16 May

Polargraph

A while ago I tried building a Polargraph at a hacker event. It kinda worked… but I ran out of time before I was able to get my head around the calibration. Since then the Ardunio Uno, motor driver shield and motors and sat in the bottom of a drawer, so a few months ago I decided it was time to finish this project properly.IMG_20150516_105012

After a lot of faffing with a related project (Eggbot/Shperebot) and the same electronics setup, I decided to bin the Arduino shield and switch to two stepstick motor drivers and assembled them on a mini breadboard stuck on top of the Arduino Uno.

This new setup was a massive improvement so I decided to design a PCB for it.

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Version 1 works but has a few mistakes and gotchas that require a few hacks. I’ll fix these issues in the v2 PCB. The PCB is sort of a mini motherboard that takes two stepper drivers (DRV8255 in the picture) and an Arduino Nano clone (using the CH430G USB to serial chip) and has connectors for the stepper motors, end stops, a servo and power to make wiring up as easy as possible.

With that done, I was able to concentrate on putting together a Polargraph.

There are plenty of Polargraph components on Thingiverse (including my first attempt at the hacker event), but most of them didn’t fit the bill or seem a bit heavy on plastics, so I designed my own motor mounts, pulley wheels, Gondola and feet to suit the parts available from Homebase (Chip board, ball chain and a 25mm dia broom handle cut up to make the feet).

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I’ll upload the models to thingiverse once I’m happy with them. Will also upload the spherebot and polargraph firmwares (tweaked for my PCB) to github.

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It has taken a while to figure out the shading calibration, but it turns out it is reasonably simple. The Polargraph controller software has a “Test pen wdiths” feature that draws 11 pixels of varying density. In my case i’m using a 0.4mm pen, so I told the controller to start as if it had a pen 0.1mm thick and increase the ‘thickness’ by 0.1mm for each pixel. This has the effect of drawing lighter and lighter pixels. I then picked the darkest pixel that wasn’t completely solid (0.3mm), told the Polargraph software that my pen wdith was 0.3mm and the square wave render of Salvador Dali is the result. (The really dark section top left is where I started this print with the pen width of 0.1mm. It wasn’t showing any difference between the dark and the really dark pixels so I canceled the print and ran this print over the top).

 

Last Updated on 26 May 2015 by Mark