Detection of concealed weapons using acoustic waves

291 Pages·2013·38.11 MB·English

by  George A Vadakkel

T U O M
HE NIVERSITY F ANCHESTER
Detection of concealed
weapons using acoustic
waves
A thesis submitted to the University of Manchester for the
degree of Doctor of Philosophy in the Faculty of Engineering and
Physical Sciences

2012

George A Vadakkel

School Of Mechanical, Aerospace And Civil Engineering
ABSTRACT
Existing weapon detection systems such as metal detectors and X-ray baggage scanners
have many drawbacks. While metal detectors can only detect metallic objects, X-ray
scanners are unsafe for use on passengers. Also, these systems can only scan people
within a short range. These limitations of detecting potentially harmful objects have led
to tragic events such as the 9/11 attack on the world trade centre and the 2008 terrorist
attack in Mumbai. Development of more advanced security systems would help in
curbing such terrorist attacks. These systems could also be used to help security officials
in tackling knife and gun related crimes in the streets. The aim of this research is to
develop a concealed weapon detection system using acoustic waves. Ideally, the system
would have large standoff distance, should be cost-effective and easy to manufacture
and would be able to detect both metal and non-metallic weapons. Different techniques
such as acoustic signature, resonance acoustic spectroscopy and acoustic imaging were
analysed. Acoustic signature techniques identify the target by comparing the acoustic
waves reflected by the target to a database of previously recorded acoustic reflections.
Resonance acoustic spectroscopy was used on the data acquired using both
experimental measurements and Finite Element simulations. A series of resonant
frequencies from the acoustic waves reflected by the concealed target were extracted
using this technique. This series of resonant frequencies that are unique to the target
were used to identify the target. Acoustic camera was used to experimentally record the
acoustic reflection from different targets. This was then used to develop images of
concealed targets. These tests were performed using commercially available array
speaker systems. The probability of improving these results using a better designed
ultrasonic or acoustic array speaker system was analysed. This was done by changing
different array design parameters and obtaining a highly focused acoustic beam. The
results from the experimental tests and Finite Element simulations proved the possibility
of using acoustic waves for concealed weapon detection. In the acoustic signature
measurements, the frequency spectra of the reflected acoustic waves were shown to be
different for different targets. The results from resonance acoustic spectroscopy showed
structural resonant frequencies in the frequency spectra that corresponded to the natural
frequency of the target. Using acoustic camera kit the image of the concealed target was
identified. The array results showed the formation of focused beams for different array
configurations. The results showed the formation of grating lobes and side lobes when
the inter-element gap became larger than the wavelength of sound waves at the
excitation frequency. Finally, a program using neural network was developed to
demonstrate how the natural frequencies from the target could be used to identify them.
This research work provides a proof of concept of different acoustic wave-based
detection and imaging techniques. It has shown the possibility of detecting concealed
targets at standoff distances. Using parametric arrays highly focused acoustic or
ultrasonic beams could be generated which could be focused on a person suspected of
carrying a weapon in a crowded environment. The sound waves reflected back could be
analysed using the resonance acoustic spectroscopic technique or one could use the
acoustic camera to generate images of targets in real-time. The use of acoustic waves
would also help in keeping the cost and complexity of the equipment to a minimum. It
also ensures that the public is not exposed to any harmful radiation. The techniques
described in this thesis would significantly support the development of a commercially
viable, robust acoustic waves based concealed weapon detection system.
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DECLARATION
“I, George.A.Vadakkel hereby declare that no part of the dissertation in part or full is
used in any other documentation submitted for a degree or qualification in this or any
other institute or University.

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ACKNOWLEDGMENT
I thank god for his blessings and for bringing me to this point in life. I would like to
express my gratitude towards my family and friends who have supported me throughout
my studies. I thank my parents Mr Aby Vadakkel and Mrs Mary Vadakkel, my sister
Mrs Anju and my wife Rose for their support and encouragement.
I take this opportunity to thank my supervisor Dr S Olutunde Oyadiji for taking me
under his wings, for his guidance and support and above all for his faith in me. I thank
him for the countless hours that he had spent advising me on my research. I also thank
him for his support and encouragement in securing the University Scholarship and the
funding towards my living expenses
My sincere gratitude and thanks to Ms. Beverley Knight, Ms Michelle Ringwood and
other MACE administrative staffs for their tireless and proactive efforts in making my
PhD run smoothly.
I would like to acknowledge the financial, academic and technical support from the
University of Manchester and its staff, particularly in the award of a Postgraduate
Research Scholarship.
I also would like to thank the Engineering and Physical Sciences Research Council
(EPSRC), The Home Office, and The London Metropolitan Police, who had originally
sponsored the project.
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COPYRIGHT STATEMENT
Copyright in text of this dissertation rests with the author. Copies (by any process)
either in full, or of extras, may be made only in accordance with the instructions given
by the author. Details may be obtained from the appropriate graduate office. This page
must form part of any copies made. Further copied (by any process) of copies made in
accordance with such instructions may not be made without the permission (in writing)
of the author.
The ownership of any intellectual property rights which may be described in this
dissertation is vested in The University of Manchester, subject to any prior agreement to
the contrary, and may not be made available for use by third parties without written
permission of the University, which will prescribe the terms and condition of any such
agreement.
Further information on the condition under which disclosure and exploitation may take
place is available from the Head of the School of Mechanical, Aerospace and Civil
Engineering.

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CONTENTS
LIST OF FIGURES
LIST OF TABLES
1. INTRODUCTION ............................................................................. 24
1.1. Background .............................................................................................. 24
1.2. Aims and objectives ................................................................................. 25
1.3. Thesis overview ....................................................................................... 26
1.4. Original contributions to knowledge ......................................................... 29
1.5. Thesis structure ........................................................................................ 30
2. LITERATURE REVIEW ..................................................................... 33
2.1. Acoustic wave based CWD systems ........................................................ 33
2.2. Resonance acoustic spectroscopy ........................................................... 35
2.3. Imaging of concealed target using an acoustic camera ............................ 36
2.4. Millimetre wave based CWD systems ...................................................... 37
2.5. Terahertz wave based CWD systems ...................................................... 39
2.6. Back scattered X-Ray waves based CWD system ................................... 40
2.7. Magnetic field based CWD system........................................................... 42
2.8. Infrared based CWD systems .................................................................. 43
2.9. CWD using data fusion ............................................................................. 43
2.10. Development of parametric arrays for CWD system ................................ 44
2.11. Summary .................................................................................................. 45
3. THEORETICAL BACKGROUND ......................................................... 46
3.1. Introduction .............................................................................................. 46
3.2. Resonance acoustic spectroscopy ........................................................... 46
3.3. Reflection coefficient ................................................................................ 47
3.4. Directional sound source using AM signals .............................................. 48
3.5. Time delay and sum beamforming algorithm for phased array
speakers ................................................................................................... 50
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3.6. Time delay and sum beamforming algorithm for acoustic
camera ..................................................................................................... 51
3.7. Finite element simulations ........................................................................ 53
3.7.1. Acoustic wave equation ................................................................................. 53
3.7.2. Finite element discretisation .......................................................................... 54
3.8. Summary .................................................................................................. 55
4. FINITE ELEMENT SIMULATION OF RESONANCE ACOUSTIC
SPECTROSCOPY ............................................................................. 56
4.1. Introduction .............................................................................................. 56
4.2. Description of the finite element simulations ............................................ 57
4.3. Mesh size ................................................................................................. 59
4.3.1. Numerical calculation..................................................................................... 59
4.3.2. Mesh convergence study ............................................................................... 60
4.4. Finite element models .............................................................................. 61
4.5. Results ..................................................................................................... 63
4.5.1. Modal analysis of the targets ......................................................................... 63
4.5.2. Sound pressure distribution ........................................................................... 70
4.5.3. Steady state Sine wave excitation ................................................................. 71
4.5.4. Chirp excitation .............................................................................................. 74
4.5.5. Continuous sinusoidal excitation at discrete frequencies: .............................. 75
4.5.6. Short pulse excitation and time gating the acoustic response ........................ 77
4.5.7. Short Time Fourier Transform of the acoustic response ................................ 78
4.6. Summary .................................................................................................. 83
5. EXPERIMENTAL TESTS TO DETECT ACOUSTIC SIGNATURE .............. 85
5.1. Introduction .............................................................................................. 85
5.2. Hardware and software ............................................................................ 86
5.2.1. Sensors and transducers ............................................................................... 86
5.2.2. Data acquisition hardware ............................................................................. 87
5.2.3. Softwares used .............................................................................................. 88
5.2.4. General layout ............................................................................................... 88
5.3. Measurement setup and targets ............................................................... 89
5.4. Testing the speakers ................................................................................ 91
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5.4.1. Response from speakers ............................................................................... 91
5.4.2. Beam cross section area ............................................................................... 94
5.4.3. Acoustic beam characteristics ....................................................................... 94
5.5. Response from unconcealed targets ........................................................ 99
5.5.1. Short Time Fourier Transform of acoustic response ...................................... 99
5.5.2. Frequency Response Function .................................................................... 104
5.5.3. Wavelet Analysis ......................................................................................... 106
5.6. Response from concealed targets .......................................................... 110
5.6.1. Short Time Fourier Transform of acoustic response .................................... 110
5.6.2. Wavelet ....................................................................................................... 112
5.7. Summary ................................................................................................ 113
6. EXPERIMENTAL VERIFICATION OF RESONANCE ACOUSTIC
SPECTROSCOPY FOR IDENTIFICATION OF TARGETS...................... 115
6.1. Introduction ............................................................................................ 115
6.2. Measurement setup................................................................................ 116
6.2.1. Equipment used........................................................................................... 116
6.2.2. Layout of the measurement setup ............................................................... 117
6.2.3. Excitation signals ......................................................................................... 118
6.2.4. The targets chosen for the experiment ........................................................ 119
6.2.5. Measurement of the natural frequencies of different targets ........................ 119
6.3. Results ................................................................................................... 120
6.3.1. Natural frequencies of targets ...................................................................... 120
6.3.2. Double differentiation and STFT of the acoustic reflections ......................... 123
6.3.3. Acoustic response spectrum from different targets ...................................... 126
6.3.4. RAS using chirp and pulse excitation signals ............................................... 129
6.3.5. Time gating the pulse response spectrum ................................................... 132
6.3.6. Extracting the natural frequencies of an intact composite plate
target ........................................................................................................... 135
6.3.7. Comparing composite plates with and without delamination ........................ 138
6.3.8. Extracting the natural frequencies of knife targets ....................................... 141
6.3.9. Tests carried out on concealed targets ........................................................ 142
6.3.10. Concealed plate -1 under chirp excitation .................................................... 143
6.3.11. Concealed plate -1 under pulse excitation ................................................... 148
6.3.12. Concealed aluminium cylinder under chirp excitation .................................. 151
6.3.13. Concealed aluminium cylinder under pulse excitation .................................. 158
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6.4. Summary ................................................................................................ 158
7. ACOUSTIC IMAGING .................................................................... 160
7.1. Introduction ............................................................................................ 160
7.2. Equipment .............................................................................................. 161
7.2.1. Speakers ..................................................................................................... 161
7.2.2. Excitation signal ........................................................................................... 162
7.2.3. Targets ........................................................................................................ 162
7.2.4. Acoustic camera .......................................................................................... 164
7.2.5. LMS Test Lab and Scadas front end ............................................................ 165
7.3. Measurement setup................................................................................ 165
7.4. Discussion of results .............................................................................. 166
7.4.1. Acoustic images generated using the electromechanical speaker .............. 166
7.4.2. Acoustic images generated using the audio spotlight speaker
system ......................................................................................................... 168
7.5. Summary ................................................................................................ 175
8. INVESTIGATING DIFFERENT DESIGN PARAMETERS OF AN
ULTRASONIC ARRAY USING NUMERICAL SIMULATIONS .............. 176
8.1. Introduction ............................................................................................ 176
8.2. Creating the FE models in Ansys ........................................................... 177
8.3. Creating the acoustic model in Abaqus .................................................. 178
8.3.1. Linear array ................................................................................................. 178
8.3.2. Curved array ................................................................................................ 180
8.4. Results from analyses of linear and curved arrays ................................. 183
8.4.1. Linear array with 100 elements and excitation from 2 kHz to 12
kHz .............................................................................................................. 183
8.4.2. Linear array with different number of transducer .......................................... 183
8.4.3. Curved array configurations ......................................................................... 185
8.4.4. Results from linear array 1 kHz to 5 kHz excitation frequencies ................... 186
8.4.5. Results from curved array simulated in Abaqus – 1 k to 20 kHz .................. 189
8.4.6. Results from curved arrays at 39 kHz excitation frequency .......................... 195
8.5. Analysis of different array parameters .................................................... 196
8.5.1. Introduction .................................................................................................. 196
8.5.2. Influence of inter element spacing on beam focusing properties .................. 197
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8.5.3. Influence of the number of array elements N on beam properties ............... 200
t
8.5.4. Geometric versus acoustic focus and its effect on the
characteristic of the acoustic beam .............................................................. 203
8.5.5. Influence of the frequency on the characteristics of the acoustic
beam ........................................................................................................... 204
8.5.6. Summary ..................................................................................................... 205
8.6. Results with a target ............................................................................... 207
8.7. Summary ................................................................................................ 209
9. DESIGN AND TESTING OF TRANSDUCER ARRAYS ......................... 211
9.1. Introduction ............................................................................................ 211
9.2. Sensitivity of 400PT160 ultrasonic transceiver as a transmitter ............. 212
9.3. Sensitivity of 400PT160 ultrasonic transceiver as receiver .................... 213
9.4. Sound attenuation through different clothing materials .......................... 214
9.5. Design of parametric transducer arrays ................................................. 216
9.5.1. Ultrasonic transceiver array ......................................................................... 216
9.5.2. Acoustic transducer array ............................................................................ 220
9.6. Test setup for ultrasonic transducer array .............................................. 220
9.7. Test setup for sound pressure measurement along the length
of the beam ............................................................................................ 223
9.8. Test setup for measuring the beam pattern and directivity ..................... 223
9.9. Results from the measurement of sound pressure at different
distances from the array ......................................................................... 225
9.10. Results from beam directivity measurements ......................................... 226
9.11. Tests done to identify target using ultrasonic transducer array .............. 227
9.12. Summary ................................................................................................ 229
10. TARGET IDENTIFICATION USING NEURAL NETWORK ................... 230
10.1. Introduction ............................................................................................ 230
10.2. Neural Network ...................................................................................... 230
10.2.1. Architecture ................................................................................................. 230
10.2.2. Working principle ......................................................................................... 232
10.2.3. Performance of the neural network .............................................................. 232
10.3. Training data generation ........................................................................ 233
10.3.1. Training dataset D1 using beam equation.................................................... 233
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