DJ GOUWS and EF VAN DER MERWE

1
THE IMPLEMENTATION OF A MAGNETIC SENSOR ON A
DYNAMIC PLATFORM
DJ GOUWS and EF VAN DER MERWE HERMANUS MAGNETIC
OBSERVATORY
2
PROPERTIES OF THE MAGNETIC FIELD

• Vector field, dependent on lat, long, alt and
  time
• Mathematically well modelled
• Relatively weak field
• It is free information
• Cannot be switched off like other manmade
• navigation aids

3
GPS DIRECTION vs MAGNETIC HEADING

• GPS direction indicates ground track angle and
• not heading
• GPS direction indicates where you were
• Time delay on velocity calculation
• Availability is not guaranteed
• Can be jammed or corrupted
• Loses lock in certain attitudes
• Must be moving

4
MAGNETOMETERS TYPES AND RANGES
Magnetic Field Strength (nT)
105
10-5
101
103
10-1
107
109
10-3
Magnetometer sensors
Search Coil (AC fields only !)
Flux-gate
Alkali Vapor
Nuclear Precession
SQUID
Hall Effect
MRS
Earths Field
5
DYNAMIC PLATFORM WITH MAGNETIC SENSOR
Xm ? Ym ? Zm ? Volt or nT
Geomagnetic field
Sources of Magnetic fields
Strapped-down configuration
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SOURCES OF MAGNETISM IN TYPICAL PLATFORMS

• Large number of magnetic sources
• Interested in the effect of the sources only at
  the
• sensor position
• Magnetic flux density ? 1/rn
• r distance between source and magnetometer
• n coefficient between 2 and 3

7
SOURCES OF MAGNETISM IN TYPICAL PLATFORMS

• Permanent (Hard Iron) magnetism
• Induced (Soft Iron) magnetism
• Magnetism of currents in electrical looms

8
PERMANENT MAGNETISM (HARD IRON MAGNETISM)
Approximation
Magnetic domains aligned in the same direction
9
FACTORS WHICH MAY AFFECT THE CONDITION OF
PERMANENT MAGNETISM

• Prolonged exposure to vibrations
• Prolonged exposure to heat
• Exposure to opposing magnetic fields
• Corrosion (oxidation) of magnetic material
• Lightning strikes on the platform

10
PERMANENT MAGNETISM COMPONENTS
RAF Manual, AP34556, Volume D
11
INDUCED MAGNETISM (SOFT IRON MAGNETISM)
Approximation
Magnetic domains aligned
Magnetic domains not aligned
12
SOFT IRON FIELD COMPONENTS

• Inducing field
• Permanent magnetism in platform
• Geomagnetic field

RAF Manual, AP34556, Volume D
13
EFFECT OF MAGNETIC PLATFORM ON MAGNETOMETER
OUTPUTS
Xm Xe P aXe bYe cZe Ym Ye Q
dXe eYe fZe Zm Ze R gXe hYe iZe
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EXAMPLE ROTATION OF A MAGNETOMETER IN THE
GEOMAGNETIC FIELD
Assumption 2 D (Horizontal) case
Xm Xe P aXe bYe Ym Ye Q dXe
eYe
15
ROTATION OF A MAGNETOMETER IN THE GEOMAGNETIC
FIELD
Magnetic North
X
Y
H 10 000 nT (0.1 Gauss)
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PERFECT MAGNETOMETER OUTPUTS IF ROTATED IN
GEOMAGNETIC FIELD
X axis Cosine graph
H 10 000 nT
Y axis Sine graph
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PERFECT MAGNETOMETER OUTPUTS IF ROTATED IN
GEOMAGNETIC FIELD
Plot X-axis vs Y-axis
H 10 000 nT
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DYNAMIC PLATFORM WITH MAGNETIC SENSOR
Strapped-down configuration
Xm ? Ym ?
Geomagnetic field
H 10 000 nT Permanent Magnetism P 2 000 nT Q
3 000 nT Induced magnetism a 0.9 b
0.8 d 0.7 e 0.6
Sources of Magnetic fields
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INFLUENCE OF PERMANENT AND INDUCED MAGNETISM ON
MAGNETOMETER OUTPUTS (THEORETICAL VALUES)
H 10 000 nT P 2 000 nT Q 3 000 nT a
0.9 b 0.8 d 0.7 e 0.6
20
INFLUENCE OF PERMANENT AND INDUCED MAGNETISM ON
MAGNETOMETER OUTPUTS (THEORETICAL VALUES)
H 10 000 nT P 2 000 nT Q 3 000 nT a
0.9 b 0.8 d 0.7 e 0.6
21
HEADING ERROR DUE TO PERMANENT AND INDUCED
MAGNETISM (THEORETICAL VALUES)
H 10 000 nT P 2 000 nT Q 3 000 nT a
0.9 b 0.8 d 0.7 e 0.6
22
IMPLEMENT CALIBRATION ROUTINE TO CORRECT FOR
EFFECTS OF PERMANENT AND INDUCED MAGNETISM
23
IMPLEMENT CALIBRATION ROUTINE TO CORRECT FOR
EFFECTS OF PERMANENT AND INDUCED MAGNETISM
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CORRECTION FOR PERMANENT AND INDUCED MAGNETISM
(THEORETICAL VALUES)
H 10 000 nT P 2 000 nT Q 3 000 nT a
0.9 b 0.8 d 0.7 e 0.6
25
CORRECTION FOR PERMANENT AND INDUCED MAGNETISM
(THEORETICAL VALUES)
H 10 000 nT P 2 000 nT Q 3 000 nT a
0.9 b 0.8 d 0.7 e 0.6
26
HEADING CORRECTION FOR PERMANENT AND INDUCED
MAGNETISM (THEORETICAL VALUES)
H 10 000 nT P 2 000 nT Q 3 000 nT a
0.9 b 0.8 d 0.7 e 0.6 Residual
error 0.8 degptp
27
THIN ROD OF INDUCED MAGNETISM FOUND DURING
MAGNETIC EVALUATIONS
Induced magnetism coefficients a most
significant b negligible c negligible d
negligible e negligible f negligible g
negligible h negligible i negligible
28
THIN RODS OF INDUCED MAGNETISM FOUND DURING
MAGNETIC EVALUATIONS
Coefficients a most significant b
negligible c negligible d less
significant e negligible f negligible g
negligible h negligible i negligible
29
INFLUENCE OF PERMANENT AND INDUCED MAGNETISM ON
MAGNETOMETER OUTPUTS (PRACTICAL VALUES)
H 10 000 nT P 2 000 nT Q 3 000 nT a
0.0500 b 0.0001 d 0.0001 e 0.0100
30
HEADING CORRECTED FOR PRESENCE OF PERMANENT AND
INDUCED MAGNETISM (PRACTICAL VALUES)
H 10 000 nT Xp 2 000 nT Yp 3 000 nT a
0.0500 b 0.0001 d 0.0001 e
0.0100 Residual error 0.012 degptp
31
CALIBRATION METHODS AVAILABLE

• External magnetic reference angle in 2D or 3-D
  
• (classical compass swing procedure)
• Attitude angles and geomagnetic field vector
  3-D
• Static calibration using calibrated coils
  system
• (e.g. Helmholtz)
• In - flight dynamic auto-calibration routines

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SUCCESFULL MAGNETOMETER IMPLEMENTATION

• Manned aircraft
• Unmanned autonomous vehicles (UAVs)
• Auto-calibration magnetic compasses
• Rocket-propelled bomb systems (rotation
  sensor)
• Magnetic proximity fuzes

33
THREE AXES HELMHOLTZ COIL CALIBRATION SYSTEM
Determination of the permanent magnetism of
weapon systems
34
SKUA COMPASS SWING PROCEDURE USING KNOWN
ATTITUDES AND MAGNETIC REFERENCE ANGLE
35
ROCKET-PROPELLED BOMB SYSTEMS USING KNOWN
ATTITUDES AND GEOMAGNETIC FIELD VECTOR
Determination of the magnetic shielding and
permanent magnetism of bomb system
36
REVOLUTION COUNTER (SEARCH COIL) FOR ARTILLERY
MUNITION
UKRAINIAN LEMI 115 LABORATORY PROTOTYPE
Dynamic range 200 1200 Hz
(12000 72000 rpm) Threshold in full
range 80 140 nTptp Dead cone angle at OTB
Test Range No screening 0.3º 5x screening
1.5º
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NO MAGIC INVOLVED WITH CALIBRATION PROCEDURES
TO COMPENSATE FOR THE MAGNETIC PROPERTIES OF
PLATFORMS