I might be missing something with a 6 cim drive...

[Oblarg]

When testing this year, we found that a new breaker can take 6 stalled CIMs and a compressor (~250 amps) for about 25 seconds before tripping. Note that this is to a large extent cumulative over the course of a match, because these breakers are thermal and they don't dump heat instantly (or even particularly quickly); that is, if you stall for 20 seconds early on, you're going to be in real trouble if you get into another pushing match 30 seconds later.

[asid61]

Actually, I heard 254 stores covers breaker in ice packs or something right before the match. Not sure how true that is.

Of course if we go 6 cim we would have a shfting gearbox. That way we would have pushing power as well.

[asid61]

Quick question, how would you stall a 6 cim drivetrain in normal usage? The wheels would lift at some point, even driving into a wall, right?
Of course, we would still build in current limitations and preferably shift.

[Dunngeon]

Originally Posted by asid61

Quick question, how would you stall a 6 cim drivetrain in normal usage? The wheels would lift at some point, even driving into a wall, right?
Of course, we would still build in current limitations and preferably shift.

Get into a pushing match with a robot, if the traction of your wheels exceeds the torque created by your robot then the motors will be in stall. (Assuming all wheels are on the ground)

Even with 2 or 4 wheels off the ground, it's possible for your wheel traction to still be greater than the torque of a 6CIM drivebase (However, unlikely)

Keep in mind that in a 6wd configuration, there are only 4 wheels in contact with the ground at any given time. It remains the same for 8wd, with only 4 wheels ever actively touching the ground (assuming center two wheels are dropped)

[BBray_T1296]

Fun fact: 6 CIMs at stall consume a total of roughly 9576 watts of electrical power, which is over 9000!


Seriously though, a robot with 6 CIMs under "normal load" output (excluding friction) a total of 2.6 Horsepower, while a 4 Cim drive does just 1.75. At exactly 40 Amps per motor the numbers are (theoretically) 3.86hp and 2.57hp, respectively.

And as everybody knows, power/weight ratio is the most important figure.

[AllenGregoryIV]

Originally Posted by BBray_T1296

Fun fact: 6 CIMs at stall consume a total of roughly 9576 watts of electrical power, which is over 9000!


Seriously though, a robot with 6 CIMs under "normal load" output (excluding friction) a total of 2.6 Horsepower, while a 4 Cim drive does just 1.75. At exactly 40 Amps per motor the numbers are (theoretically) 3.86hp and 2.57hp, respectively.

And as everybody knows, power/weight ratio is the most important figure.

That's why we ran 6 CIMs and 2 MiniCIMs this year. More power is always better.

[asid61]

So have people stalled a 6 cim drive before then?

[Oblarg]

Originally Posted by asid61

So have people stalled a 6 cim drive before then?

Yes.

With a 125lb robot (including battery and bumpers) geared 6.1:1 with 4'' HiGrip wheels, we stalled 6CIMs when simultaneously running the compressor on a not-quite-full battery, resulting in a current draw of ~250 amps.

With the compressor off and the battery completely full, the same robot spun the wheels and pulled ~180 amps.

[BBray_T1296]

Originally Posted by Oblarg

Yes.

With a 125lb robot (including battery and bumpers) geared 6.1:1 with 4'' HiGrip wheels, we stalled 6CIMs when simultaneously running the compressor on a not-quite-full battery, resulting in a current draw of ~250 amps.

With the compressor off and the battery completely full, the same robot spun the wheels and pulled ~180 amps.

But CIMs stall at 133 amps apiece...? (theoretically as always) their manufacturing specs say so. that would be 800 amps, aka very bad...

[Oblarg]

Originally Posted by BBray_T1296

But CIMs stall at 133 amps apiece...? (theoretically as always) their manufacturing specs say so. that would be 800 amps, aka very bad...

They stall at significantly less than that when you're running them off of a real battery whose effective voltage drops significantly when you start drawing lots of current from it.

It's the same reason that you can stall them at all in the first place; if you naively just calculate from the stated stall torque of the motor you'll find that your robot would have to weigh an ungodly amount to stall them at any gearing you'd see on an FRC bot.

[BBray_T1296]

Originally Posted by Oblarg

They stall at significantly less than that when you're running them off of a real battery whose effective voltage drops significantly when you start drawing lots of current from it.

It's the same reason that you can stall them at all in the first place; if you naively just calculate from the stated stall torque of the motor you'll find that your robot would have to weigh an ungodly amount to stall them at any gearing you'd see on an FRC bot.

True true. I thought about that but did not account for it.

As the resistance outside the battery decreases and approaches near zero (v=ir so 12=133r so r= ~.1ohms) (the motor stalls), the internal resistance inside the battery becomes a significant percentage of the overall resistance, so the potential difference beyond the two terminals is proportionally less than it were when the internal resistance was a small fraction of the overall resistance. Another point for Ampere's law.

[Oblarg]

Originally Posted by BBray_T1296

True true. I thought about that but did not account for it.

I, too, failed to account for it in our design this year, with the result being that we had to swap out our gearkits (which was an excruciating process due to the design of our drive) before worlds.

That's the way you learn best, though.

[Ether]

Originally Posted by BBray_T1296

As the resistance outside the battery decreases and approaches near zero (v=ir so 12=133r so r= ~.1ohms) (the motor stalls), the internal resistance inside the battery becomes a significant percentage of the overall resistance, so the potential difference beyond the two terminals is proportionally less than it were when the internal resistance was a small fraction of the overall resistance. Another point for Ampere's law.

How does what you described relate to Ampere's law?

[Answer42]

Originally Posted by Oblarg

you'll find that your robot would have to weigh an ungodly amount to stall them at any gearing you'd see on an FRC bot.

http://www.chiefdelphi.com/forums/sh...ad.php?t=89768

I don't know. You can see some pretty interesting design choices in FRC.