Sensors and Methods for Mobile Robot Positioning

60 Part I Sensors for Mobile Robot Positioning
V o
I
Magnetizing current
Time
Offset
H y
M
Magnetization
Time
a. b.
V
Sensor signal
Figure 2.26: External current pulses set and reset the direction of magnetization,
resulting in the “flipped” response characteristics shown by the dashed line. Note
the DC offset of the device remains constant, while the signal output is inverted.
(Adapted from [Petersen, 1989].)
Offset
Time
synchronous detector driven by the same square-wave source. The rectified DC voltages V H1 and VH2
are thus proportional to the measured magnetic field components H 1 and H 2 . The applied field
direction is dependant on the ratio of V to H, not their absolute values. This means that as long as the
two channels are calibrated to the same sensitivity, no temperature correction is required [Fraden,
1993].
2.4.5 Magnetoelastic Compasses
A number of researchers have recently investigated the use of magnetoelastic (also known as
magnetostrictive) materials as sensing elements for high-resolution magnetometers. The principle of
operation is based on the changes in Young’s modulus experienced by magnetic alloys when exposed
to an external magnetic field. The modulus of elasticity E of a given material is basically a measure
of its stiffness, and directly relates stress to strain as follows:
E ' F ,
(2.16)
where
E = Young’s modulus of elasticity
F = applied stress
, = resulting strain.
Any ferromagnetic material will experience some finite amount of strain (expansion or shrinkage)
in the direction of magnetization due to this magnetostriction phenomenon. It stands to reason that
if the applied stress F remains the same, strain , will vary inversely with any change in Young’s
modulus E. In certain amorphous metallic alloys, this effect is very pronounced.
Barrett et al. [1973] proposed a qualitative explanation, wherein individual atoms in the crystal
lattice are treated as tiny magnetic dipoles. The forces exerted by these dipoles on one another depend
upon their mutual orientation within the lattice; if the dipoles are aligned end to end, the opposite