What makes this work is the motors.[1] It's using 3-phase drone prop motors with no gear reduction. With direct drive, the mechanics of the joints are simple. Not too expensive, either. Each motor is about US$120.
I used to work on legged running back in the 1990s. But there were no motors good enough to build a small machine like this back then. All we had were R/C servos, gearboxes, and screw drive actuators. So I was stuck in simulation. All the early legged robots were mechanical nightmares. Springs, cables, pulleys, worm gears, harmonic drives... Now, at last, enough torque in a small package.
What you want in a leg muscle is well known - something that behaves like a spring/damper combo for which you can set the spring constant, the damping constant, and the spring's zero point. People have been trying to do that by mechanical means for decades. There were pneumatic systems, hydraulic systems, things with springs, pulleys, and wires, and a kludge called a "series elastic actuator", which is a stiff spring-damper combo in series with a screw drive actuator. They're all terrible.
"You cannot strip the teeth of a magnetic field" - GE electric locomotive sales rep, early 20th century.[2] That's why they can drop this thing and have it spring back. Try that with any of the mechanical nightmares and the results will not be good.
It's 3:1 belt drive reduction.
I was involved in building --or more accurately, rebuilding-- one. I went as far as doing a whole new design CNC-machined out of aluminum.
One of the problems with this design is that these drone motors are not designed for static operation drawing lots of current without any cooling whatsoever. The 3D printed plastic parts are pretty much insulators. We had one motor smoking. The thing could not stand due to its own weight. This (and other factors) led to thinking of a new design using similar principles, a better motor and an aluminum structure that could be used to move some of the heat away from the motors.
I was wondering about that. The big innovation from Schaft was that they liquid-cooled their motors. That allowed them to build a full size humanoid without going hydraulic.
What happened that made electric motors so much better since the 90’s? It’s not like we made groundbreaking discoveries about magnetism, right?
Neodymium-cobalt magnets and 3-phase drive in small packages. Also, a big market for high-power low-weight drone motors brought the price down.
http://moticont.com/direct-drive-linear-motors.htm
That's a nice little device. Those might make good finger actuators.
Linear motors have long been a tiny niche. I'd looked at Aura linear motors, which was the leading brand back in the 1990s, but Aura got themselves into serious legal trouble. They're back now, but mostly as an aerospace supplier. Power to weight ratio in small linear motors has seldom been good. It's not clear whether this is fundamental or just lack of engineering effort due to small demand.
If I'm understanding correctly, this is not accurate. The paper and the Github repository describe it as a "dual stage timing belt transmission with a 3:1 gear reduction on each stage".
https://github.com/open-dynamic-robot-initiative/open_robot_...
A lot.
I've broken a half dozen mounts, ripped a circuit board in half and smoked a few passives in the process. My recommendation is to start with a small motor when you're tinkering with this stuff haha.
(I'm no expert but super happy customer of mjbots btw)
That said, for the kind of work ANYmal is currently being deployed on (site inspection mainly, I believe), they're hardly doing hyper-dynamic movements, so I think selecting SEAs over quasi-direct drive was a fairly judicious choice. Just like it makes more sense to have quasi-direct drives in a highly dynamic robot like the MIT Cheetah; it depends on the use case.
[1]: https://www.researchgate.net/figure/Comparison-of-EV-electri...
Edit, also what happened that made electric motors so much better since the 90’s? It’s not like we made groundbreaking discoveries about magnetism, right?
When using a linear actuator, you need to offset it from the joint so that it generates torque (that's part of the reason we have kneecaps, actually; it increases the distance between the centre of rotation of our knee and where our leg muscles are actually pulling from, increasing the output torque). This involves more mechanical complexity in the design compared to rotary actuators, which can just be slapped onto the joint directly. That offset also takes up space, so your system is now less compact.
None of the heavier legged robots I know of use direct drive actuators; ANYmal uses Series Elastic Actuators, MIT Cheetah and (maybe) Boston Dynamics Spot use Quasi-Direct Drive actuators with a low gear ratio gearbox (sub-10:1, I believe), RealHyQ uses hydraulic actuators. Electric actuators simply can't output the amount of torque these systems need to carry themselves around, so they need some form of gearing. The linked actuators you shared are direct driven, so they would need some gearing on their output. I had trouble finding anything about gearing direct drive linear actuators, which leads me to believe it's not really a thing, because if you need high loads, you'll jump to geared linear actuators, which are really just rotary actuators with a belt or rack and pinion!
Aside from the mechanical challenges associated with using a linear actuator setup, it's (to my knowledge) not necessarily better to follow biology when it comes to robots. Wheeled robots can move faster on flat terrain than any animal, so legs aren't ideal when you have access to gas-powered engines or electric actuators. Airplanes with turbines can carry much heavier loads than birds because flapping wings just doesn't scale up to the weight of a jumbo jet with luggage and passengers. Legged robots are actually more energy efficient when they don't try mimicking an animal's gait because they don't have organic muscles arranged in the same way. Any finally, rotary actuators are a better choice than linear actuators (for now) because we don't have hyper-compact, super torque-dense linear actuators that can match our muscles.
This isn't to say that biologically inspired designs can't be better, and there's a lot of research into finding ways of replicating the natural properties of certain materials, such as the strength of silk or hardness of the shells of some crustaceans. But in the end, with the technology we have, rotary actuators provide better torque density, and a more compact and simple design than linear actuators.
[1]: I didn't cite anything here because of time constraints, but it's an amalgamation of half-forgotten knowledge from some research I've done on the topic of legged robots
General Dynamics predates Boston Dynamics by decades.
('General Dynamics' is a wonderful company name. Imagine if you'd heard it for the first time today. Cool name.)
Hmmm, just looked it up: "Boston Dynamics is an American engineering and robotics design company founded in 1992" Boston Acoustics was founded in 1979. Th A40s on my desk came out in 1986, bu6t I'm not sure how old this specific pair are, I bought them 2nd hand. I have a pair of their bigger brothers, the A60s downstairs that I bought new in 1988...
* General Dynamics
* General Atomic
* General Houses (briefly, late 1940s)
following, of course
* General Motors
* General Electric
You will be free, hackers. You will be free
https://acoup.blog/2020/03/20/collections-why-dont-we-use-ch...
I'm sure there's a lot more philosophical depth to the topic, and perhaps my quick dismissal of your criticism is equally unproductive as your comment, but I see a use-case for these (frankly super cool) robots in every day society/workplaces.
[1]: https://www.subtchallenge.com/results.html [2]: https://www.engadget.com/robot-dog-gun-ghost-robotics-sword-...
1) https://biomimetics.mit.edu/
2) https://github.com/mit-biomimetics/Cheetah-Software
edit: I don't think the cheetah plans are actually open source ?
