4 / 6 NEO Drive

I had a vague recollection of CD discussions about a drive train built with 6 motors (3 on a side) where most of the time only 4 of them were used. The 2 additional motors would be engaged for pushing matches and the like. I searched and found only this thread (Gearbox question) which indicates that this should work fine as long as the motor that is off is in coast mode. However, that was with CIMs and we are using NEOs this year.

We are considering this option but want to make sure we do not do anything that could damage the 3rd motor on each side. Alternatively, we would use all 6 all the time but limit the power most of the time.

Our questions are:

  • Would leaving one NEO off and in coast mode be safe for the motor?
  • What are your thoughts on the choice between the two options?

We had good results using a triple Mini-CIM gearbox a few years back. There is a thread about here:

NEOs have an even smaller size than the Mini-CIM and would work well but when we went to NEOs we did not see the need for more than two per side since two NEOs give us 25% more power than three Mini-CIMs and allowed us to shrink the whole package.

In my opinion, the added complexity of switching a third motor in and out of service is not worth the effort. Any potential efficiency gains will be so small as to hardly be noticed in a match that lasts only 2minutes and thirty seconds in total.

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So - I would make a little current scheduler. When you aren’t running anything else, allow full current to the motors. When you ARE running other things, drop the current to the drive motors proportionally.

Just be careful - only set the current when you really need to change them. Last time we tried this we started setting them every loop and it will saturate the can bus.

With 4 NEOs, assuming you’re gearing for a reasonable speed, you’re already pretty traction limited. This means that “engaging” additional drive motors during a pushing match doesn’t get you any more pushing power; it just decreases the current draw on each motor a bit.

If you’ve got 2 extra NEOs burning a hole in your pocket, it doesn’t hurt to throw them on the drivetrain - decreasing the strain on each motor is never a bad thing. However, if you’ve got anywhere else you want to use them, a 4 NEO drivetrain should perform incredibly similarly to a 6 NEO one.

The gear ratio is one thing I am a bit concerned about. I do not recall exactly, but the ratio currently on the drive train is around 8.5 : 1. This is not my area of expertise (programming mentor) but this seems to be geared a bit fast.

I like the idea of a current scheduler. Is your code for this in Github or some other public repository? I searched for one but only found what appears to be an old github repo for your team’s website.

Not enough people make this connection unfortunately. There are a few things that can be done to help out (i.e. have opponent robot ride up on your bumpers, decreasing their traction, and increasing yours)

Frankly unless you are building a defense bruiser bot you are better off focusing on being more nimble. But there is no harm with:

No, but it’s fairly simple. Just choose a maximum current flow you want and then use the current limiting options supplied by the appropriate vendor.

Assuming you’re running 6" wheels, that’s about 17.5 ft/s; on the fast side of drivetrains, but shouldn’t cause you too many issues. A little bit of napkin math or the JVN calculator should tell you that you’ve still got appropriate torque to break traction with 4 NEOs - gotta love brushless!

If you’re still worried, though, 2 NEOs is a pretty reasonable price to pay for peace of mind.

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