02 - 2d pa principles 05b3 f

8/4/2019 02 - 2D PA Principles 05B3 F

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Phased array technology is the ability to modifyelectronicallythe acoustic probe characteristics

Probe modifications are performed by introducingtime shifts in the signals sent to (pulse) andreceived from (echo) individual elements of anarray probe

Any UT technique for flaw detection and sizingcan be applied using phased array probes

What are Phased Arrays?


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High speed electronic scanning without movingparts

Improved inspection capabilities through software

control of beam characteristics

Inspection with multiple angles with a singleelectronically controlled probe

Many configurations: P/E, T/R, TOFD, Tandem

Greater flexibility for inspection of complex

geometries

Optimized focusing

Optimized beam angle

Why Phased Array?


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Phased Array Terminology

Active Aperture

Apodisation

Aperture

Azimuthal Scan

Beam forming

Beam Steering

Delay Laws

Focal Laws

Linear Scan

Phased Array

Sectorial Scan

Steering Aperture

Passive Aperture

Virtual Probe


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How Phased Arrays Work

Probe Design Parameters

Electronics (probe control and datacollection

Beam Forming


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A linear array (1D) isa long conventional

probe

The probe is cut intomany small elements

that are individuallyexcited

Design Parameters of

Phased Array Probes


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p g

e

H

A

PROBE PARAMETERSFrequency (f)Total number of elements in array (n)Total aperture in steering or active direction (A)Height or Elevation, aperture in mechanical or passive direction (H)Width of an individual element (e)Pitch, center-to-center distance between two successive elements (p)

Design Parameters of

Phased Array Probes


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PA Probes are based on the Composite Technology.

Signal to noise ratio obtained from composite transducers is typically10-30dB greater than obtained from piezo-ceramic probes.

Piezo-composite transducer is made by using thin rods ofceramic material embedded into a polymer.

Thin rods of ceramics

Piezzo composite

polymer

Probe Manufacturing

Composite Technology


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Piezzo composite

Elements

(thin layer of metal)

A metallic layer is deposited on the piezo-composite.

This metallic layer conforms to the element pattern and provideselectrical contacts for each element.

Probe Manufacturing

Composite Technology


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Acoustic matching

Piezo composite

Backing product

Cable up to 128 coaxialwires

The probe construction is similar to that of a conventional probe

Probe Manufacturing: Casing


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Numerous linear probe designs The probe can mechanically be focused in the passive

axis

Phased Array and transducer technology allows formany shapes - flat, curved, conical, elliptical, etc.

15L128E25.6-6

5L16E16-10

10L16E5-6

5L128E96-10C40

5L128E128-12F36

LINEAR 1D


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Ultrasonic phased arrays consist of a series ofindividual elements, each with its ownconnector, time delay circuit and A/Dconverter

Elements are acoustically insulated from eachother

Elements are pulsed in groups with pre-calculated time delays for each element

- i.e. phasing

Design Parameters Of

Phased Array Probes


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Wave front

Time

Single Trigger Pulse

Phased Array Probes


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Inclined Beam


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Focused Beam


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Linear Probes Electronic Scanning


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Linear Probes Sectorial Scanning


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The elements are purchased as an array withknown geometry

These arrays are manufactured using several

designs - each array is specifically built for theapplication, as with conventional ultrasonictransducers

Typical array designs are: Linear Matrix

Circular

Sectorial-annular

Phased Array Probes


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Wedge parameters

Velocity in wedge (vw) Wedge angle ()

Height first element (h1)

Offset first element (x1)

inc

ref

h1

Wedge (vw)

x1

Design Parameters Of

Phased Array Probes


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1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6

X =-7 .9 , Y = -8. 0 --> X = 7. 9

Y =8 .0

1

2

3

4

5

6

7

8

9

1 0

1 1

1 2

1 3

1 4

1 5

1 6

1 7

1 8

1 9

2 0

2 1

2 2

2 3

2 4

2 5

2 6

2 7

2 8

2 9

3 0

3 1

3 2

X =- 3 . 9 , Y = -1 .9 --> X = 3. 9

Y =1 . 9

2D Array Matrix1D Linear Array

Common Probe Geometries


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Common Probe Geometries

Daisy Array


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Daisy Probe Data

Polar View (End View)

Axial Plot (Side View)


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Continual or Wrapped Scanning


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Phased Array

Wave-forming Fundamentals


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Beam steering using conventional UT Probe

(on Emission) Acoustic beam generated by Huyghens

principle

Angled wedge introduces appropriatedelays during emission to generate an

angle beam

CrystalWedge

Material

Excitation pulse

Wave front

Delay

Location

A B C

AB C

Conventional Wave-forming


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Inclined Beam


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Beam steering using conventional UT probe (Reception)

Acoustic beam in wedge generated by Huyghensprinciple

Angled wedge introduces delays during reception sothat only waves in phase, yield constructiveinterference on piezoelectric crystals

CrystalWedge

Material

Received signal Delay

Location

A B C

A B C

S

Conventional Wave-forming


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Inclined Beam Receive Side


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Beam steering using phased-array probe (Emission) Acoustic beam generated by Huyghens principle

Appropriate delays introduced electronically duringemission to generate angle beam

Wave front

Time

Delay

Element

Focal law

Phased Array Wave-forming


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Beam steering using phased-array probe (Reception) Appropriate delays introduced electronically during

reception

Only signals satisfying delay law shall be in phaseand generate significant signal after summation

S



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Global Overview of Phased Array Signal Processing

For economic reasons, pulsers are usually multiplexed.

Instrumentation nomenclature such as a Focus 32/128refers to an instrument with 32 pulsers multiplexed into atotal of 128 ultrasonic channels.



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Wave-forming


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Focused Beam Receive Side


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Focused and Inclined Beam

Receive Side


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GeometricalFocal Point

Focal Law Generation

Material Velocity

Delay Law

Element DelayElement NumberElement Gain

Focal Law


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Focal Law Calculators

Native Tools

TomoView

OmniScan Program Probe

EPRI Workbook

PASS, CIVA, etc.


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Focal Law Calculators


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Phased Array Scanning


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Beam Focusing

The capability to converge the acousticenergy into a small focal spot

Allows for focusing at several depthsusing a single probe

Symmetrical (e.g. parabolic) focal laws

(time delay vs. element position)


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Unfocused Beam:

Near-field and natural divergence of

acoustic beam are determined by totalaperture A and wavelength

Near-field

Divergence (half angle , at 6 dB )

Beam dimension (at depth z)

4

2AN

A

5.0sin

A

zd

Beam Focusing


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Focused Beam :

Focusing coefficient (K) is defined as

where F : focal distanceN : near-field

Beam dimension (dst) in steering plane

at focal distance is given by

N

FK

A

Fdst

Beam Focusing


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Number of elements 10 16 32

Aperture (mm) 10 16 32

N Fresnel distance(mm) 84 216 865

Focusing depth (mm) 84 84 84

K 0.99 0.39 0.10

d (at focusing depthmm) 2.49 1.55 0.78

Linear Probe Pitch 1mm, Frequency 5 MHz

In water using a velocity of 1.48 mm/sec

Beam Focusing Theory


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Focusing 10

elements Aperture 10x 10mm

Focusing 16

elements Aperture16 x 10mm

Focusing 32

elements Aperture32 x 10mm

Beam Focusing Beam Profiles


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Mechanical Displacement

c = velocity in material

FOCUS DEPTH (PULSER)

DYNAMIC FOCUSING (RECEIVER)

Beamd

isplacement

DDF is an excellent way of inspecting thickcomponents in a single pulse. The beam isrefocused electronically on its return.

Schematic Representationof Dynamic Depth Focusing


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PA imaging without DDF PA imaging using DDF

Dynamic Depth Focusing


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Beam Steering

The capability to modify therefracted angle of the beamgenerated by the array probe

Allows for multiple angleinspections, using a single probe

Applies symmetrical (e.g. linear)focal laws


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Illustration of Sectorial(Azimuthal) Scanning


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12......

N

The ability to scan a complete sector of volume withoutany probe movement

Useful for inspection of complex geometries, or thosewith space restrictions

Combines the advantages of a wide beam and/or

multiple focused probes in a single phased array probe

Sectorial Scanning


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Beam Steering Capability

Is related to the width of an individualelement of the array

Maximum steering angle (at 6 dB), given

by

Steering range can be modified using anangled wedge

est

5.0sin


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Point A is OK because allrays are within elementalbeamwidth

Point B yields unexpectedresults because rays areoutside elemental beamwidth

Conclusion: The smallerthe element size, thebetter for steering

AB

e

est

5.0sin

Implications of Element Size

on Beam-forming


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Electronic Scanning

The ability to move theacoustic beamalong the axisof the array without anymechanical movement

The beam movement is

performed by timemultiplexing of the activegroup of elements

Scanning extent limited by: number of elements in

array number of channels in

acquisition system

Active Group


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Electronic Scanning


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Electronic combined with steering and focusing

Combined Beam Processing

W ld S i


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Weld Scanning

Conceptual animation showing weld inspection usingelectronic scanning. Emulates typical ASME-type shear waveinspection using line scan (much faster) rather than rasterscanning.A typical weld inspection requires two or more angles with

defined raster size, step size, etc. (mechanical movement inthe scan direction)There is a need to cover the weld, HAZ, any positionerrors => significant amount of scanning


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Tandem for Vertical Defects

S f S T


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For electronic scans, arrays are multiplexed using the same focal law

For sectorial scans, the same elements are used, but the focal laws arechangedFor Dynamic Depth Focusing, only the receiver focal laws arechanged in hardware

Summary of Scan Types


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Frequency

Element width (e)

Number of elements (n)

Pitch (p)

Array Selection


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Simple approach: If conventional UT uses, e.g. 10 MHz, use same

frequency for arrays

If conventional UT uses 10 mm aperture, use

similar aperture with PA (e.g. 10 elements of 1 mmwidth)

Higher frequencies (and larger apertures) mayprovide better signal/noise => tighter, optimized focalspot

Main manufacturing problems occur at highfrequencies (>15MHz) and small elements

Element Frequency (f)


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Element size (specifically e) is a key issue

As e decreases:

Beam steering capability increases

The number of elements increases rapidly

Manufacturing problems may arise

Minimum element size ~0.15-0.20 mm

Limiting factor often budget, not physics ormanufacturing

Element Size (e)


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Number of elements is a compromise between: Desired physical coverage of the probe and

sensitivity

Focusing capability

Steering capability Electronic system capability

CostExample:

An array with a large working range AND large steering

capability requires a large amount of small elements.

Such an array may exceed the electronic capability

of the system, or the budget.

Number of Elements (n)


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1 Element

2 Elements

4Elements

8 Elements

Power of the Elements


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Sectorial scans:different focal laws are applied to the same group ofelementssmaller elements needed to maximize steering capability

Typical sectorial scan would use a smaller number(e.g. 16), with a small pitch (1mm)

Design Compromise


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Number of active elements per focal law istypically 16

Maximum aperture (A max) = Pitch (p) x 16

For a high steering range, p must be small

For a good sensitivity, a large Near Zonedistance provides good focusing coefficient,therefore A must be large

The challenge is to find the best compromise

In terms of ratio p / A

Pitch / Aperture


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Typical arrays use side-by-side elements withacoustic insulation as gap

Grating lobes generally minimized by selecting

suitable element width

To reduce costs, use of a sparse array, withlarger gaps between elements is possible

Sparse arrays tend to produce strongergrating lobes - these can be minimized byusing random arrangements of the elements.

Element Positioning (p)


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Array Lobes

Far-field pattern of an array probeshows a main beam and gratinglobes at regular angular spacing

Array lobes reduce useful steeringrange and may generate multiple

images


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Fourier Transform:

Beam width (main beam,lobes) determined byaperture A

Steering width determinedby element width e

Angular position of lobesdetermined by frequency fand pitch p

Array Lobes

A

ep

Z

Fourier Transform

-z/p z/p

sinc(ex/z) sinc(Ax/z)

plobe


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Element size (e) , SideLobes will occur

e


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Array Lobes

Influence of pitch (p)(for A = fixed)

If p reduces, and nincreases

then lobe distanceincreases

and lobe amplitudedecreases

Mainlobe

Arraylobe

n=8p=9

n=12p=6

n=16p=4.5

n=20p=3.6


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Design Issues - Equivalent Apertures6 Elements (P) 1mm) 12 Elements (P) 0.4mm) 4 Elements (P)1mm) 8 Elements (P) 0.4mm)


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Design IssuesEquivalent Apertures

6 Elements p=0.4mm 3 Elements p=1mm


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Acknowledgements Written by R/D Tech - Quebec Canada

Edited by Tim Armitt at LavenderInternational NDT Consultancy Services - UK

First revision Jan 2005 by Larry EtheringtonEclipse Scientific Products Inc.

Screen images produced using the R/D TechOmniScan MX

This presentation is part of a series beingproduced by the R/D Tech CertifiedTraining Partners


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