Page 199 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Chapter 12

Several commercial robots have achieved reasonable results with narrow-beam trans-
ducers by abandoning the ring configuration altogether. Figure 12.8 is representative
of these configurations, which concentrated the coverage in the forward path of the
robot. This concentration makes sense since this is the zone from which most obstacles
will be approached.

The weakness of the forward concentration pattern is that a robot can maneuver in
such a way as to approach an obstacle without seeing it. The solid arrow in Figure
12.8 indicates one type of approach, while the dashed arrow shows the correspond-
ing apparent approach of the obstacle from the robot’s perspective.
To prevent this scenario, the robot must be restricted from making arcing move-
ments or other sensors can be deployed to cover these zones. The commercial robots
that have used this approach have typically added additional sensors to avoid the
problem.
At Cybermotion, we developed our strategy in much the same way others did. We
took our best guess at a good configuration and then let it teach us what we needed
to change or add. Unlike most of the industry, we decided to use wide-beam Piezo-
electric transducers, and to use extensive signal processing. We never had reason to
regret this decision.
All sonar transducers have a minimum range they can measure. While this minimum
range is usually less than .15 meters (half a foot) for electrostatic transducers, it is
typically as much as .3 meters (1 foot) for Piezo-electric transducers. In order for the
SR-3 to navigate from walls close to the sides of the robot, the side transducers were
recessed in slots in the sides of the robot so that it could measure distances as low as
.15 meters.

Early configurations had two forward transducers and two side transducers only. The
forward transducers were used for collision avoidance and to correct longitudinal posi-
tion. The beam patterns of these transducers were set to be cross-eyed in order to
provide double coverage in front of the robot. Because most objects in front of the
robot can be detected by both beams, the robot can triangulate on objects. In fact,
the SR-3 uses this technique to find and precisely mate with its charging plug.

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