Main breaker tripping, dead CIM

[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?

[KevinG]

Originally Posted by Al Skierkiewicz

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.

How will reducing resistance increase current draw?

[Alan Anderson]

Originally Posted by KGenson

How will reducing resistance increase current draw?

Ohm's Law.

[Chris is me]

Here's some quick data points, since this is turning into a "talk about your 6 CIM setup and your issues with the main breaker" thread - 4" wheels, 6 CIM singles, 6.1:1 gear ratio. Never tripped the breaker, playing heavy defense and whatnot. We did not use an on board compressor and we worked to minimize all other motor load to keep overall current draw low. Since this is the same ratio / setup as the OP, I would assert that they are either running a lower efficiency drive than us, a CIM is defective, or the rest of their robot draws too much current.

Something interesting we noted with 6 CIMs this year. We noticed a significant advantage in acceleration, but more notably we noticed that our actual top speed was higher than we expected. Everyone is aware of the 81% speed loss constant for a 4 CIM drive, but our 6 CIM drive's actual speed loss coefficient was around 90-91%. We measured an actual robot speed of 13.7 FPS with 4" wheels and a 6.1:1 gear ratio - 81% speed loss puts us in the upper 12 FPS range. It's possible that more motors actually results in less top speed loss.

[Thad House]

We are running 6 CIM's on our drive this year. It was one of the best decisions we have made this year. We have played two districts this year, and have never popped the main breaker once. We are running 4" wheels with a 7.4:1 ratio.

One thing I've noticed is that many teams were gearing high in the past with just 4 CIM's. With 4 CIM's its harder to pop the main breaker. As soon as you bump that up to 6 CIM's, if you run the same gearing that wouldn't pop the breaker with 4 will pop for sure with 6. 6 CIM's really does need to be geared lower then 4 in order for this not to happen. And in essense, because of the MUCH faster acceleration, the time it takes to move across the field doesnt increase much as you lower the ratio and add 2 more CIM's, but the time it takes to move 1/4 of the field speeds up alot.

[Oblarg]

Originally Posted by Chris is me

Never tripped the breaker, playing heavy defense and whatnot. We did not use an on board compressor and we worked to minimize all other motor load to keep overall current draw low. Since this is the same ratio / setup as the OP, I would assert that they are either running a lower efficiency drive than us, a CIM is defective, or the rest of their robot draws too much current.

We were running a compressor for all of Chesapeake, which certainly would increase our current draw by a lot. We do not really wish to go back to not having a compressor, though - we were that way at DC because our ball-acquisition was non-functional.

Our drive efficiency is, as far as I can tell, quite good; there's not too much friction in the KOP belt setup. Our wires are all way longer than they should be, though, and it's not particularly feasible to fix the wiring at this point in time.

[Ether]

Originally Posted by Chris is me

Something interesting we noted with 6 CIMs this year. We noticed a significant advantage in acceleration, but more notably we noticed that our actual top speed was higher than we expected. Everyone is aware of the 81% speed loss constant for a 4 CIM drive, but our 6 CIM drive's actual speed loss coefficient was around 90-91%. We measured an actual robot speed of 13.7 FPS with 4" wheels and a 6.1:1 gear ratio... It's possible that more motors actually results in less top speed loss.

There is no reason to expect the "speed loss constant" to be the same for all drivetrains and floor surfaces, as it is substantially affected by the floor surface and the type of drivetrain design (chain and sprocket, belt and pulley, direct drive, type of gearbox, type of wheels, etc) and workmanship (chain or belt tension, proper assembly and lubrication of gearbox, wheel alignment including toe-in and camber¹, wheel axial offset², etc).

¹Toe-in and/or camber of a wheel causes the wheel sprocket (or pulley) to be non-coplanar with the driving sprocket (or pulley), and thus contributes to friction between the chain (or belt) and the sprocket (or pulley). Toe-in also causes scrubbing friction with the floor surface.

²"wheel axial offset" in this context means that the wheel sprocket (or pulley) is axially offset from the plane of the driving sprocket (or pulley), causing the chain (or belt) to be non-planar, thus creating additional friction between the chain (or belt) and the sprockets (or pulleys).

[Kevin Selavko]

Its amazing to me how little the compressor demands. Last year we would drop a few volts after a match with our shooter only running for a short period of time, but this year the compressor is almost constantly running and we only drop a few tenths of a volt through the match.

[DampRobot]

Originally Posted by Al Skierkiewicz

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.

While you're right that tripping the breaker once won't make it unusable (IE, you can still turn the robot on with a breaker that has tripped), it has been observed by my team, team 254, 67, 971, 1678 and many many others that breakers that have tripped before trip at much lower currents the second time around. The reason people are telling saying to throw away after they have tripped once is because they are much easier to trip again, not because they are literally unusable. If you trip a breaker once, it will trip at a lower amperage from then on. We've also observed that different breakers trip at different current levels, but our evidence for this is only really anecdotal.

[AllenGregoryIV]

We ran Lone Star with 6 CIMs and 2 MiniCIMs on our drive plus the old Thomas Compressor running on our robot. We were on 4 omni wheels most of the time so we have very little scrub. We have a single gear 12:72 reduction when in that configuration. When we shift to traction we are on 4" traction wheels with 2" wide rough top tread and a 18:42 belt reduction off the omni-wheel shaft. We got into several pushing matches and never popped the main breaker.

[Al Skierkiewicz]

Roger,
That is anecdotal. There is nothing that happens to a breaker that trips for over current that doesn't happen during normal operation. It is possible to produce some pitting of the contacts during a high current trip but that is not likely significant unless repeated trips occur. A warm breaker will trip at lower currents whether it is new or not. That is why I encourage teams to take a full cool down period between finals matches. Warm wires, warm robot frame, motors near the breakers, etc. all will add to this phenomena.

Kevin, do you understand the Ohm's Law reference above? If not, just ask in a PM and I will explain it further if you would like.

[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.