Where am I? Sensors and Methods for Mobile Robot Positioning

52 Part I Sensors for Mobile Robot Positioning
2 ' arctan V x
V y
.
(2.11)
Another popular two-axis core design is seen in the Flux Valve magnetometer developed by
Sperry Corp. [SPERRY] and shown in Figure 2.19. This three-legged spider configuration employs
three horizontal sense coils 120 degrees apart, with a common vertical excitation coil in the middle
[Hine, 1968]. Referring to Figure 2.20, the upper and lower “arms” of the sense coil S are excited
by the driving coil D, so that a magnetizing force H developed as indicated by the arrows. In the
x
absence of an external field H , the flux generated in the upper and lower arms by the excitation coil
e
is equal and opposite due to symmetry.
When this assembly is placed in an axial magnetic field H , however, the instantaneous excitation
e
field H complements the flow in one arm, while opposing the flow in the other. This condition is
x
periodically reversed in the arms, of course, due to the alternating nature of the driving function. A
second-harmonic output is induced in the sensing coil S, proportional to the strength and orientation
of the ambient field. By observing the relationships between the magnitudes of the output signals from
each of the three sense coils (see Figure 2.20), the angular relationship of the Flux Valve with respect
to the external field can be unambiguously determined.
a
b
Figure 2.20: The Flux Valve magnetometer developed by Sperry Corporation uses a
spider-core configuration. (Adapted from [Lenz, 1990].)
When maintained in a level attitude, the fluxgate compass will measure the horizontal component
of the earth’s magnetic field, with the decided advantages of low power consumption, no moving
parts, intolerance to shock ad vibration, rapid start-up, and relatively low cost. If the vehicle is
expected to operate over uneven terrain, the sensor coil should be gimbal-mounted and mechanically
dampened to prevent serious errors introduced by the vertical component of the geomagnetic field.
2.4.2.1 Zemco Fluxgate Compasses
The Zemco fluxgate compass [ZEMCO] was used in earlier work by Everett et al. [1990] on their
robot called ROBART II. The sensor was a fluxgate compass manufactured by Zemco Electronics,
San Ramon, CA, model number DE-700. This very low-cost (around $40) unit featured a rotating
analog dial and was originally intended for 12 VDC operation in automobiles.