The hardest choices require the strongest will

The past week I’ve been looking through datasheet after datasheet in order to find several components that would fit our project.

  • Hall effect sensor:

Looking through the different types of hall sensors, I found that a switch sensor was the best fit, since we will use it to get the motor’s speed rotation, we only need to know when the magnet will be aligned with the sensor. The constraints we had on the choice of sensor were not numerous: we need a sensor with digital output, with 3.3V in input. However, one interrogation I have is whether or not the inductive power transmission will infer with the sensor. Alexis said that it was not a problem, but I am still wondering if having a highly sensitive sensor like the one Cyled chose, the TLE4964-4M, won’t increase the risk of interfering with the power transmission. I honestly don’t think it will be a serious issue, but I prefer to be sure before I validate this sensor.

Another issue I had with the position sensor was if it would be precise enough. A solution to that could have been adding an optical sensor in order to have a second measure, but the problem was that most optical sensors use infrared, which we already use for communication. In fact, simply having several magnets on the fixed part allow us to have several measurement points per rotation.

  • Infrared sensor:

Regarding the communication from the mobile part to the static part, Alexis advised us to do some research on the IrDA protocol, which is a communication protocol using infrared transceivers. There are several communication speeds available (SIR, MIR and FIR), but since we only need to transmit speed commands to the motor, the lowest speed (SIR, which corresponds to a standard serial communication speed: 115.2 Kbps) will be sufficient. I chose to use this kit with a emitter and a receiver: TFBS4650-TR1.

  • Wifi module:

For the Wifi module, I had two main choices to make:

  • the hardware interface: I started by searching what Cyled did. However, they did not have much choice regarding the hardware interface, and had to choose a USB module. On the other hand, we have a larger SOM that gives us much more choice: USB, SPI, PCIe are the main ones. I found module for each of them, but I’m still not sure which interface would be the easiest to implement.
  • the protocol version: for the 802.11 protocol there are several versions, such as 802.11b, 802.11n or 802.11ac., 802.11ac is the most recent one and the fastest, but it also consumes more power, and most importantly, we don’t need that kind of speed, since we won’t do any streaming. Consequently I choose to stick with 802.11b/g/n compatible modules.
  • Leds:

In terms of lighting, we have the same constraints as Cyled, so we will just take the same: ASMB-KTF0-0A306.

EDIT: I didn’t publish this post right after I wrote it because at the time we were waiting for answers from Alexis, and I wanted to publish it with the answers.

After the discussion, several things changed:

We will in the use a sensorless motor, as Cyled did, so this eliminates the need for a IrDA communication with the motor. No IrDA communication means we can use an optical sensor along with the Hall sensor in order to increase to precision of the speed measurement, just like Cyled did. We will use the TCUT1600X01.

As for the Wifi module, in the end we will go with an ESP32-WROOM wifi module which will communicate in UART with the PCB.

That’s it for the choice of these components. We can now start making the PCB!

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.