Quote:
Originally Posted by Chris is me
This isn't quite as black and white as you make it out to be.
Three cases to consider:
1. Less than your maximum pushing force is required to displace whatever you are pushing. In this case, your wheels don't slip, they start moving before that happens.
2. More than your maximum pushing force is required. You're already not pushing them, whatever. In fact once you start to slip you'll draw slightly less current than the max!
3. Right on the borderline. Behavior varies around here; the winner of the pushing match if head to head is usually who carried more momentum into the push.
The bigger deal that isn't modeled is the effects of voltage drop under load on pushing force and current draw. This is significant.
I think it's plenty reasonable to model at static CoF - unless you have a low gear that's mega low and you're pushing boulders (grippy boulders, made of rock, not foam), your wheels probably won't slip before you move anyway.
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The thing is, in my experience, your 'case 2' is by far the most common.
Even if your robot can move, you can (and will) still slip the wheels if your motor torque is high enough to break static friction, unless you are being very careful with the throttle. In fact, there is actually little/no difference in the first few moments between accelerating full-throttle from a dead stop, and pushing against an immovable wall. In both cases, your wheels will slip if your robot is traction-limited.
You are correct that when the wheels slip, you draw less current than the max - and it's not "slightly less," either. The difference is
significant. This is why it's so dangerous to be right near the traction-limited threshold - if you are just over it and you push, your wheels will slip and you will likely be fine; but if you're just under it, your motors will stall and you will not. There is a sharp discontinuity in the current draw you see in a pushing match between the two, and it corresponds precisely to the difference between static and dynamic wheel COF.
4464 discovered this firsthand in 2014 between our two regionals - we added some just enough weight between them to cause us to be motor-limited when our battery wasn't quite full, and the result was slew of main breaker trips in the second when in the first we had had almost no problems.