Project Logs

Collapse

• [R] 1642 brushed motor?

  kelvinA • 2 days ago • 0 comments

  I have looked into brushed motors (shown as copper-coloured lines) since their control requirements are much lower, and the CL-1642M motor is the most ideal assuming that the resistance of the coils can be calculated as [rated voltage] / [stall current]:

  The reason why it's ideal is because it has a 2mm dual shaft, making it straightforward to add both a magnetic encoder and POM gear, as well as using the lowest power out of all the commercial offerings.

  The main downside is that the supplier doesn't sell samples at all.

• [B] Custom rotor

  kelvinA • 2 days ago • 0 comments

  When I was searching for suppliers for the 4-pole rotor magnet, ABM Magnetics unexpectedly offered to create the entire rotor during conversation. Another thing that the engineers suggested was to split the rotor magnet (which was 58mm at the time, before the new 7.2 : 1 reduction):

  I communicated with our engineers and our suggestion is to divide the 58mm long magnet into 3 equal sections for the following reasons:
  
  1. Because the size of the magnet is OD8, ID3mm, and length is 58mm, the magnet is easily broken during processing;
  
  2. Dividing the magnet into three sections can reduce eddy currents and reduce the power consumption of the motor.
   

  When the magnet was shortened to 42mm, I suggested that the rotor be made from 8 magnets total (2 equal sections of 4 magnets). They're also planning to install the bearings too, so the complete solution would look something like this:

  They never mentioned price until today (I was thinking that it was going to be in the thousands but I'll stick around for the conversation anyway), but it's actually $16/rotor for 10pcs. I haven't asked about shipping price but I'm assuming it's another $50 on top.

  It seems every motor option for "new" and not "new old stock from AliExpress" is going to be £20/motor and up, and the DIY Tetoroidiv probably would be no exception since everything else about the motor (3D print, toroids and coils) cost pennies. The 5 toroids needed probably amount to £1 total, and even when printing in £40/kg PBT, I'm not sure that the overmould would even scratch 10p in cost.

  I'm also seeing if there is a possibility that I could acquire and use air-coils instead of having to coil at home.

• New motors for the 7.2 : 1 reducer

  kelvinA • 7 days ago • 0 comments

  As mentioned in this log, I need to go out and start looking for and designing shorter motors. This log describes what candidates I've found so far.

  I'm now getting under 1W of power for all shortlisted motors. If these numbers are actually accurate, it means that a solar powered Tetrescent would make sense:

  DIY Toroid

  As you can see, the difference between one pole pair and two is the difference between being the worst or best in the lineup. I have yet to see if reducing the wire diameter to 0.15mm would be worthwhile.

  There's also this graph from the paper:

  What I understand is that the flat part of the curve is current limited for continuous operation, and I believe that this relates to power dissipation. If I extrapolate and calculate the 0RPM wattage, I get 712mW. Considering that this DIY motor has a lower diameter and higher length, I'm assuming that it would have a better surface area : volume ratio and so its power dissipation would be at least on par with the one created by the authors. Hence, it should be possible that all these motors should be able to produce 300gf on the belt continuously.

  XBD-1636

  It's another Sinbad Motors offering that has a worm gear on it. I might be able to ask them to remove it, but it's not like I'm in a rush due to the listing saying it's £28/ea (and the actual price is likely higher). 

  TT-1640-24V

  So I once again asked TT motors for information regarding a 1636 motor, and I got a datasheet for a different motor lineup. Maybe it's the same thing just ever so slightly longer:

  I don't know why they didn't put this handy page in any of their listings. The 24V model was easily the best option, and from calculations, I'd only need a quarter of a watt to get the max desired force output.

  It's not all great though. It's $22 per motor.

  What about brushed motors?

  I was curious to see, since there are more brushed motors in this size range. Well, most of them have a gearing on them (17 : 1 and up) and power requirements that were too high. Even the best option I found, which didn't have gearing and had a handy double-shaft so that I could attach a magnet to the other side, used over 2.5W:

  It actually took some digging to find the datasheet for this motor. I actually just found a typical AliExpress listing that just had cool images, like this one of it's skewed rotor:

• [M] Slotless motor FEMM simulation

  kelvinA • 7 days ago • 0 comments

  During my research, I stumbled on this tutorial article that describes how to set up a BLDC motor simulation. Almost immediately, I was downloading and installing FEMM and exporting the simulation sketch from Fusion. 

  The first thing I learned was that, as far as the simulation is concerned, if the item isn't conductive and/or magnetic, it might as well be air. Thus, the overmould isn't part of these simulations because it wouldn't change the result.

  I started with at 0.2mm copper wire option, but I later found out that I can customise the material so I (eventually) renamed it to simply "Coil" and changed the wire diameter to whatever I wanted to simulate. Usually this value was 0.18mm but I did run some simulations with 0.25mm wire. 

  So I soon had my first simulation actually set up. I've been spending months on-and-off researching for a simulation solution for #Tetrinsic [gd0041] and I now had something at a time when I should actually be in bed. This is the first simulation result of the magnetic field:

  I found out how to turn on the nice colours. I also turned off the magnet to confirm that the coils are actually configured correctly:

  Fast forward, I tried quite a few configurations with 1A though the coils so that I could get the torque constant directly.

  • 2 pole pairs increased torque essentially by a factor of 2. Additionally, the flux is better contained inside the toroid than 1 pole pair.
  • 3 pole pairs had almost no torque. Allegedly, such a configuration is "unbalanced".
  • 6 coils (20T) were much weaker than 3. 
  • Diverging from 8mm rotor reduced performance.
  • 0.18mm wire seemed like the best balance of turn count and resistance.

  Effectively, 1 pole pair is not ideal. It means I need twice the current and thus 4x the power to get the same torque out as a 2-pole pair motor. I'm currently asking around on Alibaba to see what my options are.

View all 4 project logs