Page 318 - Introduction to Autonomous Mobile Robots

Planning and Navigation

Global Global Executive 303
knowledge, map

Real-time controller
behavior 1 behavior 2 behavior 3
PID motion control

Robot Hardware

Figure 6.26
An integrated planning and execution architecture in which planning is nothing more than a real-time
execution step (behavior).

robot is not kinematically symmetric, and so servoing through a particular obstacle course
may be easier in one direction than the other.
Pygmalion’s environment representation consists of a continuous geometric model as
well as an abstract topological network for route planning. Thus, if repeated attempts to
clear the obstacle fail, then the robot’s executive will temporarily cut the topological con-
nection between the two appropriate nodes and will launch the planner again, generating a
new set of waypoints to the goal. Next, using recent laser rangefinding data as a type of
local map (see figure 6.25), a geometric path planner will generate a path from the robot’s
current position to the next waypoint.
In summary, episodic planning architectures are extremely popular in the mobile robot
research community. They combine the versatility of responding to environmental changes
and new goals with the fast response of a tactical executive tier and behaviors that control
real-time robot motion. As shown in figure 6.25, it is common in such systems to have both
a short-term local map and a more strategic global map. Part of the executive’s job in such
dual representations is to decide when and if new information integrated into the local map
is sufficiently nontransient to be copied into the global knowledge base.

6.3.4.3 Integrated planning and execution
Of course, the architecture of a commercial mobile robot must include more functionality
than just navigation. But limiting this discussion to the question of navigation architectures
leads to what may at first seem a degenerate solution.
The architecture shown in figure 6.26 may look similar to the off-line planning architec-
ture of figure 6.24, but in fact it is significantly more advanced. In this case, the planner tier
has disappeared because there is no longer a temporal decomposition between the executive