Page 166 - Introduction to AI Robotics

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4.7 Exercises
ior is emergent. More complex behaviors may be constructed from primitive
behaviors, or from mixing and matching perceptual and motor components.
This supports good software engineering practices, especially low coupling
and high cohesion.
The subsumption architecture is a popular reactive system. Behaviors are
purely reflexive and may not use memory. Behaviors are arranged in layers
of competence, where the lower levels encapsulate more general abilities.
The coordination of layers is done by higher layers, which have more specific
goal-directed behaviors, subsuming lower layers. Behaviors within a layer
are coordinated by finite state automata, and can be readily implemented in
hardware.
Potential fields methodologies are another popular reactive system. Be-
haviors in potential field systems must be implemented as potential fields.
All active behaviors contribute a vector; the vectors are summed to produce
a resultant direction and magnitude for travel. Pfields provide a continu-
ous representation, which is easier to visualize than rule encoding, and are
continuous. The fields can be readily implemented in software, and parame-
terized for flexibility and reuse. The vector summation effect formalizes how
to combine behaviors, eliminating issues in how to design behaviors for sub-
sumption. The fields are often extensible to three dimensions, adding to the
re-usability and portability. In the example in this chapter, behaviors using
potential fields were able to encapsulate several layers in subsumption into
a set of concurrent peer behaviors with no layers. Ch. 5 will give examples
of how to sequence, or assemble, behaviors into more abstract behaviors.
Despite the differences, subsumption and potential fields appear to be
largely equivalent in practice. Both provide support for modularity and
niche targetability. The ease of portability to other domains is relative to
the complexity of the changes in the task and environment. Neither style
of architecture explicitly addresses robustness, although in theory, if only a
higher layer of a subsumption system failed, the lower layers should ensure
robot survivability.

4.7 Exercises

Exercise 4.1
Define the reactive paradigm in terms of a.) the SENSE, PLAN, and ACT primitives,
and b.) sensing organization.