## Early demonstrator

Since last post, all triangles have been soldered, including their power supplies. Getting them to work was a bit of a pain, since some LEDs got fried, so Alexis spent a lot of time replacing LEDs. Last Friday, we had to do a demo of our progress, and here’s a video of it:

There were five triangles we couldn’t use because they were missing a connector, but Alexis finished them this week-end.

The board Lucas is moving (far left) is the main board, with the AHRS array. The image is supposed to move to “counteract” the main board’s rotation: when it is inside, it would make the image static even when rotating the icosahedron.

Lucas used the projection algorithm he describes in this post, which works really well in my opinion. There’s still room for improvement, though: for instance, we haven’t figured the axes out yet. We’re also thinking about what to do in the final demo.

## Mechanical design

### Ranting time

There are holes in the triangles that allow us to bolt them to 3D-printed pieces. Hichem designed pieces for this in OpenSCAD to join triangles together at a shared corner, but the angles were off. We also tried using FreeCAD, but it was a huge pain to use, even for simple stuff like drawing pyramids. The problem was that when using OpenSCAD, there’s a lot of calculations to do, which are easy to get wrong, and in this case, it’s way simpler to use a constraint-based CAD package. Next, we tried FreeCAD, which was somewhere between “huge pain” and “nightmare” on the user-experience scale. Even for simple stuff like drawing pyramids, it was really unpleasant to work with, although I do concede that a) we’re not mechanical engineering students, b) we’re not familiar with mechanical CAD packages, and c) we didn’t have the time to learn FreeCAD properly (didn’t take it, either).

Enter SolveSpace. This one was a bit of a surprise: it’s a TINY (5.46MiB for the Arch package !) open-source constraint-based CAD package, as opposed to OpenSCAD which uses constructive solid geometry. Even though its UI doesn’t look very modern, SolveSpace very simple and intuitive to use, and what’s more, it doesn’t try to be as featureful as <proprietary CAD package everyone knows>: its simplicity is its best asset. There are some quirks (no loft operations, for instance) but there are often ways around them. All in all, I’m very impressed by SolveSpace: it’s a lesson in minimalism.

## A simple structure

I redesigned Hichem’s corner-piece using SolveSpace. It’s meant to join five triangles together at a corner. Here’s what it looks like:

There are hexagonal wells for the nuts. To house the electronics, I designed a plate and supports (yes, also bolted to the triangles). The supports will be glued on the plate. Here they are:

The notches in the plate are there to let the bolts protrude from the supports.

Finally, here’s a view of the whole structure:

The corner pieces have all been 3D printed as hollow pieces, which made them way faster to print, and they feel strong enough. We’ll have to laser-cut the plate once I finish its design (mostly figuring out where to put holes for velcro straps for the LiPo battery), print the supports (~20 minutes each, from my early experiments), and finally glue the supports and the plate together.

The insides look like this so far:

Way messier than the screenshots, huh ? The grey ribbons are for the SPI connections to the LEDs, and the other wires are for power. Soldering the wires to a proper power distribution board (i.e, a two-sided un-etched PCB) and shortening them will probably make this a bit more manageable.

## The last 3D printing

After several tries we finally printed a triangle with the right size for the led strip.

I also used a laser cutter but the led strip could not fit within holes again. To use the laser cutter we need someone trained to insure the safety so it was simpler for us to use the 3D printing. The main issue was the fact that we didn’t have accurate enough data. I measured with caliper and re-written the placement of holes algorithme on openscad. After several tries I printed a triangle with the right size and the data were to be very accurate (~20 micron). I took some times because each printing took 2 hours.

Lucas soldered the led strip, tied the triangle and the led strip together and we have our prototype.

Now I am working on the programming of the AHRS. We use Ultimate Sensor Fusion Solution designed by Pesky Product. It is made up of EM7180 Sensor Hub coupled with the MPU9250 IMU. The EM7180 uses a Kalman filter and provides in output quaternions.

## [bouLED] 3D printing

After we made a model of our icosahedron’s one equilateral triangle using openscad. This is the first time I used 3D printer. After two hours, the first print has finished.

As you can see, the main issue was the led strip can not fit within holes because widening holes I didn’t change the spacing’s dimension between two LEDs and two rows. So the alignment doesn’t match with our LED strip. I fixed it and I tried a new print. After two other hours, the second version is here.

The alignment is more appropriate, but it’s still not perfect. The reason remains to be determined I think we should to have more accurate data (below millimeter) and if it’s possible with 3D printer available. Tomorrow I will fix and retry another 3D printed triangle.

## A new simulator

I was trying to replace the cube in Lucas’ simulator (see here) by an icosahedron and add some kind of clue of the icosahedron’s orientation, but the Python 3D library we used, VTK, was getting on my nerves. Adding an icosahedron worked fine, but I wasn’t able to change its colors, and what’s worse, even when using the default colours (blue everywhere), one of the model’s facets stayed red, which was pretty jarring. I also added an axes widget that was supposed to rotate with the icosahedron, but to no avail: it wouldn’t rotate. One of us had to go, and it wasn’t going to be me.

Alexis sent us a script 11 days ago that displayed a colorful icosahedron with PyQtGraph, which provides a light abstraction over OpenGL. It made a nice starting point for a new simulator, with a rotating icosahedron, a fixed grid and axes. Behold !

Granted, it’s still ugly, but it works and PyQtGraph is way nicer to deal with than VTK.

## 3D modeling

We’d like our icosahedron to have 13cm equilateral triangles, which would make it fit snugly inside a 25cm transparent spherical shell for protection.

At first, we wanted to build the icosahedric display with triangular PCBs, but last week, our teachers suggested trying to 3D print a facet and put a LED strip (one the ubiquitous APA102 strips) on it, to check density, and if triangular PCBs are really necessary: perhaps we could have a PCB inside the isocahedron control LED strips glued to the facets.

Hichem and I made a model using OpenSCAD to understand how to lay the LEDs out. It’s a pretty neat piece of software for 3D declarative modeling. I really appreciated that because in this case, we had to be explicit and think about our constraints. So far, here’s what we’ve got:

The LED strips are meant to go under this. Using this model, we see that with the strips and dimensions we chose, there’s 111 LEDs per facet, so 2220 LEDs overall. That’s huge, and we’ll have to discuss whether having that many LEDs is feasible (or desirable, for that matter).