Insect-sized robots can’t get far on their own. They cannot carry heavy batteries, so they are not very strong and have little endurance. Fuels have a higher energy density than batteries, but until now it has proven difficult to get a robot moving quickly and powerfully.
Researchers from Cornell University in the United States have now succeeded in doing this, they wrote on Friday in Science. They created a micro robot that hops forward at 17 centimeters per second, jumps up 20 times its length and carries 22 times its weight in payload. It is not yet completely autonomous.
Microrobots, ranging in size from a few millimeters to a few centimeters at most, could play a role in precision agriculture, environmental monitoring or rescue operations. To be really useful, they must be able to cover a sufficient distance (hundreds of meters) with enough speed (tens of centimeters per second). They must also be so cheap that they can be deployed by the hundreds at a time. Improvement is still needed in all these areas.
Potency
Chemical fuels have a much higher energy density than batteries; for example, methane contains 55 megajoules per kilo and a lithium-ion micro battery only 1 megajoule per kilo. The main barrier to using such fuels in microrobots is that they must be converted into powerful motion within the robot itself. In a car or large robot there is plenty of space for this and strong materials can be used. Not in a micro robot.
There are already some examples of fuel-powered microrobots, but the potential of the fuel is not properly expressed mechanically – they have very little power and cannot move in a targeted manner.
The Cornell researchers therefore focused on a strong actuator – a part that sets something in motion. The actuator consists of a combustion chamber with a flexible membrane on top. The fuel – a mixture of methane and oxygen – goes through a tube to the combustion chamber. Two electrodes ignite the mixture with a spark and the heat causes gases created in the combustion chamber to expand, causing the membrane to bulge. The gas then escapes through a valve.
Promising
A single actuator is not a robot, but four actuators together are. The researchers made the four-legged robot (29 millimeters long and 1.6 grams) walk over surfaces and obstacles and jump into the air. By making the actuators move independently of each other, the robot can be steered in a certain direction. In this way he hops 17 centimeters per second. If all four legs come into action at the same time, the robot jumps 59 centimeters high. This robot performs a hundred times better than previous fuel-powered robots.
“The actuators are promising, but there are still some challenges. They are still connected to external hardware,” says Ryan Truby, assistant professor in the Robotic Matter Lab at Northwestern University, United States, in an accompanying commentary in Science. The earlier – slower – fuel-powered micro robots were autonomous.
“The fuel is still supplied here via a hose. It may be possible to bring a reservoir on board, but you must do so carefully, because the fuel is sensitive to temperature fluctuations in the environment, for example. And ultimately, some kind of battery is needed, for the spark that ignites the fuel.