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BrachioGraph – the cheapest, simplest possible pen-plotter

brachiograph.art

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Re: BrachioGraph – the cheapest, simplest possible pen-plotter

#41

Post by b15h0p »

derekenos wrote: post #30
jonah wrote: post #15

Earlier quoted context omitted.

I've seen this mechanism as a CNC router for cutting pieces out of a sheet of plywood. Seems like it would work better for that use since the feed speed of a router needs to be slower than a pen in general. This is the first Open Source one I came across with a quick search: https://www.maslowcnc.com/

I was inspired by my Maslow CNC to build this polar plotter: https://derekenos.com/project-weezel

Wow, this looks so cool. Do you have a BOM or some schematics or even source code lying around? I would be interested to try something like this myself.

Re: BrachioGraph – the cheapest, simplest possible pen-plotter

#43

Post by kragen »

myself248 wrote: post #38
kragen wrote: post #34

Earlier quoted context omitted.

The problem was that their potentiometers had too much hysteresis? That seems like something you could compensate to a significant degree in software?

It's not really predictable; you'd need a camera watching it to continuously train and compensate and improve. Which would be neat, honestly. With the lever-arms on these things, you're going to get into all sorts of details with things like the pitch-circle variation of the plastic gears, which may have been nearly-ideal when they were produced but will become uneven with wear. I would go the other way and grab any…

Theorizing a bit more, the pitch-circle variation of the gears doesn't matter at all because it's inside the control loop.

Re: BrachioGraph – the cheapest, simplest possible pen-plotter

#44

Post by kragen »

derekenos wrote: post #30
jonah wrote: post #15

Earlier quoted context omitted.

I've seen this mechanism as a CNC router for cutting pieces out of a sheet of plywood. Seems like it would work better for that use since the feed speed of a router needs to be slower than a pen in general. This is the first Open Source one I came across with a quick search: https://www.maslowcnc.com/

I was inspired by my Maslow CNC to build this polar plotter: https://derekenos.com/project-weezel

That's awesome! What things worked surprisingly well or poorly, and what problems did you run into?

Re: BrachioGraph – the cheapest, simplest possible pen-plotter

#45

Post by dekhn »

kragen wrote: post #34
sydthrowaway wrote: post #32

Earlier quoted context omitted.

Hysteresis kills this project

The problem was that their potentiometers had too much hysteresis? That seems like something you could compensate to a significant degree in software?

These are toy servos. It doesn't cost much to go up to the next level of servos, instead of attempting to correct in software.

Re: BrachioGraph – the cheapest, simplest possible pen-plotter

#46

Post by dekhn »

jonah wrote: post #16
dheera wrote: post #13

Sadly, a lot of these servos have terrible resolution, especially because a full range of motion corresponds to only a very small fraction of the duty cycle spectrum. Someone should make "good" servos that have make use of more of the duty cycle.

Do "higher quality" servos exist that could be dropped in without much modification to the design or software?

There are slightly larger servos which are much better, all the components are better, but they're expensive ($30 or so each) and beyond that, there are robotics servos (like dynamixels) but those are fairly pricey.

Re: BrachioGraph – the cheapest, simplest possible pen-plotter

#48

Post by kragen »

dekhn wrote: post #45
kragen wrote: post #34

Earlier quoted context omitted.

The problem was that their potentiometers had too much hysteresis? That seems like something you could compensate to a significant degree in software?

These are toy servos. It doesn't cost much to go up to the next level of servos, instead of attempting to correct in software.

Cost is relative; as I noted on here the other day, the minimum legal wage here is US$207 per month https://news.ycombinator.com/item?id=31830007, so even these three toy servos cost a day's wages, perhaps the equivalent of US$1000 for you. And lots of people here don't make even that much, and most people live in poorer countries than Argentina.

The fact that a better product is available at a higher cost is not relevant to the question I was asking, which is what the particular problem is with the cheap servos.

Re: BrachioGraph – the cheapest, simplest possible pen-plotter

#49

Post by dekhn »

kragen wrote: post #48
dekhn wrote: post #45

Earlier quoted context omitted.

These are toy servos. It doesn't cost much to go up to the next level of servos, instead of attempting to correct in software.

Cost is relative; as I noted on here the other day, the minimum legal wage here is US$207 per month https://news.ycombinator.com/item?id=31830007 , so even these three toy servos cost a day's wages, perhaps the equivalent of US$1000 for you. And lots of people here don't make even that much, and most people live in poorer countries than Argentina. The fact that a better product is available at a higher cost is not re…

they have little plastic gears that strip and can't hold even the tiniest weight. They are hard to mount and their rotation is noisy and crappy.

Re: BrachioGraph – the cheapest, simplest possible pen-plotter

#50

Post by kragen »

dekhn wrote: post #49
kragen wrote: post #48

Earlier quoted context omitted.

Cost is relative; as I noted on here the other day, the minimum legal wage here is US$207 per month https://news.ycombinator.com/item?id=31830007 , so even these three toy servos cost a day's wages, perhaps the equivalent of US$1000 for you. And lots of people here don't make even that much, and most people live in poorer countries than Argentina. The fact that a better product is available at a higher cost is not re…

they have little plastic gears that strip and can't hold even the tiniest weight. They are hard to mount and their rotation is noisy and crappy.

None of those seem like they should be a problem for BrachioGraph except the too-vague "crappy"? It doesn't have the servos trying to lift weight (except for the lifter servo, which has to lift the pen and popsicle sticks) or fighting resistance that can strip their little plastic gears. It mounts them with hot glue, which seems to work okay (surely it introduces uncontrolled compliance that you could reduce by gusseting them with epoxy) and doesn't require them to be quiet.

I'm trying to understand what the problem is with cheap toy servos that leads to problems for BrachioGraph, in enough detail that I could make plans to mitigate the problem.

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