cosine signals, and because the function generator 101 is weak in load capacity, the bridge amplification circuits 102 and 103 are used for carrying out power amplification on the sine signals and the cosine signals respectively to obtain the sine signals and the cosine signals with the phase difference of less than 50Hz and the excitation frequency of fexAnd is loaded on two vertically arranged U-shaped yokes 201, 202 in the magneto-optical imaging system. Under the condition of ensuring that signal distortion is small, the bridge amplification circuits 102 and 103 effectively reduce drift caused by temperature rise of an instrument, and enable output voltage to be doubled and output power to be quadrupled.
In this embodiment, the power amplification of the sine and cosine signals is performed by two bridge amplifiers, and the schematic diagram of a single bridge amplifier is shown in fig. 2, and is composed of an upper operational amplifier circuit with inverse proportion based on the operational amplifier a and a lower operational amplifier circuit with in-phase proportion based on the operational amplifier B, wherein two output ends are connected with the resistor RSNCapacitor CSNHigh frequency noise is filtered to the ground, and the cut-off frequency of the low-pass filter is calculated by using the following formula
The low-frequency sine and cosine power signals passing through the bridge amplifier are loaded on two U-shaped magnetic yokes which are vertically arranged. In order to make the load circuit have smaller impedance angle and simultaneously have overcurrent protection, a current-limiting resistor R equivalent to the impedance angle is connected in series with the magnetic yoke, so that the load impedance is higher under the condition of not causing excitation
Is small. In this embodiment, the real part impedance of the single yoke is 4 ohms, so that a 3 ohm resistor is used for current limiting while reducing the impedance angle of the inductive load, and the load current is a sinusoidal alternating current with a peak-to-peak value of 1.5A and a frequency of 0.2 Hz.
2. Magneto-optical imaging system
And applying low-frequency sine and cosine power signals to the metal magnetic material test piece by using the U-shaped magnetic yoke, and generating a low-frequency rotating magnetic field with constant size and uniform direction in the test piece according to a parameterized equation of a circle, wherein the low-frequency rotating magnetic field is used as excitation for magnetic flux leakage detection. The parameterized equation for the circle is as follows:
a is the radius of the circle, i.e. the magnitude of the magnetic field, θ represents the angle between the corresponding vector of the current point and the x-axis, i.e. the direction of the magnetic field, and θ is 2 π ft, f is the frequency of the excitation signal, t is time, and π is the circumferential ratio.
If the metal magnetic material test piece has defects, according to the magnetic leakage detection principle, the defects generate magnetic leakage fields containing defect information in different directions along with the change of time under the excitation of a rotating magnetic field. Therefore, the leakage magnetic field information of the defects in all directions can be converted into optical signals by using a magneto-optical imaging system taking red natural light as a light source, a strong light intensity transmission ratio can be ensured by using red light, interference fringes in a light path are inhibited by using high-intensity natural light instead of laser used in the traditional magneto-optical detection, and a CCD (charge coupled device) sensor is used for acquiring magneto-optical image sequences within an integral period of an excitation signal.
As shown in fig. 3, the sine and cosine power signals amplified by the bridge amplifier are applied to two perpendicular yokes 201 and 202, respectively, to generate a rotating magnetic field in the central detection region for excitation of magnetic leakage detection, and if a defect, such as a Z-type defect shown in fig. 3, exists in the metal magnetic material test piece, a magnetic leakage field is generated above the defect, and the magnetic leakage field is detected by using a magneto-optical imaging system. The magneto-optical imaging system is composed of a high-power natural light red light point light source 203, a convex lens 204, a polarizer 205, a magneto-optical film 206, a reflecting film 207, an analyzer 208, an imaging lens 209 and a CCD image sensor 210.
Firstly, a beam of uniform red natural light is generated by using a point light source 203 and a convex lens 204, and is converted into polarized light by a polarizer 205, and the polarized light impinges on a magneto-optical film 206, is reflected by a bottom reflection film 207 and is transmitted to an analyzer 208. When the leakage magnetic field exists, according to the faraday effect, the characteristic of the magneto-optical film 206 in the leakage magnetic field is changed, and the polarization direction of the linearly polarized light incident on the magneto-optical film 206 at the point where the leakage magnetic field exists is rotated by θ degrees, which is specifically calculated by using the following formula:
where i is the angle of incidence, γ is the angle of refraction in the magneto-optical film, V, n1H is the Verdet constant, refractive index and thickness of the magneto-optical film, respectively, and B is the magnetic induction intensity of the leakage magnetic field at the point of incidence.
The polarization analyzer 208 is used for converting the deflection information of the linearly polarized light into light intensity information, and the light intensity information is acquired by the CCD image sensor 210 after being imaged by the imaging lens 209. In order to prevent the spectrum leakage, a method of sampling the excitation signal in an integer period should be adopted, and in this embodiment, in order to realize rapid measurement, only a magneto-optical image sequence in a single period of the excitation signal is collected and then transmitted to a computer for processing by using a fusion algorithm.
3. Fusion algorithm module
As shown in fig. 4, for the magneto-optical image sequence in a single period of the collected excitation signal, fourier transform is performed on the amplitude response of each position pixel point in the magneto-optical image sequence, so that the magneto-optical image sequence is converted into a frequency domain. The spectrum of the magneto-optical image sequence is analyzed, and the detailed information of different types of defects can be obtained, for example, in the intersection region of m linear defects, for a rotating excitation magnetic field in a single period, when the excitation magnetic field rotates to be perpendicular to a certain linear defect, a stronger leakage magnetic field peak is generated in the region, and therefore 2m peaks exist in the leakage magnetic field single period of the m defects. Thus in the spectral image of the sequence of magneto-optical images the n multiples contain detail information in the intersection region of the n/2 linear defects. Moreover, the frequency spectrum of any periodic signal also has a peak value at the frequency multiplication part, and if the frequency multiplication can be fused, the interference of random noise on a given periodic magnetic flux leakage signal can be effectively inhibited. But at a high frequencyThe image noise corresponding to the signal is increased, so the excitation frequency f is fused by adopting frequency domain linear weightingexImage y (nf) corresponding to the amplitude of the frequency-multiplied signal of (d)ex) Fusion is performed. The specific fusion mode is to take the image y (nf) corresponding to the low-frequency signal of 1/10 before the frequency domainex) The weight increases with increasing frequency, will be y (nf)ex) Normalizing to obtain y (nf)ex1) Calculating the amplitude of the pixel point on the fused image by using the following formula:
wherein L denotes the length of the sequence of magneto-optical images,
representing the fused frequency domain image yfuseAmplitude of the middle pixel point (i, j), y (nf)ex1)i,jFor normalizing the frequency domain image y (nf)ex1) The amplitude value of the pixel point (i, j), wherein (i, j) is the horizontal and vertical coordinates of the image;
the fused frequency domain image yfuseAnd linearly converting the amplitude value of the upper pixel point into a gray value range to obtain a defect image of the metal magnetic material test piece.
The Z-type defect shown in FIG. 5 has a total length of 4mm, a total width of 3.7mm, a defect depth of 5.5mm, a slit width of 0.8mm, and a specimen thickness of 6 mm. And applying sine and cosine excitation with the peak-to-peak value of 1.5A and the frequency of 0.2Hz to the magnetic yoke, carrying out magnetic flux leakage detection from the front (hole opening) direction of the defect, and collecting a magneto-optical image sequence of the Z-shaped defect in a single period. The experimental results obtained by the frequency domain linear weighted fusion algorithm are shown in fig. 6. The method comprises the following steps of (a) obtaining a frequency domain image after fusion, and (b) obtaining a binarization result after performing binarization processing and Gaussian filtering on the frequency domain image after fusion. Compared with the magneto-optical image obtained by adopting the traditional one-way magnetic leakage detection shown in the figure 7, the magneto-optical image defect detection method has the advantages that the defect outline is more clearly shown, and the defect is that the central outline is more obvious than the side outline because the amplitude of the magneto-optical image is stronger due to the stronger magnetic leakage field at the central oblique edge of the Z-shaped defect.
Although illustrative embodiments of the present invention have been described above to facilitate the understanding of the present invention by those skilled in the art, it should be understood that the present invention is not limited to the scope of the embodiments, and various changes may be made apparent to those skilled in the art as long as they are within the spirit and scope of the present invention as defined and defined by the appended claims, and all matters of the invention which utilize the inventive concepts are protected.
Claims (3)
1. A metal magnetic material defect detection device based on image fusion is characterized by comprising
An excitation signal circuit for generating an excitation frequency of f below 50HzexThe low-frequency sine and cosine power signals are loaded on two U-shaped magnetic yokes vertically arranged in a magneto-optical imaging system;
the magneto-optical imaging system is used for applying low-frequency rotating magnetic field excitation to the metal magnetic material test piece by using two vertically arranged U-shaped magnetic yokes; the magneto-optical imaging system takes red natural light as a light source, converts leakage magnetic field information of defects in all directions into optical signals, and uses a CCD (charge coupled device) sensor to collect optical signals in an integral period of an excitation signal to form a magneto-optical image sequence;
a fusion algorithm module for performing Fourier transform on the amplitude response of each position pixel point in the magneto-optical image sequence, namely converting the amplitude response into a frequency domain to obtain frequency response, and then converting the excitation frequency f in the frequency response of all the position pixel pointsexThe n frequency multiplication responses form a frequency domain image y (nf)ex) Then, a frequency domain image corresponding to the low frequency of the front 1/10 frequency domain, namely y (nf), is takenex) N is 1,2,.. times, L/10 is normalized to obtain a normalized frequency domain image y (nf)ex1) Finally, the fused frequency domain image y is calculated by using the following formulafuseUpper pixel point amplitude:
whereinL denotes the length of the sequence of magneto-optical images,
representing the fused frequency domain image yfuseAmplitude of the middle pixel point (i, j), y (nf)ex1)i,jFor normalizing the frequency domain image y (nf)ex1) The amplitude value of the pixel point (i, j), wherein (i, j) is the horizontal and vertical coordinates of the image;
the fused frequency domain image yfuseAnd linearly converting the amplitude value of the upper pixel point into a gray value range to obtain a defect image of the metal magnetic material test piece.
2. The apparatus according to claim 1, wherein the excitation signal circuit comprises a function generator and two bridge amplifiers, wherein the function generator provides two sine signals, i.e. sine signal and cosine signal, with a phase difference of 90 degrees, and the two bridge amplifiers amplify the power of the sine signal and cosine signal respectively to obtain an excitation frequency f below 50HzexLow frequency sine and cosine power signals.
3. The apparatus of claim 1, wherein the bridge amplifier comprises an upper operational amplifier A based inverse proportion operational amplifier circuit and a lower operational amplifier B based in-phase proportion operational amplifier circuit, and the yoke is connected in series with a current limiting resistor R having a resistance L of 3 ohms.
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