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The Hybrid Deliberative/Reactive Paradigm
7
Planner. In the SFX system, the Sensing Manager would trigger the Mission
Planner. The Mission Planner presumably has the intelligence to relax con-
straints on the mission, or in practice to alert the human supervisor. This
is an example of “failing upwards,” where a lower level module fails and a
higher level, or smarter, module has to take over.
COGNIZANT FAILURE Failing upwards is also known as a cognizant failure, meaning that the fail-
ure takes some amount of awareness of the context to resolve, not that the
cognition itself has failed. Erann Gat motivated the need for cognizant fail-
ures through the “Wesson Oil” problem. 57 In old Wesson Oil TV advertise-
ments, a mother would be cooking chicken in oil when she would have to
stop to rush her son to the emergency room for treatment of a broken arm.
Ideally the mother would not only turn off the stove but remove the chicken
from the oil. In the commercial, the mother neglects to remove the chicken.
Since she is using Wesson Oil, the chicken isn’t ruined when they return.
The point of the Wesson Oil problem for robotics is what happens when a
set of reactive behaviors are turned off in the middle of a sequence? In many
cases, it may not be just a simple act of de-instantiating the behaviors and
activating new ones. There may be behaviors in the sequence that have to
execute or disaster will occur (turn off the stove) as well behaviors which
must run or lead to merely undesirable side effects (chicken sits in oil if it is
not removed). All of this means that the sequencer must know why a failure
or need to change behaviors has occurred and what the intent of the current
sequence is. Deliberation is definitely not trivial!

7.10 Summary

The Hybrid Deliberative-Reactive Paradigm can be thought of as PLAN,then
SENSE-ACT (P,S-A).The SENSE-ACT portion is always done with reactive
behaviors, while PLAN includes a broader range of intelligent activities than
just path or mission planning. Planning can be interleaved with execution;
then for the most part, the robot plans a set of behaviors which will exe-
cute for a long time. Sensing is also hybrid; the reactive portion uses local,
behavior-specific representations while the deliberative portion uses global
world models.
Architectures in the Hybrid paradigm usually encapsulate functionality
into modules. The basic modules are the mission planner, sequencer agent, be-
havioral manager, cartographer, and performance monitor. Hybrid architectures
always have portions which are recognizably deliberative and reactive. The