Popcorn-Driven Robotic Actuators
Popcorn is a cheap, biodegradable way to actuate a robot (once)
People toss around the word “novel” fairly often in robotics papers, but this right here is the definition of a novel mechanism, and it might be one of the most creative ideas I’ve seen presented at a robotics conference in a long time. This is not to say that popcorn is going to completely transform robotic actuation or anything, but it’s weird enough that it might plausibly end up in some useful (if very specific) robotic applications.
Why use popcorn to power an actuator? You can think of unpopped kernels of popcorn as little nuggets of stored mechanical energy, and that energy can be unleashed and transformed into force and motion when the kernel is heated. This is a very useful property, even if it’s something that you can only do once, and the fact that popcorn is super cheap and not only biodegradable but also edible are just bonuses.
The “pop” in popcorn happens when enough heat is applied to vaporize the moisture inside the kernel. Over 900 kPa of internal pressure causes the yummy goo inside of the kernel to explode out through the shell, expand, and then dry. Relative to the size of the original kernel, the volume of a popped piece of popcorn has increased by a factor of at least five, although it can be much more, depending on the way the kernel was heated. Because of this variability, the first step in this research was to properly characterize the popcorn, and to do this the researchers, from Cornell’s Collective Embodied Intelligence Lab, picked up some Amish Country brand popcorn (chosen for lack of additives or postharvest treatment) in white, medium yellow, and extra small white. They heated each type using hot oil, hot air, microwaves, and direct heating with a nichrome resistance wire. The extra small white kernels, which were the cheapest at the US $4.80 per kilogram, also averaged the highest expansion ratio, exploding to 15.7 times their original size when popped in a microwave.
Here’s what the researchers suggest that popcorn might be useful for in a robotics context:
• Jamming actuator. “Jamming” actuators are compliant actuators full of a granular fluid (coffee grounds, for example) that will bind against itself and turn rigid when compressed, most often by applying a vacuum. If you use popcorn kernels as your granular fluid, popping them will turn the actuator rigid. It’s irreversible but effective: In one experiment, the researchers were able to use a jamming actuator filled with 36 kernels of popcorn to lift a 100-gram weight as it popped.
• Elastomer actuator. An elastomer actuator is a hollow tube made out of an elastic material that’s constrained in one direction, such that if the tube is expanded, it will bend. Usually, these soft actuators are inflated with air, but you can do it with popcorn, too, and the researchers were able to use a trio of these actuators to make a sort of three-fingered hand that could grip a ball.
• Origami actuator. Like elastomer actuators, origami actuators are constrained in one dimension to curling as they expand, but the origami structure allows this constraint to be built into the structure of the actuator as it’s folded. The researchers used recycled Newman’s Own Organic Popcorn bags to make their origami actuators, and 80 grams of popped kernels were able to hold up a 4 kg kettlebell.
• Rigid-link gripper. Popcorn can be used indirectly as a power source by putting un-popped kernels in a flexible container in between two plates with wires attached to them. As the popcorn pops, the plates are forced apart, pulling on the wires. This can be used to actuate whatever you want, including a gripper.
It’s certainly true that you could do most of these things completely reversibly by using air instead of popcorn. But, using air involves a bunch of other complicated hardware, while the popcorn only needs to be heated to work. Popcorn is also much easier to integrate into robots that are intended to be biodegradable (DARPA has been working on this), and it’s quite cheap. It’s probably best not to compare popcorn actuators directly to other types of robotic actuators, but rather to imagine situations in which a cheap or disposable robot would need a reliable single-use actuator, to open or deploy something.
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