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White board plotter

by berkenbusch, published

White board plotter by berkenbusch Feb 26, 2014
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Summary

This is my implementation of a white board string-plotter.
The mechanical parts are designed to use inexpensive 28BYJ-48 5V stepper motors (search for them on Amazon - they sell for ~$6, including driver). More details under 'Instructions'.
UPDATE: I added links to the sources of some of the software I wrote for this project under 'Instructions'.

Instructions

All parts are modules inside the provided .scad file - the code is not exactly pretty, but should be usable.
The pen guide on the carriage in the original design was to short, so I printed an extension (just cylindrical pipes, really) and super-glued them to the existing carriage (they don't have to take any real mechanical load). The guide is sized to hold an 'Expo' brand dry erase marker.
There is a small 9g servo on the bottom of the carriage that allows the pen to be retracted, so that the plotter can move without drawing a line.
After experimenting with various types of fishing line I settled on using dental floss as string and two pieces of knitting yarn to hold the carriage such that it can easily roatate.
For electronics I am using a Teensy 3.0 microcontroller board (ARM-based, but mostly Arduino compatible - a regular Arduino would work just fine, too) and a HC-05 (check Amazon) bluetooth module as a serial pipe (yes, the plotter is wireless).
On the software side, I have implemnted two solutions:

  1. A Perl script that takes cartesian moves as inputs and generates spool-rotation steps as output that get sent to the plotter directly via bluetooth
  2. An Android app that allows you to doodle on the touch screen of your phone and then send the results to the plotter directly (again via bluetooth)
    I also created auxiliary scripts to render photographs (any jpg, really) as line drawings by turning them into grayscale images and generating a square-wave path whose frequency is modulated by the 'darkness' of the image area (see the two images attached).
    The device is rather slow, its speed is limited by how fast the stepper motors can turn without missing steps. Realistically, plotting the more complex photo in this post (w/ two faces) took about 1.5 hours.
    EDIT: For completeness' sake, here is the script that controls the plotter:
    https://www.dropbox.com/s/ug2ghe095nh04gd/wb_plotter.pl
    And this is the script used to generate the photo plots:
    https://www.dropbox.com/s/6fvj2abrmj4mfep/pixelverter.pl
    This is the Arduino (Teensy) sketch that controls the electronics:
    https://www.dropbox.com/s/4qqyd2zk95t5r01/EtchASketch.ino

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I've been watching this Thing for a while and was hoping for more traction. I want to build one but have the following concerns: I haven't looked at the scad file yet, but I don't see a model for all the parts I expect. I don't see models / STLs for the pen clips or any type of axle for the steppers etc. The bottom line is that I really would like to see pictures of the actual "thing", not just its output (the whiteboard image). A video of it in action would be inspiring as well.
I don't see any assembly diagrams or instructions, and it is unclear what motors need to be purchased. You mention the model number of a stepper on the description page, and a 9g servo in the instructions. How many steppers are used? I would think at least 2. Is it weighted? Sprung? Is it a delta configuration? I really can't tell.
I like what you've done with the Perl scripts. Very clever. Using a Teensy and BlueTooth are also super cool. I'm really excited to learn more. Thanks!

Apologies for being a little fuzzy in the instructions. Keep in mind that this design is largely a novelty rather than a truly useful plotter - it's a good way to surprise people when they see it in action, hanging from a white board...
That being said, let me try and provide some more detail on its construction:

  • the plotter uses two of the stepper motors mentioned in the description, one for each 'spool wheel'; clicking on the expand arrows on the first photo should give you a better idea of the rough layout
  • the wheels mount directly onto the shafts of the stepper motors, there is no separate axle
  • the one servo is used to move the pen away from the board and is mounted directly under the pen; it is not strictly needed if you only want to draw continuous lines
  • there are no springs, just gravity; I weighted the 'gondola' part with a single two inch washer double-side taped underneath the servo - it probably weighs about an ounce; any other way of adding some weight to bottom should work equally well - it might even work without any additional weight (I left it on there from a previous iteration of the design that did not quite work)
  • the pen clip is the very first STL (as it shows up in my browser, anyways) in the list of files; you will need to print:
    • (1) base plate
    • (1) pen clip
    • (1) gondola
    • (2) spool wheels
    • (2) mounting clips
    • (2) string guides (the things with the little eyelets for routing the string on them)
  • I seem to have forgotten to upload the STL for the cylindrical extension to the gondola - it is just simple extension of the cylindrical portion of the gondola, giving the pen more guidance - I think it is defined inside the scad file; I will upload it when I get to it...
  • I never took a video of it plotting - it is pretty slow, it probably moves on the order of a quarter inch or less per second; this is actually determined (besides geometry) by how fast you drive the steppers in the Teensy firmware; it could probably be sped up a bit, but driving them too fast may lead to missed steps
    Feel free to ask question if you try to build it and get stuck...

Ah, yes. Expanding that first photo is very telling. (I didn't think to do that). Now I can see the machine. That answers a lot of my questions and you have answered the rest! I'm looking forward to building one (or two). Thanks for your prompt response and thorough information!

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