From: engelson-sean@cs.yale.edu (Sean Philip Engelson)
Newsgroups: comp.ai,comp.robotics,comp.lang.lisp
Subject: Re: ARS MAGNA Robot Simulator now available
Date: 11 Nov 1992 16:12:31 -0500
Organization: Yale AI Mobile Robotics Project
Distribution: world
NNTP-Posting-Host: stove.ai.cs.yale.edu


In article <1992Nov10.010954.9656@netcom.com>, nagle@netcom.com (John Nagle) writes:
|> engelson-sean@cs.yale.edu (Sean Philip Engelson) writes:
|> >			    Now Available
|> >			      ARS MAGNA
|> >		     The Abstract Robot Simulator
|> >
|> >... Lately attention has turned to planning for more realistic
|> >domains in which micro-world simplifying assumptions do not hold.
|> >...simple enough to support controlled experimentation.
|> >ARS MAGNA's environment and robot models are based on
|> >current robotics research, so that the domain is reasonably realistic.
|> >At the same time, we abstracted away from many (though not all)
|> >real-world details of kinematics and motor control.  
|> 
|>       Despite being called a "robot simulator", this is a world modeller
|> without much reality.  It's a 2D grid world, and it is basically 
|> predicate-oriented,
|> not geometry-oriented.  The robot is modelled as a point with "sensors",
|> "effectors", and "internal carrying capacity", in a world with "walls"
|> and "things".  You code a planning program in "nisp" (a LISP dialect) 
|> which does such things as calling "hand-grasp-desig", commanding a
|> designated hand to grasp an object.  This succeeds or fails, depending
|> on the contents of nearby cells, and sometimes fails randomly, so the
|> planner has to be able to recover from errors.  That's the lowest level
|> of detail supported.  No kinematics, no dynamics, no gravity, no inertia,
|> no friction, etc.  


We'd like respond to Nagle's comments point by point.  A detailed
justification of the ARS MAGNA model would be too long here; a paper
is in preparation which does so, placing the simulator in the context
of other current research.  It should be noted that ARS MAGNA is
intended to simulate a world, robot, *and low-level sensing/control
systems*.  The models used are based on current work in vision, sonar,
and behavior-based control.

- Nagle's core criticism seems to be focused in the first sentence, 
where he accuses ARS MAGNA of being "without much reality".  However, 
while his distinction between "robot simulator" and "world modeller" 
may be a real one, there's clearly a continuum; where one draws the 
line (indeed, whether or not doing so is even useful) is a matter of 
opinion.  Hence this comment appears to be little more than a semantic
quibble.

- ARS MAGNA is not a "grid world" in any, except a very weak, sense.  In
Section 2.1 of the manual it is made clear that the robot does not move 
or sense on a grid, these points being where a grid approximation does the 
most violence to realism.

- As to the claim that the simulator is "basically predicate-oriented"
vs.  "geometry-oriented" (the latter is to be preferred, we gather?),
we find this claim curious.  ARS MAGNA is "basically" neither; there
are geometric aspects as well as predicate-like aspects to the system.
Please read Sections 3.2.2 and 5.4 as well as pp. 19-26 for many
examples of non-"predicate-oriented" behavior.  Perhaps there is some
other precise meaning of "predicate-oriented" of which we're unaware?

- Nagle's description of the operation "hand-grasp-desig" betrays a
misunderstanding of a fundamental concept, that of designation.  No
robot commands operate on "objects" (other than robot parts).  Rather,
the concepts of visual marking/tracking and servo control are packaged
in the concept of "designators" (see Section 3.4.1 of the manual).  A
designator is a perceptual marker containing sufficient information
for a control module to perform its function, say, to servo to some
location.  The error models here are realistic, assuming implementation 
of this reasonable low-level model.

We believe that the problem here mainly stems from a confusion about
what sorts of problems are being addressed.  If one is working on
low-level control issues, ARS MAGNA is clearly inappropriate, as it
simulates at a higher level of abstraction.  If one's domain of
interest is task-level planning or large-scale mapping, ARS MAGNA is
eminently appropriate, and, as Erann Gat mentioned, rather better than
other simulated worlds available at this time.

	Sean Engelson
	Drew McDermott

-- 
Sean Philip (Shlomo) Engelson		
Yale Department of Computer Science	
Box 2158 Yale Station			
New Haven, CT 06520			
Newsgroups: comp.ai