CN100427947C - Magnetic flux leakage detection method for large-area steel plate defects - Google Patents

Abstract

A magnetic flux leakage detection method for large-area steel plate defects. First, an excitation device is used to locally magnetize a large-area steel plate. By measuring the magnetic flux density at the magnetic poles, the magnetization saturation state inside the steel plate is calculated using a formula, and then a plurality of magnetic sensors are used. The detection probe composed of element dislocation array measures the leakage magnetic flux density at different positions on the surface of the steel plate at equal intervals with fixed lift-off values, and preprocesses the output signals of two adjacent magnetic sensitive elements such as difference, and finally converts the measured The magnetic flux density value is compared with the dynamically set detection threshold to determine the presence or absence of defects. Since the present invention can effectively judge the local magnetization saturation state of the steel plate, the magnetic circuit parameters of the magnetic flux leakage probe can be optimized; at the same time, the magnetic field leakage can be measured by using a probe composed of a magnetically sensitive element dislocation array, which can prevent missed detection and improve The coverage and spatial resolution of the measurement are improved, and the detection accuracy of large-area steel plate surface defects is guaranteed.

Description

Magnetic flux leakage detection method for large-area steel plate defects

technical field

The present invention relates to a method for non-destructive detection of defects on the inner and outer surfaces of steel plates, in particular to a method for locally magnetizing a large-area steel plate through an external magnetic field, and using the leakage magnetic field formed on the surface of the steel plate to quickly detect the defects on the inner and outer surfaces of the steel plate. It belongs to the technical field of nondestructive testing.

Background technique

In the prior art, the ultrasonic method is usually used to detect local defects and large-area corrosion thinning on the inner and outer surfaces of steel plates. Although this method has the advantage of high precision, its detection efficiency is low and it is not suitable for large-area detection.

Chinese patent application (publication number: CN1373852A) discloses a "magnetic flux leakage flaw detection method and a method for manufacturing hot-rolled steel sheets using the magnetic flux leakage flaw detection method". Inspect its internal inclusions, and use this method to produce hot-rolled steel sheets and descaled steel sheets, but this technology does not mention the detection method for internal and external surface defects of steel sheets.

Chinese utility model patent (authorized announcement number: CN 2729723Y) discloses a "DC motor driven storage tank floor magnetic flux leakage detector", the utility model consists of a notebook computer, frame, data acquisition module, battery, DC motor, reducer It is composed of a drive wheel, a magnet, a wheel, a photoelectric pulse encoder and a magnetic flux leakage sensor. The magnetic flux leakage sensor is used to dynamically collect the defects of the iron plate at the bottom of the tank, and the coordinate positioning of the defects is realized through the photoelectric pulse encoder. However, this patent lacks an effective method for judging the local magnetization saturation state of the tank bottom plate, and the structural parameters of the detector and the magnetic flux leakage sensor cannot be rationally optimized, so the detection accuracy is not high, and the driving force is relatively large in engineering applications.

Contents of the invention

The purpose of the present invention is to propose a method for detecting magnetic flux leakage of large-area steel plate defects, that is, to use the magnetic flux density value between the magnetic pole and the steel plate to judge the local magnetization saturation state of the steel plate, and to construct a magnetic flux leakage detection probe through the dislocation array of sensors. Prevent missed detections and improve measurement coverage and spatial resolution.

Technical scheme of the present invention is as follows:

1) Use an excitation device to locally magnetize a large-area steel plate, measure the magnetic flux density at the magnetic poles of the magnet, and then use the following formula (1) to calculate the magnetic flux density in the local area of the steel plate to determine the magnetization saturation state inside the steel plate,

$\frac{{\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; W</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; P</span>}}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; P</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Among them, B _M is the average magnetic flux density in the gap between the magnetic pole and the steel plate, W _M is the width of the magnet, L _P is the thickness of the steel plate, and B _P is the saturation magnetic flux density in the local area of the steel plate;

2) Measure the leakage magnetic flux density at different positions on the surface of the steel plate at equal intervals with a fixed lift-off value of 2 to 4 mm using a detection probe composed of multiple magnetic sensitive elements;

3) Firstly, differential processing is performed on the output signals of two adjacent magnetic sensitive elements, and then the output signals of the entire detection probe are superimposed, amplified and pre-filtered to filter or suppress random noise and background noise of the output signal, And amplify the jump amplitude of the defect signal;

4) According to the detection speed, the detection threshold is dynamically set to 2-10 gauss, and the presence or absence of defects is judged by comparing the measured magnetic flux density value with the detection threshold.

The present invention is also characterized in that: the magnetic sensitive elements in the detection probe described in step 2) are arranged in a dislocation array.

The magnetic flux leakage detection method for large-area steel plate defects can effectively judge the local magnetization saturation state of the steel plate, so the magnetic circuit parameters of the magnetic flux leakage probe can be optimized; at the same time, the probe composed of a dislocation array of magnetic sensitive elements can be used to detect Measuring the leakage magnetic field can prevent missed detection, thereby improving the coverage and spatial resolution of the measurement, and ensuring the detection accuracy of large-area steel plate surface defects.

Description of drawings

Figure 1 is a schematic diagram of magnetic flux leakage detection.

Fig. 2 is a schematic structural diagram of the detection probe, which shows that it is composed of a plurality of magnetically sensitive elements arranged in dislocation.

Figure 3 is a structural block diagram of a large-area steel plate defect detection system.

Among them: 1-armature, 2-magnet, 3-pole shoe, 4-large-area steel plate to be tested, 5-magnetic sensitive element, H-air gap height between magnetic pole and steel plate, δ-lift-off value of magnetic sensitive element, 6- Detection probe.

Detailed ways

The detection method provided by the present invention will be further described below in conjunction with the accompanying drawings.

First, according to the thickness of the steel plate to be tested and the standard saturation magnetic flux density value of the material used in the steel plate, use the formula (1) to calculate and select an appropriate size excitation device to magnetize the steel plate to a local magnetization saturation state; the excitation device consists of an armature 1, magnet 2, pole piece 3 composition. Figure 1 shows the detection principle diagram. The excitation device is placed on the surface of the steel plate at a certain height H, and the large-area steel plate 4 to be tested is saturated and magnetized. The magnetic sensitive element 5 is used to measure the steel plate at equal intervals with a fixed lift-off value of 2 to 4 mm. Leakage magnetic flux density at different positions on the surface; according to the description of accompanying drawing 2, a plurality of magnetic sensitive elements 5 are arranged in a dislocation array to form a magnetic flux leakage detection probe 6, so as to ensure that there is no missed detection in the scanning area, and improve the coverage and coverage of the measurement. spatial resolution. Among them, the number of magnetic sensitive elements should be arranged so that the total coverage area is larger than the measured area (as shown in Figure 2); the magnetic flux leakage detection probe scans the steel plate along the magnetization direction, and the two adjacent magnetic sensitive elements The output signal is differentially processed, and then superimposed, filtered and amplified to filter or suppress the random noise and background noise of the output signal; it is transmitted to the single-chip microcomputer through the interface circuit, and the single-chip microcomputer transmits the detection data to the computer through the buffer circuit and stores them . At the same time, according to the detection speed, the detection threshold is dynamically set to 2-10 gauss. By comparing the measured magnetic flux density value with the detection threshold value (as shown in Figure 3), if the measured magnetic flux density value is greater than the detection threshold value, then It can be judged that there are defects on the surface of the steel plate.

Example:

Select 12mm thick Q235 steel plate as the detected object, according to the saturation magnetic flux density of Q235 material and the calculation of formula (1), select the NdFeB N50 rare earth permanent magnet material with a length of 300mm, a width of 30mm, and a thickness of 25mm to form the excitation device. =5mm magnetization air gap to magnetize the steel plate to make it to local magnetization saturation; according to the description in Figure 2, 36 integrated Hall elements are dislocated to form a detection probe, wherein the interval between the magnetic sensitive areas of adjacent elements is 2mm; Use this probe to measure the magnetic field at different positions on the steel plate surface along the magnetization direction with a lift-off value of 2.5mm and a sampling interval of 2mm; Superimpose, amplify and filter the output signal of the entire magnetic flux leakage detection probe to obtain an ideal defect detection signal; according to the detection speed (0.5m/s), the detection threshold is dynamically set to 4.8 Gauss (Gauss), and the After comparing the measured leakage flux density value with the detection threshold, it is found that the measured leakage flux density value is greater than the set detection threshold value of 4.8 gauss, and it can be judged that there are defects on the surface of the steel plate.

Claims (2)

1. A large-area steel plate defect magnetic flux leakage detection method is characterized in that the method is carried out as follows: 1) Use the excitation device to locally magnetize the large-area steel plate, and measure the magnetic flux density at the magnetic poles of the magnet to use the following formula (1) to calculate the magnetic flux density in the local area of the steel plate and determine the magnetization saturation state inside the steel plate. $\frac{{\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; W</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; P</span>}}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; P</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$ Among them, B _M is the average magnetic flux density in the gap between the magnetic pole and the steel plate, W _M is the width of the magnet, L _P is the thickness of the steel plate, and B _P is the saturation magnetic flux density in the local area of the steel plate; 2) Measure the leakage magnetic flux density at different positions on the surface of the steel plate at equal intervals with a fixed lift-off value of 2 to 4 mm using a detection probe composed of multiple magnetic sensitive elements; 3) Firstly, differential processing is performed on the output signals of two adjacent magnetic sensitive elements, and then the output signals of the entire detection probe are superimposed, amplified and pre-filtered to filter or suppress random noise and background noise of the output signal, Amplify the jump amplitude of the defect signal; 4) Dynamically set the detection threshold to 2-10 gauss according to the detection speed, and compare the measured magnetic flux density value with the detection threshold to determine the presence or absence of defects. 2. The magnetic flux leakage detection method for large-area steel plate defects according to claim 1, characterized in that: the magnetic sensitive elements in the detection probe described in step 2) are arranged in a dislocation array.

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