Page 145 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Chapter 9

The first law is due to the fact that navigation is never an exact science, and the readings
from sensors will always contain some error. Furthermore, the robot may experience
stretches on which it has no active navigation and is depending entirely on odometry.

The second law is due to the fact that it is impossible to move a mass to an exact
1
position . We must accept the fact that our performance will always contain some
error, and set our goal for “good enough.”

Path planning and execution

There are two elements to the path-planning process itself. First, we must know the
path we wish to travel, and secondly, we must have a strategy for getting onto and
tracking that path. In an indoor environment, the actual paths can be composed of
simple straight lines joined by curves. For outdoor road-following robots, on the
other hand, paths will tend to be sequences of points that represent undulating
curves.

Convergence behaviors

To understand the subtleties of path following, let’s consider the simplest possible
case, a straight path to a destination. For discussion purposes, we will call the ends of
path segments nodes.

B B B

Obstacle Obstacle Obstacle

A A A

Strategy 1 Strategy 2 Strategy 3

Figure 9.1. Three strategies for getting to a destination

1 See Chapter 5, Closed Loop Controls, Rabbits and Hounds.

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