Caltech’s LEO Flying Biped Can Skateboard and Slackline

Two years of progress gives this multimodal robot impressive new capabilities

6 min read

Evan Ackerman is IEEE Spectrum’s robotics editor.

Robot with legs and propellers hovers above a rock formation

Back in February of 2019, we wrote about a sort of humanoid robot thing (?) under development at Caltech, called Leonardo. LEO combines lightweight bipedal legs with torso-mounted thrusters powerful enough to lift the entire robot off the ground, which can handily take care of on-ground dynamic balancing while also enabling some slick aerial maneuvers.

In a paper published today in Science Robotics, the Caltech researchers get us caught up on what they've been doing with LEO for the past several years, and it can now skateboard, slackline, and make dainty airborne hops with exceptionally elegant landings.

Those heels! Seems like a real sponsorship opportunity, right?

The version of LEO you see here is significantly different from the version we first met two years ago. Most importantly, while "Leonardo" used to stand for "LEg ON Aerial Robotic DrOne," it now stands for "LEgs ONboARD drOne," which may be the first even moderately successful re-backronym I've ever seen. Otherwise, the robot has been completely redesigned, with the version you see here sharing zero parts in hardware or software with the 2019 version.

The differences between the new version of LEO and the original version are largely due to the fact that the original robot was designed to use its thrusters for jumping augmentation as opposed to sustained flight. That is, the LEO prototype from 2019 did not work in the same way as the version we're writing about today, because it was designed with a different approach to hybrid mobility in mind.

To enable the new LEO to achieve sustained flight, it now has much lighter weight legs driven by lightweight servo motors. The thrusters have been changed from two coaxial propellers to four tilted propellers, enabling attitude control in all directions. And everything is now onboard, including computers, batteries, and a new software stack. I particularly love how LEO lands into a walking gait so gently and elegantly. Professor Soon-Jo Chung from Caltech's Aerospace Robotics and Control Lab explains how they did it:

Creatures that have more than two locomotion modes must learn and master how to properly switch between them. Birds, for instance, undergo a complex yet intriguing behavior at the transitional interface of their two locomotion modes of flying and walking. Similarly, the Leonardo robot uses synchronized control of distributed propeller-based thrusters and leg joints to realize smooth transitions between its flying and walking modes. In particular, the LEO robot follows a smooth flying trajectory up to the landing point prior to landing. The forward landing velocity is then matched to the chosen walking speed, and the walking phase is triggered when one foot touches the ground. After the touchdown, the robot continues to walk by tracking its walking trajectory. A state machine is run on-board LEO to allow for these smooth transitions, which are detected using contact sensors embedded in the foot.

A black bipedal robot with a round head and four thrusters standing on the ground

It's very cool how Leo neatly solves some of the most difficult problems with bipedal robotics, including dynamic balancing and traversing large changes in height. And Leo can also do things that no biped (or human) can do, like actually fly short distances. As a multimodal hybrid of a bipedal robot and a drone, though, it's important to note that Leo's design includes some significant compromises as well. The robot has to be very lightweight in order to fly at all, which limits how effective it can be as a biped without using its thrusters for assistance. And because so much of its balancing requires active input from the thrusters, it's very inefficient relative to both drones and other bipedal robots.

When walking on the ground, LEO (which weighs 2.5kg and is 75cm tall) sucks down 544 watts, of which 445 watts go to the propellers and 99 watts are used by the electronics and legs. When flying, LEO's power consumption almost doubles, but it's obviously much faster—the robot has a cost of transport (a measure of efficiency of self-movement) of 108 when walking at a speed of 20 cm/s, dropping to 15.5 when flying at 3 m/s. Compare this to the cost of transport for an average human, which is well under 1, or a typical quadrupedal robot, which is in the low single digits. The most efficient humanoid we've ever seen, SRI's DURUS, has a cost of transport of about 1, whereas the rumor is that the cost of transport for a robot like Atlas is closer to 20.

Long term, this low efficiency could be a problem for LEO, since its battery life is good for only about 100 seconds of flight or 3.5 minutes of walking. But, explains Soon-Jo Chung, efficiency hasn't yet been a priority, and there's more that can potentially be done to improve LEO's performance, although always with some compromises:

The extreme balancing ability of LEO comes at the cost of continuously running propellers, which leads to higher energy consumption than leg-based ground robots. However, this stabilization with propellers allowed the use of low-power leg servo motors and lightweight legs with flexibility, which was a design choice to minimize the overall weight of LEO to improve its flying performance.

There are possible ways to improve the energy efficiency by making different design tradeoffs. For instance, LEO could walk with the reduced support from the propellers by adopting finite feet for better stability or higher power [leg] motors with torque control for joint actuation that would allow for fast and accurate enough foot position tracking to stabilize the walking gait. In such a case, propellers may need to turn on only when the legs fail to maintain stability on the ground without having to run continuously. These solutions would cause a weight increase and lead to a higher energy consumption during flight maneuvers, but they would lower energy consumption during walking. In the case of LEO, we aimed to achieve balanced aerial and ground locomotion capabilities, and we opted for lightweight legs. Achieving efficient walking with lightweight legs similar to LEO's is still an open challenge in the field of bipedal robots, and it remains to be investigated in future work.

A rendering of a future version of LEO with fancy yellow skinsA rendering of a future version of LEO with fancy yellow skins by artist Sam Binkin.

At this point in its development, the Caltech researchers have been focusing primarily on LEO's mobility systems, but they hope to get LEO doing useful stuff out in the world, and that almost certainly means giving the robot autonomy and manipulation capabilities. At the moment, LEO isn't particularly autonomous, in the sense that it follows predefined paths and doesn't decide on its own whether it should be using walking or flying to traverse a given obstacle. But the researchers are already working on ways in which LEO can make these decisions autonomously through vision and machine learning.

As for manipulation, Chung tells us that "a new version of LEO could be appended with lightweight manipulators that have similar linkage design to its legs and servo motors to expand the range of tasks it can perform," with the goal of "enabling a wide range of robotic missions that are hard to accomplish by the sole use of ground or aerial robots."

Perhaps the most well-suited applications for LEO would be the ones that involve physical interactions with structures at a high altitude, which are usually dangerous for human workers and could use robotic workers. For instance, high voltage line inspection or monitoring of tall bridges could be good applications for LEO, and LEO has an onboard camera that can be used for such purposes. In such applications, conventional biped robots have difficulties with reaching the site, and standard multi-rotor drones have an issue with stabilization in high disturbance environments. LEO uses the ground contact to its advantage and, compared to a standard multi-rotor, is more resistant to external disturbances such as wind. This would improve the safety of the robot operation in an outdoor environment where LEO can maintain contact with a rigid surface.

It's also tempting to look at LEO's ability to more or less just bypass so many of the challenges in bipedal robotics and think about ways in which it could be useful in places where bipedal robots tend to struggle. But it's important to remember that because of the compromises inherent in its multimodal design, LEO will likely be best suited for very specific tasks that can most directly leverage what it's particularly good at. High voltage line and bridge inspection is a good start, and you can easily imagine other inspection tasks that require stability combined with vertical agility. Hopefully, improvements in efficiency and autonomy will make this possible, although I'm still holding out for what Caltech's Chung originally promised: "the ultimate form of demonstration for us will be to build two of these Leonardo robots and then have them play tennis or badminton."

Editor's note: this article has been updated to more accurately reflect the relationship between the 2019 version of LEO and the current version.

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