Tiny titan: $3 robot bicep lifts 500 grams 5,000 times, crawls pipes easily

Engineers have created a novel, flexible device apparatus that allows robots to move by contracting and expanding, much like a human muscle.

The Northwestern University team showcased their novel actuator, which they utilized to build an artificial bicep and a cylindrical, worm-like soft robot.

In experiments, the cylindrical soft robot navigated tight hairpin curves in a narrow, pipe-like environment, and the bicep successfully lifted a 500-gram weight 5,000 consecutive times.

By 3D-printing the soft actuator’s body using common rubber, the resulting robots cost about $3 in materials, excluding the small motor. This is significantly cheaper than typical rigid actuators, which often cost hundreds to thousands of dollars.

According to the researchers, the new actuator could lead to the development of inexpensive, soft, and flexible robots, making them safer and more practical for real-world applications.

The details of the team’s research were published in the journal Advanced Intelligent Systems.

Soft actuator design

Although stiff actuators have historically been the mainstay of robot design, roboticists are now investigating soft actuators as an alternative due to the rigid actuators’ restricted flexibility, adaptability, and safety.

To design soft actuators, the team takes inspiration from human muscles, which contract and stiffen simultaneously.

The team developed the new actuator by 3D-printing cylindrical structures known as “handed shearing auxetics” (HSAs) out of rubber. These HSAs, which are challenging to fabricate, possess a complex structure that allows for unique movements and properties, such as extending and expanding when twisted.

Researchers previously 3D-printed similar HSA structures using costly printers and rigid plastic resins. However, those earlier HSAs lacked the flexibility to bend or deform easily.

“For this to work, we needed to find a way to make HSAs softer and more durable. We figured out how to fabricate soft but robust HSAs from rubber using a cheaper and more easily available desktop 3D printer,” said Taekyoung Kim, a postdoctoral scholar at the University and the study’s first author, in a statement .

The team 3D-printed the HSAs using thermoplastic polyurethane, a common rubber found in cellphone cases. This made the HSAs softer and more flexible. However, the challenge was twisting them to achieve extension and expansion.

Single motor innovation

Earlier HSA actuators used servo motors to twist the materials but assembling two or four HSAs, each with its own motor, complicated fabrication and reduced flexibility. To overcome this, the researchers aimed to design a single HSA driven by one servo motor. First, they needed to develop a method for a single motor to twist a single HSA.

To address this issue, the team modified the structure to include extensible, soft rubber bellows that functioned like a revolving, pliable shaft. The actuator stretched as the motor produced torque, which is the force that rotates an object. The actuator is driven to stretch or contract by simply rotating the motor in one direction or the other.

Researchers used the bellows to construct a soft robot that could crawl with just one autonomous actuator. The actuator moved the robot ahead by pushing and pulling in a confined, winding environment that resembled a pipe .

“Our robot can make this extension motion using a single structure. That makes our actuator more useful because it can be universally integrated into all types of robotic systems,” said Kim.

Next-gen bioinspired robots

The resultant design led to the development of a compact, worm-like robot measuring 26 centimeters in length that could crawl both backward and forward at a speed of over 32 centimeters per minute.

When the actuator is fully extended, both the robot and the artificial bicep become significantly stiffer, a property that previous soft robots lacked.

According to researchers, these soft actuators function similarly to muscles by stiffening during operation. When you twist the lid off a jar, for example, your muscles tighten and become stiffer to transmit force, enabling your body to perform work.

This feature has been overlooked in soft robotics, as many soft actuators tend to get softer when in use. In contrast, these flexible actuators become stiffer as they operate.

The new actuator represents another advancement toward 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,” said Ryan Truby, a professor of materials science and engineering and mechanical engineering at the university and lead on the research, in a statement.