[E2][M][R] 16mm slotless iron lamination / coil

A project log for Tetoroidiv [gd0152]

A Ø16mm, water resistant BLDC servo with BT v5.2 and zero cogging.

kelvinakelvinA 05/18/2024 at 04:160 Comments
Image from one of the suppliers.

So, while asking multiple suppliers about custom toroid laminations, I learned some things:


So far, the lowest quote I've gotten is 8USD/ea.

I might want to consider simulating the eddy current losses with just a pure iron tube; since this is a low-speed application, they might not be too bad. At the same time though, it doesn't seem that such a tube exists; I just keep getting different kinds of steel in search results.

The foil coil method would give me the most versatility, and could potentially be used in the larger #Situation SeriouS [gd0098] motorised button without having to once again ask suppliers on Alibaba (unless I need a different length) and probably spend loads on shipping mostly air. Additionally, the foil coil could be directly wrapped around the stator unlike a traditional lamination yoke.

In other news, if I try and compute the expected torque constant to convert the BLDC made in the paper to the current design, I'm about 1/4 off:

Expected    → 75mN.m          [paper torque constant]
            x (9 / 11.5) ^2   [rotor size difference]
            x (37 / 25)       [rotor length difference]
            x (75 / 130)      [coil turns difference]
            = 39.22mN.m

Simulation  = 29.64mN.m       [SS-430 yoke]
            = 29.78mN.m       [M-22 yoke]

Difference  = 24.4%           [SS-430 yoke]

[May 19

The seller of the SS430 has a 10m listing for 40mm wide which is currently £40 (a little less than 2x the 400mm x 1m). Additionally, it's much more likely that I'd only get 13 completed yokes out of a 10m length. Thus, the expected price is £3.69/ea delivered after VAT, which is still less than the cheapest option I've come across on Alibaba (£4.39 pre-VAT pre-shipping from this supplier). Additionally, I'd have the freedom to experiment; perhaps when attempting to manufacture, I find out that 12.3mm or 12.4mm is better than 12.5mm ID.

For this to work, I'm assuming that the tiny but physical separation between turns will be enough to minimise eddy currents, as discovered in this DIY transformer core video.

This has been rendered with the yoke as a spiral.

Additionally, to make sure that there was enough exposed motor shaft, I've slightly decreased the rotor length and increased the shaft length.

The power consumption is theoretically 122mW and the Kv at 346 with 0.18mm wire (75 turns), meaning that the no-load speed at 5V would be sub 1700RPM. That could be a bit of a problem since the expected max finger speed would be 3400-3500RPM at the motor, so it might mean that power is harnessed from the fingers, causing resistance, or perhaps damage something.

I've also finally redefined the sketch so that it more accurately matches the winding process.

0.25mm wire at 41 turns would have a no-load speed under 3250RPM (460RPM at pulley) and consume 128mW. Additionally, 0.25mm wire is more popular and should be more reliable to wind due to the lower turn count. The lower resistance should also mean it's easier to generate haptic waveforms.

Front view of design with foil yoke and 0.25mm wire.