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CONTROL SYSTEMS 23
each individual herring simply watches his four immediate neighbors and reacts to their
position, speed, and movement. The net result, observed at the school level, is dramatic
and effective. Thousands of tiny brains act almost as one, and the tuna are partially frus-
trated. With luck, they go off to bother the shrimp.
The herring school is using a “distributed” control system. The school is governed
by the collective will and common actions of the individual fish. Consider some of the
advantages of a distributed control system:
Cheapness Individual control systems elements are simpler and cheaper. In this
example, we’d only have to design something simple like a herring and then repli-
cate it thousands of times (gaining economies of scale).
Reliable If the system is designed to survive the failure of portions of the sys-
tem, a few failures will not bring it down. Surely, not all the herring escape the
tuna. The school simply changes shape to heal up the hole where the eaten her-
ring once was, and life goes on.
A distributed control system does have some disadvantages:

Communication Sometimes it’s hard to communicate everything between indi-
vidual control elements. A herring at the far side of the school doesn’t know a tuna
is coming until his neighbor signals such. The panic signal spreads through the
school like a wave, but it might be too late. This form of knowledge truly is power,
and a matter of life and death.
Horsepower The individual elements within a distributed control system gen-
erally are not powerful in and of themselves. Although the collective herring
school solves the tuna problem as well as any human or computer might, the indi-
vidual herring could not match a human at math or reasoning. Distributed control
systems are often designed to solve specific problems and are not as good at field-
ing general-purpose problems. If you use a distributed control system, be very
careful that you know all the problems that it must face. If the specifications
change, your design might flounder!
If you’d like to explore a distributed model for robot control, here are some URLs
with source software and links. Just beware; you could easily spend weeks playing with
these models:
http://www.red3d.com/cwr/boids/
The following URLs consider general-purpose distributed control systems:

www-db.stanford.edu/ burback/dadl/
www-db.stanford.edu/ burback/dadl/node87.html