Page 128 - Introduction to Autonomous Mobile Robots

Perception

Figure 4.14 113
A commercially available, low-cost optical triangulation sensor: the Sharp GP series infrared
rangefinders provide either analog or digital distance measures and cost only about $ 15.

operates over a distance range of between 8 and 80 cm. It is inexpensive compared to ultra-
sonic and laser rangefinder sensors. Although more limited in range than sonar, the optical
triangulation sensor has high bandwidth and does not suffer from cross-sensitivities that are
more common in the sound domain.

Structured light (2D sensor). If one replaces the linear camera or PSD of an optical tri-
angulation sensor with a 2D receiver such as a CCD or CMOS camera, then one can recover
distance to a large set of points instead of to only one point. The emitter must project a
known pattern, or structured light, onto the environment. Many systems exist which either
project light textures (figure 4.15b) or emit collimated light (possibly laser) by means of a
rotating mirror. Yet another popular alternative is to project a laser stripe (figure 4.15a) by
turning a laser beam into a plane using a prism. Regardless of how it is created, the pro-
jected light has a known structure, and therefore the image taken by the CCD or CMOS
receiver can be filtered to identify the pattern’s reflection.
Note that the problem of recovering depth is in this case far simpler than the problem of
passive image analysis. In passive image analysis, as we discuss later, existing features in
the environment must be used to perform correlation, while the present method projects a
known pattern upon the environment and thereby avoids the standard correlation problem
altogether. Furthermore, the structured light sensor is an active device so it will continue to
work in dark environments as well as environments in which the objects are featureless
(e.g., uniformly colored and edgeless). In contrast, stereo vision would fail in such texture-
free circumstances.
Figure 4.15c shows a 1D active triangulation geometry. We can examine the trade-off
in the design of triangulation systems by examining the geometry in figure 4.15c. The mea-