Text by Evan Lerner
Ask someone to draw a picture of a cutting-edge robotic arm and he or she will likely take inspiration from the one hanging from his or her shoulder: the same muscles, elbow, wrist, and fingers, with wires and servos replacing sinew and bone.
While many advances in robotics mimic the form and function of the natural world, innovations can also come from unlikely places.
Last week at the International Conference on Robotics and Automation, Mark Yim, a professor in the School of Engineering and Applied Science, and lab member Foster Collins presented their work on a prototype robotic arm that has more in common with a jacket’s zipper than the arm in its sleeve.
It uses a flexible plastic ribbon with interlocking teeth on both edges as the basis of a new kind of linear actuator, a device made for a relatively simple task: moving back and forth in a single direction. As the ribbon is zipped into itself in a spiral, it forms a tube that can extend in a straight line, then retract into a compact, flat coil.
Mounted on a gimbal and controlled with guide wires, the zipper arm could push and pull in multiple directions with a strength that belies its minimal weight and complexity.
“The slider in your kitchen drawer has an extension ratio of 2:1—it goes from two feet to four feet. The telescoping antenna in your car is maybe 5:1,” Collins says. “Our device can achieve 12:1 and remain very lightweight. That’s where we see a real novelty.”
Yim’s ModLab develops robotic mechanisms that have relatively limited capabilities on their own, but can be combined in different ways to achieve a given task. Collins began developing the zipper arm there as a Rachleff Scholar during the summer after his sophomore year.
Other engineers have developed similar zipper-based systems, so when Yim had originally proposed the idea to Collins, he challenged him to design the mechanism to be as simple as possible. Using one motor and only one band keeps costs—and potential points of failure—to a minimum.
The single motor is also the key advantage of a linear actuator over articulated, humanoid arms. Multiple joints mean multiple motors, and the longer the arm, the stronger the motors need to be to drive it through its full range of motion. The tube shape that the band forms is one of the best in terms of strength-to-weight ratio in supporting the compressive loads that the spiral zipper would see when mounted on a gimbal with guide wires.
“We’re essentially exploiting geometry to make it stronger,” Collins says.
The next steps for the prototype arm include simplifying the mechanism that meshes the teeth of the ribbon together and further strengthening the tube it forms.
“We’re also looking forward to working with other people to figure out what other features it needs before we put it on a product,” Collins says.
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