Cheap FOCer! (VESC compatible 4.12 redesign)

Without traction control one wheel would spin on loose gravel etc and I would go nowhere. Traction control greatly improved grip. On tarmac though I don’t think it is needed.

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With two separate motors, even if one starts spinning, the other one still has power. It’s not like a differential in a car.

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Yes, however if one starts spinning then the other also loses traction on loose terrain.

That’s likely due to the fact that the terrain under the other wheel is similar terrain, based on the proximity of the two wheels to each other. It could be caused by a lower speed, which is a result of the other wheel spinning. But it is not a direct result of the other wheel spinning.

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Direct or indirect is beside the point. When one wheel starts spinning you rely on the other to propel you forward. However on low traction situations one wheel simply is not enough and also starts spinning. Traction control prevents the first wheel from breaking traction by lowering the current that goes to the motor that spins faster. You don’t have to take my word for it. Find a really steep gravel road and do tests with your mtb with and without traction control.

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Looks like the diameter is going to vary based on type and probably manufacturer. I’ll make the holes 3mm to try and compensate for variance

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What’s happening here, is this some alternate universe? AWG specifies the thickness of a cylindrical wire. It does not vary with respect to use for electrical wiring. As it specifies “thickness” of something cylindrical, it describes a circle, which can be specified by:

  • diameter or radius
  • circumference
  • area

I’m trapped in 'murica units, but I believe the rest of the world uses cross sectional area (mm^2).

This wikipedia page has a table to convert awg to diameter and cross sectional area in both 'murica and SI units.

10awg:

  • diameter
    • 0.1019 inches
    • 2.588 mm
  • area
    • 10.4 kcmil (wtf is this unit…)
    • 5.26 mm^2

(did I misunderstand the discussion and will feel stupid shortly?)

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No you’re statement is valid. That’s why I said approx 2.6mm earlier for the same reasons due to the standard of what AWG is. But no manufacturing process is perfect and there will be variance. Also the stranded stuff probably fluffs out a bit which could throw off a measurement. Either way, 3mm should allow 10awg comfortably.

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Did a bit of measurements and observation on the switch node ringing today.

So I tried 0.1uF 1206 100V ceramic cap. I measured all 3 switch nodes with a power supply voltage of 12V. The node closest to where power comes in on the rails peaks at 16V. The node furthest away peaks at 21V. Looks like I’ve got some inductance issues that increase the further you go. I may try various values of polypropylene caps to see if they improve the situation.

One of the contributing factors is the relatively low gate charge of the FETs I’m using. They’re TI CSD18535KCS and they only have a typical total gate charge 63nC vs the 236nC of the FETs from the original 4.12 hardware. Increasing the series gate resistance should help. I will seek a balance between adding caps and increasing series gate resistance.

For the PCB design, I think I need to go ahead an move the big ground track to underneath the big positive track. I’ve been advised to to this before and now I think I truly understand why. One will have to beef up those traces in a low profile fashion (like with the copper flat wire) since the exposed power track will sit right beneath the tabs of the FETs after I make this adjustment.

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No, you aren’t misunderstanding, but that is for one cylinder of solid wire. When you have one thousand small cylinders (super-stranded) all summing to the same area as the big cylinder, the total wire size increases because it also now contains air as well as conductor.

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Just whipped out some 10 awg super stranded and some calipers…smallest hole I can easily get it in is 3.2mm

When tinned its closer to 4mm

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Ok. Are all you guys measuring a freshly stripped piece of wire? Cause that’s what’s going to go through the hole in the PCB.

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IMG_20190205_224813_HHT

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You can’t pinch wire if it’s hot with solder… measure it unsquished

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But it is not pinched. Just twisted. All good, should be accurate measurement from @moon

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Yeah it’s twisted.

And no one measures shit like what you just described

I just checked here and it’s like 3.35mm

20190206_122441

20190206_122550

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OK guys. So I’ve been going hard on this ringing issue. I’m seeing a voltage spike of about 10V above the supply voltage for whatever the supply voltage may be. I’ve been successful in getting rid of the ringing after the spike but not the initial spike itself.

The next thing I intend to try is a RC snubber circuit placed on the switch nodes. There is a lot of material out there that shows how this helps and how to design it. Testing this out will be the next thing I do in the lab.

Note that I’ve improved the PCB design in Altium to attack this issue at its source. What I’m doing now is just putting band-aids on my current prototype so I can do some loaded testing without the thing frying. I’ll repeat a lot of this analysis on the version that the testers receive so that I verify that issue will have indeed been fixed.

Also, I’ll be out on vacation for the next 5 days so progress won’t happen until I get back. Cheers!

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When are you seeing this spike?

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image

Showing you guys an example of what I’m trying to get rid of. It’s that spike at the upper left corner of that square. That’s the MOSFET/DRV killer when you try run the VESC with voltages around 54V or so.

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