Why is the military’s elite research arm so interested in robots with legs? It isn’t speed.
Boston Dynamics’ Cheetah robot, funded by DARPA, made headlines after it broke its own speed record yesterday and became the first robot to run on legs faster than the fastest human, track star Usain Bolt. Cheetah got up to 28.3 miles per hour . Sure, that was on a treadmill in a lab, with an external brace to keep Cheetah from falling over; but other, much slower Boston Dynamics robots like the “Big Dog” have already solved the balance problem and can walk on their own four feet over rough ground, even ice. So the obvious next step is to combine the two technologies to build a well-balanced, fast-running robot. But why?
Cheetah is impressive and a little scary, but if the military simply wanted speed, guess what: They’ve already got killing machines that are more than 10 times faster. OK, so they fly and are called Predator and Reaper, but they are still remotely controlled killing machines that go fast. Even on the ground, wheeled vehicles go a lot faster than anything with legs, and, yes, the military is working on wheeled robots too.
Countless science-fiction fans justify legged war machines like Star Wars’ famous Imperial Walkers by saying legs would work better than wheels over rough terrain. But we already have a technology to move vehicles over broken ground, and it’s called tracks: Tanks have been using it since 1916. In fact, tracks do better than feet over soft ground, because they spread the vehicle’s weight out over a larger area, keeping it from getting stuck. A 70-ton M1 Abrams tank exerts less ground pressure per square inch than your family car, it just covers a lot more square inches.
So what’s the point of legs? Climbing.
Boston Dynamics vice-president Robert Playter explained it to me at the AUVSI robotics conference in Las Vegas, where I filmed the video above.
Wheeled vehicles have to drive over obstacles. So for a wheel to get over a vertical obstacle — like a wall or a step — its axle has to be at least as high off the ground as the top of the obstacle, which means a wheeled vehicle has to be at least twice as tall as the tallest vertical obstacle it has to cross. The math for tracks is similar; tanks can’t drive over walls higher than themselves.
But a vehicle with limbs can climb vertical obstacles higher than itself. It can do that, Playter explained, because the front leg (or arm) can reach up — above the shoulder joint where it’s attached — to grab the top of the obstacle and then pull the body over. A wheeled or tracked vehicle has to keep those wheels or tracks underneath its chassis at all times or it flips over, but with legs, “you kind of throw your center of mass around,” Playter said, which is the reason why “Dynamics” is in the company’s name in the first place.
Boston Dynamics’ RHex robot shows this climbing capability at work in real-world battlefields today. The Army Rapid Equiping Force has already bought four to use in Afghanistan, particularly to climb in and out of ditches and culverts where insurgents love to hide roadside bombs but where the wheel-driven robots used by most bomb squads cannot go. Weighing only 30 pounds and less than one foot tall, the six-legged RHex looks like a cross between a mechanical puppy and a cockroach, cute and creepy by turns. It doesn’t have the glamor of its four-legged, 28-mph cousin. But if you want to see the future of robots that walk on legs, at least in the near term, look to the cockroach, not the cheetah.