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The Hybrid Deliberative/Reactive Paradigm
7
come by the large number of robots. The other approach, characterized by
Carnegie Mellon University’s Ambler robot, proposed a single large robot
with a higher viewpoint and stability. Ambler was built by “Red” Whitaker
at the CMU Field Robotics Institute to be able to maintain a sensor platform
at a level height by stepping over the majority of rocks, but at a tremendous
penalty in size, weight, and power. In the end, planetary rover researchers
have gravitated towards wheeled vehicles with some type of articulation to
maintain stability, such as seen with Sandia National Laboratories’ Rattler.
An extension of the Ambler design philosophy was manifested in the Dante
robots. These were built to rappel down steep canyons and volcanoes on
Mars (and Earth). Dante was able to lower itself successfully most of the way
into a volcano in Antarctica, but could not climb back out. It was dropped
while being lifted out by a helicopter, twisting its frame.

7.8 Evaluation of Hybrid Architectures

In some regards, it is difficult to evaluate Hybrid architectures individually.
Each architecture is still evolving and the deliberative component is being
expanded practically daily. Returning to the four criteria set forth in the
overview in Part I, it is interesting to evaluate the architectures as a whole.
In terms of support for modularity, each architecture is highly modular.
Most are divided into layers, which are then subdivided into modules. As
the software agent programming style for AI gains in popularity, probably
more architectures will implement deliberative modules as independent spe-
cialists. AuRA and SFX clearly exhibit an organization which lends itself to
object-oriented programming. The use of specialists also enhances the ease
of portability.
Hybrids tend to have a high degree of niche targetability. The addition of
the deliberative component allows Hybrids to be used for applications not
appropriate for purely Reactive systems. However, the partitioning between
reaction and deliberation allows the reactive portion to be used alone for
purely reactive applications.
Another attractive aspect of Hybrid architectures is that they often explic-
itly attempt to ensure robustness. In the SFX and 3T architecture, modules
within the various deliberative components attempt to monitor the perfor-
mance of the reactive behaviors and either replace or adapt the configuration
as needed.