Combustible gases provide power to leaping soft robots

Researchers at Harvard have designed a soft robot that can leap upwards at four metres per second.

Soft robots have already been designed to stand, walk, wriggle under obstacles but the ability to jump could one day prove critical in allowing them to avoid obstacles during search and rescue operations. The research is described in a paper published in Angewandte Chemie.

‘Initially, our soft robot systems used pneumatic pressure to actuate,’ said Robert Shepherd, first author of the paper, former postdoctoral researcher in the Whitesides Research Group at Harvard, and now an assistant professor at Cornell. ‘While that system worked, it was rather slow – it took on the order of a second. Using combustion, however, allows us to actuate the robots very fast.’

Pneumatic robots are connected to tubing that pumps air, whilst the jumping robots are connected to tubes that deliver a precisely controlled mix of methane and oxygen. Using high-voltage wires embedded in each leg of the robot, researchers deliver a spark to ignite the gases, causing a small explosion that sends the robot into the air.

Among the key design innovations that allowed the combustion system to work, Shepherd said, was the incorporation of a valve into each leg of the robot.

‘We flow fuel and oxygen into the channels, and ignite it,’ Shepherd said in a statement. ‘The heat expands the gas, causing the flap to close, pressurizing the channel and causing it to actuate. As the gas cools, the flap opens and we push the exhaust out by flowing more gas in. So we don’t need to use complex valve systems, all because we chose to mould a soft flap into the robot from the beginning.’

Using combustion to power a soft robot – moulded using soft silicone – came with a number of critical questions, such as whether the soft silicone used to create the robots would survive.

‘It’s a lot more powerful, but the question we had to answer was whether it was compatible — were the temperatures compatible — with this system,’ Shepherd said. ‘What we were able to show is, because the duration of the explosion is so short, the energies absorbed by the robot are small enough to be compatible with soft robots. What’s more, the temperature of the robot increases by, on average, less than one kelvin.’

While he hopes to see internal combustion systems developed that can allow robots to walk or even run, Shepherd said jumping made sense as a starting point.

‘Because it releases so much energy so fast, it made sense for jumping to be the first ‘gait’ we explored with this system,’ he said. ‘The next step now is to learn how we can use this combustion system for other gaits, like running or even walking.’