REACTIVE PARADIGM

 REACTIVE PARADIGM

The Reactive Paradigm was a reaction to the Hierarchical Paradigm, and
led to exciting advances in robotics. It was heavily used in robotics starting
in 1988 and continuing through 1992. It is still used, but since 1992 there
has been a tendency toward hybrid architectures. The Reactive Paradigm
was made possible by two trends. One was a popular movement among AI
researchers to investigate biology and cognitive psychology in order to examine
living exemplars of intelligence. Another was the rapidly decreasing
cost of computer hardware coupled with the increase in computing power.
As a result, researchers could emulate frog and insect behavior with robots
costing less than $500 versus the $100,000s Shakey, the first mobile robot,
cost.
The Reactive Paradigm threw out planning all together (see Figs. I.3b and
I.5). It is a SENSE-ACT (S-A) type of organization. Whereas the Hierarchical
Paradigm assumes that the input to a ACT will always be the result of a
PLAN, the Reactive Paradigm assumes that the input to an ACT will always
be the direct output of a sensor, SENSE.
If the sensor is directly connected to the action, why isn’t a robot running
under the Reactive Paradigm limited to doing just one thing? The robot has
multiple instances of SENSE-ACT couplings, discussed in Ch. 4. These couplings
are concurrent processes, called behaviors, which take local sensing
data and compute the best action to take independently of what the other
processes are doing. One behavior can direct the robot to “move forward 5
meters” (ACT on drive motors) to reach a goal (SENSE the goal), while another
behavior can say “turn 90 ” (ACT on steer motors) to avoid a collision

with an object dead ahead (SENSE obstacles). The robot will do a combination
of both behaviors, swerving off course temporarily at a 45 angle to
avoid the collision. Note that neither behavior directed the robot to ACT with
a 45 turn; the final ACT emerged fromthe combination of the two behaviors.
While the Reactive Paradigm produced exciting results and clever robot
insect demonstrations, it quickly became clear that throwing away planning
was too extreme for general purpose robots. In some regards, the Reactive
Paradigm reflected the work of Harvard psychologist B. F. Skinner in
stimulus-response training with animals. It explained how some animals
accomplished tasks, but was a dead end in explaining the entire range of
human intelligence.
But the Reactive Paradigm has many desirable properties, especially the
fast execution time that came from eliminating any planning. As a result,
the Reactive Paradigm serves as
HYBRID DELIBERA- the basis for the Hybrid Deliberative/Reactive

Paradigm, shown in Fig.I.3c. The Hybrid Paradigm emerged in the 1990’s and
continues to be the current area of research. Under the Hybrid Paradigm, the
robot first plans (deliberates) how to best decompose a task into subtasks
(also called “mission planning”) and then what are the suitable behaviors to
accomplish each subtask, etc. Then the behaviors start executing as per the
Reactive Paradigm. This type of organization is PLAN, SENSE-ACT (P, S-A),
where the comma indicates that planning is done at one step, then sensing
and acting are done together. Sensing organization in the Hybrid Paradigm
is also a mixture of Hierarchical and Reactive styles. Sensor data gets routed
to each behavior that needs that sensor, but is also available to the planner

for construction of a task-oriented global world model. The planner may
also “eavesdrop” on the sensing done by each behavior (i.e., the behavior
identifies obstacles that could then be put into a map of the world by the
planner). Each function performs computations at its own rate; deliberative
planning, which is generally computationally expensive may update every
5 seconds, while the reactive behaviors often execute at 1/60 second. Many
robots run at 80 centimeters per second.