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4 The Reactive Paradigm
The major theme of this chapter is that all reactive systems are composed
of behaviors, though the meaning of a behavior may be slightly different in
each reactive architecture. Behaviors can execute concurrently and/or se-
quentially. The two representative architectures, subsumption and potential
fields, are compared and contrasted using the same task as an example. This
chapter will concentrate on how architecture handles concurrent behaviors
to produce an emergent behavior, deferring sequencing to the next chapter.

4.2 Attributes of Reactive Paradigm

The fundamental attribute of the reactive paradigm is that all actions are
BEHAVIORS accomplished through behaviors. As in ethological systems, behaviors are a
direct mapping of sensory inputs to a pattern of motor actions that are then used
to achieve a task. From a mathematical perspective, behaviors are simply a
transfer function, transforming sensory inputs into actuator commands. For
the purposes of this book, a behavior will be treated as a schema, and will
consist of at least one motor schema and one perceptual schema. The mo-
tor schema contains the algorithm for generating the pattern of action in a
physical actuator and the perceptual schema contains the algorithm for ex-
tracting the percept and its strength. Keep in mind that few reactive robot
architectures describe their behaviors in terms of schemas. But in practice,
most behavioral implementations have recognizable motor and perceptual
routines, even though they are rarely referred to as schemas.
The Reactive Paradigm literally threw away the PLAN component of the
SENSE, PLAN, ACT triad, as shown in Fig. 4.3. The SENSE and ACT com-
ponents are tightly coupled into behaviors, and all robotic activities emerge
as the result of these behaviors operating either in sequence or concurrently.
SENSE-ACT The S-A organization does not specify how the behaviors are coordinated
ORGANIZATION and controlled; this is an important topic addressed by architectures.
BEHAVIOR-SPECIFIC Sensing in the Reactive Paradigm is local to each behavior, or behavior-
(LOCAL) SENSING specific. Each behavior has its own dedicated sensing. In many cases, this is
implemented as one sensor and perceptual schema per behavior. But in other
cases, more than one behavior can take the same output from a sensor and
process it differently (via the behavior’s perceptual schema). One behavior
literally does not know what another behavior is doing or perceiving. Fig. 4.4
graphically shows the sensing style of the Reactive Paradigm.
Note that this is fundamentally opposite of the global world model used
in the hierarchical paradigm. Sensing is immediately available to the be-