|Build Your Own Combat Robot|
Basic Stamp Kits
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Robots at Work
the excitement of building your own champion battling bot! Build a
powerful robot for full-blown competition with the help of this
authoritative robot resource. This team of experts gives you an inside
look at the innovative new world of robotic combat, explaining the
origins of the sport as well as all the elements that go into
constructing a fighting robot.
Chapter 3: Robot Locomotion
In this chapter, we'll concentrate on locomotion methods that are easy to construct and most effective for large robots and combat machines. We'll also discuss the drawbacks of some methods for combat robot applications. Several methods of locomotion have been successfully used in combat and other large robots. These are legs, tank-type treads, and various other configurations and styles of wheels. Yes, some really cool machines have used other means to get across the floor, but "cool" and effective are sometimes very different.
Legs are often one of the first types of locomotion we envision when we think of robots. For most people, "robot" means a walking bot like C3P0 in Star Wars or Robby from Forbidden Planet. However, we must remember that these creatures were just actors wearing robot suits to make them appear as walking machines. Walking is actually a difficult task for any creature to perform, whether its human or humanoid. It takes babies nine months or longer to master the act, and for several years after that they're tagged with the title of "toddler." A child's brain is constantly learning and improving this complex process each day. Bipedal (two legs) walking is really controlled falling - stop in the middle of taking a step and we'd fall over. Impede the process with a few beers too many, and our built-in accelerometers (our ears' semi-circular canals) feed us wrong information and we stumble.
Robots with Legs
Bipedal robots have been constructed, and a few Japanese companies are demonstrating these in science news shows. Most robotics experimenters, however, soon learn the complexities of two-legged robots and quickly move to quadrupeds (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 opposite 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 experimentation 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 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 locomotion 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 appendices in this book.
That early experience 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 only way to 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."
Tank Treads: The Power of a Caterpillar Bulldozer in a Robot
Many first-time robot builders are drawn to treads because they look so menacing. Treads come in two basic sizes:, massive off-road and toy sizes, and there is no similarity between the two. The toy variety is just a rubber ring with "teeth" molded into the rubber. The larger off-road-size treads consist of a series of interconnected metal plates, supported by a row of independently sprung idler wheels. The construction of interconnected plate treads is complex and should be left to experts with large machine shops. Peter Abrahamson has built a very impressive 305 pound robot named, Ronin. The aluminum tank treads were custom machined for this robot. Each side of Ronin can rotate relative to one another thus improving the overall traction capability of this robot. Figure 3-2 shows a photo of Ronin climbing a log.
Bot experimenters usually opt for the rubber tracks removed from a child's toy bulldozer, and then start piling batteries, extra motors, sensors, and arms onto the new machine. When the first test run is started, the rubber tips of the tread surface begin to bend as they push onto the floor. The robot chugs along just fine until it has to make a turn. If the operator happens to be monitoring the current drawn by the drive motors, he'll see a sharp increase as the turn begins. This is one of the major drawbacks of tank-style treads; they must skid while making a turn, and energy is wasted in this skid. Only the center points of each "track" are not skidding in a turn. For this reason, many robotics engineers opt not to use tank-style treads in their machines.
However, the efficiency of the propulsion system is a less significant factor in combat robots than in other types of bots. Because a combat robot's "moment of truth" is limited to a 3 to5-minute match, builders can easily recharge or install new batteries between matches, making the issue of wasted energy less of a consideration. With this fact in mind, many builders opt for tank-style treads, so let's examine another feature of treads: they're complex and hard to mount.
The toy rubber ring tank tread seems anything but complex. It's just a toothy rubber ring strung between two pulleys. The experimenter with his toy bulldozer treads might be so preoccupied with the current draw of his drive motors or with maneuvering the machine that he doesn't notice one of the treads working its way off the drive spindle. And if the tread slips off your heavyweight bot in a robot combat match, chances are you'll lose.
Building Tank Treads for a Robot
Now you have four spindles to mount both belts for a complete robot base, two rubber and two aluminum. After assembly, you find that the new drive spindles work pretty well. The rough ground surface of the spindle does a decent job of gripping the smooth rubber belt's surface. After trying the base out on the floor, you find that the turning is erratic and decide that you need a row of idler wheels to keep the entire length of each belt firmly on the floor. Your friend patiently machines for you 10 idler wheels on which you mount to a series of spring-loaded lever arms. Wow, this robot is beginning to be a bit complicated! After a few tries on your garage floor, you begin to notice that the teeth are wearing down. You smile at your creation and decide to put it away. It was a good learning experience....
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