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.
The sketch (A) and photos (B-D) show the prototype actuator, made of silicone sheets, wrapped around nichrome coils filled with popcorn kernels. When current is applied, the kernels pop and jam against each other, creating a relatively rigid rod, which can lift a 100-gram weight (E).
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 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 curl 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 unpopped 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, for example.
For more details, we spoke with Cornell researcher Steven Ceron via email.
IEEE Spectrum: Where did you get the idea to use popcorn in all of these different ways? Why do you think nobody has tried this before?
Steven Ceron: My advisor, Kirstin Petersen, was actually the one that came up with the idea several weeks before I joined the lab, and she was thinking of a new type of soft/compliant actuator that would be biodegradable and demonstrate large changes in mechanical properties. It’s definitely a new concept for biodegradable actuators, and at first glance it might seem silly to use something as random as popcorn, but as we show in the paper, the mechanical properties drastically change upon actuation and the expansion can be quite large, and these characteristics can be used to our advantage.
How much variability is there between the unpopped and popped characteristics of different varieties of popcorn, and how does this affect the way that popcorn can be optimally used in robotics?
About the same temperature is needed to pop the different types of kernels. The final size of the popped kernels, however, can vary drastically depending on the method of heating. For example, using our current setup for directing hot air to the popcorn kernels, we get a volumetric expansion ratio of about 5, but with a microwave oven we can get up to 15 times the original volume. The amount of expansion is also going to affect the amount of force that is exerted when the kernel pops, so this means we could have a good amount of control over the exerted force simply by choosing the heating method.
What are some potential applications where the irreversible nature of popcorn’s state change wouldn’t be a problem?
We could imagine a scenario where we need an isolated chamber to be hollow at first and must then be filled with a material that, for example, could serve as thermal insulation. The kernels would be able to flow as a granular fluid into the desired site and then pop upon heating within the chamber. If the chamber needs to be emptied, then we could simply flow water through it to dissolve the popcorn and push it out.
The paper mentions that you’d like to design “stand-alone popcorn-driven robots.” Can you describe how those robots might work, what they would look like, and what they’d be able to do?
We have thought of different possibilities, but one interesting demonstration would be a small robot that can hold a packed chamber of kernels. The robot would be able to rapidly/locally heat a single kernel and push it out of the robot, simultaneously filling an open region with popped kernels and propelling the robot forward. We would be able to fill empty isolated spaces (maybe for thermal insulation, or added structural support) without having to open up the area.
What kind of response did the paper get at ICRA?
People were genuinely interested in the topic and what the possible applications of this novel robotic actuator could be. It’s definitely the first work of its kind, so we really hope that it will motivate others to consider non-traditional materials, even something as random as popcorn, when developing new types of actuators and robotic devices.
What are you working on next?
I did this work when I first arrived at Professor Petersen’s lab, so I have been exploring different projects throughout the field. I am switching gears now to study how we can leverage embodied intelligence and adaptability in soft robots to reduce the cost and control complexity required for a large robot collective.