Page 196 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Sensors, Navigation Agents and Arbitration

4. Deploy sensors to face in the directions where useful data is richest. Gener-
ally, this is from the sides forward.
5. Allow the maximum flexibility possible over the data acquisition process and
signal-processing process. Avoid fixed thresholds and filter parameters.

6. Never discard useful information. For example, if the data from a sonar
transducer is turned into a range reading in hardware, it will be difficult or
impossible to apply additional data filters or adaptive behaviors later.

7. Consider how sensor systems will interact with those of other robots.
8. Sensor data processing should include fail-safe tests that assure that the robot
will not rely upon the system if it is not operating properly.

9. Read sensor specifications very carefully before committing to one. A sensor’s
manufacturer may state it has resolution up to x and ranges out to y with
update rates as fast as z, but it may not be possible to have any two of these at
the same time.
Since Polaroid transducers have been the most popular sonar sensors for mobile ro-
bots, it is instructive to look at some of the configurations into which they have been
deployed. Figure 12.6 shows a configuration that resulted from the recognition of the
importance of focusing on the forward-direction. This configuration has sacrificed
the rear-looking elements of the ring configuration in favor of a second-level oriented
forward.
While this configuration provides better coverage in a collision avoidance role, it
still suffers from gaps in coverage. The most glaring gap is that between the bottom
beam and ground level. There is also a gap between the beams as seen in the top view.
Since the beams are conical in shape, the gaps shown between beams is only repre-
sentative of coverage at the level of the center of the transducers. The gap is worse
at every other level.

While it would appear that the gaps in the top view are minimal, it should be remem-
bered that a smooth surface like a wall will only show up at its perpendicularity to
the beams. In the patterns of Figure 12.6, there are 12 opportunities for a wall to be
within the range of the transducers and not be seen. For this reason, when two half-
rings are used, the lower set of transducers is usually staggered in azimuth by one-half
the beam-width angle of the devices.
Notice that this configuration has 26 transducers to fire and read. If the range of the
transducers was set to 3 meters (10 feet), then the round-trip ping-time for each

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