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Mineo, Carmelo and Pierce, Stephen Gareth and Nicholson, Pascual Ian

and Cooper, Ian (2014) Robotic path planning for non-destructive testing

of complex shaped surfaces. In: E-Book of Abstracts, 41st Annual

Review of Progress in Quantitative Nondestructive Evaluation

Conference. Center for Nondestructive Evaluation, Boise (USA). ,

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Robotic Path Planning for Non-Destructive Testing of

Complex Shaped Surfaces

Carmelo Mineo1, a)

, Stephen Gareth Pierce1,

Ben Wright2, Pascual Ian Nicholson2

, Ian Cooper2

1University of Strathclyde, Department of Electronic and Electrical Engineering, George Street,

Glasgow, G1 1XW, UK 2TWI Technology Centre (Wales), Harbourside Business Park, Port Talbot, SA13 1SB, UK

a)carmelo.mineo@strath.ac.uk

Abstract. The requirement to increase inspection speeds for non-destructive testing (NDT) of composite aerospace parts

is common to many manufacturers. The prevalence of complex curved surfaces in the industry provides significant

motivation for the use of 6 axis robots for deployment of NDT probes in these inspections. A new system for robot

deployed ultrasonic inspection of composite aerospace components is presented. The key novelty of the approach is

through the accommodation of flexible robotic trajectory planning, coordinated with the NDT data acquisition. Using a

flexible approach in MATLAB, the authors have developed a high level custom toolbox that utilizes external control of

an industrial 6 axis manipulator to achieve complex path planning and provide synchronization of the employed

ultrasonic phase array inspection system. The developed software maintains a high level approach to the robot

programming, in order to ease the programming complexity for an NDT inspection operator. Crucially the approach

provides a pathway for a conditional programming approach and the capability for multiple robot control (a significant

limitation in many current off-line programming applications). Ultrasonic and experimental data has been collected for

the validation of the inspection technique. The path trajectory generation for a large, curved carbon-fiber-reinforced

polymer (CFRP) aerofoil component has been proven and is presented. The path error relative to a raster-scan tool-path,

suitable for ultrasonic phased array inspection, has been measured to be within ± 2mm over the 1.6 m2 area of the

component surface.

INTRODUCTION

In civil aerospace manufacturing, the increasing deployment of composite materials demands a high integrity

and traceability of NDT measurements. Modern components increasingly present challenging shapes and geometries

for inspection. Using traditional manual inspection approaches produces a time-consuming bottleneck in industrial

production environments [1] and hence provides the fundamental motivation for increased automation.

The combined use of Modern Computer-Aided Design (CAD) and Computer-Aided Manufacturing (CAM) now

allows large items to be produced easily from one piece of raw material (either through traditional subtractive

approaches, or with more recent additive manufacturing processes [2]). As a result, large components with complex

geometries are becoming very common in modern structures, and the aerospace industry is a typical field, where

wide complex shaped parts are very frequently used. Moreover the use of composite materials, which are

notoriously challenging to inspect [3], is becoming widespread in the construction of new generations of civilian

aircraft. To cope with future demand projections for these operations, it is therefore essential to overcome the

current NDT bottleneck.

A fundamental issue with composites manufacturing compared to conventional light alloy materials lies in the

process variability. Often parts that are designed as identical, will have significant deviations from CAD, and also

suffer from inherent but different part to part spring-back out of the mould. This presents a significant challenge for

precision NDT measurement deployment which must be flexible to accommodate these manufacturing issues. For

these reasons, NDT inspection is often performed manually by technicians who typically have to move appropriate

probes over the contour of the sample surfaces. Manual scanning requires trained technicians and results in a very

slow inspection process for large samples. The repeatability of a test can be challenging in structures where complex

setups are necessary to perform the inspection (e.g. orientation of the probe, constant standoff, etc.) [4]. While

manual scanning may remain a valid approach around the edges of a structure, or the edges of holes in a structure,

developing reliable automated solutions has become an industry priority to drive down inspection times. The

fundamental aim of automation within the inspection process is to minimize downtimes due to the higher achievable

speed, and in parallel to carry out 100% inspection coverage of the sample, including all edge areas.

Semi-automated inspection systems have been developed to overcome some of the shortcomings with manual

inspection techniques, using both mobile and fixed robotic platforms. The use of linear manipulators and bridge

designs has for a number of years provided the most stable conditions in terms of positioning accuracy [5, 6]. The use

of these systems to inspect parts with noncomplex shapes (plates, cylinders or cones) is widespread; typically, they

are specific machines which are used to inspect identically shaped and/or sized parts.

More recently, many manufacturers of industrial robots have produced robotic manipulators with excellent

positional accuracy and repeatability. In the spectrum of robot manipulators, some modern robots have suitable

attributes to develop automated NDT systems and cope with the challenging situations sought by the aerospace

industry [7]. They present precise mechanical systems, the possibility to accurately calibrate each joint, and the

ability to export positional data at frequencies up to 500Hz.

Some applications of 6-axis robotic arms in the NDT field have been published during the last few years and

there is a growing interest in using such automation solutions with many manufacturers within the aerospace sector [1, 7-10]. Despite these previous efforts, there remain challenges to be addressed before fully automated NDT

inspection of composite parts becomes commonplace. The key challenges include generation and in-process

modification of the robot tool-path, high speed NDT data collection, integration of surface metrology measurements,

and overall visualization of measurement results in a user friendly fashion. Collaborations driving this vision include

the IntACom project, developed by TWI Technology Centre (Wales) on behalf of their sponsors over a period of 3

years. The project objective has been to achieve a fourfold increase in the throughput of aerospace components [1].

Additionally the UK RCNDE consortium conducts research into integration of metrology with NDT inspection [11, 12]. Both these consortia have identified the requirement for optimal tool path generation over complex curved

surfaces. The current paper describes a novel approach to flexible robotic toolpath generation using a purposely

developed MATLAB based path-planning software platform.

INVESTIGATED PATH PLANNING APPROACHES FOR NDT APPLICATIONS

Traditional Approach

Six-axis robotic arms have traditionally been used in production lines to move the robot end-effector from one

position to a new position for repetitive assembly and welding operations. In this scenario, where the exact trajectory

between two points in the space is not too important, the teach pendant of a robot is used to manually move the end-

effector to the desired position and orientation at each stage of the robot task. Relevant robot configurations are

recorded by the robot controller and a robot program is then written to command the robot to move through the

recorded end-effector postures. More recently, accurate mechanical joints and control units have made industrial

robotic arms flexible and precise enough for finishing tasks in manufacturing operations [13]. Robotic manipulators

are highly complex systems and the trajectory accuracy of a machining tool has a huge impact on the quality and

tolerances of the finished surfaces. As a result, many software environments have been developed by non-robot

manufacturers, academic researchers and also by the robot manufacturers themselves, in order to help technicians

and engineers to program complex robot tasks [14]. The use of such software platforms to program robot movements

is known as Off-Line Programming (OLP). It is based on the 3D virtual representation of the complete robot work

cell, the robot end-effector and the samples to be manipulated or machined. OLP was first achieved within the

IntACom

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d parts of intere

can be set into

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oftware is respo

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xisting software

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n is an interactiv

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teach pendant, ar

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surface of the p

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ding to the inspe

by a two-way

th, and reception

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terest into the Sa

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of flexibility, the

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ls in the Robots

embled robot ce

ries.

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it is widely use

on of the part of

; the operator

l Centre Point (

ed into the start-

ntains the neces

est. Three main t

veral characteris

(scanning step,

that allows stre

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tion of feedback

aced with the so

rrection. Moreo

le pathway towar

king envelope.

tecture based on

the software co

rary. The probes

Robot Cell Libr

arate libraries ex

ots Library or to

cells defined in

raries.

scene of the pa

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sed for rapid pr

of interest withi

can select up

t (TCP) to the

-up module.

cessary algorithm

in tool-path type

eristic settings th

ep, speed, offsets

streaming of the

dback. Since th

ck coordinates,

software platfor

reover, the com

ards automatic

on four module

contains five lib

bes, sensors and

ibrary can conta

exist for the en

to the environm

in the Robot Ce

path-planning p

at was chosen b

prototyping and

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p to 10 referenc

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thms to generate

pes are being im

s that allow a go

sets, etc…).

the command

the upgraded

s, it paves the

form to obtain

ommunication

tic recognition

ules: start-up,

libraries. The

nd tools to be

ntain multiple

environments

nments in the

Cells Library.

project. The

n because it is

and computer-

l model of the

ence points of

. The relative

ate the desired

implemented:

good level of

The ev

other CAD

robotic ar

by the Un

KCT. The

point with

unsuitable

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inspection

of 100 mm

Finally

format are

execute th

approach

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control un

controller.

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minimum.

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situations,

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mm/s. The toolp

FIGURE 4. Sn

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are generated fo

the generated t

ch the starting po

ne merely contai

unit, shown by

ler.

new path-plann

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multiple robot s

he same externa

m. The maxim

lation cycle. Fo

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usual for NDT

ns, generating s

be time consum

odule provides

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ially been based

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ce code does no

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the new softw

based on a geom

he recorded sim

for a given datum

lpath simulation

Snapshots of the

t module has be

for each tool-

d tool-path; the

point of the insp

tains 6 coordina

by the schema g

nning approach

ltiple packets of

t synchronizatio

rnal server com

imum misalignm

For example, f

um path mismat

s that use digital

T operators to d

g specific tool-p

uming and not v

es a full simula

in the robot cell

ed on open-sour

in 2011 [18]. How

not allow the se

e in 8 differen

tware applicatio

eometric approa

simulation video

tum surface. Th

ion of RoboNDT

the simulation vid

of the

been developed

-path. The firs

he second file co

inspection and re

inates (x, y, z, A

a given in Figur

ch paves the way

of coordinates

tion. Sending co

omputer enable

gnment would

for robots run

atch would be e

tal I/O signals fo

to double check

paths for all t

t very practical

ulation capabilit

ell. The implem

ource MATLAB

owever, a funda

selection of the

rent configuratio

tion that has a

roach, was there

deo acquired fr

The tool-path is

DT allows accur

video and of the re

he example target

ed to output the

irst file contains

contains the ne

retract from the

, A, B, C) to dr

ure 2, imports b

ay to supportin

in each line o

command coor

bles the path sy

d remain within

running in a 12

e equal to 1.2 m

for synchroniza

ck some suspect

ll the areas of in

cal. A MATLAB

ility of the prog

ementation of th

B code, the KU

damental proble

the robot configu

ations). This inc

a target to be a

erefore develope

from the Robo

is a raster scan

curate time appro

e real inspection to

et surface.

he results of the

ins all comman

necessary point

the endpoint. Th

drive the roboti

ts both files and

ting inspection t

e of the first co

ordinates simult

synchronization

thin the distanc

12 millisecond

mm. This wors

ization purposes.

ect areas of a pa

f interest throug

AB based path

ogrammed robo

f the full inverse

KUKA Control T

blem was observ

figuration (a six

inconvenience

e as flexible as

ped and implem

boNDT softwar

n with 50 mm p

proximation of s

n tool-path, for the

he computations

and coordinates

ints to set the in

These two files

otic arm to a sp

nd sends the co

n techniques tha

command file c

ultaneously to m

tion mismatch t

nce covered by

nds interpolatio

orst case scenari

es.

part, after an in

ugh commercia

th-planning mo

bot path togethe

rse kinematic m

l Toolbox (KCT

erved with the u

ix-axis robot can

made the KC

possible. A n

lemented. Figure

are and of the

pitch, executed

of scanning time

the datum surface

ns. Two text file

tes that the rob

initial and fina

es have very sim

specific pose. T

command data t

that require mult

e can become th

o multiple robot

h to be maintai

by the robots

tion cycle and

ario is much imp

initial inspectio

cial path-plannin

odule has been

ther with any

model of the

CT) produced

e usage of the

can reach any

KCT function

new inverse

ure 4 presents

the real robot

ted at a speed

me.

e

files in ASCII

obot needs to

inal motion to

simple syntax;

. The external

ta to the robot

ultiple robots.

the principle

bot controllers

tained to the

ts in a single

nd moving at

improved over

tion. For such

ning software

een purposely

developed

centre poi

returning

interpolate

simulation

controlling

acquisition

FI

The I

arms are p

To eva

measurem

distance m

the acquis

full surfac

The la

the sensor

used trian

ed to be integra

point (TCP) data

g to the point

lated to generate

ion of a raster s

ling the robot ar

tion module.

FIGURE 5. Scre

V

IntACom inspe

presented in T

evaluate the accu

ement probe wa

e measurement p

uisition module

face scan. The ca

laser sensor wa

sor span (accord

iangulation to m

grated into the

ata, received fro

int of interest a

ate the desired t

r sub-scan, befo

arm via the UD

Screenshot of the in

VALIDATI

spection system

Table 1.

TABL

Payload

Maximum

Max. rea

Max. spe

Number

Position

Controlle

Protectio

ccuracy of gene

was mounted on

t probe was cho

le. It was used t

e capability to co

was established

ording to dimen

measure distanc

e main GUI. Th

from the robot d

t and executing

d type of sub-sc

efore execution.

DP/IP Ethernet

e integrated path-

TION EXPE

em comprises tw

BLE 1. KUKA K

oad

imum total load

. reach

. speed

ber of axes

tion repeatabilit

roller

ection classificat

nerated paths fr

onto the end ef

hosen [20]. The l

d to continuousl

control the laser

ed as the robot to

ensional specifi

ance. It had a 10

The path-planni

t during the initi

ing what is call

scan tool-path:

on. Therefore th

net connection e

-planning modul

PERIMENTS

two KUKA KR

KR16 L6-2 princ

oad

bility

fication

from the MATL

effector of one

e laser distance

usly monitor the

ser sensor via th

t tool, setting its

cifications of sen

101.6mm detect

nning software a

nitial scan, in or

alled a “sub-sc

th: raster, segme

the execution o

n established by

dule during the sim

TS – PATH A

KR16 L6-2 robo

incipal specificati

KR16 L6-

6 kg

36 kg

1911 mm

2 m/s

6

<±0.05 mm

KR C4

IP 65

TLAB toolbox,

ne of the two ro

ce meter is a me

the probe separa

the RS232 seria

its TCP along th

sensor and hold

ection span with

re add-on is able

order to generat

scan”. The ori

ment or single p

n of the sub-sca

by the external c

simulation of the

ACCURAC

bot arms. The m

ations [19].

-2

m

mm

ox, a precision n

robot arms. An

metrology device

aration from the

rial communicat

g the laser beam

older). The selec

ith 79mm stand

ble to use the o

rate a specific to

original robot tr

e point. Figure 5

scan is carried o

al control compo

he sub-scan tool-p

CY

main features

n non-contact las

An acuity AR20

ice that was inte

the sample surfa

cation was explo

m at the central

lected laser dist

ndoff to the mid

e original tool

c tool-path for

t trajectory is

e 5 shows the

d out through

ponent of the

path.

of the robot

laser distance

200-100 laser

integrated into

ace during a

ploited.

ral position of

istance meter

iddle of span

and a reso

frequency

Raster

contour of

of 1.6m2.

constant s

command

cycles of

data samp

The la

Figure 6 s

speeds. Th

± 2mm.

During

due to fi

manufactu

principal s

A scan

IntACom

using ultra

Figure

inspection

water jet n

of the sam

across the

seen. The

flaw has b

esolution of 30.5

cy was equal to

ter scan paths w

r of the main ski

. Three separa

t speeds of 100

nd positions to

of 12 millisecon

mples for each ro

laser sensor dat

shows the reco

The results show

FIGURE

ing manufacture

fibre deviation

ctured compone

al source of erro

can image of a

m prototype sys

ltrasound pulse-

ure 7 shows the

ion system. The

et nozzles that pr

ample. The stan

the C-scan and

he sample conta

s been sized to b

0.5µm. The mea

to 1250Hz.

s were generated

skin of a carbon

rate tool-paths

100, 200 and 3

to the robot con

onds. The samp

robot interpolat

data was mapped

ecorded maps of

how that the dev

RE 6. Maps show

ure, as the comp

ion, residual str

nent can be sev

rror of the experi

a curved reinfo

system and a su

-echo phased a

the Time of Fli

he IntACom rob

t provide suitabl

tandoff between

d equal to 0.6m

ntains some tape

o be 5mm wide a

easurement lase

ted through the

on fibre compos

ths were genera

300mm/s, resp

ontroller and rec

pling rate of th

lation cycle.

ped to the surfac

of difference be

eviation of the m

owing distance de

mposite part is r

stress and strai

several millimetr

eriment.

ND

nforced wing sk

suitable tool-pa

d array inspectio

Flight (TOF) C

robotic arms de

able water colum

en the water jet

.6mm. The skin

ape insert defects

e and the bigges

laser spot size w

he MATLAB ba

site material ae

erated to travel

espectively. The

received actual

the laser sensor

rface scan by co

between measu

e measured dista

deviation from th

s removed from

train [21]. The d

etres for large c

NDT RESUL

skin, manufactu

path generated

tion through a 5M

C-Scan of the

deploy end-effe

umns to support

et nozzle and the

in thickness var

ects, as indicated

gest 15mm.

was smaller tha

based software

l aerospace wing

el along equally

he C++ code o

al TCP coordina

sor was set to 25

comparing with

sured and theore

istance from the

theoretical TCP f

m the mould, th

e difference bet

e composite sam

ULTS

ctured with intr

ed by the Robo

a 5MHz – 64 ele

he large curved

ffectors carrying

ort the ultrasonic

the sample was

varies across the

ted by the black

than 250µm and

are and used to

inglet. The surfa

ally spaced traje

e of the acquisi

inates over the

250 Hz, resultin

ith the interpolat

oretical TCP for

he theoretical TC

P for different rob

, the part can exh

between the CA

samples [22] and

ntroduced flaws,

boNDT software

element probe (0

ed surface obta

ing ultrasonic tr

nic beams from

as set to 8mm.

the sample and

ck ovals in figur

and the maximum

to finely follow

rface of interest

ajectories (10mm

isition module

e Ethernet inter

lting in an avera

lated feedback c

for the three diff

TCP spans a ran

robot speeds.

exhibit a spring

CAD dimensio

nd this was foun

ws, was obtaine

are. The part w

(0.6 mm pitch).

btained with the

transducers, m

m the probes to

. The resolution

d the stiffeners

gures 7 and 8. T

um sampling

w the curved

st had an area

mm pitch) at

transmitted

terface within

erage of three

k coordinates.

different robot

range between

ng-back effect

sions and the

und to be the

ined using the

t was scanned

h).

the prototype

mounted into

to the surface

ion is uniform

ers are clearly

. The smallest

Figure

the collec

the screen

potential d

In mod

the inspec

structure.

complex p

present si

increasing

themselve

FI

ure 8 shows a cl

lected data throu

een; the A-Scan

al defects.

odern aerospac

pection and ver

re. Traditional N

x part shapes em

significant cha

ingly demanding

lves to bespoke

FIGURE 7. C-Sc

close up of the

rough visualizati

an section is at

FIG

ace manufacturi

verification proc

l NDT methods

employed in ae

hallenges. Trad

ng reduced cyc

ke Cartesian or C

Scan of the main

he first group of

ation of B-Scan

at the bottom.

FIGURE 8. Close

CO

turing, the increa

rocedures requi

ds such as ultra

aerospace struct

raditional manua

cycle times for t

r Cartesian plus

ain skin surface of

of defects. The

ans and the A-S

m. The B-Scan

ose-up of one of th

CONCLUSIO

reasing use of co

quired to ensure

ltrasonic testing

uctures, combin

nually delivered

r the inspection

lus rotation stag

of the aerospace

he GUI of the In

Scans. The B-

n is very usefu

f the defected reg

IONS

f composite mate

ure safe deploy

ing are fundame

ined with comp

red NDT is tim

ions undertaken.

tage mechanical

ce composite wing

IntACom softw

-Scan is given

eful to size the

egions.

aterials is introd

loyment of com

mental to such

mplex material p

time consuming

en. Although so

cal scanners, the

inglet.

ftware lets the u

en on the right h

e C-Scan featu

roducing new ch

omponents in t

h inspection. H

al properties of c

ing and manufa

some part geom

there are many i

e user analyze

t hand side of

atures and the

challenges in

the finished

However the

f composites,

ufacturers are

ometries lend

y instances of

complex geometry that make the use of 6 axis robot positioners highly attractive. Most existing off line

programming approaches are geared towards manufacturing processes, and lack the required flexibility for

application to delivery of NDT measurements. In particular the lack of conditional programming capability has been

identified as one of the key shortcomings in existing software. This is fundamentally important for 2 reasons. Firstly

composites manufacturing has higher part variability and deviation from CAD than encountered with traditional

materials. Secondly NDT processes often require local measurement refinement. These shortcomings have

motivated the authors to develop new MATLAB based robotic programming software specifically geared towards

NDT path planning applications.

The new software has been tailored to the generation of raster scan paths for inspection of curved surfaces by 6-

axis industrial robots, and in its current form represents the first iteration of a system designed to overcome the

issues with current OLP packages. The software is intended to be flexible and extensible to future systems and robot

developments. The output tool-path is not specific to robot programming language or hardware, so could be

deployed across a range of platforms. It has been explained how the execution of the calculated path by a robotic

arm, externally controlled through a C++ external control unit, can be beneficial for NDT inspections. The

developed NDT robot toolbox will ultimately assist NDT technicians to move from a component CAD file to the

actual physical inspection, without the need to use multiple pieces of software not optimized for robotic NDT

inspections. The commercial driver for this work is the need to increase NDT inspection throughput. This has been

identified clearly as a bottleneck in existing composite parts manufacture in aerospace industries. The new software

will contain specific functions tailored to generate several types of tool-paths for raster scans, segment scans or

single point inspections. The inclusion of an evaluation module, incorporating the inverse kinematics, allows the

user to have a real-time preview of the calculated path for a successive safe execution in the real robotic

environment.

Comparative metrology experiments have been undertaken to evaluate the real path accuracy of the toolbox

when inspecting a curved 1.6 m2 surface using a KUKA KR16 L6-2 robot. The results obtained via a precision laser

range meter have shown that the deviation of the distance between the theoretical TCP and a large aerofoil curved

surface spans a range between ± 2mm; the dominant source of this error being the deviation of the part from the

CAD model. For the current NDT application, this level of accuracy is sufficient as the intended NDT delivery is

accomplished using a water jet coupling approach. However more demanding NDT inspection requiring higher path

accuracy will demand individual component surface metrology measurement prior to NDT inspection.

In the future a fully developed software solution will support different types of robotic arm and robotic

environments. Additionally, the possibility to define custom robotic cells and equip the robotic end-effector with

several different probes is the aim for a more versatile software platform for robot deployed NDT. The ultimate goal

of the authors remains the simultaneous management of command coordinates, robot positional feedback and NDT

data by an integrated server application running on an external computer. This paves the way for the introduction of

intelligent novelty factors for the application of robotic NDT inspections. On-line monitoring and data visualization,

real-time path correction and versatile path amending approaches are just some of the possible opportunities.

ACKNOWLEDGEMENTS

This work was developed in partnership with TWI Technology Centre (Wales), University of Strathclyde

(Glasgow), the Prince of Wales Innovation Scholarship Scheme (POWIS) and by IntACom, a project funded by

Welsh Government, TWI, Rolls-Royce, Bombardier Aerospace and GKN Aerospace. Additional support was

provided with assistance from UK Research Centre in NDE (EP/F017332/1) and EPSRC Equipment Grant “New

Imaging Systems for Advanced Non-Destructive Evaluation” (EP/G038627/1).

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