Every time we come back from a robotics conference thinking, “Okay, that’s it, people are out of ideas, there are no more unique ways of getting robots to move,” someone comes along and proves us wrong with something completely unexpected and cool. More than once, that someone has been David Zarrouk, who came up with the world’s fastest inchworm robot and this robot, which can drive forward and steer left and right using just one motor.
In a paper recently published in the journal Bioinspiration & Biomimetics, Zarrouk describes his latest innovative robot: SAW, or Single Actuator Wave-like robot, “a novel bioinspired robot which can move forward or backward by producing a continuously advancing wave.” Basically, SAW moves around by doing the worm nonstop. Funky.
As far as I can tell, real worms don’t actually get around by doing the worm. The worm (the dance move, that is) is a large-amplitude continuously advancing sine wave, which is also how SAW moves. Earthworms, on the other hand, move in a more stretchy/grippy/contract-y way (a longitudinal wave). The kind of wave locomotion that SAW uses (a transverse wave) is much more common in ocean animals, and at very small scales (like tiny organisms that swim using flagella). The appeal is that it’s a very simple motion, leading to a “minimalistic approach and high performance,” although figuring out how to make it work was quite a challenge, as Zarrouk told us:
“I had been developing robots with minimalistic approach and high performance for many years. During my undergraduate studies (in 2004), I had a project in which I tried to replicate wave locomotion to mimic miniature biological systems. I realized back then that all you need is a single motor, but how you do it is a pretty difficult challenge. During my Ph.D. I developed a worm robot, but realized that wave locomotion could be excellent for self-propelled systems in the digestive system, and that a simple design with one motor will allow to shrink it enough to locomote inside the intestines.
A couple of years ago, as I was teaching about springs (in a mechanical design course), I realized that the side projection of a spring in one direction is a circle and the other is a sine wave. Interestingly, if the spring rotates, the projection becomes an advancing wave. I figured out that this would be the simplest design I have ever thought of and started the design and building.
SAW’s design is simple, but that’s only because it’s also clever. A single motor rotates a rigid helix (the robot’s “spine”), which is embedded inside a series of rigid 3D-printed plastic links that are flexible in one dimension, kind of like a bicycle chain. As the helix rotates, the links move vertically up and down while also rotating as they flex. Where they contact the ground, the links push off as they rotate, propelling the robot forward.
Since SAW’s method of propulsion really works in only one dimension (it can go forward and backward), some wheels are necessary to help the robot steer, but they’re passive, not propulsive: All they do is turn. The largest version of the robot (the colorful one) has a top speed of 57 centimeters per second, or 5.6 wavelengths per second (which is a unit of speed I don’t think I’ve ever used before). Since each individual link doesn’t move against the ground all that much, the robot doesn’t have many problems with sliding as it wiggles, except occasionally as it gets going under hard acceleration.
At first glance, it looks like the speed of the robot is firmly constrained by how fast the wave travels, but the researchers have been able to boost the speed by 13 percent over the wave speed by adding sets of spine-type things to each link. The spines increase the effective rotational speed where each link contacts the ground, meaning that over six wave cycles, the robot is able to move forward by seven wavelengths. You can see the spines on some versions of the robot in the video if you look closely, and I would guess that they provide some additional traction benefits, which is helpful for exploring outside.
The current iteration of SAW is very simple to build, lightweight, fast, efficient, rugged, and easy to scale both up and down. Zarrouk told us that the robot “almost doesn’t require any maintenance,” which is very unusual for an almost completely 3D-printed prototype that’s this dynamic. There’s still plenty of room for improvement, though: The steering system is not the most, er, elegant, so the researchers are planning to attach two SAWs together via an articulated joint.
Perhaps the biggest challenge going forward is to develop a way to change the pitch and amplitude of the wave on the fly. That would allow the robot to adapt to different terrain and more effectively climb through pipes of different diameters. Zarrouk tells us that they’ll also be analyzing the locomotion of SAW-type robots over compliant and slippery surfaces as well as in viscous liquids (to mimic the conditions that bacteria swim in). Long-term, SAW may be reduced in size enough to find a comfortable new home in your intestines for filming and taking biopsies, which sounds like fun. Fun for the robot, that is. Probably not fun for you.
“Single Actuator Wave-like Robot (SAW): Design, Modeling, and Experiments,” by David Zarrouk, Moshe Mann, Nir Degani, Tal Yehuda, Nissan Jarbi, and Amotz Hess from Ben Gurion University in Israel, was recently published in the journal Bioinspiration & Biomimetics.