magnetic ordinance detection by christopher fenton
TRANSCRIPT
Magnetic Ordinance Detection
By Christopher Fenton
Goals
• Analyze feasibility of magnetic ordinance detection methods, specifically with IED detection in Iraq in mind
• If feasible, build working prototype
• Successfully detect something metallic
Different Approaches to Object Detection
• Traditional Metal Detectors
• Ground-Penetrating Radar
• Magnetic Detectors
Magnetic Detection Approaches
• Balanced-Loop– Detects change in B-field over time– Covers large areas
• Magnetometers– Measures absolute B-field– Covers small areas
Balanced Loops• First use of Magnetic “Indicator Loops” for
harbor defense in 1915 by British in WWI, adopted by U.S. in 1942 during WWII
• Can only detect moving magnetic disturbances
• Typically large and immobile (>1.6 km^2)
• Abandoned for harbor defense in favor of SONAR following WWII
Balanced Loops in Action
Old detector station in Nahant, MA
Magnetometers
• First invented in 1833 by Carl Gauss
• Can detect magnitude and direction of magnetic field
• Small and lightweight
• Still used for geological surveying and “Magnetic Anomaly Detectors”
Magnetometers in Action
Magnetometer Array used for UXO detection
MicroMag3 3-axis Magnetometer
Circuit model of sensor used in MicroMag3 (Sensor inductance changes with external B-field)
Approach: Magnetometer Array
• Sensors are small (~1”x1”), cheap ($50) and easy to handle – > Even small loops are several m^2
• Insensitive to scanning speed and tilt– > For loops, tilt and speed need to be precisely
monitored
• Arrays can be scaled to arbitrary width for wide-area scanning– > Magnetometers give point measurements, but can
be expanded to cover wide areas like loops do
The MAGNETube
MAGNETube Setup
• 3 x MicroMag3 3-axis SPI magnetometers– Sensors mounted 15” apart– Calibrated so Earth’s B-field = 1 = 0.48568G
• 2 x Picaxe 18X microcontrollers– Expandable through “daisy-chaining”
• 1 Laptop running “Listener” software and outputting to CSV file for analysis in Microsoft Excel®
Setup
A B C
How is the magnitude computed?
1. X, Y, and Z values for all 3 sensors are sent to laptop
2. Calibration offset is subtracted from each direction
3. Magnitude = √(X^2 + Y^2 + Z^2)
4. Magnitude is scaled from 150-200 range to approximately equal “1” in Earth’s B-field
5. Sensor: 1=.48568 Gauss in Los Angeles
Test 1: 80 lbs of Iron
Location: Erdem’s Apartment
Target: 80 lbs of iron weights in a plastic trashcan
Test 1: 80lbs of IronPeak Magnitude vs. Distance
-0.1
-0.05
0
0.05
0.1
0.15
0.2
14 20 26 36
Distance from ground (inches)
Dif
fere
nce f
rom
backg
rou
nd
A
B
C
Conclusion: Readily detectable if directly above pile, drops off quickly
Possibly due to misalignment of sensor during test
Test 2: 4” Brass Artillery Shell
Test 2: 4” Brass Artillery Shell
Magnitude vs. Distance*
0
0.2
0.4
0.6
0.8
1
1.2
1 3 5 7 9 11 13 15 17 19 21 23 25
Ticks
Sca
led
Mag
nit
ud
e
A
B
C
Magnitude vs. Distance
0
0.2
0.4
0.6
0.8
1
1.2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Magnitude
Dis
tan
ce A
B
C
Test 2: 12” above groundBackground: 12” above ground
Conclusion: Brass has no magnetic signature. Only bolts were detectable, and only then at close range.
Test 3: Neodymium Magnets (high sensitivity simulation)
Large 3”x6” Neodymium magnet
Test 3: N.D. Magnet
Peak Magnitude vs. Distance
-2
0
2
4
6
8
10
12
6 12 18 24 30 36
Distance (inches)
Dif
fere
nce
fro
m
Bac
kgro
un
d
A
B
C
Conclusion: Magnet is easily detectable at a reasonable range
Test 4: Attenuation in Water
Test 4: Submerged N.D. Magnet
Magnitude Vs. Water Depth @ 18"
0
0.5
1
1.5
2
0 5 10 15
Water Depth (inches)
Dif
fere
nce
fro
m
Bac
kgro
un
d
Conclusion: Water has no attenuation effect on magnetic field
Future Improvements
• Use faster microcontroller with on-board FPU (~3X improvement in sampling rate)
• Add wireless serial link for easier calibration and field-use
• Experiment with distortion detection vs. simple magnitude detection
• Use higher-sensitivity magnetometers and higher-density array
• Compare vs. traditional metal detector
Conclusion
• Undocumented hardware failure-modes can be extremely difficult to fix
• Magnetic detection appears to be a valid method (and is apparently in-use)
• A simple array can be constructed for less than $250
• With more time, the current design could be greatly improved