## Choosing a shape

As stated in this previous post, we have to compare different PCB dispositions to avoid blind spots. We soon came to realization that we would not be able to get rid of blind spots with our design, but it is possible to study the different ideas we came up with in order to mitigate this issue.

That’s why we created a python program that can simulate :

• blind spots created by PCBs hiding each other
• the variations of brightness due to the fact that LEDs can be seen at an angle (which is what created the dark center zone in CyL3D, which we are trying to get rid of)

## Stairs : an iteration over CyL3D’s design

To avoid the issue of a dark zone due to all LEDs facing the observer at an angle in the center zone, an idea was to make sure that all LEDs were not facing the same direction. There came the stairs idea:

After modelling this idea in python we get:

As you can see, there are a lot of deadzones, particularly in the central viewing axis.

Imagine a configuration with one vertical PCB with LEDs on both sides, facing the radial direction. Wait, don’t. I modeled it so you don’t have to :

This creates a 2D cylindrical POV display, with dark zones on the sides, which is in our opinion much better than a center dark zone. To create a 3D POV display, we “only” need to add concentric cylinders. The question is now: how serious is the problem of PCBs hiding each other and how to avoid it.

## Spiraling down the rabbit hole

The idea of spirals seems natural. PCBs can laid out with an angular offset when compared to the adjacent PCB. For balancing reason, we alternate the PCBs with a central symmetry in order to create a double spiral.

The major issue is that the adjacent PCBs tend to hide each other in large portions of space, due to the fact that they are aligned and closed to each other. We can also notice that the resulting shape is far from being symmetric (which is expected with a spiral).

## Some arithmetics

In order to shake things up a little more, we can keep the idea of a constant angular offset, but with a twist.

Let us take a circle split in n angular positions. If you choose a starting point and then jump from position to positions by skipping k positions, you will reach all positions if, and only if, n and k are coprime. This gives us a more evenly spread disposition.

For the sake of convenience, and because it is a nice name, we shall name this configuration “modulo forest”.

Deadzones tend to be much more spread out, which seems more acceptable. Further investigation has to be done in order to choose a definitive shape.

## Touch

Touch is an ambitious project which objective is to create a new kind of display device.
Want to know more ? Look here (or click on the menu above)!

## Power and data transmission

One of the issues that LitSpin raises is power transmission between moving and static parts. One idea that came to mind was induction but we didn’t know whether integrated solutions existed and if they did, would they work with our power requirements.
Würth Elektronik offers a plug and play development kit that allows for 200W of output power which solves our power transmission issues.

Since the coils work using resonent induction, W.E. integrated frequency modulation in order to allow I²C data transmission. This could allows us to put the wifi module on a static PCB and get rid of the signal drops that were present on previous projects with spinning wifi modules.

The rotating assembly would become completely wireless while avoiding the issues that rotation at a relatively high speed (for the size of the system) can create.

## Design Crisis

We discovered our first model contained a lot of issues. The outermost PCB often hide PCB behind them as you can see below. This results in some voxels being invisible. So we decided to create a simulator on Python to find the invisible areas. We will use it to determine the optimal configuration for our display. This configuration has several parameters such as the number of PCB, their position and the arrangement of the LED on one or both sides.

This a an example with 8 double-sided PCB in double spiral and 100 angular resolution. The blue dots are the visible voxels. The red dot is the user point of view and the green dot is the rotation center.

## Meet LitSpin

Our goal is to build a 3D POV (Persistance Of Vision) display with no black zone in the middle.

Last week we mainly discussed the technical limits of our project. See here for more information.

Thinking about it, we discovered a number of challenges that we will have to face:

• Our project will have a spinning part and a base, and we will have to find a way to transfer power and information between both parts:

Concerning the power transfer, we thought of induction, but the main constraint is the available power. We will need between 50 and 100W of power for the PCB, and there are not many induction transmitters that can transmit that much power.

As for the information, we only need to measure the speed of the motor and to send speed commands, so a Hall effect sensor for the measurement and an infrared sensor for sending commands should do the trick.

• Our targeted resolution brings a few more constraints:

We want a framerate of 30fps, and with a radial resolution of 512 pixels the blink frequency for the leds will have to be at least 30kHz. Since we need a PWM resolution on 9 bits, we need a minimal PWM frequency of 7.7MHz, which is a serious constraint for the choice of the led drivers. And this is valid only if we don’t multiplex the leds, because multiplexing will increase that frequency.

That’s all we discussed, because we also had to do a first draft of the PSSC for last Friday’s presentation. We will continue this week to discuss these technical challenges.