6 CIMs and 4 MiniCIMs - Will they stay?

[EricH]

Originally Posted by Wing

Before they allowed 6 CIMs, several teams used to fry FP motors as a 5th and 6th motor in their drivetrains due to similar specs.

Fixed that for you. FPs in the drivetrain will probably end up running at or near stall quite a bit, and they don't like stalling.

[ttldomination]

Originally Posted by EricH

Fixed that for you. FPs in the drivetrain will probably end up running at or near stall quite a bit, and they don't like stalling.

I'm not sure about this, but there was a discussion and quite a few teams came out in support of this. I think the argument was along the lines of, "they end up doing more work than stalling so it's fine." I don't quite remember.

Either way, I've heard of quite a bit of success from this. At the very least, it's worth exploring.

- Sunny G.

[EricH]

Originally Posted by ttldomination

I'm not sure about this, but there was a discussion and quite a few teams came out in support of this. I think the argument was along the lines of, "they end up doing more work than stalling so it's fine." I don't quite remember.

I know of a couple of very, very good teams that did FP+2 CIMs in the drive for one year and then swore never to do it again. Something about having to replace the FPs--I know one of those teams had to do it on Thursday at a regional and it was a long job, or something like that.

[MichaelBick]

Originally Posted by EricH

I know of a couple of very, very good teams that did FP+2 CIMs in the drive for one year and then swore never to do it again. Something about having to replace the FPs--I know one of those teams had to do it on Thursday at a regional and it was a long job, or something like that.

I know 254 and 968 did it 2008 and had problems. We used 550s this year and ran into no problems whatsoever.

Also anyone doing a 6 motor drive needs to make sure they have a traction limited gear

[Cory]

Originally Posted by MICHAELABICK

I know 254 and 968 did it 2008 and had problems. We used 550s this year and ran into no problems whatsoever.

Also anyone doing a 6 motor drive needs to make sure they have a traction limited gear

We ran 2 CIM's and 2 FP's in 2006. These were the super awesome 300W FP's that were only rated for 6V, IIRC.

It was a nightmare. We smoked too many motors to count and our drive was unreliable at the worst possible times as a result. We did indeed swear we would never use only 2 CIM's in a drive again (Lunacy notwithstanding).

[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?

[theawesome1730]

We used all 6 CIMs and all 4 Minis and absolutely loved it. I really hope they continue the use of all 4 Minis even if they drop back to 4 CIMs

[coalhot]

I like the idea of 6 CIMS and 4 MiniCims. However, if FIRST does decide to stay with them, I'd like to see them ramp up the battery size and PDB.

Maybe they'll do that with the new control system. Who knows?

[Zebra_Fact_Man]

I sure hope so.