For the last several years, Harvard has been developing a robot bee. They’ve done some impressive work: their sub-paper-clip-sized, 100-milligram flapping-wing micro aerial vehicle is fully controllable down to a stable autonomous hover. It’s still tethered for power, and there’s no onboard autonomous control, but the robot flaps its wings and flies like an insect, which is awesome.
Tiny robotic bugs have lots of potential for search and rescue, surveillance, and exploration, but what’s been all the rage recently is adaptive multi-modal robotics: robots that can creatively handle a combination of terrains, making them much more versatile. With some exceptions, robots are usually pretty bad at this, and with some exceptions, humans and animals are too. There are ground robots that can handle water, and a few flying robots that aren’t totally helpless on the ground, but so far, we haven’t seen much in the way of flying robots that are good swimmers.
Yesterday at IROS, Harvard researchers presented a paper describing how they managed to get their robotic bee to swim, which I’m pretty sure is not a thing that even real bees are known for doing. With no hardware modifications at all, Harvard’s RoboBee can fly through the air, crash land in the water, and turn into a little submarine. You know what that means: nowhere is safe from robot bees.
Harvard’s RoboBee.Photo: Wyss Institute/Harvard University
Things to keep in mind about these videos: RoboBee is small enough to sit on the tip of your finger, and light enough that you’d barely feel it if it was. When it flies (or swims), it’s doing so under full control: a motion capture system tracks its position, and sends trajectory commands to the robot. This works in both air and water, and RoboBee’s method of entry (a pitch over, dive, crash, and sink) is deliberate. Also, the hovering at the beginning of the first video below looks a little bit wonky, but that’s because RoboBee still has some water on its wings from previous tests (a more stable hover is shown in that same video, after the diving sequence, and the second video below has more details about the experiments).
The key realization here is that swimming is actually a lot like flying: in both cases, you’re trying to propel yourself through a fluid by moving a wing (or fin) back and forth. To fly (and particularly to hover) you need to do this very quickly, but to swim, it’s a much more relaxed motion. It’s fundamentally the same motion, though, and you can achieve it with the same basic hardware. In the case of RoboBee, to fly in air it flaps its wings at 120 Hz, while to swim in water it flaps its wings at just 9 Hz. Otherwise, three axis torque control is very similar, meaning that the robot can be steered around in the water, too.
One unique problem that RoboBee has with the water entry is that it’s so small that the surface tension of the water is enough to keep it from submerging. This is part of the reason that it has to crash land in water [right] (it also needs to have its wings treated with a surfactant to help it sink). A fully loaded RoboBee (with a battery on board) might be heavy enough to avoid this problem, but at this point, it’s still an issue. Also still an issue is the whole water-air transition, which seems like it’s significantly more difficult than going from air to water, but we’ve been assured that the researchers will be tackling this in future work.