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Executive Chapter 6

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

Robot Hardware

Figure 6.24
A two-tiered architecture for off-line planning.

this architecture that contains all tactical decision-making as well as frequent updates of the
robot’s short-term memory, as is the case for localization and mapping.
It is interesting to note the similarity between this general architecture, used in many
specialized forms in mobile robotics today, and the architecture implemented by Shakey,
one of the very first mobile robots, in 1969 [115]. Shakey had LLA (low-level actions) that
formed the lowest architectural tier. The implementation of each LLA included the use of
sensor values in a tight loop just as in today’s behaviors. Above that, the middle architec-
tural tier included the ILA (intermediate-level actions), which would activate and deactivate
LLA as required based on perceptual feedback during execution. Finally, the topmost tier
for Shakey was STRIPS (Stanford Research Institute Planning System), which provided
global look ahead and planning, delivering a series of tasks to the intermediate executive
layer for execution.
Although the general architecture shown in figure 6.23 is useful as a model for robot
navigation, variant implementations in the robotics community can be quite different.
Below, we present three particular versions of the general tiered architecture, describing for
each version at least one real-world mobile robot implementation. For broader discussions
of various robot architectures, see [26].

6.3.4.1 Off-line planning
Certainly the simplest possible integration of planning and execution is no integration at
all. Consider figure 6.24, in which there are only two software tiers. In such navigation
architectures, the executive does not have a planner at its disposal, but must contain a priori
all relevant schemes for traveling to desired destinations.
The strategy of leaving out a planner altogether is of course extremely limiting. Moving
such a robot to a new environment demands a new instantiation of the navigation system,