Page 194 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Sensors, Navigation Agents and Arbitration

The disadvantages of lidar systems are their cost, significant power consumption
(typically over 10 watts), planar detection patterns, and mechanical complexity.
Early lidar systems were prone to damage from shock and vibration; however, more
recent designs like the Sick LMS appear to have largely overcome these issues. Early
designs also tended to use ranging techniques that yielded poor range-resolution at
longer ranges. This limitation is also far less prevalent in newer designs.

Radar imaging
Radar imaging systems based on synthetic beam steering have been developed in re-
cent years. These systems have the advantage of extremely fast refresh of data over a
significant volume. They are also less susceptible to some of the weather conditions
that affect lidar, but exhibit lower resolution. Radar imaging also exhibits specular
reflections as well as insensitivity to certain materials. Radar can be useful for both
collision avoidance and feature navigation, but not as the sole sensor for either.

Video
Video processing has made enormous strides in recent years. Because of the popular-
ity of digital video as a component of personal computers, the cost of video digitizers
and cameras has been reduced enormously.
Video systems have the advantages of low cost and modest power consumption. They
also have the advantage of doubling as remote viewers. The biggest disadvantages of
video systems are that the environment must be illuminated and they offer no inher-
ent range data. Some range data can be extrapolated by triangulation using stereoscopic
or triscopic images, or by techniques based on a priori knowledge of features.
The computational overhead for deriving range data from a 2D camera image de-
pends on the features to be detected. Certain surfaces, like flat mono-colored walls
can only be mapped for range if illuminated by a structured light. On the other
hand, road following has been demonstrated from single cameras.

GPS

The Global Positioning System (GPS) is a network of 24 satellites that orbit the
earth twice a day. With a simple and inexpensive receiver, an outdoor robot can be
provided with an enormously powerful navigation tool. A GPS receiver provides
continuous position estimates based on triangulation of the signals it is able to receive. If
it can receive three or more signals, the receiver can provide latitude and longitude. If it

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