Pulse Compression - Soft

Pulse-Compression Ultrasonic Technique
for Non Destructive Testing Inspection
of forged Steel with High Attenuation
Marco Ricci, Pietro Burrascano
University of Perugia, CADETlab
{marco.ricci,pietro.burrascano}@unipg.it
Outlines:

Goals & Motivations

State of the art: Pulse-Echo
Our proposal: Pulse-Compression



↳
Basic Theory
↳ Pros & Cons
↳ Lab. Set-up
Custom Pulse-Compression Device
↳ Hardware and Software details
↳
Comparison with Pulse-Echo Device
Perspectives and Conclusions
Main Goal
Improve the inspection capability of ultrasonic
NDT of large forgings, in particular by:
4 Allowing even smaller defects to be detected
4 Gather as much information as possible
4 Performing test at the early stages of the
hardening process
Motivations
Guarantee high-quality of the samples
Improve the working cycle
State of the art:
Ultrasonic Forgings Inspection
Ultrasonic NDT to assess forgings' integrity
4 Pulse-echo is the standard method,
AVG-Distance Gain Size curves to calculate the
minimum detectable defect
4
4
For energy industry applications:
4
100 % scanning coverage is required and the tolerance
is quite low
4 the capability to detect defects as small as 1mm in
large structures (diameter D~3 m) is desirable
State of the art:
Pulse-Echo (PuE) Method
1) Transducer:
1 transducer acts both as
Tx and Rx
Probe
2) Input Signal:
A short high-voltage pulse
x(t)=V(t) excites the
probe
3) Output Signal:
4) Defect detection:
Analysis of the multiple
echoes/paths enclosed
in the reflectogram
5) Defect Sizing:
Defect
voltage signal y(t)
(the reflectogram)
received by the probe
Backwall
comparison between
echoes, backwall echoes
and theoretical crosssection curves:Distance
Gain Size (DGS-AVG)
curves
State of the art:
Limitations of Pulse-Echo method
in large forgings inspections
Geometrical ( ) and physical
attenuation ( f2-4)
 Low Energy of output
signal= Low SNR
Above a certain frequency, 2nd
or even 1st back-wall echoes
cannot be detected
NO defect sizing
To increase ultrasonic
penetration lower frequencies
are used at the cost of reducing
the inspection resolution
 Trade off-between SNR
and Resolution
Technical Goal:
Optimization of the SNR
Technical Goal:
Optimization of the SNR

Increase of the defect detection
How to Enhance the SNR?
Higher level excitation signal:
The gain is limited by the transducer breakdown
voltage as well as by the onset of non-linear
phenomena

How to Enhance the SNR?
Higher level excitation signal:
The gain is limited by the transducer breakdown
voltage as well as by the onset of non-linear
phenomena

Optimization of the transducers:
focused transducers, DGS based-design, more
efficient materials, etc → It needs development
and test, flexible beam focusing with phase-array
probes can be a solution

How to Enhance the SNR?
Proposed Solution:
Exploit signal processing

Pulse-Compression
→ wide applicability
→ works with different HW
(i.e. phased array)
→ great flexibility
Signal Processing for
Ultrasonic Inspection
Starting Point: Pulse-echo  measurement of the
impulse response h(t) of a linear system
Voltage Pulse
Impulse exc. (t)
h(t) completely
characterizes the
Input-Output
relation of a
Linear System
Forgings under Test
Linear System
x(t)
Reflectogram
Impulse Response h(t)
SUT
h(t)
y(t)=x(t)h(t)
Output = Convolution between
Input and Impulse Response
Signal Processing for
Ultrasonic Inspection
Starting Point: Pulse-echo  measurement of the
impulse response h(t) of a linear system
Voltage Pulse
Impulse exc. (t)
Forgings under Test
Linear System
Reflectogram
Impulse Response h(t)
 If we are able to retrieve the impulse
response h(t) with a higher SNR we improve
the ultrasonic inspection !!
Pulse Compression:
How it works?
Sample
Under Test
Pulse Compression:
How it works?
Sample
Under Test
x(t) is a coded
signal
Pulse Compression:
How it works?
Sample
Under Test
x(t) is a coded
signal
y(t)=h(t) x(t) is the
output signal
Pulse Compression:
How it works?
Sample
Under Test
x(t) is a coded
signal

y(t)=h(t) x(t) is the
output signal
With a proper choice of
x(t),h(t) is retrieved by
correlating the output with
the input h*(t)=y(t)x(t)
PuC Vs PuE:
Pros & Cons
Main Pros: In PuC the duration TPuC of the input signal x(t) is
independent from the bandwidth B (i.e. the resolution) and
it can be arbitrary long → SNRPuC  ENPuC  T
For Pulse-Echo TPuE x B~1 → SNRPuCE  ENPuE  1/B
→ with Pulse Compression we can keep constant resolution
and increase SNR by using low voltage signals
EX. 5MHz
Bandwidth:
ENPuE = ENPuC
TPuE = 0.1 s;
TPuC = 30 s;
PuC Vs PuE:
Pros & Cons
How Long can be the coded signal?
Usually TPuC ~ Impulse response duration Th→
The longer is h(t) the higher is the SNR gain →
Large forgings take great advantage from PuC
Ex. 2 backwall echoes in D=3m → Th 2 ms=TPuC
nd
For B=5MHz TPuE 0.1-0.2 s →TPuC/TPuE =10000-20000
For B=1MHz TPuE 0.5-1 s →TPuC/TPuE =2000-4000
The SNR gain can be very high!!
Pulse Compression:
Pros & Cons
Main Cons:
●
two separated probes should be used, one
Tx and one Rx;
●
AVG-DGS curves should be calculated for
two-probes Geometry
●
Digital Signal Processing is requested

What we have done?
Early Experimental Set-Up
✔ 2 Probes in Pitch-Catch configuration:
side-by-side along a circumference;
✔ Coded Signal is a linear chirp
(AMPLITUDE ± 10 V)provided by an
Arbitrary Waveform Generator National
Instruments PXI 100MS/s
✔ Output signal amplified by a Low Noise
Amplifier
✔ Analog-to-Digital Converter National
Instruments PXI 100 MS/s
✔ All the process is managed by a Virtual
Instrument realized with Labview

hardware cost ~ 15000 €
Procedure Characterization
Input: Linear Chirp with
Spectral shaping, Amplitude ± 12V
Time
Frequency
Benchmark D=3.1 m
rotor
Output: peak-to-peak < 1mV
high-frequencies attenuation
Time
Frequency
Results with
Pulse Compression
Results with
Pulse-Echo:
MDD>3mm
Minimum
Detectable
Defect<2 mm
Custom Prototype
assembled with off-the-shelf components (<2 k€)
Panel Industrial PC Function Generator &
Low Noise
With Win OS
Digital Oscilloscope Variable Amplifier
Function
Generator
USB
port
Amplified
Input Channel
Direct Input
Channel
Power
Switch
Power Fan
Supply
Coarse
Amplifier
Gain Control
Custom Software
Virtual Instrument
Measurement Panel
Reflectogram
Analysis
Automatic Report
Generation
and
3D Rendering
Defect
Sizing
AVG-DGS computation
for arbitrary geometry
Comparison with
Commercial Devices
Extensive measurement campaign on Forgings with
different diameters and at different hardening cycle steps
together with Società delle Fucine
Probes KrautKramer B2S f0=2MHz, Bandwidth = 2MHz
Comparison of SNR levels
Comparison with
Commercial Devices
Extensive measurement campaign on Forgings with
different diameters and at different hardening cycle steps
together with Società delle Fucine
Probes KrautKramer B2S f0=2MHz, Bandwidth = 2MHz
Comparison of Minimum
Detectable Defect
MDD [mm]
Comparison of SNR levels
Comparison with
Commercial Devices
Probes KrautKramer B2S
f0=2MHz, Bandwidth = 2MHz
Probes Olympus V109
f0=5MHz, Bandwidth = 4MHz
Pulse Compression Device achieves better results for all
forgings and significant inspection capability enhancement
when starting SNR is low (large dimensions, coarse grain,
early hardening process)
Further Developments
Hardware
Circuit Design & Realization from basic components :
→ reduction of the costs, optimization of the characteristics
→ Multi-channel for Phased Array probes
Two main directions:
→ reduce space for portable instrumentation
→ high-performance devices for automatic inspection system
Different PC solution:OpenSource OS, FPGA, etc...
→ Lower Costs for Higher Performances
Software
→ Digital Filtering for further SNR increase
→ Signal optimization
→ BeamSteering & Beam Focusing for Phased Array
Conclusions
Pulse Compression improves the inspection
sensitivity with respect to Pulse-Echo method.
Large forgings inspection is particularly suitable
for Pulse Compression
The method can be applied to various set-up:
phased-array, EMAT probes, air-coupled, etc.
Work in progress
Custom device for PulseCompression &
PhasedArray
Application of the apparatus to EddyCurrent and
Thermography NDT
References
[1] N.A.H.K. Rao, "Investigation of a pulse compression technique for medical ultrasound: a
simulation study", Medical and Biological Engineering and Computing 32, (2), 181-188 (1994).
[2] M. O'Donnell, "Coded excitation system for improving the penetration of real-time phasedarray imaging systems", IEEE Trans. on Ultr., Ferr. and Freq. Contr 39, 341-351,(1992).
[3] Y. Iizuka, "High signal-to-noise ratio ultrasonic testing system using chirp pulse
compression", Insight 40, (4), 282-285 (1998)
[4] T.H. Gan, D.A. Hutchins, D.R. Billson and D.W. Schindel,"The use of broadband acoustic
transducers and pulse-compression techniques for air-coupled ultrasonic imaging",
Ultrasonics, 39, (3), 181-194 (2001);
[5] P. Pallav, T.H. Gan, D.A.Hutchins, Elliptical-Tukey chirp signal for high-resolution, aircoupled ultrasonic imaging, IEEE Trans. on Ultr., Ferr. and Freq. Contr, 54,(8), 1530-40,
(2007).
[6] T. Misaridis, J.A. Jensen,"Use of Modulated excitation Signals in Medical Ultrasound. Part I:
Basic Concepts and Expected Benefits",IEEE Trans. on Ultr., Ferr. and Freq. Contr. 52, (2),
177-191 2005; & "Part II: Design and Performance for Medical Imaging Applications", 192-207
(2005);
[7] M. Ricci, L. Senni, P. Burrascano, “Exploiting Pseudorandom Sequences to Enhance Noise
Immunity for Air-Coupled Ultrasonic Nondestructive Testing”. IEEE Trans. On Instrumentation
and Measurement 61(11), 2905-2915 (2012).
[8] M. Ricci, L. Senni, P. Burrascano, R. Borgna, S. Neri, M. Calderini "Pulse-compression
ultrasonic technique for the inspection of forged steel with high attenuation." Insight 54 (2),
91-95 (2012).
[9] D. Hutchins,P. Burrascano, L. Davis, S. Laureti, M. Ricci “Coded waveforms for optimised
air-coupled ultrasonic nondestructive evaluation”, Ultrasonics 54 (7), 1745-1759 (2014).
Thanks for your
attention
Marco Ricci, Pietro Burrascano
Dept. Of Engineering, University of Perugia,
e-mail:
{marco.ricci,pietro.burrascano}@unipg.it
Some numbers:
Which levels of SNR?
EX.:1mm defect on a Rotor with D=3m, 2 MHz probe
4 Geometric attenuation for a 1—mm defect at the axis ~ 95 dB
4 Phys. Attenuation of the medium ~ 2db/m → 3 db on the axis
4 Losses due to coupling and transducer efficiency ~ 30 dB
Total attenuation ~130 dB:
by exciting the probe with 500 V,
a 1-mm defect returns an echo
of ~150 V.
SNR required > 65 db
(~ 1/2000 of 1st backwall echo)
Pulse-echo devices could not
have the necessary SNR
What we have done:
Main Pros:
The coded signal is a Chirp Signal → linear
swept of frequency in the desired bandwidth →
optimal energy transfer
Main Cons:
→ two separated probes in pitch-catch
configuration (but it is possible to use dualelement probes or phased-array too)
→ numerical tool for calculate AVG-DGS curves
for arbitrary geometry
→ optimization of the computational cost (FFT
based algorithms)
Pulse Compression
Coded Signals
Discrete phase modulation → Binary Spreading Sequences
(m-sequences, Chaotic-BSS, Golay, Barker, Legendre....)
Continuous phase modulation → Chirp, PseudoChirp...
Linear Chirp
input signal:
st=tsin 2  [ f0 tt]
matched filter
 t= tsin2 [f0 t]
t,  t amplitude modulation functions
2
B0 t
B0 t
t=
−
phase modulation function
2T0
2
B0 nominal bandwidth , T0 signalduration
Usually (t)= (t), but they can be different
Pulse Compression: Typical signals
Input Coded Signal
Ideal Output Signal
True Output Signal
Ideal Reflectogram
Additive Gaussian Noise
Estimated Reflectogram
Early Experimental Set-Up
I. National Instruments PXI System :
● Embedded PC;
● Arbitrary Waveform Generator PXI5412 100MS/s
● Digital Oscilloscope PXI-5105 60 MS/s
II. Linear Amplifier
III. Probes in Pitch-Catch configuration
IV. Virtual Instrument
to manage hardware
and to perform signal
processing and
analysis
IV
I
II
III

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