It is interesting to note that the use of predicate logic and recursion by Strips favors languages like Lisp and PROLOG. These languages were developed by AI researchers specifically for expressing logical operations. These lan-guages do not necessarily have good real-time control properties like C or C++. However, during the 1960’s the dominant scientific and engineering language was FORTRAN IV which did not support recursion. Therefore, re-searchers in AI robotics often chose the lesser of two evils and programmed in Lisp. The use of special AI languages for robotics may have aided the split between the engineering and AI approaches to robotics, as well as slowed down the infusion of ideas from the the two communities. It certainly dis-couraged non-AI researchers from becoming involved in AI robotics.
The Hierarchical Paradigm tends to encourage monolithic programming, rather than object-oriented styles. Although the NHC decomposes the plan-ning portion of intelligence, the decomposition is strictly functional. In par-ticular, NHC and RCS don’t provide much guidance on how to build modu-lar, reusable components.
2.7 Summary
The Hierarchical Paradigm uses aSENSE thenPLAN thenACT (S,P,A). It organizes sensing into a global data structure usually called a world model that may have an associated knowledge base to contain a priori maps or knowledge relevant to a task. Global data structures often flag that an
ar-chitecture will suffer from the frame problem. The Hierarchical Paradigm was introduced in the first mobile robot, Shakey. Strips is an important plan-ning technique that came out of the Shakey project at SRI, which focused on thePLANprimitive in robotics. Concepts and terms which emerged that continue to play an important role in defining robotics are: preconditions, the closed and open world assumptions, and the frame problem. Hierar-chical systems have largely fallen out of favor except for the NIST Realtime Control Architecture. The decline in popularity is due in part to its focus on strong niche targetability at the expense of true modularity and portability.
However, as will be seen in the following chapters, insights from biology and cognitive science have led to paradigms with more intuitive appeal. One of-ten overlooked property of most hierarchical architectures is that they of-tend to support the evolution of intelligence from semi-autonomous control to fully autonomous control.
2.8 Exercises
Exercise 2.1
Describe the Hierarchical Paradigm in terms of a) theSENSE, PLAN, ACTprimitives and b) sensing organization.
Exercise 2.2
Define the following terms in one or two sentences. Give an example of how each arises in a robotic system:
a. precondition
b. closed world assumption c. open world
d. frame problem Exercise 2.3
Consider the frame problem. Suppose the World Model for a Strips-based robot con-sisted of 100 facts. Each fact requires 1KB of memory storage. Every time a new object is added to the world model, it increases the model by 100 (a linear increase). One object, 100 facts, 100KB of storage; two objects, 200 facts, 200KB. How many objects would fill 64KB of memory?
Exercise 2.4
Redo the above exercise where the number of facts in the world model doubles every time a new object is added (exponential). One object, 100 facts, 1KB, two objects, 200 facts, 200KB, three objects, 400 facts, 400KB. Which is a more reasonable rate to assume the world model would increase at, linear or exponential?
Exercise 2.5
Describe the mission planner, navigator, pilot organization of the Nested Hierarchical Controller. Write down how it would handle the problem in Sec. 2.2.2.
Exercise 2.6
List 2 advantages and disadvantages of the Hierarchical Paradigm.
Exercise 2.7
Solve the following navigation problem using Strips. Return to the world in Sec 2.2.2.
The robot will move to the boxB1and pick it up.
a. Add a new operatorpickupto the difference table.
b. Use the world model, difference table, difference evaluator to construct a plan.
Failed preconditions and new subgoals should be shown after each step.
c. Show the changes in the world model aftereachoperator is applied.
Exercise 2.8
Name and evaluate one representative Hierarchical architecture in terms of: support for modularity, niche targetability, ease of portability to other domains, robustness.
Exercise 2.9 [World Wide Web]
Search the web for interactive versions of Strips and experiment with them.
2.9 End Notes
A robot that did take a cross-country trip.
Robot vehicles do in fact need special authorization to drive on public roads. In 1996, the Carnegie Mellon University Navlab vehicle project led by Dean Pomerleau steered itself (the driver handled the gas pedal and brakes) over 90% of the way across the USA from Washington, DC, to Los Angeles in the “No Hands Across America”
trip. The Navlab (a modified Saturn station wagon) was reportedly pulled over by the Kansas State Highway Patrol for driving an experimental vehicle without permis-sion. The entire trip was placed in jeopardy, but eventually the Navlab was allowed to continue, and the team and vehicle appeared on the David Letterman show in Los Angeles. As impressive as the Carnegie Mellon feat was, a group of German researchers under the direction of Ernst Dickmanns and Volker Graefe have been fielding even more advanced autonomous highway driving vehicles since 1988.
Shakey.
It can be debated whether Shakey is really the first mobile robot. There was a tortoise built by Grey Walter, but this was never really on the main branch of AI research. See Behavior-Based Robots10for details.
Robot name trivia.
Regardless of how Shakey got its name, SRI continued the tradition with Shakey’s successor being calledFlakey, followed by Flakey’s successor,Erratic.
Strips.
The description of Strips and the robot examples are adapted fromThe Handbook of Artificial Intelligence, A. Barr and E. Feigenbaum, editors, vol. 1, William Kaufmann, Inc., Los Altos, CA, 1981.
Jim Albus.
Jim Albus is one of the statesmen of robotics. Although his RCS architecture is a rep-resentative of the Hierarchical Paradigm, Albus was heavily by cognitive studies—so much so that he wroteBrains, Behaviors and Robots(1981). What’s odd is that ex-ploiting biological and cognitive studies is commonly associated with the Reactive Paradigm. The RCS architecture was never meant to be totally static, and in recent years, it has begun to resemble what will be referred to as a model-oriented style of the Hybrid Paradigm.