Sensors and Methods for Mobile Robot Positioning

58 Part I Sensors for Mobile Robot Positioning
Fe
Indium
arsenide
Fe
Fe Indium
arsenide
Figure 2.25: A pair of indium-arsenide-ferrite Hall-effect sensors (one
shown) are positioned between flux concentrating wings of mumetal in
this early Motorola prototype. (Adapted from [Wiley, 1964].)
Fe
sensed magnetic heading. Excellent response linearity was reported down to flux densities of 0.001
Gauss [Willey, 1962].
Maenaka et al. [1990] report on the development of a monolithic silicon magnetic compass at the
Toyohashi University of Technology in Japan, based on two orthogonal Hall-effect sensors. Their use
of the terminology “magnetic compass” is perhaps an unfortunate misnomer in that the prototype
device was tested with an external field of 1,000 Gauss. Contrast this with the strength of the earth’s
magnetic field, which varies from only about 0.1 Gauss at the equator to about 0.9 Gauss at the poles.
Silicon-based Hall-effect sensors have a lower sensitivity limit of around 10 Gauss [Lenz, 1990]. It
is likely the Toyohashi University device was intended for other than geomagnetic applications, such
as remote position sensing of rotating mechanical assemblies.
This prototype Hall-effect magnetometer is still of interest in that it represents a fully selfcontained
implementation of a two-axis magnetometer in integrated circuit form. Two vertical Hall
cells [Maenaka et al., 1987] are arranged at right angles (see Figure 2.25) on a 4.7 mm² chip, with
their respective outputs coupled to a companion signal processing IC of identical size. (Two separate
chips were fabricated for the prototype instead of a single integrated unit to enhance production
yield.) The sensor and signal processing ICs are interconnected (along with some external variable
resistors for calibration purposes) on a glass-epoxy printed circuit board.
The dedicated signal-processing circuitry converts the B-field components B x and B y measured by
the Hall sensors into an angle 2 by means of the analog operation [Maenaka et al., 1990]:
2 ' arctan B x
B y
(2.15)
where
2 = angle between B-field axis and sensor
B x = x-component of B-field
B y = y-component of B-field.
The analog output of the signal-processing IC is a DC voltage which varies linearly with vector
orientation of the ambient magnetic field in a plane parallel to the chip surface. Reported test results
show a fairly straight-line response (i.e., ± 2 percent full scale) for external field strengths ranging
from 8,000 Gauss down to 500 Gauss; below this level performance begins to degrade rapidly
[Maenaka et al., 1990]. A second analog output on the IC provides an indication of the absolute value
of field intensity.