Hacker News Reader

Comment by tdeck | Hacker News Reader

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.

View on HN

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.

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.

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.

Reply on news.ycombinator.com