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.
BrachioGraph – the cheapest, simplest possible pen-plotter
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Re: BrachioGraph – the cheapest, simplest possible pen-plotter
#12The world needs more projects like this. Put a smile on my face, thank you!
Re: BrachioGraph – the cheapest, simplest possible pen-plotter
#13Sadly, 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.
Re: BrachioGraph – the cheapest, simplest possible pen-plotter
#14Earlier quoted context omitted.
I would recommend a Cricut machine. Far less maintenance and everyone in the family can use.
For the HN crowd, I'll suggest an AxiDraw over Cricut. Cricut tried adding a subscription fee just for people to continue to use the machines they already bought[1]. It was reversed over public outcry, but the fact stands that they're capable of bricking your machine with a dumb policy decision. The AxiDraw has a high build quality, and you can use their open source software to drive it or other software that uses th…
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Re: BrachioGraph – the cheapest, simplest possible pen-plotter
#15I 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/
Re: BrachioGraph – the cheapest, simplest possible pen-plotter
#16Sadly, 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?
Re: BrachioGraph – the cheapest, simplest possible pen-plotter
#17"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 later use to DIY things that don't exist yet.
Re: BrachioGraph – the cheapest, simplest possible pen-plotter
#18Sadly, 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?
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 microcontrollers are usually 8-12 bits for the entire 0%-100% range.
Re: BrachioGraph – the cheapest, simplest possible pen-plotter
#19I haven't tried this, but "under 15 euros" seems a little optimistic. The servos cost US$2.75 each [0] and you need three of them plus a fairly beefy 5-volt power supply --- the USB ports on your laptop are not going to cut it. But the big issue is the Pi. The cheapest Raspberry Pi I can find locally (that isn't a Pico) is a Raspberry Pi 400, which is US$92 [1]. For some reason a bare Pi 4B is US$230 [2], and even a 3B is US$130 [3].
I had the impression that the Pi didn't have hardware PWM, but the pigpio library used [4] says it supports hardware PWM and thus servo pulses on pins 0-31. (Shows what I know.) So if your Linux process gets delayed because the CPU is busy, it might cause arm movements to pause, but it won't command the servo to move to a wildly different position.
Still, I feel like an Arduino or Pico sort of thing would be a better fit. Much cheaper and no worries about pauses. A BluePill costs about US$11 [5] and an ESP32 is about US$7.10 [6]. That's still not quite cheap enough to hit US$15 but it might fit into 15 euros. Can the BrachioGraph software be hacked to run in MicroPython?
(I calculated all these prices using AR$207/US$ but that was the price yesterday; apparently today the peso is worth 7% less and it's AR$222/US$, so adjust accordingly.)
The PantoGraph [7] looks more appealing to me because its parallel kinematics remove the need to move heavy servomotors around.
[0]: https://articulo.mercadolibre.com.ar/MLA-619943977-mini-serv...
[1]: https://articulo.mercadolibre.com.ar/MLA-905167031-computado...
[2]: https://articulo.mercadolibre.com.ar/MLA-876846669-raspberry...
[3]: https://articulo.mercadolibre.com.ar/MLA-1143260129-raspberr...
[4]: http://abyz.me.uk/rpi/pigpio/index.html
[5]: https://articulo.mercadolibre.com.ar/MLA-737177975-modulo-de...
[6]: https://articulo.mercadolibre.com.ar/MLA-916790826-nodemcu-e...
Re: BrachioGraph – the cheapest, simplest possible pen-plotter
#20"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…
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.