Advantages and Disadvantages

Advantages and Disadvantages

Robots built in the time period before 1990 typically had a Hierarchical style
of software organization. They were generally developed for a specific application
rather than to serve as a generic architecture for future applications.
The robots are interesting because they illustrate the diversity and scope of
applications being considered for mobile robots as far back as 15 or 20 years
ago.
The primary advantage of the Hierarchical Paradigm was that it provides
an ordering of the relationship between sensing, planning, and acting. The
primary disadvantage was planning. Every update cycle, the robot had to
update a global world model and then do some type of planning. The sensing
and planning algorithms of the day were extremely slow (and many still
are), so this introduced a significant bottleneck. Notice also that sensing and
acting are always disconnected. This effectively eliminated any stimulusresponse
types of actions (“a rock is crashing down on me, I should move
anywhere”) that are seen in nature.
The dependence on a global world model is related to the frame problem.
In Strips, in order to do something as simple as opening a door, the robot had
to reason over all sorts of details that were irrelevant (like other rooms, other
doors). NHC and RCS represent attempts to divide up the world model into
pieces best suited for the type of actions; for example, consider the roles of
the Mission Planner, Navigator, and Pilot. Unfortunately, these decomposi-

tions appear to be dependent on a particular application. As a result, robotics
gained a reputation as being more of an art than a science.
Another issue that was never really handled by architectures in the Hierarchical
Paradigm was uncertainty. Uncertainty comes in many forms, such
as semantic (how close does NEXTTO mean anyway?), sensor noise, and actuator
errors. Another important aspect of uncertainty is action completion:
did the robot actually accomplish the action? One robotics researcher said
that their manipulator was only able to pick up a cup 60% of the attempts;
therefore they had to write a program to check to see if it was holding a cup
and then restart the action if it wasn’t. Because Shakey essentially closed its
eyes during planning and acting, it was vulnerable to uncertainty in action
completion.