EFFORTS TOWARDS THE DESIGN AND DEVELOPMENT OF AN ELECTROMAGNETIC 
INDUCTION SENSOR OPTIMIZED FOR DETECTION AND DISCRIMINATION OF  
UNEXPLODED ORDNANCE 
 
Except where reference is made to the work of others, the work described in this thesis is 
my own or was done in collaboration with my advisory committee. This thesis does not 
include proprietary or classified information. 
 
 
_________________________________________ 
Jithendra N Palasagaram 
 
Certificate of Approval: 
 
________________________   _______________________  
Sadasiva M. Rao                                      Lloyd S. Riggs, Chair 
Professor                     Professor 
Electrical and Computer Engineering   Electrical and Computer Engineering 
 
 
 
________________________   _________________________ 
Ramesh Ramadoss     Stephen L. McFarland 
Assistant Professor                Dean 
Electrical and Computer Engineering   Graduate School 
 
EFFORTS TOWARDS THE DESIGN AND DEVELOPMENT OF AN ELECTROMAGNETIC 
INDUCTION SENSOR OPTIMIZED FOR DETECTION AND DISCRIMINATION OF  
UNEXPLODED ORDNANCE 
 
Jithendra N Palasagaram 
 
A Thesis 
Submitted to 
the Graduate Faculty of 
Auburn University 
in Partial Fulfillment of the 
Requirements for the  
Degree of 
Master of Science 
 
Auburn, Alabama 
May 11, 2006 
 
 iii
EFFORTS TOWARDS THE DESIGN AND DEVELOPMENT OF AN ELECTROMAGNETIC 
INDUCTION SENSOR OPTIMIZED FOR DETECTION AND DISCRIMINATION OF  
UNEXPLODED ORDNANCE 
 
Jithendra N Palasagaram 
Permission is granted to the Auburn University to make copies of this thesis at its 
discretion, upon the request of individuals or institutions and at their expense. The author 
reserves all publication rights. 
 
 
       _______________________ 
       Signature of Author 
 
       _______________________ 
       Date of Graduation 
 
 
 iv
VITA 
Jithendra N Palasagaram, son of Nagarajeswara Rao P, and Rajamma P, was born 
on November 15, 1979 in Miryalguda, India. After finishing high school from Crescent 
Jr. College in 1997, he attended Bangalore University, India where he received his 
Bachelor of Engineering (Electronics and Communication) degree in 2001. He entered 
graduate school at Auburn University in Fall of 2003 as a graduate research assistant, and 
later went on to accept a fellowship through Auburn University.    
 
 
 
 
 
 
 
 
 
 
 
 v
THESIS ABSTRACT 
EFFORTS TOWARDS THE DESIGN AND DEVELOPMENT OF AN ELECTROMAGNETIC 
INDUCTION SENSOR OPTIMIZED FOR DETECTION AND DISCRIMINATION OF  
UNEXPLODED ORDNANCE 
 
Jithendra N Palasagaram 
Master of Science, May 11, 2006 
(B. E., Golden Valley Institute of Technology, Bangalore University, 2001) 
  
 101 Typed Pages 
Directed by Dr. Lloyd S. Riggs 
This thesis provides a detailed description of efforts towards the development of a time 
domain electromagnetic induction (EMI) system optimized for detection and 
discrimination of Unexploded Ordnance (UXO). A time domain (or pulsed) EMI system 
with good discrimination capability must be able to accurately capture a UXO target?s 
late-time response.  Here, late-time response is defined as the time period starting a few 
hundred microseconds after the transmitter current turn off and extending to 
approximately 20 milliseconds or to just before the beginning of the next transmitter 
current pulse. A pulsed EMI system must have sufficient bandwidth, especially at the 
lower end, to accurately measure a UXO target?s late time response. EMI system 
components include transmitter and receiver coils and corresponding transmitter and 
 vi
receiver coil amplifiers as well as the data acquisition system (analogue-to-digital 
converter). This thesis addresses EMI system component design and tradeoffs necessary 
to achieve the bandwidth and sensitivity for good UXO detection and discrimination 
performance. 
Field measurements were taken at the Blossom Point Test Site in Maryland in 
order to evaluate the detection and discrimination performance of the pulsed EMI sensor.  
Although overall performance was good, several deficiencies were noted. In particular, 
the low 3dB frequency of the receiver coil amplifier was inadvertently set too high 
(above 1 kHz) which resulted in a prematurely truncated late-time response. In short, we 
did not observe the expected long exponential ?tails? indicative of larger ferrous test 
targets. This problem has recently been corrected using a current-to-voltage converter as 
the first amplification stage following the receiver coil.  The current-to-voltage converter 
has very low input impedance so that the system lower 3 dB frequency is fixed solely by 
the resistance-inductance ratio of the receiver coil (currently set to 80 Hz).     
Testing also indicated a deficiency in system sensitivity. This problem has been 
attributed to insufficient receiver coil moment; specifically, the area of each half of the 
currently employed bucking receiver coil is too small.  Future plans are to replace each 1 
ft
2
 half of the bucking receiver coils with a vertically stacked arrangement of two 1 m
2
 
coils, one above and the other below the transmitter coil. The moment of the new coils 
will increase by a factor of 10 over that of the old and thus overall sensitivity should 
increase by the same factor. Additionally, future plans include purchasing a 1 kW 
transmitter coil amplifier capable of delivering over 40 amps at rated voltage (22.5 V 
rms). The transmitter coil amplifier used in the Blossom Point tests delivered a peak 
 vii
current of less than 20 amps so the new amplifier, all other system parameters held 
constant, should improve overall sensitivity by more than 3 dB.  
Of course sensitivity can be improved by either increasing the received signal 
level or by reducing noise. A ?boot strapped? differential amplifier has been tested that 
has significantly improved noise performance compared to the previously used single 
ended amplifier. Furthermore, testing is currently underway to evaluate a three 
operational amplifier configuration that should even further reduce noise yet still have 
very low input impedance and thus very good low frequency performance.  
Lastly, during the Blossom Point tests some problems were encountered with the 
test cart wheel bearings. At the very least we plan to replace the ?home made? PVC 
wheel bearings with manufactured bearings. We learned about the source of the 
manufactured bearings from a colleague at a recent UXO workshop. 
In addition to the description of efforts towards the development of an EMI sensor 
optimized for detection and discrimination of UXO, a brief description of the efforts 
towards the development of a Microelectromechanical Systems (MEMS) Capacitive 
Pressure Sensor Array fabricated using Printed circuit processing techniques is also 
presented in this thesis. MEMS capacitive pressure sensors are usually fabricated using 
silicon micromachining techniques. In this work, novel Liquid Crystal Polymer (LCP) 
based capacitive pressure sensors fabricated using printed circuit processing techniques 
are presented. The design, fabrication and experimental characterization of the sensor are 
presented. 
 viii
ACKNOWLEDGEMENTS 
My deepest respect and appreciation goes to my advisor, Dr. Lloyd S. Riggs for 
his guidance, support and encouragement provided in my journey towards the Master?s 
Degree in Engineering. I will forever be grateful for his endless advice, incredible 
patience, generosity and friendship. I am also grateful to my committee member and co-
advisor Dr. Ramesh Ramadoss for his guidance and support.  
I would like to thank all my research team members Sailaja, Raghav, Harish, and 
Kerry for their friendship, support and all the stimulating discussions. I would also like to 
thank Bill for his help in developing the electronic hardware of the system. 
I take the opportunity to thank Mr. Charles Ellis, Mr. William Baugh, Ms. Linda 
Barresi and Mr. Joe Haggerty, for their support and help. 
I would like to thank my father and mother, my sisters Prasanna and Jyotsna, and my 
brother-in-laws Soma Sekhar and Chakrapani for their enduring love, immense moral 
support and encouragement throughout my life.  
 
 
 
 
 ix
Style manual of journal used Graduate School: Guide to preparation and 
submission of theses and dissertations_________________________________________ 
 
Computer software used Microsoft Office XP_____________________________ 
 
 
 
 
 
 
 
 
 
 
 
 
 x
TABLE OF CONTENTS 
 
LIST OF FIGURES .......................................................................................................... xii 
LIST OF TABLES............................................................................................................ xv 
1  INTRODUCTION .......................................................................................................... 1 
2  THEORY OF ELECTROMAGNETIC INDUCTION SENSOR................................... 4 
2.1   Introduction............................................................................................................ 4 
2.2   EMI System Configuration .................................................................................... 4 
2.3   Circuit Model for a CW EMI System .................................................................... 5 
2.4   Circuit Model for a Time Domain EMI System .................................................... 9 
2.4.1 Coupling Between the Transmitter and Buried Object............................... 10 
2.4.2 Voltage at the Receiver- Open Circuit Case............................................... 14 
2.5   Summary .............................................................................................................. 17 
3  HARDWARE DESIGN AND IMPLEMENTATION ................................................. 19 
3.1   Transfer Function Characterization of the CW EMI System............................... 19 
3.2   Optimal Number of Transmitter Coil Turns: Case 1 V
OUT
/V
S
 Measurement...... 21 
3.3   Receiver coil and Receiver coil amplifier considerations for Case 1: V
OUT
/V
S
 
Measurement................................................................................................................ 23 
3.4   Optimal Number of Transmit Coil Turns: Case 2 Object Current Measurement 24 
3.5   Receiver coil and Receiver coil amplifier considerations for Case 2: Object     
Current Measurement................................................................................................... 25 
3.6   Block Diagram of a Pulsed EMI System ............................................................. 27 
3.7   Transmit Current Generation ............................................................................... 29 
3.8   Transmitter and Receiver Coil Construction ....................................................... 32 
3.9   Receiver signal Conditioning............................................................................... 33 
3.10 Data Acquisition .................................................................................................. 35 
3.11 Summary .............................................................................................................. 38 
4  PULSED EMI MEASUREMENTS ............................................................................. 39 
4.1   Laboratory Measurements.................................................................................... 39 
4.2   Field Measurements ............................................................................................. 44 
4.2.1 Pit Measurements........................................................................................ 44 
4.2.1.1 Response of 4 inch Carbon Steel Sphere ....................................... 45 
 
 xi
4.2.1.2 Response of the 105 mm shell ....................................................... 47 
4.2.1.3 Response of 81 mm mortar ............................................................ 48 
4.2.1.4 Response of 40 mm shell ............................................................... 49 
4.2.1.5 Response of Cylinder E.................................................................. 49 
4.2.1.6 Response of Clutter........................................................................ 50 
4.2.2 Dynamic Testing......................................................................................... 51 
4.2.2.1 Testing using MTADS trailer ........................................................ 52 
4.2.2.2 Testing using Custom made Cart ................................................... 53 
4.3   Summary .............................................................................................................. 54 
 
5  CONCLUSION............................................................................................................. 56 
A  FIELD MEASUREMENTS AT THE BLOSSOM POINT TEST SITE..................... 58 
A.1   Pit Measurements ................................................................................................ 58 
A.1.1 Data collected for 4 inch Carbon Steel Sphere.......................................... 61 
A.1.2 Data collected for 105 mm shell................................................................ 62 
A.1.3 Data collected for 81 mm mortar............................................................... 63 
A.1.4 Data collected for 40 mm shell.................................................................. 64 
A.1.5 Data collected for Cylinder E .................................................................... 65 
A.1.6 Data collected for Clutter........................................................................... 66 
A.2   Dynamic Testing ................................................................................................. 67 
A.2.1 Testing using MTADS trailer .................................................................... 68 
A.2.2 Testing using Custom made Cart............................................................... 69 
 
B  MICROELECTROMECHANICAL SYSTEMS (MEMS) CAPACITIVE   
PRESSURE  SENSOR ARRAY FABRICATED USING PRINTED CIRCUIT 
PROCESSING TECHNIQUES........................................................................................ 71 
B.1   Introduction ......................................................................................................... 71 
B.2   Fabrication and Assembly ................................................................................... 73 
B.2.1 Sensor Configuration ................................................................................. 73 
B.2.2 Sensor fabrication ...................................................................................... 74 
B.2.3 Lamination Process.................................................................................... 76 
B.3.1 Design ........................................................................................................ 77 
B.3.2 Experimental Characterization................................................................... 78 
B.3.2.1 Scanning Acoustic Microscope Measurements............................. 78 
B.3.2.2 Measurement Setup ....................................................................... 80 
B.3.2.3 Capacitance-to-voltage conversion circuit .................................... 80 
B.3.2.4 Results and Discussion.................................................................. 82 
B.4   Conclusion........................................................................................................... 83 
 
BIBLIOGRAPHY............................................................................................................. 84 
 
 xii
 LIST OF FIGURES 
 
2.1 Schematic of a typical EMI system .............................................................................. 4 
2.2 Magnetic Coupled Circuit used to represent a Continuous Wave (CW) 
Electromagnetic Induction Circuit...................................................................................... 6 
2.3 Frequency response of a simple first order target....................................................... 10 
2.4 Circuit model describing the charge up and decay of object currents ........................ 12 
2.5 Normalized transmitter current versus normalized time............................................. 12 
2.6 Object current versus time .......................................................................................... 13 
2.7 Normalized object current vs the ratio of shutoff time to object time constant.......... 14 
2.8 Representative plots for (a) Transmitter Current (b) Object Current (c) Object 
Coupled Receiver Voltage and (d) Direct Coupled Receiver Voltage. Dotted vertical   
line marks the time t=T..................................................................................................... 16 
3.1  Transfer function characteristics of a high pass filter: (jf/10)/ (1+jf/10)................... 22 
3.2  Operational amplifier connected in non-inverting configuration to the receiver     
coil. The parasitic capacitance of the receiver coil distorts measurements of the     
object?s response............................................................................................................... 24 
3.3 Operational amplifier connected in an inverting configuration as a                    
current-to-voltage converter.............................................................................................. 25 
3.4 Plot of break frequency of the receiver coil as a function of number of turns ........... 26 
3.5 A modified feedback network that will extend the low frequency response of the 
current-to-voltage converter.............................................................................................. 27 
3.6 General block diagram of a pulse EMI system........................................................... 28 
3.7 Circuit Schematic for the Transmitter......................................................................... 29 
3.8 (a) Transmit current pulse 800?s duration, 50Hz repetition rate.                                        
(b) The current turn-off time is approximately 5 ?s. ........................................................ 30 
3.9 (a) Components of the pulser on a printed circuit board, (b) Front end of the pulser 
box, (c) Back end of the pulser box .................................................................................. 31 
 
 xiii
3.10 Square transmitter and rectangular ?figure 8? receiver coil. The transmitter coil 
consist of 10 turns, R=917 m?, L=357 ?H (measured at 1 kHz). The receiver coil  
consist of 14 turns per coil, total R=832 m?, L=335 ?H (measured at 1 kHz.). Spacing 
between turns on both transmitter and receiver coils is approximately 0.06". ................. 32 
3.11 Schematic of the Receiver Amplifier........................................................................ 34 
3.12 Frequency Response plot of the Receiver Amplifier from Pspice............................ 34 
3.13 Frequency Response plot of the Receiver Amplifier measured using an     
HP89410A Vector Signal Analyzer.................................................................................. 35 
3.14 Photo of pulse EMI System. (1) Tektronix Oscilloscope (2) Battery power supply 
for the ?pulser? (3) Transmitter coil (4) receiver coil (5) Pulser and (6) Gagescope PC . 37 
4.1 Example of a q-coil (5 inch copper loop with 18 AWG copper wire)........................ 39 
4.2 Theoretical, FEM, and Measured Time Domain response of 18 AWG 5"            
copper loop........................................................................................................................ 40 
4.3  UXO test objects consisting of 12 cylinders outlined in Table 4.1 ........................... 41 
4.4 Measured decays for ?2 series? cylinders identical except for wall thickness ........... 43 
4.5 FEM decays for ?2 series? cylinders identical except for wall thickness................... 43 
4.6 Targets used in Pit test ................................................................................................ 44 
4.7 Picture of the 4 inch Carbon Steel Sphere from the Pit testing .................................. 45 
4.8 Background Response for the sphere tests.................................................................. 45 
4.9  Response of 4 inch carbon steel sphere with background subtraction. Sphere is 
relatively close to the transmitter and receiver coils......................................................... 46 
4.10 ?Zoomed-in? response of 4 inch carbon steel sphere with background subtraction. 46 
4.11  Response of 105 mm shell with background subtraction ........................................ 47 
4.12  ?Zoomed-in Response? of 105 mm shell with background subtraction as a    
function of position........................................................................................................... 48 
4.13  ?Zoomed-in? response of 81 mm mortar with background subtraction .................. 48 
4.14  ?Zoomed-in? response of 40 mm shell with background subtraction ..................... 49 
4.15 Depth of Cylinder E with respect to the top of the Platform for Test1..................... 50 
4.16  ?Zoomed-in? response of Cylinder E with background subtraction........................ 50 
4.17 Picture of the Clutter Item from Pit Testing ............................................................. 51 
4.18 Response of Clutter Item with background subtraction (zoomed in) ....................... 51 
4.19 A section of the data from LaneC_Pass4 for the MTADS test................................. 52 
4.20 A section of the data from LaneC_Pass1.................................................................. 53 
4.21 Response of Figure 4.20 zoomed on the area shown by the dotted rectangle .......... 54 
 xiv
A.1 Schematic of the stationary testing platform (a) Top View (b) Side View................ 59 
A.2 Testing platform for the stationary testing................................................................. 60 
A.3 Some of the targets used in the pit test....................................................................... 61 
A.4 Layout of the NRL ordnance classification test site at Blossom Point...................... 68 
A.5 Dynamic Testing of EMI sensor with MTADS trailer .............................................. 69 
A.6 Dynamic Testing of EMI sensor with Custom made cart.......................................... 70 
B.1 LCP based MEMS capacitive pressure sensor configuration?????????.74 
B.2 Photographs of various layers of the pressure sensor array (a) LCP substrate with      
a common bottom electrode, (b) LCP spacer with circular holes, and (c) LCP top layer  
with copper top electrodes. ............................................................................................... 75 
B.3 Metal blocks with alignment holes and pins used in the lamination process............. 76 
B.4 Sequence of the three layers of the pressure sensor during lamination ..................... 76 
B.5 Press used for thermo-compression bonding ............................................................. 77 
B.6 Photograph of a 5 x 3 capacitive pressure sensor array ............................................. 79 
B.7 Scanning Acoustic Microscope (SAM) picture showing a portion of the pressure 
sensor array ....................................................................................................................... 79 
B.8 Experimental setup for testing LCP based pressure sensors...................................... 80 
B.9 Block diagram of the MS3110 IC used in the single-variable mode ......................... 81 
B.10 Output voltage from MS3110 UCR for an example pressure sensor....................... 82 
B.11 Relative capacitance change versus applied pressure for an example pressure   
sensor ................................................................................................................................ 83 
 
 
 
 
 
 xv
LIST OF TABLES 
3.1 Comparison of Data acquisition devices..................................................................... 36 
4.1  Physical properties of UXO test objects.  Length (L), diameter (D), and wall ......... 42 
A.1 4 inch Carbon Steel Sphere........................................................................................ 62 
A.2 Data Acquired for the 105 mm shell.......................................................................... 63 
A.3  Data Acquired for the 81 mm mortar........................................................................ 64 
A.4 Data Acquired for the 40 mm shell............................................................................ 65 
A.5 Data Acquired for the Cylinder E .............................................................................. 66 
A.6 Data Acquired for the Clutter..................................................................................... 67 
A.7 Details of the Dynamic Test using MTADS trailer ................................................... 69 
A.8 Details of the Dynamic Test using the cart................................................................ 70 
 
 
 
 
 
 
 
 
 
 
 1
CHAPTER 1 INTRODUCTION 
Unexploded ordnance (UXO) is explosive ordnance that, due to some malfunction 
remains undetonated. Usually, they are long thin rod like structures, which can range in 
size from 20mm shells to 2000 lb bombs. UXO may be clearly visible, hidden in 
undergrowth, or buried beneath the ground. 
UXO is recognized as one of the world?s most serious and dangerous 
environmental problems. The presence of UXO has an economic impact on the livelihood 
of many people across the world and also deters the reconstruction and development 
efforts of war torn countries. The majority of the casualties caused by UXO are 
comprised of civilians and children, who work for livelihood in areas which were once 
battlefields and /or military testing areas. 
According to Landmine Monitor Report 2004 [1], around two-thirds of the 
countries in the world ? 121 countries ? are affected to some extent by the 
landmine/UXO problem.  New landmine/UXO casualties were reported in 66 countries. 
1,833 of these deaths were children, approximately 23% of the total casualties reported. 
Less than 14 percent of reported casualties were identified as active military personnel. 
These numbers do not take into account the many unreported casualties, as innocent 
civilians are killed or injured in remote areas away from any form of assistance or means 
of communication. 
 2
In many Mine/UXO affected countries the assistance available to address the 
needs of survivors is inadequate. Most of the survivors do not have access to some of the 
basic needs like food security, access to water, roads, adequate housing, and a way to 
earn an income.  The social and economic impact of Mines/UXO on the lives of these 
people and their countries serve to emphasize the need for humanitarian landmine/UXO 
removal [1]. 
The United States Department of Defense estimates that there are nearly 15 
million acres of ranges contaminated with UXO [2]. In recent years there has been a 
considerable effort to clear these UXO contaminated areas. However, current methods 
used are time consuming, expensive and present a great risk to the personnel involved. 
Funding has been provided by the Environmental Security Technology Certification 
Program (ESTCP), and the Strategic Environmental Research and Development Program 
(SERDP) to address the technology needed to improve the detection and discrimination 
of landmine/UXO items and also ensure the safety of the personnel involved. 
With a SERDP funded grant, our research team at Auburn University is involved 
in the development of an Electromagnetic Induction Sensor optimized for UXO 
detection, and classification. Previous work at Auburn University and elsewhere has 
shown that EMI has the potential for detection and classification [3], [4].  
This thesis will address the key topics related to an EMI sensor optimized for the 
purpose of detection and discrimination of UXO. The focus is to concentrate on 
developing low frequency (late time) EMI techniques in the characterization of thin and 
thick-walled UXO like items and UXO. This research is divided into four parts: In 
CHAPTER 2, a brief description of the theory of an EMI system is presented followed by 
 3
a description of the two types of EMI systems: Time Domain (Pulsed) EMI and 
Frequency Domain (Continuous Wave) EMI sensor. In CHAPTER 3, the issues related to 
the design of the transmitter and receiver coils, the signal processing circuitry, and data 
acquisition hardware are presented. In CHAPTER 4, data collection of the desired targets 
in the lab, and also the data obtained from the field measurements carried out at the Naval 
Research Laboratory?s Blossom Point Test Site in Maryland are presented and analyzed. 
A detailed description of the field tests is presented in APPENDIX A. 
CHAPTER 5 summarizes the research work followed by suggestions for future 
efforts.
In addition to the work described above, a brief description of a MEMS capacitive 
pressure sensor array fabricated using printed circuit processing techniques is presented 
in this thesis. MEMS capacitive pressure sensors are usually fabricated using silicon 
micromachining techniques [20]. In this work, novel Liquid Crystal Polymer (LCP) 
based capacitive pressure sensors fabricated using printed circuit processing techniques 
are presented. The advantages of using LCP include low cost, versatility of fabrication 
such as low temperature thermal bonding, mechanical flexibility, less moisture absorption 
compared to other polymer films used in MEMS processes [21]. Further details are 
provided in APPENDIX B. 
 
 4
CHAPTER 2   THEORY OF ELECTROMAGNETIC INDUCTION SENSOR 
2.1 Introduction 
In the previous chapter, the need for the development of an EMI system for 
detection and discrimination of UXO was discussed. This chapter provides the reader 
with the basic concepts of an EMI system. This chapter is organized as follows: The 
configuration and principle of a basic EMI system [5], [6] are discussed. Next, the 
different types of EMI systems available are presented followed by a mathematical 
analysis of the two types of systems.  
2.2 EMI System Configuration 
A typical Electromagnetic Induction (EMI) System consists of a transmitter coil, 
receiver coil, and the buried metallic object. The transmit coil when excited, radiates a 
primary magnetic field in the surrounding environment, which in accordance with 
Faraday?s law, induces an emf in the buried object as well as the receiver coil. 
 
 
Figure 2.1 Schematic of a typical EMI system
 5
This emf induced in the object generates eddy currents which radiate a secondary 
magnetic filed. The secondary magnetic field, in turn, induces a voltage in the receiver 
coil. Therefore, the total voltage induced in the receiver coil consists of two parts: the 
direct coupled voltage from the transmitter coil and the secondary voltage due to the 
object eddy currents. Figure 2.1 illustrates this basic concept.  
EMI systems can be classified into one of two categories based on the type of 
excitation of the transmitter coil [7]. An EMI system which uses short duration current 
pulses to excite the transmitter coil is referred to as a pulsed EMI system. In one type of 
pulsed EMI system, the current pulses are increased to some maximum value and then 
turned-off abruptly. On the other hand a Continuous wave (CW) EMI system uses 
sinusoidal currents to excite the transmitter coil. Sinusoidal frequencies are in the range 
from below 1Hz to seldom higher than a few hundred kilohertz.   
 The choice of a pulsed or CW EMI system depends on many factors and 
the best method to use for discriminating UXO from clutter is a topic of considerable 
interest to the UXO community [8].  
2.3 Circuit Model for a CW EMI System 
Figure 2.2 shows the magnetically coupled circuit model of an EMI system, 
which is derived from the physical model of Figure 2.1. The transmit coil is represented 
by an equivalent self inductance and resistance, L
T
 and R
T
 respectively. Similarly the 
object can be represented by an inductance Lo and resistance Ro. This representation of 
the object is accurate only for a simple first order object such as a loop of thin copper 
wire (radius of the wire much less than the radius of the loop), and does not hold for more 
complex objects like the sphere and cylinder. Similar to the transmitter coils, the 
 6
inductance and resistance of the receiver coil are represented by L
R
 and R
R
, respectively. 
Furthermore, the input impedance of the receiver amplifier is Z
R
, the transmitter current is 
I
T
, the object current is I
O
, and the receiver current is I
R
. The mutual coupling between the 
transmitter and the object, object and the receiver coil, and the transmitter and the 
receiver are denoted as M
TO
, M
OR
, and M
TR
, respectively. The mutual coupling M
TR
 
induces a voltage V
DIRECT
 at the receiver coil. It is desirable to reduce V
DIRECT
 as much as 
possible so that the output voltage is due to the object only. A figure-8 receiver coil also 
known as bucking coil can be used to minimize the direct coupling M
TR
 and therefore, the 
undesirable voltage V
DIRECT.
 
 
Figure 2.2 Magnetic Coupled Circuit used to represent a Continuous Wave (CW) 
Electromagnetic Induction Circuit 
 
A figure-8 coil consists of two N-turn coils wound in opposite direction, 
connected in series such that flux passing through them will produce an equal but 
opposite voltage in each coil. Therefore, the net voltage across the coil pair will, 
theoretically, be zero.  
 7
Simple circuit theory can be used to derive a mathematical solution of the 
magnetically coupled circuit of Figure 2.2. The mathematical model allows one to define 
the fundamental operation of an EMI system. The receiver coil voltage in terms of the 
transmitter coil current can be obtained using Faraday?s law of induction [5] and simple 
circuit theory.  
Assuming sinusoidal excitation, the voltage V
1
 induced in the object loop as a 
result of coupling between the object and the transmit coil is given by 
TTO
IMjV ??=
1
 
                                              (2.1)
The voltage V
DIRECT
 induced in the receiver coil due to the mutual coupling 
between the transmitter and the receiver coil is given by,  
TTRTTRDIRECT
IMjI
dt
d
MV ??=?=
(2.2)
By Lenz?s law a current I
O
 flows in the object loop and produces a secondary 
voltage in opposition to the induced voltage V
1
 [9]. The voltage drop across the object 
loop V
11 
is given by      
 ()
OOO
ILjRV ?+?=
11
                                            (2.3)
Assuming that Z
R
 is infinity (open circuit), and applying Kirchoff?s voltage law, yields 
0
111
=+VV  
                                          (2.4)
The expression for the object current can be obtained by substituting equations (2.2) & 
(2.3) into (2.4) and re-arranging terms, or 
T
OO
TO
O
I
LjR
Mj
I
?
?
+
?=  
                                        (2.5)
 8
The voltage V
OBJECT
 induced in the receiver circuit due to the object currents may be 
written as, 
OOROBJECT
IMjV ??=  
                                             (2.6)
The response of the system to the object can be obtained by comparing the eddy 
current induced receiver voltage to the receiver voltage due to the primary field in the 
absence of the object, or, 
2
2
1 ?
??
+
+
?=
j
ML
MM
V
V
TRO
ORTO
DIRECT
OBJECT
 
                                       (2.7)
where 
OO
RL?? = is the response parameter of the system. 
Mutual coupling between two circuits i and j, M
ij
, can be represented in terms of 
the coupling coefficients of the two circuits k
ij
, as 
jiijij
LLkM =  
                                            (2.8)
where L
i
 and L
j
 are the self inductances of circuits i and j respectively, and  1?
ij
k  
Using (2.8), equation (2.7) may be written as, 
2
2
1 ?
??
+
+
?=
j
k
kk
V
V
TR
ORTO
DIRECT
OBJECT
 
                                       (2.9)
In (2.9), the term )1()(
22
??? ++ j  known as the response function, depends 
only on the frequency and the electrical properties of the loop [10]. The term 
TRORTO
kkk  
may be referred to as a coupling coefficient, the numerator, 
ORTO
kk  is the coupling from 
the transmitter to the receiver through the object where as the denominator, 
TR
k  is the 
direct coupling between transmitter and receiver. Coupling is independent of frequency 
 9
and only depends on the relative sizes, geometry, and the positions of the object with 
respect to the transmitter and receiver coils.  
Figure 2.3 is a plot of the response function )1()(
22
??? ++ j , with respect to 
the response parameter? . As ? tends to zero, the real part of the response function tends 
to zero resulting in a purely imaginary response given by, 
 
TR
ORTO
O
O
DIRECT
OBJECT
k
kk
R
Lj
V
V ?
?=  
                                           (2.10)
Equation (2.10) is referred to as the resistive limit. As ? tends to infinity the 
imaginary part of the response function approaches zero and the real part asymptotically 
approaches unity. Under this condition, the response is given by 
TR
ORTO
DIRECT
OBJECT
k
kk
V
V
?=  
                                           (2.11)
which is referred to as the inductive limit. At the crossover frequency? =1 or f
crossover 
=
0
2 LR
O
? , the real and imaginary parts of the response function are equal and the 
magnitude is approximately 3 dB below its asymptotic value. It should be noted that the 
general form of the response presented above is only valid for simple non permeable first 
order objects. (e.g. a thin copper loop of wire).  
2.4 Circuit Model for a Time Domain EMI System 
A time-domain (pulsed) EMI system differs from the CW EMI system only in the 
type of waveform used to excite the transmitter coil. Therefore, the magnetically coupled 
circuit model of Figure 2.2 can be used to model the pulsed EMI system. As the name 
suggests, in a pulsed EMI system, the transmitter coil may be excited by current pulses of 
short duration that increase to some maximum value and then abruptly fall to zero. 
 10
 
Figure 2.3 Frequency response of a simple first order target 
For the pulsed EMI system a shunt resistance is connected across the transmit coil. This 
helps to ensure that the current turns off without oscillations or ringing. 
The voltage induced in the object is proportional to the time rate of change of the 
transmitter current, so, generally speaking, it is desirable to turn off the transmitter 
current as rapidly as possible (without oscillations). 
2.4.1 Coupling Between the Transmitter and Buried Object 
A simple switched circuit model shown in Figure 2.4 can be used to illustrate the 
interaction between the transmitter current and the object current. In accordance with 
Faraday?s Law of Induction [5], if a current I
T
 flows through the transmitter coil then the 
voltage induced in the object, V
OBJ
 is the product of the mutual coupling between the 
transmitter and the object, and the time rate of change of the transmitter current, or 
dt
dI
MV
T
TOOBJECT
?=  
                                       (2.12)
As illustrated in Figure 2.5 a transmitter current I
T
 is defined, which has an initial 
value, I
I
, for all times t<0, and for times 0<t<T, linearly decreases until it is zero at t =T. 
 11
The voltage induced in the object is zero for time t<0, since the transmitter current I
T
(t) is 
constant during this time period. While the switch is in position A, the transmitter current 
decays linearly (0?t?T), and the voltage induced in the object is constant and is equal to 
TIM
ITO
. During this period, the object eddy currents exponentially increase as shown 
in Figure 2.6. The object current, I
O
(t), when the switch is in position A can be written as, 
O
t
O
ekktI
??
+=
21
)( 
                                            (2.13)
where ?
O
=L
O
/R
O
 is the time constant of the object. 
At time t =0, the object current is zero and (2.13) yields 
21
)0( kktI
O
+==  or  
  k
1
= -k
2
                                             (2.14)
If the switch were left in position A indifinitely, the current would asymptotically 
approach 
OOBJECT
RV so 
1
)( k
R
V
tI
O
OBJECT
O
==?=  
                                          (2.15)
Using (2.14) and (2.15) in (2.13) yields, 
)1()(
O
t
O
ITO
O
e
TR
IM
tI
??
?=  
(2.16)
which is the charge up current of the object, valid only for 0 ? t ? T. 
The object current reaches a maximum value at time T, given by, 
)1()(
O
T
O
ITO
O
e
TR
IM
TtI
??
?==  
                           (2.17)
At time t = T, the voltage induced in the object becomes zero, as the transmitter 
current no longer changes. Therefore, the peak object current in (2.17), starts to decay 
 12
exponentially toward zero, starting at time t = T. When the switch in Figure 2.4 is in 
position ?B? ( ??? tT ), the object current is given by,  
)()1()(
)(
Ttuee
TR
IM
TtI
OO
TtT
O
ITO
O
??==
??? ??
(2.18)
where u(t) is the unit step function. 
 
 
Figure 2.4 Circuit model describing the charge up and decay of object currents 
 
Figure 2.5 Normalized transmitter current versus normalized time 
 13
The exponential build up and the subsequent exponential decay of the object 
current is as shown in Figure 2.6. 
The peak object current in (2.17) can also be expressed as, 
O
T
O
ITO
O
O
T
e
L
IMT
I
O
?
?
?
)1(
)(
?
?
=  
                                   (2.19)
If 
O
ITO
L
IM
is taken as a constant, then the peak object current at time t = T takes 
the form, 
x
e
xy
x?
?
=
1
)( 
(2.20)
with 
O
Tx ?=  
The equation in (2.20), by L?Hopital?s rule, is maximum when x=T/?
O
=0 or when 
the current shutoff time is much less than the time constant of the object, T<< ?
o
.  
 
Figure 2.6 Object current versus time 
Figure 2.7  shows a plot of the normalized peak current as a function of the ratio 
of turnoff time to the object time constant. From Figure 2.7, it is evident that the peak 
object current is maximum when x = 0 or when T<< ?
o 
or
 
?
o
>>T. 
 14
x (t/?
O
)
 
Figure 2.7 Normalized object current vs the ratio of shutoff time to object time constant. 
Therefore, the turn off time of the transmitter current should be much less than the 
object time constant to maximize the object current. 
This presents an interesting possibility in that the transmitter coil current turn off time T 
can be adjusted to supress the response of ?small? objects with fast decay rates in favor of 
?large? objects with slow decay rates. This might be of value when one wishes to 
suppress the response form small surface clutter while simultaneously maintaining good 
sensitivity to larger, more deeply buried objects. Of course, the converse is not true, if the 
turn off time is fast enough to retain sensitivity to small objects then the system must also 
have good sensitivity to large objects.  
2.4.2 Voltage at the Receiver- Open Circuit Case 
The voltage at the receiver coil can be obtained in accordance with Faraday?s 
Law, by simply differentiating the object current and multiplying by the mutual coupling 
between the object and the receiver coil, or  
dt
tdI
MV
O
OROR
)(
=  
                                          (2.21)
 15
If T/?
o
<<1, then a Taylor series expansion of (2.16) provides a linear 
approximation, t
TL
IM
tI
O
ITO
O =)(
~
 for the object current during the time 0 ? t ? T, which is 
shown in Figure 2.8 (b) as the dashed line. 
Substituting )(
~
tI O in (2.21) gives, 
TL
IMM
tV
O
ITOOR
OR
=)( , 0?t?T 
                                            (2.22)
Similarly substituting (2.18) in (2.21) for t ? T, gives 
OO
TtT
OO
IORTO
OR
ee
TR
IMM
tV
??
?
)(
)1()(
???
??= ,  t?T 
               (2.23)
Using a Taylor series expansion in (2.23), the voltage induced in the receiver coil 
for t?T can be written as 
O
t
OO
IORTO
OR
e
L
IMM
tV
?
?
?
?=)( , t?T 
                                (2.24) 
It is interesting to note that the ratio of the voltages in (2.22) and (2.24), is ?
o
/T and since 
?
o
/T >>1, the receiver voltage drops rapidly from the maximum value to the lower limit at 
t=T, and then the exponential decay starts, as shown in Figure 2.8 (c). This analysis 
agrees with the analysis given in [5]. There is also a direct coupled voltage at the receiver 
coil due to the coupling between the transmitter and receiver. The direct coupled voltage 
can be given as 
?
?
?
?
?
?
?
>
??
<
=?=
Tt
Tt
T
IM
t
tI
dt
d
MV
ITR
TTRDIRECT
0
0
00
)(
 
(2.25) 
 16
 
Figure 2.8 Representative plots for (a) Transmitter Current (b) Object Current (c) Object 
Coupled Receiver Voltage and (d) Direct Coupled Receiver Voltage. Dotted vertical line 
marks the time t=T. 
 
 17
The direct coupled receiver voltage is shown in Figure 2.8 (d). For the case of a 
bucking receiver coil, the transmitter coil couples equally and oppositely to each half of 
the receiver coil resulting in a zero net output voltage at the receiver when there is no 
object. If there is any imbalance between the two, however small, then there will be a 
component of the output voltage due to direct coupling.  
If the residual coupling between the transmitter coil and the receiver coil is 
denoted as 
r
TR
M  (where the superscript r represents residual) then 
dt
tdI
MtV
Tr
TR
r
)(
)( ?=
?
 
                                           (2.26)
The sign of the direct-coupled voltage depends upon the winding direction of the 
halves of the receiver coil with respect to that of the transmitter coil and upon which half 
has the larger direct coupling. Therefore it is possible for the direct-coupled voltage to 
either add 
or subtract from the object coupled receiver voltage for times 0?t?T.   
2.5 Summary 
An EMI system consists of three basic elements a transmitter coil, receiver coil, 
and the metallic object. In a CW EMI system, sinusoidal current is used to excite the 
transmitter coil with frequencies in the range from below 1Hz to seldom higher than a 
few hundred kilohertz. In a pulsed EMI system short current pulses flow through the 
transmitter coil and then turn off abruptly. In both the cases, the current in the transmitter 
coil radiates a field that induces a current in both the metallic object and the receiver coil. 
Additionally, the induced object-current radiates a secondary magnetic field which also 
induces a current in the receiver coil. If the receiver coil inductance is halved and the 
 18
transmitter coil is assumed to couple equally but oppositely to each half, then the net 
output voltage at the receiver due to direct coupling will be zero. 
In the case of pulsed EMI system, it is beneficial to construct a transmitter current 
which is as large as possible before ?turn off? but turns of rapidly without oscillations. A 
short current turn-off time generates a large object current. Furthermore, by controlling 
the current turn-off time, the response from objects that are not of interest like small 
metallic ?clutter? can be suppressed in favor of objects of interest.  
In the next chapter, the frequency domain transfer function obtained in section 2.3 
is extended to take into account the physical characteristics of the transmitting coil, the 
receiver coil, and the receiver coil amplifier. Also, the design and development of EMI 
hardware based on the principles discussed in this chapter will be examined. 
 
 
 
 
 
 
 
 
 
 19
CHAPTER 3 HARDWARE DESIGN AND IMPLEMENTATION 
In this chapter, first, the mathematical analysis of the frequency domain EMI 
system presented in the last chapter is extended to take into account the physical 
characteristics of the transmitter coil, receiver coil, and receiver coil amplifier. Next, a 
block diagram of a pulsed EMI system is presented, followed by a description of the 
electronics used to generate the transmitter current, and also the signal amplifier for the 
receiver coil. The final section in this chapter addresses the data acquisition hardware. 
3.1 Transfer Function Characterization of the CW EMI System 
Referring to the Figure 2-2 in chapter 2, the loop on the left represents the 
transmitter circuit (parameters subscripted with T), and the one on the right is the receiver 
circuit (parameters subscripted with R), while the middle loop represents the object to be 
detected (parameters subscripted with an O). The CW source drives a current I
T
 through 
the transmitter coil resistance and inductance R
T
 and L
T
 respectively. The coupling 
between the transmitter coil and object, object and receiver coil are denoted as M
TO
 and 
M
OR
 respectively. In general there is also a coupling between the transmitter and the 
receiver coil denoted as M
TR
, but it can be kept to a minimum by using a figure-8 
receiver coil, which is a pair of coils wound in a ?bucking configuration?. The object and 
receiver coil inductance and resistance pairs are respectively (L
O
, R
O
) and (L
R
, R
R
). 
If the coupling between the circuits is weak then the object current may be written 
as [11], 
 20
T
OO
TO
O
I
LjR
Mj
I
?
?
+
=  
  (3.1)
Also, the transmitter current is just the source voltage divided by the impedance of the 
transmitter coil or 
)(
TT
S
T
LjR
V
I
?+
=  
                                   (3.2)
Substituting (3.2) into (3.1) yields 
))((
TTOO
STO
O
LjRLjR
VMj
I
??
?
++
=  
(3.3)
The receiver current is the voltage induced in the receiver coil divided by the sum 
of the impedance of the receiver coil and the load impedance or 
RRL
OOR
R
LjRZ
IMj
I
?
?
++
=  
                             (3.4)
which may be rewritten using (3.3) as   
))()((
RRTTRRL
STOOR
R
LjRLjRLjRZ
VMjMj
I
???
??
++++
=
(3.5)
The output voltage is obtained by the product of the receiver current and the load 
impedance as 
))()((
RRTTRRL
STOORL
OUT
LjRLjRLjRZ
VMjMjZ
V
???
??
++++
=  
         (3.6)
The frequency-dependent transfer function of the system 
SOUT
VV is given as  
))()((
RRTTRRL
TOORL
S
OUT
LjRLjRLjRZ
MjMjZ
V
V
???
??
++++
=  
(3.7)
 21
If we consider the case where the receiver coil is open circuited, the load 
impedance Z
L
 is infinite, then (3.7) would simplify to 
))((
RRTT
TOOR
S
OUT
LjRLjR
MjMj
V
V
??
??
++
=
                          (3.8)
By defining the object and the transmitter coil break frequencies 
OOO
LR=?  
and 
TTT
LR=? respectively,   (3.8) may be written as  
 
 
The frequency-dependent terms (rightmost two fractions) of (3.9) are high-pass filter 
terms, with break frequencies of ?
T
 and ?
O
 corresponding to the transmitter coil and the 
object respectively. Referring to Figure 3.1, as the operating frequency exceeds the break 
frequency, the response approaches unity and at the break frequency, the magnitude of 
the response is 0.707 or 3 dB below its high frequency asymptotic value. As the operating 
frequency approaches zero, the magnitude of the response decreases by 20 dB each time 
the frequency decreases by a factor of 10 (20 dB per decade roll-off). The phase of the 
high-pass filter starts out at 90 degrees well below the break frequency, is 45 degrees at 
the break frequency and approaches zero degrees well above the break frequency.  
3.2 Optimal Number of Transmitter Coil Turns: Case 1 V
OUT
/V
S
 Measurement 
Theoretically the information regarding the object can be obtained by the 
measurement of the transfer function V
OUT
/V
S
 given in (3.9). This would certainly be the 
case if the high-pass transmitter coil term in (3.9) were unity over all frequencies of 
interest. If the object frequency of interest is below the transmitter coil break frequency, 
then the response of the object would be attenuated by 20 dB each time the frequency 
)1()1(
O
O
T
T
RT
TOOR
S
OUT
j
j
j
j
LL
MM
V
V
??
??
??
??
++
=
                       (3.9)
 22
decreases by a factor of 10. For this reason, the transmitter coil break frequency should 
be somewhere near the lowest frequency of interest. 
 
Figure 3.1  Transfer function characteristics of a high pass filter: (jf/10)/ (1+jf/10).  
Top figure is the magnitude response and bottom figure is the phase response. 
 
The transmitter coil?s break frequency is ?
T
 =R
T
/L
T
 and R
T
 and L
T
 are 
proportional to N
T
 and N
T
2
 respectively so that ?
T
 is inversely proportional to N
T
 where 
N
T
 is the number of turns of wire used to make the transmitter coil. So, the transmit coil 
should have enough turns to bring the transmit coil break frequency close to the lowest 
frequency of interest. But with the increase in the number of transmitter coil turns N
T,
 
there is one more tradeoff that is probably important to consider. If we consider the object 
to be an N
O
 turn coil (q-coil), from (3.9), V
OUT
/V
S, 
is proportional to M
OR
M
TO
/L
T
L
O
 and 
M
OR
 and M
TO
 are proportional to the product N
O
N
R
 and N
T
N
O
 respectively but L
T
 and L
O 
are proportional to N
T
2 
and N
O
2 
respectively so that M
OR
M
TO
/L
T
L
O
 is proportional to 
N
R
/N
T 
(just the ratio of receiver to transmitter turns). 
 
 23
So, for a fixed number of receiver coil turns N
R
, the amplitude of the response is 
inversely proportional to N
T
. Therefore, there is a tradeoff between system voltage gain 
and system bandwidth. As N
T
 increases, the system bandwidth increases by lowering the 
transmitter coil break frequency but at the same time the overall gain of the system, 
N
R
/N
T
 is also decreased. 
3.3 Receiver coil and Receiver coil amplifier considerations for Case 1: V
OUT
/V
S
 
      Measurement
 
 
The voltage induced in the receiver coil due to the eddy currents that flow on and 
within the object is usually small and therefore needs to be amplified. Generally, an 
operational amplifier is used to boost the weak signals induced into the receiver coil. In 
Figure 3.2, an operational amplifier connected in the non-inverting configuration is 
shown. It provides a voltage gain, V
out
/V
in
=1+R
2
/R
1
. The gain of the amplifier can be 
changed by adjusting the values of the resistors R
2
 and R
1
. The input impedance of the 
operational amplifier is very high, ideally infinite, but usually on the order of several 
mega ohms. Unfortunately, due to the high input impedance of the amplifier, a frequency 
is reached where significant currents will begin to flow in the parasitic coil capacitance, 
designated as C
R
 in Figure 3.2. A resonant circuit is formed by L
R
, R
R
 and C
R
 of the 
receiver coil, due to which the input voltage V
in
 will increase proportionally with 
frequency at 20 dB/dec up-to the first self-resonance frequency of the receiver coil 
(which is around 10 kHz). This in-turn distorts the output voltage, which is no longer a 
faithful representation of the object response. The problems associated with the first self-
resonance frequency of the receiver coils can be overcome by using a current-to-voltage 
converter [12], [13]. 
 24
 
Figure 3.2  Operational amplifier connected in non-inverting configuration to the receiver 
coil. The parasitic capacitance of the receiver coil distorts measurements of the object?s 
response. 
 
3.4 Optimal Number of Transmit Coil Turns: Case 2 Object Current Measurement 
The current flowing in the object due to the emf induced from the transmitter coil 
can be expressed in terms of the transmitter current, by re-arranging (3.1) as, 
O
O
T
O
TO
O
j
j
I
L
M
I
??
??
+
=
1
 
(3.10)
The object response will not suffer distortion as long as the transmitter current I
T 
is 
frequency independent. But the transmitter current is given by (3.2) which may be written 
in terms of the transmit coil break frequency, ?
T
=L
T
/R
T
 as, 
T
TS
T
j
RV
I
??+
=
1
 
                                  (3.11)
For this case, since the transmitter current contains a low-pass filter term, the transmitter 
coil break frequency should be much higher than that of the object, ?
T
>>?
O
. Since ?
T
 is 
inversely proportional to the number of turns of the transmitter coil, N
T
, the number of 
turns on the transmitter coil should be small. But, with fewer transmitter coil turns, N
T
, 
 25
the coupling to the object will be low. So, there is a trade-off between bandwidth and 
coupling. More turns on the transmitter coil will lower the 3 dB break frequency, but a 
series resistance can be added to the transmitter coil to compensate for that. But, this can 
be done only if we increase the source voltage V
S
 or equivalently the source power.  
3.5 Receiver coil and Receiver coil amplifier considerations for Case 2: Object Current  
      Measurement 
To eliminate the problems associated with the first resonance frequency of the 
receiver coil, a current-to-voltage converter is used as shown in Figure 3.3. It consists of 
an operational amplifier connected in an inverting configuration. The output voltage of 
the current-to-voltage converter is V
OUT
=-I
R
R
F
, where I
R 
is the receiver coil current and 
R
F 
is the feedback resistance that sets the gain. The receiver coil current is nothing more 
than the voltage induced in the receiver coil divided by the impedance seen by the source. 
 
Figure 3.3 Operational amplifier connected in an inverting configuration as a current-to-
voltage converter 
 
Therefore, the output of the operational amplifier can be written as, 
O
R
R
R
ORF
OUT
I
j
j
L
MR
V
?
?
?
?
?
?
?
?
+
?=
??
??
1
                                     (3.12)
 26
where ?
R
=R
R
/L
R
 is the break frequency of the high-pass filter term in parenthesis. The 
break frequency of the high-pass filter term is inversely proportional to the number of 
turns on the receiver coil, N
R
. Therefore, the break frequency can be set to lower value by 
increasing the number of turns. Figure 3.4 shows the plot of the break frequency of the 
receiver coil as a function of number of turns for a 1/3 m square figure-8 receiver coil 
using 18 AWG magnetic wire.  From Figure 3.4, it can be seen that the break frequency 
decreases considerably with increase in the number of turns up to a certain point, after 
which it decreases but only slowly. Therefore, an optimum number of turns can be 
chosen to set the break frequency closer to the frequency of interest. There is an 
important difference between measuring the object current and measuring a voltage 
proportional to the object response.  
0
500
1000
1500
2000
2500
10203040
Number of Turns on Receive Coil (N
R
)
Brea
k Freq
uen
cy, f
R
 (Hz)
 
Figure 3.4 Plot of break frequency of the receiver coil as a function of number of turns 
In the case of object current measurement using a current-to-voltage converter, 
the input resistance of the current-to-voltage converter is very low, so the parallel 
capacitance of the receiver coil is approximately short circuited at low frequencies. This 
causes the self-resonance of the receiver coil to shift to a much higher frequency for the 
 27
inverting op-amp configuration compared to that of the non-inverting op-amp 
configuration of Figure 3.3. As described in [13] it is even possible to compensate for the 
receiver coil roll-off below the break frequency, ?
R
 by replacing R
F
 in Figure 3.3 with the 
network shown in Figure 3.5. According to Prance et. al. [13]  below the corner (3 dB) 
frequency, f
corner
=1/RC, the gain of the current-to-voltage converter increases with a first 
order response (20 dB/decade) until the limit set by the bypass resistor RF is reached. 
This 20 dB/decade increase will exactly cancel the 20 dB/decade roll off below the 
receive coil break frequency ?
R
. Also limiting the low frequency gain to a fixed value at 
DC serves to suppress amplifier drift. 
 
Figure 3.5 A modified feedback network that will extend the low frequency response of 
the current-to-voltage converter 
 
3.6 Block Diagram of a Pulsed EMI System 
A block diagram of a pulsed EMI system is shown in Figure 3.6. It consists of a 
square wave generator, which is used to ?pulse? the voltage controlled switch. When the 
generated signal is ?high?, the switch closes and allows current I
T
 to flow through the 
transmitter coil for the duration of the pulse. When the pulse waveform falls below a 
characteristic ?off? voltage the switch is opened. As discussed in the preceding chapter, 
 28
the transmit current must be allowed to reach a steady maximum value.  Therefore, it is 
important that the duration of the pulse on-time is long enough for the transmitter current 
to reach a steady maximum value. It is also important that the off time of the current 
pulse is long enough to permit the receiver to capture the object?s response which may be 
of significant amplitude for tens of milliseconds.  
As stated earlier, flux from eddy currents that flow on and within the object 
produce a magnetic field that induces a voltage in the receiver coil. The receiver coil 
detects the object. The filtering and amplification section at the output of the receiver coil 
consists of a second order low pass filter, which converts the current in the receiver coil 
into voltage with some gain and also reduces, via low pass filtering, the high frequency 
noise.  
 
Figure 3.6 General block diagram of a pulse EMI system 
 
Next the signal is passed to the Data Acquisition hardware, which is basically an 
Analog-to-Digital Converter (ADC). Different kinds of ADC?s were used for data 
acquisition, and a brief description of each will be presented later in this chapter. 
 29
3.7 Transmit Current Generation 
The circuit model for the transmitter section of the pulsed EMI system is shown in 
Figure 3.7. A PIC Microcontroller PIC16F88 is used to generate a pulse waveform with 
programmable period and duty cycle. The duration of the pulse is set to about 800 ?s 
with a repetition frequency of 50 Hz. The microcontroller provides a TTL output, which 
is fed to a MOSFET/IGBT driver chip IXDN404PI from IXYS Corp. which boosts the 
TTL signal from the microcontroller to a +12 V output signal. The MOSFET driver IC 
has low output impedance, which is essential for a faster turn off of the IGBT [14]. An 
Insulated Gate Bipolar Transistor (IGBT) IRG4PH40K from International Rectifiers Inc. 
is used as the switch in the transmitter current generation circuit. The output of the driver 
is connected to the gate of the IGBT through a resistor R_Series.  
 
Figure 3.7 Circuit Schematic for the Transmitter 
This resistor along with the output impedance of the MOSFET driver and the 
intrinsic internal gate capacitance of the IGBT controls the turn on and turn off times of 
the IGBT [14].  
 30
(a)
(b)
 
Figure 3.8 (a) Transmit current pulse 800?s duration, 50Hz repetition rate. (b) The 
current turn-off time is approximately 5 ?s. 
 
The shunt resistor R_Shunt across the transmitter coil in Figure 3.7 is used to 
dampen the oscillations in the transmitter current. As shown in Figure 3.8 (a), when the 
IGBT is on, the transmitter current exponentially increases to a maximum value of 
approximately 18A. The maximum current value was determined by the choice of a 24V 
power supply.  
 31
When the IGBT turns off, the current falls to zero in approximately 5?s, as shown 
in Figure 3.8 (b). 
Output to the Data 
Acquisition System
Signal from 
Receiver 
Coil
Trigger Signal to the Data 
Acquisition System
Signal to 
Transmitter Coil
(c)
(b)
Push-Button switches 
for Period and Duty 
Cycle adjustment
Control switch for 
transmitter coil power
(a)
IGBT
MOSFET  
Driver
Receiver 
Amplifier
PIC 
Microcontroller
 
Figure 3.9 (a) Components of the pulser on a printed circuit board, (b) Front end of the 
pulser box, (c) Back end of the pulser box 
 
Figure 3.9 shows the components of the system packaged into a portable unit, referred to 
as the ?pulser?. External connectors are provided for the 24V and 12V power supplies 
 32
and for the transmit coil. Another connector provides access to the IGBT gate signal 
which is used as the trigger signal for Data Acquisition. A safety switch that interrupts 
the power supply to the coil, and the push button switches for adjusting the period and 
duty cycle of the pulse, are all mounted on the front of the box. 
3.8 Transmitter and Receiver Coil Construction 
Taking into account the design considerations discussed in the earlier sections, a 
1m?1m square transmitter coil was constructed with 18 AWG magnetic wire wound 
around a wooden frame. In general, the field strength at the center of the coil is inversely 
proportional to the radius of the coil, and the field strength along the axis of the coil  
 
Figure 3.10 Square transmitter and rectangular ?figure 8? receiver coil. The transmitter 
coil consist of 10 turns, R=917 m?, L=357 ?H (measured at 1 kHz). The receiver coil 
consist of 14 turns per coil, total R=832 m?, L=335 ?H (measured at 1 kHz.). Spacing 
between turns on both transmitter and receiver coils is approximately 0.06". 
 33
decreases faster as the radius of the coil decreases. A ?rule of thumb? that takes into 
account these opposite trends states that a coil sees a target approximately one coil 
diameter deep. UXO targets buried deeper than 1m are usually not considered dangerous 
and so a square coil of 1m diameter was considered to be appropriate for this application. 
Figure 3.10 shows a diagram of the coils with dimensions marked, along with the 
final product and relevant electrical characteristics. A ?figure 8? shape (sometimes 
referred to as a bucking configuration) was chosen for the receive coil. As explained 
earlier, each loop of the figure eight configuration, couples equally but oppositely to the 
transmitter coil producing zero output in the absence of a target.  As shown in Figure 
3.10, the receive coils were fixed to a plexiglas sheet which rests on a larger thicker 
sheet.  Guides were placed on either side of the receive coil sheet allowing the receiver 
coil to slide only along one axis.  The plexiglas sheet holding the receiver coils is 
adjusted while monitoring the receiver coil voltage on an oscilloscope.  The receiver coils 
are fixed at the position that produces a minimum voltage. 
3.9 Receiver signal Conditioning 
For the work presented in this thesis, a second order low pass filter designed using 
an operational amplifier in the inverting configuration is used as the receiver signal 
conditioning circuit. The schematic of the filter is shown in Figure 3.11. Since there is 
very little target information at higher frequencies, the cut-off frequency of the filter was 
set to 10 kHz, and the gain was set to 20 dB. The receiver amplifier shown in Figure 3.11 
was simulated in Pspice to determine its frequency response characteristics. The 
frequency response of the designed practical circuit was obtained using an HP89410A 
 34
Vector Signal Analyzer. The frequency response plots of the filter are shown in Figure 
3.12 and Figure 3.13 respectively. 
 
 
Figure 3.11 Schematic of the Receiver Amplifier 
 
 
Figure 3.12 Frequency Response plot of the Receiver Amplifier from Pspice  
 35
 
Figure 3.13 Frequency Response plot of the Receiver Amplifier measured using an 
HP89410A Vector Signal Analyzer 
 
The receiver amplifier circuit shown in Figure 3.11 has some drawbacks. The 
input impedance looking into the inverting input of the amplifier is nothing but the 
resistor R1. This resistor is in series with the receiver coil, and adds to the resistance of 
the receiver coil. The additional resistance causes the break frequency of the high pass 
filter term in the receiver coil response to shift upwards, which reduces the bandwidth of 
the system. As discussed earlier, a current-to-voltage converter has very low input 
impedance and does not affect the low frequency cut-off of the receiver coil.  
3.10 Data Acquisition 
The output voltage of the receiver coil amplifier for different targets was fed to an 
analog-to-digital converter (ADC) for data logging and analysis purposes. Table 3.1 
presents the different data acquisition devices that were investigated and presents a 
comparison of parameters such as sampling rate, resolution, and range of the ADC?s.  
 36
Table 3.1 Comparison of Data acquisition devices 
Name of ADC Sampling Rate Resolution Range Remarks 
Compuscope (CS) 
1602  from Gage  
Applied Inc. 
100kS/s-2.5 
MS/s 
 
16-bit ?500 mV -?10 V 
Plugs into 
the PCI slot 
of a desktop 
computer. 
DAQcard-6062E 
from  National 
Instruments 
500 kS/s 12-bit ?50 mV - ?10 V 
Plugs into 
the PCMCIA 
slot of a 
Laptop 
computer. 
ADC-216 from 
PICO Technologies 
333 kS/s 16-bit ?10 mV - ?20 V 
Plugs into 
the parallel 
port of a PC. 
cRIO with cRIO-
9215 from National 
Instruments 
100 kS/s 16-bit ?10 V Plugs into 
the Ethernet 
port of a PC. 
 
The CS1602 is a 16-bit ADC which is installed on a desktop computer as it is a 
PCI bus card. It has 2 differential analog inputs, an external trigger and clock inputs. The 
GageScope data acquisition software helps in displaying and storing the data acquired by 
CS1602. It provides control of the sampling rate, input range, and trigger level.  
The DAQcard-6062E is a 12-bit ADC which can be installed in the PCMCIA slot 
of a laptop. It has 16 analog inputs, 2 analog outputs, and 8 digital I/O lines. LabVIEW 
software from National Instruments can be used to store the data acquired using the DAQ 
card. 
The ADC216 is a 16-bit data acquisition device, which can be plugged into the 
parallel port of a desktop or a laptop. It has 2 analog inputs, and an external trigger input. 
Picoscope software from PICO technologies can be used to program the device.  
 37
Compact Reconfigurable I/O (cRIO) is a real-time embedded controller from 
National Instruments. It has a 200 MHz Pentium class processor that reliably and 
deterministically executes the LabVIEW Real-Time applications. It has a reconfigurable 
embedded chasis, which contains the reconfigurable I/O (RIO) Field Programmable Gate 
Array (FPGA) core. The RIO core is connected to the cRIO embedded real-time 
controller through a local PCI bus interface. It comes with many different types of I/O 
modules for data acquisition and storage. It connects to a computer through either an RS-
232 serial port or an Ethernet port. The information provided in Table 3.1 for the cRIO is 
for one of the I/O modules, cRIO-9215, which is an analog input module. The 
Compuscope1602 card with Gagescope software was used for laboratory as well as 
stationary field measurements. For field measurements which required mobility, the 
DAQcard-6062E was used as the data acquisition device. These measurements are 
discussed in detail in the next chapter. The complete EMI system is shown in Figure 3.14. 
 
Figure 3.14 Photo of pulse EMI System. (1) Tektronix Oscilloscope (2) Battery power 
supply for the ?pulser? (3) Transmitter coil (4) receiver coil (5) Pulser and (6) Gagescope 
PC 
 38
3.11 Summary 
The pulse system consists of four major parts; the pulser, sensor coils, signal 
conditioning, and finally the data acquisition hardware.  The pulser was built into a 
portable unit, capable of switching a 18 A (peak) transmit current ?off? in approximately 
5?s.  Care was taken in the construction of the sensor coils, with concentrated efforts to 
eliminate parasitic capacitance, obtain precise symmetry to reduce direct coupling 
between transmitter and receiver coils, and build the coils large enough to detect large 
metallic objects at expected UXO target depths.  Low-pass filtering and amplification of 
the response at the receiver coil increased SNR by rejecting responses not of interest and 
by amplifying the desired signal.  After low-pass filtering and amplification (signal 
conditioning), the analog signal is converted to digital form and stored using the data 
acquisition hardware and the accompanying software. 
In the next chapter, first, the data collected in the laboratory for a variety of 
targets is examined and compared with the analytical data in an effort to quantify the 
performance of the pulse EMI system. Next, data collected at the Naval research 
Laboratory?s (NRL) Blossom Point test site is analyzed to examine the detection and 
discrimination capabilities of the pulse EMI sensor system.  
 
 39
CHAPTER 4   PULSED EMI MEASUREMENTS 
In this chapter the laboratory measurements obtained from the pulsed EMI system 
for targets such as copper loops, and UXO-like cylindrical targets are compared with the 
analytical response in an effort to quantify the performance of the system.  Next, the field 
measurements taken at the Blossom Point Test Site, in Maryland are analyzed to 
demonstrate the detection and discrimination capability of the pulsed EMI sensor.  
4.1 Laboratory Measurements 
As mentioned before, the purpose of the laboratory measurements was to ensure 
that the pulsed EMI system is working properly. Specifically, the measured response of 
test targets is compared with their analytically derived counterpart and to the response 
obtained from finite element modeling (FEM) [7]. The first target chosen for comparison 
purposes was the wire loop (sometimes referred to as a q-coil). As shown in Figure 4-1, 
the q-coil consists of a wire of thickness a, and loop radius b, constructed such that 
a<<b.   
 
Figure 4.1 Example of a q-coil (5 inch copper loop with 18 AWG copper wire) 
 40
Referring to (2.25), for time t>T, the receiver coil voltage takes the form of a 
simple exponential decay or  
()
O
t
OR
KetV
??
?=  
(4.1)
where K is a constant and the object time constant is given by, ?
O
=L
O
/R
O
. The 
object inductance is given by  
?
?
?
?
?
?
?
?
?
?
?
?
?
= 75.1
8
ln
a
b
bL
O
?  
(4.2)
where a is the wire radius, b the mean loop radius, and ? is the effective 
permeability of the wire [1]. 
The resistance of the object is given by, 
A
L
R
O
?
=  
(4.3)
with L the wire length, A its cross-sectional area, and ? is the conductivity of the wire [2]. 
 
Figure 4.2 Theoretical, FEM, and Measured Time Domain response of 18 AWG 5" 
copper loop 
 
L, a, and b were measured with a micrometer for precise calculation of the time constant 
of the loop.  The accuracy of the EMI system was evaluated by comparing the three 
 41
decay rates obtained from calculation, measurement, and FEM modeling. Figure 4-2 
shows a plot of the time response of the copper loop obtained from calculations, FEM 
modeling, and measurements. It can be seen from the figure that the agreement among 
the three responses is excellent. The next targets used to validate the performance of the 
pulsed EMI system are the UXO like cylinders. The cylinders are categorized into four 
series based on the dimensions and each series is further subdivided into three types 
based on cylinder wall thickness. The wall thickness plays an important part in 
discrimination. Solid cylinders and cylinders with ?? wall thickness are used to model 
ordnance while cylinders with ?? wall thickness might be representative of clutter. The 
cylinder test target set is shown in Figure 4-3 and their dimensions are given in Table 4.1. 
 
Figure 4.3  UXO test objects consisting of 12 cylinders outlined in Table 4.1 
Figure 4.4 shows the measured time domain response of the ?2 series? (3? 
diameter ? 12? height) cylinders with same diameter and height but different wall 
thicknesses. The response has a steep slope in the early time and a gradual slope later. 
The early time response corresponds to higher frequencies that do not penetrate the 
object. 
 42
Table 4.1  Physical properties of UXO test objects.  Length (L), diameter (D), and wall 
                   thickness. 
Steel Cylinders 
Target 
Wall Thickness 
(in) 
L (in) D (in) 
1A 0.25 8 2 
1B 0.5 8 2 
1C solid 8 2 
2A 0.25 12 3 
2B 0.5 12 3 
2C solid 12 3 
3A 0.25 16 4 
3B 0.5 16 4 
3C solid 16 4 
4A 0.25 24 6 
4B 0.5 24 6 
4C solid 24 6 
 
Therefore, not surprisingly, the responses of the three cylinders are almost 
identical in early time. As the fields penetrate deeper into the object at late time (low 
frequencies), the response of the otherwise visually identical targets should be different. 
However, it is very difficult to measure the late time response as the signal is 
overwhelmed by noise in late time. Hence, measured data only out to 17 ms is shown in 
Figure 4.4. In order to better understand the nature of the response beyond 17 ms, the 
FEM responses [7] of the cylinders out to 40 ms are compared in Figure 4.5. 
 43
 
Figure 4.4 Measured decays for ?2 series? cylinders identical except for wall thickness 
 
Figure 4.5 FEM decays for ?2 series? cylinders identical except for wall thickness 
From Figure 4.5 it is observed that there is a clear difference in the decay 
behavior depending on the wall thickness. The response of cylinder 2A decays faster and 
has a pure exponential decay starting at about 5 ms. Whereas cylinder 2B goes into the 
pure exponential regime at around 20 ms and the exponential portion of the response for 
cylinder 2C would have been noticeable, if the response was extended beyond 40 ms. So, 
 44
it is very important to obtain the late time response of the targets to be able to 
discriminate among different targets.   
4.2 Field Measurements 
The purpose of visiting the Blossom Point test site in Maryland was to 
demonstrate the detection and discrimination capabilities of the pulsed EMI sensor. The 
tests were done in August 2005, for a period of 4 days. Two types of tests were carried 
out at the test site. The first one was a stationary test or Pit test of the EMI sensor which 
was carried out on a platform over a pit. The second type is a dynamic test where the 
EMI sensor is mounted on a flat fiberglass trailer and pulled by a truck down a lane. 
APPENDIX A gives the details of the two types of tests. In the following sections of this 
chapter, results obtained from the two tests are presented.  
4.2.1 Pit Measurements 
Section A.1 of APPENDIX A gives the details of the setup for the pit test. Figure 
4.6 shows some of the targets used in the pit test. 
 
Figure 4.6 Targets used in Pit test  
The responses of different targets were obtained for different target depths, and also 
different target positions with respect to the center of the transmitter and receiver coils. 
 45
4.2.1.1 Response of 4 inch Carbon Steel Sphere 
The first target measured in the pit testing is a 4 inch carbon steel sphere. The 
details of the tests carried out for the 4 inch carbon steel sphere are given in Table A.1. 
Figure 4.7 shows a photograph of the 4 inch Carbon Steel Sphere. Figure 4.8 presents 
data in File1 from Test1 in Table A.1, when the coils are far away from the sphere. This 
data can be referred to as background response since the target is so far away from the 
coils.  
 
Figure 4.7 Picture of the 4 inch Carbon Steel Sphere from the Pit testing 
 
Figure 4.8 Background Response for the sphere tests 
 46
Figure 4.9 shows the data of File 8 from Test1 for the carbon steel sphere with 
background subtraction. Figure 4.10 shows the zoomed in data indicated by the dotted 
box in Figure 4.9.  
From these plots it can be observed that when the coils are far away from the 
target, there is no exponential decay observed, and when the coils move closer to the 
target, the long tail of the exponential decay becomes evident. 
 
Figure 4.9  Response of 4 inch carbon steel sphere with background subtraction. Sphere 
is relatively close to the transmitter and receiver coils 
 
 
Figure 4.10 ?Zoomed-in? response of 4 inch carbon steel sphere with background 
subtraction 
 47
4.2.1.2 Response of the 105 mm shell 
The next target measured was the 105 mm shell. For the 105 mm shell different 
measurements were taken with horizontal and vertical orientations of the target with 
respect to the axis of the coils, as well as different target depths and locations with 
respect to the center of the coils. Table A.2 summarizes the data acquired for the 105 mm 
shell. Figure 4.11 shows a plot of the response of the 105 mm shell with background 
subtraction. The response shown is of File12 from Test1, with the incident magnetic field 
in the transverse direction with respect to the axis of the target. Figure 4.12 shows the 
response corresponding to data files 11 and 12 with background subtraction from Test1, 
and the data is zoomed in to look at the exponential decay of the target. For the data 
presented here, the number of data averages was set to 25. 
 
Figure 4.11  Response of 105 mm shell with background subtraction 
 48
 
Figure 4.12  ?Zoomed-in Response? of 105 mm shell with background subtraction as a 
function of position 
 
4.2.1.3 Response of 81 mm mortar 
The next target tested was an 81 mm mortar. For the 81 mm mortar, 
measurements were taken with horizontal and vertical orientations of the target with 
respect to the axis of the coils. Table A.3 summarizes the data acquired for the 81 mm 
mortar. For the data acquired for the 81 mm mortar, 25 data averages were taken. Figure 
4.13 shows the data of File 7 with background subtraction for the 81 mm mortar with the 
incident magnetic field along the axis of the target. 
 
Figure 4.13  ?Zoomed-in? response of 81 mm mortar with background subtraction 
 49
4.2.1.4 Response of 40 mm shell 
The next target tested was a 40 mm shell. For the 40 mm shell, measurements 
were taken with horizontal and vertical orientations of the target with respect to the axis 
of the coils. For the data acquired for the 40 mm shell, 25 data averages were taken. 
Table A.4 summarizes the data acquired for the 40 mm shell. Figure 4.14 shows the data 
of File 7 with background subtraction for the 40 mm shell with the incident magnetic 
field along the axis of the target. 
 
Figure 4.14  ?Zoomed-in? response of 40 mm shell with background subtraction 
4.2.1.5 Response of Cylinder E 
The next target tested was Cylinder E. For the Cylinder E, measurements were 
taken with horizontal and vertical orientations of the target with respect to the axis of the 
coils. For each orientation measurements were taken for two different target depths with 
respect to the coils. Figure 4.15 shows a picture of cylinder E along with a schematic 
showing the height of the cylinder from the receiver coil for Test1. 
 50
 
Figure 4.15 Depth of Cylinder E with respect to the top of the Platform for Test1 
Table A.5 summarizes the data acquired for Cylinder E. For the data acquired for 
Cylinder E, 50 data averages were taken. Figure 4.16 shows the data of File 11 from Test 
4 with background subtraction for Cylinder E with the incident magnetic field along the 
axis of the target. 
 
Figure 4.16  ?Zoomed-in? response of Cylinder E with background subtraction 
4.2.1.6 Response of Clutter 
The next target tested was the clutter item. The clutter item was placed at a 
position, and the coils were moved in both the X- and Y-directions. For the data acquired 
for the clutter item, f 50 data averages were taken. Table A.6 summarizes the data 
acquired for the clutter item. Figure 4.17 shows a picture of the clutter item used for these 
 51
measurements. Figure 4.18 shows the data of File 13 with background subtraction for the 
clutter item. 
 
Figure 4.17 Picture of the Clutter Item from Pit Testing 
 
Figure 4.18 Response of Clutter Item with background subtraction (zoomed in) 
4.2.2 Dynamic Testing 
Dynamic testing of the EMI sensor was performed at the NRL Ordnance 
Classification Test Site at Blossom Point to evaluate how motion noise affects the 
performance of the sensor. The test field at blossom point had a 30 m X 100 m area 
divided into 5 lanes. Lanes are annotated A, B, C, D, and E for convenience, and each 
lane had 15 targets spaced 6 m from one another. A layout of the test grid is shown in 
 52
Figure A.4 . The dynamic tests were carried out using the MTADS trailer as well as a 
custom made cart. For both type of measurements, data was taken by moving the coils 
over Lane C shown in Figure A.4 [16]. The data acquisition for the dynamic test was 
carried out using a Laptop computer with DAQcard-6062E from National Instruments in 
the PCMCIA slot. The specifications of the data acquisition device are given in Table 3.1 
in chapter three. For the dynamic test, 5 million data points were acquired as the coils 
were moved down Lane C. 
4.2.2.1  Testing using MTADS trailer 
The first part of the dynamic tests was carried out using the MTADS trailer. The 
details of the setup for this test are given in section A.2.1. Table A.7 gives the details of 
the 4 tests carried out using MTADS trailer. Figure 4.19 shows a section of the response 
of pass 4 from the MTADS test.  
 
Figure 4.19 A section of the data from LaneC_Pass4 for the MTADS test 
From the data it is observed that a good representation of the late time exponential 
decay of the target is not obtained. And also a lot of noise was observed in the MTADS 
data. The data acquired was not averaged and this could be one of the reasons for poor 
 53
data quality. Also for the MTADS test, the height of the targets with respect to the 
transmitter and receiver coils was more. Due to this reason, the signal level was very low 
and noise dominated the data obtained. Smooth function in Matlab was used to reduce the 
noise in the data. Methods to achieve good signal-to-noise ratio at heights of up to 1 m as 
well as to reduce the effects of external noise are discussed in Chapter 5.  
4.2.2.2 Testing using Custom made Cart 
Another dynamic test was carried out using a custom made cart. The details of the 
setup for this test are given in section A.2.2. Table A.8 gives the details of the 3 tests 
carried out using the cart. Figure 4.20 shows a section of the response from pass 1 from 
the cart test.  
 
Figure 4.20 A section of the data from LaneC_Pass1 
Observe that the cart-data signal level is larger than that obtained when the system 
was mounted on the MTADS platform. When the coils were mounted on the cart they 
were closer to the ground than when on the MTADS platform and for this reason higher 
signal-to-noise ratio was obtained. Just like the MTADS data, the cart data was not 
averaged and so, the smooth function in Matlab was used to reduce noise in the data. But, 
 54
even for the cart test, the exponential decay is not observed late into time, and methods to 
improve data fidelity in late time are discussed in Chapter 5.  
 
Figure 4.21 Response of Figure 4.20 zoomed on the area shown by the dotted rectangle 
4.3 Summary 
In this chapter, first the testing of the pulsed EMI sensor carried out in the 
laboratory was presented. Responses of different targets were presented, and the q-coils 
were analytically modeled and compared with actual responses.  Also, the response of the 
steel cylinders was compared with the response obtained from an FEM analysis of the 
same. Good qualitative agreement between measured and FEM computed cylinder data 
helped to establish the performance bounds of the pulsed EMI system. Next, field 
measurements carried out at the Blossom Point Test Site in Maryland were presented.  
In the field measurements stationary and dynamic tests of the pulsed EMI sensor 
were carried out. Responses of a variety of targets such as a 105 mm shell, 81 mm 
mortars etc. were presented. At this time, no analytical model exists for direct comparison 
to the measured data. From the data collected in the stationary tests, it is observed that the 
late time exponential decay of the targets was not obtained. This deficiency can be 
 55
attributed to the fact that the low frequency corner of the receiver coil amplifier was 
inadvertently set to high thus chocking off the late time response. From the dynamic tests, 
it was observed that due to low signal level, motion-induced noise dominated the data. To 
improve overall sensor performance, the receiver coil amplifier needs to be redesigned 
and the overall sensitivity of the system must also be improved.  
In the next chapter, the research will be summarized, followed by suggestions for 
improvement and a brief overview of the work currently being done to improve 
performance. 
 
 56
CHAPTER 5   CONCLUSION 
This thesis summarized the efforts towards the development of a Time-Domain 
(Pulsed) EMI system for detecting buried metallic objects and for discriminating between 
UXO and metallic clutter. Improved techniques are needed to overcome bandwidth and 
sensitivity limitations of the current system.  
The initial chapter provided an overview of the theoretical concepts of a basic 
EMI system. A magnetically coupled circuit model was used to provide the analytical 
modeling of time- and frequency-domain EMI systems. 
In Chapter 3, the frequency domain transfer function obtained in chapter two was 
extended to take into account the characteristics of the transmitting coil, the receiver coil, 
and the receiver coil amplifier. The trade-off between system dynamic range and system 
gain was defined. Also, the design and development of the electronics, and the data 
acquisition hardware for a pulsed EMI system were presented. A method of measuring 
the current in the receiver coil was compared to a voltage measurement technique and 
advantages and disadvantages of the two methods were compared. 
Measurements carried out in the lab as well as the field measurements at the 
Naval Research Laboratory?s Blossom Point test site in Maryland were described in 
Chapter 4. 
 57
In the lab measurements a variety of test targets such as a simple loop, and carbon 
steel cylinders were tested. The laboratory measurements were compared with the 
response obtained from Finite Element Modeling (FEM). The FEM analysis showed that 
important information exists for discrimination purposes in the late time response. In the 
field, stationary as well as dynamic tests were carried out to evaluate the performance of 
the sensor. A variety of targets such as the 4 inch carbon steel sphere, 105 mm shell, 81 
mm mortar etc., were tested. From the stationary tests, it was observed that a good 
representation of the late time exponential decay of the targets was not obtained. This can 
be attributed to problems with the receiver coil amplifier. From the dynamic tests, it was 
observed that due to low sensitivity, noise dominated the data.  
Currently work is in progress to overcome the problems discussed above. To 
increase the sensitivity of the sensor, larger receiver coils are being constructed. As the 
cross section of the receiver coils increases, the sensitivity of the sensor to deeply buried 
targets also increases. Next, the receiver amplifier used for these measurements is 
replaced by a current- to-voltage converter with very low input impedance. This 
decreases the lower break frequency of the receiver coils thereby increasing the 
bandwidth of the system as discussed in Chapter 3. To reduce the external noise effects as 
well as minimize the input offset effects, a differential current-to-voltage converter can 
be used. Initial measurements using a differential current-to-voltage converter have 
provided encouraging results. Additional tests at Blossom Point test site are planned for 
January 2005 in order to objectively evaluate recently completed sensor modifications. 
 
 
 58
APPENDIX A FIELD MEASUREMENTS AT THE BLOSSOM POINT TEST SITE 
 The purpose of visiting the Blossom Point test site in Maryland was to 
demonstrate the detection and discrimination capabilities of the pulsed EMI sensor. The 
tests were done in August 2005, for a period of 4 days. Two types of tests were carried 
out at the test site. The first one was a stationary test or Pit test of the EMI sensor which 
was carried out on a platform over a pit. The second type is a dynamic test where the 
EMI sensor is mounted on a flat fiberglass trailer and pulled by a truck down a lane. 
A.1 Pit Measurements 
Figure A.1 shows a schematic of the configuration of the pit measurement system. 
The apparatus consists of a test bed constructed using fiberglass material over a pit (or 
hole) in the ground. The test bed has a rectangular cross section with four long bars in the 
north-south direction and many horizontal cross pieces in the east-west direction. The 
outer north-south bars have a total of 40 holes numbered as hole #1, hole #2 etc. The 
holes are spaced 10 cm apart up to hole #34, and are spaced 20 cm thereafter. A 
horizontal platform sits on top of the north-south beams and can be moved along them in 
10 cm or 20 cm increments. The platform has holes that can be aligned with the holes on 
the north-south bars and pegs are put through the holes to fix the platform to a known 
position. The platform has an additional small square platform on top of it that can move 
in the east-west direction. The frame consisting of transmitter and receiver coils is placed 
on top of the platform and held in place using non-metallic clamps. 
 59
x
y
z
N
W
S
E
 
Figure A.1 Schematic of the stationary testing platform (a) Top View (b) Side View  
The target is placed on a hole on an east west cross member and then the platform 
containing the coils is moved along the x-(east-west) and y-(north-south) directions 
relative to the stationary target. The distance from the center of the coils to the point 
where the target is placed is 68 cm when the platform is over hole #22, and the height of 
this point from the top of the square platform is 38 cm. The location (x, y, z) of the center 
of the platform relative to the hole in the east-west directed cross member is known. 
 60
Furthermore, the location of a target relative to the hole in the cross member has been 
measured and therefore the location of the target relative to the transmitter and receiver 
coils can be easily computed.  
The ?pulser? box which contains the pulse-generation circuitry along with the 
receiver coil signal conditioning circuitry, the oscilloscope which is used to monitor the 
current through the transmitter coil, and the batteries to supply power to the pulser, are 
placed on a wooden platform at one end of the test bed as shown in Figure A.2  
 
Figure A.2 Testing platform for the stationary testing 
The output of the receiver signal conditioning circuit is fed to a Desktop 
Computer which contains a CS-1602 A/D card from Gage Applied Technologies and the 
Gagescope software which comes with the A/D card was used to monitor and save the 
target response data. The responses of different targets were obtained for different target 
depths (distance from target to center of coils), and also different target positions with 
respect to the center of the transmitter and receiver coils. 
 61
 
Figure A.3 Some of the targets used in the pit test 
A.1.1 Data collected for 4 inch Carbon Steel Sphere 
The first target measured in the Pit testing is a 4 inch carbon steel sphere. Three 
different tests were done using the 4 inch carbon steel sphere and these tests are referred 
to as Test1, Test2, and Test3 respectively.  For each test, the target was placed at some 
position and the platform with the transmitter and receiver coils was moved in the Y 
direction as indicated in Figure A.1. Data was collected at each platform position. Table 
A.1 shows the position of the target with respect to the center of the coils for all the three 
tests.  
 
 
 
 
 
 62
Table A.1 4 inch Carbon Steel Sphere  
4 inch Carbon Steel Sphere Tests 
Position of the Target with respect to the center of the coils 
Test1 Test2 Test3 Data File 
X(cm) Y cm) Z(cm) X(cm) Y(cm) Z(cm) X(cm) Y(cm) Z(cm)
File 1 0 +98 33 0 +95.5 42.5 0 +95.5 43.5 
File 2 0 +88 33 0 +85.5 42.5 0 +85.5 43.5 
File 3 0 +78 33 0 +75.5 42.5 0 +75.5 43.5 
File 4 0 +68 33 0 +65.5 42.5 0 +65.5 43.5 
File 5 0 +58 33 0 +55.5 42.5 0 +55.5 43.5 
File 6 0 +48 33 0 +45.5 42.5 0 +45.5 43.5 
File 7 0 +38 33 0 +35.5 42.5 0 +35.5 43.5 
File 8 0 +28 33 0 +25.5 42.5 0 +25.5 43.5 
File 9 0 +18 33 0 +15.5 42.5 0 +15.5 43.5 
File 10 0 +8 33 0 +5.5 42.5 0 +5.5 43.5 
File 11 0 -8 33 0 -5.5 42.5 0 -5.5 43.5 
File 12 0 -18 33 0 -15.5 42.5 0 -15.5 43.5 
File 13 0 -28 33 0 -25.5 42.5 0 -25.5 43.5 
File 14 0 -38 33 0 -35.5 42.5 0 -35.5 43.5 
File 15 0 -48 33 0 -45.5 42.5 0 -45.5 43.5 
File 16 0 -58 33 0 -55.5 42.5 0 -55.5 43.5 
File 17 0 -78 33 0 -65.5 42.5 0 -65.5 43.5 
File 18 0 -98 33 0 -75.5 42.5 0 -75.5 43.5 
 
A.1.2 Data collected for 105 mm shell 
The next target measured was the 105 mm shell. For the 105 mm shell different 
measurements were taken with horizontal and vertical orientations of the target with 
respect to the axis of the coils, as well as different target depths and locations with 
respect to the center of the coils. Table A.2 summarizes the data acquired for the 105 mm 
shell. For all the tests except Test3, the number of data averages was set to 25 in the data 
acquisition software, and for Test3 50 averages were used. 
 
 
 
 63
Table A.2 Data Acquired for the 105 mm shell 
105 mm shell measurements 
Orientation of the incident magnetic field with respect to the primary axis of the 
target and position of the target with respect to the center of the coils respectively
Transverse Axial 
Test1 Test2 Test3 Test4 Test5 
Data 
File 
X 
(cm) 
Y 
(cm) 
Z 
(cm) 
X 
(cm)
Y 
(cm)
Z 
(cm)
X 
(cm)
Y 
(cm)
Z 
(cm)
X 
(cm)
Y 
(cm) 
Z 
(cm) 
X 
(cm) 
Y 
(cm)
Z 
(cm)
File1 0 +144.7532.45 0 +143.241.6 -65 -81 53 0 +127.25 52 -4 +13367.7
File2 0 +134.7532.45 0 +133.241.6 -65 -41 53 0 +117.25 52 -4 +12367.7
File3 0 +124.7532.45 0 +123.241.6 -65 -1 53 0 +107.25 52 -4 +11367.7
File4 0 +114.7532.45 0 +113.241.6 -65 +39 53 0 +97.25 52 -4 +10367.7
File5 0 +104.7532.45 0 +103.241.6 -65 +79 53 0 +87.25 52 -4 +93 67.7
File6 0 +94.75 32.45 0 +93.2 41.6 -25 +79 53 0 +77.25 52 -4 +83 67.7
File7 0 +84.75 32.45 0 +83.2 41.6 -25 +39 53 0 +67.25 52 -4 +73 67.7
File8 0 +74.75 32.45 0 +73.2 41.6 -25 -1 53 0 +57.25 52 -4 +63 67.7
File9 0 +64.75 32.45 0 +63.2 41.6 -25 -41 53 0 +47.25 52 -4 +53 67.7
File10 0 +54.75 32.45 0 +53.2 41.6 -25 -81 53 0 +37.25 52 -4 +43 67.7
File11 0 +44.75 32.45 0 +43.2 41.6 +15 -81 53 0 +27.25 52 -4 +33 67.7
File12 0 +34.75 32.45 0 +33.2 41.6 +15 -41 53 0 +17.25 52 -4 +23 67.7
File13 0 +24.75 32.45 0 +23.2 41.6 +15 -1 53 0 +7.25 52 -4 +13 67.7
File14 0 +14.75 32.45 0 +13.2 41.6 +15 +39 53 0 -7.25 52 -4 -13 67.7
File15 0 +4.75 32.45 0 +3.2 41.6 +15 +79 53 0 -17.25 52 -4 -23 67.7
File16 0 -4.75 32.45 0 -3.2 41.6 +55 +79 53 0 -27.25 52 -4 -33 67.7
File17 0 -14.75 32.45 0 -13.2 41.6 +55 +39 53 0 -37.25 52 -4 -43 67.7
File18 0 -24.75 32.45 0 -23.2 41.6 +55 -1 53 0 -47.25 52 -4 -53 67.7
File19 0 -34.75 32.45 0 -33.2 41.6 +55 -41 53 0 -57.25 52 -4 -63 67.7
File20 0 -44.75 32.45 0 -43.2 41.6 +55 -81 53 0 -67.25 52 -4 -73 67.7
File21 0 -54.75 32.45 0 -53.2 41.6 +95 -81 53 0 -77.25 52 -4 -83 67.7
File22 0 -64.75 32.45 0 -63.2 41.6 +95 -41 53 0 -87.25 52 -4 -93 67.7
File23 0 -74.75 32.45 0 -73.2 41.6 +95 -1 53 0 -97.25 52 -4 -103 67.7
File24 0 -84.75 32.45 0 -83.2 41.6 +95 +39 53 0 -107.25 52 -4 -113 67.7
File25 0 -94.75 32.45 0 -93.2 41.6 +95 +79 53 0 -117.25 52 -4 -123 67.7
File26 0 -104.75 32.45 0 -103.2 41.6 ? ? ? 0 -127.25 52 -4 -133 67.7
 
A.1.3 Data collected for 81 mm mortar 
The next target tested was an 81 mm mortar. For the 81 mm mortar, measurements were 
taken with horizontal and vertical orientations of the target with respect to the axis of the 
 64
coils. Table A.3 summarizes the data acquired for the 81 mm mortar. For the data 
acquired for the 81 mm mortar, 25 averages were taken.  
Table A.3  Data Acquired for the 81 mm mortar 
81 mm mortar measurements 
Orientation of the incident magnetic field with respect to the primary axis of the 
target and position of the target with respect to the center of the coils 
respectively 
Transverse Axial 
Data File 
X (cm) Y (cm) Z (cm) X (cm) Y (cm) Z (cm) 
File 1 0 +125 35.05 0 +88.5 56 
File 2 0 +115 35.05 0 +78.5 56 
File 3 0 +105 35.05 0 +68.5 56 
File 4 0 +95 35.05 0 +58.5 56 
File 5 0 +85 35.05 0 +48.5 56 
File 6 0 +75 35.05 0 +38.5 56 
File 7 0 +65 35.05 0 +28.5 56 
File 8 0 +55 35.05 0 +18.5 56 
File 9 0 +45 35.05 0 +8.5 56 
File 10 0 +35 35.05 0 -8.5 56 
File 11 0 +25 35.05 0 -18.5 56 
File 12 0 +15 35.05 0 -28.5 56 
File 13 0 +5 35.05 0 -38.5 56 
File 14 0 -5 35.05 0 -48.5 56 
File 15 0 -15 35.05 0 -58.5 56 
File 16 0 -25 35.05 0 -68.5 56 
File 17 0 -35 35.05 ? ? ? 
File 18 0 -45 35.05 ? ? ? 
File 19 0 -55 35.05 ? ? ? 
File 20 0 -65 35.05 ? ? ? 
File 21 0 -75 35.05 ? ? ? 
File 22 0 -85 35.05 ? ? ? 
 
A.1.4 Data collected for 40 mm shell 
The next target tested was a 40 mm shell. For the 40 mm shell, measurements 
were taken with horizontal and vertical orientations of the target with respect to the axis 
 65
of the coils. For the data acquired for the 40 mm shell, 25 averages were taken. Table A.4 
summarizes the data acquired for the 40 mm shell.  
Table A.4 Data Acquired for the 40 mm shell 
40 mm shell measurements 
Orientation of the incident magnetic field with respect to the primary axis 
of the target and position of the target with respect to the center of the coils 
respectively 
Transverse Axial 
Data File 
X (cm) Y (cm) Z (cm) X (cm) Y (cm) Z (cm)
File 1 0 +98 23.2 0 +88 24.05 
File 2 0 +88 23.2 0 +78 24.05 
File 3 0 +78 23.2 0 +68 24.05 
File 4 0 +68 23.2 0 +58 24.05 
File 5 0 +58 23.2 0 +48 24.05 
File 6 0 +48 23.2 0 +38 24.05 
File 7 0 +38 23.2 0 +28 24.05 
File 8 0 +28 23.2 0 +18 24.05 
File 9 0 +18 23.2 0 +8 24.05 
File 10 0 +8 23.2 0 -8 24.05 
File 11 0 -8 23.2 0 -18 24.05 
File 12 0 -18 23.2 0 -28 24.05 
File 13 0 -28 23.2 0 -38 24.05 
File 14 0 -38 23.2 0 -48 24.05 
File 15 0 -48 23.2 0 -58 24.05 
File 16 0 -58 23.2 ? ? ? 
 
A.1.5 Data collected for Cylinder E 
The next target tested was Cylinder E. For the Cylinder E, measurements were 
taken with horizontal and vertical orientations of the target with respect to the axis of the 
coils. For each orientation measurements were taken for two different target depths with 
respect to the coils. Table A.5 summarizes the data acquired for Cylinder E. For the data 
acquired for Cylinder E, 50 averages were taken. 
 
 
 66
Table A.5 Data Acquired for the Cylinder E 
Cylinder E measurements 
Orientation of the incident magnetic field with respect to the primary axis of the 
target and position of the target with respect to the center of the coils respectively
Transverse Axial 
Test1 Test2 Test3 Test4 
Data 
File 
X(cm
) 
Y (cm) 
Z(cm
) 
X (cm) Y(cm)
Z(cm
) 
X(cm) Y(cm)
Z(cm
) 
X(cm) 
Y(cm
) 
Z 
(cm)
File 1 0 +163.5 41.81 0 +137.8 54.81 0 +130.5 58.94 0 131 45.24
File 2 0 +153.5 41.81 0 +127.8 54.81 0 +120.5 58.94 0 +121 45.24
File 3 0 +143.5 41.81 0 +117.8 54.81 0 +110.5 58.94 0 +111 45.24
File 4 0 +133.5 41.81 0 +107.8 54.81 0 +100.5 58.94 0 +101 45.24
File 5 0 +123.5 41.81 0 +97.8 54.81 0 +90.5 58.94 0 +91 45.24
File 6 0 +113.5 41.81 0 +87.8 54.81 0 +80.5 58.94 0 +81 45.24
File 7 0 +103.5 41.81 0 +77.8 54.81 0 +70.5 58.94 0 +71 45.24
File 8 0 +93.5 41.81 0 +67.8 54.81 0 +60.5 58.94 0 +61 45.24
File 9 0 +83.5 41.81 0 +57.8 54.81 0 +50.5 58.94 0 +51 45.24
File 10 0 +73.5 41.81 0 +47.8 54.81 0 +40.5 58.94 0 +41 45.24
File 11 0 +63.5 41.81 0 +37.8 54.81 0 +30.5 58.94 0 +31 45.24
File 12 0 +53.5 41.81 0 +27.8 54.81 0 +20.5 58.94 0 +21 45.24
File 13 0 +43.5 41.81 0 +17.8 54.81 0 +10.5 58.94 0 +11 45.24
File 14 0 +33.5 41.81 0 +7.8 54.81 0 +0.5 58.94 0 +1 45.24
File 15 0 +23.5 41.81 0 -7.8 54.81 0 -0.5 58.94  -1 45.24
File 16 0 +13.5 41.81 0 -17.8 54.81 0 -10.5 58.94 0 -11 45.24
File 17 0 +3.5 41.81 0 -27.8 54.81 0 -20.5 58.94 0 -21 45.24
File 18 0 -3.5 41.81 0 -37.8 54.81 0 -30.5 58.94 0 -31 45.24
File 19 0 -13.5 41.81 0 -47.8 54.81 0 -40.5 58.94 0 -41 45.24
File 20 0 -23.5 41.81 0 -57.8 54.81 0 -50.5 58.94 0 -51 45.24
File 21 0 -33.5 41.81 0 -67.8 54.81 0 -60.5 58.94 0 -61 45.24
File 22 0 -43.5 41.81 0 -77.8 54.81 0 -70.5 58.94 0 -71 45.24
File 23 0 -53.5 41.81 0 -87.8 54.81 0 -80.5 58.94 0 -81 45.24
File 24 0 -63.5 41.81 0 -97.8 54.81 0 -90.5 58.94 0 -91 45.24
File 25 0 -73.5 41.81 0 -107.8 54.81 0 -100.5 58.94 0 -101 45.24
File 26 0 -83.5 41.81 0 -117.8 54.81 0 -110.5 58.94 0 -111 45.24
File 27 0 -93.5 41.81 0 -127.8 54.81 0 -120.5 58.94 0 -121 45.24
File 28 0 -103.5 41.81 0 -137.8 54.81 0 -130.5 58.94 0 -131 45.24
 
A.1.6 Data collected for Clutter 
The next target tested was the clutter item. The clutter item was placed at a known 
position, and the coils were moved in both the x- and y- directions. For the data acquired 
 67
for the clutter item 50 averages were used. Table A.6 shows the position of the target 
with respect to the center of the coils. 
Table A.6 Data Acquired for the Clutter 
Clutter measurements 
Position of the target with respect to the axis of the coils
Data File 
X (cm) Y (cm) Z(cm) 
File 1 +80 -80 40 
File 2 +80 -40 40 
File 3 +80 0 40 
File 4 +80 +40 40 
File 5 +80 +80 40 
File 6 +40 +80 40 
File 7 +40 +40 40 
File 8 +40 0 40 
File 9 +40 -40 40 
File 10 +40 -80 40 
File 11 0 -80 40 
File 12 0 -40 40 
File 13 0 0 40 
File 14 0 +40 40 
File 15 0 +80 40 
File 16 -40 +80 40 
File 17 -40 +40 40 
File 18 -40 0 40 
File 19 -40 -40 40 
File 20 -40 -80 40 
File 21 -80 -80 40 
File 22 -80 -40 40 
File 23 -80 0 40 
File 24 -80 +40 40 
File 25 -80 +80 40 
 
A.2 Dynamic Testing 
The dynamic testing of the EMI sensor was performed at the Blossom Point test 
site to evaluate how motion noise affects the performance of the sensor. The test field at 
blossom point had a 30 m?100 m area divided into 5 lanes. The lanes are named A, B, C, 
D, and E for convenience, and each lane had 15 targets spaced approximately 6 m from 
 68
one another. A layout of the test grid is shown in Figure A.4. The dynamic testing was 
performed using the MTADS trailer as well as a custom made cart. For both type of 
measurements, data was taken by moving the coils over Lane C shown in Figure A.4.  
 
Figure A.4 Layout of the NRL ordnance classification test site at Blossom Point 
A.2.1 Testing using MTADS trailer 
The MTADS trailer is made of non metallic material and an all-terrain vehicle is 
used to tow the MTADS trailer. Figure A.5 shows the setup for dynamic testing of the 
EMI sensor using MTADS trailer. The EMI sensor coils were placed on the trailer at the 
center, and were fixed in place to prevent them from moving. The pulser box and the 
power supply for it were placed at one end of the trailer. A Laptop with NI DAQcard-
6062E in the PCMCIA slot was used for data acquisition. Labview code was developed 
and used for acquiring the data. 
 69
            
            
Figure A.5 Dynamic Testing of EMI sensor with MTADS trailer 
The data acquisition was started at the beginning of the lane, and stopped at the end of the 
lane. The measurements were repeated four times and Table A.7 gives the details of the 
tests for each pass. 
Table A.7 Details of the Dynamic Test using MTADS trailer 
Pass Direction Pulse Rep. rate (ms) 
1 S to N 10 
2 N to S 10 
3 S to N 10 
4 N to S 10 
 
A.2.2 Testing using Custom made Cart 
The cart for performing the dynamic tests of the EMI sensor is made of non 
metallic fiberglass material called EXTREN. Figure A.6 shows the set up for dynamic 
testing of the EMI sensor using the cart.  
 70
 
Figure A.6 Dynamic Testing of EMI sensor with Custom made cart 
The cart was moved along Lane C in the test grid and data was acquired. The 
measurements were repeated three times, and Table A.8 gives the details of the tests. 
Table A.8 Details of the Dynamic Test using the cart 
Pass Direction Pulse Rep. rate (ms) 
1 N to S 20 
2 S to N 20 
3 N to S 10 
 
 
 
 
 
 71
APPENDIX B MICROELECTROMECHANICAL SYSTEMS (MEMS) CAPACITIVE  
PRESSURE SENSOR ARRAY FABRICATED USING PRINTED  
CIRCUIT PROCESSING TECHNIQUES 
Micro Electro-Mechanical Systems (MEMS) based capacitive pressure sensors 
are typically fabricated using silicon micromachining techniques. In this work, novel 
Liquid Crystal Polymer (LCP) based capacitive pressure sensors, fabricated using printed 
circuit processing techniques, are reported. LCP exhibits good dimensional stability, 
material flexibility, high chemical resistance, and extremely low moisture absorption, 
which makes it suitable for MEMS applications. The pressure sensor consists of a 
cylindrical cavity formed by a sandwich of LCP substrate, LCP spacer layer with circular 
holes and LCP top layer. The bottom electrode and the top electrode of the capacitive 
pressure sensor are defined on the top surface of the LCP substrate and the bottom 
surface of the top LCP layer, respectively. A typical pressure sensor with a diaphragm 
radius of 1.6 mm provides a total capacitance change of 0.7627 pF for an applied 
pressure in the range of 0-170 kPa.  
B.1 Introduction 
A large variety of bulk micromachined and surface micromachined pressure 
sensors have been developed for industrial and biomedical applications [17]-[19]. For 
bulk micromachining, single crystal silicon has been the predominant material of choice 
whereas polysilicon and low stress nitride are the two notable thin films used for surface 
micromachining [20]. 
 72
In recent years, there has been considerable interest in the development of 
Microelectromechanical systems (MEMS) fabricated using printed circuit board (PCB) 
processing techniques combined with specialized micromachining techniques such as 
laser machining, plasma etching etc. [21]-[22]. The advantages of the PCB based MEMS 
approach include low cost, compatibility with organic substrate based Multi-chip module 
laminate (MCM-L) technology [23], ease of integration with embedded passives  [24] 
and surface mount electronics,  suitability for batch fabrication in large panels and high 
volume manufacturing. A large array of pressure sensors can be batch fabricated using 
roll-to-roll flexible printed circuit processing systems for large area applications.   
Two types of pressure sensors commonly used in MEMS applications are: 
piezoresistive type and capacitive type. In this work, capacitive type pressure sensors 
have been developed using LCP. Capacitive pressure sensors have higher measurement 
sensitivity, decreased temperature sensitivity, and reduced power consumption, and better 
stability compared with other types of pressure sensors [25]. Capacitive pressure sensors 
operate on the principle of pressure induced deflection of a metallized flexible membrane 
and the subsequent measurement of capacitance between this flexible membrane and a 
fixed bottom plate [26].  
In the past, pressure sensors have been fabricated using kapton Polyimide films, 
ceramic tape etc. Sung-Pil Chang et. al. have used a combination of micromachining and 
lamination techniques for fabrication of a capacitive pressure sensor using kapton 
polyimide film [27]. Michael A. Fonseca et. al. have used lamination techniques for the 
fabrication of a capacitive pressure sensor using low-temperature cofireable ceramic 
(LTCC) [28]. In this appendix, novel Liquid Crystal Polymer (LCP) based capacitive 
 73
pressure sensors fabricated using conventional printed circuit processing techniques are 
presented. The proposed pressure sensors are suitable for integration with system-on-
package (SOP) type Microsystems fabricated using low-cost MCM-L technologies [29]. 
Liquid crystal polymer is a thermoplastic material with unique structural and physical 
properties. The advantages of LCP include low cost, versatility of fabrication such as low 
temperature thermal bonding, mechanical flexibility, and less moisture absorption 
compared to other polymer films used in MEMS processes [21].  
The remainder of the appendix is organized as follows: fabrication and assembly 
processes for the proposed LCP based pressure sensors are discussed in Section B.2, 
design and experimental characterization of the pressure sensor array are presented in 
Section B.3, and finally concluding remarks are outlined in Section B.4.  
B.2 Fabrication and Assembly 
B.2.1 Sensor Configuration 
The configuration of the LCP based capacitive pressure sensor is shown in Figure 
B.1. It consists of three layers: 30-mil thick high-temperature LCP substrate (R/flex 3850 
available from Rogers Corporation) with 0.5-micron thick copper metallization, 2-mil 
thick high-temperature LCP top layer (R/flex 3850) with 0.5-micron thick copper 
metallization, and 2-mil low-temperature LCP spacer layer (R/flex 3600). The melting 
temperature of the high-temperature LCP substrate and the LCP top layer is 315?C. The 
melting temperature of the low-temperature LCP spacer is 280?C. The pressure sensor 
consists of a cylindrical cavity formed by a sandwich of the top layer (with the top 
electrode), the spacer layer (with the circular hole) and the substrate (with the bottom 
electrode). The bottom electrode and the top electrode inside the cavity form a parallel 
 74
plate capacitor configuration. The spacer layer provides the spacing between the substrate 
layer and the top layer. Consequently, the thickness of the spacer layer determines the 
initial gap between the two electrodes of the sensor. The air gap between the electrodes is 
49 ?m.  
The circular portion of the top LCP layer located above the circular hole 
(diameter 2a) of the spacer layer serves as the movable diaphragm of the pressure sensor. 
At atmospheric pressure, the pressure sensor provides an initial capacitance, which 
corresponds to the undeflected position of the movable diaphragm. When the pressure 
(outside the sensor cavity) is increased above the atmospheric level, the diaphragm 
deflects downward increasing the capacitance of the sensor from the initial value. 
Top 
electrode
Bottom 
electrode
LCP Top Layer (2 mils)
LCP Top 
Layer
Circular Diaphragm
Electrode
49 ?m
2a
LCP Spacer Layer (2 mils)
LCP Substrate (30 mils)
 
Figure B.1 LCP based MEMS capacitive pressure sensor configuration 
B.2.2 Sensor fabrication  
The sensor fabrication process involves two major steps. In the first step, the 
metallization layers on the top surface of the LCP substrate and the bottom surface of the 
top LCP layer are patterned by photolithography and wet etching processes to form the 
 75
bottom electrode and the top electrode of the capacitive pressure sensor, respectively. In 
the second step, circular holes of various diameters in the range of 1-3 mm are machined 
in the LCP spacer layer using a deep reactive ion etching (DRIE) technique.  
Common bottom 
electrode
Circular holes
Top 
Electrodes 
(a)
Interconnection 
line
(b)
(c)
 
Figure B.2 Photographs of various layers of the pressure sensor array (a) LCP substrate 
with a common bottom electrode, (b) LCP spacer with circular holes, and (c) LCP top 
layer with copper top electrodes. 
 
The circular segment of the top LCP layer located above the circular hole of the 
spacer layer forms the movable diaphragm of the pressure sensor. Therefore, the diameter 
 76
of the movable circular membrane corresponds to the diameter of the circular hole in the 
spacer layer. Figure B.2 shows photographs of the three layers of the pressure sensor.  
B.2.3 Lamination Process 
The lamination process involves alignment of the three layers followed by a 
thermo-compression bonding process. Two stainless steel blocks with alignment holes 
and pins (shown in Figure B.3) are used to align the three layers. The sequence of the 
three layers after alignment is shown in Figure B.4. 
 
Figure B.3 Metal blocks with alignment holes and pins used in the lamination process 
Thermo-compression bonding was performed using a Carver Press consisting of 
two rectangular aluminum platens shown in Figure B.5. The fixture consisting of two 
stainless steel blocks, along with the aligned layers, is placed between the press platens. 
Platens? heating is regulated by a temperature controller with a thermocouple.  
 
Figure B.4 Sequence of the three layers of the pressure sensor during lamination 
Thermo-compression bonding is performed at a pressure of 150 psi and at a 
temperature of 280?C, which is the melting temperature of the LCP spacer layer. During 
the bonding process, both temperature and pressure are maintained for 30 min and the  
 77
 
Figure B.5 Press used for thermo-compression bonding 
assembly is cooled down to room temperature before the pressure is released. The 
fabricated pressure sensor array is shown in Figure B.6. The fabrication and assembly 
processes employed here are described in [22]. 
B.3 Design and Characterization 
B.3.1 Design 
The principle of operation of the capacitive type pressure sensor reported here is 
based on the pressure induced deflection of a flexible LCP membrane with electrode and 
the subsequent measurement of capacitance between the electrode on this deflecting 
membrane and a fixed bottom electrode on the substrate. For analytical modeling [27], 
[30], several assumptions have been made: (a) stretching of the membrane has been 
neglected, since the membrane will not be undergoing deflections large compared with its 
thickness; (b) residual stress in the membrane has been neglected; (c) the thickness of the 
metallic electrode on the membrane has been neglected, since this thickness is small 
compared with the membrane thickness; (d) electric field fringing effects of the parallel 
plate configuration have been neglected, since the gap between the top electrode and the 
 78
fixed bottom electrode is small compared with their lateral extents. Under these 
conditions, if the deflection at the center of the membrane is w
0
, t is the thickness of the 
membrane, d
1
 is the initial (undeflected) gap distance between the fixed bottom electrode 
and the top electrode, and P is the uniform applied pressure on the membrane, then the 
deflection of a circular membrane with fully clamped perimeter as a function of radius, 
w(r), is given by: 
2
2
0
1)(
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?=
a
r
wrw
 
(B.1)
where a is the radius of the plate (0?r?a) and the deflection at the center of the plate is 
given by  
D
Pa
w
64
4
0
=
 
(B.2)
In equation (B.2), D is the flexural rigidity of the membrane which is given by: 
()
2
3
112 v
Et
D
?
=
 
(B.3)
where E is the elastic modulus and ? is the Poisson ratio of the membrane.  
These design equations have been used for designing various pressure sensors and 
the experimental characterization of an example pressure sensor is discussed. 
B.3.2 Experimental Characterization 
B.3.2.1 Scanning Acoustic Microscope Measurements 
Photograph of the fabricated 5 x 3 pressure sensor array prototype is shown in 
Figure B.6. The prototype consists of various pressure sensors with diaphragm diameters 
in the range of 2 mm to 4.8 mm. The diameter of the circular holes in the LCP spacer 
 79
layer for various pressure sensors has been measured using a Scanning Acoustic 
Microscope (SAM). 
 
Figure B.6 Photograph of a 5 x 3 capacitive pressure sensor array 
Circular hole of 
diameter ~ 3 mm
Interconnection line
Top Electrode
 
Figure B.7 Scanning Acoustic Microscope (SAM) picture showing a portion of the 
pressure sensor array 
 
An SAM picture of a portion of the pressure sensor array is shown in Figure B.7. 
A pressure sensor with an approximate diaphragm diameter of 3 mm is indicated in 
Figure B.7. The top electrodes and the interconnection lines of the sensor array defined 
on the bottom surface of the top LCP layer (refer to c)) can also be seen in the SAM 
picture. 
 80
B.3.2.2 Measurement Setup 
The measurement setup for testing the capacitive pressure sensors is shown in 
Figure B.8. It consists of two components: a custom made pressure chamber and the 
signal conditioning circuitry. The pressure chamber is made of Teflon with dimensions of 
9.5??8.5??3?. It has a pressure inlet connected to a pressure gauge and an outlet 
connected to the safety valve. As shown in Figure B.8, the pressure sensor is placed 
inside the chamber for characterization. When the pressure inside the chamber is above 
the atmospheric pressure, the movable diaphragm deflects downward, thereby increasing 
the capacitance between the top and bottom electrodes. The capacitance change is 
measured using a Capacitance-to-voltage conversion circuit discussed in the next section. 
Pressure Chamber
Safety Valve
Pressure sensor array
Pressure Gauge
Inlet
Outlet
MS3110BDPC 
Evaluation Board
 
Figure B.8 Experimental setup for testing LCP based pressure sensors 
B.3.2.3 Capacitance-to-voltage conversion circuit  
A universal capacitive read-out (UCR) IC MS3110 is used in conjunction with its 
evaluation board MS3110BDPC from Microsensors Inc. as the read-out circuit. The 
evaluation board allows programming of the read-out IC through a parallel port cable 
 81
connected to a PC. The theory of operation of the chip can be described using the block 
diagram shown in Figure B.9 [31]. The MS3110 outputs a voltage proportional to the 
difference between the two capacitances CS1IN and CS2IN connected to the input 
terminals. CS1 and CS2 are the internal trim capacitances of the chip. The chip can be 
used either in differential mode or single-variable mode. In the single-variable mode, the 
sensor being tested is connected to the input terminal CS2IN of the chip and the internal 
trim capacitor CS1 on the other input terminal of the chip is used to compensate for any 
offset in the output voltage corresponding to the capacitance in the initial state of the 
pressure sensor. The initial capacitance of the sensor includes parasitic capacitance and 
the up state capacitance between the sensor electrodes. The internal trim capacitor CS1 
can be varied from 0.2pF-10pF.  The output voltage can be expressed as a function of the 
sensing capacitances CS1
T
 and CS2
T
 given by [31] 
( ) VREFCCSCSPVGAINV
FTT
+?= 12*14.1*252*0
(B.4)  
where CS1
T
=CS1, CS2
T
=CS2IN+CS2, V2P25=VREF is the reference input voltage, 
which is DC 2.25 V.  
 
Figure B.9 Block diagram of the MS3110 IC used in the single-variable mode  
 
 82
B.3.2.4 Results and Discussion 
An example pressure sensor with diaphragm radius of 1.6mm is discussed here. 
Figure B.10 shows a plot of voltage output from the MS3110 UCR circuit. The voltage 
change over an applied pressure range of 0-170 kPa is 0.763V. The sensitivity is 4.8 
mV/kPa
 
and the linearity error is 6.1%.  Linearity error is defined as the percentage ratio 
of deviation of the sensor output from a straight-line relationship over the desired 
pressure range to the full-scale span of the pressure sensor. In this case, the deviation was 
obtained using a least-squares method, and the full scale span of the sensor is defined as 
the difference between the output voltage at the maximum and minimum pressure, 
respectively, in the desired operating pressure range. The corresponding capacitance 
change of the pressure sensor was calculated using equation (4). Figure B.11 shows the 
relative capacitance change with the applied pressure. The total capacitance change of the 
sensor is 0.7627pF for an applied pressure in the range of 0-170 kPa. 
 
Figure B.10 Output voltage from MS3110 UCR for an example pressure sensor. 
 83
 
Figure B.11 Relative capacitance change versus applied pressure for an example pressure 
sensor 
 
B.4 Conclusion 
A MEMS capacitive pressure sensor array fabricated using low cost printed 
circuit processing techniques is reported. An example pressure sensor with diaphragm 
radius of 1.6 mm that provides a total capacitance change of 0.7627 pF for an applied 
pressure in the range of 0-170 kPa is discussed. The sensitivity of the pressure sensor is 
4.8 mV/kPa. The proposed pressure sensors are suitable for integration with system-on-
package (SOP) type Microsystems fabricated using low-cost MCM-L technologies. 
 
 84
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