Page 289 - Introduction to AI Robotics

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The Hybrid Deliberative/Reactive Paradigm
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The reactive component is also divided into two layers. In this case, the
STRATEGIC layers reflect strategic versus tactical behaviors, defined below. As noted ear-
TACTICAL BEHAVIORS lier, just about every hybrid architecture has its own method of combining
behaviors in the reactive portion. AuRA uses potential field combination,
whereas SFX uses a filtering method. The SFX philosophy is similar to sub-
sumption: there are some fundamental behaviors that should override other
behaviors. The difference is that in SFX, it is the “lowest” tactical behaviors
which do the subsuming, not the “higher” strategic ones.
The idea of a tactical behavior is best seen by example. Recall the case
of obstacle avoidance with potential fields. The use of a repulsive field was
simple, but could lead to a global minima, where the repulsion could can-
cel out any other motive fields such as move-to-goal. The NaTs 132 solution
was to use the vector created by that other field as an input to the avoid-
obstacle field. That vector would lead to a tangential field in addition to the
repulsive field, resulting in an avoid-obstacle field which repulses the robot
towards the direction it was heading in to begin with. In this case, the move-
to-goal field was offering a strategic command; like a general, it was telling
the troops to move ahead. But the avoid-obstacle field was like an infantry
man; it was trying to go in the direction given by the general but not hit
anything. The avoid-obstacle behavior filters the strategic direction (given
by move-to-goal) with the immediate tactical situation (the presence of an
obstacle).
Another example of a tactical behavior in SFX is speed-control. Speed-
control in AuRA and many architectures is a by-product of the mechanism
used for combining behaviors. The emergent speed in AuRA is the mag-
nitude of the vector summed from all the active behaviors. In SFX, speed-
control is considered a separate behavior. The safe velocity of a robot de-
pends on many influences. If the robot cannot turn in place (in effect, turns
like a car), it will need to be operating at a slow speed to make the turn with-
out overshooting. Likewise, it may need to go slower as it goes up or down
hills. These influences are derived from sensors, and the action is a template
(the robot always slows down on hills), so speed control is a legitimate be-
havior. But the other behaviors should have some influence on the speed as
well. So these other, strategic behaviors contribute a strategic speed to the
speed-control behavior. If the strategic speed is less than the safe speed com-
puted from tactical influences, then the output speed is the strategic speed.
But if the tactical safe speed is lower, the output speed to the actuator is the
tactical speed. Tactical behaviors serve as filters on strategic commands to