BracioGraph: a Raspberry Pi-powered pen plotter

It’s MagPi Tuesday, technically, but we’re still going to use #MagPiMonday to make sure we don’t miss out on the cool projects you’ve been making over the weekend, and to also freak everyone out when they see it on Twitter and think that today is yesterday. Take a look at this charmingly inaccurate pen plotter driven by Raspberry Pi, which Rosie Hattersley wrote about in the latest issue of The MagPi.

Many of us remember being shown, or even making, pantograph pen plotters from the days before graphing tools were part of a standard home or office computing feature set. Open-source computing guru Daniele Procida recalled just such a device when considering a low-tech example of what Raspberry Pi could be used for while planning for the PyCon Namibia conference. The result, BrachioGraph, which sketches portraits and reproduces images of objects, was an instant hit, inspiring a fan following of keen makers. 

The project builds on similar principles to those of a pantograph plotter

Noting the contrast between the resources typically available at conference events in the UK and Europe and those in African host nations, Daniele says: “It’s one thing doing things with robotics if you’ve got access to 3D printers, laser cutters, and off-the-shelf parts and components; quite another if you’re an undergraduate at the University of Namibia, or a high school student and probably have to share a laptop or computer.” Combined with Daniele’s own interest, but lack of experience in robotics, he decided “to do something that was as affordable as possible, but also meaningful and also accessible, that didn’t require an absolute minimum of materials and knowledge and facilities.”

Motion capture

Playing around with servos and Raspberry Pi was the entry point for Daniele’s plotting device project. Moving them around was easy enough, but he was keen to see a practical result: “What can we actually do?” With nostalgia for pantographs and XY aerial plotters in common, Daniele explains he found “their ability to reproduce something we do with our hands intrinsically fascinating. When you see a pen moving around, it looks like it has a purpose”. 

From bitmap to plot via vectorisation, the Prague Conservatory concert hall

Although servo motors can be used to drive a device along straight lines, translating the movements and hand-drawn lines on a page is more about angles. The question, Daniele says, was whether he could turn movements into lines on a page. “Of course you can, because your own arm, if you’re drawing with a pen, also doesn’t have motors that go in straight lines. It just uses angles.” He began by modelling the design on that of a pantograph, which faithfully mimics the movement of the human arm. A BrachioGraph, meanwhile is an ‘arm plotter’. 

Copycats encouraged

The BrachioGraph can be set up to run on its own, making it an ideal conference exhibit. “At one conference, I was very embarrassed because I borrowed a table in the sponsors’ hall and just left something running, plotting.” When he returned, the crowd was three or four deep around the table where the BrachioGraph was in action, taking away all the attention from sponsors who bought tables.

Given an image to copy, the BrachioGraph is able to reproduce it

Daniele says that, after lots of design drafts and experimentation, the most important thing is not that he “built this thing, and it works,” but that it’s reproducible by other people and with household materials such as ice cream sticks, or cardboard and pencil, or clothes.

“The nicest thing for me is that, every so often, somebody will email me and say, ‘Oh, it was my 12-year-old niece’s birthday and, when I was visiting, I took a Raspberry Pi and three servo motors, and then we spent the weekend ‘making a BrachioGraph’.”

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