As the skies increasingly fill with buzzing, hovering drones, spare a thought for the humble hopping robot. Hopping is a common form of locomotion among insects and amphibians, and it is nearly two orders of magnitude more energy-efficient than flying. A mosquito must constantly spend energy to stay aloft; a flea works only when it jumps.

That efficiency is the pitch behind DirectHop, a roughly 1-gram robot built by engineers at the University of Washington. It fits in the palm of a hand, measures about 50 millimetres across, can perform multiple hops in sequence and jumps high enough — using a small ramp as a launch aid — to clear a standard stair step.

## Escaping the all-or-nothing spring Most hopping robots are modelled on fleas and are literally spring-loaded: they compress and release parts of an exoskeleton-like mechanism to launch themselves. Springs and latches travel impressively far, but a discharging spring is an “all or nothing” action, so jump distance is hard to calibrate. They are also complex to build and operate at small scales.

DirectHop removes the spring entirely. A tiny electric motor is attached to a tower by a length of fishing line. When the motor accelerates, it spools the line quickly and hoists itself up the tower, pulling the rest of the robot into the air; three hinged legs extend to keep the motor aligned with the robot's foot.

“We found that a very small electric motor can accelerate fast enough to power a jump directly, the way a frog's leg muscles directly power its hop,” said lead author Hanquan (John) Wang, a graduate research assistant in mechanical engineering. “We also discovered that by adjusting the current going to the motor, we could make the robot jump specific distances.” The team reports a jump-height accuracy of about one centimetre.

## Landing, righting, reloading Because the robot tends to tumble in the air, the researchers gave it a roll cage. By driving the motor back down the tower, the team shifts the robot's centre of gravity and rolls it back onto its foot — a manoeuvre inspired by box turtles, which right themselves with a similarly shaped shell. The researchers say it succeeds about 90 percent of the time and prepares the robot for another jump.

“In my mind, the three hardest challenges for hopping robots are the ability to vary jump length, self-right and reload for multiple hops,” said senior author Sawyer Fuller, a UW associate professor of mechanical engineering. “DirectHop clears all three of those hurdles with a totally new design, and we're well on our way to tackling other challenges like onboard power, steering and navigation.”

## From a lab prototype to disposable swarms The current prototype has clear limits: power arrives through wires, and it cannot steer or orient itself beyond random twitching. The team is working on onboard solar cells and a battery, a vibration motor to spin the robot in place, tiny retractable feet to adjust hop angle, and an onboard camera and electronics.

“One of our goals is fully autonomous stair climbing,” Fuller said. “A camera looks at the stair, the processor figures out how high the robot has to jump, it steers itself into position, jumps, self-rights, steers again and repeats.”

Wang estimates a fully featured DirectHop could cost around $10 given how small and cheap the components are. At that price the robots could act as semi-disposable complements to larger machines. “We could dispatch 100 or even 1,000 of these robots into the field,” he said. “If some are lost or disabled, it's not really a failure. The team of them can complete the task.” Field uses could include finding gas leaks in refineries, monitoring water and fertiliser use on farms, or exploring other planets.

The work is funded by the National Science Foundation. The team presents DirectHop on 30 September at the International Conference on Intelligent Robots and Systems (IROS).

Sources