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Comment by Torkel | Hacker News Reader

It has the wrong kind of motors to be interesting. Servos are a dead end, imo.

The interesting robot projects now are the ones embracing brushless motors with encoders and dynamic walking gaits. It’s the same components that are used in drones.

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A brushless motor with an encoder and a feedback loop is a servo. It's just not a crappy integrated RC model servo motor assembly.

The (ab)use of the term "servo" is a major pet-peeve of mine.

Sure. I should have written "rc-servo". But in this context it was not ambiguous and the point stands - anything that uses rc-servos is a dead end. Personally I'm also skeptical of series elastic actuators - it seems like a half-assed solution where the proper solution is to embrace full direct-drive digital control of electrical <-> force conversion.

The "problem" I've seen with a lot of walking robot projects is, where an RC car needs 2 motors and a quadcopter 4, most "interesting" walking robots need 12 - which means they instantly have to start making compromises due to cost and weight.

This robot ain't exactly large - but it's already needed $450 worth of Dynaixel AX-12A servos.

We have a hexapod (18 Dynapixels) at work ... apart from giving demos to visitors it's not used at all. Gait patterns and (inverse) kinematics are a solved problem ... dynamic gaits are becoming more mainstream now, but those require motors that are reasonably backdrivable. Maybe the Dynapixels can be configured to do that, but I think the high gear ratio will prevent that.

I'm interested in Robotics (I made a simple ping-pong ball collector a while back https://sendcat.com/dl/D9DI15MAOjhKMFJYLJLADj64ywyU2qGWaKMSl...). Are there places I should go or things I should read to find out more about the latest developments in robotics in general, or the use of brushless motors with encoders/dynamic walking gaits specifically?

Nice build :) Have you checked out EasyEDA + jlcpcb? It can improve the electronics aspects of robot projects.

I searched a bit now to see if any new projects have shown up. Came across this one: https://hackaday.io/project/167845-mjbots-quad Could maybe be interesting, but I haven't looked into it.

Here is a chinese motor manufacturer that makes more sane motors: http://www.gyems.cn/product.html

Getting a drone or gimbal working properly from scratch is a lot of work. And these dynamic walking systems are more complex endeavors because there is less info out there. So I would recommend to find some project with people who seems talented and has a nice atmosphere and then get building :)

edit: Ratholed on this, here is a nice blog with inspiration (scroll for lots of awesome) https://build-its-inprogress.blogspot.com/

> Nice build :) Have you checked out EasyEDA + jlcpcb? It can improve the electronics aspects of robot projects.

Thanks! Yeah, the electronics is a bit gross. This build was probably 6 years ago perhaps? I got it to the point that it worked and then it was kinda "done" and all that was left was polish, but it was really a toy so there wasn't much point and I left it.

Planning on building something for slug and snail control (NW UK, they can be a problem), and cat/badger/squirrel scaring in my garden (they dig up things). Will likely make use of PCBs with this so will check out the links.

> Here is a chinese motor manufacturer that makes more sane motors: http://www.gyems.cn/product.html

So I did some digging and found this guy:

https://www.youtube.com/user/SkyentificTube/videos

He does a some testing of some Turnigy motors (a bit like this: https://hobbyking.com/en_us/turnigy-aerodrive-sk3-6374-149kv..., there's a whole range) which looked cool, and he was using an odrive controller https://odriverobotics.com/ which also looked pretty handy, so there's some stuff to check out there, but he also did some tests with some gyems motors and they looked great:

https://www.youtube.com/watch?v=smQJeg87ff8

> dynamic walking systems are more complex

Given the speed of slugs and snails I am going to probably go for a less dynamic movement style. Less force, smaller motors, longer battery life. Though I like the idea of trying to program dynamic movement. The physical build for my more static movement style shouldn't be too hard to do.

> https://build-its-inprogress.blogspot.com/

Thanks, this is great!

Just going to share a YouTube channel (Skyentific) that I have learned a ton from when it comes to the considerations of sizing and power consumption when selecting motors for robotics. https://www.youtube.com/channel/UCcgqJ1blFKqbC2bWGY4Opmg

Servos (hobbyist level) are a dead end in this application, but even marginal quality BLDCs are quite a bit more expensive. These servos can be swapped out for BLDCs in a retrofit, so I think this is a good starting point that can be expanded on.

Can you do without an encoder for legs? Maybe attach an IMU like you do for gimbals? Or measure current?

No. For these motors to do their job at these low speeds you need to know where the windings are relative to the magnets so you can apply a controlled force instantaneously.

Btw, this is true for gimbals too, and good gimbals use sensors.

Could you provide some examples of these projects?

Stanford Doggo: https://nathankau.com/Research https://github.com/Nate711/StanfordDoggoProject

Here are two:

MIT Cheetah: https://biomimetics.mit.edu/

Unitree/laikago: http://www.unitree.cc/

Here is an open-source, 3D-printable quadruped with (almost) direct-drive actuation:

https://open-dynamic-robot-initiative.github.io

Paper: https://arxiv.org/pdf/1910.00093.pdf

Videos: https://www.youtube.com/watch?v=MkXU-5Oupg4

Why are they a dead end?

Too slow, too weak, too inefficient, too much backlash, not backdrivable, no energy regeneration, no built in force sensing.

Add sloppy controls to the list.

They aren’t sexy, isn’t that it — you want a sexy robot and RC servos don’t cut it

It's not that rc-servos are not sexy. It't that they suck. For a number of specific reasons well listed in other posts here.

RC-servos are very cheap and very easy to use, so there are many projects using them. But there is no way to make a good robot out of them. It's a local maximum.

All true. But they are cheap, available, and have well understood limitations. Which means you can design and build your project quickly and cheaply, then upgrade to brushless motors with encoders or whatever when you have the time and funds. I think from that aspect they are a great way to get a project started without flinging cash at it.

IMO the only inherent issues with hobby-style servos are lack of feedback (force and position) and lack of direct torque control. The rest of the complaints are features of all cheap positioning elements and aren't unique to hobby servos.

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