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Robots Get Flexible and Torqued Up With Origami Wheels

Wheels that fold and unfold enable complex functionality from simple structures

3 min read

Robots Get Flexible and Torqued Up With Origami Wheels

Origami, the art of folding pieces of paper to create shapes, is an appealing concept for robotics because you can transform two dimensional materials into three dimensional structures that are inherently flexible, or, as a roboticist would say, "deformable." What's more, structures that fold and unfold enable all kinds of interesting functionality that would otherwise only be possible with systems that are much more complex.

The approach can be particularly useful in designing wheels for robots, and earlier this month at the IEEE International Conference on Robotics and Automation (ICRA) two research groups presented origami-inspired wheel systems that allow mobile robots to be nimbler and stronger. 

One of the groups was from Seoul National University's BioRobotics Laboratory, led by Professor Kyu-Jin Cho. Researchers there have designed a clever robotic wheel based on one of the most famous origami patterns, the magic ball pattern. 

The wheel they created can change its radius by deforming its shape. This is a useful trick to be able to perform, since a wheel with a large radius is better at climbing over things, while a wheel with a smaller radius is better at squeezing under things, as the robot demonstrates:

The wheel and hooks together can deform from a minimum diameter of 55 millimeters to a maximum diameter of 120 millimeters, which is a substantial range, especially considering that the transformation only requires one single actuator per wheel.

"Fabrication of Origami Wheel Using Pattern Embedded Fabric and Its Application to a Deformable Mobile Robot," by Dae-Young Lee, Ji-Suk Kim, Jae-Jun Park, Sa-Reum Kim, and Kyu-Jin Cho from Seoul National University, was presented this month at ICRA 2014 in Hong Kong.

In another project presented at ICRA, researchers at the Harvard Microrobotics Lab, led by Professor Robert Wood, teamed up with the group from Seoul National University to design origami wheels that can automatically expand and shrink.

The goal in this case wasn't just allowing the robot to climb over some things and squeeze under other things. Since there's a direct relationship between the radius of a wheel and the torque that it can exert, a wheel that can deform can also act as an automatic, continuously variable transmission.

First, a word (make that words) about transmissions: in the most general sense, a transmission is the thing that turns the power output of a motor into some combination of speed and torque. Usually, the way a transmission works is that you can trade torque for speed (or vice versa) while optimizing the efficiency of your motor: in automobiles, a low gear gives you lots of torque but limits your top speed, while a high gear gives up torque to let you go faster.

Most automobiles have discrete gearboxes, where you can choose from some number of fixed gear ratios. Inevitably, this means that most of the time, the transmission is not optimized for what you want to do. A continuously variable transmission, on the other hand, offers an infinite number of gear ratios over a fixed range. 

Unlike most automobiles, most robots don't bother with transmissions at all, because they're heavy and complicated. This means that robots have to be designed to either move fast (which doesn't require a lot of torque), or haul stuff (which does), but they're not great at doing both.

The researchers designed an origami wheel that can do both, by simply changing its radius, and thanks to its origami design, it does this by itself. It's a completely automatic continuously variable transmission, which allows a small robot to either move fast or haul stuff without requiring any additional complexity beyond an origami wheel:

Photos: Harvard Microrobotics Lab/Seoul National University's BioRobotics Laboratory
Images (a) and (b) show the origami wheel in high speed, low torque configuration; (c) and (d) show the origami wheel in low speed, high torque configuration.

There are three robots in the video below. There's one with big fixed wheels (high speed, low torque), one with small fixed wheels (low speed, high torque), and then one with origami wheels that can expand and shrink. No adjustment is necessary: if the origami wheels have too much load on them to rotate, they stall, and as the wheel hubs continue to turn, the wheels collapse, which increases their torque until they can move:

To reiterate, this transmission is both continuously variable and completely automatic. The wheel can adopt any effective gear ratio in the range between its minimum and maximum diameters, and it does this passively like a spring, as it responds to loads on it by shrinking its diameter until it achieves the maximum diameter at which it can consistently rotate. 

The cost of the origami wheel is 70 percent of the cost of a fixed diameter wheel, less whatever it costs to hire someone who knows what they're doing to fold it up for you. The researchers suggest that it would also be ideal for applications where weight and volume are issues, like interplanetary rovers, as the wheel can be folded up and then deploy itself.

"A Passive, Origami-Inspired Continuously Variable Transmission," by Samuel M. Felton, Dae-Young Lee, Kyu-Jin Cho, and Robert J. Wood, from Harvard University and Seoul National University, was presented this month at ICRA 2014 in Hong Kong.

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