One of the biggest challenges with swarms of robots is manufacturing and deploying the swarm itself. Even if the robots are relatively small and relatively simple, you’re still dealing with a whole bunch of them, and every step in building the robots or letting them loose is multiplied over the entire number of bots in the swarm. If you’ve got more than a few robots to handle, it starts to get all kinds of tedious.
The dream for swarm robotics is to be able to do away with all of that, and just push a button and have your swarm somehow magically appear. We’re not there yet, but we’re getting close: At IROS this month, researchers from the Wyss Institute for Biologically Inspired Engineering at Harvard presented a paper demonstrating an autonomous collective robotic swarm that can be manufactured in a single flat composite sheet. On command, they’ll rip themselves apart from each other, fold themselves up into origami structures, and head off on a mission en masse.
Here’s how the bots look before the self-assembly process:
Image: Wyss Institute/Harvard
There are four robots in this sheet, and they’re pretty darn near two dimensional. Want more robots? No problem, just make the sheet bigger. The sheet itself consists of six layers, which are all automatically laser machined: A pre-stretched polystyrene, or PSPS, layer (a kind of shape-memory polymer) in the center, sandwiched between layers of copper circuits etched into polyimide sheets, with paper substrates for support. The PSPS is the magical stuff: When heated above 100° C (which can be done by running a 2.5-ampere current through the copper circuitry), it shrinks, which is what powers the robots’ self-folding behaviors.
One of the robots after detaching itself from the others and self-folding. Image: Wyss Institute/Harvard
Otherwise, each robot consists of some discrete electrical components that have to be placed by hand, but according to co-author Michael Tolley, “we foresee straightforward ways to automate these steps.” Self-folding robots that use shape-memory polymers have been done before, but the challenge with them is to accurately control the folding. To address that issue, the Harvard researchers came up with a clever feedback-controlled assembly technique by using phototransistors and infrared LEDs to precisely measure the fold angles, “greatly improving the repeatability of self-folding,” says Tolley, who now leads UC San Diego’s Bioinspired Robotics and Design Lab.
The final thing that sets these self-folding bots apart is their ability to go from a single continuous sheet to a swarm of discrete robots. Self-folding joints that are designed to prevent folding causes the PSPS to instead rip itself apart, allowing each robot to split off by itself, where its vibration motors can help it buzz along flat surfaces seeking sources of light. Here’s a video of everything in action:
Martin Nisser, first author on the paper, explains how it all happens:
To expedite the manufacturing process of large robot collectives, the robots are fabricated as part of a single connected composite by bonding together layers of structural material, flexible circuit boards and shape memory polymer (SMP) that connects individual robots in the collective.
When large currents are applied to certain resistive circuits on the circuit board, they dissipate heat, raising the temperature of the SMP in the adjacent layer and causing it to contract. If this heated SMP bridges a leg hinge, its contraction forces the structure to fold in a pre-defined direction dictated by cuts in the supporting paper structure. By controlling this process with the help of on-board sensors, the robots are able to accurately fold their legs to specific angles in order to stand themselves up.
Prior to this self-folding, the same technique is first used to detach individual robots in the collective from each other by designing the interfaces between individual robots to resemble the leg hinges— but with a key difference. By limiting the strength of the SMP that bridges neighboring robots, the contractile forces generated during heating is not strong enough to cause folding, resulting in the SMP pulling itself apart instead which detaches the robots from each other. This technique allows the mechanical separation and self-folding of individual robots to be achieved using the same materials and manufacturing processes.
A group of four robots (left) are fabricated on a single composite sheet held together by a layer of shape memory polymer (SMP). The robots autonomously detach from one another (middle) by heating the SMP layer with an electrical current, and then self-fold their “legs,” standing up in the process. The self-folded robots (right) use vibration motors and embedded sensors to slowly navigate. Image: Wyss Institute/Harvard
These particular robots are fairly limited in what they can do, with a top speed of 10 mm/s (only on smooth horizontal surfaces) and a battery life of just over 3 minutes. They’re also not the most robust, likely because each of them has 75 components that all have to be hand-soldered. But the researchers say there are plenty of ways of automating this process, and once they’ve got that figured out, they’ll be able to roll out sheets and sheets of these little guys, which could be handy in a number of applications. As Tolley explains:
The complexity of these machines is limited only by one’s imagination. In other work, we have demonstrated linkage mechanisms for a walking robot, and others in Prof. Wood’s lab have demonstrated bee-inspired flying robots using similar fabrication techniques. I believe this work will lead the way to the rapid, automated fabrication and deployment of low-cost, customized robotic systems for search-and-rescue, medical, and other applications.