阅读说明：本技术 一种精准控制声场幅值与偏振方向的多波聚焦方法 (Multi-wave focusing method for accurately controlling amplitude and polarization direction of sound field ) 是由 林莉 马志远 解曦宇 金士杰 于 2019-11-13 设计创作，主要内容包括：一种精准控制声场幅值与偏振方向的多波聚焦方法,属于超声无损检测领域,该方法根据介质已知横波声速c<Sub>s</Sub>与纵波声速c<Sub>l</Sub>建立声场指向性函数来描述介质内任意一点的声场,对介质表面一阵元施加两个激励脉冲,两脉冲的时间差由目标焦点的声程与介质声速确定,两脉冲的激励幅值由声场指向性函数与目标焦点处的聚焦波幅值和偏振方向角γ确定。通过改变脉冲的激励时间差t<Sub>0</Sub>、幅值,可实现目标焦点处横波与纵波的同时聚焦,对介质内任意点声场幅值与偏振方向进行精准控制。本方法无需预模拟,单阵元即可多波聚焦,克服了利用单一波聚焦的缺点,实现较大扫描区域聚焦同时可进行多偏振方向扫描,有效提高信噪比和分辨率,对实际工程运用具有意义。(A multi-wave focusing method for accurately controlling the amplitude and polarization direction of a sound field belongs to the field of ultrasonic nondestructive detection, and the method is based on the known transverse wave sound velocity c of a medium s Velocity c of longitudinal wave l Establishing a sound field directivity function to describe a sound field of any point in a medium, applying two excitation pulses to an array element on the surface of the medium, wherein the time difference of the two pulses is determined by the sound path of a target focus and the sound velocity of the medium, and the excitation amplitudes of the two pulses are determined by the sound field directivity function, the focus wave amplitude at the target focus and the polarization direction angle gamma. By varying the excitation time difference t of the pulses 0 And the amplitude can realize simultaneous focusing of transverse waves and longitudinal waves at a target focus, and accurately control the amplitude and the polarization direction of a sound field at any point in a medium. The method can focus multiple waves by a single array element without pre-simulation, overcomes the defect of single wave focusing, realizes the focusing of a larger scanning area and can scan in multiple polarization directions simultaneously, and is effectiveThe signal-to-noise ratio and the resolution ratio are improved, and the method has significance for practical engineering application.)

1. A multi-wave focusing method for accurately controlling the amplitude and the polarization direction of a sound field is characterized by comprising the following steps:

(1) establishing a sound field directivity theoretical formula, wherein the width of an excitation array element is 2a, theta is an azimuth angle of a target focus relative to an origin, namely tan theta is x/y, and displacement potentials of longitudinal waves and transverse waves at a point (x, y) with a distance r from the center of the array element are as follows:

wherein

wherein ξ is k in equations (3) and (4), respectively_psin theta and k_ssinθ；

(2) Known medium of transverse and longitudinal wave sound velocity c_sAnd c_lExcitation time difference t of two pulses for array element transmission_o＝r/c_s-r/c_lExcitation signal transmitted by array elementThe first pulse in the sign is denoted as an S-wave and the second pulse as a P-wave;

the displacement function of the S-wave is then:

the displacement function of the P-wave is:

wherein D is the duration of a pulse, q_sIs the amplification of the S wave, q_pIs the amplification factor of the P wave;

(3) in a solid medium, the two waves respectively excite corresponding transverse waves and longitudinal waves, the transverse wave excited by the S wave is named as S-S wave, the longitudinal wave excited by the P wave is named as P-P wave, and according to the principle of time reversal method, the excitation time difference t is set₀Under the condition, the S-S wave and the P-P wave are focused and synthesized at the target focal point (x, y), and are derived according to the formula in the step (1):

the displacement function of the S-S wave at the focus point in the X direction is as follows:

the displacement function in the Y direction is:

the displacement function of the P-P wave in the X direction is as follows:

the displacement function in the Y direction is:

wherein, t_s＝r/c_s、t_p＝r/c_lAt the focus point, there is t₀+t_p＝t_sThe phase of the displacement function of the two pulses is the same;

the displacement function of the focused wave in the X direction is:

the displacement function in the Y direction is:

the polarization angle of the focused wave at the target focus is gamma, the unit is degree, and the tangent value of the polarization angle is the ratio of the displacement value in the X direction and the displacement value in the Y direction at the target focus; the focused wave amplitude at the target focus is the vector superposition sum of the X-direction displacement value and the Y-direction displacement value;

(4) based on the steps (2) and (3), according to the formulas (12) and (13), by continuously changing q_sAnd q is_pThe accurate control of the sound polarization direction and amplitude is realized for any point in the medium; according to the polarization direction and amplitude of the target focus, corresponding q is deduced according to a formula_sAnd q is_pObtaining the waveform of the array element stress excitation pulse; and transmitting the excitation pulse, wherein the phases of the S-S wave and the P-P wave at the target focus are consistent, so that focusing is realized.

Technical Field

The invention relates to a multi-wave focusing method for accurately controlling the amplitude and the polarization direction of a sound field, and belongs to the field of ultrasonic nondestructive testing.

Background

In ultrasonic detection, a focused acoustic beam is often used for detection, acoustic energy is more concentrated by using the focused acoustic beam, sound pressure on a central axis is enhanced, meanwhile, directivity of the acoustic beam can be improved, and the method is favorable for improving flaw detection sensitivity, resolution and signal to noise ratio. In conventional ultrasonic focus detection, it is common to use the velocity of a single type of wave packet to calculate the time delay for focus transmit and focus receive processing. This focusing approach only focuses one type of wave packet, resulting in a detection direction that is limited to the main propagation direction of that type of wave. Ultrasonic multi-wave focusing is an ultrasonic detection method which utilizes the time reversal self-adaptive focusing characteristic to simultaneously focus wave packets of a plurality of components. When sound waves are transmitted in a solid medium, different types of waves such as longitudinal waves and transverse waves exist, and when transverse waves and longitudinal waves in the medium are utilized for multi-wave focusing, not only can richer medium and defect information be obtained, but also large-range focusing can be realized. On the other hand, on the basis of multi-wave focusing, the sound polarization direction and amplitude at the target focus can be controlled randomly within a certain angle range by changing the excitation intensity of longitudinal waves and transverse waves, so that scanning in multiple polarization directions is formed, and multi-angle scanning and detection of the shape and direction of the defect are realized.

The method belongs to a novel idea, and at present, Zhang Bixing et al in the aspect of multi-wave focusing applies excitation displacement vibrating in a preset direction at a preset focusing point based on numerical simulation to obtain an excitation signal to be loaded on an excitation array element, loads the signal on the excitation array element for excitation, and realizes multi-wave focusing at the preset focusing point. The control of the polarization direction of the method needs to obtain the excitation signal through pre-simulation, and the pre-simulation needs to be carried out once every time the polarization direction is changed. If the medium in the experiment has deviation from the medium parameter in the numerical simulation, the method has errors when being used for carrying out the actual experiment. The method is characterized in that two transducers are used, the sound polarization direction at a preset focusing point is directly calculated by a vector synthesis method, then the time delay and excitation pulse of the two transducers are controlled to control the sound polarization direction, and compared with the former method, pre-simulation is not needed. However, this method is very sensitive to the position of the transducer, and when the position of the transducer deviates a little from the preset position, the sound polarization direction deviates from the preset direction or is no longer linear polarization, which may result in poor focusing effect.

Disclosure of Invention

The method aims to provide a multi-wave focusing method for accurately controlling the amplitude and the polarization direction of a sound field. The transverse waves and the longitudinal waves are utilized to carry out multi-wave focusing, so that the accurate control of the sound polarization direction and the amplitude of the focused waves is realized. Compared with the two methods, the method calculates the sound polarization direction through a formula from the principle angle and controls the sound polarization direction without pre-simulation; and the single-array element probe is used for detection, so that the influence of the position of the array element on the focusing effect is avoided. The method is realized by changing the excitation time difference t of the pulse₀And the excitation amplitude can realize simultaneous focusing of transverse waves and longitudinal waves at a target focus, and accurately control the amplitude and the polarization direction of a sound field at any point in a medium. The focusing scanning of a plurality of polarization directions can be realized by controlling the acoustic polarization, a good focusing effect can be obtained in a large scanning area, the signal-to-noise ratio and the resolution ratio are effectively improved, and the method has significance for practical engineering application.

A multi-wave focusing method for accurately controlling the amplitude and the polarization direction of a sound field comprises the following steps:

(1) establishing a sound field directivity theoretical formula, wherein the width of an excitation array element is 2a, theta is an azimuth angle of a target focus relative to an origin, namely tan theta is x/y, and displacement potentials of longitudinal waves and transverse waves at a point (x, y) with a distance r from the center of the array element are as follows:

wherein

k_pAnd k_sWave numbers, phi, of longitudinal and transverse waves, respectively_pAnd phi_sDisplacement potentials of longitudinal waves and transverse waves are respectively, P (theta) and S (theta) are directivity factors of the longitudinal waves and the transverse waves, the expressions are shown as (3) and (4), and a is half of the size of the transducer array element in the x direction;

wherein ξ is k in equations (3) and (4), respectively_psin theta and k_ssinθ；

(2) Known medium of transverse and longitudinal wave sound velocity c_sAnd c_lExcitation time difference t of two pulses for array element transmission_o＝r/c_s-r/c_lThe first pulse in the excitation signal transmitted by the array element is represented as S wave, and the second pulse is represented as P wave;

the displacement function of the S-wave is then:

the displacement function of the P-wave is:

wherein D is the duration of a pulse, q_sIs the amplification of the S wave, q_pIs the amplification factor of the P wave;

(3) in a solid medium, the two waves respectively excite corresponding transverse waves and longitudinal waves, the transverse wave excited by the S wave is named as S-S wave, the longitudinal wave excited by the P wave is named as P-P wave, and according to the principle of time reversal method, the excitation time difference t is set₀Under the condition, the S-S wave and the P-P wave are focused and synthesized at the target focal point (x, y), and are derived according to the formula in the step (1):

the displacement function of the S-S wave at the focus point in the X direction is as follows:

the displacement function in the Y direction is:

the displacement function of the P-P wave in the X direction is as follows:

the displacement function in the Y direction is:

wherein, t_s＝r/c_s、t_p＝r/c_lAt the focus point, there is t₀+t_p＝t_sThe phase of the displacement function of the two pulses is the same;

the displacement function of the focused wave in the X direction is:

the displacement function in the Y direction is:

the polarization angle of the focused wave at the target focus is gamma, the unit is degree, and the tangent value of the polarization angle is the ratio of the displacement value in the X direction and the displacement value in the Y direction at the target focus; the focused wave amplitude at the target focus is the vector superposition sum of the X-direction displacement value and the Y-direction displacement value;

based on the steps (2) and (3), according to the formulas (12) and (13), by continuously changing q_sAnd q is_pThe accurate control of the sound polarization direction and amplitude can be realized for any point in the medium; according to the polarization direction and amplitude of the target focus, corresponding q is deduced according to a formula_sAnd q is_pObtaining the waveform of the array element stress excitation pulse; and transmitting the excitation pulse, wherein the phases of the S-S wave and the P-P wave at the target focus are consistent, so that focusing is realized.

The invention has the advantages that: the multi-wave focusing method for accurately controlling the amplitude and the polarization direction of the sound field can arbitrarily control the amplitude and the sound polarization direction of the focused wave, and realize the focusing of transverse waves and longitudinal waves at any positions in a medium. The method has the advantages of concentrated sound energy, good directivity and high sensitivity and resolution in focusing detection, can realize focusing scanning of a plurality of polarization directions by controlling sound polarization, can obtain good focusing effect in a larger scanning area, and has practical significance in defect orientation identification in ultrasonic detection.

Drawings

FIG. 1 is a model diagram of numerical simulation in an embodiment.

Fig. 2 is a diagram of the excitation pulses that should be applied to the array elements.

Fig. 3 is a graph of displacement results received by the probe.

Fig. 4 is an acoustic polarization diagram according to the results.

FIG. 5 is a polar diagram of acoustic polarization under varying conditions.

Detailed Description

(1) Arranging a single array element transducer on the surface of a medium, wherein the width of the array element is 1mm, the center of the array element is taken as a (0,0) point, the position of a target focus is (-30mm ), a model diagram is shown in figure 1, and the sound path r from the center of the transducer to the target focus is calculated to be 42.426mm, the material medium is steel, c_s＝3140m/s、c_lAt 5770m/s, using t₀＝r/c_s-r/c_lCalculating the excitation time difference t of two pulses for array element transmission_oThe preset amplitude at the target focus is 0.00890 μm for 6.159 μ s, with a polarization direction of 110 °.

(2) Corresponding P (theta) and S (theta) are calculated from the sound field directivity function, and then q is derived from the given polarization angle and incidence angle according to equations (12) and (13) in the summary of the invention_s/q_pAt a set amplitude, q is calculated as 0.886_s＝0.828，q_p0.935. This results in the waveform of the excitation pulse that should be applied to the array element, as shown in figure 2.

(3) So that the array element excites the pulse, the order of theoretically receiving the wave packet at the target focus is: two probes are arranged at a target focus and are respectively used for receiving displacement in the X direction and the Y direction, the overall waveform received by the probes is as shown in figure 3, the overall waveform conforms to a theoretical result, polarization analysis is carried out on the focused wave, a polarization pole diagram is shown in figure 4, the polarization angle is 109.68 degrees, and the amplitude of the focused wave is 0.00885 microns. Compared with the theoretical value, the polarization angle error is 0.3%, and the polarization amplitude error is 0.6%.

(4) Modifying the polarization angle to 120 DEG, and recalculating q_s＝0.883，q_p0.573. The re-simulated polarization diagram is shown in FIG. 5, the polarization angle is 119.78 °, the focused wave amplitude is 0.00886 μm, and the error is within the allowable range.

(5) According to the above steps, by continuously changing q_sAnd q is_pThe value of (3) can be used for scanning a focus point in multiple polarization directions, the calculated S wave and P wave excitation conditions are input into an array element, S-S waves and P-P waves can be received at a target focus point, the two waves are focused at the moment, the particle polarization direction and the amplitude of the focused waves are the same as theoretical values, double control of the sound polarization direction and the amplitude is realized, and the value has a certain value for practical application of ultrasonic focusing detection.