2.Phased Array Probe Characteristics 相控陣探頭特性

Contemporary phased array probes for industrial NDT applications are

typically constructed around piezocomposite materials, which are made of

many tiny,thin rods of piezocomposite embedded in a polymer matrix. While

they can be more challenging to manufacture, composite probes typically

offer a 10dB to 30dB sensitivity advantage over piezoceramic probes of

otherwise similar design.

Segmented metal plating is used to divide the composite strip into a number of electrically separate elements that can be pulsed individually. This segmented element is then incorporated into a probe assembly that include a protective matching layer, a backing, cable connections and a housing(see Figure 2-17).

用于工業(yè)NDT的現(xiàn)代相控陣探頭一般由壓電復(fù)合材料構(gòu)建，具體地說就是許多細(xì)小、極薄的壓電陶瓷棒被嵌在聚合物矩陣中。雖然制造這種探頭會(huì)復(fù)雜一些，但是與在其它方面設(shè)計(jì)相似的壓電陶瓷探頭相比，這種復(fù)合材料探頭在一般情況下可提供的靈敏度會(huì)高出10dB 到 30dB。

分成小段的金屬鍍層用于將條狀的復(fù)合材料分割成若干可單獨(dú)接收電子脈沖激勵(lì)的晶片個(gè)體。這個(gè)被分割成小段的晶片被裝入探頭組合件中。探頭組合件包含一個(gè)保護(hù)性匹配層、一個(gè)背襯層、線纜連接器以及一個(gè)外殼（參見圖2-17）。

2.7 Beam Focusing with Phased Array Probes 相控陣探頭的聲束聚焦
Sound beams can be focused like light rays, creating an hourglass-shaped beam that tapers to a minimum diameter at a focal point and then expands once past that focal point(See figure 2-26).
聲束可以像光線一樣聚焦，生成一個(gè)如沙漏形狀的聲束，這條成錐形的聲束在聚焦點(diǎn)處直徑達(dá)到最小，然后從這個(gè)焦點(diǎn)處再次擴(kuò)散向更遠(yuǎn)處傳播（見圖2-26）。

2.4 Phased Array Wedges相控陣楔塊
Phased array probe assemblies usually include a plastic wedge. Wedge are used in both shear wave and longitudinal wave applications, including straight beam linear scans. These wedges perform basically the same function in phased array systems as in conventional single element flaw detection, coupling sound energy from the probe to the test piece in such a way that it mode converts and/or refracts at a desired angle in accordance with snell’s law.
相控陣探頭的組合件通常包括一個(gè)塑料楔塊。楔塊用于利用橫波和縱波的檢測應(yīng)用中，其中包括垂直聲波的線性掃查。這些用于與相控陣系統(tǒng)的楔塊與常規(guī)單晶片缺陷探測中使用的楔塊基本上具有相同的功能。用戶需要耦合從探頭到測試試樣之間的聲能，以根據(jù)斯涅爾定律在適當(dāng)?shù)慕嵌任恢檬孤暡ㄟM(jìn)行模式轉(zhuǎn)化和/或折射。
Zero-degree wedges are basically flat plastic blocks that are used for coupling sound energy and for protecting the probe face from scratch or abrasion in straight linear scans and low-angle longitudinal wave angled scans(see Figure 2-20).
零度楔塊是一些平面塑料塊，在垂直線性掃查和小角度縱波掃查中，起到耦合聲能作用和保護(hù)探頭面不受刮擦或磨蝕（見圖2-20）。

圖2-17 相控陣探頭橫截面圖

只要平時(shí)有時(shí)間 就會(huì)摘抄一些和大家分享。

相控陣探頭根據(jù)以下基本參數(shù)從功能上被分成不同的類型：
類型：大多數(shù)相控陣探頭屬于角度聲束類型，與塑料楔塊、平直塑料靴（即零度楔塊）或延遲塊一起使用。此外，還有直接接觸式探頭和水浸式探頭。
頻率：超聲缺陷探測一般使用2MHz到10MHz之間的頻率，因此大多數(shù)相控陣探頭都屬于這個(gè)頻率范圍。此外，還有頻率更低或更高的探頭。使用常規(guī)探頭，穿透性會(huì)隨著頻率的降低而增加，而分辨率及聚焦銳利度會(huì)隨著頻率的升高而增強(qiáng)。
晶片數(shù)量：最常用的相控陣探頭一般有16到128個(gè)晶片，有些探頭的晶片多達(dá)256個(gè)。隨著晶片數(shù)量的增多，聲波聚焦與電子偏轉(zhuǎn)的能力會(huì)增強(qiáng)，同時(shí)檢測所覆蓋的區(qū)域也會(huì)擴(kuò)大，然而探頭和儀器的成本費(fèi)用也會(huì)增加。每個(gè)晶片被單獨(dú)脈沖激勵(lì)，以創(chuàng)建希望得到的波前。因此這些晶片排列方向的維度通常被稱為主動(dòng)方向或偏轉(zhuǎn)方向。

晶片尺寸：隨著晶片寬度的減少，聲束電子偏轉(zhuǎn)的性能會(huì)增加，但是要覆蓋大區(qū)域就需要有更多的晶片，因此費(fèi)用也會(huì)增加

相控陣探頭的維度參數(shù)一般被定義如下(見圖2-18)：

圖2-20 零度楔塊

2.4 Phased Pulsing 相控陣脈沖
Whenever waves originating from two or more sources interact with each other, there are phasing effects leading to an increase or decrease in wave energy at the point of combination. When elastic waves of the same frequncy meet in such a way that their displacements are precisely synchronized(in phase, or zero-degree phase angle), the wave energies add together to create a large amplitude wave(see Figure 2-21a).
如果來自兩個(gè)或兩個(gè)以上聲源的聲波交織在一起，就會(huì)在聲波相交處產(chǎn)生聲能增強(qiáng)或削弱的相位效果。相同頻率的彈性波相交時(shí)，如果他們的位移完全相同（即相位重合，或相位角為零度），則它們的聲能會(huì)累加在一起，創(chuàng)建一個(gè)波幅更大的聲波（見圖2-21a）。
If they meet in such a way that their displacements are exactly opposite(180 degree out of phase),then the wave energies cancel each other(see Figure 2-21c).At phase angles between 0 degree and 180 degree, there is a range of intermediate stages between full addition and full cancellation(see 2-21b).

如果他們的相位正好相反（相位完全不重合，或相位角為180度），則他們的聲能會(huì)互相抵消（見圖2-21c）。如果他們的相位角處于0度到180度之間，則聲能的變化處于完全累加與完全抵消的中間狀態(tài)（見圖2-21b）。

By varying the timing of the waves from a large number of sources,it’s possible to use these effects to both steer and focus the resulting combined wavefront. This is an essential principle behind phased array testing.
通過使每個(gè)聲波從它們各自聲源發(fā)出的時(shí)間各不相同，可以利用上述的相位效果，對(duì)所產(chǎn)生的組合波前進(jìn)行偏轉(zhuǎn)和聚焦。這就是相控陣檢測的基本原理。

如果使用常規(guī)探頭，這種建設(shè)性和抵消性干涉效果會(huì)創(chuàng)建近場和遠(yuǎn)場區(qū)域，以及這些區(qū)域中逐漸變化的聲壓。此外，常規(guī)角度聲束探頭使用單個(gè)晶片向楔塊中發(fā)射一束聲波。根據(jù)楔塊的形狀，這束聲波波前上的各個(gè)點(diǎn)會(huì)有不同的時(shí)間延遲。這些延遲屬于機(jī)械延遲，而相控陣檢測中所用的延遲為電子延遲。根據(jù)惠更斯原理，當(dāng)波前觸到底面時(shí)，會(huì)表現(xiàn)為一系列點(diǎn)源。根據(jù)斯涅爾定律，從這些點(diǎn)發(fā)出的理論上的球面波會(huì)互相作用，形成一個(gè)帶有一定角度的單一聲波。

在相控陣檢測中，通過控制相位，產(chǎn)生可預(yù)見性的聲能增強(qiáng)效果和抵消效果，可形成和偏轉(zhuǎn)超聲波束。以不同時(shí)間延遲觸發(fā)單個(gè)晶片或晶片組會(huì)生成一系列的點(diǎn)源聲波，而這些聲波會(huì)匯集成以所選角度傳播的單一波前（見圖2-22）。以電子方式得到的延遲與借助常規(guī)楔塊得到的機(jī)械延遲相似，不同的是，在相控陣檢測中還可以通過改變延遲的方式進(jìn)一步控制聲束的偏轉(zhuǎn)。通過積極干涉，這種組合波的波幅會(huì)比形成組合波的每個(gè)單個(gè)波的波幅高出很多。同理，不同的延遲被應(yīng)用于陣列的每個(gè)晶片所接收的回波。這些回波累加在一起，得到一個(gè)具有單一角度/或單一聚焦值得統(tǒng)一聲束。除了可以改變主要波前的方向，這種將多個(gè)單聲束組合在一起的方式還可以使聲束在近場內(nèi)的任何一點(diǎn)聚焦。


In conventional transducer, constructive and destructive interference effects create the near-field and far field zones and the various pressure gradients therein. Additionally, a conventional angle beam transducer uses a single element to launch a wave in a wedge. Points on this wavefront experience different delay intervals due to the shape of the wedge. These are mechanical delays, as opposed to the electronic delays employed in phased array testing. When the wavefront hits the bottom surface it can be visualized through Huygens’ principle as a series of point sources. The theoretically spherical waves from each of these points interact to form a single wave at an angle determined by Sell’s law.

In phased array testing, the predictable reinforcement and cancellation effects caused by phasing are used to shape and steer the ultrasonic beam. Pulsing individual elements or groups of elements with different delays creates a series of point source waves that combine into a single wavefront that travels at a selected angle (see Figure 2-22).This electronic effect is similar to the mechanical delay generated by a conventional wedge, but it can be further steered by changing the pattern of delays. Through constructive interference, the amplitude of this combined wave can be considerably greater than the amplitude of any one of the individual waves that produce it. Similarly, variable delays are applied to the echoes received by each element of the array. The echoes are summed to represent a single angular and/or focal component of the total beam. In addition to altering the direction of the primary wavefront, this combination of individual beam components allows beam focusing at any point in the near field.

通常將晶片分組進(jìn)行脈沖發(fā)射，每組包括4到32個(gè)數(shù)量不等的晶片。通過增加孔徑的方法，可以減少不希望發(fā)生的聲束擴(kuò)散，完成更清晰的聚焦，從而有效地提高靈敏度。

回波被不同的晶片或晶片組接收，并在必要時(shí)進(jìn)行時(shí)間偏移計(jì)算，以補(bǔ)償這些變化著的楔塊延遲，然后再進(jìn)行匯總。與常規(guī)單晶探頭實(shí)際上將觸及被測區(qū)域的所有聲束的效果融合在一起不同的是，相控陣探頭可以根據(jù)回波到達(dá)每個(gè)晶片的時(shí)間和波幅，在空間上對(duì)返回的波前進(jìn)行分揀。儀器軟件對(duì)聲束進(jìn)行處理時(shí)，會(huì)將每個(gè)返回的聚焦法則認(rèn)作是以某個(gè)具體角度、從線性聲程的某個(gè)點(diǎn)、和/或從某個(gè)具體的聚焦深度反射回來的聲束。然后，回波信息可以幾種形式的任何一種顯示出來。

被稱為聚焦法則計(jì)算器的軟件根據(jù)探頭和楔塊的特性以及被測材料的幾何形狀和聲學(xué)屬性，確定向每組晶片發(fā)射脈沖的特定延遲時(shí)間，以通過聲波的互相作用生成想要的聲束形狀。

Elements are usually pulsed in groups of 4 to 32 in order to improve effective sensitivity by increasing aperture, which reduces unwanted beam spreading and enables sharper focusing.

The returning echoes are received by various elements or groups of elements and time-shifted as necessary to compensate for varying wedge delays and then summed. Unlike a conventional single element transducer, which effectively merges the effect of all beam components that strike its area, a phased array probe can spatially sort the returning wavefront according to the arrive time and amplitude at each element.When processed by instrument sofeware,each returned focal law represents the reflection from a particular angular component of the beam, a particular point along a linear path, and/or a reflection from a particular focal depth. The echo information can then be displayed in any of several standard format.

Software known as a focal law calculator establishs specific delay times for firing each group of elements in order to generate the desired beam shape through wave interaction,taking into account probe and wedge characteristics as well as the geometry and acoustical properties of the test material.  

The beam can be dynamically steered through various angles,focal distances, and focal spot sizes in such a way that a single probe assembly is capable of examining the test material across a range of different perspectives.

聲束可以不同的角度、不同的焦距以及不同的焦點(diǎn)大小被動(dòng)態(tài)偏轉(zhuǎn)，這樣做所要達(dá)到的目標(biāo)是單個(gè)探頭組合件可以通過一系列不同視角檢測到整個(gè)被測材料。

任何超聲檢測系統(tǒng)的響應(yīng)都取決于對(duì)以下因素的綜合考慮：所用的探頭、所用的儀器類型及其設(shè)置、被測材料的聲學(xué)特性。相控陣探頭所做的響應(yīng)與任何其它NDT超聲探頭所做出的相應(yīng)一樣，即與探頭設(shè)計(jì)參數(shù)（如頻率、大小、機(jī)械阻尼）相關(guān)，也與用于驅(qū)動(dòng)探頭的激勵(lì)脈沖的參數(shù)相關(guān)。

有四個(gè)重要的探頭參數(shù)會(huì)相互作用，對(duì)探頭的性能產(chǎn)生影響。
頻率：如前面章節(jié)中所述，檢測頻率對(duì)近場長度和聲束擴(kuò)展有極大的影響。實(shí)際上，與低頻率相比，高頻率會(huì)產(chǎn)生更好的信噪比，因?yàn)楦哳l率可以提供銳利度更高的聚焦，從而得到更集中、更優(yōu)化的焦點(diǎn)。同時(shí)，當(dāng)頻率提高時(shí)，聲波在任何檢測材料中的穿透能力都會(huì)降低，因?yàn)椴牧系乃p性會(huì)隨著頻率的增高而增強(qiáng)。 在檢測應(yīng)用中如果聲程很長，或測試材料具有極高的衰減性或散射性，就需要使用較低的頻率。一般來說，我們所提供的工業(yè)相控陣探頭的頻率在1 MHz 到 15 MHz 之間。

晶片大小：隨著陣列中單個(gè)晶片尺寸的減小，聲束電子偏轉(zhuǎn)能力會(huì)得到增強(qiáng)。商業(yè)探頭的實(shí)際最小晶片尺寸一般接近0.2mm。然而，如果晶片尺寸大于二分之一波長，則會(huì)產(chǎn)生不希望出現(xiàn)的較強(qiáng)的旁瓣。

晶片數(shù)量：隨著陣列中單個(gè)晶片數(shù)量的增加，探頭所能覆蓋的物理范圍、靈敏度、聚焦能力以及電子偏轉(zhuǎn)能力也相應(yīng)增加。然而，使用較大的陣列經(jīng)常會(huì)伴隨出現(xiàn)系統(tǒng)的復(fù)雜性及費(fèi)用較高等問題。

晶片間距和孔徑：晶片間距是指兩個(gè)相鄰單個(gè)晶片之間的距離；孔徑是指得到脈沖激勵(lì)的一組單個(gè)晶片的總和尺寸（虛擬孔徑）。要優(yōu)化電子偏轉(zhuǎn)的范圍，需要較小的晶片間距。要得到最佳靈敏度、最小的聲束擴(kuò)散、較強(qiáng)的聚焦，則需要較大的孔徑。當(dāng)今的相控陣儀器通常支持孔徑最多為16個(gè)晶片的聚焦法則。更為高級(jí)的系統(tǒng)最多會(huì)支持含有32個(gè)晶片或者64個(gè)晶片的孔徑。