The RAP System
A robot acting in the real world must use flexible plans.
Actions will sometimes fail to produce their desired effects and unexpected
events will sometimes demand the robot shift its attention from one
task to another. A plan is
usually construed as a list of primitive robot actions to be executed
one after another but in a complex domain a plan must be structured
to cope effectively with the myriad unpredictable details it will
encounter during execution. However, adding structure to a plan
involves more than augmenting the primitive plan representation; it
requires adopting a situation-driven model of interaction with the world.
Situation-driven execution assumes that a
plan consists of tasks with three major components: a satisfaction
test, a window of activity, and a set of execution methods that are
appropriate in different circumstances. Execution of such a plan
proceeds by selecting an unsatisfied task and choosing a method to
achieve it based on the current world state. A task may be executed
as many times as necessary to keep it satisfied while it is active.
The RAP system proposes a plan and task representation based on
program-like reactive action packages, or RAPs.
A plan consists of RAP-defined goals, or tasks, at a variety of
different levels of abstraction and the RAP system attempts to
carry out each task in turn using different methods in different situations
and dealing with common problems and simple interruptions.
Within the
system, execution monitoring becomes an intrinsic part of the
execution algorithm, and the need for separate replanning on failure
disappears. RAPs are more than just programs that run at execution
time, however, they are also hierarchical building blocks for plan
construction. The RAP representation is structured to make a task's
expected behavior evident for use in planning as well as in execution.
The RAP execution system described includes a sensor memory,
representation language and interpreter. Examples and experiments
demonstrate a wide range of adaptive system behavior.
More Information
The RAP System /
R. James Firby /
firby@cs.uchicago.edu
March, 1995