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7.9 Interleaving Deliberation and Reactive Control
An evaluation of Hybrid architectures would not be complete without ask-
ing the question, “so what’s the difference between what Shakey could do (if
it had had infinite computation power) and what a Hybrid can do?” Speed of
execution aside, it is interesting to ponder whether the only fundamental dif-
ference is how Hierarchical and Hybrids achieve the same ends. Hybrids cer-
tainly explicitly reflect more of the principles of software engineering (mod-
ularity, coherence, design for reuse, etc.). The two paradigms also certainly
reflect different attitudes toward world modeling. In the Hybrid Paradigm,
global models are used only for symbolic functions. The frame problem ei-
ther doesn’t exist or is minor because 1) execution is reactive and therefore
well-suited for unstructured environments, and 2) software agents can use
agent-specific abstractions to exploit the structure of an environment in or-
der to fulfill their particular role in deliberation. Global models are generally
closed world, but the world is “closed” at the deliberative level. The robot
can think in terms of a closed world, while it acts in an open world. Another
major philosophical difference is the role of planning and execution. Under
STRIPS, Shakey planned every move, down to the lowest level of granu-
larity, and had problems confirming that an action had been accomplished.
It should be noted that modern planners often produce only partial plans,
then execute that part of the plan, note the results and plan the next step.
It is not clear whether a SENSE, PLAN, ACT paradigm with a more suit-
able planner would produce a more intuitively appealing robot architecture.
The final distinction between the Hierarchical and Hybrid Paradigms may
be the influence of biology. The Hybrid Paradigm has its roots in ethology,
and provides a framework for exploring cognitive science. The Hierarchical
Paradigm is less clearly cognitively plausible, although both share the same
cognitive motivation.

7.9 Interleaving Deliberation and Reactive Control

The primary contribution of a Hybrid architecture is to provide a template
for merging deliberation and reaction. However, interleaving the two func-
tions is often task dependent. In navigation, there is the issue of interleaving
path planning and reactive execution of the path. Another issue is monitor-
ing the progress of behaviors and terminating behaviors correctly.
One of the obvious task domains for a mobile robot is navigation, simply
getting around in the world. Many robot architectures have well-developed
modules for handling navigational tasks. In the early 1990’s, navigation