The device, termed an actuator, has been utilized to create both a cylindrical, worm-like soft robot and an artificial bicep. In practical tests, the soft robot adeptly maneuvered through narrow, pipe-like spaces, and the bicep successfully lifted a 500-gram weight 5,000 consecutive times without failure.
By 3D-printing the soft actuator body using common rubber, researchers have significantly reduced costs. Excluding the small motor that drives the actuator, the robots cost approximately $3 in materials, a stark contrast to the typically high costs of rigid actuators.
The research, which presents a potential for developing cost-effective and practical soft robots for real-world applications, was published on July 8 in the journal Advanced Intelligent Systems.
"Roboticists have been motivated by a long-standing goal to make robots safer," explained Ryan Truby of Northwestern, the study's lead. "If a soft robot hit a person, it would not 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 are inexpensive, we potentially could use more of them in ways that, historically, have been too cost prohibitive."
Truby, who is the June and Donald Brewer Junior Professor of Materials Science and Engineering and Mechanical Engineering at Northwestern's McCormick School of Engineering, directs The Robotic Matter Lab. Taekyoung Kim, a postdoctoral scholar in Truby's lab and the study's first author, led the research alongside Ph.D. candidate Pranav Kaarthik.
Robots Mimicking Living Organisms
Rigid actuators have traditionally been integral to robot design, but their limited flexibility and safety concerns have driven research towards soft actuators. Truby's team drew inspiration from human muscles, which simultaneously contract and stiffen.
"How do you make materials that can move like a muscle?" Truby asked. "If we can do that, then we can make robots that behave and move like living organisms."
To create the new actuator, the team 3D-printed cylindrical structures called "handed shearing auxetics" (HSAs) using rubber. These complex structures, which expand when twisted, were previously made from rigid plastic resins using expensive printers, limiting their flexibility and deformability.
"For this to work, we needed to find a way to make HSAs softer and more durable," said Kim. "We figured out how to fabricate soft but robust HSAs from rubber using a cheaper and more easily available desktop 3D printer."
The team printed the HSAs from thermoplastic polyurethane, a common rubber, but faced the challenge of achieving the necessary twisting motion for actuation.
Earlier versions of HSA actuators used multiple motors to achieve extension and expansion, complicating their design and reducing softness. To improve this, the researchers aimed to develop an actuator driven by a single motor.
Simplifying the Actuation Process
Kim incorporated a soft, extendable rubber bellows to the actuator, functioning like a deformable, rotating shaft. The motor's torque caused the actuator to extend, simplifying the actuation process.
"Essentially, Taekyoung engineered two rubber parts to create muscle-like movements with the turn of a motor," Truby said. "While the field has made 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 make."
This innovation enabled the creation of a crawling soft robot propelled by the actuator's motions. The robot could navigate a winding, constrained environment simulating a pipe.
"Our robot can make this extension motion using a single structure," Kim said. "That makes our actuator more useful because it can be universally integrated into all types of robotic systems."
Stiffening Like Human Muscles
The worm-like robot, measuring 26 centimeters in length, could crawl at a speed of over 32 centimeters per minute. The actuator also provided stiffness when fully extended, a feature lacking in many soft robots.
"Like a muscle, these soft actuators actually stiffen," Truby said. "If you have ever twisted the lid off a jar, for example, you know your muscles tighten and get stiffer to transmit force. That's how your muscles help your body do work. This has been an overlooked feature in soft robotics. Many soft actuators get softer when in use, but our flexible actuators get stiffer as they operate."
Truby and Kim believe their actuator marks a step towards more bioinspired robots.
"Robots that can move like living organisms are going to enable us to think about robots performing tasks that conventional robots can't do," Truby said.
Research Report:A Flexible, Architected Soft Robotic Actuator for Motorized Extensional Motion
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