[Animats]

Very nice.

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.

[1] https://store.tmotor.com/goods.php?id=774

[2] https://news.ycombinator.com/item?id=24288264

[robomartin]

> no gear reduction

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.

[Animats]

cooling

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.

[JabavuAdams]

Thanks, that's great info. Am I mistaken in remembering that the video shows the thing standing? Is it actually not capable of this? Or is there some very short time-limit?

[robomartin]

I am sure it is. However, the design uses very expensive custom boards to save a lot of weight. The build I was involved with used Odrives and that was enough to have issues. Pre-stretching the belts is also important.

If I had to guess I would suggest that, while 3D printing is a great enabling technology one of the issues it suffers from is that of tolerances. I'll preface this by saying I did not take any measurements at all on the parts as printed. If I did, I would not be surprised to discover issues that, in the aggregate, could lead to performance issues.

[neotriple]

The robot definitely stands. In fact, it can do pretty dynamic things:

https://www.youtube.com/watch?v=YJnbFJ-8Fkg

That said, the platform does have its faults and requires a lot of hardware maintenance, but it certainly can stand and move around (walk around).

[robomartin]

The one I was involved with could never jump like that. My guess is that umbilical, at a minimum, carries power. Based on what I know, I cannot imagine the robot doing what is shown on these videos while loaded with the two battery packs that are part of the design.

Please keep in mind that this is a single data point. I am sure there have been many successful builds. It's a nice design. I was not intimately involved in the original build of the robot we dealt with. I was involved in a complete tear-down and reconfiguration to make it work. As I mentioned, this led to the design of a lightweight quadruped that is fully CNC machined and uses more powerful and capable customized drone motors.

[neotriple]

I'd be curious to learn more about the motor selection for the complete tear down and rebuild. One of the biggest buys of the Solo (in the article) design is the 'high bandwidth' current/torque control, and the price/performance seemed (at the time) to be pretty great.

[teraflop]

> It's using 3-phase drone prop motors with no gear reduction.

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_...

[Animats]

Aw, you're right. We're still not there on tiny direct drive. I've seen direct drive robots with large "pancake" motors, and thought they were driving those little guys that way. But no. They still need a lot of reduction. I'm surprised that they bounce as well as they do while back-driving through 9:1

[jcims]

I bought a couple 'moteus' motor controllers from Josh Pieper at mjbots to build a small two axis gimbal to point an antenna. It was my first foray into any kind of hardware project of this nature and I honestly didn't fully respect the amount of force these small brushless motors can generate. My head is thinking 'these things run on batteries and meant to spin at 3000 rpm, how much torque can they have?'

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)

[joreilly]

I'm not sure any of the big legged robots are using direct drive actuation yet; MIT Cheetah and Cheetah mini use quasi-direct drives with a sub-10:1 gear ratio for super high proprioceptive sensing. ANYmal actually uses series-elastic actuators, since they allow the actuator to operate at a higher speed and lower output torque, thus making them more efficient than a quasi-direct drive setup found in other quadrupeds [1]. They also allow for some energy reuse, since they can release energy stored by compressing the spring. The downside is, like you said, reduced control bandwidth, since the spring element will behave optimally under a given set of conditions and, going outside that operating range, will either be more stiff or less than is ideal.

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...

[bglazer]

What do you think about these direct linear actuators [0]? They seem super simple, and like a closer match to the way biological muscles work. Is there some reason why rotary (?) actuators are the default choice for robotics?

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?

[0] http://moticont.com/direct-drive-linear-motors.htm

[TaylorAlexander]

Well those are voice coil motors, similar to a subwoofer driver in construction. I don't know how efficient those are, but I did release some info on a brushless linear motor I designed. My linear motor design is like a regular rotary brushless motor but rolled out and made tubular. Maybe better power density than voice coil motors, tho I am not sure. The motor worked great, but I got busy with other things and never commercialized it. I have some details on github.

https://github.com/tlalexander/open_linear_motor

[kbob]

Neodymium magnets and brushless DC motors. The brushes and commutator have been replaced by a micro controller and set of power MOSFETs.

[joreilly]

I can't give an answer which would pass a proper roboticist without them wrinkling their nose, but my guess is that rotary actuators are more simple and space efficient to implement, and can generate the necessary high torques for these systems.

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