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ROBOT MOBILITY
by Tom Carroll
Recently, a young kid in my neighborhood came up to me and excitedly
told me about several robots that he’d seen on television that could snake
along the ground like a mechanical serpent and one that could climb up
the side of buildings like Spiderman. We got to talking about how robots
move around. The single feature that distinguishes a robot from a
computer or any other sort of electro-mechanical device is its ability to
move — whether the movement is an appendage or a movable base.
Most of us want our creations to move about our house, yard or in some
location. To accomplish this, we have to select and build a method of
mobility. For now, and I’ll concentrate on land vehicles only.
Robot Legs
The earliest robots of fiction used legs for
mobility and were humanoid in form; they were
anthropomorphic or ‘man-formed.’ It did not take
robot experimenters long to discover that the
technology required to develop a real walking
robot was just not available in the early 20th
century. People have
tried to make successful
walkers for years but
most seem to end up
with overly large feet or
“C” shaped feet that
spread across a footprint
that’s wide enough past
the center of gravity to
prevent the robot from
toppling over.
Accelerometers, gyros,
and powerful variable-speed motors were not
available in those days.
Today, there are very
successful walkers such
FIGURE 1. Big Dog from
Boston Dynamics.
76 SERVO 04.2010
as Hubo, Asimo, and Big Dog.
When I was a kid, I built a walking robot from
an Erector Set, jukebox, and other junk parts. I
carefully trimmed the length of the leg segments
(there were no knee joints) so that the resonance
of the leg lengths would match the speed of the
fixed speed AC motor. I finally got the robot to
walk without falling over, but only if I held onto
the power cord on start-up and stopping. When it
started or stopped walking, it would always keel
over.
University labs certainly built better walkers in
those days, but they still were not the dynamically
balanced machines we have today. Take, for
example, Marc Raibert’s ‘Big Dog’ shown in
Figure 1. Walking robots have become quite
popular in recent years, but I’m going to
concentrate on other types of “mobile” robots in
this article.
Early Robots Used Wheels
The first true experimental mobile robots used
a wheeled platform base. This type of system
seemed to be the logical choice as virtually every
land-based machine that moved across a floor or
the ground used wheels (except for other methods
