Wheels Are Better Than Feet for Legged Robots

As much as we like to go on about bio-inspired robots (and we do go on about them), there are some things that nature hasn't quite figured out yet. Wheels are almost one of those things-while some animals do roll, and have inspired robots based on that rolling, true wheeled motion isn't found in nature above the microscopic level. When humans figured out how useful wheels were, we (among other things) strapped them to our feet to make our motion more efficient under certain conditions, which really showed nature who was boss. Our smug wheeled superiority hasn't lasted very long, though, because robots are rapidly becoming more skilled with wheels than we can ever hope to be.

The key difference between a human on roller skates and a robot on actuated wheels is that the robot, if it's engineered properly, can exert control over its wheels with a nuance that we'll never be able to match. We've seen this in action with Boston Dynamics' Handle, Handle, although so far, Handle hasn't seemed to take full advantage of the fact that it's got legs, too. To understand why wheels and legs together are such a game-changer for robotic mobility, we can take a look at ANYmal, which seamlessly blends four legs and four wheels together with every movement it makes.

The really cool thing here is that ANYmal is dynamically choosing an optimal hybrid gait that's a fusion of powered rolling and legged stepping. It's doing this blind," without any camera or lidar inputs, just based on the feel of the terrain underneath its wheels. You can see how it transitions seamlessly between rolling and stepping, even mid-stride, based on how much utility the wheeled motion has on a per-leg basis-if a wheel stops being efficient, the controller switches that leg to a stepping motion instead, while maintaining coordination with the other legs. Overall, this makes ANYmal move more quickly without reducing its ability to handle challenging terrain, and reduces its cost of transport since rolling is much more efficient than walking.

For more details, we spoke with Marko Bjelonic from ETH Zurich.

IEEE Spectrum: Are there certain kinds of terrain that make these ANYmal's gait transitions particularly challenging?

Marko Bjelonic: Aperiodic gait sequences are automatically found through kinematic leg utilities without the need for predefined gait timings. Based on the robot's current situation, each leg can reason on its own when it is a good time to lift off the ground. Our approach works quite well in rough terrain, but more considerable obstacles, e.g., stairs, are challenging.

How much of a difference do you think incorporating sensors to identify terrain would make to ANYmal's capability?

Our submitted publication is only based on proprioceptive signals, i.e., no terrain perception is used to make gait transitions based on the perceived environment. We are surprised how well this framework already works on flat and uneven terrain. However, we are currently working on an extension that considers the terrain upfront for the robot to plan the stepping sequences. This terrain-responsive extension is capable of handling also larger obstacles like stairs.

My experience shows me that the current version of ANYmal with actuated wheels improves mobility drastically. And I believe that these kinds of robots will outperform nature first. There is no animal or human being that can exploit such a concept." -Marko Bjelonic, ETH Zurich

How many degrees of freedom do you think are optimal for a hybrid robot like ANYmal? For example, if the wheels could be steerable, would that be beneficial?

It is a nice challenge to have no steerable wheels, because then the robot is forced to explore hybrid roller-walking motions. From an application perspective, it would be beneficial to have the possibility of steering the wheels. We already analyzed the leg configuration and the amount of actuation per leg and found that no additional degrees of freedom are necessary to achieve this. We can rotate the first actuator, the hip adduction/abduction, and without increasing the complexity, we increase the robot's mobility and add the possibility of steering the wheels.

What are the disadvantages of hybrid mobility? Why shouldn't every legged robot also have wheels?

Every legged robot should have wheels! I think it's going to be more common in the future. There are currently only a few hybrid mobility concepts out there, e.g., the roller-walking ANYmal, the CENTAURO robot, and Handle from Boston Dynamics. The additional degrees of freedom and missing counterparts in nature make designing locomotion capabilities for wheeled-legged robots more challenging. This is one reason why we do not see more of these creatures. But I am sure that more concepts will follow with the current advancements in this field.

What are you working on next?

We are working on an artistic framework enabling the robot more complex motions on the ground and over challenging obstacles. The challenge here is how to find optimal maneuvers for such high-dimensional problems and how to execute these motions on the real robot robustly.

Whole-Body MPC and Online Gait Sequence Generation for Wheeled-Legged Robots," by Marko Bjelonic, Ruben Grandia, Oliver Harley, Cla Galliard, Samuel Zimmermann, and Marco Hutter from ETH Zurich, is available on arXiv.