Bumblebees are clumsy fliers. It is estimated that a foraging bee bumps into a flower about once every second, damaging its wings over time. But despite having many small tears or holes in their wings, bumblebees can still fly.
Aerial robots, on the other hand, are less resilient. Punch a hole in a robot’s wing motors or cut off part of its propeller, and there’s a good chance it’ll be grounded.
Because of the durability of bumblebees, MIT researchers have developed repair techniques that allow a bug-sized aerial robot to sustain severe damage to the actuators, or artificial muscles, that power the its wings — but still fly effectively.
They optimized these artificial muscles so that the robot can better isolate defects and overcome small damage, such as small holes in the actuator. In addition, they demonstrated a new laser repair method that can help the robot recover from severe damage, such as a fire that burns the device.
Using their techniques, a damaged robot could maintain flight-level performance after one of its artificial muscles was pierced with 10 needles, and the actuator still functioned after a large hole was burned through it. Their repair methods allowed a robot to continue flying even after the researchers cut off 20 percent of its wing tip.
This could make swarms of small robots better able to perform tasks in difficult environments, such as conducting a search mission in a collapsed building or dense forest.
“We have spent a lot of time understanding the dynamics of soft, artificial muscles and, through both new fabrication methods and a new understanding, we can demonstrate a level of resilience to injury 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 catch up on,” said Kevin Chen , the D. Reid Weedon, Jr. Assistant Professor in the Department of Electrical Engineering and Computer Science (EECS), the head of the Soft and Micro Robotics Laboratory in the Research Laboratory of Electronics (RLE), and the senior author of the paper on these latest advances.
Chen wrote the paper with co-lead authors and EECS graduate students Suhan Kim and Yi-Hsuan Hsiao; Younghoon Lee, a postdoc; Weikun “Spencer” Zhu, a graduate student in the Department of Chemical Engineering; Zhijian Ren, an EECS graduate student; and Farnaz Niroui, the EE Landsman Career Development Assistant Professor of EECS at MIT and an RLE member. The article will appear on Science Robotics.
Robot repair techniques
The small, rectangular robots developed in Chen’s lab are about the same size and shape as a microcassette tape, though a robot is about the size of 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 layers of elastomer that are sandwiched between two razor-thin electrodes and then combined into a squishy tube. When voltage is applied to the DEA, the electrodes compress the elastomer, which flaps the wing.
But microscopic imperfections can cause sparks that burn the elastomer and cause the device to fail. About 15 years ago, researchers discovered that they could prevent DEA failures from a small defect using a physical phenomenon known as self-clearing. In this process, applying a high voltage to the DEA disconnects the local electrode around a small defect, separating that failure from the rest of the electrode so that the artificial muscle can still function.
Chen and his collaborators used this self-clearing process in their robot repair techniques.
First, they optimized the concentration of carbon nanotubes that make up the electrodes in DEA. Carbon nanotubes are very strong but very small rolls of carbon. Having fewer carbon nanotubes in the electrode improves self-clearing, as it reaches higher temperatures and burns more easily. But it also reduces the power density of the actuator.
“At a certain point, you can’t get enough energy from the system, but we need a lot of energy and power to fly the robot. We had to find the optimal point between these two constraints — optimize the self-clearing property under the constraint that we still want the robot to fly,” Chen said.
However, even an optimized DEA will fail if it suffers severe 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 carefully cut the outer contours of a large defect with a laser, causing minor damage around the perimeter. Then, they can use self-clearing to burn off the partially damaged electrode, isolating the larger defect.
“In a way, we are trying to do surgery on muscles. But if we don’t use enough power, then we can’t do enough damage to isolate the defect. On the other hand, if we use too much power, the The laser will cause severe damage to the actuator that cannot be cleared,” Chen said.
The team soon realized that, when “operating” such small devices, it was very difficult to observe the electrode to see if they had successfully isolated a defect. Drawing on previous work, they incorporated electroluminescent particles into the actuator. Now, if they see light shining, they know that part of the actuator is working, but the dark patches mean they’ve successfully isolated those areas.
Flight test success
Once they perfected their techniques, the researchers performed tests on the damaged actuators — some were pierced with multiple needles while others had holes burned into them. They measured how well the robot performed in wing flapping, take-off, and hovering experiments.
Even with damaged DEAs, the repair techniques allowed the robot to maintain its flight performance, with altitude, position, and attitude errors slightly deviating from the undamaged robot. With laser surgery, a DEA that would have been damaged beyond repair was able to regain 87 percent of its function.
“I have to give it to my two students, who worked really hard when they flew the robot. Flying the robot by ourselves was very difficult, not to mention now that we accidentally broke it,” Chen said.
These repair techniques make the tiny robots more stable, so Chen and his team are now working on teaching them new functions, such as landing on flowers or flying on a swarm. They are also developing new control algorithms to make robots fly better, teach robots to control their yaw angle so they maintain a constant heading, and enable robots to carry a small circuit, with the long-term goal of bringing its own source of power.
This work was funded, in part, by the National Science Foundation (NSF) and a MathWorks Fellowship.