For no particular reason, this spring I rigged one up with motors and a computer interface, to see if I could get it to draw arbitrary pictures. I got the whole thing about 95% done back in April, then last week I finally finished it up and took some pictures.

The Details
The basic parts are: the dome (from eBay), PICAxe microcontroller (from here), two 5.25" floppy drives (Wacky Willy's surplus), a servo, and few various electronic parts. The knobs on the dome are replaced driven by stepper motors from the floppy drives, and the pic uses the floppy controller boards to move the motors in either direction. The floppy interface is simple, one line on the connector controls the direction, and another causes the motor to step when toggled. This site explains further. The steppers could also easily be hooked directly to the pic (getting rid of the floppy control boards), but this way uses less I/O lines. The servo is attached to where the draw on/off lever was, and moves in or out to control that aspect.

The downside of using the floppy-control boards is that they automatically move the motors a bit when powered on (to move the head to the back of the drive, I imagine). This results in an unwanted line if the drawing toggle is on, so I had to add a big relay to control power to the boards. This allows the pic to move the drawing pin out of the way before powering the boards on.

The right knob moves the pin up and down, and the left knob rotates the whole thing. The stepper isn't strong enough to directly drive the left knob, so I had to find a gear for it. After scrounging at the surplus store, I came up with a gear from an old air pump that worked nicely.

The whole rig is removable, since you need to shake the dome to clear the image. It is held together with a mix of gravity, super glue, epoxy, and alligator clips where the epoxy broke and I never bothered to fix it.

On the software side, commands are streamed over the serial port from a PC, and the PICAxe controller carries them out. The commands are "right X", "left X" (where X is a number of steps), "draw on", "draw off", and "down one line". The PIC takes care of translating those commands to servo position changes and toggling pins to actually move things. It also accounts for taking extra steps when the direction changes, in order to compensate for play in the gears (this is a toy, after all), and does some initialization to center the pointer vertically.

The host PC does most of the hard image-conversion stuff. I take a black-and-white image (~800x300), and convert it to a ppm file. A ppm is a very simple image format for lazy programmers- it just lists the RGB values of each pixel in ASCII numbers. I have a perl script that parses through the ppm file, and converts it to commands, essentially drawing a line where there are dark pixels and skipping areas where there are light pixels. Another perl script sends the commands over the serial port, filling the pic's buffer and waiting while it does the drawing.

The images are all drawn similar to a plotter- it writes one line across, drops down, writes another, etc. The reason for this (other than it being very easy to program) is that the vertical control knob is not as accurate as the horizontal, especially if the pin is drawing. It tends to stick in some places, and move easier in others. If the pin is not drawing, the vertical control is much more accurate. So whenever the vertical moves, the pin is retracted, and the printing can only be done horizontally.

Overall, it takes around a day to draw an image on the entire globe. The faster the drawing happens, the less well the silver dust is scraped, and the less clean the image, so the steppers aren't operating at their maximum speed. It also has to go back and forth over each drawn line to get reasonably sharp edges, which slows things down even more.

At first, the results weren't great. The Doodle Dome I got off eBay is a bit scratched up, and the constrast in any case between the light gray of the background and the dark gray of the image is not great. Then, while trying to take pictures of it, I discovered that backlighting it makes the contrast much better. All of these pictures are taken with a desk lamp just behind the dome. I suppose I could install a light source inside it, but I don't plan on it.

This is the hardware:

The overall setup- Doodle Dome, motors, floppy controllers, breadboard with PIC, power supply.

Close-up of the motors hooked up to the dome.

The first image I worked on was a world map. For those who are curious, the appropriate projection for a map image file to do this is Plate carrée. I had just figured out the backlighting technique, so the photos aren't as good as they could be:

After that, I did a drew a few geometric designs- these required very little effort on my part, I just made some quick images in Paint Shop Pro, and set them to draw all day while I was at work.

Then, I decided to try text. I printed the first hundered or so digits of pi in an image (inverting the colors on half, to get a cool spiral affect when viewed from above), and drew that:

Last, I tried a photograph. I found a fairly high-contrast image (a picture of Mt. Shasta, which I took a few summers ago), and used my image editor to dither it to black and white. This produced lots of small lines for the dome to draw, and it took a day and a half to finish. This is the result:

As you can see, the results were mixed- the mountain area came out as well as could be expected, but the bottom half (the road) is just a dark blob. In a certain range, the different gray values just don't translate as well. Some of this is because the image is dark on the bottom all the way around, stopping the backlight from getting in, and some is the general inaccuracy of drawing for lines around that size.

Sam Wintermute, Spring / Summer 2005