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New Unimorph SMA-Based Actuator for Microrobotics

Washington State University produced two insect-like robots that are the tiniest, lightest, and fastest fully functional micro-robots ever made: the mini-bug and the water strider.

The WaterStrider weighs 55 milligrams and can move at 6 millimeters per second. Image Credit: Bob Hubner, WSU Photo Services

These future miniature robots could be employed for tasks including robotically assisted surgery, environmental monitoring, artificial pollination, search and rescue, and microfabrication.

In the International Conference on Intelligent Robots and Systems proceedings of the IEEE Robotics and Automation Society, the researchers reported on their work. The water strider weighs 55 milligrams, while the mini-bug weighs only eight milligrams. Both have a maximum speed of six millimeters per second.

That is fast compared to other micro-robots at this scale although it still lags behind their biological relatives.

Conor Trygstad, Study Lead Author and PhD Student, School of Mechanical and Materials Engineering, Washington State University

An ant can weigh up to five milligrams and travel at about one meter per second.

The tiny actuators in the small robots are what give them their mobility. Trygstad was able to reduce the actuator to the lowest known size—less than a milligram—by using a novel production process.

The actuators are the smallest and fastest ever developed for micro-robotics.

Néstor O. Pérez-Arancibia, Flaherty Associate Professor in Engineering, School of Mechanical and Materials Engineering, Washinton State University

Shape memory alloy, the material used in the actuator, is capable of changing shape when heated. Because it remembers and then reverts to its original shape, it is known as “shape memory.” These alloys lack any spinning or moving parts, as opposed to a standard motor that would propel a robot.

Trygstad added, “They are very mechanically sound. The development of the very lightweight actuator opens up new realms in micro-robotics.

Due to their excessive slowness, shape memory alloys are not often utilized for large-scale robotic movement. However, in the case of the WSU robots, the actuators consist of two extremely small wires, each measuring 1/1000 of an inch in diameter, composed of shape memory alloy.

The wires can be readily heated or cooled with a tiny amount of electricity, enabling the robots to move their feet or flap their fins up to 40 times per second. Additionally, the actuator demonstrated the ability to lift over 150 times its weight in preliminary testing.

Additionally, the SMA technique uses a relatively small amount of heat or power to move robots in comparison to other methods.

The SMA system requires a lot less sophisticated systems to power them,” Trygstad further added.

Trygstad, an ardent fly fisherman, has long studied water striders and wishes to investigate their movements better. While the WSU water strider robot moves by flapping its wings, the natural insect moves by rowing its legs, which is one of the reasons why the actual thing can move so quickly.

The goal is to create a robot that moves both just below and above the water’s surface, much like a water strider, by mimicking the movements of another insect. They are also attempting to employ catalytic combustion or small batteries to make their robots completely autonomous and untethered from a power supply.

Journal Reference:

Trygstad, C. K., et. al. (2023) A New 1-mg Fast Unimorph SMA-Based Actuator for Microrobotics. IEEE. doi:10.1109/IROS55552.2023.10342518.


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