Introduction: Ommatid Spherical Display: Constructing the Enclosure and Optical Globe
This is the pièce de résistance of my artist residency at Pier 9: an interactive spherical LED display with 76 touch-sensitive illuminated facets. IR detectors on each facet react to touch and facilitate tactile interaction.
This Instructable details the physical construction: a companion Instructable explains the electronic design and programming.
Here's a video that will have to do until I can make a better one:
Update: All design files and code (still in progress) can be found on github at https://github.com/headrotor/ommatid
Step 1: Squaring the Base
For the base I started with a block of figured eucalyptus (Eucalyptus obliqua). This was obviously cut to be a bowl blank, but it was a little split along the grain and had been rejected. So I felt sorry for it. But it was a nice size -- like a thick hardback book with interesting grain.
None of the sides were parallel or even very flat so the first thing I did was to smooth down the flattest side with a hand plane to get it close to flat. Once that was flattish, I ran the block through the drum sander a few times to get the top and bottom sides parallel sides, flipping it and turning it 90 degrees every time. Once the top abd bottom were flat and parallel, I squared up the sides on a table saw
Step 2: Forming and Hollowing the Base
The base is the enclosure for the control computer so I needed to excavate some wood to make room. First I found the center and used a 1" Forstner bit to drill a hole for the central shaft from the bottom. I drilled this most of the way through, to about 1/4" from the top. I then used the Forstner bit to hollow out an enclosure for the electronics, a little less deep than the central hole. I cleaned up the inside edges with a chisel, and finally drilled a 3/8 inch hole through the top in the center of the block for the threaded tube. The 1" hole surrounding it gave a place to seat a 1" washer. To get a consistent rounded edge, I ran all edges of the block through the router with a rounding bit.
Step 3: Making the Riser
I made the riser out of walnut stock on the wood lathe. I'm a wood turning novice so I chose an easy thing to do: a straight conical taper. This turned out to be challenging as the turning tools are small diameter, so I cheated a little bit by using a sanding block to find the high spots. When I was done I put a 3/8" drill in the tailstock chuck to drill the central hole; I finished it by drilling from the other side in the drill press.
The first conical riser (in the first picture above) was about six inches long. I realized after a rough assembly with the optical sphere that it was too tall and not a good proportion, so I put it back on the lathe and shortened it to about 3" high and gave the profile a slight curve.
Step 4: Lathing the Aluminum Spacer
I wanted a separation between the different woods of the walnut riser and the eucalyptus base, so I fabricated a spacer from aluminum. Using a lathe it was easy to make a ring of the exact outer diameter of the riser with a center 3/8" hole for the threaded tube. I used autofeed on the final cutting pass for a uniform finish of fine grooves.
Step 5: Designing the Optical Sphere
I designed the optical sphere using Autodesk Inventor. The sphere is based on an interior icosahedron -- a 20-sided regular polyhedron. Each face of the icosahedron is an equilateral triangle.
I subdivided each triangular face into four identical triangles, and projected them out to the surface of a sphere. I used the shell command to make the thin opaque walls, and subtracted these from the solid to leave the transparent light pipes. I made a number of prototypes to test the material shapes, visible and IR light interaction, and to check the mechanical and optical matching with the interior electronics.
I then assembled the icosahedron from 19 of these sides plus a special side that had a hole for the base.I originally was going to print them separately and attach them, but realized it would be far easier just to print two hemispheres!
I designed in spaces for small rare-earth magnets to hold the hemispheres together, and installed the magnets with polarities such that the hemispheres would mate only in the proper configuration.
Step 6: Fabricating the Optical Sphere
Except for my first attempt where the printer ran out of resin some way through, printing went off without a hitch. I removed the support material and wet-sanded the exterior with 600 grit sandpaper for a smooth satin finish. (Though I could have polished it to transparency, which would have helped with the IR sensing, I left it a little matte to help diffuse the color LEDs)
Step 7: Finishing and Assembly
I finished the wooden parts with a good deal of hand-sanding, finishing up with 600 grit sandpaper and a linseed oil finish rubbed in with fine Scotchbrite. After the parts were finished, they went together easily just like a table lamp. I attached the base, aluminum disk, and riser with washers and 3/8" nuts. I then fabricated a triangular washer from a round one that perfectly fit the bottom triangular face of the lower hemisphere. I then attached that with another nut, leaving some of the threaded tube in the interior for a bus board.
Step 8: Installing the Sphere Electronics
I fabricated 19 printed circuit boards, one for each interior face except the bottom. After thesewere stuffed, programmed and tested, there was the small matter of installing them and wiring them together. Each PCB has six electrical connection s to be made: 5V power, ground, LED data in, LED data out, and two wires for the RS-485 differential serial data. The LED data lines are daisy-chained: each PCB is connected to its upstream and downstream neighbors. All the other connections are busses in that every board is connected to the same four wires.
To help connect the busses, I made a bus board out of protoboard that attaches to the threaded rod. This also has sockets to mate with connectors for the top of the globe.
Step 9: Control Electronics in the Base
The hollow area inside the base holds the control electronics: a Raspberry Pi embedded computer, a Fadecandy board to drive the LEDs, and an RS-485 interface to read the IR sensor data.
Step 10: Fabrication Files
Here are the .stl files for the 3-d printed dome. (Thanks to commenter kbuilds for the request!) These were designed for the Objet printers at Pier 9 which can print in two kinds of resin: transparent and opaque. So each half of the sphere has two .stl files corresponding to the clear and opaque parts. The bottom half is identical to the top half save for the bottom triangular segment has a 1" diameter hole for the pedestal and three small ventilation holes.
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