Build your own pen plotter for less than $15 • The Register

Ubuntu Summit BrachioGraph is a DIY pen plotter made up of a Raspberry Pi Zero, a couple of $2 servo motors, and household parts like clothespins and bulldog clips, and is controlled in Python.

One positive side of the Ubuntu Summit was that, unlike some of the more dignified events from the commercial side of the industry, here people showcased their own personal passion projects. BrachioGraph is one of them and was brought to you by its creator, Daniele Procida, who works full-time as Technical Lead at Canonical.


BrachioGraph in action. Image courtesy of Daniele Procida

Nowadays, if you want a printed copy of something big, like a a construction diagram, there are large format printers that will accommodate you, but this task used to fall to XY plotters. Decades ago your author himself had one attached to an acorn Archimedes. As Procida put it, “You don’t really see them anymore, but it was nice to watch them work.”

They were also quite a complex kit – they actually featured more than once in On Call. There are still hobbyist versions: AxiDraw, for example, whose MiniKit budget is around $325.

So Procida decided to build his own… “But all my tools were for repairing bikes.” So he bought some cheap $2 servomotors from a Chinese sales site and used household items for the rest: some popsicles (popsicles) , a clothespin (clothespin), a staple, a sheet of cardboard and a spare Raspberry Pi Zero. “I had this in my hands and wanted to do something with it.” he explained. “I like to have limits because whenever you hit a limit, you have a challenge.”

He titled the result brachiographfrom ancient Greek βραχίων (brakhīón), arm, and γράφω (gráphō) to write: “arm scribe”. (That vulture has since been a dope for a classic nod Asterix and Obelix.) Instead of an XY mechanism, the brachiograph has a shoulder motor, an elbow motor, and a rudimentary wrist to lift the pen from the paper.

The result is that it can only draw curves: as Procida put it, “It’s much more difficult to draw a straight line with the arm than to walk in one.”

To do this, he had to come back with some help from high school trigonometry and Python numpy Library. The result is limited: it can only be drawn over a fairly small, curved area of ​​a piece of card or paper, and its lines are shaky. Each of the three servomotors has a limited range of motion of approximately 120°. He told us, “You can get about 150 degrees, but it gets very, very inaccurate. Anything beyond that and they burn out. I destroyed so many of them. Never a single Raspberry Pi – they are so heavily abused! – but the servos, loads.”

He recommends the basic $2 servos: apparently more expensive ones actually give worse results. (Beware of even cheaper motors: there are those for around $1, but they’re fake and don’t work.) A big problem is that the devices have a non-linear response, and he had to take that into account in his Python code. You can calibrate your particular setup by drawing some pre-programmed lines, measuring the results with a ruler, and entering the resulting values ​​into the code, which will improve the output.

The code can extract bitmap files line by line, and for a more artistic drawing, you can stick a charcoal pencil in the clothespin for a sketchy result.

As you might expect from the person responsible for Ubuntu documentation, the project and associated code are remarkably well-described, especially for a project marked as version 0.1. It was great fun to see the device in action and that Registration number FOSS Desk plans to build one.

Youtube video

You can see one in action in the video above. ® Build your own pen plotter for less than $15 • The Register

Rick Schindler

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