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A new approach to artificial ‘muscles’ for safer, softer robots: For Journalists

A new approach to artificial ‘muscles’ for safer, softer robots: For Journalists

  • New robotic actuator stretches and contracts like a biological muscle
  • Researchers demonstrate actuator in crawling soft robot and artificial biceps
  • Crawling robot bends in tight spaces; artificial biceps lift weights
  • The soft actuator stiffens like human muscle, a feature historically lacking in soft robotics

EVANSTON, Ill. — Northwestern University engineers have developed a new, soft, flexible device that allows robots to move by expanding and contracting just like human muscles.

The researchers used this to demonstrate their new device, called an actuator, to create a cylindrical, worm-like soft robot and an artificial bicep. In experiments, the cylindrical soft robot navigated tight, sharp bends in a narrow tube-like environment, and the biceps were able to lift a 500-gram weight 5,000 times in a row without failing.

Because the researchers 3D-printed the body of the soft actuator using a common rubber, the resulting robots cost about $3, excluding the small motor that enables the actuator to change shape. That’s in sharp contrast to the typical hard, rigid actuators used in robotics, which typically cost hundreds to thousands of dollars.

The new actuator could be used to develop inexpensive, soft, flexible robots that are safer and more practical for real-world applications, the researchers said.

The research published Monday (July 8) in the journal Advanced Intelligent Systems.

“Roboticists are inspired by a long-standing goal of making robots safer,” Northwestern said Ryan Truby“If a soft robot hits a human, it doesn’t hurt as much as if it were hit by a hard, rigid robot,” said lead author Dr. Richard Mann, Ph.D., who led the study. “Our actuator could be used in robots that are more practical for human-centered environments. And because they’re cheap, we can use more of them in ways that have historically been too costly.”

Truby is Northwestern’s June and Donald Brewer Young Professor of Materials Science and Engineering and Mechanical Engineering McCormick School of Engineeringwhere he manages Robotic Matter LaboratoryTaekyoung Kim, a postdoctoral researcher in Truby’s lab and first author of the paper, led the research. Pranav Kaarthik, a doctoral candidate in mechanical engineering, also contributed to the work.

Robots that ‘act and move like living organisms’

While rigid actuators have long been a cornerstone of robot design, their limited flexibility, adaptability, and safety have led roboticists to explore soft actuators as an alternative. To design soft actuators, Truby and his team are taking inspiration from human muscles that contract and stiffen simultaneously.

“How do we make materials that can move like muscles?” Truby asked. “If we can do that, we can make robots that act and move like living organisms.”

To develop the new actuator, the team 3D-printed cylindrical structures called “hand-cut auxetics” (HSA) from rubber. Difficult to manufacture, HSAs feature a complex structure that enables unique movements and properties. For example, when bent, HSAs stretch and expand. Although Truby and Kaarthik have 3D-printed similar HSA structures for robots in the past, they had to use expensive printers and hard plastic resins. As a result, their previous HSAs could not bend or deform easily.

“For this to work, we needed to find a way to make the HSAs softer and more durable,” Kim said. “We figured out how to make soft but sturdy HSAs out of rubber using a desktop 3D printer that is cheaper and more readily available.”

Kim printed the HSAs from thermoplastic polyurethane, a common rubber often used in cellphone cases. While this made the HSAs much softer and more flexible, one challenge remained: How to bend the HSAs to flex and expand them.

Previous versions of HSA soft actuators used common servo motors to bend materials into extended and stretched states. However, the researchers achieved successful actuation after stacking two or four HSAs, each with its own motor. Building the soft actuators in this manner presented manufacturing and operational challenges and also reduced the softness of the HSA actuators.

The researchers aimed to design a single HSA driven by a single servo motor to build an improved soft actuator. But first, the team needed to find a way to have a single motor rotate a single HSA.

Simplifying the ‘entire pipeline’

To solve this problem, Kim added a soft, extensible, rubber bellows to the structure, which acts like a deformable, rotating shaft. When the motor provides torque—an action that causes an object to rotate—the actuator extends. Turning the motor in one direction or the other causes the actuator to extend or contract.

“Essentially, Taekyoung designed two rubber pieces to create muscle-like movements with the rotation of a motor,” Truby said. “While the field has produced soft actuators in more cumbersome ways, Taekyoung greatly simplified the entire pipeline with 3D printing. Now we have a practical soft actuator that any roboticist can use and build.”

The bellows provided enough support for Kim to build a self-propelled, crawling soft robot from a single actuator. The actuator’s push and pull motions propel the robot forward through a tortuous, constrained environment simulating a pipe.

“Our robot can do this extension movement using a single structure,” Kim said. “This makes our actuator more useful because it can be universally integrated into any type of robotic system.”

The missing piece: muscle stiffness

The resulting worm-like robot was compact (just 26 centimeters long) and crawled both forward and backward at a speed of just over 32 centimeters per minute. Truby noted that when the actuator was fully extended, both the robot and its artificial biceps became more rigid—another feature that previous soft robots had failed to achieve.

“These soft actuators actually stiffen like a muscle,” Truby said. “For example, if you’ve ever turned the lid of a jar, you know your muscles are tense and become stiffer to transmit force. That’s how your muscles help your body work. This has been an overlooked feature in soft robotics. Many soft actuators soften during use, but our flexible actuators become stiffer as they work.”

Truby and Kim say their new actuator represents another step toward more biologically inspired robots.

“Robots that can move like living organisms will allow us to think about what robots can do that traditional robots can’t do,” Truby said.

The study, “A flexible, architected soft robotic actuator for linear, servo-driven motion,” was supported by Truby’s Young Researcher Award from the Office of Naval Research and the Northwestern Center for Engineering and Sustainability Resilience.