Re: pic: Did Somebody Say Defense Game?
 

Adding an extra motor per side does increase the theoretical maximum available torque. However, traction limitations and battery limitations do not allow that torque to be fully utilized.

Re: pic: Did Somebody Say Defense Game?
 

Correct, you would have to be able to feed the motor the extra current to increase the torque. So basically you increase the system stall torque by increasing motors, but can you feed it the extra current?

If two drivetrains exactly the same in traction, weight, gear ratios were in a push off/breaker cook race and the only difference is the motor count (lets say a 4 cim versus a 6 cim), the 4 cim drivetrain will pop its breakers faster. The 6 cims will have less current going through the individual 40a breakers (total flow divided by 6 instead of 4), and slightly less through the main breaker (because they should be running at a slightly higher efficiency).

My assumption why people pop breakers with 6 cims faster than 4 cims is because they think they can push while geared high, when really both systems are limited by their breakers. So you would need the correct gearing in either case to be a good pushing robot.

Re: pic: Did Somebody Say Defense Game?
 

There's a limited amount of pushing force that you can have before the wheels start slipping. So if you have two CIMs geared really slow, or 15 CIMs geared really slow, once the wheels slip, you won't see any difference. However, with more CIMs, your traction limited speed will be faster, which is useful for playing defense. If you've got a crazy high traction robot that moves really slowly, it'll be hard to catch up to somebody.

Re: pic: Did Somebody Say Defense Game?
 

I agree on the 40 amp breakers. Once those things heat up, they're really prone to tripping.

With 6 full size CIMs on drive, you can trip the main doing full forward to full reverse. It's scary to think it could happen in a competition. Also, once you've driven for a minute or so, you can get the battery voltage low enough to cause the radio to drop out. Those CIMs can draw 100 amps at stall. It'll take a few seconds for the 40 amps to start tripping, but the 120 amp breaker trips quickly when you try to put 600 amps through it....

Re: pic: Did Somebody Say Defense Game?
 

Can you explain this one, I don't fully understand.

Re: pic: Did Somebody Say Defense Game?
 

in order to get to traction limited, no matter how many CIMs you have, you need a certain amount of torque at the wheels. Let's just say for my robot, it's 16 times the torque I can get directly, with no gear reduction from a single CIM motor. So, I would have a 16:1 gear reduction with a single CIM to be traction limited.

If I had two CIMs, I would need to have 16 times the torque from one CIM, or because their torque adds together, I would need 8 times the torque from two CIMs. So, I would put these two CIMs in an 8:1 gear reduction.

The second, 8:1 gear reduction with 2 CIMs will have the same amount of torque at the wheels (16 times the torque directly from a CIM) as the 16:1 with a single CIM, but the 8:1 will go twice as fast.

What you said in your previous post was correct, you need proper gearing to take advantage of your motors.

Re: pic: Did Somebody Say Defense Game?
 

I think that a brown-out and re-set of the cRIO is a very likely drivetrain induced failure. Not just the breakers blowing. I don't know which one, 6cim or 4cim, would be more likely to trigger a brown-out, though I think a brown-out or low-voltage reset is much more likely than blowing a main breaker.

Full disclosure: I have not a lot of experience blowing 40A breakers, or main breakers for that matter. I've always geared transmissions on robots a little conservatively, prioritizing robustness over ultimate performance. One robot team 95 made had issues with that, a full 4-wheel swerve in 2003, that could drain a battery down to the point of brown-out half-way through a match.

Re: pic: Did Somebody Say Defense Game?
 

Excellent, thank you! A more visual explanation helps me, I miss things when I cant visualize them.

Re: pic: Did Somebody Say Defense Game?
 

We ran a 6 motor, 2" wide wheel 6WD last year. We had troubles popping the main breaker but not the 40A breakers. Because your motors are sharing the load the current is actually LESS per motor. Therefor it is actually HARDER to trip the 40A breakers.

Re: pic: Did Somebody Say Defense Game?
 

Tripping the 120 amp? Never had that happen... Anyone who has, what caused it (that wasn't a short :P)? Your drivetrain?

Popping that is quite a scary thought...

Re: pic: Did Somebody Say Defense Game?
 

What do you think the root cause of the high current load was from? Pushing, turning, hard accels?

Re: pic: Did Somebody Say Defense Game?
 

The only time we experienced the main breaker popping was when we went from full speed forward to full speed backwards. Adding in a small ramp prevented us from tripping the main again.

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Hence our use of a 24:1 Low Gear + 4in wheels on this design.

We've already got our programmers working on this exact thing, we've shredded far too many AM toughbox gears in the past to not know better. :rolleyes:



Oh, and for those of you that might not have noticed, I did update the OP with some specs.

Re: pic: Did Somebody Say Defense Game?
 

This is not a universal, blanket statement. It's less correct than the statement you were referring to actually, which is pretty much always true. With a 6 motor drive, all other things equal, each individual motor will draw less current than each individual motor in a 4 motor system. This makes it harder to trip the 40 amp resetting breakers, which tend to trip first in a 4 motor drive. The tradeoff is that, at certain gear ratios, you are more likely to trip the 120A breaker. The most vulnerable edge case is an extended pushing match when geared above ~10 feet per second. A low gear for a 6 motor 2 speed drive can certainly be geared low enough to prevent this from basically ever happening in a 2 minute long match.

This tradeoff is something teams should carefully analyze and play with when deciding to use a 6 motor drive. Study the breaker spec sheets, both 40A and 120A. Experiment with different motors or robot cooling systems. Run a shifter and push in low gear. There are a lot of ways to try and mitigate this potential pitfall.

(By the way, consider running a low gear that is traction limited with each motor drawing, say, 30 amps instead of 40, if you are extra paranoid)

Re: pic: Did Somebody Say Defense Game?
 

Or at ~25-30 amps if your team 703. Loving this drive train. :)

They could push for the entire season and not break a sweat with the motors, breakers, or batteries.