A solid foundation

Last week we met with Alain Croullebois, the go-to guy for mechanical questions at the school. We went to his workshop with the motor we received during the holidays : this Turnigy Multistar 4225-390Kv 16 Poles Multi-Rotor Outrunner. We discussed what the physical structure of the Phyllo could look like.

Here is a global diagram of what we came up with :

The mechanical structure of the whole Phyllo

The Base

For the base of our Phyllo, he suggested stacking two 30cmx30cm aluminium plates 8mm thick, with a separation of 6cm between the two plates maintained by 2cm wide pillars at each corner.
The space between the plates would hold the battery, the motor, and the fixed PCB.

The fixed PCB needs to be able to communicate via IRdA with the main rotating PCB, so we plan to bore a hole in the top plate to allow the IRdA transceiver on the fixed PCB to see through.

We’re not thrilled by the size of the plates, they seem too big to us. We’d like to see if we could take 20cmx20cm plates instead. That way we could later hide this square base behind a cylindrical cover to give the appearance of a circular socle on which the Phyllo sculpture would rotate.

A Phyllo with a square base and another with a circular base

The 6cm height separation might also be excessive, we went with that because we weren’t yet sure how much place would be needed for the battery and the PCB (we need a 4S LiPo battery, so the dimensions will roughly be 15cm x 5cm x 5cm). We might reduce this height a bit later on.

Rotation and power transmission

The drive shaft would extend above the base from the motor through a hole in the top plate, fitted with ball bearings to stabilise it. Alain Croullebois offered to design a shaft piece made to be affixed to the rotor of our motor, with an 8mm diameter.

We also need to transmit current to the rotating part. This requires to have to lines of current running from the fixed to the rotating part, one for VCC and one for GND. 

The idea we came across with Alexis is to transmit one of these through the shaft via ball bearings, and to use another set of ball bearings isolated from the shaft to transmit the other line to a wire that will rotate alongside the shaft. It is very similar to this method the RoseAce project used a few years ago. We might wrap the wire around the shaft or otherwise fix it to keep it from flapping around.

Current transmission to the rotating part

We plan on doubling the ball bearings for each line of current, to reduce micro-interruptions as much as possible.

The question of the thickness of the wires also came up. On average the rotating part doesn’t need much power, but since we use the LEDs with infrequent but intense flashes, this results in high peak currents. Our current configuration (see what I did there ?) calls for 50A peak currents at 5V when we turn the LEDs on for a 100µs flash. The thing is, the initial load will mainly be handled by capacitors on the rotating part, since our 5V regulators won’t have time to react fast enough. But they’ll probably have time to react before the end of the flash, so we plan to transmit as many Amperes to the rotating part as we reasonably can. 

With a 12V power supply, this means we need to be able to transmit around 21A to the rotating part. According to this chart, this means we need an AWG16 wire, with a cross section of 1.29 mm.

The rotating sculpture

Finally, we discussed how to fix the 3D-printed sculpture to the rotating part. Since the shaft conducts current, we’ll fix a 25cm wide non-conducting PVC disc to the shaft. The 3D-printed Phyllo sculpture (20cm diameter) will then be fixed to the PVC disc.

The center of the PVC disc will need to have holes or apertures to allow the main rotating PCB to communicate via IRdA with the fixed PCB.

In order to make the shaft accessible to easily fix or detach the rotating part, Alain Croullebois plans to have a gap of several centimeters between the top of the base and the rotating PVC disc. Some of this space is also necessary to fit all the ball bearings we need to transmit the current. Aesthetically speaking, we’d like to reduce this gap as much as possible, but we’ll take what we can get.

What next ?

Last friday, we 3D-printed a first full scale Phyllo sculpture : 

Full-scale 3D-printed Phyllo sculpture

We’ll show it to Alain Croullebois when we meet with him again to refine our plan to fix it to the rotating PCV disc. 

We also plan to see whether we can reduce the size of the aluminium plates as discussed above.

We’ll keep you posted 🙂

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