Main breaker tripping, dead CIM

[Oblarg]

After having nearly no problems with our drive for all of Greater DC, our robot started tripping our main breaker in matches today (problem arose as the day went on, in our last two matches we tripped the breaker about halfway through). Upon testing, we discovered a few things:

1) From a cold start, it takes ~20 seconds of pushing against a wall to trip our main breaker.
2) The wires going to the breaker from the battery heated up noticeably during this time.
3) One of our CIMs was badly damaged; the front ring was able to turn independently from the body of the CIM.

We replaced the CIM (and for good measure replaced the breaker), but are wondering why the problem cropped up so suddenly and if the dead CIM could have been exacerbating it. Any thoughts? We have not been able to test since replacing the parts, unfortunately.

We're running a 6-CIM modified KOP drive with HiGrip wheels geared for 14 fps.

[Ether]

Originally Posted by Oblarg

...geared for 14 fps.

At CIM free speed?

Or 80% of CIM free speed?

Or something else?

[Oblarg]

Originally Posted by Ether

At CIM free speed?

Or 80% of CIM free speed?

Or something else?

6.11:1 gearing with 4'' (probably now more like 3.75'' due to wear) wheels.

[sanddrag]

A dead CIM certainly could be the source of the problem.

[jeremylee]

Originally Posted by Oblarg

6.11:1 gearing with 4'' (probably now more like 3.75'' due to wear) wheels.

We also had 4" higrip kit setup. We were 7.08:1 in high gear with 3 cim ball shifters (6 full cims), and we never needed low gear even when we played heavy D as we never had any main trips through our two regionals.

[aryker]

Another thing that could compound the issues stated above is that when CIMs break, they sometimes short to their chassis, which in turn can short to your robot's chassis depending on how you have the CIM mounted. This in turn can cause some strange issues with other components. I've seen this happen several times before, where an electrically hot chassis can be traced back to a broken CIM.

[Maximillian]

My team also had a problem with our breaker tripping with our 6 CIM drive train. It only became a problem in the later matches when we were playing heavy defense or were getting heavily defended ourselves. What we ended up doing was cooling down our main breaker right before our matches. We used a canned computer air duster held upside down and then sprayed it into the opening of the breaker. This got it cold enough that it wouldn't trip in the match for the most part. Its not the best solution but it ended up working decently well for us.

[Oblarg]

Well, swapping out the dead CIM unfortunately did not fix the issue; we were plagued by main breaker trips for all of our matches today.

After our last match, we went out to the practice field and tested a few things; during this time, I noticed that with a perfectly topped-up battery and no compressor running, we'd slip the wheels while pushing (as intended) and only draw 180 amps, which isn't a problem; this caused no heating of the wires and no breaker trip.

If the battery was slightly low and/or the compressor was running, we'd be unable to slip the wheels and our CIMs would stall, drawing 250 amps and causing massive heating and an eventual main breaker trip.

I'd imagine the problems we're having now, as opposed to at Greater DC, are due to an amalgam of factors; due to our robot's mechanisms being in a state of semi-operability, we weren't running our compressor during matches at DC. Moreover, our robot has gained ~6 lbs since then, and our gearboxes (they're WCP 3CIM SS gearboxes, so they're open) have been steadily accruing a layer of carpet fluff mixed in with the grease (we didn't have time to fully clean them out between matches).

It seems with our gearing/wheel COF/weight we're right at the boundary of what's operable. Does anyone have any advice (other than ditching a bunch of weight and cleaning out the gearboxes) for ensuring we're not tripping our breaker at worlds?

[Ian Curtis]

I was always under the impression that once a main breaker had tripped you should replace it since it took less energy for subsequent trips. Is that actually true?

[Pat Fairbank]

Originally Posted by Ian Curtis

I was always under the impression that once a main breaker had tripped you should replace it since it took less energy for subsequent trips. Is that actually true?

We've observed this on multiple occasions, and my understanding is that the tripping action is caused by the bending of a bimetal strip inside the breaker. I imagine that the strip could be subject to plastic deformation near the trip point and wouldn't have the same characteristics afterwards.

Whenever we trip a main breaker in competition (twice so far this year) it gets thrown into the nearest trash can and a new one goes on the robot. As Marcus said, between matches with tight turnaround we cool it down with compressed air.

[NotInControl]

Originally Posted by Ryan Dognaux

You could try implementing a voltage ramp in software. We did this in-between Central Illinois & St. Louis and never had another issue.

We did some testing on our practice drive at our school and found we were able to force the robot's radio or CRIO to reboot using 6 full sized CIMs geared similarly to what you described. Once we implemented the voltage ramp the problem was solved and we couldn't re-create the symptoms.

+1 for voltage ramp. Doing this will lower the probability of all of your motors drawing peak current. We too do this on all of our 6-cim robots.

What language do you use? I am sure many teams on here can provide an example for the implementation in your programming language.

Originally Posted by Pat Fairbank

Whenever we trip a main breaker in competition (twice so far this year) it gets thrown into the nearest trash can and a new one goes on the robot. As Marcus said, between matches with tight turnaround we cool it down with compressed air.

I know im side tracking here, but I am curious, do you know what was causing you to trip the main breaker?

[Pat Fairbank]

Originally Posted by NotInControl

I know im side tracking here, but I am curious, do you know what was causing you to trip the main breaker?

Usually it's stalling a six-CIM drivetrain while trying to get out of a pin or T-bone, sometimes exacerbated by having had another match right before for the breaker to heat up.

Perhaps it's time for FIRST to consider distributing a higher-amperage breaker in the Kit of Parts? With all the motor power available to teams now, this seems like a pitfall that a lot of teams are falling into.

[Ryan Dognaux]

Originally Posted by Oblarg

It seems with our gearing/wheel COF/weight we're right at the boundary of what's operable. Does anyone have any advice (other than ditching a bunch of weight and cleaning out the gearboxes) for ensuring we're not tripping our breaker at worlds?

You could try implementing a voltage ramp in software. We did this in-between Central Illinois & St. Louis and never had another issue.

We did some testing on our practice drive at our school and found we were able to force the robot's radio or CRIO to reboot using 6 full sized CIMs geared similarly to what you described. Once we implemented the voltage ramp the problem was solved and we couldn't re-create the symptoms.

[Oblarg]

Originally Posted by Ryan Dognaux

We did some testing on our practice drive at our school and found we were able to force the robot's radio or CRIO to reboot using 6 full sized CIMs geared similarly to what you described. Once we implemented the voltage ramp the problem was solved and we couldn't re-create the symptoms.

How did you implement the voltage ramp?

If we were to use the simple slew rate limiter described by Ether, do you have any idea of what a sensible value for the limit should be? I'm not sure how fast the main loop is executing, and it seems it depends critically on that. We're coding in java.

[MrRoboSteve]

Another thing to check is whether the terminals on the main breaker are tight. I saw two teams this year with circuit breaker trips with a root cause of loose wires at the breaker.

I agree with the suggestion that you replace the breaker if it has been tripping regularly.