Where am I? Sensors and Methods for Mobile Robot Positioning

62 Part I Sensors for Mobile Robot Positioning
in conjunction with any magnetoelastic expansion or contraction of its coating. The output beam from
this fiber-optic cable is combined in a light coupler with the output beam from the uncoated reference
fiber and fed to a pair of photodetectors.
Constructive and destructive interferences caused by differences in path lengths associated with
the two fibers will cause the final output intensity as measured by the photodetectors to vary
accordingly. This variation is directly related to the change in path length of the coated fiber, which
in turn is a function of the magnetic field strength along the fiber axis. The prototype constructed by
Lenz [1990] at Honeywell Corporation measured 10×2.5 centimeters (4×1 in) and was able to detect
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fields ranging from 10 Gauss up to 10 Gauss.
Researchers at the Naval Research Laboratory (NRL) have developed a prototype magnetoelastic
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magnetometer capable of detecting a field as small as 6×10 Gauss [Brizzolara et al., 1989] using the
tunneling-tip approach. This new displacement sensing technology, invented in 1982 at IBM Zürich,
is based on the measurement of current generated by quantum mechanical tunneling of electrons
across a narrow gap (Figure 2.29). An analog feedback circuit compares the measured tunnel current
with a desired value and outputs a drive signal to suitably adjust the distance between the tunneling
electrodes with an electromechanical actuator [Kenny et al., 1991]. The instantaneous tunneling
current is directly proportional to the exponential of electrode displacement. The most common
actuators employed in this role are piezoelectric and electrostatic, the latter lending itself more readily
to silicon micro-machining techniques.
Tip
Cantilever
Surface
Figure 2.29: Scanning tunneling microscopy, invented at IBM Zürich in
1982, uses quantum mechanical tunneling of electrons across a barrier
to measure separation distance at the gap. (Courtesy of T. W. Kenny,
NASA JPL).
The active sense element in the NRL magnetometer is a 10 centimeter (4 in) metallic glass ribbon
made from METGLAS 2605S2, annealed in a transverse magnetic field to yield a high
magnetomechanical coupling [Brizzolara et al., 1989]. (METGLAS is an alloy of iron, boron, silicon,
and carbon, and is a registered trademark of Allied Chemical.) The magnetoelastic ribbon elongates
when exposed to an axial magnetic field, and the magnitude of this displacement is measured by a
tunneling transducer as illustrated in Figure 2.30.
An electrochemically etched gold tip is mounted on a tubular piezoelectric actuator and positioned
within about one nanometer of the free end of the METGLAS ribbon. The ribbon and tip are
electrically biased with respect to each other, establishing a tunneling current that is fed back to the
piezo actuator to maintain a constant gap separation. The degree of magnetically induced elongation
of the ribbon can thus be inferred from the driving voltage applied to the piezoelectric actuator. The
solenoidal coil shown in the diagram supplies a bias field of 0.85 oersted to shift the sensor into its
region of maximum sensitivity.