CN111307945B - An imaging method and device for detecting near-surface defects of ballastless track based on ultrasonic array - Google Patents

Abstract

The invention discloses an imaging method and device for detecting near-surface defects of a ballastless track based on an ultrasonic array, wherein the imaging method comprises the steps of exciting a receiving transducer by using a pulse signal, transmitting an ultrasonic signal with the frequency in a range of 1-2.5 MHz to the inside of the ballastless track by using an ultrasonic array probe, acquiring a diffusion field signal in the ballastless track by using an ultrasonic array sensor in a spontaneous self-receiving mode, and transmitting the diffusion field signal to a computer; and passively extracting a green function from the received signals by adopting MATLAB software through a computer, carrying out cross-correlation on the diffusion field signals, reconstructing the green function among the array elements, acquiring undelayed response among the array elements, recovering early defect information submerged by noise, and then carrying out ballastless track near-surface defect imaging according to a full-focus imaging algorithm. The invention can clearly display the information of the near-surface defect of the track plate, and can provide a timely early warning and powerful assessment means for the daily track plate damage detection of the high-speed rail.

Description

An imaging method for detecting near-surface defects of ballastless track based on ultrasonic array and its device

technical field

The invention relates to an imaging method and device for detecting near-surface defects of a ballastless track based on an ultrasonic array, and belongs to the technical field of track nondestructive testing.

Background technique

Due to the advantages of high-speed railways such as fast speed, strong transport capacity, safety and punctuality, high-speed railways have become one of the public transportation methods vigorously developed in my country at this stage, and high-speed railways, especially high-speed passenger railways, will definitely be the future development trend. With the rapid development of high-speed railways, ballastless track, as a main track structure, is composed of ballastless track slab, CA mortar layer, support layer and foundation bed, and its application scope is becoming wider and wider. However, in recent years, more and more diseases have appeared in the offline structure of the high-speed rail, including the off-line structural clearance, through cracks, and voids in the CA mortar layer. The main reason is that, on the one hand, the ballastless track will be squeezed and impacted during the high-speed and heavy-load operation of the train, which may cause various defects such as incompactness, cracks or cavities inside, and damage layers or honeycomb hemp layers on the outside; on the other hand, the ballastless track itself may have defects due to problems in the construction process and construction experience in the pre-production process; in addition, natural disasters such as rain and snow erosion and environmental temperature changes will also cause defects. The existence of defects will seriously affect the bearing capacity and durability of the ballastless track, and will cause the structure of the ballastless track to fail. The stability and smoothness of the ballastless track and the offline structure of the high-speed railway cannot be guaranteed. The stability and smoothness are precisely the important prerequisites for ensuring the fast and safe operation of the high-speed railway, which will directly affect the normal operation of the train and the personal safety of passengers.

However, at present, the detection of near-surface defects on ballastless tracks in my country mainly relies on manual static detection technology. Since the effective skylight time for rail transit to be used for line maintenance and maintenance is only 2 to 3 hours, and the threads of high-speed railways are very long, using existing detection methods not only consumes a lot of manpower and material resources, but also is very inefficient. The cost of detection and maintenance is very high, and it cannot meet the needs of track safety early warning.

Contents of the invention

In view of the above-mentioned problems and demands in the prior art, the object of the present invention is to provide an imaging method and device for detecting near-surface defects of ballastless track based on ultrasonic array, so as to realize efficient, non-destructive and real-time detection of near-surface defects of ballastless track, and provide timely early warning and strong guarantee for the safe operation of high-speed rail.

To achieve the above object, the present invention adopts the following technical solutions:

An imaging method for detecting near-surface defects of a ballastless track based on an ultrasonic array, comprising the following steps:

a) Use the pulse signal to stimulate the receiving transducer to excite and transmit the ultrasonic signal with a frequency in the range of 1-2.5MHz to the ballastless track through the ultrasonic array probe. The ultrasonic array sensor adopts the spontaneous and self-receiving mode to obtain the diffuse field signal in the ballastless track and transmit the diffuse field signal to the computer. Specifically, it means: first use the pulse signal to stimulate the transmitting end of the receiving transducer to transmit an ultrasonic signal with a frequency in the range of 1-2.5MHz, and transmit it to the ballastless track from the transmitting end of the ultrasonic array sensor, and then receive it from the ultrasonic array sensor The terminal collects the diffuse field signal and transmits it to the pulse signal to excite the receiving end of the receiving transducer, and then the pulse signal excites the receiving end of the receiving transducer to transmit the received diffuse field signal to the computer;

b) The computer uses MATLAB software to passively extract the Green's function from the received signal, cross-correlate the diffusion field signal, reconstruct the Green's function between the array elements, obtain the undelayed response between the array elements, restore the early defect information submerged by noise, and then perform near-surface defect imaging on the ballastless track based on the total focus imaging algorithm.

One embodiment, said step b) specifically includes the following operations:

1) Data processing:

First of all, during the propagation of the ultrasonic signal inside the ballastless track, the ultrasonic signal interacts with the defect target, and after scattering and multiple reflections, an approximately uniform sound field will be formed. The all-source integral of the cross-spectrum is equal to the difference between the frequency-domain causal Green's function and the non-causal Green's function between r1 and r2:

G(r ₁ ,r ₂ ,ω)-G ^* (r ₁ ,r ₂ ,ω)=-2iω∫G(r ₁ ,r,ω)G(r ₂ ,r,ω)dv (1);

In formula (1), the left side of the equation is the difference between the causal and non-causal functions of the Green’s function in the frequency domain between the two receiving array elements r1 and r2, and the right side of the equation represents all the sound source integrals of the cross-correlation of the sound fields at the receiving array elements r1 and r2, specifically, G(r₁,r₂,ω) represents the causal Green's function between r1 and r2 in the frequency domain, * represents the conjugate complex number, G^*(r₁,r₂,ω) represents the non-causal Green's function between r1 and r2 in the frequency domain, i represents the imaginary unit, i²=-1, ω represents the angular frequency of the transmitted signal, r is any point in the diffuse field integration confined space V, which represents any defect target, which can be regarded as a noise source, G(r₁, r, ω) represents the frequency domain Green's function propagation formula between r and r1, G(r₂, r, ω) represent the frequency-domain Green's function propagation formula between r and r2, and dV represents the differential of V in the confined space;

Then, assuming that the intensity of the diffuse field noise source is uniformly distributed in space and independent of each other, the power spectral density |q(ω)| ₂ is also independent of the position of the noise source:

<q(r ₁ , ω)*q(r ₂ , ω)>=δ(r ₁ -r ₂ )|q(ω)| ² (2);

Formula (2) represents that the expected value of the cross power spectrum of signals at r1 and r2 in the frequency domain is equal to the power spectral density of the noise source between r1 and r2. Specifically, in formula (2), <q(r ₁ , ω)*q(r ₂ , ω)> represents the expected value of the cross power spectrum of signals at r1 and r2 in the frequency domain, q(r ₁ , ω) represents the field strength at r1, q(r ₂ , ω) represents the field strength at r2, and <> represents the statistical The average expected value, * represents the conjugate complex number, δ(r ₁ -r ₂ ) represents the cross power spectrum of the signal between r1 and r2 is independent of the position of the noise source;

At the same time, the cross-correlation between the sound fields at the positions of the two receiving elements r1 and r2 in the frequency domain is:

<P(r ₁ , ω)P*(r ₂ , ω)>=|q(ω)| ² ∫G(r ₁ ,r)G*(r ₂ ,r)dv (3);

In the formula (3), p(r ₁ , ω) represents the sound field at the receiving array element r1, p(r ₂ , ω) represents the sound field at the receiving array element r2 _, <p( _{r 1} ,ω)*p(r ₂ ,ω)> represents the cross-correlation function between the sound fields at the positions of the two receiving array elements r1 and r2, * represents the conjugate complex number; , r) represent r2 and r frequency-domain Green's function propagation formula;

Then by the formulas (1) and (3):

(G(r ₁ , r ₂ , ω)-G ^* (r ₁ , r ₂ , ω))|q(ω)| ² =-2iω<P(r ₁ ,ω)P ^* (r ₂ ,ω)>(4);

In formula (4), the left side of the equation is the Green’s function G(r ₁ , r ₂ , ω) and its corresponding time reversal, that is, the corresponding conjugate operation in the frequency domain. After multiplying these two terms by the power spectral density of random noise, the result is equal to the cross-correlation result of the two receiving array elements r1 and r2 in the diffusion field on the right side of the equation, and * represents the conjugate complex number;

Then, the time domain expression corresponding to formula (4) is obtained. The frequency domain 2iω corresponds to the time domain 2d/dt. According to the convolution theorem, the product in the frequency domain corresponds to the convolution in the time domain, thus:

In formula (5), G(r ₁ ,r ₂ ,t) represents the Green’s function between r1 and r2 in the time domain, G(r ₁ ,r ₂ ,-t) represents the time reversal of G(r ₁ ,r ₂ ,t), which corresponds to the conjugate operation in the frequency domain, * represents the convolution operation, Represents the cross-correlation operation, C _q (t) represents the autocorrelation result of the noise q(t) in the diffuse field, d/dt represents the derivative of t, p(r ₁ ,t) represents the sound field at the receiving element r1 in the time domain, and p(r ₂ ,t) represents the sound field at the receiving element r2 in the time domain;

The results of formulas (4) and (5) show that the sound fields p(r ₁ , t) and p(r ₂ ,t) of the receiving array elements in the diffuse field are cross-correlated and derived, and the result is equal to the Green's function response between the two receiving array elements. According to the principle of reciprocity of sound waves, the Green's function response between the two receiving array elements is symmetric on the time axis;

2) Full focus imaging:

Ultrasonic array sensors have mutually independent linear arrays, not only capable of dynamic focusing to achieve B-scan and S-scan, but also have a powerful full-matrix capture function, which provides a variety of imaging methods for off-line data processing;

The ultrasonic array sensor is composed of a number of ultrasonic sensors in an array. The ultrasonic sensor used to transmit signals is called a transmitting sensor, and the ultrasonic sensor used to receive signals is called a receiving sensor. Each transmitting sensor corresponds to a transmitting element, and each receiving sensor corresponds to a receiving element. Assume that there is a defect P(x, y) on the near surface of the ballastless track. Taking a pair of transmitting and receiving sensors in the ultrasonic array sensor as an example, set it to be the i-th array element to transmit and the j-th array element to receive. The time values from element to P(x,y) are different, that is, there is a phase difference. Set the coordinates of the i-th transmitting element as (x_Ti,y_Ti), the time of flight to P(x,y) is Ti, and the coordinates of the jth receiving element are set as (x_Rj,y_Rj), the flight time to P(x,y) is T_j, the total flight time T can be obtained_ijfor:

In formula (6), c is the constant longitudinal wave velocity of the measured ballastless track, and its value is 5900m/s;

Then the size of the array is an ultrasonic phased array of n*n transceiver combination pairs, and the size I _ij of a pixel value corresponding to any array element is:

Because the accumulation of pixel values at the defect is much higher than that of the normal area, it presents different colors during imaging, so that the defect area and the size and shape of the defect can be quickly and accurately identified.

An imaging device for detecting near-surface defects of a ballastless track based on an ultrasonic array, comprising a pulse signal to excite a receiving transducer, an ultrasonic array sensor, and a computer, wherein the pulse signal excites a transmitting port of the receiving transducer to be connected to a transmitting end signal in the ultrasonic array sensor, the pulse signal excites a receiving port of the receiving transducer to be connected to a receiving end signal in the ultrasonic array sensor, and the pulse signal excites a receiving port of the receiving transducer to be connected to a computer signal.

An embodiment also includes the mobile power unit, which includes the mobile power unit includes the mobile power shell and the power body located in the mobile power shell. The power supply body is connected to the power connection with the pulse signal stimulating the receiver and the ultrasonic array sensor. There are several openings that are adapted to the receiving transducer with a pulse signal to stimulate the receiver.

In a preferred solution, handles are provided at both ends of the housing of the mobile power supply.

An embodiment further includes a mobile communication control unit, and the mobile communication control unit is signal-connected with the pulse signal excitation and reception transducer.

In a preferred solution, the mobile communication control unit includes a communication unit and a control unit, the communication unit has a wireless connection function, the control unit is signal-connected to the communication unit, the control unit is signal-connected to the pulse signal excitation and receiving transducer, and the communication unit is wirelessly connected to the computer.

In a preferred solution, the mobile communication control unit is arranged on the top of the mobile power supply casing.

Compared with the prior art, the present invention has the following beneficial technical effects:

1) The ultrasonic array sensor adopted in the present invention has the advantages of high energy and small attenuation. Ultrasonic energy will hardly leak, and it is better incident into the interior of the material to be tested and interacts with defects to form a scattering signal that is beneficial to detection. In addition, each array element in the ultrasonic array sensor can be excited individually or in combination, which can realize focusing and deflection functions, and can better collect diffusion field signals, thereby enabling the present invention to use diffusion field signals to detect slab-type ballastless tracks. The processing and analysis of near-surface defects effectively improves the detection efficiency;

2) When the present invention processes and images the signal, the computer first uses the principle of cross-correlation calculation between the diffusion field signals of the receiving array elements to reconstruct the Green's function, obtains the undelayed response between the array elements, and extracts the early defect scattering information submerged by noise, and then uses the frequency-domain synthetic aperture focusing imaging technology and the time-domain full-focus imaging method to superimpose the signal amplitude at the defect, realize the visualization of the plate-type ballastless track defect, and improve the imaging accuracy and clarity. Fast speed and high precision;

In summary, the present invention can realize efficient, non-destructive, accurate and real-time detection of near-surface defects of ballastless track, can provide timely early warning and strong guarantee for the safe operation of high-speed rail, and can provide strong support for subsequent track maintenance work; therefore, compared with the prior art, the present invention has significant progress and application value.

Description of drawings

Fig. 1 is a schematic structural diagram of pulse signal excitation and receiving transducers, ultrasonic array sensors, wireless power supply units, and mobile communication control units in an imaging device based on ultrasonic array detection of near-surface defects in ballastless tracks provided by the embodiment;

2 is a structural schematic diagram of another perspective of the pulse signal excitation receiving transducer, ultrasonic array sensor, wireless power supply unit, and mobile communication control unit in the imaging device for detecting near-surface defects of ballastless track based on ultrasonic array provided by the embodiment;

FIG. 3 is a schematic structural diagram of an imaging device for detecting near-surface defects of ballastless tracks based on an ultrasonic array provided in an embodiment;

The state diagram of the imaging device described in the embodiment of FIG. 4 during operation;

Fig. 5 is the schematic diagram of omni-focus imaging in the embodiment;

Fig. 6 is the sectional view of detection track plate described in the embodiment;

Figure 7 is an all-focus imaging diagram of the track plate described in the embodiment;

The symbols in the figure are shown as follows: The symbols in the figure are shown as follows: 1. The pulse signal excites the receiving transducer; 2. The ultrasonic array sensor; 21. The signal transmitting/receiving port of the ultrasonic array sensor; 3. The mobile power supply unit;

Detailed ways

The technical solution of the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments.

Shown in conjunction with Fig. 1 to Fig. 4: a kind of imaging device based on ultrasonic array detection near-surface defect of ballastless track provided by the present invention, comprises pulse signal excitation receiving transducer 1, ultrasonic array sensor 2 and computer 6, described pulse signal excites the transmission port of receiving transducer 1 to be connected with the transmitting end signal in ultrasonic array sensor 2, the receiving port of described pulse signal excitation receiving transducer 1 is connected with the receiving end signal in ultrasonic array sensor 2, and the receiving port of described pulse signal excitation receiving transducer 1 is connected with computer signal.

The ultrasonic array sensor 2 can be a commercially available product. For example, a contact ultrasonic array sensor can be used, which has the advantages of high energy and small attenuation. The array elements can be excited individually or in combination to achieve focusing and deflection functions. In addition, it can be seen from the above that the ultrasonic array sensor 2 in this embodiment has a transmitting end and a receiving end, and has the function of self-sending and self-receiving, that is, among the ultrasonic sensors forming the ultrasonic array sensor 2, some are transmitting sensors for transmitting signals, and the rest are receiving sensors for receiving signals. Due to the different propagation speeds of ultrasonic waves in media with different densities, when they pass through the interface between two different media, phenomena such as reflection and scattering will occur. Because the ballastless track is a concrete component, it is a porous heterogeneous composite material composed of sand, cement, stones, etc., and the pulse signal excites the ultrasonic pulse wave emitted by the receiving transducer 1. When it encounters defects when propagating in concrete, it will be diffracted, and scattering and reflection will occur at the defect interface, resulting in a significant reduction in the amplitude of the sound wave energy when it reaches the receiving sensor. In this application, the use of ultrasonic array sensor 2 can not only be more efficient The collection of diffuse field signals can also superimpose the received diffuse field signals, so that the reflected diffuse field signal has the largest radiation energy in a certain direction, while the total radiation energy in other directions is small, so as to realize the focusing effect on the diffuse field signal, enhance the useful diffuse field signal, and suppress the interference signal, thus ensuring the accuracy of signal acquisition and detection results.

In addition, the imaging device described in this embodiment also includes a mobile power supply unit 3. The mobile power supply unit 3 includes a mobile power supply housing 31 and a power supply body (not shown) located in the mobile power supply housing 31. The power supply body is electrically connected to the pulse signal excitation and reception transducer 1 and the ultrasonic array sensor 2 respectively. Transmitting and receiving signals), the ultrasonic array sensor 2 is vertically arranged in the mobile power supply housing 31, and the signal transmitting/receiving port 21 of the ultrasonic array sensor 2 passes through the mobile power supply housing 31 and is located under the mobile power supply housing 31 (to facilitate the ultrasonic array sensor 2 to track detection). The mobile power supply unit provides mobile power for the remaining components of the imaging device. In addition, the pulse signal excitation and receiving transducer 1 and the ultrasonic array sensor 2 are collectively integrated in the mobile power supply housing 31, which effectively simplifies the overall structure and footprint of the device, thereby facilitating the overall movement of the device.

In addition, handles 312 are provided at both ends of the mobile power supply housing 31 , so as to facilitate the transportation and installation of the mobile power supply unit 3 and the device as a whole through the handles 312 .

In addition, the imaging device described in this embodiment further includes a mobile communication control unit 4 , and the mobile communication control unit 4 is signal-connected with the pulse signal excitation and reception transducer 1 . Specifically, described mobile communication control unit 4 comprises communication unit and control unit, and described communication unit has wireless connection function (for example, bluetooth connection function, can be connected with computer 6 bluetooth), to realize and carry out wireless communication connection with remote terminal (not shown in the figure), described control unit (can be commercially available single-chip microcomputer, controller etc.) is connected with communication unit signal, and described control unit excites receiving transducer 1 signal connection with pulse signal, stimulates the switch of receiving transducer 1 with pulse signal automatically.

In addition, the mobile communication control unit 4 is arranged on the top of the mobile power supply casing 31, and can be fixedly installed on the top of the mobile power supply casing 31, that is, the components in the imaging device except the computer 6 are all integrated in the mobile power supply casing 31, which simplifies the overall structure of the device.

The imaging method for detecting near-surface defects of ballastless track using the imaging device of the present invention is as follows:

a) Place the imaging device on the ballastless track 5 (as shown in Figure 4), adjust the position, first use the pulse signal to excite the transmitting end of the receiving transducer 1 to transmit an ultrasonic signal with a frequency in the range of 1 to 2.5 MHz, and transmit it from the transmitting end of the ultrasonic array sensor 2 to the inside of the ballastless track 5, then collect the diffusion field signal from the receiving end of the ultrasonic array sensor 2 and transmit it to the pulse signal to excite the receiving end of the receiving transducer 1, and then the pulse signal excites the receiving end of the receiving transducer 1 to transmit the received diffuse field signal to the computer 6 ;

b) The computer 6 uses MATLAB software to passively extract the Green’s function from the received signal, cross-correlate the diffusion field signal, reconstruct the Green’s function between the array elements, obtain the undelayed response between the array elements, restore the early defect information submerged by noise, and then perform the near-surface defect imaging of the ballastless track according to the total focus imaging algorithm, specifically including the following operations:

1) Data processing:

First of all, during the propagation of the ultrasonic signal inside the ballastless track, the ultrasonic signal interacts with the defect target (as shown in Figure 6). After scattering and multiple reflections, an approximately uniform sound field will be formed. This sound field is a diffuse field. As shown in step a), the diffuse field signal in the ballastless track can be obtained through the ultrasonic array sensor. Set any two receiving array elements in the ultrasonic array sensor as r1 and r2 respectively. The integral of all sound sources of the sound field cross-spectrum at r1 and r2 is equal to the difference between the frequency-domain causal Green's function and the non-causal Green's function between r1 and r2:

G(r ₁ ,r ₂ ,ω)-G ^* (r ₁ ,r ₂ ,ω)=-2iω∫G(r ₁ ,r,ω)G(r ₂ ,r,ω)dv (1);

In formula (1), the left side of the equation is the difference between the causal and non-causal functions of the Green’s function in the frequency domain between the two receiving array elements r1 and r2, and the right side of the equation represents all the sound source integrals of the cross-correlation of the sound fields at the receiving array elements r1 and r2, specifically, G(r₁,r₂,ω) represents the causal Green's function between r1 and r2 in the frequency domain, * represents the conjugate complex number, G^*(r₁,r₂,ω) represents the non-causal Green's function between r1 and r2 in the frequency domain, i represents the imaginary unit, i²=-1, ω represents the angular frequency of the transmitted signal, r is any point in the diffuse field integration confined space V, which represents any defect target, which can be regarded as a noise source, G(r₁, r, ω) represents the frequency domain Green's function propagation formula between r and r1, G(r₂, r, ω) represent the frequency-domain Green's function propagation formula between r and r2, and dV represents the differential of V in the confined space;

Then, assuming that the intensity of the diffuse field noise source is uniformly distributed in space and independent of each other, the power spectral density |q(ω)| ₂ is also independent of the position of the noise source:

<q(R ₁ , ω)*q(r ₂ , ω)>=δ(r ₁ -r ₂ )|q(ω)| ² (2);

Formula (2) represents that the expected value of the cross power spectrum of the signal at r1 and r2 in the frequency domain is equal to the power spectral density of the noise source between r1 and r2. Specifically, in formula (2), <q(r ₁ , ω)*q(r ₂ , ω)> represents the expected value of the cross power spectrum of the signal at r1 and r2 in the frequency domain, q(r ₁ , ω) represents the field strength at r1, q(r ₂ , ω) represents the field strength at r2, and <> represents the statistical The average expected value, * represents the conjugate complex number, δ(r ₁ -r ₂ ) represents the cross power spectrum of the signal between r1 and r2 is independent of the position of the noise source;

At the same time, the cross-correlation between the sound fields at the positions of the two receiving elements r1 and r2 in the frequency domain is:

<P(r ₁ ,ω)P ^* (r ₂ ,ω)>=|q(ω)| ² ∫G(r ₁ ,r)G ^* (r ₂ ,r)dv (3);

In the formula (3), p(r ₁ ,ω) represents the sound field at the receiving array element r1, p(r ₂ ,ω) represents the sound field at the receiving array element r2 _, <p( _{r 1} ,ω)*p(r ₂ ,ω)>represents the cross-correlation function between the sound fields at the positions of the two receiving array elements r1 and r2, * represents the conjugate complex number; , r) represent r2 and r frequency-domain Green's function propagation formula;

Then by the formulas (1) and (3):

(G(r ₁ , r ₂ , ω)-G ^* (r ₁ , r ₂ , ω))|q(ω)| ² =-2iω<P(r ₁ ,ω)P ^* (r ₂ ,ω)>(4);

In formula (4), the left side of the equation is the Green’s function G(r ₁ , r ₂ , ω) and its corresponding time reversal, that is, the corresponding conjugate operation in the frequency domain. After multiplying these two terms by the power spectral density of random noise, the result is equal to the cross-correlation result of the two receiving array elements r1 and r2 in the diffusion field on the right side of the equation, and * represents the conjugate complex number;

Then, the time domain expression corresponding to formula (4) is obtained. The frequency domain 2iω corresponds to the time domain 2d/dt. According to the convolution theorem, the product in the frequency domain corresponds to the convolution in the time domain, thus:

In formula (5), G(r ₁ ,r ₂ ,t) represents the Green’s function between r1 and r2 in the time domain, G(r ₁ ,r ₂ ,-t) represents the time reversal of G(r ₁ ,r ₂ ,t), which corresponds to the conjugation operation in the frequency domain, * represents the convolution operation, Represents the cross-correlation operation, Cq(t) represents the autocorrelation result of the noise q(t) in the diffuse field, d/dt represents the derivative of t, p(r ₁ ,t) represents the sound field at the receiving element r1 in the time domain, and p(r ₂ ,t) represents the sound field at the receiving element r2 in the time domain;

The results of formulas (4) and (5) show that the sound fields p(r ₁ , t) and p(r ₂ ,t) of the receiving array elements in the diffuse field are cross-correlated and derived, and the result is equal to the Green's function response between the two receiving array elements. According to the principle of reciprocity of sound waves, the Green's function response between the two receiving array elements is symmetric on the time axis;

2) Full focus imaging:

Ultrasonic array sensors have mutually independent linear arrays, not only capable of dynamic focusing to achieve B-scan and S-scan, but also have a powerful full-matrix capture function, which provides a variety of imaging methods for off-line data processing;

The ultrasonic array sensor is composed of a number of ultrasonic sensors in an array. The ultrasonic sensor used to transmit signals is called a transmitting sensor, and the ultrasonic sensor used to receive signals is called a receiving sensor. Each transmitting sensor corresponds to a transmitting element, and each receiving sensor corresponds to a receiving element. As shown in Figure 5, assuming that there is a defect P(x, y) on the near surface of the ballastless track. Taking a pair of transmitting and receiving sensors in the ultrasonic array sensor as an example, it is set that the i-th array element transmits and the j-th array element receives. The time value from each receiving signal element to P(x,y) is different, that is, there is a phase difference. Set the coordinates of the i-th transmitting element as (x_Ti,y_Ti), the time of flight to P(x,y) is Ti, and the coordinates of the jth receiving element are set as (x_Rj,y_Rj), the flight time to P(x,y) is T_j, the total flight time T can be obtained_ijfor:

In formula (6), c is the constant longitudinal wave velocity of the measured ballastless track, and its value is 5900m/s;

Then the size of the array is an ultrasonic phased array of n*n transceiver combination pairs, and the size I _ij of a pixel value corresponding to any array element is:

Since the accumulation of pixel values at the defect is much higher than that of the normal area, different colors are presented during imaging, so that the defect area and the size and shape of the defect can be quickly and accurately identified (as shown in Figure 7).

To sum up, it can be seen that the present invention can realize the detection and evaluation of near-surface defects of ballastless track, can solve the problem of low efficiency of the current track, and can greatly reduce manpower and work intensity, and provide defect visualization services, which can provide strong support for railway safety maintenance; in addition, the present invention is simple in structure, easy to use, and can greatly reduce maintenance costs; therefore, compared with the prior art, the present invention has significant progress and application value.

Finally, it is necessary to point out that: the above is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto, any changes or replacements that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be covered within the scope of protection of the present invention.

Claims (5)

1. An imaging method for detecting the near-surface defect of a ballastless track based on an ultrasonic array is characterized in that an imaging device comprises a pulse signal excitation receiving transducer, an ultrasonic array sensor and a computer, wherein a transmitting port of the pulse signal excitation receiving transducer is in signal connection with a transmitting end in the ultrasonic array sensor, a receiving port of the pulse signal excitation receiving transducer is in signal connection with a receiving end in the ultrasonic array sensor, and a receiving port of the pulse signal excitation receiving transducer is in signal connection with the computer; the ultrasonic array sensor consists of 24 ultrasonic sensors in total of 8 rows and 3 columns, and each array element in the ultrasonic array sensor is excited independently or excited in a combined way, so that the focusing and deflection functions can be realized;

the imaging method comprises the following steps:

a) Firstly, utilizing a pulse signal to excite a transmitting end of a receiving transducer to transmit an ultrasonic signal with the frequency in the range of 1-2.5 MHz, transmitting the ultrasonic signal to the inside of a ballastless track by the transmitting end of an ultrasonic array sensor, then acquiring a diffusion field signal by a receiving end of the ultrasonic array sensor and transmitting the diffusion field signal to a receiving end of the pulse signal to excite the receiving transducer, and then transmitting the received diffusion field signal to a computer by the receiving end of the pulse signal to excite the receiving transducer;

b) Passively extracting a green function from a received diffusion field signal by adopting MATLAB software, carrying out cross-correlation on the diffusion field signal, reconstructing the green function among array elements, acquiring undelayed response among the array elements, recovering early defect information submerged by noise, and then applying a frequency domain synthetic aperture focusing imaging technology to realize near-surface defect imaging of the ballastless track by adopting a time domain full focusing imaging method to enable signal amplitude of a defect to be overlapped; step b) comprises the following operations:

1) And (3) data processing:

firstly, because the ultrasonic signal is interacted with a defect target in the process of propagating in the ballastless track, an approximately uniform sound field is formed after scattering and multiple reflections, the sound field is a diffusion field, as shown in the step a), the diffusion field signal in the ballastless track is obtained through an ultrasonic array sensor, any two receiving array elements in the ultrasonic array sensor are set to be r1 and r2 respectively, the two receiving array elements r1 and r2 are in a closed curved surface space, and then the integral of all sound sources of the sound field cross spectrum at the positions of the receiving array elements r1 and r2 is equal to the difference between a frequency domain causal green function and a non-causal green function between the positions of r1 and r 2:

G(r ₁ ，r ₂ ，ω)-G ^* (r ₁ ，r ₂ ，ω)＝-2iω∫G(r ₁ ，r，ω)G(r ₂ ，r，ω)dv (1)；

in equation (1), the left side of the equation is the causal and non-causal of the frequency domain green function between the two receive elements r1 and r2The difference between the fruit functions, to the right of the equation, represents the integral of all sound sources of the cross-correlation of the sound fields at the receiving array elements r1 and r2, in particular, G (r ₁ ，r ₂ ω) represents a causal green's function between r1 and r2 in the frequency domain, represents the complex conjugate, G ^* (r ₁ ,r ₂ ω) represents a non-causal green's function between r1 and r2 in the frequency domain, i represents imaginary units, i ² = -1, ω represents the angular frequency of the transmitted signal, r is any point in the diffusion field integration enclosed space V, represents any defect target, is a noise source, G (r) ₁ R, ω) represents the frequency domain green function propagation formula between r and r1, G (r) ₂ R, ω) represents the frequency domain green's function propagation formula between r and r2, dV represents the derivative of the enclosed space V;

then, assuming that the diffuse field noise source intensities are spatially uniformly distributed and uncorrelated with each other, the power spectral density |q (ω) | ₂ While being independent of the location of the noise source:

<q(r ₁ ，ω)*q(r ₂ ，ω)>＝δ(r ₁ -r ₂ )|q(ω)| ² (2)；

equation (2) represents that the expected value of the cross-power spectrum of the signal at the frequency domains r1 and r2 is equal to the power spectral density of the noise source between r1 and r2, specifically, in equation (2),<q(r ₁ ，ω)*q(r ₂ ，ω)>representing the expected value, q (r ₁ ω) represents the field strength at r1, q (r) ₂ ω) represents the field strength at r2,<>represents the expected value of the statistical average, represents the complex conjugate, δ (r) ₁ -r ₂ ) The cross-power spectrum representing the signal between r1 and r2 is independent of the location of the noise source;

meanwhile, the cross correlation between sound fields at the positions of two receiving array elements r1 and r2 in the frequency domain is as follows:

<P(r ₁ ，ω)P ^* (r ₂ ，ω)>＝|q(ω)| ² ∫G(r ₁ ，r)G ^* (r ₂ ，r)dv (3)；

in the formula (3), p (r) ₁ ω) represents the sound field at the receiving element r1, p (r) ₂ ω) represents the sound field at the receive array element r2,<P(r ₁ ，ω)P ^* (r ₂ ，ω)>representing a cross-correlation function between sound fields at the positions of two receiving array elements r1 and r2, representing conjugate complex numbers; g (r) ₁ R) represents r1 and r frequency domain green's function propagation formula, G (r) ₂ R) represents a green's function propagation formula representing r2 and r frequency domain;

then, from formulas (1) and (3):

(G(r ₁ ，r ₂ ，ω)-G ^* (r ₁ ，r ₂ ，ω))|q(ω))|q(ω)| ² ＝-2iω<P(r ₁ ，ω)P ^* (r ₂ ，ω)> (4)；

in equation (4), the left side of the equation is the Green function G (r ₁ ,r ₂ ω) and its corresponding time reversal, corresponding to the conjugate operation in the frequency domain, multiplying these two terms by the power spectral density of the random noise, the result of which is equal to the cross-correlation result of the received array elements r1 and r2 of the two in the spreading field on the right side of the equation, representing the conjugate complex number;

then, a time domain expression corresponding to the formula (4) is obtained, the frequency domain 2iω corresponds to the time domain 2d/dt, and according to the convolution theorem, the product in the frequency domain corresponds to the convolution in the time domain, thereby obtaining:

in the formula (5), G (r) ₁ ,r ₂ T) represents the green's function between r1 and r2 in the time domain, G (r) ₁ ,r ₂ (t) represents G (r) ₁ ,r ₂ Time reversal of t) corresponding to the conjugate operation in the frequency domain, representing a convolution operation,representing cross-correlation operations, C _q (t) represents the autocorrelation result of the noise q (t) in the diffused field, d/dt represents the derivative of t, p (r) ₁ T) represents the sound field at the receiving array element r1 in the time domain, p (r) ₂ T) represents the sound field at the receiving array element r2 in the time domain;

the results of equations (4) and (5) indicate that the receive array in the diffusion fieldMeta-sound field p (r) ₁ T) and p (r) ₂ T) mutually correlating and deriving, the result of which is equal to the green function response between the two receiving array elements, and the green function response between the two receiving array elements has symmetry on the time axis according to the reciprocity principle of the sound wave;

2) Time domain full focus imaging:

the ultrasonic array sensor has mutually independent linear arrays, can realize B scanning and S scanning by dynamic focusing, has a full matrix capturing function, and provides various imaging methods for offline data processing;

the ultrasonic array sensor is formed by a plurality of ultrasonic sensors in an array, wherein the ultrasonic sensor for transmitting signals is called a transmitting sensor, the ultrasonic sensor for receiving signals is called a receiving sensor, each transmitting sensor corresponds to one transmitting array element, each receiving sensor corresponds to one receiving array element, the near surface of the ballastless track is provided with a defect P (x, y), taking a pair of transmitting-receiving sensor in the ultrasonic array sensor as an example, the pair of transmitting-receiving sensor in the ultrasonic array sensor is set to be the ith array element for transmitting, the jth array element for receiving, the time value from each transmitting array element of signals to P (x, y) of each receiving signal is different for all arrays in the ultrasonic array sensor, namely, a phase difference exists, and the coordinates of the ith transmitting array element are set to be (x, y) _Ti ,y _Ti ) The flight time to P (x, y) is Ti, and the coordinates of the j-th receiving array element are set as (x) _Rj ,y _Rj ) The time of flight to P (x, y) is T _j Obtaining the total flight time T _ij The method comprises the following steps:

in the formula (6), c is the constant longitudinal wave speed of the ballastless track to be tested, and the value of c is 5900m/s;

then the array size is an ultrasonic phased array of n x n transceiver combination pairs, and one pixel value corresponding to any one array element is I _ij The method comprises the following steps:

since the pixel value accumulation at the defect is much higher than that of the normal region, different colors are presented during imaging, thereby identifying the defect region and the size and shape of the defect.

2. The imaging method of claim 1, wherein: the imaging device further comprises a mobile power supply unit, the mobile power supply unit comprises a mobile power supply shell and a power supply body arranged in the mobile power supply shell, the power supply body is respectively electrically connected with the pulse signal excitation receiving transducer and the ultrasonic array sensor, the pulse signal excitation receiving transducer is arranged in the mobile power supply shell, a plurality of openings matched with the pulse signal excitation receiving transducer are uniformly formed in the side part of the mobile power supply shell, the ultrasonic array sensor is vertically arranged in the mobile power supply shell, and a signal transmitting/receiving port of the ultrasonic array sensor penetrates through the mobile power supply shell and is located below the mobile power supply shell.

3. The imaging method as claimed in claim 2, wherein: handles are arranged at two ends of the mobile power supply shell.

4. The imaging method of claim 1, wherein: the imaging device also comprises a mobile communication control unit which is in signal connection with the pulse signal excitation receiving transducer.

5. The imaging method as claimed in claim 4, wherein: the mobile communication control unit comprises a communication unit and a control unit, wherein the communication unit has a wireless connection function, the control unit is in signal connection with the communication unit, the control unit is in signal connection with the pulse signal excitation receiving transducer, and the communication unit is in wireless connection with a computer.