[Animats]

3 DOF per leg, so it needs 12 motors and controllers. Getting that under $1000 is nice.

Here's the US$18 motor: [1] Those things are getting really cheap. He did have to rewind it, though, for more turns with thinner wire. The manufacturer mentions that you can order with "custom Kv", which means you might be able to get a different winding from the factory if you order a reasonable quantity. Especially if you tell them that makes them "robot motors".

Motor overheating might be a problem. The dog, just standing, has its motors stalled under load, converting power to heat. Drones don't do that. Temperature feedback would help if this thing has to operate for extended periods. Remember yesterday's article on humanoid robots and their cooling problems.

The motor controller is nice too, and cheap at $49. Needed fixes to the firmware, but that's not surprising at the price. High performance motor controllers used to cost about $1000.

Repurposed drone technology has done wonders for legged robots. We're not quite at the point where limb drive hardware is off the shelf, but it's way better than it used to be.

[1] https://www.xntyi.com/tyi-5008-kv335/kv400-high-speed-brushl...

[Tade0]

> Motor overheating might be a problem. The dog, just standing, has its motors stalled under load, converting power to heat.

The Nao robot[0] had this exact problem and of course no way to fail gracefully. I recall checking its basic functions with my lab partner in college. I looked away for a moment and that was when it went down hard. Me and the other guy locked eyes in an "oh fuck" moment, as the robot was expensive and our thesis supervisor went through quite a lot of paperwork to have it funded. Fortunately it was intact and none of us mentioned this incident to anyone.

[0] https://en.wikipedia.org/wiki/Nao_(robot)

[regularfry]

It doesn't have to stall to stand still. Or squat, at least. With that leg layout it can safely rest against its backstops when the motors switch off. The drive motors, anyway. The hip motors probably still need to hold vertical balance, but that's intermittent, not a stall load.

[regularfry]

Oh, just thought in addition to this: you could define a standing locked position, too, if you drive the capstans to full rotation inwards so the knees only just reverse, and put extra backstops there. Not sure what that would do on uneven ground, I think you'd need to balance centrally over three legs and let the fourth hang loose.

[barrenko]

Could I pursuade you to expand on "Repurposed drone technology has done wonders for legged robots." ? Thanks!

[cyanf]

For legged, you want high torque and backdriveability (for shock absorption).

Agricultural drone motors like the eaglepower 8308 are ideal.

They’re cost effective, (~$80 from aliexpress) & you can pair them with a 3d printed cycloidal drive to fulfill both requirements.

Industry actuators are an order of magnitude more expensive than this.

Extra: If you go down this path, you’ll need a driver. The Xdrive is frequently recommended, but there’s a clone that’s significantly cheaper: https://makerbase3d.com/product/makerbase-xdrive-mini-high-p...

[lastdong]

This is pretty cool, thanks for sharing. I wish there was a (mostly) 3d printable Cara mini, but I’ll start with Cara.

[regularfry]

The layout is doable with hobby servos, but you'd need to patch in current sensing for that bit of the feedback. It's not terribly difficult conceptually but it's an extra complication that most servo power distribution boards don't give you.

You can also strap a capstan to the servo axle, if that's your thing. I've prototyped that myself in the past. You can go surprisingly far with an FDM printer, an SG90, and some dyneema bowstring. One thing I haven't tried is modding one for continuout rotation to get around the way the capstan drive limits the output angle you can achieve - I was happy reducing from ~180deg to ~45deg for what I was doing - but that's relatively well-trodden ground. Might pull that project out of the storage box it's languishing in at some point.

[Animats]

> The layout is doable with hobby servos

Mostly we're past that. Robotics with hobby servos sucks. Been there, done that, robot arm is in electronics recycling bin right now.

[regularfry]

I agree it's not great but for scale and convenience it's good. If this was going to just get scaled down, what motors would you pick?

[cyanf]

Motor: https://www.aliexpress.us/item/3256804897857573.html?gateway...

The 90KV version is what you want.

[JKCalhoun]

"The 90KV version is what you want."

You mean the "KV90 color" of course, ha ha.

That we live in an age where "agricultural drone" is even a word pairing…

[coolvision]

thanks for the Xdrive pointer, I just got samples of tinymovr and odrive controllers (with FOC), they seem nice but are more expensive.

[franciscop]

Look at the original video/article, they used drone motors for the robot dog, by reusing the rotor/stator and rewinding the coils manually.

[JKCalhoun]

"Temperature feedback would help if this thing has to operate for extended periods."

Rather than thermistors all over the place, perhaps an onboard program could calculate motor temperature by integrating current sent to each over time—assumed some degree of cooling (and perhaps here a single temperature sensor might measure ambient temperature of the environment… or could just assume "indoor temperature").

[fennecfoxy]

I would rather just have temperature sensors over the place. More reliable.

[Animats]

For a toy, you might just have a simple thermal fuse.[1] ($0.78 from DigiKey, plus "tariff may apply if shipping to the United States". About $0.15 on Alibaba, if you order enough.). For a working machine for field use, you want feedback to the controller, so it can slow down or rest or something when it overheats.

[1] https://www.digikey.com/en/products/detail/cantherm/RT121/28...

[rmast]

Train it like Disney’s Olaf where it can adjust how it walks/stands to keep motor temperature under control.

[Gracana]

That’s how industrial VFDs do motor overload protection, they just keep track of heat accumulation (based on current) vs dissipation (based on fan speed) and fault when a threshold is reached. Probably there’s more nuance to it, but that’s the gist of it.

[rmast]

If you read the epilogue, they weren't able to achieve the under $1000 price goal. Total cost ended up being around $1,450. Pretty good price reduction compared to CARA 1.0 though.

Hypothetically if I were to want a quadrupedal robot to experiment with it's not an impulse buy/build, but getting closer to that point... whereas $3000+ is a hard pass (e.g. Apple Vision Pro territory).

[regularfry]

It's $1450 if you discount the construction time, as ever. Which ordinarily wouldn't be worth commenting on, but in this case it means rewinding 12 motors which just sounds like an exercise in tedium and hand pain.

[15155]

Only because they didn't know how to ask the vendor to do it for them.

I guarantee this vendor would be delighted to make them to spec at a 1ku volume, max. Rewinding isn't even a meaningful SKU distinction or line retool, it's a configuration parameter.

At 12 motors per product, it's easy to hit MOQ.

[drzaiusx11]

There are inventor programs that'll literally ship you to Shenzhen to build connections to manufacturing sites and even provide you with a liaison, etc. I only know this because I was once in a program that did exactly this.

[modeless]

Mind linking some?

[drzaiusx11]

HAX Ventures (Formerly HAXLR8R) https://hax.co/

TroubleMaker (Conquer! program) https://troublemakershenzhen.com/member/apprentice-membershi...

Seeed Maker Camp https://www.seeedstudio.com/blog/2024/01/15/introducing-make...

I was an alumni of HAXLR8R before the rename.

[Animats]

Rewinding drone motors for high torque and lower speed seems to be a popular thing. Here's the commercial process of machine winding motors like that.[1] That's a medium-volume machine, loaded and unloaded by hand, and adjustable for different wire and motor geometry. Here's a hobbyist version of a similar winder.[2] And another automatic hobbyist winder. Those wind under uniform tension, and with wire positioning, so you get a smooth, tight winding. This matters if you're going to use the motor much. Doesn't matter as much for short-lived toys.

There should soon be enough convergence that low cost, high torque, low speed models are off the shelf items. It's a great time to be building robots. Used to be all uphill on the parts side.

[1] https://www.youtube.com/watch?v=cMk2qFFcSho

[2] https://www.youtube.com/watch?v=486nUU2FjGU

[3] https://www.youtube.com/watch?v=e-tAsM0K7Sg

[dezgeg]

Reminder that this was a student project.

[squarefoot]

Not my field so I'm likely using the wrong name for the concept, anyway, don't stepping (bistable) brakes exist? I mean, one pulse to one pin engages and one pulse to another pin disengages, just like step relays, with the current consumption being zero when it's not operated so that the robot can be kept indefinitely in resting position without wasting energy to keep motors stalled.

[Animats]

Stepping motors are not good at stationary power consumption. They use power even with no load on them.

Brakes for robot joints are common in industrial robots. They're usually part of the emergency stop system. If power fails, the controller crashes, or someone pushes the emergency stop button, spring-driven brakes lock all major joints to stop all motion.[1] That might be useful in a quadruped, which can park without active balance.

[1] https://www.techbriefs.com/component/content/article/28812-c...