Page 190 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Sensors, Navigation Agents and Arbitration

Figure 12.3 shows two cases of normal reflection, one from a flat surface and one from a
convex object. Also shown is a case of retroreflection from a corner. Retroreflection can
occur for a simple corner (two surfaces) or a corner-cube. In the case of a simple
corner such as in Figure 12.3, the beam will only be returned if the incidence angle
in the vertical plane is approximately 90 degrees.

Unless the wall surface contains irregularities larger than one-half the wavelength of
the sonar frequency (typically 1 to 3 cm), sensors 2, 3, and 5 will “go specular” and
not return ranges from it. Interestingly, beam 2 returns a phantom image of the post
due to multiple reflections.

Wall Corner

Sonar
Wall
Ring

Phantom
Post of post

Figure 12.4. Sonar “map” as returned by narrow-beam sonar ring

Figure 12.4 approximates the sonar data returned from our hypothetical environment in
Figure 12.3. If the data returned by lidar (Figure 10.2) seemed sparse, then the data from
sonar would appear to be practically nonexistent.

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