Page 350 - Robot Builder's Bonanza

AN OVERVIEW OF LEGGY ROBOTS 319

Legged robots aren’t for beginners. If you’re just starting out, first hone your skills by
G constructing a couple of wheeled bots. This applies whether you buy a ready- made kit of parts
or build it from scratch. Walking robots require greater precision and attention to detail— when
improperly constructed, they rattle apart, may simply not work, or fall over in a crashing heap.

NUMBER OF LEGS

This concept was introduced in Chapter 2, “Anatomy of a Robot,” and Chapter 20, “Moving
Your Robot,” but it bears repeating here. The most common forms of legged bots are:

Bipeds, which walk on two legs. In a true bipedal robot, one leg lifts up to make a step.
Balance can be tricky when the robot has just one foot on the ground, making this type
among the hardest to master. The alternative is the “shuffling” walking robot common
in toys whose legs don’t actually lift up and down.
Quadrupeds, meaning four legs. The more sophisticated four- legged bots demonstrate a
variety of walking styles, some mimicking animals. These can be tricky to build, because
each leg needs three separate joints— otherwise known as degrees of freedom, or
DOF— in order to keep balance and make turns.
Hexapods, for which six legs provide excellent balance and mobility. Among walking
robots, they’re the most common and, despite the extra legs, the easiest to build.
And, of course, there are examples of robots with other appendage counts. One- legged
robots, or hoppers, look like pogo sticks without a rider. They move around by bouncing.
Robots with eight or more legs, or multipods, include a wide variety of wild and crazy
designs. These include segmented bots, which behave (and somewhat look) like snakes,
worms, or caterpillars. Each segment is controlled by one or more motors not unlike the joints
in a walking robot.
A few experimental walking robots employ an odd number of legs. If four are too few, and
six too many, why not build a bot with five legs? And finally, hybrid robots may combine legs
with wheels or tracks. An example is a 4WD robot that has two legs in the front. The legs can
help to lift the robot up steps and over obstacles and, if jointed properly, can serve as manip-
ulators. Pretty rad stuff.

STATIC VERSUS DYNAMIC BALANCE
Balance is the ability of the robot to remain upright when it’s standing on its legs. Balance can
be static or dynamic.
• Static balance means the legs provide a natural stability, using any (or a combi-
nation) of several techniques. In four- and six- legged bots, the most common
static balance comes from always having at least three legs on the ground at the
same time, forming a tripod stance. With all types, static balance is improved by
having a low center of gravity; much of the weight is at least 50 percent below
= leg on ground
the overall height of the robot.
• Dynamic balance means the robot uses sensors to keep itself upright. When the bot feels
it’s starting to tip over, the sensors activate one or more motors to shift the robot’s weight
one way or another. As the weight shifts, the tilt is negated, and the robot is kept upright.

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