6 CIMs and 4 MiniCIMs - Will they stay?

[Pratik Kunapuli]

Originally Posted by cbale2000

Does anyone know of an instance where this has EVER happened on a FIRST robot?

Yes. We had 6 CIM's for our drive train, and in our first event of the year (Hatboro Horsham), we got into a few pushing matches and popped our main breaker more than twice. When all of our CIM's stalled, they pulled about 780-800 amps for more than a few seconds, which is definitely enough to trip the main breaker.

[Jared Russell]

Originally Posted by Pratik341

Yes. We had 6 CIM's for our drive train, and in our first event of the year (Hatboro Horsham), we got into a few pushing matches and popped our main breaker more than twice. When all of our CIM's stalled, they pulled about 780-800 amps for more than a few seconds, which is definitely enough to trip the main breaker.

Worth noting that after replacing the main breaker I do not think we ever had this problem again.

[Tom Line]

Originally Posted by cgmv123

I only see the extra motors staying if the available power increases (bigger battery/main breaker).

I disagree.

This is one of those things that teams need to be aware of and design around, as part of the design challenge. I think it's much more real-world to actually have some imposed design constraints rather than having FIRST provide us a system that can be plugged together and won't fail (like Lego League). Even FTC needs to be careful about motor loads.

[Pratik Kunapuli]

Originally Posted by Jared341

Worth noting that after replacing the main breaker I do not think we ever had this problem again.

We were also conscientious that it was possible to trip the main breaker and didn't push for that long the rest of the season.

[Nuttyman54]

Originally Posted by Andrew Lawrence

Maybe someone from 971 can answer this, but what gear were they in (and what speed)? Do you have video of the match?

We had this issue a few times this year. We are traction limited in low gear, but not in high gear. The instances where this happened occurred when our driver failed to shift down soon enough. Because of the 2 extra CIMS, the breaker blew much quicker than our driver was used to reacting to shift down. If we do a 6 CIM drive next year, there will likely be some mechanical and/or programming solution to prevent this from happening.

[apalrd]

Originally Posted by cbale2000

Does anyone know of an instance where this has EVER happened on a FIRST robot?

It's happened with only 4-motor drives as well.

I personally tripped it in 4 matches during my time as a driver. We also tripped it twice this year. In both cases we had a 4-motor 2-speed drive that was traction limited in low gear.

In other cases, we've seen battery cables heat up and become sticky, and also melted a few robot-side battery connectors (they're only rated for 50a) on practice robots (even with cool-down times between batteries). We used 4-gauge robot-side battery cables to hopefully improve this, and did not melt anything this year.

[thefro526]

Originally Posted by cbale2000

Does anyone know of an instance where this has EVER happened on a FIRST robot?

As Jared and Pratik said above, it's happened to us at least once this season. Took a good bit a defense to pop it, but it can happen. There are a handful of other teams that have complained of popping breakers as well, most of which are using high traction drives with 6 CIMs or 4 CIMs and some additional motor.

Looking back, I'm fairly sure that prior to this year electrical shorts were the most/more common cause of the 120a breaker tripping, but I'm not sure.

Originally Posted by AlexH

you just gear to break traction before you trip the breaker.

There's a bit more to it than that. In a perfect world, when you break traction between the drive wheel and surface (carpet) the motor would then essentially spin at free speed and draw little to no current (see 2009) but with carpet that isn't the case.

More often than not, a wheels static COF is what's being used for current draw calculations rather than it's dynamic COF - which makes sense for seeing if you're traction limited. Most of the time, when people design something to be traction limited, it's so that they know that they won't have an issue in the off chance that their motors are stalled due to some interaction with an immovable object for a reasonably short period of time. Usually, this is all you need to do as long (more than a few seconds) instances of high load like that are pretty uncommon in recent FRC games.

There are occasions where you need to go a bit deeper into the issue than that. Once your wheels have broken traction with the carpet, the wheels Dynamic COF starts getting important. Most wheels have a Dynamic COF that's significantly lower than it's Static COF, but there are some wheels that don't so it's best to try and dig up some hard numbers first. Anyway, once you're out of the traditional traction limited realm and Dynamic COF starts coming into play, you'll see that the motor is still under a decent amount of load and will draw the appropriate amount of current. Depending on the setup, this can be (probably is) enough of a current draw to start tripping breakers (either 40A or the 120A main) after a few seconds.

One more thing to be aware of: The weight/normal force on your robot's drive wheels may increase during a pushing match. This is especially true for low bumper robots pushing high bumper robots as the bumpers tend to ride onto one another. You might be traction limited at normal weight, but what if an additional 60lbs or so is on top?