When we think of bipedal humanoid robots, we tend to think of robots that aren’t just human-shaped, but also human-sized. There are exceptions, of course—among them, a subcategory of smaller humanoids that includes research and hobby humanoids that aren’t really intended to do anything practical. But at the IEEE International Conference on Robotics and Automation (ICRA) last week, roboticists from Carnegie Mellon University (CMU) are asked an interesting question: What happens if you try to scale down a bipedal robot? Like, way down? This line from the paper asking this question sums it up: “Our goal with this project is to make miniature walking robots, as small as a LEGO Minifigure (1-centimeter leg) or smaller.”
The current robot, while small (its legs are 15-cm long), is obviously much bigger than a Lego minifig. But that’s okay, because it’s not supposed to be quite as tiny as the group's ultimate ambition would have it. At least not yet. It’s a platform that the CMU researchers are using to figure out how to proceed. They’re still assessing what it’s going to take to shrink bipedal walking robots to the point where they could ride in Matchbox cars. At very small scales, robots run into all kinds of issues, including space and actuation efficiency. These crop up mainly because it’s simply not possible to cram the same number of batteries and motors that go into bigger bots into something that tiny. So, in order to make a tiny robot that can usefully walk, designers have to get creative.
Bipedal walking is already a somewhat creative form of locomotion. Despite how complex bipedal robots tend to be, if the only criteria for a bipedal robot is that it walks, then it’s kind of crazy how simple roboticists can make them. Here’s a 1990-ish (!) video from Tad McGeer, the first roboticist to explore the concept of passive dynamic walking by completely unpowered robots placed on a gentle downward slope:
The above video comes from the AMBER Lab, which has been working on efficient walking for large humanoids for a long time (you remember DURUS, right?). For small humanoids, the CMU researchers are trying to figure out how to leverage the principle of dynamic walking to make robots that can move efficiently and controllably while needing the absolute minimum of hardware, and in a way that can be scaled. With a small battery and just one actuator per leg, CMU’s robot is fully controllable, with the ability to turn, start, and stop on its own.
“Building at a larger scale allows us to explore the parameter space of construction and control, so that we know how to scale down from there,” says Justin Yim, one of the authors of the ICRA paper. “If you want to get robots into small spaces for things like inspection or maintenance or exploration, walking could be a good option, and being able to build robots at that size scale is a first step.”
“Obviously [at that scale] we will not have a ton of space,” adds Aaron Johnson, who runs CMU’s Robomechanics Lab. “Minimally actuated designs that leverage passive dynamics will be key. We aren't there yet on the LEGO scale, but with this paper we wanted to understand the way this particular morphology walks before dealing with the smaller actuators and constraints.”
Scalable Minimally Actuated Leg Extension Bipedal Walker Based on 3D Passive Dynamics, by Sharfin Islam, Kamal Carter, Justin Yim, James Kyle, Sarah Bergbreiter, and Aaron M. Johnson from CMU, was presented at ICRA 2022, in Philadelphia.