New drone has legs for landing gear, enabling efficient launches

Most drones on the market are rotary-wing quadcopters, which can conveniently land and take off almost anywhere. The problem is they are less energy-efficient than fixed-wing aircraft, which can fly greater distances and stay airborne for longer but need a runway, a dedicated launcher, or at least a good-fashioned throw to get to the skies.

To get past this limit, a team of Swiss researchers at the École Polytechnique Fédérale de Lausanne built a fixed-wing flying robot called RAVEN (Robotic Avian-inspired Vehicle for multiple ENvironments) with a peculiar bio-inspired landing gear: a pair of robotic bird-like legs. “The RAVEN robot can walk, hop over obstacles, and do a jumping takeoff like real birds,” says Won Dong Shin, an engineer leading the project.

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The key challenge in attaching legs to drones was that they significantly increased mass and complexity. State-of-the-art robotic legs were designed for robots walking on the ground and were too bulky and heavy to even think about using on a flying machine. So, Shin’s team started their work by taking a closer look at what the leg mass budget looked like in various species of birds.

It turned out that the ratio of leg mass to the total body weight generally increased with size in birds. A carrion crow had legs weighing around 100 grams, which the team took as their point of reference.

The robotic legs built by Shin and his colleagues resembled a real bird’s legs quite closely. Simplifications introduced to save weight included skipping the knee joint and actuated toe joints, resulting in a two-segmented limb with 64 percent of the weight placed around the hip joint. The mechanism was powered by a standard drone propeller, with the ankle joint actuated through a system of pulleys and a timing belt. The robotic leg ended with a foot with three forward-facing toes and a single backward-facing hallux.

There were some more sophisticated bird-inspired design features, too. “I embedded a torsional spring in the ankle joint. When the robot’s leg is crouching, it stores the energy in that spring, and then when the leg stretches out, the spring works together with the motor to generate higher jumping speed,” says Shin. A real bird can store elastic energy in its muscle-tendon system during flexion and release it very rapidly during extension for a jumping takeoff. The spring’s job was to emulate this mechanism, and it worked pretty well—“It actually increased the jumping speed by 25 percent,” Shin says.

In the end, the robotic legs weighed around 230 grams, way more than the real ones in a carrion crow, but it turned out that was good enough for the RAVEN robot to walk, jump, take off, and fly.

Crow’s efficiency

The team calculated the necessary takeoff speed for two birds with body masses of 490 grams and a hair over 780 grams; these were 1.85 and 3.21 meters per second, respectively. Based on that, Shin figured the RAVEN robot would need to reach 2.5 meters per second to get airborne. Using the bird-like jumping takeoff strategy, it could reach that speed in just 0.17 seconds.

How did nature’s go-to takeoff procedure stack up against other ways to get to the skies? Other options included a falling takeoff, where you just push your aircraft off a cliff and let gravity do its thing, or standing takeoff, where you position the craft vertically and rely on the propeller to lift it upward. “When I was designing the experiments, I thought the jumping takeoff would be the least energy-efficient because it used extra juice from the battery to activate the legs,” Shin says. But he was in for a surprise.

“What we meant by energy efficiency was calculating the energy input and energy output. The energy output was the kinetic energy and the potential energy at the moment of takeoff, defined as the moment when the feet of the robot stop touching the ground,” Shin explains. The energy input was calculated by measuring the power used during takeoff.

“It turned out that the jumping takeoff was actually the most energy-efficient strategy. I didn’t expect that result. It was quite surprising”, Shin says.

The energy cost of the jumping takeoff was slightly higher than that of the other two strategies, but not by much. It required 7.9 percent more juice than the standing takeoff and 6.9 percent more than the falling takeoff. At the same time, it generated much higher acceleration, so you got way better bang for the buck (at least as far as energy was concerned). Overall, jumping with bird-like legs was 9.7 times more efficient than standing takeoff and 4.9 times more efficient than falling takeoff.

One caveat with the team’s calculations was that a fixed-wing drone with a more conventional design, one using wheels or a launcher, would be much more efficient in traditional takeoff strategies than a legged RAVEN robot. “But when you think about it, birds, too, would fly much better without legs. And yet they need them to move on the ground or hunt their prey. You trade some of the in-flight efficiency for more functions,” Shin claims. And the legs offered plenty of functions.

Obstacles ahead

To demonstrate the versatility of their legged flying robot, Shin’s team put it through a series of tasks that would be impossible to complete with a standard drone. Their benchmark mission scenario involved traversing a path with a low ceiling, jumping over a gap, and hopping onto an obstacle. “Assuming an erect position with the tail touching the ground, the robot could walk and remain stable even without advanced controllers,” Shin claims. Walking solved the problem of moving under low ceilings. Jumping over gaps and onto obstacles was done by using the mechanism used for takeoff: torsion springs and actuators. RAVEN could jump over an 11-centimeter-wide gap and onto an obstacle 26-centimeter-high.

But Shin says RAVEN will need way more work before it truly shines. “At this stage, the robot cannot clear all those obstacles in one go. We had to reprogram it for each of the obstacles separately,” Shin says. The problem is the control system in RAVEN is not adaptive; the actuators in the legs perform predefined sets of motions to send the robot on a trajectory the team figured out through computer simulations. If there was something blocking the way, RAVEN would have crashed into it.

Another, perhaps more striking limitation is that RAVEN can’t use its legs to land. But this is something Shin and his colleagues want to work on in the future.

“We want to implement some sensors, perhaps vision or haptic sensors. This way, we’re going to know where the landing site is, how many meters away from it we are, and so on,” Shin says. Another modification that’s on the way for RAVEN is foldable wings that the robot will use to squeeze through tight spaces. “Flapping wings would also be a very interesting topic. They are very important for landing, too, because birds decelerate first with their wings, not with their legs. With flapping wings, this is going to be a really bird-like robot,” Shin claims.

All this is intended to prepare RAVEN for search and rescue missions. The idea is legged flying robots would reach disaster-struck areas quickly, land, traverse difficult terrain on foot if necessary, and then take off like birds. “Another application is delivering parcels. Here in Switzerland, I often see helicopters delivering them to people living high up in the mountains, which I think is quite costly. A bird-like drone could do that more efficiently,” Shin suggested.

Nature, 2024.  DOI: 10.1038/s41586-024-08228-9