Page 126 - Introduction to Autonomous Mobile Robots

Perception

Figure 4.12 111
Typical range image of a 2D laser range sensor with a rotating mirror. The length of the lines through
the measurement points indicate the uncertainties.

In figure 4.11 the schematic of a typical 360 degrees laser range sensor and two exam-
ples are shown. figure 4.12 shows a typical range image of a 360 degrees scan taken with
a laser range sensor.
As expected, the angular resolution of laser rangefinders far exceeds that of ultrasonic
sensors. The Sick laser scanner shown in Figure 4.11 achieves an angular resolution of
0.5 degree. Depth resolution is approximately 5 cm, over a range from 5 cm up to 20 m or
more, depending upon the brightness of the object being ranged. This device performs
twenty five 180 degrees scans per second but has no mirror nodding capability for the ver-
tical dimension.
As with ultrasonic ranging sensors, an important error mode involves coherent reflection
of the energy. With light, this will only occur when striking a highly polished surface. Prac-
tically, a mobile robot may encounter such surfaces in the form of a polished desktop, file
cabinet or, of course, a mirror. Unlike ultrasonic sensors, laser rangefinders cannot detect
the presence of optically transparent materials such as glass, and this can be a significant
obstacle in environments, for example, museums, where glass is commonly used.

4.1.6.2 Triangulation-based active ranging
Triangulation-based ranging sensors use geometric properties manifest in their measuring
strategy to establish distance readings to objects. The simplest class of triangulation-based