BrachioGraph – the cheapest, simplest possible pen-plotter | Hacker News Reader

BrachioGraph – the cheapest, simplest possible pen-plotter

brachiograph.art

209 pointsTomte49 comments

"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.

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?

Wouldn’t any spring work for application of downforce? Not that I’m immediately clear on how to attach that to the brachiograph, but for purchasable machinery it seems it should be relatively easy to solve.

The arm from the BrachioGraph shoulder to the elbow is a spring. I mean, not just in the sense that literally every solid object is a spring, but in the sense that it has a large compliance; the derivative of force with respect to equilibrium position is small. But if you want constant downforce, at least at low accelerations, the best thing is a weight, and almost all the downforce on the BrachioGraph end effector appears to be from the weight of the lifter motor.

Love that analogy at the end, very well said

Thanks!

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?

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.

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.

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

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

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 £100-500 ($120-620) range.

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 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...

Yeah, screw Cricut and their policies. When they sued Sure Cuts-A-Lot into removing support for Cricut machines, I completely swore off their heretofore-interesting hardware.

Get any old Roland clone. You can often score an old USCutter on Craigslist for similar prices, the hardware is way more durable, and it's all simple parts you can replace without vendor lock-in. They speak HPGL and there's a million softwares to drive 'em, including Inkcut which sorta sucks but it's free.

Don't buy a Cricut. They are an insanely shitty company. The Oracle of vinyl cutting.

Get a Graphtec Silhouette Portrait 3. £180 and you can use open source software with it.

This one works well:

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

I would recommend an old Graphtec or Roland one. They are designed for this and work extremely well in good condition. The downsides are you may have to wait a while to find one on an auction site, and may have to work to either find proper pens or attach ordinary pens.

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.

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.

I've used my 3D printer as a plotter, you can get pretty good results by first printing a pen adapter and then using this Inkscape plugin: https://github.com/soukupak/inkscape-unicorn-for-marlin

https://github.com/bdring/midTbot_esp32 is retired now but contains lots of good ideas.

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?

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.

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.

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.

The world needs more projects like this. Put a smile on my face, thank you!

Wait a second, how'd they upload a video to Vimeo? I thought Vimeo shut down.

Edit: Well I guess I just made that up somehow. I swore I read something about people moving their content because Vimeo's over.

You we're probably thinking about recent leave due to pricing change.

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

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?

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?

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 broken 3d printer, rip off the hot-end and stick a pen in its place. It's slower by default but a thousand times more precise, and you can tune up the speed now that you're no longer whipping around a heavy extruder or waiting for plastic to flow.

Hysteresis is, in general, predictable, though sometimes compensating for it is impossible. Are you speaking from experience in measuring SG90 hysteresis and gear wear or are you theorizing?

(I'm theorizing, because I think theorizing is valuable, but I want to be careful to distinguish theoretical predictions from reports from experience.)

The broken 3-D printer idea is brilliant. Unfortunately, right now there's only one broken 3-D printer for sale near me on MercadoLibre, and it's an SLA printer, so it only has one degree of freedom of positioning. However, it won't be a thousand times more precise; typically RepRap FDM printers like the Prusa or Creality have positioning errors on the order of 0.1 mm, and a thousand times less precise than that would be an error of 100 mm. The BrachioGraph drawings seem to have an error of about 2 mm, only about 20 times worse than a bog-standard RepRap, and actually, 20 is less than 1000.

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

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.

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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