The Use of Pulse Compression Technique in Non-Destructive ... · THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW Muhammad Khalid Rizwan1, Hamed Malekmohammadi1,

10th International Symposium on NDT in Aerospace

1 License: https://creativecommons.org/licenses/by/4.0/

The Use of Pulse Compression Technique in

Non-Destructive Testing: A Review

Muhammad Khalid RIZWAN 2, Hamed MALEKMOHAMMADI 1, Stefano LAURETI 1,

Pietro BURRASCANO 2, David A. HUTCHINS 3, Gui Yun TIAN 4, Junzhen ZHU 4,

Qiuji YI 4, Marco RICCI 1 1 University of Perugia, Perugia, Italy

2 Universita degli Studi di Perugia, Terni, Italy 3 University of Warwick, Warwick Manufacturing Group (WMG), Coventry, UK

4 Newcastle University, Newcastle upon Tyne, UK

Contact e-mail: [email protected]

Abstract

Non-Destructive Testing (NDT) refers to an extended group of techniques used both in

research and industry, which are exploited for inspecting, characterizing and evaluating

various kinds of materials and goods. Eddy Current Testing (ECT), Ultrasonic Testing (UT)

and InfraRed Thermography (IRT) are among the most employed NDT methods. In the said

methods, a short duration delta-like signal provided by different types of transducers is

typically employed to excite the sample under test (SUT) within an extended bandwidth.

Features of interest, e.g. presence of flaws and inclusions inside the SUT, can be extracted

by analyzing the system’s impulse response both in time and frequency domains. However,

the maximum achievable Signal-to-Noise Ratio (SNR) is directly related to the source power.

This limit can be overcome by using coded excitations together with Pulse Compression

(PuC) technique. In fact, in PuC the frequency spectrum of the coded excitation can be

tailored to suit the investigation of a given sample, while the SNR can be increased almost

arbitrarily by enhancing the signal time duration. Finally, the system’s impulse response is

retrieved by applying the PuC algorithm on the acquired data. In this paper, the use of PuC

technique in IRT, UT, ECT will be reviewed and discussed.

https://creativecommons.org/licenses/by/4.0/

THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW

Muhammad Khalid Rizwan1, Hamed Malekmohammadi1, Stefano Laureti1, Pietro Burrascano1, David A. Hutchins2, Gui Yun Tian3, Junzhen Zhu3, Qiuji Yi3, Marco Ricci4

1Department of Engineering

University of Perugia, Italy

2School of Engineering

University of Warwick, UK

3School of Engineering

Newcastle University, UK

4DIMES Department

University of Calabria, Italy


Outline→ Background

→ Limits of PuE approach

→ Pulse Compression (PuC)

• Basic principle

• Signals for PuC

• Choice of the Signals

→ Application in NDT

• PuC Ultrasonic

• PuC Thermography

• PuC Eddy Current Testing

→ Conclusions

→ References


BackgroundGeneral procedures in Ultrasonic, Eddy Current, Thermography NDT inspections are based on

measuring the response ℎ(𝑡) of the sample under test (SUT) to an impulsive excitation which

approximates a delta 𝛿(𝑡) like function (e.g. Pulse Echo)

ℎ(𝑡)𝛿(𝑡)


Background

• In circuit theory, the response to a delta 𝛿(𝑡) excitation is called impulse response h(t) and it describes the input-output relation of the system as

𝑦 𝑡 = 𝑥(𝑡)⨂ℎ(𝑡)

Measuring ℎ 𝑡 allows the inspection of the sample as well as the calculation of the response of the SUT to any other possible excitation


Limits of the Pulse-echo approach

→ Maximum achievable SNR is not sufficient for the defect detection and characterization in the presence of noise or high attenuation

→ Aim is to measure the impulse response even in the presence of noise, to enhance sensitivity of the system

How?

→ Increase energy of the excitation pulse Amplitude of the pulse; constrained by the system and the transducer’s limitations

→ Averages Long time of measurements


Enhance SNR HOW?

Proposed Solution

Pulse Compression

An alternative technique to increase the SNR of the measurement setup

increase the time bandwidth product increase the time duration of

the excitation pulse rather than the amplitude

❖ Flexible both in time and frequency shaping

❖Widely applicable

❖ Various techniques to implement


➢ Pair of signals whose correlation is a delta 𝛿(𝑡) like function, i.e.,

𝑔 𝑡 ∗ Ψ 𝑡 = መ𝛿(𝑡)

➢ Excite the Linear (and in some cases nonlinear) system with one of the signals, 𝑔 𝑡

➢ Impulse response of the unknown system can be estimated when the output is

convolved to the second signal Ψ 𝑡 in the pair, i.e.,

𝑦 𝑡 ∗ Ψ 𝑡 = 𝑔 ∗ ℎ 𝑡 ∗ Ψ 𝑡 = ℎ 𝑡 ∗ መ𝛿 𝑡 ≃ 𝐶ℎ(𝑡)

➢ SNR is maximized when Ψ 𝑡 = 𝑔 −𝑡 matched filter theory

Pulse Compression (PuC)- basic principle


Pulse Compression- basic principle

𝑔(𝑡)

𝑦 𝑡 = [ℎ ∗ 𝑔] 𝑡 + 𝑛(𝑡) ℎ 𝑡 ≃𝐶 × ℎ 𝑡 + 𝑛(𝑡)LTI

system

Matched

Filter

ℎ(𝑡) 𝛹 𝑡 = 𝑘𝑔(−𝑡 + 𝑇)𝑛 𝑡noise

Excitati

on

signal

𝑔(𝑡)Matche

d

filter

𝛹 𝑡

Estimat

ed

A-scan

ℎ 𝑡

☺ High SNR Required SNR Gain can be achieved by increasing the time duration of the excitation pulse without effecting the bandwidth

Bandwidth is independent of TPuC

☺ Flexibility in time and frequency domains shaping

☺ No hardware complexity for implementation

Advantages


Signals for PuC

➢ Ideally, the signal should prevail the property of autocorrelation as;

𝑅𝑔𝑔 = 𝛿(𝑡)

➢ In Real cases, the closer the correlation 𝑅𝑔𝑔 = መ𝛿(𝑡)~𝛿(𝑡), the

more accurate is the estimation of the impulse response, ℎ(𝑡)~ℎ(𝑡)

➢ Practically, frequency spectrum of the selected signal should be wide

enough to cover the bandwidth of the sample under test


Applications of PuC in NDT

→ Wave Phenomenon Ultrasonic NDT

▪ SNRGain is significantly improved widely applicable

▪ h(t) consists of several reflections/paths

▪ Due to propagation, duration of impulse response, Th is much longer than the required time

resolution δT, i.e., Th >> δT ThB >>1 SNRGain is high

→ Diffusion Phenomenon EDDY Current, Thermography

▪ SNRGain is reasonable, but other advantages exist

▪ h(t) is smooth and exponentially decaying

▪ Time duration of the impulse response Th is greater than time resolution δT, but reasonable


Choice of the Signals

Swept frequency Signals:

Chirps (both linear and nonlinear)

Chirps are the widely used signals in PuC

NDT Easy control over the bandwidth and almost a flat spectrum

Coded Signals:

Binary codes, mapped into continuous waveforms (bits in the form of square wavelets), e.g. Multi-Length Sequences (MLS), Golay Complementary Sequences, Barker Codes, etc.


PuC Ultrasonic NDT: Schematic of the Setup

Signal for PuC

Signal Generation: Arbitrary waveform Generator

Transmission Transducer

Sample Under Test

Reception Transducer

Low Noise Amplifier

Data Acquisition:Digital Oscilloscope

Post Processing and Display Unit

Back wall EchoDefect

Acquired Signal Impulse Response 𝒉(𝒕) of the SUT


PuC Ultrasonic NDT: Results comparison to PuE

▪ PuC (using a Linear chirp as anexcitation signal) and PuE UTinspections of a steel forging,1.5m long and diameter of0.6m

▪ Using the same transducers,V108-VEDIOSCAN fromOlympus

▪ AWG Vp-p;❖ in case of PuE 200V

❖ in case of PuC 24V

SNRGain at Defect

SNRGain is enhanced significantly in case of PuC


PuC Ultrasonic NDT: Results

▪ Echograms of PuC UTinspection of forgings

▪ SNRGain is significantlyhigh, because theduration of the signal(4m) is much higherthan the resolution (inmm)

▪ Exploit the advantage ofbroad bandwidth of aLinear Chirp

▪ Obtain the optimumfrequency of Inspection


PuC Thermography, Setup

Schematic of the PuC based thermographic Experiment [1]

The Setup at our Lab (the NDT Laboratory, Terni) [2]

[1],[2] Diagram and the setup from presentation of Hamed Malekmohammadi,

Comparison of optimization strategies for the improvement of depth detection capability of Pulse-Compression Thermography, QIRT-2018

o Heating Source: 8 x 50W LED chips

o Thermal Camera: Xenics Onca-MWIR-InSb IR

o Signal Generation/Acquisition: LabviewTM

o Camera in Reflection mode, 40 FPS


PuC Thermography, Results

▪ CFRP sample with defectsat different depths

▪ Impulse Response ℎ 𝑡 isslow, SNRGain is not veryhigh

▪ Binary Sequences, BarkerCode is used for PuC


PuC Eddy Current Inspections, Setup

Time Amplitude and Phase response


➢ Comparison of Time and Frequency domain analysis for the defect detection, using Linear chirps (varying duration of the signal)

➢ A surface defect of (0.1x0.4x3 mm3), covered with another sound plate of thickness = 2 mm

PuC Eddy Current Inspections, Results

➢ SNRGain is better because the time resolution of impulse response is in between the UT and thermographic NDT

➢ In this case, time domain analysis outperforms the frequency domain analysis in terms of defect detection

➢ One can obtain more robustness of the system to the lift off variations, while applying time or frequency phase analysis


• PuC procedures enhances the sensitivity of NDT techniques,

• As discussed, the PuC methods,➢ Applied in UT inspection of large structures, improved SNR has resulted in better spatial and temporal

resolution➢ Applied in Eddy currents testing, lift off compensation can be achieved via time and frequency phase

analysis➢ Applied in thermographic, thermograms with improved SNR and fidelity can be achieved

• The PuC method can easily be incorporated to the NDT systems with simple hardware

• Flexibility over the time and frequency domain shaping of the signal, spectral density of the excitation pulse can developed well matched with the bandwidth of the system

• One can exploit the broadband of the PuC procedure to obtain a multifrequency defect evaluation system, to gain the optimum frequency of inspection for a specified SUT

• An NDT system with improved SNR gain opens further application of the method for the inspection of highly attenuating materials and structures, like CFRP and GFRP

• Performance of the PuC NDT system can further be improved combing other advanced signal and image processing techniques

Conclusions


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2. Pollakowski, Martin, and Helmut Ermert. "Chirp signal matching and signal power optimization in pulse-echo mode ultrasonic nondestructive testing." IEEE transactions on ultrasonics, ferroelectrics, and frequency control 41.5 (1994): 655-659

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6. Callegari, Sergio, et al. "From chirps to random-FM excitations in pulse compression ultrasound systems." 2012 IEEE International Ultrasonics Symposium. IEEE, 2012.

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10. Ricci, Marco, et al. "Exploiting non-linear chirp and sparse deconvolution to enhance the performance of pulse-compression ultrasonic NDT." Ultrasonics Symposium (IUS), 2012 IEEE International. IEEE, 2012.

11. Oelze, Michael L. "Bandwidth and resolution enhancement through pulse compression." IEEE transactions on ultrasonics, ferroelectrics, and frequency control 54.4 (2007).

12. Silipigni, Giuseppe, et al. "Optimization of the pulse-compression technique applied to the infrared thermography nondestructive evaluation." NDT & E International 87 (2017): 100-110.

13. Laureti, S., et al. "Comparative study between linear and non-linear frequency-modulated pulse-compression thermography." Applied Optics 57.18 (2018): D32-D39.

14. Laureti, S., et al. "The use of pulse-compression thermography for detecting defects in paintings." NDT & E International 98 (2018): 147-154.15. Betta, Giovanni, et al. "An experimental comparison of multi-frequency and chirp excitations for eddy current testing on thin defects." Measurement 63 (2015): 207-220.

16. Betta, Giovanni, et al. "On the use of complex excitation sequences for eddy current testing." Proceeding of 19th IMEKO TC4 Symposium, Barcelona, Spain. 2013.

References


Acknowledgements

This research work has been supported from the European

Union’s Horizon 2020 research and innovation programm under

the Marie Skłodowska-Curie grant agreement No 722134 –

NDTonAIR

www.ndtonair.eu

http://www.ndtonair.eu/

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The Use of Pulse Compression Technique in Non-Destructive ... · THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW Muhammad Khalid Rizwan1, Hamed Malekmohammadi1,