To Give a Robot Cheetah-Like Handling, Engineers Added a Tail

It’s always cool when engineers mimic the biological world, and the little car above highlights one of my favorite examples yet: In order to turn faster without tipping over, researchers at the University of Cape Town added a cheetah-like tail to the back end of the car.

Machines can have wide-ranging advantages over living creatures, such as edges in power and endurance. But biological creatures do have one pretty major trump card: hundreds of millions of years’ worth of evolutionary R&D. In this case, the team was inspired by the way cheetahs’ tails act as a counterbalance to help them steer through tight turns at high speed. As Amir Patel and Martin Braae note in their YouTube demo, adding the tail to counteract body roll allowed their robot car to more than double its speed through a 30° turn from 3.1 m/s to 7.5. m/s.

The work was presented at IROS 2013, a robotics and intelligent systems conference hosted by IEEE and RSJ. Writing at IEEE Spectrum’s robotics blog, Evan Ackerman explains that the bot—named Dima, which was “derived from a Sotho word that means ‘flash of lightning’”—was inspired by landmark research presented by UC Berkeley engineers at last year’s IROS involve the use of tails in lizard locomotion.

The Berkeley team did an exhaustive biomechanic analysis of how lizards’ tails play into their own body motion, as observed with the help of high-speed cameras. Details of their work was published in Nature as well as IEEE Xplore, but the short version is that lizards do use their tails to aid in balance and movement. One key finding was that lizards can move their tails to correct for pitch when jumping; for example, if a lizard slips while jumping and ends up flying nose-up towards a target, it can and will flick its tail upward to bring its nose down. The end result was a daredevil robot that uses a tail to help fly true.

But instead of using a tail to control for pitch, Patel and Braae’s Dima uses a tail to counteract yaw, which might have more immediate practical implications—unless cars start jumping more often. Any vehicle making a turn will roll (yaw) towards the outside of the turn. It’s simple physics, and the reason race cars have stiff suspensions to counteract that roll. Less roll means all four tires are used more effectively, which means more grip and more speed. 

At about the 1:30 mark, you can see a cheetah using its tail as a rudder in slow motion.

It’s also the reason we lean into turns while running or surfing. We’re counteracting the mid-turn forces caused by conservation of momentum. Cheetahs lean while they run, too, but they’ve also got added advantage: Their long tails. The kinetics of cat tails have been well studied, and videos show that cheetahs’ long tails act as a rudder when making tight turns, helping pull them in their desired direction.

Patel and Braae added their own version of a tail, which acts as a counterbalance to Dima’s yaw in high-speed turns. The reason I suggest this is more immediately practical, at least from an automotive standpoint, than a pitch-controlling tail is that controlling yaw is already a major focus of vehicle safety engineering.

Remember when all those Ford Explorers were flipping over? Automakers sure do. Eliminating roll means that not only are cars less likely to flip when they’re out of control, but also that they easier to control at the limit in the first place. If you’ve ever seen videos of enormous cars from the 60s and 70s trying to make quick maneuvers, you’ll recognize that massive body roll leads to horribly imprecise and dangerous handling.

Nü-metal aside, this is a great comparison of sloppy old suspension technology and how high body roll is dangerous.

The best solution is to lower a car’s center of gravity, which helps decrease a car’s roll moment, or the theoretical lever arm that causes a car to roll from side to side. Because not all cars can be low to the ground—Americans still love SUVs and all—things like adaptive yaw control and active suspensions have been developed to keep cars planted in unsafe situations.

But what about another step beyond? Adding huge tails to cars may help handling, but also means adding a large flying weight to the rear end of your mom’s BMW—hardly safe in traffic. Yet the concept could still live on. Instead of a long lever arm, why not add in counterbalancing in the form of a spinning flywheel?

Torque is not to be messed with.

Flywheels are already making their way into sports car for kinetic-energy recovery systems (KERS), and perhaps it’s possible that a flywheel designed to rotate against the yaw axis of a car could help stabilize a vehicle. It’s almost assuredly impractical for a passenger car, but perhaps not for one on a race track.

Ackerman closed his story about Dima by asking if a tail could be legal for Formula 1 cars. Although a tail would be restricted by chassis dimension rules, it’s perhaps not impossible. Imagine a track that has more right turns than left. If a car’s KERS flywheel was set up to rotate against the yaw caused by right turns—and perhaps even eliminated for left turns—it might gain a precious time advantage. Of course, this is all completely hypothetical, but we’re already talking about robot cars with cheetah tails, so why not?

@derektmead