Dan Kohn's Blog

From IEEE Spectrum

By Evan Ackerman
Posted 29 Jun 2017 | 13:00 GMT

Last December, Duncan Haldane (whose research on incredibly agile bioinspired robots we’ve featured extensively in the past) ended up on the cover of the inaugural issue of Science Robotics with his jumping robot, Salto. Salto had impressive vertical jumping agility, and was able to jump from the ground onto a vertical surface, and then use that surface to change its direction with a second jump. It was very cool to watch, but the jumping was open-loop and planar, meaning that two jumps in a row was just about all that Salto could manage.

Haldane mentioned to us in December that future work on Salto could include chaining together multiple jumps, and in a paper just accepted to the 2017 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), he and co-author Justin Yim at UC Berkeley’s Biomimetic Millisystems Lab, led by Professor Ronald Fearing, show the improvements that they’ve made over the last six months. Thanks to some mechanical fine-tuning and the clever addition of a pair of thrusters, the new Salto-1P is jumping longer, faster, and higher than ever. Prepare to be amazed.

We’ve seen other jumping robots over the years, but Salto-1P takes the cake. Watch this:

Salto is short for “Saltatorial Locomotion on Terrain Obstacles,” a reference to saltatorial animals, which are adapted to locomotion by jumping. Kangaroos and rabbits are a few saltatorial animals that you’re probably familiar with, but Salto was particularly inspired by the galago, or bushbaby, which has a vertical jumping agility that no other animal can match. The galago is able to manage this thanks to a rather clever bit of leg design which uses variable mechanical advantage, leveraging the shape of their leg to amplify the force that their muscles can deliver. For all the details on the jumping ability of the original Salto (and how it’s different from other jumping robots), be sure and read our very in-depth article about it, because this article is focused on the new and upgraded Salto-1P.

The original Salto was able to control its pitch through the use of a rotating inertial tail: By spinning the tail one way, the robot could pitch itself in the other direction. This worked very well, but only in one plane, which made Salto difficult to control. Salto-1P is, according to Haldane, essentially “Salto with half of a mini-quadrotor glued to it.”

Those two little thrusters are able to control Salto-1P’s yaw and roll: When they’re thrusting in different directions, the robot yaws, and when they both thrust in the same direction, the robot rolls. Combined with the tail, that means Salto-1P (which only ways 98 grams) can stabilize and control itself in three dimensions, even in mid-air, which is what allows it to chain together so many jumps. Other hardware modifications include a deeper crouch than the original Salto, which allows more energy to be transferred from the jumping motor into the spring, giving it the highest vertical jumping agility of any battery powered robot at 1.83 m/s.

Haldane says one issue that came when they redesigned the leg mechanism to allow the robot to jump higher is that, as he puts it, “Salto lost its friendly and forgiving nature.” The robot would occasionally “fire pieces of itself across the room when the motor tore the leg-mechanism apart.” They had to do revise the design to keep everything in one piece. The video below is a compilation of Salto-1P’s “little acts of self-destruction”:

The software that Salto-1P is running to make all of this work is an adaptation of Marc Raibert’s hopping controller from 1984. Raibert’s 3D One-Leg Hopper weighed 170 times more than Salto-1P, and can’t jump nearly as high, but fundamentally the algorithm works just as well on Salto as it did on Raibert’s hopper more than 30 years ago. However, controlling Salto-1P involves some unique challenges, because the robot spends so little time on the ground. In fact, 92 percent of the time, the robot is in the air, which means that you really have to control it in the air, which is why the tail and thrusters are necessary (as opposed to control through the leg and foot).

This results in enormous accelerations (on the order of 14 g’s), and to put this in context, Haldane compares Salto-1P to a cheetah: The robot has “a lower duty cycle than a single cheetah limb at top speed,” he says, adding: “Imagine a cheetah running at top speed using only one leg, and then cut the amount of time that leg spends on the ground in half. That’s the duty factor of Salto-1P.”
“Imagine a cheetah running at top speed using only one leg, and then cut the amount of time that leg spends on the ground in half. That’s the duty factor of Salto-1P.” —Duncan Haldane, UC Berkeley

It’s important to note that when you see Salto-1P bouncing around in the video, it’s doing so untethered, but not completely autonomously: There’s a bunch of stuff going on in the background to get it to perform the way it does. The path it follows relies on motion capture, with an offboard computer (though not a particularly powerful one) receiving tracking data and wirelessly sending control commands to the robot.

“Motion capture is an easy way to track the robot that freed us up to work more on the robot mechanics,” co-author Justin Yim explains. “It’s also useful for gathering performance data since we can very closely track Salto-1P throughout its hopping.” It’s also worth noting that Salto-1P isn’t doing a lot of sensing on its own, and it’s still able to handle all those obstacles at the end of the video, which is impressive.