Experience With 6 Cim Drive train

[jeremylee]

Great! 6 cims driving 3 cim ballshifters geared 12ft/sec and 5.5 ft/sec. Kit drivetrain upgraded to 8wd long. Traction limited in high, estimate of around 200 amps spinning in high using drivetrain calculator on WCP website. Played tough D in high a lot at central il regional, no trips. No trips at Wisconsin either. Standard wiring sizes, noticed some heating in wiring and battery, no failures, no major concerns. 6 cim acceleration made for some great defensive capability.

We will seriously consider 6 cim single speed next year at 12 ft/sec unless game dictates otherwise.

Heres a short video showing some highlights from cental IL for those interested:
http://m.youtube.com/watch?v=IQqoqUNYt_c

Edit: To clarify, we almost never used low gear all year.

[Dunngeon]

CV Robotics has used single stage 6-CIM gearboxes for the last two seasons. Our experience has been great with this drive type.

While our boxes are geared for 17.5 FPS theoretical, our actual speed is ~16 FPS. We really like the increased acceleration we get, even at a higher speed of 17.5.

Electrically, we popped the thermal breaker three times this year.... twice it happened immediately after we had been in a prolonged pushing match (8-12 seconds) which is a bad idea in any case.
The other time was a fluke incident, one of the nuts on the PDB came loose and it induced a huge resistance into the system, nearly melting the plastic on the standard size power leads.

Other than that, our system remains relatively cool throughout operation with the exception of the breaker and Rear CIM plates. We have no dedicated cooling, and this year we did nothing to cool the robot between matches. However, next year we are planning on introducing some protocol to cool our main breaker and CIM's which get to 46 C/114 F occasionally.

We didn't run an on-board compressor last year, and all of our other motors were used intermittently. This gave us some extra time before the breaker thermally tripped because of reduced current demand.

Overall, we consider our 6-CIM drive to have been integral to our success the last two years. Our plan is to continue with 6-CIM unless it isn't useful for the game. Ie. If the increased acceleration/torque isn't useful next year

We recommend it

[BBray_T1296]

Originally Posted by Joe G.

• Coded our drivetrain to automatically drop to 4 CIMs when running our shooter winch or intake motors, with the driver given a trigger to override this in high-pressure situations

Does this mean you were running (at least one pair of motors) in coast mode?

I suppose there probably isn't a huge difference between 4 and 6 brakes, so it doesn't matter I guess

[Gregor]

Originally Posted by Joe G.

That wasn't an option for us because our gear box was constructed single stage and already pushing geometric limits for that configuration, our robot was not set up to accept a COTS single speed gearbox, and manufacturing our own wasn't an option for the same reasons as for shifters. That's the quick solution though!

I'm sure you already considered them, but in the event you didn't know about them, did you look at the 11 (or 13 tooth if you were running 14 tooth CIM pinion gears) tooth gears to drop in to replace the 12 tooth pinion gears?

[Juan Martinez]

Originally Posted by inkling16

Is this the gearbox you used? If so, how did you achieve a 1.5x spread?

My bad I meant 18 and 8, I was thinking about 2 different robots

[Joe G.]

Originally Posted by BBray_T1296

Does this mean you were running (at least one pair of motors) in coast mode?

I suppose there probably isn't a huge difference between 4 and 6 brakes, so it doesn't matter I guess

Yes, two motors were on coast.

I'm sure you already considered them, but in the event you didn't know about them, did you look at the 11 (or 13 tooth if you were running 14 tooth CIM pinion gears) tooth gears to drop in to replace the 12 tooth pinion gears?

We were running 11 tooth pinions at each of our events.

[Rob Stehlik]

There are some interesting ideas here for dealing with main breaker trips.

I think the best solution is to gear more conservatively and enjoy the extra acceleration. For short distances, slower is faster. The last two years we have run a 6 CIM single speed drive geared for around 11-12 fps, and have never tripped the main breaker. We have used a spreadsheet to help optimize our gear ratio, and this speed seems to be a sweet spot for single speed drives. It provides great acceleration, and the current demands are low enough not to trip the main breaker. Most of the teams I have talked to about breaker problems were geared higher than 12 fps.

[DarkRune]

4159 used an amazing 6 CIM. Low of 7, high of around 17. The motors got hot during violent matches, and we tripped the breaker when we tried to push in high gear. Our low gear had an amazing amount of torque, we were able to 2-3 robots at a time no problem.

[who716]

Team 716 used our first ever six cim drive and it was absolutely awesome. I couldn't ask for anything better, As the driver I didn't really see the need for the two speed so we only used a one speed, which ended up being about 15 fps. (we decided to use only the single because this game was based strictly on time so you did not want to get into a pushing match but instead wanted to zip around everyone.) It turned out though that we could still push 80% of the robots that we went against. in regards to your question, I would assume you would want to avoid an extended pushing match because it wouldn't be worth the time lost.

[Chris is me]

6 CIM drive, single speed geared for 13 FPS actual (measured) speed. 6.11:1 on 4" Colsons, if you want to do the math yourself. Treaded them for regional 2, plain for regional 1.

We had no problems tripping our breaker all season, however we did design our entire robot around preserving current. We modeled how much air our pistons might use and went with 4 plastic air tanks in order to leave the compressor offboard. Most teams only leave the compressor offboard if they have weight problems, but I think current draw is a much more important reason to consider leaving it off. To keep current draw low we used relatively few motors across the rest of the robot. Other than the drive we had just one BAG motor for the intake, one mini-CIM to drive the arm, which we partially balanced, and one mini-CIM to retract the winch, which we geared down significantly to reduce the load on the motor.

We never did take off two motors to compare the performance of a 4 vs 6 CIM drive, so all of these opinions are pretty worthless... but purely subjectively, the 6 CIM drive seemed to accelerate faster and felt more responsive than a 4 CIM drive. Interestingly, with regard to the 40 amp breaker, we found that when the 6 CIMs stalled against a heavy load, the 40 amp breakers tripped less quickly than with a 4 CIM drive. Obviously part of this is because the load is distributed across more motors, but the drive was fully stalled so each motor should have been at its max current draw. What we later realized is that with more motors, the voltage drop was greater, resulting in the motors drawing less current at stall (and also having a lower stall torque at the lower voltage). Thus, the 40 amp breakers are less of a constraint in our particular configuration with 6 CIMs than they are with 4.

[Oblarg]

Originally Posted by Chris is me

We modeled how much air our pistons might use and went with 4 plastic air tanks in order to leave the compressor offboard. Most teams only leave the compressor offboard if they have weight problems, but I think current draw is a much more important reason to consider leaving it off.

This is consistent with my own observations. We had the same gearing/wheel size this year (HiGrip wheels instead of Colsons, though), and had no problems with the breaker except when the compressor was running.

Interestingly, we found that the compressor (and, by extension, any other power-consuming device on the robot) caused a twofold problem: not only does it draw current, but it lowers the effective voltage of the battery which can result in stalling a drive that would otherwise be traction-limited. This is crucially important, since if you're going to last in an extended pushing match you absolutely must be traction-limited.

Re: having less problems with the 40A drive breakers, we experienced this, too. Unfortunately, we found that unless you're exceedingly careful with current draw, at aggressive gearing going from 4 to 6 CIM SS basically results in swapping problems with the 40A breakers for problems with the main breaker. We plan to do a two-speed drive next season.

Originally Posted by Rob Stehlik

There are some interesting ideas here for dealing with main breaker trips.

I think the best solution is to gear more conservatively and enjoy the extra acceleration. For short distances, slower is faster. The last two years we have run a 6 CIM single speed drive geared for around 11-12 fps, and have never tripped the main breaker. We have used a spreadsheet to help optimize our gear ratio, and this speed seems to be a sweet spot for single speed drives. It provides great acceleration, and the current demands are low enough not to trip the main breaker. Most of the teams I have talked to about breaker problems were geared higher than 12 fps.

I've always read about how y'all prefer to gear more conservatively with your 6 CIM drives to abuse the acceleration boost, yet whenever I play around with acceleration and time to distance calculators I find that the extra motors become decreasingly more useful the closer you get to the "sweet spot" you have determined. At higher speeds - regardless of target distance - my calculators show that the extra motors can save a tenth of a second or so (nothing significant), and at lower speeds (especially around your sweet spot) the calculator shows improvements of a few hundredths of a second at best. Could you go into detail as to how you determine how much time you save with the extra motors to travel a set distance, and what tools you use? I feel I'm doing something wrong with the calculators I have due to the disagreements my data is showing compared to yours, but I cannot figure out what it is.

[Thad House]

We were geared for about 10 fps with 3 CIMs throughout competition season. It was great up until we were facing alliances with high speed bots, and could not get passed them. So the sweet spot is a good sweet spot, until you are facing teams which are faster and can get in front of you faster then you can move. That is why we geared up to 15 fps for IRI, which was probably too fast for a single speed bot, but the increased speed really was needed to be competitive at a high level of play. In other years, that 10-12 sweet spot would be great, but this year with the wide open field it just was not fast enough.

[Gregor]

Originally Posted by Thad House

We were geared for about 10 fps with 3 CIMs throughout competition season. It was great up until we were facing alliances with high speed bots, and could not get passed them. So the sweet spot is a good sweet spot, until you are facing teams which are faster and can get in front of you faster then you can move. That is why we geared up to 15 fps for IRI, which was probably too fast for a single speed bot, but the increased speed really was needed to be competitive at a high level of play. In other years, that 10-12 sweet spot would be great, but this year with the wide open field it just was not fast enough.

I seem to recall you popping your main breaker in one of your elimination matches. Was it a main breaker blowing? Did you track down the cause of that?

[Thad House]

Originally Posted by Gregor

I seem to recall you popping your main breaker in one of your elimination matches. Was it a main breaker blowing? Did you track down the cause of that?

We actually popped it in all 3 matches. We are pretty sure the first time was because our driver started pushing for too long, and that gearing does not do well pushing. After that, I think it had worn our breaker, because the 2 matches after that the breaker blew pretty early in the match. So we learned 2 lessons. If you pop the breaker, replace it immediately. And if you want to gear that high, it would be a good idea to have shifters. We are looking at maybe running shifters this fall, swiching between 10 fps in the sweet spot, where we never blew once, and 15+ fps for the sprints when we need to.