Tuesday, October 4, 2022

Soft actuators give flying microrobots a performance boost

In recent years, we have seen how robotics has advanced in an impressive way. Fortunately, not everything is robot dogs; the Massachusetts Institute of Technology (MIT) has been developing various inventions in the area for a long time, and one of them is aerial microrobots.

MIT researchers have developed insect-sized robots that can zip around with bug-like agility and resilience, which could eventually perform different tasks. The soft actuators that propel these microrobots are very durable, but they require much higher voltages than similarly-sized rigid actuators.

Now, researchers have pioneered a fabrication technique that enables them to build soft actuators that operate with 75% lower voltage than current versions while carrying 80% more payload. These soft actuators are like artificial muscles that rapidly flap the robot’s wings.

According to the researchers, this new fabrication technique produces artificial muscles with fewer defects, which dramatically extends the lifespan of the components and increases the robot’s performance and payload.

The team’s rectangular microrobot weighs less than one-fourth of a penny and has four sets of wings that are each driven by a soft actuator. These muscle-like actuators are made from layers of elastomer that are sandwiched between two very thin electrodes and then rolled into a squishy cylinder. When voltage is applied to the actuator, the electrodes squeeze the elastomer, and that mechanical strain is used to flap the wing.

The more surface area the actuator has, the less voltage is required. So the MIT team built these artificial muscles by alternating between as many ultrathin layers of elastomer and electrode as they could. For the first time, researchers were able to create an actuator with 20 layers, each of which is 10 micrometers in thickness, by reinventing the parts of the fabrication process.

“In this process, air comes back into the elastomer and creates a lot of microscopic air bubbles. The diameter of these air bubbles is barely 1 micrometer, so previously, we just sort of ignored them. But when you get thinner and thinner layers, the effect of the air bubbles becomes stronger and stronger. That is traditionally why people haven’t been able to make these very thin layers,” explains Kevin Chen, senior author of the paper.

The researchers came to the conclusion that the vacuum processing of the elastomer, which has not yet solidified, allows you to get rid of the bubbles. The scientists also optimized the electrodes, which consist of carbon nanotubes that are about 1/50,000 the diameter of a human hair. The more nanotubes the actuator has, the higher the actuator’s power output is.

During liftoff experiments, the 20-layer actuator, which requires less than 500 volts to operate, exerted enough power to give the robot a lift-to-weight ratio of 3.7 to 1, so it could carry items that are nearly three times its weight. They also demonstrated a 20-second hovering flight, which Chen says is the longest ever recorded by a sub-gram robot. The 20-layer actuator was still working smoothly after being driven for more than 2 million cycles, far outpacing the lifespan of other actuators.

The team plans to continue testing fabrication techniques in a clean room where it won’t have to contend with dust in the air when creating the actuator layers. While Chen is thrilled about producing 10-micrometer actuator layers, his hope is to reduce the thickness to only 1 micrometer, which would open the door to many applications for these insect-sized robots.

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