Untethered Minirobot Takes to the Air

Sometimes, size does mean everything. For engineers building miniaturized robots, the reward of overcoming obstacles such as energy supply and millimeter-scale actuators is huge. Recently, a team of engineers at the University of California Berkeley has developed one of the smallest flying robots developed to date. Smaller than a penny, with a wingspan of just 9 millimeters, this tiny bot was developed with the significant potential for applications like artificial pollination.  

The team said they studied the flight of bees and their incredible control and flexibility. Knowing they wanted to develop a robot that was untethered and controllable, their design incorporates a propeller with a stabilizing outer ring. 

“Because of its rotating propeller structure, it has this interesting gyroscopic effect that actually increases the stability of the robot,” said Liewei Lin, distinguished professor of mechanical engineering at Berkeley. “It’s just like when you're riding a bicycle. You don't need any support when you try to turn because the bicycle provides stability and actually self-balances using its center point of gravity, so it will not fall.” 

At the micro-scale, the physics of flight shifts dramatically to a low Reynolds number regime where resistant forces dominate, making it incredibly difficult for tiny robots to generate efficient lift and overcome disproportionately high drag. This necessitates innovative aerodynamic designs, explained Wei Yu, co-first author of the study and a graduate student in Liwei Lin’s lab.

"The geometry is actually optimized for working in a low Reynolds number of about 2,500,” Wei Yu said. “Since we are developing this for possible applications in agriculture, you can imagine that those very small insects like bees can fly freely in between leaves and flowers to collect nectar and bring it into another flower. They can accomplish this because they have very good flexibility and controllability even in viscous environments.” 

While remarkably tiny, this robot also exhibits impressive resilience and maneuverability, particularly in its ability to course correct and recover from collisions. Unlike many miniature robots that might be easily destabilized by impacts, its design allows it to absorb minor bumps and interactions with obstacles, quickly reorienting itself to its desired flight path. This intrinsic self-correction mechanism minimizes the need for complex, heavy onboard sensors and active control systems, further contributing to its minimalist and robust design.

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The biggest obstacle for the team was achieving optimal power without the added weight of bulky batteries. To solve this, the team took inspiration from the agility of small insects, then devised an innovative system: they affixed a pair of permanent magnets to the robot's propeller and balance ring. These magnets interact with an external, single-axis alternating magnetic field, allowing the untethered robot to fly. This setup generates a continuous magnetic torque, driving the propeller's rotation and providing the necessary lift for flight, all while circumventing the need for heavy onboard power sources and complex electronics.

They used a Helmholtz coil setup for generating the magnetic field and tested two coils versus one. 

“Two coils were used to generate a uniform magnetic field and proved that the robot could operate,” said Fanping Sui, co-first author who recently received a doctorate in engineering at UC Berkeley. “But we also tested using a single coil and then figured out how to control it in a non-uniform magnetic field.” 

As expected, the single coil system caused the robot's spinning speed to be unsteady, but when they made the magnetic field switch directions at a high operating frequency, the spinning speed became much more consistent, only varying by less than 10 percent.

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The next goal for the team, who are designing a whole suite of small insect inspired robots – from a cockroach-inspired robot that can withstand immense pressure to swarming robots that can work together—is to go even smaller, to the size of a gnat. 

"We already thought about making another one that's ten times smaller, but at that scale we don't even have the commercial magnet,” Lin said. “We probably need to use an electroplating process to put a magnetic material onto it."

There’s also the possibility that if they can make it this small, they may be able to harness environmental magnetic fields.  

“The magnetic force is proportional to the cubic scale of this flying robot,” Yu said. “The drag force is proportional to the surface area, indicating a new possibility that we could use radio waves or actual magnetic waves to actuate it.” 

By creatively leveraging external magnetic fields for power and stability, the team has not only pushed the boundaries of untethered flight but also laid crucial groundwork for a future where miniature robots could revolutionize fields from agriculture to environmental monitoring. As they continue to miniaturize and explore new actuation methods, these insect-inspired innovations promise a future where swarms of tiny, resilient robots could perform complex tasks, seamlessly integrating into our world and addressing some of our most pressing global challenges.

Cassandra Kelly is a technology writer in Columbus, Ohio.