Here is a diagram summarizing the architecture of a Phyllo, to allow you to have a global vision :
This week, we spend a lot of time trying to choose the components. Here are the solutions we explored and the decisions we took.
Motor and ESC
All the animations we will display are based on two or three basic animations : aging petals, de-aging petals and possibly fixed images. Here are simulations to visualize those animations :
Aging and de-aging petals
In order to display those animations without skipping any petal (have a look at this instructable to remember the “skipping petals” part) while flashing the LEDs at 30Hz (ie 30 frames per seconds), the Phyllo has to rotate at :
Furthermore, we are going to use a brushless motor (less wear-and-tear, less noise) controlled by an ESC. We have chosen not to use the same motor than J. Edmark (KV135) but a motor with a higher KV, in order to consume less power. A higher KV motor has a weaker torque at the beginning, so we will have to choose a precise ESC (some of them are controlled in I2C or SPI).
For now, we have chosen:
- Turnigy Multistar 4225-390Kv 16Pole Multi-Rotor Outrunner for the motor
- VESC 6+ for the ESC
For the ESC, we will first test if the ESC CyLED used last year is able to start our motor. If not, we will use the VESC 6+.
The LEDs will require a lot of power, but for very short bursts: they will require less than 1W on average. The motor require at most 75W. We would like to have 1h autonomy, so the battery has to provide 80000mWh. The motor we choose requires a 6S battery. Therefore the capacity must be more than 3622mAh. We have chosen this one : Batterie Lipo 6S 4500mAh.
First we wanted to have a wireless transmission to avoid the ugly effect of having a metal bar coming out of the top of the Phyllo, as CyLED did last year. After extensive research, we found that this type of energy transmission is currently used almost exclusively for mobile phone charging. Therefore, output power is usually far lower than our needs (20W against 120W). The few products with better output power are essentially coming out of China without any documentation.
We also considered the possibility of building our own wireless transmission to get an appropriate output power. However, it is too hard to get a satisfactory calibration and we do not have the time required.
The following idea was the slip ring. However it has a big flaw: it wears down quickly. We imagined to have one connection on the ball bearer of the motor. Then, we thought that we could put a second one as a ring on the axis of the motor to completely get rid of the slip ring.
This ball bearer has no other use than electricity transmission. Here the insulating layer enables the ball bearer to stick to the axis. It is also necessary since the axis already bears the current from the ball bearer of the motor.
We will need at least a 3.3V regulator and a 5V regulator, to regulate the output voltage of the battery and of the power transmission device. We have considered 2 switching regulators : LT1765 and LM2678.
LEDs and light pipes
We haven’t received all the components yet... While we were working on the components choices, Vlaya has started to design some tests for the LEDs.
We plan to use a LED driver for each spiral arms. The drivers need to have enough output channels to drive all the RGB LEDs of the spiral without multiplexing. A Phyllo has a different number of clockwise and anticlockwise spirals, so we can take as many drivers as the smaller spiral number, in which case they need to have more channels, or as many drivers as the bigger number, and they can have fewer output channels.
The drivers cannot be interrupted in the middle of a PWM cycle, therefore the clock of the internal PWM of the drivers needs to be fast enough that a PWM cycle is shorter than approximately 100µs. That is the time the LEDs can be flashed, if they stay on for longer periods the image will blur due to the Phyllo’s rotation.
Moreover we need to be able to communicate fast enough with the drivers to send the color values of each LED to the drivers in the time between each frame (1/30 of a second). And we need to be able to simultaneously enable and disable all the drivers.
Finally the output current must be high enough to power our LEDs with an acceptable brightness.
We have chosen these LED drivers : THL3502
- 24 channels, allowing up to 8 RGB LEDs per driver
- PWM 10 MHz and 8-bit brightness control, making for 25.6 µs PWM cycles
- 10 Mbps I2C serial input, allowing us to send colors for 150 RGB LEDs in 240 µs
- Output current : 100 mA
With the I2C interface, we cannot simultaneously fire and disable all the drivers. If the delay gets too big, a solution could be to separately control the current output to the LEDs with transistors.
We will need Wifi to communicate with a Phyllo from a computer, and probably to communicate between Phyllos. At first, we wanted to use an ESP32 because it’s simple to use and easy to find help online. But after discussion with Alexis, we will use a AMW006.
We would like to be able to upload complex animations frames on a Phyllo and maybe allow users to do the same. To provide the necessary storage space, we will use an SD card : JAE_ST1W008S4ER1500.
Angular position sensor
To be able to display neat and clear animations in a precise direction, we must be able to detect the alignment of the motor with its original position with an accuracy of less than half a degree (to have distinct images like J. Edmark). To do so, we will use a Hall effect sensor (TLE4964) and we planned an optical sensor (the same than Cyled : TCUT1600X01) just in case, even if theoretically, the Hall sensor is precise enough. The idea is to get a pulse every rotation and use these to deduce motor speed, and then integrate the speed, coupled with the once-a-rotation pulse, to deduce the angular position.
To communicate inside a Phyllo between the fixed part and the rotating one, we will use IrDA. We have chosen these transceivers : TFDU4101-TT3.
The IrDA Design Guide requires a transmission distance of 0cm to at least 1m and an emission angle of at least 30 °. Thus, if we place the transceivers close to the axis of rotation, they will be able to communicate during most of a turn, so no worries about the transmission speed.
If we can not place them close enough to the axis of rotation (because of the wide motor diameter), we can reduce their directivity by putting a diffuser so that the LED illuminates the entire interior of the phyllo (or at least most of it) and the two transceivers can communicate all the time. It will not be necessary that it affects the communication of the other Phyllos.
If we cannot place them close to the axis of rotation nor diffuse enough light, and we are obliged to place one at the periphery of the PCB: we place one near the center facing outwards, the other on the periphery oriented towards the center. They will be able to communicate during 2.7ms minimum. In this case, we must take elbow transceivers like TFDU4101-TR3, probably faster ones…
To be continued…
The following post will detail our ideas about detection of other Phyllos and communication between them. And Monday, we have a meeting with Alexis to decide on a solution. We will keep you updated :)