[HN Gopher] BrachioGraph - the cheapest, simplest possible pen-p...
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BrachioGraph - the cheapest, simplest possible pen-plotter
Author : Tomte
Score : 134 points
Date : 2022-06-24 15:54 UTC (7 hours ago)
(HTM) web link (www.brachiograph.art)
(TXT) w3m dump (www.brachiograph.art)
| bluenose69 wrote:
| I think the big difference between this and the pen plotters I
| used for many years is that the latter can lift the pen. Even so,
| I could not take my eyes off the video of this little toy thing
| scribbling away so diligently. What a clever thing this is.
|
| PS. the old calcomp I used back in the early 80s was quite fun to
| watch. For one thing, the motors were so strong that they shook
| the table upon which the plotter sat. For another, graphical
| terminals were quite rare then, so often you didn't know what the
| plot would be like until it started to be drawn.
| bufferoverflow wrote:
| This lifts the pen, you can clearly see the third motor at the
| very tip that's using a white piece of plastic to lift.
| superb-owl wrote:
| 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
| peppertree wrote:
| I would recommend a Cricut machine. Far less maintenance and
| everyone in the family can use.
| paulgb wrote:
| 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
| their published USB spec. EMSL are good people, but even if
| they weren't, they physically aren't able to remotely brick a
| machine once they ship it to you.
|
| [1] https://www.utahbusiness.com/cricut-announced-a-new-
| subscrip...
| [deleted]
| Jyaif wrote:
| This one works well:
|
| https://fr.aliexpress.com/item/1005002484805942.html?spm=a2g...
| gilleain wrote:
| Yes, I looked into this a few months ago. The cheapest I could
| find were 'robot' plotters that actually move across the paper.
| Not as accurate as I would like.
|
| The alternatives seemed in the PS100-500 ($120-620) range.
| acadavid79 wrote:
| 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.
| kragen wrote:
| What were the most challenging parts, and what kinds of
| problems did you run into and have to resolve?
| dheera wrote:
| 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.
| jonah wrote:
| Do "higher quality" servos exist that could be dropped in
| without much modification to the design or software?
| dheera wrote:
| 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.
| kragen wrote:
| 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.
| dheera wrote:
| 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.
| kragen wrote:
| 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.
| aidenn0 wrote:
| 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 had a simple spring-loaded solenoid
| for lifting the pen.
|
| Biggest downside is speed is limited by how fast gravity can keep
| a releasing cable taut.
| jonah wrote:
| 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/
| gtm1260 wrote:
| "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.
| kragen wrote:
| 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.
| hwers wrote:
| Love that analogy at the end, very well said
| swatcoder wrote:
| 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.
| kragen wrote:
| 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?
| sydthrowaway wrote:
| 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.
| kragen wrote:
| Were they noisy, or did they quantize motion, or were they too
| weak, or what?
| vardump wrote:
| The world needs more projects like this. Put a smile on my face,
| thank you!
| kragen wrote:
| I 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...
|
| [7]: https://www.brachiograph.art/how-to/pantograph.html
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