Page 60 - Modern Robotics Building Versatile Macines

40 Modern Robotics

upside-down pendulums mounted on moving carts. They developed
mathematical equations that specified the ranges of motion in which
balance could be maintained.
Using related ideas, Ralph Mosher at General Electric developed
walking devices that would be controlled by a human operator. The
operator would be placed in a harness such that his or her walking
motions would be translated into powered walking movement by
the machine. In 1968, Mosher demonstrated his “walking truck”
for the military.

Robot Kangaroos

Building a dynamic walking robot involves several different systems.
Besides controlling the specific motion of each leg joint, some sort
of master controller must determine the sequence in which the legs
move. In turn, these instructions must take account of feedback that
informs the robot when it is getting out of balance (and in what
direction). Finally, there has to be a sort of “strategic planner” that
determines the robot’s desired destination, examines possible foot-
holds (places to step down), and picks the best route.
Raibert and his team at the Leg Laboratory began with a very
simple proof-of-concept: a robot “pogo stick.” It consisted of a com-
puter-controlled piston that determined how far the robot could stride
forward without losing its balance and another control that deter-
mined how much “spring” or “bounce” could propel the leg from one
stride to the next. By combining legs, Raibert and his team later built
two-legged (bipedal) and four-legged (quardapedal) robots.
They got their idea for the robot from one of nature’s most accom-
plished hoppers: the kangaroo! They studied the structure of the
animal’s bones and muscles and analyzed its hopping gait.
One special thing about Raibert’s approach is that it did not
require exhaustive calculations or some sort of central control
system. As long as each leg (or coordinated leg pair) was kept in
balance, the robot as a whole remained in balance. Nevertheless,
the filmed trials of the robots include blooper reels featuring the
worst stumbles and crashes of the early prototypes. But progress
continued. By 1984, a quadrupedal robot could trot across the