CN112129831A

CN112129831A - Crack detection system and method for safety production

Info

Publication number: CN112129831A
Application number: CN202011030855.0A
Authority: CN
Inventors: 张瑞蕾; 唐彦东; 李海君; 刘京会; 倪欣宇; 陈金梅
Original assignee: College Of Disaster Prevention Technology
Current assignee: College Of Disaster Prevention Technology; Institute of Disaster Prevention
Priority date: 2020-09-27
Filing date: 2020-09-27
Publication date: 2020-12-25

Abstract

The system comprises an excitation circuit module, a planar coil sensor and a processor, wherein the planar coil sensor and the surface of a material to be detected form a preset angle; the excitation circuit module is configured to apply a high-frequency alternating current sinusoidal signal to an excitation coil of the planar coil sensor, and the processor is configured to judge the surface crack condition of the detected material according to the collected voltage at two ends of a detection coil of the planar coil sensor; the method and the device have the advantages that the whole life cycle detection of the key parts of the parts is realized, and the stress state and the damage condition of the mechanical parts in the working state can be detected and evaluated in situ.

Description

Crack detection system and method for safety production

Technical Field

The disclosure relates to the technical field of crack detection, and in particular relates to a crack detection system and method for safety production.

Background

The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.

With the development of industrial technology, mechanical equipment increasingly bears the effects of high temperature, high pressure and high load, and the problem of fatigue damage caused in the dynamic load and stress concentration environment is increasingly prominent. The fatigue damage of mechanical parts is closely related to the indexes of the performance of weighing equipment, such as the service life, the operating efficiency, the working reliability and the like of a machine, and the safety of lives and properties of people is threatened. Statistics show that mechanical structures that fail due to fatigue account for about 80% of failed structures. Among them, crack propagation caused by microscopic damage of materials is an important cause of accident outbreak. How to reliably evaluate the early damage of the in-service equipment and prevent the occurrence of major accidents is a critical problem which needs to be solved urgently in the technical field of mechanical testing.

The state of the mechanical part is generally characterized by the physical and mechanical properties of the material, such as residual stress variation, thickness distribution of a deformation layer, micro-damage degree and the like. The factors have obvious influence on important indexes such as fatigue resistance, service life and reliability of mechanical parts, and are always the problem of important research in the field of mechanical manufacturing detection. For the physical and mechanical properties of the material, the material has been provided with more perfect experimental analysis means, such as analyzing the residual stress by using an X-ray diffraction method, analyzing the thickness of a deformation layer by using a microhardness instrument, analyzing the micro-damage state of the material by using a metallographic method, and the like. The methods can realize quantitative accurate analysis, have high accuracy, but cannot meet the requirements of nondestructive, online, quick and piece-by-piece detection.

The inventor of the present disclosure finds that the existing nondestructive testing methods such as ultrasonic testing, eddy current testing, acoustic emission testing, magnetic memory testing, etc. have been widely applied in early fatigue damage testing, and these methods all have certain limitations, and a common problem is that it is difficult to implement effective monitoring on the key parts of the machine operation during the machine operation, and only periodic testing can be performed during the disassembly and assembly period; in addition, the detection methods can only accurately detect materials with planar surfaces, but are difficult to detect the complex curved surface gaps such as gear gaps with high precision, and have unsatisfactory detection effects on internal structure changes of the materials such as early fatigue damage, profit concentration and the like, so that missing detection is easily generated.

Disclosure of Invention

In order to overcome the defects of the prior art, the disclosure provides a crack detection system and method for safety production, so that the full-life cycle detection of key parts of parts is realized, and the in-situ detection and evaluation of the tiny stress state and damage condition of mechanical parts in a working state can be realized.

In order to achieve the purpose, the following technical scheme is adopted in the disclosure:

a first aspect of the present disclosure provides a crack detection system for safety production.

A crack detection system for safety production comprises an excitation circuit module, a planar coil sensor and a processor, wherein the planar coil sensor and the surface of a material to be detected form a preset angle;

the excitation circuit module is configured to apply a high-frequency alternating current sinusoidal signal to an excitation coil of the planar coil sensor, and the processor is configured to judge the surface crack condition of the detected material according to the collected voltages at two ends of the detection coil of the planar coil sensor.

As some possible realization modes, the excitation circuit module comprises a sinusoidal signal generating circuit and a power amplifying circuit, and generates a high-frequency excitation signal with adjustable frequency.

As some possible implementation manners, the device further comprises an amplifying and filtering circuit module and an amplitude discrimination circuit module, and the amplifying and filtering circuit module and the amplitude discrimination circuit module are used for detecting the voltage at two ends of the detection coil of the planar coil sensor.

As some possible realization modes, the direction of the plane coil sensor probe is vertical to the surface of the material to be measured.

As some possible implementations, the planar coil sensor includes a flexible PCB substrate, an excitation coil and a detection coil, and the excitation coil and the detection coil are respectively disposed at different layers of the PCB substrate.

As some possible implementations, the line width of the detection coil is smaller than the line width of the excitation coil.

As some possible implementation manners, the excitation coils and the detection coils both comprise at least two layers, the excitation coils of the layers are connected through via holes in the PCB substrate, and the detection coils of the layers are connected through via holes in the PCB substrate.

As possible realization modes, the PCB substrate is a substrate material coated with copper foil, and patterns of the exciting coil and the detecting coil are etched on the substrate material by adopting a PCB multi-layer board process.

As some possible implementation manners, the excitation coils of each layer at least comprise six, the six excitation coils are sequentially connected in series, the detection coils of each layer at least comprise six, the six detection coils are mutually independent, and each detection coil is connected with different pins through a lead wire.

A second aspect of the present disclosure provides a crack detection method for safety production.

A crack detection method for safety production, using the detection system according to the first aspect of the present disclosure, comprising the steps of:

applying an alternating current signal with a preset size to the planar coil sensor through the excitation circuit module;

and scanning in a direction perpendicular to the crack by adopting a planar coil sensor with a plurality of excitation coils and detection coils, and carrying out curved surface imaging processing on a plurality of voltage signals to obtain the size and the position of the crack.

Compared with the prior art, the beneficial effect of this disclosure is:

1. according to the system and the method, the planar coil sensor is arranged on the tested piece, so that the whole life cycle detection of the key part is realized, and the stress state and the damage condition of the mechanical part in the working state can be detected and evaluated in situ.

2. The system and the method disclosed by the disclosure adopt the flexible array planar coil sensor, adopt the flexible printed circuit board process and adopt the etching method to manufacture the flexible substrate material, so that the sensor has very good flexibility, can be freely bent or folded, has flexible and various structural forms and can conveniently detect parts with complex surface geometric shapes.

3. The basic structure of the flexible array planar coil sensor is composed of a plurality of units, each unit comprises an excitation coil and an induction coil, the sensor array can be adjusted according to the surface shape of a material to be detected, and a differential excitation or differential response detection mode can be formed; the combination of various arrangement modes and detection modes can effectively avoid the interference of a magnetic field and improve the accuracy and stability of the test.

4. Compared with the traditional nondestructive detection method, the flexible array sensor detection technology has the remarkable characteristics and has the real early, quick and early warning significance of nondestructive detection; by applying the detection technology of the flexible array sensor and developing a detection instrument according to the basic principle of the eddy current, the early cracks of the test piece can be rapidly and qualitatively evaluated by recording the distribution condition of the induced voltage value of the tested piece.

Drawings

The accompanying drawings, which are included to provide a further understanding of the disclosure, illustrate embodiments of the disclosure and together with the description serve to explain the disclosure and are not to limit the disclosure.

Fig. 1 is a schematic view of a planar coil sensor detection system provided in embodiment 1 of the present disclosure.

Fig. 2 is a schematic diagram of a sine function represented by a unit circle provided in embodiment 1 of the present disclosure.

Fig. 3 is a schematic diagram of a power amplification circuit provided in embodiment 1 of the present disclosure.

Fig. 4 is a circuit diagram of an OPA690 operational amplifier provided in embodiment 1 of the present disclosure.

Fig. 5 is a diagram of MPY634 pin provided in embodiment 1 of the present disclosure.

Fig. 6 is a schematic diagram of a frequency multiplier provided in embodiment 1 of the present disclosure.

Fig. 7 is a schematic view of a planar coil structure provided in embodiment 1 of the present disclosure.

Fig. 8 is a schematic diagram of an ideal model for Wheeler inductance calculation provided in embodiment 1 of the present disclosure.

Fig. 9 is a schematic diagram of an ideal model of a planar rectangular spiral coil provided in embodiment 1 of the present disclosure.

Fig. 10 is a schematic diagram of an actual model of a planar rectangular spiral coil provided in embodiment 1 of the present disclosure.

Detailed Description

The present disclosure is further described with reference to the following drawings and examples.

It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.

It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to the present disclosure. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.

The embodiments and features of the embodiments in the present disclosure may be combined with each other without conflict.

Example 1:

as shown in fig. 1, an embodiment 1 of the present disclosure provides a crack detection system for safety production, including:

(1) the excitation part is composed of a sinusoidal signal generating circuit and a power amplifying circuit, generates a high-frequency excitation signal with adjustable frequency and supplies the high-frequency excitation signal to the excitation coil;

(2) the signal processing part is mainly composed of an amplifying filter circuit and an amplitude discrimination circuit and is used for processing voltage signals at two ends of the detection coil and improving the identification degree of the characteristic signals;

(3) and (3) signal acquisition, wherein an acquisition card is adopted to transmit signals to a computer so as to facilitate MATLAB software and the like to perform data processing.

Specifically, the method comprises the following steps:

sinusoidal signal generation circuit design

Direct Digital Frequency Synthesis (DDS) is widely used in recent years as an ideal signal generator, can output various signals such as sine waves, triangular waves and square waves, has the characteristics of adjustable output signal Frequency, phase and amplitude, and is widely applied to the fields of communication, high-end technologies, military radar technologies and the like.

The DDS is based on the generation of a sine function, and from the phase, different voltage amplitudes are given by different phases, i.e. phase-sine amplitude conversion, and finally, the frequency required by filtering and smoothing output is obtained, and the basic concept of the DDS synthesis signal is established by taking the generation of a sine function with an amplitude of 1 as an example, as shown in fig. 2. And theta (t) is an included angle between the rotation of the R by taking the original point as the center and the positive direction of the x axis, S is the projection of the R on the y axis, the value range of S is from +1 to-1 in the rotation process of the R, and the value range of theta (t) is changed from 0 to 360 degrees, so that a sine function S is Rsin theta (t).

When R is continuously rotated around the origin, the waveform of S will be a perfect sine wave. When R is rotated stepwise in phase increments of equal steps, instead of being rotated continuously, the waveform of S is affected by the phase increments and the steps. It can be imagined that when the phase increment is large and the number of steps is small, which means that there are few points to construct the function S, the waveform of S will be a stepped sine wave; when the phase increment is smaller, the step number is larger, and the waveform of S is closer to the actual sine function. In addition, the smaller the step length is, the faster the speed of one rotation is, and therefore the higher the frequency of S is; conversely, when the step size is increased, the frequency is decreased, and finally the output is in the form of square wave.

Power amplifier circuit design

In the eddy current testing circuit, in order to meet the attenuation of the signal during transmission, the signal generated by the signal generating device is required to have larger power. In order to obtain a sufficiently large high frequency output power, a high frequency power amplifier must be employed. The transistor is an amplifying element in the amplifying circuit, and obtains larger power output by amplifying the current of the input signal, and the part of energy is mainly from direct current. In this case, the transistor can be regarded as a control element, which controls the power supply U_CCThe energy supplied to obtain a signal of greater energy at the collector output.

In this embodiment, a high-frequency power amplifier circuit is designed by using the current amplification effect of a transistor, and fig. 3 shows a basic ac amplification circuit of a common emitter of a transistor. Wherein the collector power supply U_CCBesides providing energy for the output signal, the collector junction is ensured to be in reverse bias so as to enable the transistor to play an amplifying role; collector load resistor R_CThe change of the collector current is mainly changed into the change of voltage so as to realize voltage amplification; base resistance R_BHas the functions of making the emitting junction in forward bias and providing base current I with proper size_BSo that the amplifying circuit obtains a proper working point, and the resistance value is generally dozens of kilohms to hundreds of kilohms; coupling capacitor C₁And C₂On the one hand, plays a role in blocking, C₁For interrupting the direct current path between the amplifying circuit and the signal source, and C₂The circuit is used for cutting off a direct current path between the amplifying circuit and the load, so that the amplifying circuit and the load have no direct current communication and are not influenced mutually, and on the other hand, the circuit plays a role in alternating current coupling, so that alternating current signals can smoothly pass through the amplifying circuit and are communicated through alternating current paths among the signal source, the amplifying circuit and the load.

Operational amplifier circuit design

Even under the high-frequency condition, the voltage signal generated by the coil sensor is still weak, and the signal amplification is particularly necessary. In this embodiment, an OPA690 integrated amplifier chip is used. The OPA690 belongs to a voltage feedback operational amplifier, and an internal integrated circuit is stable, and has the characteristics of high bandwidth, stable unit gain, large output power and the like. The relationship between gain and bandwidth of the OPA690 is shown in table 1, and the operating frequency range of the circuit designed in this example is 1MHz to 10MHz, and it can be seen from the table that the OPA690 can satisfy the requirement of the problem in terms of bandwidth.

Table 1: gain vs. bandwidth (in the case of + -5V supply)

The designed operational amplifier circuit is shown in fig. 4 according to the characteristics and circuit requirements of the OPA690 chip. Compared with other operational amplifier chips, the OPA690 needs to pay attention to the following matters during the use process: for high-frequency signals, the amplification factor should not exceed 10 times, otherwise, the interference noise is large, the signal distortion is more serious, and R in the figure_FThe resistance value should be between 200 omega and 1.5K omega; if impedance matching is required then R is adjusted_MIs connected in parallel to R_GFor adjusting the input impedance. Simultaneously, the three resistances are determined, and then R is determined_BIs equal to the DC resistance to ground of the input and inverting input terminals to determine R_BA value of (d), which enables input dc offset to be minimized; the 0.01uF capacitor is used for voltage coupling, and waveform distortion caused by second harmonic output can be reduced.

The designed signal amplification circuit is an inverse proportion operation circuit, namely, an input signal is introduced from an inverse input end, and the voltage amplification factor formula is as follows:

in the circuit, besides using the OPA690 as an operational amplifier, a UA741 chip is also used, and the principles are basically consistent, so that the details are not repeated.

Amplitude discrimination circuit design

From previous analysis, we have known that the existence of surface defects of a metal test piece affects the amplitude and the phase of the voltage of the detection coil, and therefore, it is very important to obtain amplitude information and phase information. Phase sensitive detection is the most common way of modulation and has great advantages. Its features are not only the amplitude but also the phase of original modulated signal. The subject is to realize a phase-sensitive detection function by using a multiplier and obtain a characteristic signal by the assistance of other circuits.

MPY634 is a wide-band, high-precision, four-quadrant analog multiplier produced by BURR-BROWN of America, has a bandwidth of 10MHz, has a precision of +/-5% in a four-quadrant range, has excellent characteristics, is convenient to use, does not need external elements, and often does not need external adjustment.

The MPY634 pin diagram is shown in fig. 5, and includes three differential input signals X, Y, Z, which can implement operations such as multiplication, division, squaring, and opening, and can also constitute circuits with other functions, such as voltage-controlled amplifiers, filters, and oscillators. The MPY634 has excellent high-frequency characteristics, can be used in the fields of intermediate frequency, radio frequency, video and the like, and can complete various functions of frequency mixing, frequency doubling, modulation, demodulation and the like.

MPY634 has a transfer function of

Wherein: a-open loop gain of the output amplifier (dc gain about 80 dB); SF-magnification factor, internally calibrated to 10V by laser, adjustable between 3V and 10V by means of an external resistor; x, Y, Z-input voltage, full-width input voltage value is the same as SF, and maximum can be + -1.25 SF.

If the open-loop gain of the output amplifier is infinity, the parenthesis in the above equation is zero, and if Z is set₁＝V_out，Z₂When 0, then:

in this embodiment, a frequency multiplier is designed by using MPY634, and a schematic diagram is shown in fig. 6. X of multiplier₁、Y₁The same signal, Z, being input together₁And V_outAre connected to each other by X₂、Y₂、Z₂Are all grounded; SF is suspended, and the amplification factor at this time is that the SF subjected to laser accurate correction in the integrated circuit is 10V, and the error is 0.1% or less.

At this time, the multiplier principle can be understood as such. When the circuit is connected, if X₁＝X₂Asin (2 π ft + φ), then

From V_outIt can be seen that the signal output by the multiplier can be seen as being made up of two parts: a direct current signal and an alternating current signal. The frequency of the alternating current signal is one time of the frequency of the input signal, and the direct current signal only contains amplitude information in the input signal. According to the eddy current principle, the existence of cracks can have great influence on the amplitude of signals, and the defect condition of the surface of the test piece can be obtained by observing the amplitude change of the signals. Therefore, it is important to extract amplitude information from the detection signal. In the embodiment, the RC low-pass filter is adopted to filter the ac signal output by the multiplier, and the dc component is retained, so that the requirement of the high-frequency signal on the detection system is also reduced.

In this embodiment, the excitation circuit is mainly characterized in that the singlechip AT89S51 controls the AD7008 to generate a sinusoidal signal with adjustable frequency, and after the signal is filtered, the sinusoidal signal is power-amplified by the two-stage transistor 2N 2219. The detection circuit mainly amplifies signals by an OPA690, an MPY634 multiplier and a filter circuit extract signal amplitude information, and finally the UA741 amplifies the extracted signals.

Probe device of planar coil sensor

In the experiment, a planar coil array is manufactured by adopting a PCB multi-layer board process, and a coil pattern is etched on a base material coated with copper foil. The coil is processed by adopting a PCB process, so that the consistency of the coil can be improved, and the precision of the sensor can be improved. Meanwhile, the coil array can be designed according to the condition of the tested piece, and large-area eddy current detection can be realized.

The schematic diagram of the designed planar coil unit is shown in fig. 7, the excitation coil and the detection coil are respectively arranged on different layers of the PCB, so that on one hand, the line width of the excitation coil can be increased, and larger excitation current can be applied to increase excitation energy; on the other hand, the line width of the detection coil can be reduced, and the number of turns of the coil is increased so as to improve the inductance value of the detection coil; in addition, the interference of the excitation signal to the detection signal can be reduced, and a better detection effect is obtained.

Each coil sensor unit consists of an excitation coil and a detection coil, wherein the excitation coil and the detection coil are respectively provided with two layers and are connected through a through hole, so that the number of turns of the coil in unit area and the inductance value are improved. The sensor adopts an eddy current double-coil detection mode, applies a high-frequency alternating current sinusoidal signal to an exciting coil during detection, and judges the surface damage condition of a detected material by analyzing the voltage at two ends of a detection coil.

The actually manufactured planar coil array structure comprises six excitation coils and six detection coils, wherein the six excitation coils are connected in series, the detection coils are mutually independent, and pins of the coils are led to specified positions through lead wires, so that the connection and wiring between the coils and a circuit board are facilitated. The length and width of the plane of the probe are 40mm 17.7mm, the length and width of a single excitation coil are 6.1mm 7.1mm, and the line spacing is 0.1 mm; the length and width of each single detection coil is 5.8 mm/6.7 mm, and the line spacing is 0.1 mm; the distance between two adjacent coils is also 0.1 mm.

Inductance calculation analysis

The size of the inductance is one of the key elements for designing the planar coil, has great influence on the selection of the frequency and the amplitude of the excitation signal, and also has relation to the strength of the surrounding magnetic field of the coil in the electrified state, and theoretically, the larger the inductance of the coil is, the better the inductance of the coil is. Therefore, it is very necessary to estimate the coil inductance before coil processing.

Inductance calculation method 1

According to the Wheeler equation, the theoretical value of the planar spiral inductance L can be calculated by the following formula:

wherein:

d_avg＝(d_in+d_out)/2

ρ＝(d_out-d_in)/(d_out+d_in)

l is coil inductance and has the unit of H; n is the number of coil turns; d_in，d_outAs shown in fig. 8, the unit is m; mu.s₀Permeability in vacuum, equal to 4 π × 10^-7H/m; k is a coefficient related to the coil layout shape, and values thereof are shown in table 2. In this embodiment, K is taken when calculating the inductance₁＝2.34，K₂＝2.75。

Table 2: relationship between K value and coil layout shape

The planar coil array designed in this embodiment is designed by using a PCB multi-layer board process, and the parameters and the calculation results are shown in table 3.

Table 3: wheeler formula inductance calculation

Inductance calculation method two

For an ideal planar rectangular spiral coil, it should have equal number of turns n and size c in the x and y directions, as shown in fig. 9. When the thickness of the coil is very small and approaches zero, the inductance L of the ideal plane rectangular coil in vacuum₀The following formula can be used for calculation:

wherein n is the number of turns of the coil, and c is the width of the helical coil; mu.s₀Permeability in vacuum, equal to 4 π × 10^-7H/m；

For a square coil, x₁＝y₁Double integration in the above equation maySo as to be accurately solved by converting the polar coordinate into a double integral form. For convenience of calculation, when x₁When < c, the solution formula can be simplified as:

wherein x is₂＝x₁+c，η＝x₁/x₂≤1。

For the calculation of the inductance of the square coil, when eta is less than 0.5, the accuracy of the above formula is about 0.01%. For other shapes of coils, such as rectangular coils, the inductance value can also be estimated using the above equation.

In the coil model designed in this embodiment, on the basis of fig. 10, the right angle is changed into the round chamfer, so that the loss of the inductance at the corner can be avoided. When the inductance value was estimated, the calculation was performed using the graph shown in fig. 9, and the coil parameters and the calculation results are shown in table 4.

Table 4:

in this embodiment, an inductance value and a quality factor of each coil are measured by using a ZJ2776 type inductance measuring instrument (with a measurement accuracy of 0.05%), and when a given level value is 1.0V, the inductance value and the quality factor value are continuously read 15 times, and by averaging the 15 measurements, the inductance value of the excitation coil is 0.6580 μ H and the quality factor is 0.0110. Since the excitation coil is formed by connecting 6 coils in series, the inductance of a single excitation coil is 0.1097 muH. For an excitation coil with a line width of 0.50mm, a figure of merit of 0.0152 was calculated and the single excitation coil inductance value was 0.2943 μ H in magnitude. The inductance values and quality factor values of the six detection coils are measured by the same method.

The inductance values of the six detection coils are not different greatly, and the absolute value difference between the maximum value and the minimum value is 0.08988 mu H. Analysis of the reasons for the differences: on the one hand, the lead lengths may be different, and on the other hand, the processing precision is difficult to be absolutely consistent. When comparing with the theoretical calculation value, the average value of the six detection coil inductances was used and calculated as 2.1843 μ H.

The calculated value and the measured value of the inductance are compared and analyzed, so that the difference between the calculated values of the inductance of the two formulas is not large, and when the number of turns of the coil is small, the difference between the measured value and the theoretical value is small; for the detecting coils with more coil numbers, a certain difference exists between an actually measured value and an inductance value, the difference between the actually measured value and a calculation result of the first method is 16%, and the difference between the actually measured value and a calculation result of the second method is 15%, which are both within 20%, and belong to effective data.

The actually designed planar coil is different from an ideal coil model to a certain extent, so that an absolutely appropriate formula is not available in inductance calculation, and according to the analysis, when the inductance value of the planar coil is estimated, the calculation results of the two formulas are reasonable and can be adopted.

Example 2:

the embodiment 2 of the present disclosure provides a crack detection method for safety production, and the detection system according to the embodiment 1 of the present disclosure includes the following steps:

Specifically, the method comprises the following steps:

crack detection experiment

In the embodiment, the aluminum plate and the 16 manganese steel plate are used as detection objects, and the crack detection effect of the planar coil sensor on two different materials is analyzed. The surface cracks of the metal test piece adopt an electric spark corrosion mode, and the size of the prefabricated cracks is shown in a table 5. The length, depth and width of the 8 cracks are not completely the same, so that the influence of different parameters on the detection result can be analyzed conveniently.

Table 5: and (5) pre-forming the size of the crack.

In the experiment, an alternating current signal with certain frequency and amplitude is applied to an exciting coil, and the signal is collected in a computer and further processed by MATLAB software.

Analysis of crack detection result of non-ferromagnetic test piece

Influence of crack size on detection results

And applying a sinusoidal signal with the frequency of 4MHz and the voltage peak value of 5V to the excitation signal, and scanning the test piece in the direction vertical to the crack by using a planar coil sensor to obtain a signal curve. Comparing the characteristic signals corresponding to four groups of

cracks

1 and 5, 2 and 6, 3 and 7, 4 and 8 with the same width and depth, wherein the longer the crack is, the more obvious the characteristic signals are; comparing two groups of crack characteristic signals of 2, 3, 4, 6, 7 and 8 with the same length and width, wherein the larger the crack depth is, the more obvious the characteristic signals are; comparing the characteristic signals of the two groups of

cracks

1, 2, 5 and 6 with the same length and depth shows that the wider the crack is, the more obvious the characteristic signal is.

It can be found that the planar coil sensor probe is more sensitive to changes in crack length and width, and less sensitive to changes in depth, but still discernable. This is because the eddy current is concentrated on the surface of the test piece due to the skin effect, and the permeability of the eddy current decays exponentially with increasing depth, so that the change in the length and width of the crack on the surface of the test piece has a great influence on the surface eddy current density, and the smaller the eddy current density with increasing depth, the smaller the influence on the magnetic field is. Either factor makes the signature more visible, whether from crack length, width or depth. This is because the larger the crack is, the larger the influence on the eddy current distribution of the test piece is, the larger the change in the impedance of the detection coil is, and the higher the amplitude of the obtained defect signal is.

Influence of excitation frequency on detection result

The detection frequency in eddy current detection determines the sensitivity of flaw detection to a great extent, and the detection frequency needs to be selected according to respective characteristics of different test pieces. In this embodiment, the detection results at the frequencies of 2MHz, 3MHz, and 4MHz are analyzed without changing the parameters such as the amplification factor of the detection circuit.

Under the condition that the peak value of the excitation signal is 5V, the frequency of excitation is changed, other conditions are not changed, a crack with the length of 10mm is taken as an object, and the detection result under different frequency conditions is analyzed.

The amplitude of the signal becomes significantly larger as the frequency becomes larger. The difference (absolute difference between the voltage value at the defect-free position and the maximum voltage value of the characteristic signal) of the characteristic signals corresponding to the four cracks under different frequency conditions is shown in table 6, and it can be seen from the degree of the voltage change of the characteristic signals that the higher the frequency is, the more obvious the characteristic signals are.

Table 6: characteristic signal voltage difference/mV (5V for U) under different frequency conditions

The coil can be regarded as formed by connecting an inductor and a capacitor in series, the frequency of a signal determines the impedance of the coil, and in a certain frequency range, the higher the frequency is, the higher the impedance is, the larger the amplitude of the signal is, and the change of a characteristic signal is more obvious. Therefore, in eddy current inspection, it is necessary to adjust the excitation frequency so that the defect generates the largest impedance change, and the best inspection result is obtained, for different test pieces and defect inspection.

Influence of excitation amplitude on detection result

Generally, increasing the excitation current can increase the detection sensitivity. In the embodiment, under the condition that the frequency of the excitation signal is 2MHz, the amplitude of the excitation is changed, other conditions are not changed, a crack with the length of 10mm is taken as an object, and the detection results under different excitation amplitude conditions are analyzed.

The amplitude of the detection signal is obviously larger as the amplitude of the excitation signal is larger. The difference values of the characteristic signals corresponding to the four cracks under different frequency conditions are shown in table 7, and it can be seen from the degree of voltage change of the characteristic signals that the larger the peak value of the excitation signal is, the more obvious the characteristic signals are.

Table 7: characteristic signal voltage difference/mV (f 2MHz) under different excitation amplitude conditions

Influence of scanning direction on detection result

Among the eddy current testing, the distribution of test piece surface vortex has very big influence to the testing effect, adjusts the vortex distribution on aluminum plate surface through the direction that changes the probe in this embodiment, and the analysis scans the influence of direction to the testing result.

Cracks of 10mm by 1.0mm by 0.2mm were detected at an excitation frequency of 2MHz and a voltage peak of 5V. The scanning direction of the probe and the crack is changed,

the included angles of the probe scanning direction and the crack are respectively scanned from 30 degrees, 45 degrees, 60 degrees and 90 degrees, and the scanning is carried out in the directions

The variation of the characteristic signal is the largest at 90 degrees (perpendicular to the crack direction), and the variation degree of the characteristic signal is smaller with the decrease of the angle, so that the variation trend of the characteristic signal can be more obviously seen from the voltage difference in table 8. It can be seen that the scanning direction also has a great influence on the detection result, and therefore, in the eddy current detection, it is necessary to adjust the scanning direction for an unknown crack to obtain a better result.

Table 8: characteristic signal voltage difference/mV under different scanning direction conditions

Array scanning imaging analysis

In the above experiment, only the detection results of a single coil were analyzed, and the analysis results of array scanning are discussed below. Six coils are adopted to scan the cracks in parallel, the experimental objects are two cracks of 10mm x 1.0mm x 0.2mm (No. 5, length x depth x width) and 10mm x 1.0mm x 0.1mm (No. 6, length x depth x width) on the surface of the aluminum plate, the excitation condition is alternating current signals of 4MHz and 5V, and 6 detection signals are collected at the same time for imaging processing. Because the length of the crack is limited, not every coil is in contact with the crack, only the middle two coils are in contact with the crack during scanning, more signals are in contact with the crack than the other coil, and other coils without contact cannot observe characteristic signals. Since it is difficult for the six-way processing circuit to adjust the voltages of the respective coils to be absolutely uniform, there is also a difference in the voltages of the respective coils when there is no defect.

In order to visualize the scan results stereoscopically, the 6 signals are subjected to curved surface imaging processing. In the processing, in order to reduce the difference of each coil, the voltage contour line is processed to be zero, and the size of the crack can be reflected more clearly through imaging analysis, and the position of the crack on the surface of the test piece can also be judged. Therefore, the planar coil array sensor can be used for scanning the surface of the metal test piece.

In the embodiment, the planar coil sensor probe and the detection device thereof are used for detecting the prefabricated cracks on the surface of the non-ferromagnetic material (aluminum plate), and the influence of a plurality of parameters on the detection result is analyzed. According to experimental data analysis, no matter the size of the crack, the excitation frequency, the excitation amplitude, the scanning direction and the like have great influence on the detection sensitivity: the larger the crack, the easier it is to detect; within a certain frequency range, the increase of the frequency is beneficial to improving the detection sensitivity of the surface cracks; the improvement of the excitation amplitude is also beneficial to crack detection; scanning in a direction perpendicular to the crack gives the best detection results.

In addition, the surface crack detection of the ferromagnetic material (16 manganese steel) is also analyzed, and the direction of an internal magnetic domain of the ferromagnetic material can be changed and the external magnetic field intensity can be enhanced through the excitation effect of the neodymium iron boron alloy permanent magnet. The test results confirmed that the planar coil sensor can not only detect non-ferromagnetic materials, but also detect cracks on the surface of ferromagnetic materials, and can detect cracks of 5mm × 0.3mm × 0.1mm (length × depth × width).

And finally, scanning the cracks in parallel by using six coil sensors, and carrying out imaging processing on the data, wherein the results show that the planar array sensor can effectively scan the surface of the metal material, so that scanning in a certain area is realized.

The above description is only a preferred embodiment of the present disclosure and is not intended to limit the present disclosure, and various modifications and changes may be made to the present disclosure by those skilled in the art. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present disclosure should be included in the protection scope of the present disclosure.

Claims

1. A crack detection system for safety production is characterized by comprising an excitation circuit module, a planar coil sensor and a processor, wherein the planar coil sensor and the surface of a material to be detected form a preset angle;

2. The crack detection system for safety production as claimed in claim 1, wherein the excitation circuit module comprises a sinusoidal signal generating circuit and a power amplifying circuit for generating a high frequency excitation signal with adjustable frequency.

3. The crack detection system for safety production as claimed in claim 1, further comprising an amplifying and filtering circuit module and an amplitude discrimination circuit module for detecting a voltage across the detection coil of the planar coil sensor.

4. The crack detection system for safety production as claimed in claim 1, wherein the orientation of the planar coil sensor probe is perpendicular to the surface of the material to be measured.

5. The crack detection system for safety production as claimed in claim 1, wherein the planar coil sensor comprises a flexible PCB substrate, an excitation coil and a detection coil, the excitation coil and the detection coil being respectively disposed at different layers of the PCB substrate.

6. The crack detection system for safety production as claimed in claim 1, wherein the detection coil has a smaller line width than the excitation coil.

7. The crack detection system for safety production as claimed in claim 1, wherein the excitation coils and the detection coils each comprise at least two layers, the excitation coils of the respective layers are connected through via holes in the PCB substrate, and the detection coils of the respective layers are connected through via holes in the PCB substrate.

8. The crack detection system for safety production as claimed in claim 1, wherein the PCB substrate is a copper clad substrate material, and patterns of the excitation coil and the detection coil are etched on the substrate material using a PCB multi-layer board process.

9. The crack detection system for safety production as claimed in claim 1, wherein the excitation coils of each layer include at least six, the six excitation coils are connected in series in turn, and the detection coils of each layer include at least six, the six detection coils are independent of each other, and each detection coil is connected to a different pin by a lead wire.

10. A crack detection method for safety production, characterized in that, with the detection system according to any one of claims 1-9, it comprises the following steps: