Comment by charleszyong

6 days ago

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).

32 comments

charleszyong

Reply
tdeck  6 days ago

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.

  • 6510  6 days ago

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

    • nine_k  6 days ago

      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.

      5 replies →

  • charleszyong  5 days ago

    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.

slow_typist  5 days ago

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.

taneq  5 days ago

> 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?

  • charleszyong  5 days ago

    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.

    • taneq  3 days ago

      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?

fragmede  6 days ago

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

  • charleszyong  6 days ago

    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.

DidYaWipe  6 days ago

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

  • charleszyong  5 days ago

    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.