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4 The Reactive Paradigm

Figure 4.5 “Veteran” robots of the MIT AI Laboratory using the subsumption archi-
tecture. (Photograph courtesy of the MIT Artificial Intelligence Laboratory.)

to walk, avoid collisions, and climb over obstacles without the “move-think-
move-think” pauses of Shakey.
The term “behavior” in the subsumption architecture has a less precise
meaning than in other architectures. A behavior is a network of sensing and
acting modules which accomplish a task. The modules are augmented finite
state machines AFSM, or finite state machines which have registers, timers,
and other enhancements to permit them to be interfaced with other modules.
An AFSM is equivalent to the interface between the schemas and the coor-
dinated control strategy in a behavioral schema. In terms of schema theory,
a subsumption behavior is actually a collection of one or more schemas into
an abstract behavior.
Behaviors are released in a stimulus-response way, without an external
program explicitly coordinating and controlling them. Four interesting as-
pects of subsumption in terms of releasing and control are:

LAYERS OF 1. Modules are grouped into layers of competence. The layers reflect a hi-
COMPETENCE erarchy of intelligence, or competence. Lower layers encapsulate basic
survival functions such as avoiding collisions, while higher levels create