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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

#21

Post by kragen »

swatcoder wrote: post #20
kragen wrote: post #17

Earlier quoted context omitted.

Why, what are the super fiddly parts? It's very generally true that, when you have money and a functioning economy, buying a product to do some well-understood task requires less effort than building something to do it yourself. Similarly, it's easier to look up the answer to a well-known math problem than to solve it yourself. The advantage of DIYing existing things is usually just that you build competency you can…

I haven't tried this, but I would expect that maintaining precise and steady down-force is especially tough. The axidraw does a lot to make that work well. Sloppy x/y motion (as you'd also expect here) can just be treated as a "lofi" aesthetic, but sloppy z can mean having whole sections fail to draw or your pen digging into the surface and producing unrecoverable tears and drags of the paper.

I think the AxiDraw has a hard time maintaining precise down-force because it's so good at maintaining precise position. Maintaining a precise position requires either precise negative feedback (which the AxiDraw doesn't have) or high rigidity, and the AxiDraw does have (comparatively) high rigidity.

But the BrachioGraph has very little rigidity, so (I infer) most of the weight of the pen-lifting servo is supported by the point of the pen, and almost none of the weight of anything else. So I don't think that will be a problem, as long as your gravity field is constant. But maybe someone who's built one can comment?

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

#22

Post by kragen »

acadavid79 wrote: post #9

I built the brachiograph a few years ago. I wanted to show my kids (8 years old and 6 years old) cool things the could build by plugging a raspberry, some servos and readily available materials. We were impressed on how good we could reproduce one of those shopping mall cartoons of my daughter. It was a fun build.

What were the most challenging parts, and what kinds of problems did you run into and have to resolve?

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

#23

Post by kragen »

sydthrowaway wrote: post #11

WHen I built this I thought there was a fancy algorithm that I could use to improve the quality of the drawing.. but no, the SG90 servos are hopelessly crap.

Were they noisy, or did they quantize motion, or were they too weak, or what?

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

#24

Post by kragen »

dheera wrote: post #18
jonah wrote: post #16

Earlier quoted context omitted.

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

I mean, there are digital servos that are much better, and need I2C or SPI to control, that's another story. But analog servos could easily change the PWM range to span e.g. 1% to 99% instead of the silly 8%-10% or whatever it is now, and software would be trivial to change, the PWM pins on microcontrollers can do any duty cycle. It would be an easy 10X resolution improvement because the PWM settings on these microco…

SPI is a lot easier to bitbang than PWM though. You can speak SPI with a couple of debounced toggle switches and an indicator LED+resistor.

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

#25

Post by dheera »

kragen wrote: post #24
dheera wrote: post #18

Earlier quoted context omitted.

I mean, there are digital servos that are much better, and need I2C or SPI to control, that's another story. But analog servos could easily change the PWM range to span e.g. 1% to 99% instead of the silly 8%-10% or whatever it is now, and software would be trivial to change, the PWM pins on microcontrollers can do any duty cycle. It would be an easy 10X resolution improvement because the PWM settings on these microco…

SPI is a lot easier to bitbang than PWM though. You can speak SPI with a couple of debounced toggle switches and an indicator LED+resistor.

Maybe, but they already have PWM controllers in them, it's just they don't use the full range so you lose a few bits. By just using the full range it would be incredibly trivial change in software to just use the full PWM range again.

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

#26

Post by kragen »

dheera wrote: post #25
kragen wrote: post #24

Earlier quoted context omitted.

SPI is a lot easier to bitbang than PWM though. You can speak SPI with a couple of debounced toggle switches and an indicator LED+resistor.

Maybe, but they already have PWM controllers in them, it's just they don't use the full range so you lose a few bits. By just using the full range it would be incredibly trivial change in software to just use the full PWM range again.

Maybe, if the controllers are internally digital. If they're internally analog requiring their deviation from linearity to be insignificant over the 1%-99% range of PWM values rather than the 8%-10% range is likely to require extra complexity.

Now that Padauk has microcontrollers that cost less than most discrete transistors it might seem like an obvious improvement to redesign such controllers to be internally digital --- but Padauk's microcontrollers were out of stock this year, and discrete transistors weren't.

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

#27

Post by hwers »

kragen wrote: post #17
gtm1260 wrote: post #6

"simplest" in the sense of the simplest bill of materials, but definitley not a simple path towards plotting an image. Anyone who's tried to build one of these and actually have it plot reasonably well will know that it's super fiddly to get working. I'd save my time and just buy a proper axidraw, if you actually want to plot anything.

Why, what are the super fiddly parts? It's very generally true that, when you have money and a functioning economy, buying a product to do some well-understood task requires less effort than building something to do it yourself. Similarly, it's easier to look up the answer to a well-known math problem than to solve it yourself. The advantage of DIYing existing things is usually just that you build competency you can…

Love that analogy at the end, very well said

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

#28

Post by Jyaif »

superb-owl wrote: post #2

I've wanted a pen plotter for so long, but mainly to draw sharp geometrical figures. I don't think this would scratch the itch but I'm hopeful something will appear for <$100

This one works well:

https://fr.aliexpress.com/item/1005002484805942.html?spm=a2g...

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

#30

Post by derekenos »

jonah wrote: post #15
aidenn0 wrote: post #5

I wonder how this compares to a two-wheel plotter; I saw one of those demoed and it made surprisingly crisp drawings for how simple it was. If you're not familiar, take an inclined drawing table, have a wheel at each top corner with cable (or string) running to the pen, making a triangle. Using a servo to retract or release the cable will change the length of that side, moving the point of the triangle. The one I saw…

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
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