A Compact, Versatile Robot Capable of Maneuvering Tight Spaces with a Heavy Payload

Andrew Corselli

Swimming robots play a crucial role in mapping pollution, studying aquatic ecosystems, and monitoring water quality in sensitive areas such as coral reefs or lake shores. However, many devices rely on noisy propellers, which can disturb or harm wildlife. The natural clutter in these environments — including plants, animals, and debris — also poses a challenge to robotic swimmers.

Now, researchers in the Soft Transducers Lab and the Unsteady flow diagnostics laboratory in EPFL’s School of Engineering, and at the Max Planck Institute for Intelligent Systems, have developed a compact and versatile robot that can maneuver through tight spaces and transport payloads much heavier than itself. Smaller than a credit card and weighing 6 grams, the nimble swimming robot is ideal for environments with limited space like rice fields, or for performing inspections in waterborne machines. The research has been published in Science Robotics.

“In 2020, our team demonstrated autonomous insect-scale crawling robots, but making untethered ultra-thin robots for aquatic environments is a whole new challenge,” said EPFL Soft Transducers Lab Head Herbert Shea. “We had to start from scratch, developing more powerful soft actuators, new undulating locomotion strategies, and compact high-voltage electronics.”

Here is an exclusive Tech Briefs interview, edited for length and clarity, with former EPFL researcher Florian Hartmann, now a Research Group Leader at the Max Planck Institute for Intelligent Systems in Stuttgart, Germany.

Tech Briefs: What was the biggest technical challenge you faced while developing this swimming robot?

Hartmann: The greatest challenge was to create propulsion with an undulating fin, using only a single artificial muscle. Our first approach, similar to many other larger swimming robots, was to use multiple artificial muscles (or actuators) to produce a traveling wave in a soft fin. However, we could not achieve our ambitious size goals of developing a robot smaller than 5 centimeters, following this approach. A larger number of artificial muscles requires space and more complex control, hence larger electronics circuit boards. We wanted to simplify the system as much as possible, but achieving complex motion, as seen in nature. We did overcome this challenge by developing miniaturized artificial muscles and specifically by tuning their resonance behavior. The rapid oscillations of the artificial muscle could now excite a traveling wave in a soft fin, which resulted in propulsion.

Tech Briefs: Herbert Shea is quoted in the article I read as saying, “In 2020, our team demonstrated autonomous insect-scale crawling robots, but making untethered ultra-thin robots for aquatic environments is a whole new challenge.” My question is: How does this swimming robot compare to the 2020 robot?

Hartmann: The new swimming robot and the 2020-crawling robot have similar size of around 4-5 centimeters and both use artificial muscles (although different kinds) that convert high voltage signals into motion. Their application area is however very different, water versus land. Swimming requires larger actuation strokes and the driving electrical signals need to be well shielded from contact with water. Therefore, we had to completely replace the technology for the artificial muscles of the robot.

Tech Briefs: Can you please explain in simple terms how the swimming robot works?

Hartmann: The locomotion of the robot is enabled by a pair of undulating fins, similar to a manta ray or a marine flatworm. The motion of each fin is generated by a single artificial muscle that is oscillating in the front of the fin. The oscillations of the artificial muscles are driven by repeatedly switching a high voltage (500 volts) signal on and off. The energy is sourced from a battery, from which we amplify the voltage using a custom-built miniature high voltage power supply.

Tech Briefs: Do you have any plans for further research?

Hartmann: We now head the development toward operation under water. We are redesigning the power supply and actuators to allow for longer operation times, underwater operation, and hope to test a new version of the robot in natural environments soon.

Context: For applications that require operation under water, the robot would need improved artificial muscles and a reworked power supply. When we tested the propulsion mechanism under water, we realized that effective oscillation requires more complex control signals to drive the artificial muscles. Our miniaturized power supply is not yet capable of driving complex fin motion underwater. In a next iteration we will slightly increase the size of the robot to make space for more electronic components, capable of generating arbitrary control signals.

Tech Briefs: Is there anything else you’d like to add that I didn’t touch upon?

Hartmann:

The development of the robot took over 2.5 years, starting in March 2021.
We call the robot SOMIRO (used internally but not in the publication), which is an acronym for Soft Milli Robot. (The name was also used for the related project funded by the European Union’s Horizon 2020 research and innovation program, https://www.somiro.eu)
The size of the robot is 45 mm length and 55 mm width. Its fins are less than 1 mm thin. We used this size to make an untethered and autonomous version of the robot, but the technology and principles for locomotion work also on smaller scales. The smallest robot (tethered and not autonomous) was 25 mm in length.

Tech Briefs: Do you have any advice for researchers aiming to bring their ideas to fruition (broadly speaking)?

Hartmann: For everyone developing their own robot hardware, it is absolutely crucial to first figure out reliable fabrication processes. In particular, when making robots on the small-scale, fabrication is challenging and requires creative solutions for reliable manufacturing.