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4 The Reactive Paradigm
designer can create a library of behaviors and schemas to choose from, with
no implicit reliance on a lower layer.
Neither architecture presents systems which could be called genuinely ro-
bust. The layering of subsumption imparts some graceful degradation if an
upper level is destroyed, but it has no mechanisms to notice that a degrada-
tion has occurred. The finite state mechanisms in the docking behavior show
some resilience, but again, only for situations which can be anticipated and
incorporated into the state diagram. As with animals, a reactive robot will
also do something consistent with its perception of the world, but not always
the right thing.

4.6 Summary

Under the Reactive Paradigm, systems are composed of behaviors, which
tightly couple sensing and acting. The organization of the Reactive Paradigm
is SENSE-ACT or S-A,with no PLAN component. Sensing in the Reactive
Paradigm is local to each behavior, or behavior-specific. Each behavior has
direct access to one or more sensors independently of the other behaviors. A
behavior may create and use its own internal world representation, but there
is no global world model as with the Hierarchical Paradigm. As a result,
reactive systems are the fastest executing robotic systems possible.
There are four major characteristics of robots constructed under the Reac-
tive Paradigm. Behaviors serve as the basic building blocks for robot actions,
even though different designers may have different definitions of what a be-
havior entails. As a consequence of using behaviors, the overall behavior
of the robot is emergent. Only local, behavior-specific sensing is permitted.
The use of explicit representations in perceptual processing, even locally, is
avoided in most reactive systems. Explicit representations of the world are
often referred to as maintaining the state of the world internally, or internal
state. Instead, reactive behaviors rely on the world to maintain state (as ex-
emplified by the gripper controlling whether the robot was looking for soda
cans or for the recycling bin). Animal models are often cited as a basis for
a behavior or the architecture. Behaviors and groups of behaviors which
were inspired by or simulate animal behavior are often considered desir-
able and more interesting than hacks. Finally, reactive systems exhibit good
software engineering principles due to the “programming by behavior” ap-
proach. Reactive systems are inherently modular from a software design
perspective. Behaviors can be tested independently, since the overall behav-