Main breaker tripping, dead CIM

[Ether]

Perhaps something like this.

[Oblarg]

Originally Posted by Ether

Perhaps something like this.

Amusingly, that's essentially exactly what my naive approach would have been, software-wise. Thanks.

I'm still partially leaning towards swapping out our gearkit to 7:1 or 8:1; we've got two spare gearboxes which should expedite the process greatly (much easier to switch out the whole gearbox than to change the gearkits of the ones on the bot). We can probably absorb the hit in top speed without too much trouble, as our defensive strategy is very much based on positioning and pushing.

[Ether]

Proper gearing is the correct solution.

[Rob Stehlik]

Originally Posted by Oblarg

I'm still partially leaning towards swapping out our gearkit to 7:1 or 8:1; we've got two spare gearboxes which should expedite the process greatly (much easier to switch out the whole gearbox than to change the gearkits of the ones on the bot). We can probably absorb the hit in top speed without too much trouble, as our defensive strategy is very much based on positioning and pushing.

Originally Posted by Ether

Proper gearing is the correct solution.

I agree. Changing the gear ratio is the right way to go. Don't think of it as losing top speed, think of how you are gaining acceleration. For short moves and evading defense, acceleration is what you want. We gear more conservatively than most, and haven't tripped the main breaker once in the last two seasons with a 6 CIM drive.

[Oblarg]

Originally Posted by Rob Stehlik

We gear more conservatively than most, and haven't tripped the main breaker once in the last two seasons with a 6 CIM drive.

Could you tell me your robot weight, wheel COF, and gearing?

[Rob Stehlik]

Here are some specs for our robot this year
weight 115 lbs
wheels 5" Colson
coefficient of friction approx 0.8-0.9 (not sure)
motors 6 CIM
gear ratio 10.4 : 1
top speed 10-11 ft/s
time to traverse field 5.1s

[Ether]

Originally Posted by Rob Stehlik

time to traverse field 5.1s

Is that a calculated number or did you measure it?

Full throttle from a dead stop with a freshly charged battery and only the drive motors powered?

[Rob Stehlik]

Originally Posted by Ether

Is that a calculated number or did you measure it?

It's calculated. I threw that number in there to emphasize that top speed isn't the most important parameter.

[Ether]

Originally Posted by Rob Stehlik

It's calculated.

If I may ask: How did you calculated it?

[Rob Stehlik]

Originally Posted by Ether

If I may ask: How did you calculated it?

For the past three years we have been using a spreadsheet that was made by the lead mentor of 294. It's probably posted on CD somewhere, but a quick search didn't find it for me. Anyway, the spreadsheet takes inputs such as weight, wheel diameter, motor parameters, gear ratio, wheel coF, etc, and produces distance vs time graphs among other things. We have it to be an effective tool for optimizing your gear ratio based on the distance you expect to be driving most frequently during matches.

[Monochron]

Originally Posted by Rob Stehlik

For the past three years we have been using a spreadsheet that was made by the lead mentor of 294. It's probably posted on CD somewhere, but a quick search didn't find it for me. Anyway, the spreadsheet takes inputs such as weight, wheel diameter, motor parameters, gear ratio, wheel coF, etc, and produces distance vs time graphs among other things. We have it to be an effective tool for optimizing your gear ratio based on the distance you expect to be driving most frequently during matches.

Off topic, but if you do happen to find that again, I would love to take a look at it.

[Ether]

Originally Posted by Monochron

Off topic, but if you do happen to find that again, I would love to take a look at it.

Me too.

There are many FRC drivetrain spreadsheets and models out there, and they are indeed useful - as long as their limitations are understood and taken into account.

Most of them contain at least one fudge factor to close the gap between theory and practice. Unfortunately, one size does not fit all: the correct factor(s) for one drivetrain design may be quite wrong for another. And it's not just the type of drivetrain that can substantially affect performance; craftsmanship also plays a major role.

[Bill_B]

Originally Posted by Ether

Perhaps something like this.

Good for testing. Not legal to have any custom circuits not going through the main breaker in competition. The millivolts across the breaker is very close to the current passing through it in amperes.

I found that putting a good sized digital voltmeter display on an open PDB port and visible during test driving goes a long way toward understanding the electrical dynamics of your machine. Panel mount meters are 5 or 6 dollars. Seeing the meter drop into single digit range is eye opening indeed. I know the guys were working on coding that withheld the shot motor if the compressor was running after they saw the system voltage drop when it started.

[Al Skierkiewicz]

OK, once again for effect, six motor drives are likely to trip the main breaker. Every report I have received of repeated trips of the main breaker were on six motor drives. Yes they are cool and yes they may achieve your design criteria but they come at a cost. Do you want to go fast or play an entire match? When the #6 wire is hot, that is your robot telling you something is wrong. Listen to it. To address some things from previous posts. Yes the breaker is a thermal device so when the wire heats up, some of that heat is conducted to the breaker and that adversely affects the trip point. Once cool, it returns to normal operation. A trip uses the same procedure as the red button on top of the breaker, so it is not a requirement to change out a breaker once it trips. Otherwise you would have to change it out every time you use it. Use ramping software will help quite a bit but will do nothing in a pushing match. Your results will vary with the type and number of wheels you have on the ground and how much you turn during a match. Add to this the fact that FRC robots are already drawing excessive currents that shorten battery life. If you are drawing currents that trip the breaker, consider that your battery life is cut by more than half. Do not be surprised when a battery suddenly appears to be discharged when you are on the field.

[Samwaldo]

We are using 6cims in our Vexpro single speed, single reduction gearboxes. First time using 6 drive CIMs, and we will never go back. We havent had any problems yet, and we push all the time (and 95% win). I asked our electrical lead and she did notify me that at a few points the wires around the main breaker warmed up. We have never tripped the main breaker though even after 4 quarterfinal matches in Groton with very little break time. We have no software assistance either. Reading this thread worries me though.

Should we mount a large cooling fan above it just like we do for our compressor?