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:
From a cold start, it takes ~20 seconds of pushing against a wall to trip our main breaker.
The wires going to the breaker from the battery heated up noticeably during this time.
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.
Damage to the CIM could certainly increase its power draw either through increased drag or due to an internal short.
The breaker is rated to trip in 20 sec with an aprox 200% over current situation. The breaker is thermal so short bursts of much higher current will cause the temp to rise and it to trip eventually. If enough heat is building up in the breaker to cause it to trip the wire will certainly be heating up as well. Extended use at near trip conditions will eventually cause fatigue of the internal materials and eventually lead to an early trip situation.
At peak power, ~2600 rpm a CIM draws 68 amps and as it speed decreases that current increases to 133 amps at stall assuming a full constant 12v is applied.
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.
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.
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.
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?
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.
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.
I think you could be geared a little fast. If its not too hard to swap, a 50:24 gearkit and some 14T or 13T pinions would fix it, though you lose some top speed. Checkout WCP website for ratio options.
It seems to be true in our case. Switching out the breaker solved almost all our problems. We also copied the poofs and used a upside down can of compressed air to freeze the main breaker before a match.
1: easy: set a maximum current flow lower than 100%
2: more challenging: limit current to CIMs at a gearbox when it is not turning and you’ve been at high power for a bit. This requires distinguishing “starting at rest” from “been pushing for a while”.
3: more challenging: like 2, but turn off other motors/compressor on robot when in pushing situations
4: more challenging: like 2, but implement this as a “current budget” where you estimate current flows and heat dissipation, with the latter decaying over time.
5: easy: like 1, exception implement a driver controlled “pushing mode” on your robot, that lowers maximum current levels
6: more challenging: like 2, but only when driver selects “pushing mode”
Next year’s control system should open up some interesting options in this scenario.