Transcript Magnetic Ordinance Detection

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
Circuit model of sensor used in MicroMag3
(Sensor inductance changes with external
B-field)
MicroMag3 3-axis Magnetometer
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 Iron
Peak Magnitude vs. Distance
Difference from
background
0.2
0.15
0.1
A
0.05
B
0
C
-0.05
14
20
26
36
-0.1
Distance from ground (inches)
Possibly due
to
misalignment
of sensor
during test
Conclusion: Readily detectable if directly above pile, drops off quickly
Test 2: 4” Brass Artillery Shell
Test 2: 4” Brass Artillery Shell
Background: 12” above ground
Test 2: 12” above ground
Magnitude vs. Distance*
1.2
1.2
1
1
0.8
A
0.6
B
0.4
C
0.2
0
Scaled Magnitude
Distance
Magnitude vs. Distance
0.8
A
0.6
B
0.4
C
0.2
0
1
2
3
4
5
6 7
8
9 10 11 12 13 14 15 16 17
Magnitude
1
3
5
7
9
11
13
15
17
19
21
23
25
Ticks
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
12
Difference from
Background
10
8
A
6
B
4
C
2
0
-2
6
12
18
24
30
36
Distance (inches)
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"
Difference from
Background
2
1.5
1
0.5
0
0
5
10
15
Water Depth (inches)
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