Page 141 - Introduction to Autonomous Mobile Robots

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Figure 4.21 Chapter 4
The Cheshm robot uses three monochrome cameras as its only ranging sensor for obstacle avoidance
in the context of humans, static obstacles such as bushes, and convex obstacles such as ledges and
steps.

A significant advantage of the horizontal sum of differences technique [equation (4.21)]
is that the calculation can be implemented in analog circuitry using just a rectifier, a low-
pass filter, and a high-pass filter. This is a common approach in commercial cameras and
video recorders. Such systems will be sensitive to contrast along one particular axis,
although in practical terms this is rarely an issue.
However depth from focus is an active search method and will be slow because it takes
time to change the focusing parameters of the camera, using, for example, a servo-con-
trolled focusing ring. For this reason this method has not been applied to mobile robots.
A variation of the depth from focus technique has been applied to a mobile robot, dem-
onstrating obstacle avoidance in a variety of environments, as well as avoidance of concave
obstacles such as steps and ledges [117]. This robot uses three monochrome cameras placed
as close together as possible with different, fixed lens focus positions (figure 4.21).
Several times each second, all three frame-synchronized cameras simultaneously cap-
ture three images of the same scene. The images are each divided into five columns and
three rows, or fifteen subregions. The approximate sharpness of each region is computed
using a variation of equation (4.22), leading to a total of forty-five sharpness values. Note
that equation (4.22) calculates sharpness along diagonals but skips one row. This is due to
a subtle but important issue. Many cameras produce images in interlaced mode. This means