Bumblebees are clumsy fliers. It is estimated that a foraging bee bumps into a flower once per second, which damages its wings over time. However, even though there are many small slits or holes in the wings, bumblebees can still fly.
On the other hand, aerial robots are not very durable. Switch a robot’s wing motors or cut off part of its motor and chances are pretty good it will ground.
Inspired by the resilience of bumblebees, MIT researchers have developed a repair technique that allows a bug-sized aerial robot to sustain severe damage to the motors or artificial muscles that power its wings, but still fly effectively.
They optimized these artificial muscles so that the robot can better isolate defects and cope with minor damage, such as small holes in the motor. Additionally, they demonstrated a new laser repair method that can help a robot recover from severe damage, such as a fire that incinerates the device.
Using their technique, the damaged robot was able to maintain level flight after one of its artificial muscles was punctured with 10 needles, and the engine was still able to function after a large hole was burned into it. Their repair methods enabled the robot to continue flying even after researchers cut off 20 percent of its wing tip.
This could make swarms of tiny robots better at performing tasks in harsh environments, such as conducting a search mission through a collapsing building or a dense forest.
“We’ve spent a lot of time understanding the dynamics of soft, artificial muscle, and with both a new preparation method and a new understanding, we can show a level of resilience to damage that’s comparable to insects. We are very excited about it. But insects are still superior to us in the sense that they can lose up to 40 percent of their wings and still fly. We still have some work to do,” says Kevin Chen, D.D. Reed Whedon. is a junior assistant professor in the Department of Electrical Engineering and Computer Science (EECS), head of the Soft and Microrobotics Laboratory in the Research Laboratory of Electronics (RLE), and senior author of a paper on these recent advances.
Chen wrote the paper with co-authors and EECS graduate students Suhan Kim and Yi-Hsuan Hsiao; Yanghoon Lee, postdoc; Weikun “Spencer” Zhu, PhD, Department of Chemical Engineering; Zhijian Ren, EECS PhD student; and Farnaz Nirui, EE Landsman Assistant Professor of Career Development, EECS and RLE Fellow at MIT. The article will appear Scientific robotics.
Robot repair techniques
The tiny, rectangular robots being developed in Chen’s lab are about the same size and shape as a microcassette tape, although a single robot weighs barely more than a paper clip. The wings at each corner are powered by dielectric elastomer actuators (DEAs), which are soft artificial muscles that use mechanical forces to rapidly flap the wings. These artificial muscles are made from strips of elastomer that are sandwiched between two razor-thin electrodes and then rolled into a fragile tube. When voltage is applied to the DEA, the electrodes compress the elastomer that wraps the sleeve.
But microscopic flaws can cause sparks that burn the elastomers and cause the device to malfunction. About 15 years ago, researchers discovered they could prevent DEA failures from a single tiny flaw by using a physical phenomenon known as self-cleaning. In this process, applying a high voltage to the DEA disconnects the local electrode around the small defect, isolating that failure from the rest of the electrode, so the artificial muscle still works.
Chen and his colleagues applied this self-cleaning process to their robot repair technique.
First, they optimized the concentration of carbon nanotubes that comprise the DEA electrodes. Carbon nanotubes are super-strong but extremely small coils of carbon. Having fewer carbon nanotubes in the electrode improves self-cleaning because it reaches higher temperatures and burns more easily. But this also reduces the power density of the actuator.
“At some point you won’t be able to get enough energy out of the system, but we need a lot of energy and power to fly the robot. We had to find the sweet spot between these two constraints to optimize the identity. – property clearing under the constraint that we still want the robot to fly,” Chen says.
However, even an optimized DEA will fail if it suffers from serious damage, such as a large hole that lets too much air into the device.
Chen and his team used a laser to overcome the large defects. They use a laser to precisely cut the outer contours of a large defect, causing little damage around the perimeter. They can then burn off a slightly damaged electrode through self-cleaning, isolating the larger defect.
“In a way, we’re trying to do muscle surgery. But if we don’t use enough force, we can’t do enough damage to isolate the defect. On the other hand, if we use too much force, the laser will cause serious damage to the actuator that will not be cleaned,” says Chen.
The team soon realized that when “working” on such small devices, it was very difficult to observe the electrode to determine whether they had successfully isolated the defect. Building on previous work, they incorporated electroluminescent particles into the engine. Now, if they see a light shining, they know that part of the engine is working, but the dark spots mean they’ve successfully isolated those areas.
Flight test success
As they perfected their technique, the researchers conducted experiments with damaged engines; some were pierced with multiple needles and others had holes burned into them. They measured how well the robot performed wing flapping, flying and hovering experiments.
Even with damaged DEAs, the repair technique enabled the robot to maintain its flight performance with altitude, attitude, and position errors that deviated very little from those of the undamaged robot. With laser surgery, the DEA, which might not have recovered, was able to regain 87 percent of its performance.
“I have to hand it to my two students who did a lot of hard work when they were flying the robot. It’s very difficult to make the robot fly by itself, let alone now that we’re intentionally damaging it,” Chen says.
This repair technique makes the tiny robots sturdier, so Chen and his team are now working to teach them new functions, such as landing on flowers or swarming. They’re also developing new control algorithms to help the robots fly better, teaching the robots to control their tilt angle so they can move continuously, and enabling the robots to wear a small chain to carry its own for a longer-term purpose. Source of nutrition.
This work is funded in part by the National Science Foundation (NSF) and a MathWorks Fellowship.