Watch a person walking and you see them swaying from side to side with each step to keep balanced. Try race walking and see how exaggerated you must twist your body to speed up walking. While walking, we always strive to keep our cen-ter of gravity over one foot if only for a fraction of a second. If you count the number of joints and motions in a person’s leg, you’ll realize that these joints are multi-axis joints—not just single-axis joints that we might have in a robot. Many 42
human joints have three degrees of freedom (DOF), in that they can move fore and 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 ofMechadonbuilt by Mark Setrakian.Mechadonweighs 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.
FIGURE 3-1 Mechadon,the
largest walking combat robot
ever built.
(courtesy of Peter Abrahamson)
True Story: Christian Carlberg and Minion
“I have been building mechanical devices since I was a kid,” says Christian Carlberg, founder and captain of Team Coolrobots. Christian is well-known for robot designs like OverKill, Minion, and Dreadnought. “Erector Sets, Lincoln Logs, LEGOs,” he adds, “I used them all.”
That earlyexperience with building toys paid off for Christian, who further honed his mechanical skills at Cornell through mechanical competitions (“build an electric motor in a couple of hours with these common house hold items,” he says). But LEGOs were—and remain—important. “If you can’t build the premise of your robot with LEGOs then it’s not simple enough to withstand the BattleBox.”
What competition stands out in Christian’s mind?
“My favorite fight was the Super Heavyweight rumble for the first season of Comedy Central’s BattleBots.”
Minion’s story actually begins in September of 1999, when BattleBots announced the new Super Heavyweight class. “The idea of building a 325 pound robot really appealed to me, especially considering it was a brand new weight class and there wouldn’t be a lot of competition.”
For that event, BattleBots placed ten 300-pound robots into a box for five minutes.
“I was driving Minion for that fight,” Christian recalls. “As the fight progressed it was clear that Minion was the strongest robot in the BattleBox. I was pushing three robots at a time, slamming other robots up against the wall. It was so much fun and totally worth all the hours spent on building the robot.”
Indeed, Team Coolrobots exudes bravado about Minion’s power. “Minion will not break or be broken. The onlywayto defeat Minion is to overpower it. This used to be impossible but has been known to happen.” Christian admits that there’s a secret to that raw locomotive power. “The weapon was always last on my list of priorities. You can still win as long as you are moving, which is why the frame and drive train will always be a higher priority for me.”