Where am I? Sensors and Methods for Mobile Robot Positioning

Chapter 2: Heading Sensors 51
excitation H-field is parallel or perpendicular
to the external B-field being measured. Alternative
excitation strategies (sine wave,
square wave, sawtooth ramp) also contribute
to the variety of implementations seen in the
literature. Hine [1968] outlines four different
classifications of saturable inductor magnetometers
based on the method of readout
(i.e., how the output EMF is isolated for
evaluation):
Drive
winding
Sensing
winding 1
Toroidal
core
Sensing
winding 2
C
C
C
C
Fundamental frequency.
Second harmonic.
Peak output.
Pulse difference.
Drive
winding
Figure 2.18: Two channel ring core fluxgate with
toroidal excitation. (Adapted from [Acuna and Pellerin,
1969].)
Unambiguous 360-degree resolution of
the earth’s geomagnetic field requires two
sensing coils at right angles to each other. The ring-core geometry lends itself to such dual-axis
applications in that two orthogonal pickup coils can be configured in a symmetrical fashion around
a common core. A follow-up version developed by Gordon and Lundsten [1970] employed a toroidal
excitation winding as shown in Figure 2.19. Since there are no distinct poles in a closed-ring design,
demagnetization effects, although still present [Stuart, 1972], are less severe. The use of a ring
geometry also leads to more complete flux linkage throughout the core, implying less required drive
excitation for lower power operation, and the zero offset can be minimized by rotating the circular
core. For these reasons, along with ease of manufacture, toroidal ring-core sensors are commonly
employed in many of the low-cost fluxgate compasses available today.
The integrated DC output voltages V and V of the orthogonal sensing coils vary as sine and
x y
cosine functions of 2, where 2 is the angle of the sensor unit relative to the earth’s magnetic field.
The instantaneous value of 2 can be easily derived by performing two successive A/D conversions
on these voltages and taking the arctangent of their quotient:
S
120
o
H e
S
Primary
Hx sin wt
P
H e
S
Figure 2.19: The Sperry Flux Valve consisted of a common drive winding P in the center of
three sense windings S symmetrically arranged 120 E apart. (Adapted from [Hine, 1968].)