NEW DIRECTION FOR INCHWORM ROBOTS
Many inchworm robots lack turning capabilities, but the use of a helical actuator could change that.
Written by Kayt Sukel
Joel Quarnstrom and Yujiang Xiang discussing the inchworm robot assembly. Photo: Adrian Toquothty, Oklahoma State University
MANY ROBOTICISTS LOOK TOWARD Mother Nature to help guide their designs. Over the past decade, many engineering teams have leveraged the simple movements of the inchworm, a small caterpillar with legs only along the front and back of its body, which moves in a unique, looping motion. Its efficient gait allows it to easily adapt to different types of terrain, traversing obstacles, climbing structures, and squeezing into small spaces. And many researchers have suggested such inchworm-inspired robots could be used in a wide range of applications, from assisting with minimally invasive drug delivery to helping dock spacecraft.
Many of the traditional inchworm designs have leveraged prismatic actuators to help them achieve the caterpillar’s tell-tale locomotive style.
“The main inchworm motion is the robot moving back and forth or even up and over this prismatic actuator,” said Joel Quarnstrom, a doctoral candidate in Yujiang Xiang’s Biodynamics Optimization Laboratory at Oklahoma State University. “While many of the inchworm robots in the literature have a lot of interesting features—they can stick to the wall and move over different terrain—most aren’t able to turn.”
Other inchworm designs have limited actuation power and slow speeds. Quarnstrom, who typically works on biomedical exoskeletons in his research, decided to try to come up with a new design with the potential to overcome these obstacles. It relies on a helical actuator, a type of hydraulic actuator with a helix-shaped piston that can convert fluid hydraulic pressure into linear motion. He hypothesized the use of this type of actuator would allow the robot to extend to more than twice its contracted length, giving it expanded movement capabilities.
A depiction of the simulated inchworm at different points in a continuous turning locomotion. Image: Joel Quarnstrom/Yujiang Xiang/Oklahoma State University
“While many of the inchworm robots in the literature have a lot of interesting features—they can stick to the wall and move over different terrain—most aren’t able to turn.”
– Joel Quarnstrom, doctoral candidate, Biodynamics Optimization Laboratory, Oklahoma State University
An illustration of the helical actuator, which consists of a set of rings spaced along its length. Each ring is connected by three rods that have ball joints on each end. Image: Joel Quarnstrom/Yujiang Xiang/Oklahoma State University
“If you think about most prismatic actuators, they work like hydraulic pistons. There’s going to be a kind of housing and then there will be the piston part, going in and out,” he said. “The piston always has to fit inside the housing to work, so it can never get larger than twice its original length. If you want robots that can go through tight spaces and be able to turn, being able to have a significant change in size is important so it can do those things.”
Quarnstrom’s prototype can contract and expand its body segments to move forward or to turn, with each segment supported by a helical actuator. The actuators help to transform the rotation of stepper motors into the so-called measuring motion of the worm. Each segment also has spiky feet, angled in such a way so that during each contraction and extension phase, only one of the feet is moving.
“I went through a lot of different prototypes,” Quarnstrom said. “I was able to achieve about a 1:3 contraction ratio. It’s got a non-linear relationship between the input rotation and the output displacement.”
During testing, the team sent the helical actuator-driven inchworm robot across multiple surface types. Video: Oklahoma State University
While this design did allow the robot to move forward, turning proved to be trickier than anticipated.
“The friction on the ground was a lot less controllable than I thought it would be,” he explained. “I could probably do another study simply on the friction characteristics of the foot design that I had and the different types of ground.”
While Quarnstrom does not currently have plans to continue refining his prototype—his doctoral work is focused on exoskeletons—understanding the different mechanisms to help robots move is important, he said. There are many ways that biological organisms move and with more detailed understanding of the trigonometry of those unique mechanisms, engineers will surely be inspired to create new and innovative designs.
“I got into engineering initially because I think mechanisms are really cool,” he said. “And there’s still a lot we can learn about them. I hope that more people become interested in mechanisms and will continue to try different complicated mechanisms and analyze the kinematics so we can translate them into new things.”
Kayt Sukel is a technology writer and author in Houston.
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