Update on the choices in components for Litspin. All of the main components have been chosen. Here are the ones we landed on :
System on Module
After looking at the schematics for CyL3D, we saw that they used around 40 pins on the FPGA IO. This meant that we could use the same Aries Embedded SoM based on a Cyclone V combined with two ARM cores.
For our LED drivers, we’ve chosen the Texas Instruments TLC5957. They give us a high enough input data rate and PWM reference frequency as well as enough channels to drive our LEDs.
When you try to rotate at a relatively high speed, centrifugal force comes with the territory. The issue is that PCBs are not the most robust pieces in a mechanical system. We need to find a way to make them stronger to hold them in place so they don’t fly away and/or break and find their way into a bystander’s face.
We tried to figure out how much the PCBs would flex. Thankfully, Wikipedia exists and our problem is a classic strength of material case. We have an issue similar to beam theory’s cantilever beam case with a spread out force. Wikipedia has an article with the relevant formula giving us the information we need.
the last formula is the one giving the vertical flex in relation to x. In this formula, w is the force per length, E is Young’s Modulus for the material the beam is made of, I is the second moment of area for the beam and L is the beam’s length.
With this formula, we can deduce that a PCB will flex about 2.5cm if left on its own. Needless to say, in this formula, an assumption is made that it would not break. We need to find a way to reduce flex as much as possible.
A path to solutions
We explored ways to make the PCBs stronger. For now, the best solution we have is small steel beams on each side of the PCB. We estimate that flex would then go down to about mm. There might be a better solution but we have not found it yet and mm of flex over a cm PCB does not seem to be that much and might work.
As referenced in a previous post we need a way to wirelessly transfer power from the static part to the rotating part of our system. We found a 200W dev kit from Würth Elektronik but it seemed over-powered for our needs. Another integrated solution exists but is only good for 60W. Therefore we needed to estimate how much power our rotating system would need.
The bulk of our needs is in the LEDs. Considering 1280 LEDs and a multiplexing factor of 1:4, we would consume at most the power of 320 LEDs at a time. According to the specifications, an LED could use 60mA@3.3V at most. This gives us a max power consumption of 63W just for the LEDs. Even though this is a worst case scenario, it seemed to us that using a 60W power supply would be too risky, even more with all of the other components (SoM, wifi module etc..) and taking into account a DC-DC converter efficiency of 95%.
Since we couldn’t find another solution for wireless power transmission between 60W and 200W, we stuck with the 200W option, a using only coils and making our on driving circuitry would require us to be experts in induction power transmission and power supply design.
Motor and ESC choice
Our search for a BLDC motor took us all over the e-bike, drone and RC car and boat world. At first, we found a motor at Alien Power Systems but couldn’t find an ESC to match so they directed us towards Maytech. They make both ESCs and BLDC motors. We need a motor with enough torque to start the rotation considering our system will be quite heavy compared to an RC car or a drone propeller.
Getting both the motor and ESC from the same brand made it so we didn’t have to solder connectors or bother with pinout issues. It would allow us to have better support down the line should we have a technical issue. We then chose a Maytech MTVESC50A for our ESC which gives plenty of power for the Maytech 5055 1100W 70KV motor.
In order to better understand how to make LitSpin a coherent and functionnal device, we created a diagram of its architecture .
The following represents how the different modules will communicate and work together. Technical details will come later as choices in components and communication buses are made. This is not supposed to be a drawing so it is not representative of how LitSpin will look but it should give an idea of how it will work.
Basically, there will be two main systems. A static system that will have an electric motor and what is needed to control it : a control board that will communicate with an Electronic Speed Controller, the accompanying BLDC motor, an IR receiver that will get information from the rotating system and the induction power transmitter.
The rotating system will contain the bulk of the project. The DC-DC power supply converting 24V to 5V, our SoM, LED drivers and multiplexing circuits and other modules. The SoM will contain both an ARM processor and an FPGA element to control the drivers and multiplexing circuits.
In order to have one driver per 64 LEDs, we will be multiplexing our LED outputs 1:4. Each PCB will then consist of 32 LEDs on each side, one TLC5957 driver and the multiplexing circuitry.
As for communication, the rotating system will have a Wifi module that a user will be able to connect to and an IR LED to send motor control information to the static PCB.
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.