Surprisingly, one of the trickiest parts of building a robot bee isn’t making it fly, or stopping it from crashing into things, but perching. Perching saves a lot of energy, especially if you’re relying on the constant buzzing of your wings to keep you from falling. Perching also means that you need to carry the extra weight of your perching "limbs" around with you.
A new paper from the Harvard Microrobotics Lab, published in the journal Science, describes how the researchers used static electricity as a temporary "glue" to allow tiny flying robots to perch anywhere. This is especially useful for small drones, because they have relatively little battery power, and perching can dramatically extend the time between charges. Birds just use their talons to either grab or teeter on any available perch, but robots, especially tiny ones, have other problems.
They need to carry heavy landing gear (the equivalent of talons), which negate the benefits to battery life, and they need to see out specific kinds of perch. A ‘bot with magnetic feet needs a ferrous metal perch, suction might require smooth surface, spikes need a soft surface, and so on. "The methods [that animals] use to perch," co-author Kevin Ma told Harvard News, "like sticky adhesives or latching with talons, are inappropriate for a paperclip-size microrobot, as they either require intricate systems with moving parts or high forces for detachment."
Static electricity, by contrast, can stick pretty much anything to anything else using an electrical charge. It sticks balloons to ceilings, polystyrene packing peanuts to pretty much anything they touch, and now it can stick robobees to their perches.
The Harvard team came up with a tiny drone design that has feet at one end, for regular landings, and a thin disk at the other, which can be charged in order to stick to any available surface—power lines, the bottom of a ledge, an overhanging branch. Once the static charge is set, the ultra-light robobee can hang indefinitely, continuing its observations until it needs to move elsewhere. The static charge is occasionally topped off, in order to make up for dissipation (a balloon will eventually drop off the ceiling as its charge dissipates).
The mechanism requires around 1,000 times less power to stick than it does to fly. This, as you can imagine, extends the mission time many times over.
Our method enables repeatable transitions from flight to perching, as well as transitions from attachment to stable hovering flight on overhanging surfaces consisting of wood, glass, or a natural leaf.
While this switchable electrostatic adhesion isn’t strong enough for larger devices, it works great for insect-sized flying machines. But the small scales involved bring their own problems. For instance, the team found that the tiny propellor driving the robobee’s flight suddenly develops 40% more lift when it gets close to a ceiling. This sudden spurt of speed was enough to make the ‘bot bounce off before the electrostatic adhesion could stick. The answer was to mount the electrostatic disk on a small block of polyurethane foam, to act as a damper.
The result is a robobee that weighs in at around 100mg, similar to the weight of a real bee.
Because the robobee’s static charge plate sits on the top of the body, it can currently only hang, but v2.0 will be designed to stick anywhere, making it even more flexible. The team lists several uses for its tiny robots—"providing a bird’s-eye view of a disaster area, detecting hazardous chemical or biological agents, or enabling secure signal transmission in ad hoc communication networks"—but these perching, insect-sized drones would also be perfect for deploying listening and watching devices. In which case, they will be vulnerable to exactly the same countermeasure as a mosquito perched on your bedroom walls, waiting for you to fall asleep: A good slap with a paper magazine.