[CyL3D] LED Rooting

Quick post.

I finished placing and routing the LEDs, one each 3.4mm (not minimal but required to ease soldering).

We have less space and MOSFETs are going to take more space than I originally thought. Although, if we use less MOSFETs, it should be fine.

Still, we won’t have much free space at the top of the panel for the LED drivers. Since they are quite small, I think we can put them on the two sides.

Here is the updated routing idea:

[LASMO] PCB progress

Externals Clocks

In my previous article, I talk about the STM32F7’s and the STM32F1’s clock.  However, according to the STM32CubeMX and after talked with Alexis, we will use for each controller a 8MHz external clock (for consumption questions) and rise the frequency using the PLL until 216 MHz for the STM32F7 and 72MHz for the STM32F103. According the oscillator design guide for STM32, we have 

with CL the load capacity of the clock wich is 18pF here and Cs the stray capacitance of the printed circuit board and connections wich is 10pF here. So, we have the construction for each controller : 

Also, we have a ESP32devKitC which don’t must to have an external clock because the ESP32 WROOM integrated has one.

XLR connection

We also work an the XLR connection but due to the phantom and the symetrisation we had some problem with the architerture, so we decided to use a jack connector. We talked about an architecture but we change again for RCA connectors which Pierre will talk about.   

DAC to LASER 

The drivers of the laser work with a balanced analog input ranging from 0 to 5V. Since the DAC of the MAX outputs a single-ended signal ranging from 0 to 3.3V , this is an issue we had to resolve. 

We basically had to convert an analog single-ended signal to a other one, and amplify it with a 5/3.3 ratio. We achieved this by using an ADA4500-2 amplifier with two resistances, as represented below.

This circuit is composed of a non-inverting amplifier (taking the out of the MAX5105 as entry), which, with the resistances of 1kΩ and 1.96 kΩ, multiplies the signal by 1+1/1.96 = 1.51 = 5/3.3 (approximately). Then an inverting amplifier with a gain of 1, create aa signal which is going to the input of the laser

After talking with Sam, we decided to delete the DAC_OUT signals from the MAX5101  to the F103 microcontroller because their are not essential. So, we configure the HREF of the MAX5101 at 5V, and not at 3.3 V. So, we don’t need this architecture for the lasers’ inputs.

Project architecture

We also decided about the architecture of our project and about the workflow. I put the configurations files of the STM32F4 controller that we use in TP in order to begin to code with the board that we have. Let’s begin !

[bouLED] MARG

I worked on the programming of the MARG. We use Ultimate Sensor Fusion Solution designed by Pesky Product. It is made up of an EM7180 Sensor Hub coupled with an MPU9250 IMU. The EM7180 outputs quaternions. We tested with the Lucas’ simulation.

Simulation

We judged this model will be enough for us and bouLED. So we will definitely use it for the design of our PCB.

Booting the FPGA & Schematization of the USB3320

For the past days, I have been working on the schematics of our ULPI transceiver, the USB3320 and when my colleagues were working on other parts of the schematics, I had to focus on other stuff such as making our SOM work and improving our system architecture.

Making our Cyclone V SOM work was a breath of air compared to the schematic part. Following the Quick Start Guide of our development kit, I had to make sure the jumpers were setup correctly (and they were – not sure if it was the factory default or if Alexis did his magic before giving us the devkit) and configure the UART0 interface. Once everything was set and connected on /dev/ttyUSB0, I simply plugged in the devkit and it booted correctly.

You can see the booting log on this pastebin post

As we would like to be able to use Wi-Fi USB dongles, we are currently adding host USB 2.0 support. Unfortunately, we must use the ULPI interface to communicate with our USB devices. Thus, we have to use a ULPI-USB transceiver.

ULPI works with a 60MHz clock and I am unsure for now if our FPGA can generate such an accurate reference. However, the USB3320 can work with a external oscillator and it has its own PLLs to generate a 60MHz clock from a 12MHz reference clock.

Below is a work in progress of the schematic of the USB3320 in our system. It also includes a Linear Voltage Regulator as the transceiver also needs a 1.8V DC supply. 

What is definitely missing are the appropiate capacitors and resistors that I have not added everywhere yet.

USB3320 Schematic (WIP)

[bouLED] LED layout redux and component choice

LED layout

We settled for a layout with 78 LEDs per facet, with a slightly modified border clearance. It looks pretty great on the simulator:

Don’t pay attention to that triangle with white LEDs, it’s coplanar with the z=0 plane, as a reference. You’re seeing the icosahedron from above. The important thing is that the layout looks regular enough, without any weird stuff in the corners, where 5 triangles meet.

Next step: build a paper version of bouLED. Behold !

That thing is HUGE. It was a pain to build using only paper and scotch tape, but it’s really useful to get a better idea of how to design the inner structure. On the flip side, I messed the dimensions up when printing the triangles, they’ve got 21cm sides instead of 18.5cm. The triangles I showed in my last post had the right scale, though (but the LED-to-side margins are slightly different). Notwithstanding my clumsiness while building this, it gives a good idea of what the end result will look like.

Power supply

Choosing a battery

With Lucas, we thought that by limiting the LED brightness, we’d be able to downscale the power supply a bit. The 3D printed prototype triangle (103 LEDs) was already unbearably bright when drawing 1A in white, so we’ll avoid drawing over 20A for all the triangles, at 5V. The main board will run on 3.3V, and 3A is way more than enough for it. That’s 110 watts.

A 4S LiPo battery has a 14.8V output voltage, and for a 110W output it’d spit 7.4A out. That’s a very conservative discharge current for LiPo batteries, a 4S 3000mAh 35C battery like this one can deliver more than 90 amps continuously and its peak current is probably even higher.

With 3Ah, this battery could supply bouLED power (at full brightness !) for 24 minutes. We could have picked one with a beefier capacity, at the expense of its weight. This one weighs 308g.

Voltage regulator choice

Alexis suggested using a small regulator per facet (and another one for the main board), saying that’s it’s way easier than finding a big regulator.

What we need is a buck converter: it reduces voltage but increases current. The part Alexis suggested is a LT1765 from Analog Devices: it’s rated for voltage inputs up to 25V and the output current limit is guaranteed by the datasheet to be above 3A. It has fixed-voltage versions that can output 5V or 3.3V without having to add an external voltage divider.

My previous calculations for LiPo sizing didn’t take into account the fact that a buck regulator transmits constant (without taking efficiency into account, of course) power but at a different voltage. Basically, I did them as if I had a linear regulator. Now, the battery is one third of the price of the one I mistakenly chose, same for the weight.

[bouLED] Physical structure

Hichem got the MARG working last week-end, so we tested it with our simulation (see his post) and confirmed this model would suit us and go on our central PCB. With this and the voltage regulators chosen, we have almost all we need to design the PCBs. The triangular ones may be simpler to design, but before we want to know precisely the physical structure of bouLED.

For ease of conception and of battery management, it should be possible to open the icosahedron. One way to do it is to make five faces removable, as shown on this simple model.

Made with Tinkercad

The inner armature would support the electronics and the LiPos, ideally keeping the MARG in the center of the icosahedron. Thus the stick supporting the removable part should be moved, and probably replaced with something stronger. The faces would be connected to the central part with JST-like connectors, easy to plug and unplug.

We want to chain the triangles without using too long wires, and we want a removable part. This will dictate how we’ll position each face and how we’ll place the connectors on it, while keeping all triangular PCBs identical. The LED repartition we think we’ll use surely is even enough not to be considered when choosing the face’s orientations.

[CyL3D] Schematics + routing continued

I’ve almost completed the schematics of all the LED panels. Once done, I will start placing and routing it.

I tried routing a small number of LEDs to check the estimations I made in the previous post. I forgot to take the soldering pad into account, so I only managed to place a LED each 3.2mm.

Anyway, this distance is too small and we risk soldering issues. Based on the vertical distance between last year’s SPIROSE LEDs, we will put a LED each 3.4mm (1.2mm between each).

Based on this, the LED panel will take roughly 11cm x 14cm. Since the PCB is 19cm x 19cm, we have plenty of space for other components. I will try to fully route one LED panel and its drivers to check if it is possible.

For now, here is an idea of how I was hopping to do the routing:

Technically, all components (except for the power supply, which I haven’t checked) fit in. What I need to figure out is if routing is feasible,

[CyL3D] Decoupling of the SoM

Altera made a spreadsheet named “Power Delivery Network” designed to determine an optimum number, type and value of decoupling capacitors to achieve the target impedance. I select the VRM type (Switcher), the supply voltage (3.3V), maximum current (2.58A), transient current (I typed 30%, but it seems extremely complicated to obtain a value without having the programming of the FPGA already done) and ripple tolerance (5% given the specifications of the FPGA and knowing that we don’t have transceivers for PCIe, which is the only bank requiring at most 3% ripple). For the spreading inductance, BGA Via inductance and plane capacitance profile I left the default values because I don’t know how to estimate that.

Auto decoupling

The tool looks up the capacitors it has in its database and tries to find the optimal combination for our target. With the preceding values it outputs 2x 0.01µF, 2x 0.047µF, 1x 1µF and 1x 10µF capacitors, with their respective ESR, ESL and mounting inductance (Lmnt).

Auto decoupling

Development board decoupling

Development board decoupling: capacitors

If I take the configuration of the development board of our SoM, they are using 4x 10µF and 9x 100nF capacitors. If I use the capacitors pre-configured in the Altera tool, it shows that the resulting impedance is higher than the one I got with my inputs, but given that the development board has to be designed for any configuration of the FPGA, their decoupling capacitors are certainly adapted to more extreme power draws than what we will be experiencing. So, I think it would be a good idea to keep what Aries did with their development board.

Development board decoupling: impedance

[bouLED] Choosing a LED layout

I changed the LED layout algorithm to allow us to set a number of LEDs for each row. To improve regularity, the algorithm only takes the bottom row’s number of LEDs, and the row above this has one less LED, and so on. There’s also a “margin” setting for the space between the outermost LEDs and the triangle’s edges.

Here are all the layouts I generated, printed on paper with a 1:1 scale, and a 5 cents coin for scale.

We’d appreciate your feedback on this: which variant is your favourite ? You can click on the images to display them in their full size.

[CyL3D] Motor’s First Turns

In order to determine how much current the motor pulls, we had to make it turn with a load and monitor its consumption. The perfect load was in hand, we took last year’s LED pannel, as ours will have approximatly the same weight and dimension, therefore giving a good estimate on our power consumption.

We took the same brushless motor as last year, a MEGA Acn 22/20/4. Which is connected to a motor controller, a EZRUN Max 10 SCT. It is rated to be able to supply up to 120A, so we could not take this value to size our power supply.  So I tested the whole system to see how much would it drain.

First I had to understand how the controller worked. The datasheet is not very helpful, as it only says to put a RC receiver on the input wires. So I emulated one with an arduino. A RC receiver uses PPM signal, and is waiting for a 1 to 2ms pulse every 20ms. 1ms being the command for neutral throttle and 2ms for full throttle.

In order for the motor controller to acknowledge that you sent a RC signanl, you need to start by sending neutral throttle command. It is a security measure to prevent the motor turning on when it has just been plugged in. 

After came my first attempt with no load on the motor. It ran fine and the controller pulled around 1.5 to 2A. Then Itried with last year’s pannel, and it would not start rotating at the lowest speed, or be all jittery. My teacher told me that last year, in order to start the rotation, they sent a high speed command and backed down to lower one when it started. I did so and now the whole structure is rotating.

LED pannel rotating.
Slowed down 16 times.

I filmed it slow motion to be able to determine the speed it’s rotating at. With the almost lowest setting it is rotating just under 20rps. At this speed it is pulling 4A under 12V. I tried making it rotate faster, so I just left it at the high speed command I use to start it up. I did not go well. Leaving a high speed command made the controller pull more than 20A because it was unable to match the requested speed. So the power outlet shut down.

In order to get the whole system to high speed I proceeded gradually. Letting it establish at low speed for seconds and increasing the speed slowly.

An other problem rose up, at low speed it was not that wobbly. The structure was only a bit unstable. At high speed it is a whole different story. The metal structure would move around the table and produce more noise. We will have to be very carefull about the placement of the component and plan to be able to put masses and the lower rotating part to balance the whole structure, as you would on a car tire.