Page 62 - Build Your Own Combat Robot

Chapter 3:
Robot Locomotion
human joints have three degrees of freedom (DOF), in that they can move fore and 43
aft, move side to side, and rotate.
Bipedal robots have been constructed, and a few Japanese companies are dem-
onstrating these in science news shows. Most robotics experimenters, however,
soon learn the complexities of two-legged robots, and quickly move to quadru-
peds (four legs)—and then just as quickly to hexapods (six legs) for their inherent
stability. Sony has sold many of its popular AIBO dogs and cats with four legs, and
the same for the much cheaper i-Cybie; but these machines have many motors for
each leg and are not being attacked by killer robots, as are combat robots.
Hexapods are a popular robot style for robotics experimenters because, with
six legs, the robot can keep three feet on the floor at all times—thus presenting a
stable platform that won’t tip over. Compare this with a quadruped, which can lift
one leg and easily tip over, depending upon the location of its center of gravity.
The six-legged “hex-walkers,” as they are sometimes called, can be programmed
to have their fore and aft legs on one side of the body and the center leg on the op-
posite side all raise and take a step forward, while the other three “feet” are on the
floor. In the next step, the other three legs raise and move forward, and so on.
More complex walking motions needed for turning use different leg combinations
selected by an on-board microcontroller. Each leg can use as few as two axes of
motion or two DOF, and some builders have used two model airplane R/C servos
to control all six legs. These types of robots are excellent platforms for experimen-
tation and for carrying basic sensors, but they are difficult to control and might
present an added complexity for a combat robot’s operator.
Although many of the robot organizations you’ll find on the Internet focus a lot
of attention on the construction of legged robots, the basic fragile nature of legs
makes them an extra challenge for builders of combat robots. Don’t get us
wrong—walking combat robots have been built, and some have done very well in
competition. If you want to build a legged combat robot, go for it. Many popular
robot competitions, including BattleBots and BotBash, even allow an extra
weight advantage for walking bots. Figure 3-1 shows a photo of Mechadon built
by Mark Setrakian. Mechadon weighs in at 480 pounds. This robot is the largest
and most impressive walking robot ever built for any combat robot event. The robot
can roll over, and it can crush its opponents between its legs
If you’re a beginning-level robot builder, you’ll probably find it easiest to work
with one of the more battle-proven methods of locomotion when designing and
constructing your combat robot. Since we’re assuming that a lot of our readers are
still at the beginner level, we’ll be focusing on other, less complicated forms of loco-
motion for competition robots. If you’re interested in learning more about walking
robots, many Web sites and reference books can provide helpful information. Some
of our recommended books and sites are listed in the appendixes in this book.