Page 295 - Introduction to AI Robotics

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The Hybrid Deliberative/Reactive Paradigm
tered robotics and made a distinct contribution. Their architectures have a
more top-down, symbolic flavor than managerial and state-hierarchies. One
hallmark of these architectures is that they concentrate symbolic manipu-
lation around a global world model. However, unlike most other Hybrid
architectures, which create a global world model in parallel with behavior-
specific sensing, this global world model also serves to supply perception to
the behaviors (or behavior equivalents). In this case, the global world model
serves as a virtual sensor.
The use of a single global world model for sensing appears to be a throw-
back to the Hierarchical Paradigm, and conceptually it is. However, there
are four practical differences. First, the monolithic global world model is
often less ambitious in scope and more cleverly organized than earlier sys-
tems. The world model is often only interested in labeling regions of the
sensed world with symbols such as: hallway, door, my office, etc. Second,
perceptual processing is often done with distributed processing, so that slow
perceptual routines run asynchronously of faster routines, and the behav-
iors have access to the latest information. In effect, the “eavesdropping”
on perception for behaviors is an equivalent form of distributed processing.
Third, sensor errors and uncertainty can be filtered using sensor fusion over
time. This can dramatically improve the performance of the robot. Fourth,
increases in processor speeds and optimizing compilers have mitigated the
processing bottleneck.
Two of the best known model-oriented architectures are the Saphira archi-
tecture developed by Kurt Konolige with numerous others at SRI, and the
Task Control Architecture (TCA) by Reid Simmons which has been extended
to do multi-task planning with the Prodigy system. The Saphira architecture
comes with the ActivMedia Pioneer robots.

7.6.1 Saphira

SAPHIRA The Saphira architecture, shown in Fig. 7.9, has been used at SRI on a vari-
ety of robots, including Shakey’s direct descendents: Flakey and Erratic. The
motivation for the architecture stems from the basic tenet that there are three
COORDINATION, keys to a mobile robot operating successfully in the open world: coordina-
COHERENCE, tion, coherence,and communication. 77 A robot must coordinate its actuators
COMMUNICATION
and sensors (as has been seen through in the Reactive Paradigm), but it must
also coordinate its goals over a period of time (which is not addressed by the
Reactive Paradigm). Whereas the motivation for coordination is compatible
with reactivity, coherence is an explicit break from the Reactive Paradigm.