Sensors and Methods for Mobile Robot Positioning

50 Part I Sensors for Mobile Robot Positioning
Figure 2.16: a. Ideal B-H curve.
b. Some minor hysteresis in the actual curve results in a residual non-zero
value of B when H is reduced to zero, known as the retentivity. (Adapted from
Halliday and Resnick, 1974; Carlson and Gisser, 1981).
capability of the core is substantially reduced.
Consider again the cylindrical rod sensor presented in Figure 2.17, now in the absence of any
external magnetic field B . When the drive coil is energized, there will be a strong coupling between
e
the drive coil and the sense coil. Obviously, this will be an undesirable situation since the output signal
is supposed to be related to the strength of the external field only.
One way around this problem is seen in the Vacquier configuration developed in the early 1940s,
where two parallel rods collectively form the core, with a common sense coil [Primdahl, 1979] as
illustrated in Figure 2.17. The two rods are simultaneously forced into and out of saturation, excited
in antiphase by identical but oppositely
wound solenoidal drive windings. In this
fashion, the magnetization fluxes of the two
drive windings effectively cancel each other,
with no net effect on the sense coil.
Bridges of magnetic material may be
employed to couple the ends of the two coils
together in a closed-loop fashion for more
complete flux linkage through the core. This
configuration is functionally very similar to
the ring-core design first employed in 1928
by Aschenbrenner and Goubau [Geyger,
1957]. An alternative technique for decoupling
the pickup coil from the drive coil is to
arrange the two in an orthogonal fashion. In
practice, there are a number of different
implementations of various types of sensor
cores and coil configurations as described by
Stuart [1972] and Primdahl [1979]. These
are generally divided into two classes, parallel
and orthogonal, depending on whether the
Sensitive
axis
S
D
Core
D
S
Sense
coil
Figure 2.17: Identical but oppositely wound drive
windings in the Vacquier configuration cancel the net
effect of drive coupling into the surrounding sense coil,
while still saturating the core material. (Adapted from
[Primdahl, 1979].)