The Multimodal Autonomous Drone (S-MAD) is a fixed-wing drone that has a few bird-like tricks up its sleeve. You can, for example, fly it like a glider through a room or open space, but when it approaches a flat surface the drone quickly changes configuration and lands flat with its little spiky teeth digging in to keep it from falling. In short, this is one of the scariest robotic behaviors I’ve seen since Big Dog galumphed its way into our nightmares.
The S-MAD uses something called microspines to attach itself to rough surfaces. The spines are essentially hardened steel spikes that grip small bumps in a surface from two directions — “the opposed-grip strategy for microspines is just like a human hand grasping a bottle of water, except that while humans require some macroscopic curvature to get our fingers around both sides of an object, the microspines can go deep into the micro-features of a rough surface and latch on those tiny bumps and pits,” said researcher Hao Jiang of Stanford. These spines are already being used on multi-rotor drones, but this is the first time they’ve been used on a fixed-wing device. The plane now lands on surfaces 100 percent of the time, an impressive feat for such a drone.
With these microspines, the plane can flatten itself against a wall and perch there, gathering data and scanning the environment. When it’s ready to move on, it releases the spines and flies off into the wild blue. Researchers Dino Mehanovic, John Bass, Thomas Courteau, David Rancourt and Alexis Lussier Desbiens from the University of Sherbrooke decided to connect these spines with a fixed-wing drone and had to create a new way to essentially stop the plane in mid-air — something birds are quite adept at — and settle on a surface. The system switches from plane to helicopter in a split second, allowing it to flatten against the wall instantly. From Spectrum:
There are several tricks to this. The first trick is the pitch-up maneuver, which turns the fixed-wing airplane into a temporary helicopter of sorts, relying entirely on the propellor for lift generation (the thrust to weight ratio is 1.5) while the wings provide enough of a control surface to cancel out the torque. At that point, the UAV can approach the wall as slowly as you like (using a laser rangefinder for wall detection), which leads to the second trick: maximizing the “zone of suitable touchdown conditions,” or making sure that the approach is slow enough and steady enough that you can perch reliably with little hardware (sensing and otherwise). And the third trick is having a perching system, legs and microspines in this case, that are flexible enough to achieve a robust perch even if the aircraft isn’t doing exactly what you’d like it to be doing.
This is obviously a proof of concept, but it could be used in situations where long-distance glides terminate in a permanent perch at some high point for data collection. After the data is collected, the plane can essentially fall off the surface and fly back by righting itself, climbing to altitude and gliding home.