Page 195 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Chapter 12

can receive four or more signals, it can provide altitude as well. Some services are
even available that will download maps for the area in which a receiver is located.
The biggest disadvantage to GPS is the low resolution. A straight GPS receiver can
only provide a position fix to within about 15 meters (50 feet). In the past, this was
improved upon by placing a receiver in a known location and sending position cor-
rections to the mobile unit. This technique was called differential GPS. Today there
is a service called WAAS or Wide Area Augmentation System. A WAAS equipped
receiver automatically receives corrections for various areas from a geostationary
satellite. Even so, the accuracy of differential GPS or a WAAS equipped GPS is 1 to
3 meters. At this writing, WAAS is only available in North America.
The best accuracy of a WAAS GPS is not adequate to produce acceptable lane tracking
on a highway. For this reason, it is best used with other techniques that can mini-
mize the error locally.
Thus, the advantages of the GPS include its low cost, and its output of absolute posi-
tion. The biggest disadvantages are its accuracy and the fact that it will not work in
most indoor environments. In fact, overhead structures, tunnels, weather, and even
heavy overhead vegetation can interfere with GPS signals.

Guidelines for selecting and deploying navigation and
collision avoidance sensors
The preceding discussion of various sensor technologies has provided some insight
into the most effective use of navigation sensors. Following is a summary of some
useful guidelines:
1. Try to select sensors that can be used for multiple functions. The SR-3
Security robot, for example, uses sonar for collision avoidance, navigation,
camera direction, and even short-range intrusion detection.

2. Determine the sources of false data that the sensor will tend to generate in
the environments where the robot will be used, and develop a strategy ahead
of time to deal with it.

3. For battery-operated robots, power consumption is a prime concern. The
average power consumed by the drive motor(s) over time can easily be
exceeded by the power draw of sensors and computers. If this is allowed to
happen, the robot will exhibit a poorer availability and a smaller return on
investment to its user.

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