Page 116 - Introduction to Autonomous Mobile Robots

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Perception
Rate gyros have the same basic arrangement as shown in figure 4.4 but with a slight
modification. The gimbals are restrained by a torsional spring with additional viscous
damping. This enables the sensor to measure angular speeds instead of absolute orientation.

Optical gyroscopes. Optical gyroscopes are a relatively new innovation. Commercial use
began in the early 1980s when they were first installed in aircraft. Optical gyroscopes are
angular speed sensors that use two monochromatic light beams, or lasers, emitted from the
same source, instead of moving, mechanical parts. They work on the principle that the
speed of light remains unchanged and, therefore, geometric change can cause light to take
a varying amount of time to reach its destination. One laser beam is sent traveling clockwise
through a fiber while the other travels counterclockwise. Because the laser traveling in the
direction of rotation has a slightly shorter path, it will have a higher frequency. The differ-
ence in frequency ∆f of the two beams is a proportional to the angular velocity Ω of the
cylinder. New solid-state optical gyroscopes based on the same principle are build using
microfabrication technology, thereby providing heading information with resolution and
bandwidth far beyond the needs of mobile robotic applications. Bandwidth, for instance,
can easily exceed 100 kHz while resolution can be smaller than 0.0001 degrees/hr.

4.1.5 Ground-based beacons
One elegant approach to solving the localization problem in mobile robotics is to use active
or passive beacons. Using the interaction of on-board sensors and the environmental bea-
cons, the robot can identify its position precisely. Although the general intuition is identical
to that of early human navigation beacons, such as stars, mountains, and lighthouses,
modern technology has enabled sensors to localize an outdoor robot with accuracies of
better than 5 cm within areas that are kilometers in size.
In the following section, we describe one such beacon system, the global positioning
system (GPS), which is extremely effective for outdoor ground-based and flying robots.
Indoor beacon systems have been generally less successful for a number of reasons. The
expense of environmental modification in an indoor setting is not amortized over an
extremely large useful area, as it is, for example, in the case of the GPS. Furthermore,
indoor environments offer significant challenges not seen outdoors, including multipath
and environmental dynamics. A laser-based indoor beacon system, for example, must dis-
ambiguate the one true laser signal from possibly tens of other powerful signals that have
reflected off of walls, smooth floors, and doors. Confounding this, humans and other obsta-
cles may be constantly changing the environment, for example, occluding the one true path
from the beacon to the robot. In commercial applications, such as manufacturing plants, the
environment can be carefully controlled to ensure success. In less structured indoor set-
tings, beacons have nonetheless been used, and the problems are mitigated by careful
beacon placement and the use of passive sensing modalities.