Engineers at Northwestern University have developed a new soft, flexible device that moves robots by expanding and contracting — just like human muscles.
To demonstrate their new device, called an actuator, the researchers used it to build a cylindrical, worm-like soft robot and an artificial bicep. In experiments, the cylindrical soft robot navigated the tight, hairpin curves of a narrow pipe-like environment, and Bicep was able to continuously lift a 500-gram weight 5,000 times without failure.
Because the researchers 3D printed the body of the soft actuator using a common rubber, the resulting robot costs about $3, excluding the small motor that changes the shape of the actuator. This is in stark contrast to the typical stiff, rigid actuators used in robotics, which often cost hundreds to thousands of dollars.
The researchers said the new actuator could be used to develop cheaper, softer, flexible robots that are safer and more practical for real-world applications.
The research was published Monday (July 8) in the journal Advanced intelligent system.
“Roboticists are motivated by a long-standing goal of making robots safer,” said Northwestern's Ryan Turby, who led the research. “If a soft robot hits a person, it won't hurt as much as a hard, rigid robot. Our actuators can be used in robots that are more practical for human-centric environments. And , because they are cheap, we can potentially use more of them in ways that have historically been very cost-prohibitive.”
Turby Jon and Donald Brewer are junior professors of materials science and engineering and mechanical engineering in Northwestern's McCormick School of Engineering, where they direct the Robotic Materials Lab. Taekyung Kim, a postdoctoral scholar in Turby's lab and first author on the paper, led the research. Pranav Karthik, PhD Mechanical Engineering candidate also participated in the work.
Robots that 'behave and move like living things'
Although rigid actuators have long been the cornerstone of robot design, their limited flexibility, adaptability, and safety have led roboticists to explore soft actuators as an alternative. To design the soft actuators, Terbi and his team took inspiration from human muscles, which simultaneously contract and stiffen.
“How do you make a material that can move like a muscle?” asked Turby. “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 “handshearing actuators” (HSAs) out of rubber. Difficult to fabricate, HSAs form a complex structure that enables unique movement and properties. For example, when bent, HSAs expand and expand. Although Turbi and Karthik had printed similar HSA structures for robots in the past, they were bound to use expensive printers and hard plastic resins. As a result, their previous HSAs could not be easily diverted or distorted.
“For this to work, we need to find a way to make HSAs softer and more durable,” Kim said. “We figured out how to produce soft but strong HSAs from rubber using a cheap and more readily available desktop 3D printer.”
Kim printed the HSAs from thermoplastic polyurethane, a common rubber often used in cell phone cases. While this made HSAs much softer and more flexible, one challenge remained: how to bend HSAs to expand them.
Previous versions of HSA soft actuators used conventional servo motors to bend the material into extended and stretched positions. But the researchers achieved success only after assembling two or four HSAs — each with its own motor — together. Thus, the construction of soft actuators presented fabrication and operational challenges. It also reduced the flexibility of the HSA actuators.
To create a better soft actuator, the researchers aimed to design an HSA powered by a servo motor. But first, the team needed to find a way to make a single motor into a single HSA.
Simplifying the 'whole pipeline'
To solve this problem, Kim added a soft, expandable, rubber bellows to the structure that acted like an inflexible, rotating shaft. As the motor delivers torque — an action that causes an object to rotate — the actuator is extended. Simply turning the motor in one direction or the other extends or contracts the actuator.
“Basically, Taekyoung created two rubber parts to create a muscle-like movement with the twist of the motor,” Turbi said. “While the field has made soft actuators in more cumbersome ways, Taekyoung has greatly simplified the entire pipeline with 3D printing. Now, we have a practical soft actuator that any roboticist can use and build.”
The vines provided enough support for Kim to build a single-actuator crawling soft robot that moved on its own. Pushing and pulling motions of the actuator propel the robot through a winding, confined environment in the form of a pipe.
“Our robot can make this extended motion using a single structure,” Kim said. “This makes our actuator more useful because it can be universally integrated into all kinds of robotic systems.”
The missing piece: muscle stiffness
The resulting insect-like robot was compact (only 26 centimeters long) and crawled — backwards and forwards — at a speed of only 32 centimeters per minute. Turby noted that both the robot and the artificial bicep stiffen when the actuator is fully extended. This was another feature that previous soft robots had failed to achieve.
“Like a muscle, these soft actuators are actually hard,” Turby said. “If you've ever twisted a jar lid, for example, you know that your muscles are stiff and hard to transmit force. That's how your muscles help your body work. This has been an overlooked feature in soft robotics. Many soft actuators become soft when in use, but our flexible actuators become stiff during operation.”
Turby and Kim say their new actuator provides another step toward more bio-inspired robots.
“Robots that can move like living things will enable us to think about robots performing tasks that traditional robots can't,” Turby said.
The study, “A flexible, architected soft robotic actuator for linear, servo-driven motion,” was supported by a Turby Young Investigator Award from the Office of Naval Research and the Northwestern Center for Engineering and Sustainability Resilience.