Page 114 - Introduction to Autonomous Mobile Robots

Perception

Figure 4.3 99
Digital compass: Sensors such as the digital/analog Hall effect sensor shown, available from Dins-
more (http://dinsmoregroup.com/dico), enable inexpensive (< $ 15) sensing of magnetic fields.

tain two such semiconductors at right angles, providing two axes of magnetic field (thresh-
olded) direction, thereby yielding one of eight possible compass directions. The
instruments are inexpensive but also suffer from a range of disadvantages. Resolution of a
digital Hall effect compass is poor. Internal sources of error include the nonlinearity of the
basic sensor and systematic bias errors at the semiconductor level. The resulting circuitry
must perform significant filtering, and this lowers the bandwidth of Hall effect compasses
to values that are slow in mobile robot terms. For example, the Hall effect compass pictured
in figure 4.3 needs 2.5 seconds to settle after a 90 degree spin.
The flux gate compass operates on a different principle. Two small coils are wound on
ferrite cores and are fixed perpendicular to one another. When alternating current is acti-
vated in both coils, the magnetic field causes shifts in the phase depending on its relative
alignment with each coil. By measuring both phase shifts, the direction of the magnetic
field in two dimensions can be computed. The flux gate compass can accurately measure
the strength of a magnetic field and has improved resolution and accuracy; however, it is
both larger and more expensive than a Hall effect compass.
Regardless of the type of compass used, a major drawback concerning the use of the
Earth’s magnetic field for mobile robot applications involves disturbance of that magnetic
field by other magnetic objects and man-made structures, as well as the bandwidth limita-
tions of electronic compasses and their susceptibility to vibration. Particularly in indoor
environments, mobile robotics applications have often avoided the use of compasses,
although a compass can conceivably provide useful local orientation information indoors,
even in the presence of steel structures.

4.1.4.2 Gyroscopes
Gyroscopes are heading sensors which preserve their orientation in relation to a fixed ref-
erence frame. Thus they provide an absolute measure for the heading of a mobile system.