Main breaker tripping, dead CIM

[Ether]

Proper gearing is the correct solution.

[Joe Ross]

Originally Posted by Oblarg

The crucial bit we noticed is that at our gearing, our breaker is fine unless we stall the CIMs, i.e. while slipping the wheels we have no problem, and the CIMs do not stall on a fully topped-up battery if the compressor is not running.

If you work with a fully charged battery, you may be able to get more headroom by optimizing your electrical system to minimize voltage drop. Make sure that all wires are the minimum length necessary, and as large as possible. Make sure all electrical connections are tight.

[Mike Schreiber]

A tripped breaker cost us the Waterford district. As was said above once it's tripped trash it.

http://www.cooperindustries.com/cont...UITBREAKER.pdf

Heat is a huge factor as can be seen in the spec sheet, if it starts the match hot you're in trouble. We run fans on ours between matches. We've now implemented some type of monitoring with our drive wheel encoder counts to limit our current draw, not exactly sure what the details were, I'll see if I can get someone who knows more about it to post. I think it basically says, if you're stalling either downshift or reduce power to motor.

[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?

[KevinG]

Would upping the gauge of the wires running from the PD board help at all? Say running 10 gauge from the PD board to the motor controllers, and keeping the 12 gauge lines coming out of the CIMs as short as possible? Is there a software solution that might also help, say by turning off a CIM or reducing power when a stall condition is detected? Do any of the motor controllers have a way of reporting current draw or load back to the main system?

[cgmv123]

Originally Posted by KGenson

Would upping the gauge of the wires running from the PD board help at all?

Not with the breaker tripping. You'll lose less voltage, but 120 amps is 120 amps.

Say running 10 gauge from the PD board to the motor controllers, and keeping the 12 gauge lines coming out of the CIMs as short as possible?

Your battery will drain less quickly, but as I said 120 amps is 120 amps.

Is there a software solution that might also help, say by turning off a CIM or reducing power when a stall condition is detected?

That's an idea, but it would be hard to implement without impacting performance.

Do any of the motor controllers have a way of reporting current draw or load back to the main system?

Jaguars with CAN do, and you can add external current monitors to any motor controller.

[Oblarg]

Thanks for all the advice. We've decided the safest/best option is to switch our gearing to 7.44:1 (or possibly 8:1 - any thoughts on which of these two would be better?) - we'll lose some top speed over where we were at DC, but gain a bit of pushing power and not compromise our ability to play offense. The only other solution that doesn't compromise drive performance would be to take off our roller arm and stop using the compressor, to put us back in the situation we were in at Greater DC, and we don't think that is acceptable. After all, we were only in that situation at Greater DC due to massive failure of multiple parts on the arm; it's ironic that the hardware failures, in hindsight, ultimately helped us, but we do not wish to intentionally remove most of the robot that our students spent so much effort building.

In the future, we will likely pursue 2-speed 6CIM drives; I am sold on the 6CIM drive after our performance at DC this year (we were a major contributor on the winning alliance due to our drive power and little else), but I had not realized how much strain it puts on the FRC batteries.

[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?

[KevinG]

Originally Posted by Oblarg

Thanks for all the advice. We've decided the safest/best option is to switch our gearing to 7.44:1 (or possibly 8:1 - any thoughts on which of these two would be better?) - we'll lose some top speed over where we were at DC, but gain a bit of pushing power and not compromise our ability to play offense. The only other solution that doesn't compromise drive performance would be to take off our roller arm and stop using the compressor, to put us back in the situation we were in at Greater DC, and we don't think that is acceptable. After all, we were only in that situation at Greater DC due to massive failure of multiple parts on the arm; it's ironic that the hardware failures, in hindsight, ultimately helped us, but we do not wish to intentionally remove most of the robot that our students spent so much effort building.

In the future, we will likely pursue 2-speed 6CIM drives; I am sold on the 6CIM drive after our performance at DC this year (we were a major contributor on the winning alliance due to our drive power and little else), but I had not realized how much strain it puts on the FRC batteries.

The compressor draws a hellacious amount of current, especially when it starts. Would it be possible for you to replace the compressor with additional stored air? Another option, and this would be a fair amount of programming, would be to selectively run the compressor when the total draw isn't all that high.

Other than that I agree that gearing things down would help significantly. At least for this year speed is less important than control and power (IMO).

[Al Skierkiewicz]

Originally Posted by KGenson

Would upping the gauge of the wires running from the PD board help at all? Say running 10 gauge from the PD board to the motor controllers, and keeping the 12 gauge lines coming out of the CIMs as short as possible? Is there a software solution that might also help, say by turning off a CIM or reducing power when a stall condition is detected? Do any of the motor controllers have a way of reporting current draw or load back to the main system?

Changing from #12 to #10 and making everything shorter will increase your current draw, not decrease it. As you increase current demand, there is a voltage drop increase across everything in the path, wires, PD, breakers, speed controllers, and connectors. Jaguars when using the CAN bus can report current at the input to the controller. Be advised that the Jags have an over current and an under volt fault. Just like putting fans on a closed motor, trying to cool the breakers externally does little to change the trip point.
The compressors draw significant current when starting but in general are one of the least demand motors we use. The old Thomas KOP compressor would draw 25-27 amps at start and will run at 12 amps. The new Viair starts at something like 20 amps and runs at a little over 10.