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Comment by charleszyong | Hacker News Reader

So true. Every time I solder surface mount components, I always wish I could have a steady hand. Sadly, this arm doesn't have that kind of accuracy. The output shaft of the servos we use has about 1 degree of wiggle room and the mechanical structure adds more.

To get better accuracy, if sticking with this kind of RC servo, it's basically required to have two servos per joint to preload each other to kill that wiggle room. It's something I've been calculating, but I just can't figure out a way to offer it at a good price.

Interestingly, for arms that are popular in academia, even when the price goes to $10k (like ARX or Trossen), the wiggle room is still there (better, but still there).

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I was recently trying to get better angular accuracy with servos and minimize backlash. One option that kind of worked was to have a pulley on the servo shaft which wound a string attached to a spring to add mechanical bias.

But I ended up giving up and going with 400 step stepper motors instead. They're larger, draw more current, and the drive circuitry is more complicated (it can't get simpler than a PWM servo after all). But they're accurate and significantly quieter.

I know nothing about the topic but could one build some kind of 3d pantograph to scale down the motions?

What you need is to scale down the tolerances. To remove the wiggle room.

One of the solutions that does not add a bias that I remember is two identical flat gears on the same axis with a spring that tries to rotate them one relative to another. This removes the wiggle room between this composite gear and the next, regular gear. The motor may have wiggle room, but the gears (which carry angle sensors, don't they?) move without wiggling, and react immediately as you reverse the direction. The load is limited though: the beating surface is twice as small, and the friction is higher.

have to share the noob thought because it is funny: You could attach an unbalanced wheel to a motor and induce a vibration to maximize wiggling and frequency across the available w-room.

I sometimes wonder if it makes more sense to just use those yellow gearboxes, everyone seem to start with SG90s only to reimplement most of the servo part.

Here is that animation you've never asked for.

https://img.go-here.nl/unwiggle.gif

One could put those in series too and get even less range of motion in exchange for less wiggling.

One could also duplicate the contraption on both sides. Then could replace the arm with cables (under tension) and control motion further down the arm.

Furthermore it seems you could remove the motors from the moving parts?

Ill let myself out.

Motors inside the big body and limbs actuated by series of strings is like many arthropods are powered. Evolution approves!

You could also control the tension of the wires and have accuracy on demand without the wear. I have this mental image of a darts player moving his arm back and forwards to smooth out the tension.

Here's how to do it:

https://www.youtube.com/watch?v=GCHXNcpq3OA

Check his channel for servo motor modifications.

Oh, I think I actually saw this! But it's a very manual process with a lot of small parts to assemble, and I want to eventually design a product I can manufacture myself so I didn't pursue it.

I understand. But you could sell a normal version and an enhanced version for those who need the accuracy.

Backlash is a real problem. I’m working with the supplier to prototype a small batch with 20 to 30% less backlash, though this may come at a slightly higher price and potentially a shorter lifespan.

If cost isn’t a concern, harmonic drives combined with brushless servos are excellent. I have a few harmonic drive units, and they’re truly amazing.

There are cheaper options for soldering SMD-parts on prototype boards. Developing and teaching robot arms to do it would give a good demonstrator but economically it’s a disaster. And mass production is already highly automated.

even something twice the price ($438) would still be a great deal. Mind telling us something about your pricing strategy trade-off consideration matrix?

Design for manufacturing is one thing. I did it a lot when building micro gas turbines in college. Sometimes changing the design or manufacturing process will make it 10x faster to make one while not compromising the performance.

The second thing is low margin. When people are pricing hardware, they usually plan a 50% to 100% margin to offset various costs that happen in the real world. From what I've heard, in extreme cases, some products cost around $100 while they are being sold at close to $1000. I believe in the Prusa printer approach: you design a good product and price it a little bit above cost. So the company grows with the community.

Deep down, there are so many times that I wish I could afford a fancy tool like a Milwaukee drill or a Mitutoyo caliper. And in extreme cases, I really wish I could have a HAAS UMC-400 or even a KERN Micro HD+. Now that I can set the price, I really wish I could make someone get what they want without breaking their bank.

that's really great, thanks for sharing!

For folks curious about why these kinds of margins are built in for hardware products, I found these videos informative:

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

https://m.youtube.com/watch?v=IdAT_SIRK8c&pp=QAFIAQ%3D%3D

https://m.youtube.com/watch?v=63pnz-z_2sU&pp=QAFIAQ%3D%3D

As a customer, thin margins scare the crap out of me. It means you probably won’t be around in a few years.

Seems like this stuff is mostly built on open source, so no need to be too scared.

that's more scary, not less! Sure, if the worst happens, since it's open source, we'll have the source, but being open source means they've got to figure out a more complicated business model than make thing, sell thing, profit. there are some success stories but also a lot of failures.

I guess I should clarify that it's open source, and seems to be essentially forked from HuggingFace's SO-101, so it's probably not too hard to fall back to that stuff (with fewer features).

That’s not at all reassuring to someone who wants the benefit of the product, not the obligation to become an expert on the product.

I’ll buy one of these for sure, but I would cheerfully spend 3x the price if it meant being sure of support and repair and software updates for a few years.

I have bins full of exciting devices that no longer integrate, many of which have open source communities, which I just don’t have the time to deal with.

This is based on the open-source HuggingFace/LeRobot SO-101, so it'll probably be possible to fall back to that. Or you can just get an SO-101. But I think this is attempting to draft on that ecosystem, and this is cheaper than Alibaba parts to build an SO-101.

It’s worth reading the history behind Raspberry Pi. It depends on the team. With the right product market for, this strategy has and will work. In my view, the Vassar team got the pricing strategy right to do something even bigger than Raspberry Pi if they can scale at this price point for the low end model.

For software support, the software is open source, so the community can keep it going even if we’re no longer around (hopefully not anytime soon).

For repairs, the components are inexpensive enough to simply swap in a new one, and you can always find replacements on AliExpress.

> it's basically required to have two servos per joint to preload each other to kill that wiggle room

Couldn’t you preload with some form of spring?

The advantage of servos is that they can provide constant torque preloading, allowing the preload to be kept low (otherwise, the servo will overheat) but still sufficient.

A spring might also be an option if designed properly. I’ll probably give it a try in July.

You can use a spring with long travel and low K (like a very soft torsional spring, preloaded several turns, or a spring connected by a wire to a drum on the joint) to reduce the change in torque across the range, if that helps?

Well, based on that info, I guess usability for pick-&-place of surface-mount components on PCBs is out of the question.

Yeah… the accuracy isn’t good enough. A gantry system is probably needed.

There was one robot startup (Haddington Dynamics) figured it out how to do it at a higher price. Sadly they've been acquired and shifted directions I think.

Yep, a gantry is exactly what I was imagining too.

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