New repair techniques allow microrobots to recover their flight performance after suffering severe damage to the artificial muscles that power their wings. – Zoo House News

Bumblebees are clumsy fliers. It is estimated that a forager bee encounters a flower about once per second, damaging its wings over time. Despite many tiny cracks or holes in their wings, bumblebees can still fly.

Flying robots, on the other hand, are not so resilient. Drill holes in the robot’s wing motors or hack off part of its propeller and chances are pretty good it’ll end up grounded.

Inspired by the robustness of bumblebees, MIT researchers have developed repair techniques that allow an insect-sized flying robot to sustain severe damage to the actuators, or artificial muscles, that power its wings — yet still fly effectively.

They optimized these artificial muscles to enable the robot to better isolate defects and overcome minor defects such as tiny holes in the actuator. In addition, they demonstrated a novel laser repair method that can help the robot recover from serious damage such as a fire that scorches the device.

Using their techniques, a damaged robot was able to maintain flight-level performance after one of its artificial muscles was hit by 10 needles, and the actuator was still able to work after a large hole was burned in it. Their repair methods allowed a robot to continue flying even after the researchers clipped 20 percent of its wing tip.

As a result, swarms of tiny robots could be better able to perform tasks in difficult environments, such as B. Performing a search mission through a collapsing building or a dense forest.

“We’ve spent a lot of time understanding the dynamics of soft, artificial muscles, and through both a new manufacturing method and new understanding, we’re able to demonstrate 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 catching up to do,” says Kevin Chen, the D. Reid Weedon, Jr. assistant professor in the Department of Electrical Engineering and Computer Science (EECS), director of the Laboratory for Soft and Microrobotics in the Research Laboratory for Electronics (RLE) and senior author of the article about these latest advances.

Chen co-wrote the work with co-lead authors and EECS 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 doctoral student; and Farnaz Niroui, EE Landsman Career Development Assistant Professor of EECS at MIT and a member of the RLE. The article appears in Science Robotics.

Robotic Repair Techniques

The tiny, rectangular robots being developed in Chen’s lab are about the same size and shape as a microcassette, although one robot weighs little 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 flap the wings rapidly. These artificial muscles are made of layers of elastomer sandwiched between two wafer-thin electrodes and then rolled into a squishy tube. When voltage is applied to the DEA, the electrodes compress the elastomer, causing the wing to flap.

But microscopic imperfections can cause sparks that burn the elastomer and lead to device failure. About 15 years ago, researchers found they could prevent DEA failures due to a tiny defect using a physical phenomenon known as self-cleaning. In this process, applying a high voltage to the DEA separates the local electrode around a small defect and isolates that defect from the rest of the electrode, allowing the artificial muscle to continue functioning.

Chen and his collaborators used this self-cleaning process in their robot repair techniques.

First, they optimized the concentration of the carbon nanotubes that make up the electrodes in the DEA. Carbon nanotubes are super strong but extremely small rolls of carbon. Fewer carbon nanotubes in the electrode improve self-cleaning as it reaches higher temperatures and burns off more easily. However, this also reduces the power density of the actuator.

“At a certain point you won’t be able to get enough power out of the system, but we need a lot of power and power to fly the robot. We had to find the sweet spot between these two constraints — self-optimizing – clearing lots on the condition that we still want the robot to fly,” says Chen.

But even an optimized DEA will fail if it takes serious damage, like a large hole that allows too much air into the device.

Chen and his team used a laser to fix larger defects. They carefully cut with a laser along the outer contours of a large defect that causes minor damage to the perimeter. Then they can burn off the easily damaged electrode by self-cleaning and isolate the larger defect.

“In a way, we’re trying to operate on muscles. But if we don’t apply enough force, we can’t do enough damage to isolate the defect. The laser causes severe damage to the actuator that cannot be eliminated,” says Chen.

The team quickly found that when “operating” on such tiny devices, it is very difficult to observe the electrode to see if they have successfully isolated a defect. Based on previous work, they incorporated electroluminescent particles into the actuator. Now, if they see lights glowing, they know part of the actuator is operational, but dark spots mean they’ve successfully isolated those areas.

Flight test successful

After perfecting their techniques, the researchers ran tests on damaged actuators — some had been hit by lots of needles, while others had burned holes in them. They measured how well the robot performs in flapping wing, launch, and hover tests.

Even with damaged DEAs, the repair techniques allowed the robot to maintain its flight performance, with errors in altitude, position, and attitude that varied only very slightly from those of an undamaged robot. With laser surgery, a DEA that would have been beyond repair could restore 87 percent of its performance.

“I have to leave it to my two students who did a lot of hard work flying the robot. Flying the robot alone is very difficult, not to mention we are now intentionally damaging it,” says Chen.

These repair techniques make the tiny robots much more resilient, so Chen and his team are now working to teach them new functions, like landing on flowers or flying in a swarm. They’re also developing new control algorithms to help the robots fly better, teaching the robots to control their yaw angle so they can maintain a constant heading, and empowering the robots to carry a tiny circuit with the longer-term goal of making its own carry energy source.

This work is funded in part by the National Science Foundation (NSF) and a MathWorks Fellowship.