Nested Hierarchical Controller

Nested Hierarchical Controller

As shown in Fig. 2.5, the Nested Hierarchical Controller architecture has
components that are easily identified as either SENSE, PLAN, or ACT. The
robot begins by gathering observations from its sensors and combining those
observations to form the World Model data structure through the SENSE
activity. The World Model may also contain a priori knowledge about the
world, for example, maps of a building, rules saying to stay away from the
foyer during the start and finish of business hours, etc. After the World
Model has been created or updated, then the robot can PLAN what actions
it should take. Planning for navigation has a local procedure consisting of
three steps executed by the Mission Planner, Navigator, and Pilot. Each of
these modules has access to the World Model in order to compute their portion
of planning. The last step in planning is for the Pilot module to generate
specific actions for the robot to do (e.g., Turn left, turn right, move straight at
a velocity of 0.6 meters per second). These actions are translated into actuator
control signals (e.g., Velocity profile for a smooth turn) by the Low-Level
Controller. Together, the Low-Level Controller and actuators form the ACT
portion of the architecture.
The major contribution of NHC was its clever decomposition of planning
into 3 different functions or subsystems aimed at supporting navigation: the
Mission Planner, the Navigator, a MISSION PLANNER nd the Pilot. As shown in Fig. 2.6, the Mis-
NAVIGATOR
PILOT
sion Planner either receives a mission from a human or generates a mission

NHC has several advantages. It differs from Strips in that it interleaves
planning and acting. The robot comes up with a plan, starts executing it,
then changes it if the world is different than it expected. Notice that the decomposition
is inherently hierarchical in intelligence and scope. The Mission
Planner is “smarter” than the Navigator, who is smarter than the Pilot. The
Mission Planner is responsible for a higher level of abstraction then the Navigator,
etc. We will see that other architectures, both in the Hierarchical and
Hybrid paradigms, will make use of the NHC organization.
One disadvantage of the NHC decomposition of the planning function is
that it is appropriate only for navigation tasks. The division of responsibilities
seems less helpful, or clear, for tasks such as picking up a box, rather than
just moving over to it. The role of a Pilot in controlling end-effectors is not
clear. At the time of its initial development, NHC was never implemented
and tested on a real mobile robot; hardware costs during the Hierarchical
period forced most roboticists to work in simulation.