2.Phased Array Probe Characteristics 相控阵探头特性

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).

用于工业NDT的现代相控阵探头一般由压电复合材料构建，具体地说就是许多细小、极薄的压电陶瓷棒被嵌在聚合物矩阵中。虽然制造这种探头会复杂一些，但是与在其它方面设计相似的压电陶瓷探头相比，这种复合材料探头在一般情况下可提供的灵敏度会高出10dB 到 30dB。

分成小段的金属镀层用于将条状的复合材料分割成若干可单独接收电子脉冲激励的晶片个体。这个被分割成小段的晶片被装入探头组合件中。探头组合件包含一个保护性匹配层、一个背衬层、线缆连接器以及一个外壳（参见图2-17）。

2.7 Beam Focusing with Phased Array Probes 相控阵探头的声束聚焦
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).
声束可以像光线一样聚焦，生成一个如沙漏形状的声束，这条成锥形的声束在聚焦点处直径达到最小，然后从这个焦点处再次扩散向更远处传播（见图2-26）。

2.4 Phased Array Wedges相控阵楔块
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.
相控阵探头的组合件通常包括一个塑料楔块。楔块用于利用横波和纵波的检测应用中，其中包括垂直声波的线性扫查。这些用于与相控阵系统的楔块与常规单晶片缺陷探测中使用的楔块基本上具有相同的功能。用户需要耦合从探头到测试试样之间的声能，以根据斯涅尔定律在适当的角度位置使声波进行模式转化和/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).
零度楔块是一些平面塑料块，在垂直线性扫查和小角度纵波扫查中，起到耦合声能作用和保护探头面不受刮擦或磨蚀（见图2-20）。

图2-17 相控阵探头横截面图

只要平时有时间 就会摘抄一些和大家分享。

相控阵探头根据以下基本参数从功能上被分成不同的类型：
类型：大多数相控阵探头属于角度声束类型，与塑料楔块、平直塑料靴（即零度楔块）或延迟块一起使用。此外，还有直接接触式探头和水浸式探头。
频率：超声缺陷探测一般使用2MHz到10MHz之间的频率，因此大多数相控阵探头都属于这个频率范围。此外，还有频率更低或更高的探头。使用常规探头，穿透性会随着频率的降低而增加，而分辨率及聚焦锐利度会随着频率的升高而增强。
晶片数量：最常用的相控阵探头一般有16到128个晶片，有些探头的晶片多达256个。随着晶片数量的增多，声波聚焦与电子偏转的能力会增强，同时检测所覆盖的区域也会扩大，然而探头和仪器的成本费用也会增加。每个晶片被单独脉冲激励，以创建希望得到的波前。因此这些晶片排列方向的维度通常被称为主动方向或偏转方向。

晶片尺寸：随着晶片宽度的减少，声束电子偏转的性能会增加，但是要覆盖大区域就需要有更多的晶片，因此费用也会增加

相控阵探头的维度参数一般被定义如下(见图2-18)：

图2-20 零度楔块

2.4 Phased Pulsing 相控阵脉冲
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).
如果来自两个或两个以上声源的声波交织在一起，就会在声波相交处产生声能增强或削弱的相位效果。相同频率的弹性波相交时，如果他们的位移完全相同（即相位重合，或相位角为零度），则它们的声能会累加在一起，创建一个波幅更大的声波（见图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).

如果他们的相位正好相反（相位完全不重合，或相位角为180度），则他们的声能会互相抵消（见图2-21c）。如果他们的相位角处于0度到180度之间，则声能的变化处于完全累加与完全抵消的中间状态（见图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.
通过使每个声波从它们各自声源发出的时间各不相同，可以利用上述的相位效果，对所产生的组合波前进行偏转和聚焦。这就是相控阵检测的基本原理。

如果使用常规探头，这种建设性和抵消性干涉效果会创建近场和远场区域，以及这些区域中逐渐变化的声压。此外，常规角度声束探头使用单个晶片向楔块中发射一束声波。根据楔块的形状，这束声波波前上的各个点会有不同的时间延迟。这些延迟属于机械延迟，而相控阵检测中所用的延迟为电子延迟。根据惠更斯原理，当波前触到底面时，会表现为一系列点源。根据斯涅尔定律，从这些点发出的理论上的球面波会互相作用，形成一个带有一定角度的单一声波。

在相控阵检测中，通过控制相位，产生可预见性的声能增强效果和抵消效果，可形成和偏转超声波束。以不同时间延迟触发单个晶片或晶片组会生成一系列的点源声波，而这些声波会汇集成以所选角度传播的单一波前（见图2-22）。以电子方式得到的延迟与借助常规楔块得到的机械延迟相似，不同的是，在相控阵检测中还可以通过改变延迟的方式进一步控制声束的偏转。通过积极干涉，这种组合波的波幅会比形成组合波的每个单个波的波幅高出很多。同理，不同的延迟被应用于阵列的每个晶片所接收的回波。这些回波累加在一起，得到一个具有单一角度/或单一聚焦值得统一声束。除了可以改变主要波前的方向，这种将多个单声束组合在一起的方式还可以使声束在近场内的任何一点聚焦。


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.

通常将晶片分组进行脉冲发射，每组包括4到32个数量不等的晶片。通过增加孔径的方法，可以减少不希望发生的声束扩散，完成更清晰的聚焦，从而有效地提高灵敏度。

回波被不同的晶片或晶片组接收，并在必要时进行时间偏移计算，以补偿这些变化着的楔块延迟，然后再进行汇总。与常规单晶探头实际上将触及被测区域的所有声束的效果融合在一起不同的是，相控阵探头可以根据回波到达每个晶片的时间和波幅，在空间上对返回的波前进行分拣。仪器软件对声束进行处理时，会将每个返回的聚焦法则认作是以某个具体角度、从线性声程的某个点、和/或从某个具体的聚焦深度反射回来的声束。然后，回波信息可以几种形式的任何一种显示出来。

被称为聚焦法则计算器的软件根据探头和楔块的特性以及被测材料的几何形状和声学属性，确定向每组晶片发射脉冲的特定延迟时间，以通过声波的互相作用生成想要的声束形状。

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.

声束可以不同的角度、不同的焦距以及不同的焦点大小被动态偏转，这样做所要达到的目标是单个探头组合件可以通过一系列不同视角检测到整个被测材料。

任何超声检测系统的响应都取决于对以下因素的综合考虑：所用的探头、所用的仪器类型及其设置、被测材料的声学特性。相控阵探头所做的响应与任何其它NDT超声探头所做出的相应一样，即与探头设计参数（如频率、大小、机械阻尼）相关，也与用于驱动探头的激励脉冲的参数相关。

有四个重要的探头参数会相互作用，对探头的性能产生影响。
频率：如前面章节中所述，检测频率对近场长度和声束扩展有极大的影响。实际上，与低频率相比，高频率会产生更好的信噪比，因为高频率可以提供锐利度更高的聚焦，从而得到更集中、更优化的焦点。同时，当频率提高时，声波在任何检测材料中的穿透能力都会降低，因为材料的衰减性会随着频率的增高而增强。 在检测应用中如果声程很长，或测试材料具有极高的衰减性或散射性，就需要使用较低的频率。一般来说，我们所提供的工业相控阵探头的频率在1 MHz 到 15 MHz 之间。

晶片大小：随着阵列中单个晶片尺寸的减小，声束电子偏转能力会得到增强。商业探头的实际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长，则会产生不希望出现的较强的旁瓣。

晶片数量：随着阵列中单个晶片数量的增加，探头所能覆盖的物理范围、灵敏度、聚焦能力以及电子偏转能力也相应增加。然而，使用较大的阵列经常会伴随出现系统的复杂性及费用较高等问题。

晶片间距和孔径：晶片间距是指两个相邻单个晶片之间的距离；孔径是指得到脉冲激励的一组单个晶片的总和尺寸（虚拟孔径）。要优化电子偏转的范围，需要较小的晶片间距。要得到最佳灵敏度、最小的声束扩散、较强的聚焦，则需要较大的孔径。当今的相控阵仪器通常支持孔径最多为16个晶片的聚焦法则。更为高级的系统最多会支持含有32个晶片或者64个晶片的孔径。