abandon all hope ye robots who enter
Design News Magazine
Float like a butterfly, sting like a... robot
Joseph Ogando, Materials Editor
Power transmission, materials, fastening challenges drive dueling-robot design
Never underestimate the appeal of metallic carnage. That's the idea behind BattleBots, the weekly television series that pits dueling robots against one another. With their builders pulling the strings via radio controls, these robotic fighters go mano a mano for three minutes and attempt to batter each other into submission. If only one robot works at the end, it wins. If both BattleBots are still standingor more likely, rollingjudges award a decision to whichever robot inflicts the most damage.
Not surprisingly, BattleBots take a beating during these mechanical slugfests. The robots all sport weaponsanything from a simple hammer or spike to circular saws. The current heavyweight titleholder, Carlo Bertocchini, recalls the time his robot, BioHazard, had an eight-inch gash torn through its 3/16-inch titanium armor by another robot's saw-like spinning disc. "That disc moves about 300 mph on its edges. When it hit my robot, sparks flew everywhere," Bertocchini says, noting that the attack launched the 208-lb Biohazard several feet into the air. BattleBots also have to withstand collisions with arena walls and other robots. In the heavyweight division, robots weighing more than 200 lbs commonly collide head on at 15 mph. One former heavyweight champ, Gage Cauchois, reports that he even tests his robot's ability to "take a punch" with some full-speed dumpster tilting. And, as if the other robots weren't bad enough, BattleBots also compete amidst hazards in the ring: circular saws, spikes, and hammers.
By the time a BattleBot gets ready to rumble, its builder has typically spent months on design and makeshift manufacturing, cobbling the robot together from a combination of custom-fabricated and off-the-shelf components. And the builders, who may or may not be engineers, have to juggle many of same design constraints found in many a commercial engineering project. BattleBot rules, for example, impose strict weight limits. The builders also struggle with packaging limitations and powertrain design. Weapons and fighting style drive many of engineering decisions, according to Bertocchini. "A ramming robot might need more speed than a robot with an impact weapon," he says. "And that has design implications from the size of the motors to the overall construction of the BattleBot." BattleBot builders even have their own take on life-cycle engineering, with an acceptable lifespan for many components defined as a three-minute BattleBot bout. Finally, because most builders still compete as hobbyists, they work hard to avoid needless cost and complexity.
"The great thing about BattleBots is that there are many ways of solving the same engineering problems," says Jason Dante Bardis, a BattleBot competitor who's working on his doctorate in mechanical engineering. Here's a look at the very different engineering approaches taken by three contenders in the heavyweight division.
Behind the robot: Built by Carlo Bertocchini, a mechanical designer for Raychem Corp., Biohazard won last year's heavyweight competition.
Specs: Weighing in at 208 lbs and measuring 36 × 48 × 6.25 inches, this robot epitomizes the "low-slung box" approach.
Weapon: A scooping arm unfolds from a shallow recess in the top of the robot and lets Biohazard lift competitors, either to flip or shove them into ring hazards and walls.
Locomotion: BioHazard's entire drivetrain squeezes into a pair of 3 × 3-inch-square aluminum extrusions. Each extrusion, one for each side of the robot, houses a two-stage, 6:1 chain reduction and three 4-inch-diameter wheels. Bertocchini decided to use extrusions mostly for the sake of rigidity. "Using a single structural member for the drivetrain prevents any flexing that could misalign the sprockets," he points out. Two 3.5-hp electric motors and a 24-V battery power Biohazard. "I rewound the motors and re-machined the commutators for a better performance profile," Bertocchini says, explaining that his battle strategy needs to slightly favor forward motion.
Engineering challenges: "The weapon was the trickiest thing to engineer," says Bertocchini. Made from a four-bar linkage, the scooping arm has to unfold from its nearly flat space to an extended length of 18 inches. "It takes a tremendous amount of force to move the fully-loaded arm," he says, citing an average of 1,400-lb average mechanical advantage through its range of motion. Bertocchini drives the arms with two 3.5-inch linear actuators. Though rated to only 400 lbs, the actuators underwent extensive modificationsincluding a new set of needle bearings and beefed up stanchionsto triple its load capabilities. The actuator works in conjunction with what Bertocchini describes as a "complicated bell crank-torsion shaft mechanism." Squeezed between arm and actuators, this mechanism bears the brunt of the load and had to be machined from a specialty steel alloy developed for aerospace applications. (Aeromet from Carpenter Technologies). Bertocchini prototyped the crank mechanism in pre-hardened P-20 but found that this conventional steel wouldn't hack it. "It didn't fail, but it did flex," he recalls.
Finding the right materials for Biohazard also took some doing. With a 3 × 4-ft area to cover, Bertocchini needed a material with a good strength-to-weight ratio. He picked 3/16-inch-thick titanium. "I needed a high-tech material to stay within the weight limitation," he says, noting that he got away with magnesium on earlier, sparsely armored versions of Biohazard. Underneath the robot, he installed alumina ceramic tiles to protect the undercarriage from the BattleBot's circular saws. "Hardened steel would have weighed at least 5 times as much," he guesses.
Sponsors: PTC contributed Pro/Engineer software. W.M. Berg chipped in with precision mechanical components, like the sprockets.
Check out Biohazard's webstie at www.robotbooks.com.
Behind the robot: Newly built for this month's heavyweight competition, Towering Inferno has yet to see any action. But its builder, Jason Dante Bardis, a doctoral candidate in mechanical engineering at Univ. of California Santa Barbara, has built an small army of fighting robots over the past 10 years.
Specs: Measuring 28.5 inches high, 40 inches long and more than 40 inches wide, Towering Inferno lives up to its name—the first part of it anyway. But stature doesn't set the robot apart as much as a design that even Bardis describes as "weird." In contrast to the more common robot-as-box approach, Towering Inferno consists of two wheel modules joined by a 42-inch chromoly shaft. Bardis builds the wheel modules by attaching arc sections to the exterior of prism-shaped aluminum and polycarbonate "pods." Bardis briefly considered cylindrical wheel modules, but instead chose the structural strength and visual impact of a triangular cross section. Because he couldn't run wires between pods that rotate relative to one another, each hollow pod houses a complete powertrain and can move independently on the shaft—and a set of self-aligning bearings—for tank-style steering.
Towering Inferno tips the scales right at the 210-lb weight limit. Bardis says he felt comfortable playing so close to the limit in part because he designed the robot in Pro/E and used the CAD software to keep a running tally of weight as he went through the design and building process. "Pro/E got me within one-tenth of a pound," he says.
Weapon: Bardis hopes to pound his way to victory with a pair of sharpened Home Depot hammers mounted on 32-inch arms that rotate about the same shaft as the wheel modules and normally trail behind the wheels. During an attack, the hammer swings over the top of the robot. Unlike some earlier designs, which depended on robot momentum to swing the hammer down, Towering Inferno can swing its hammers from a dead stop. In fact, when Bardis is feeling really mean, he can even flail the hammers by modulating the motors.
Locomotion: In an innovative design twist, the hammers also do double duty as a drivetrain component. "The weapons are the means of locomotion," Bardis says. Rather than driving the wheels directly, Bardis instead uses Towering Inferno's 6:1 chain reduction to drive the hammers into the floor. As the hammers push downward, Bardis explains, they set up a moment about the tube, rotating the wheels and propelling the robot in the direction opposite the hammers. Bardis himself describes his hammer drive as "a bit weird," but there's method behind his weirdness: Integrating the weapons and drive systems keeps component count and weight in check.
Towering Inferno runs on a pair of 24-V, 1.33-hp motors and a 24-V Hawker lead-acid batteries. Bardis says he likes these batteries because they combine two important aspects for a BattleBot power source: "They pump out a lot of amps and charge quickly." The first attribute allows the motors to operate at or near stall—a common condition as robots try to push each other around the ring. The second attribute lets Towering Inferno top off its batteries in the scant 20 minutes that separate BattleBot rounds.
Engineering challenges: Coming up with Towering Inferno's hammer-drive mechanism took up most of Bardis' initial engineering time. To make sure the concept would work, he first prototyped the robot in large LEGO blocks. Once he knew the basics would work, he set out to design the biggest chain reduction that would fit between each pod's motor and drive shafts. "I wanted as big a reduction as possible to get good acceleration and torque on the hammers," he says. "Because a lot of these contests turn into shoving matches, acceleration and torque are key."
Sponsors: Bell Everman handled all the CNC machining, including some relatively complex brackets that hold the prism pods together. Specialty Tool chipped in fasteners and tools. And PTC contributed a Pro/ENGINEER license.
Vlad the Impaler
Bio: The heavyweight champ in 1998, Vlad the Impaler is the creation of Gage Cauchois, a lighting designer turned robot master. In this year's heavy weight competition, Cauchois will compete with a revamped version of the same robot. "It's called Vlad 2001," he says.
Specs: At 208 lbs, Vlad consists of a steel frame clad with T6 aluminum and 3/16-inch polycarbonate sheet. The robots box-like housing, which is put together with bolts and screws, rides around on 12-inch polyurethane wheels. Except for stronger 7075 aluminum armor, Vlad 2001 uses similar materials and basic construction. The new design, however, has no dedicated top or bottom. Symmetrical about its horizontal axis, the robot runs equally as well if it's flipped completely over, according to Cauchois.
Weapon: Vlad gets its punch and its name from a front-mounted pneumatic lever arm whose sharpened prong threatens to pierce or flip any robot that gets in the way. A pair of CO2 cylinders and a cable-and-pulley mechanism allow the lever to lift up to 400 lbs, Cauchois reports. On Vlad 2001, the prong still works as a lever arm, but it's double acting so it can work regardless of which side of the robot is up. Attached at the robot's centerline, the new prong weapon dips downward to form a wedge when the lifting arm isn't in use. With a potential to flip over other BattleBots, the wedge gives Vlad a second means of attack.
Locomotion: A pair of 300-A, 3-hp PMDC motors and three Hawker dry-cell batteries drive Vlad—with the help of a 6:1 chain reduction. The revamped Vlad will be more powerful with custom-built 7-hp motors from a vendor that Cauchois won't reveal.
Engineering challenges: Win or lose, Vlad often takes a beating as two robots try to disable each other in an enclosed ring. "During a competition, the robots experience huge amounts of vibration, and they have to endure repeated impacts," says Cauchois. Much of the vibration originates in Vlad's chain drive. In a common BattleBot strategy, Cauchois deliberately picks undersized drive components in order to squeeze them into the robot's body and to make weight. Cauchois then runs those components well above their recommended speeds to give Vlad enough power and speed to win. Vlad's 3/4-inch, eight-tooth sprocket, for example, runs at speeds up to 4,000 rpm rather than a suggested maximum of 1,800 rpm.
"The resulting vibration goes right through the whole machine," he says. To sidestep the loosening caused by all the vibration as well as impact from other robots, Cauchois applies threadlockers like Rocky Balboa's cut man slathers on Vaseline. Medium-strength threadlockers secure all the bolts that hold the motor to the frame and also all the screws that hold the drive-train components in place. Retaining compounds keep the drive sprocket on its shaft. And Cauchois uses a wicking grade threadlocker on the screws that join armor plates. "Just about every place I could use a threadlocker or a retaining compound, I did," he says. "And nothing inside Vlad has ever come loose."
Cauchois has also found a way to dissipate the heat that plagues BattleBot motors. Some excess heat results from prolonged periods of operating motors at their stall torque. Even more comes from the common BattleBot strategy of "overvolting" motorsrunning 12-V motors at 24V to gain power at the expense of longevity. To cool Vlad's overburdened motors, Cauchois installs cooling fans he built from thrusters designed for radio-controlled airplanes. Describing the throughput of the cooling fans, Cauchois says these thrusters "are powerful enough to fly a two-pound airplane vertically."
Sponsors: Loctite Corp., which certainly shares Cauchois' fondness for threadlockers, sponsors Vlad.
Design News 04/23/2001
All content © Jason Dante Bardis and the Infernolab, 1999-2003