Creating artificial muscles for safer, softer robots

PHOTO COURTESY OF RYAN TRUBY/TAEKYOUNG KIM/NORTHWESTERN UNIVERSITY

Northwestern University engineers developed a soft, flexible device making robots move by expanding and contracting – just like a human muscle.

To demonstrate their new actuator, researchers created a cylindrical, worm-like robot and an artificial bicep. The cylindrical soft robot successfully navigated tight, hairpin curves of a pipe-like environment, and the bicep lifted a 500g weight 5,000 times in a row.

Researchers 3D printed the body of the soft actuator using a common rubber, resulting in a robot that cost about $3 in materials, excluding the small motor that drives the actuator’s shape change, a sharp contrast to typical actuators used in robotics, which cost hundreds to thousands of dollars. The actuator could be used to develop inexpensive, soft, flexible robots that are safer and more practical for real-world applications.

“Roboticists have been motivated by a long-standing goal to make robots safer,” says Northwestern’s Ryan Truby, the June and Donald Brewer Junior Professor of Materials Science and Engineering and Mechanical Engineering at Northwestern’s McCormick School of Engineering. Truby led the study and directs the Robotic Matter Lab. “If a soft robot hit a person, it wouldn’t hurt nearly as much as getting hit with a rigid, hard robot. Our actuator could be used in robots that are more practical for human-centric environments. And, because they’re inexpensive, we potentially could use more of them in ways that, historically, have been too cost prohibitive.”

Robots that behave and move like living organisms

While rigid actuators have long been the cornerstone of robot design, their limited flexibility, adaptability, and safety drive the exploration of soft actuators. Truby and his team take inspiration from human muscles, which contract and stiffen simultaneously.

“How do you make materials that can move like a muscle?” Truby asks.

The team 3D printed rubber cylindrical structures called handed shearing auxetics (HSAs). HSAs embody a complex structure enabling unique movements and properties. Although similar HSA structures had been 3D printed before, they required expensive printers and rigid plastic resins that couldn’t bend or deform easily.

Printing the HSAs from thermoplastic polyurethane made HSAs much softer and more flexible, but one challenge remained: how to twist HSAs to get them to extend and expand.

Previous versions of HSA soft actuators used common servo motors to twist the materials into extended and expanded states. But researchers only achieved successful actuation after assembling multiple HSAs together, each with its own motor. This method presented fabrication and operational challenges and reduced the softness of HSA actuators.

To build an improved soft actuator, the researchers aimed to design a single HSA driven by one servo motor. First, the team needed to make a single motor twist a single HSA.

Simplifying ‘the entire pipeline’

To solve this problem, Taekyoung Kim, a postdoctoral scholar in Truby’s lab, added soft, extendable, rubber bellows to the structure performing like a deformable, rotating shaft. As the motor provided torque, the actuator extended.

The bellows added enough support for Kim to build a crawling soft robot from a single actuator that moved on its own. The pushing and pulling motions of the actuator propelled the robot through a winding, constrained, pipe-like environment.

The resulting worm-like robot was 26cm long, crawling at a speed of just more than 32cm/min. Truby noted both the robot and artificial bicep become stiffer when the actuator is fully extended, another property previous soft robots were incapable of.

Truby and Kim say their new actuator provides another step toward more bioinspired robots.

Northwestern University McCormick School of Engineering
https://www.mccormick.northwestern.edu