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Engineers at Northwestern University have developed a new soft, flexible device that makes robots move by expanding and contracting — just like a human muscle.
To demonstrate their new device, called an actuator, the researchers used it to create a cylindrical, worm-like soft robot and an artificial bicep. In experiments, a cylindrical soft robot navigated the tight, hairpin curves of a narrow, pipe-like environment, and the bicep was able to lift a 500-gram weight 5,000 times in a row without failure.
Since the researchers 3D printed the body of the soft controller using regular rubber, the resulting robots cost about $3 in materials, except for the small motor that powers the controller’s shape change. This is in stark contrast to the typical stiff, rigid actuators used in robotics, which often cost hundreds to thousands of dollars.
The new actuator could be used to develop cheap, soft, flexible robots that are safer and more practical for real-world applications, the researchers said.
“The roboticists were motivated by the long-term goal of making robots safer,” said Northwestern’s Ryan Truby, who led the study. “If a soft robot were to hit a human, it wouldn’t hurt nearly as much as if a stiff, hard robot were to hit them. Our actuator could be used in robots that are more practical for human-centric environments. And because they’re cheap, we could potentially use them in more ways that have historically been too expensive.”
Truby is the June and Donald Brewer Junior Professor of Materials Science and Engineering and Mechanical Engineering at Northwestern’s McCormick School of Engineering, where he directs The Robotic Matter Lab. The research was led by Taekyoung Kim, a postdoctoral fellow in Truby’s lab and the paper’s first author. Pranav Kaarthik, Ph.D candidate, also contributed to the work.
Robots that “behave and move like living organisms”
While hard actuators have long been a cornerstone of robot design, their limited flexibility, adaptability, and safety have prompted roboticists to explore soft actuators as an alternative. In designing the soft actuators, Truby and his team were inspired by human muscles that contract and stiffen at the same time.
“How do you make materials that can move like a muscle?” Truby asked. “If we can do that, we can make robots that behave and move like living organisms.”
To develop the new actuator, the team 3D printed cylindrical structures called “handed shearing auxetics” (HSA) from rubber. Difficult to manufacture, HSAs embody a complex structure that allows for unique movements and properties. For example, HSAs lengthen and stretch when twisted. Although Truby and Kaarthik have previously 3D printed similar HSA structures for robots, they had to use expensive printers and rigid plastic resins. As a result, their previous HSAs could not bend or deform easily.
“To make it work, we had to find a way to make HSA softer and more durable,” Kim said. “We figured out how to make soft but robust HSA from rubber using a cheaper and more readily available desktop 3D printer.”
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Kim printed the HSA from thermoplastic polyurethane, a common rubber often used in cell phone cases. Although this made the HSAs much softer and more flexible, one problem remained: how to twist the HSAs to stretch and expand.
Previous versions of HSA soft drives used conventional servo motors to twist materials into an extended and stretched state. But the researchers achieved a successful launch only after building two or four HSAs – each with its own engine – together. Creating soft actuators in this way presents manufacturing and operational challenges. It also reduced the softness of the HSA drives.
To create an improved soft actuator, the researchers focused on designing a single HSA driven by a single servo motor. But first, the team needed to find a way to make a single motor twist a single HSA.
To demonstrate the new controller, the researchers used it to create a worm robot that could move in tight spaces. | Credit: Northwestern University
Simplifying the “Whole Pipeline”
To solve this problem, Kim added a soft, stretchable rubber bellows to the structure that acted as a deformable rotating shaft. When the motor provided torque—the action that causes the object to rotate—the actuator extended. By simply turning the motor in one direction or the other, the actuator extends or retracts.
“Taekyoung basically engineered two rubber parts to make muscle-like movements by turning the motor,” Truby said. “While the field created soft actuators in more difficult ways, Taekyoung greatly simplified the entire pipeline using 3D printing. Now we have a practical soft actuator that any roboticist can use and make.”
The bellows gave Kim enough support to build a crawling soft robot from a single actuator that moved on its own. Push and pull movements of the actuator propelled the robot forward through a coiled, confined environment simulating a pipeline.
“Our robot can perform this extension motion using a single structure,” Kim said. “This makes our actuator more useful because it can be universally integrated into all types of robotic systems.”
The missing piece: muscle stiffness
The resulting worm-like robot was compact (measuring only 26 centimeters in length) and crawled – backwards and forwards – at a speed of just over 32 centimeters per minute. Truby noted that both the robot and the artificial bicep are stiffer when the actuator is fully extended. This was another feature that previous soft robots could not achieve.
“Like a muscle, these soft actuators actually stiffen,” Truby said. “For example, if you’ve ever unscrewed a jar lid, you know that your muscles tense up and stiffen to transmit the force. Muscles thus help your body work. This has been an overlooked feature in soft robotics. Many soft actuators soften in use, but our flexible actuators are stiffer in operation.”
Truby and Kim say their new actuator represents the next step toward bio-inspired robots.
“Robots that can move like living organisms will allow us to think about robots doing tasks that conventional robots can’t do,” Truby said.
Editor’s note: This article was republished from Northwestern University.
