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2 The Hierarchical Paradigm
ing the 1970’s and 1980’s worked on either computer vision related issues,
trying to get the robots to be able to better sense the world, or on path plan-
ning, computing the most efficient route around obstacles, etc. to a goal lo-
cation.

2.4 Representative Architectures

As mentioned in Part I an architecture is a method of implementing a para-
digm, of embodying the principles in some concrete way. Ideally, an archi-
tecture is generic; like a good object-oriented program design, it should have
many reusable pieces for other robot platforms and tasks.
Possibly the two best known architectures of the Hierarchical period are
the Nested Hierarchical Controller (NHC) developed by Meystel 93 and the
1
NIST Realtime Control System (RCS) originally developed by Albus, with
its teleoperation version for JPL called NASREM.

2.4.1 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 por-
tion 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 actua-
tor 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 Mission Planner,the Navigator,and the Pilot. As shown in Fig. 2.6, the Mis-
NAVIGATOR sion Planner either receives a mission from a human or generates a mission
PILOT