Experience With 6 Cim Drive train

[BBray_T1296]

Originally Posted by Infinity2718

Just my opinion, but for the short duration of FRC matches, the low gears on 3 CIM gearboxes have way too high of reduction for robot movement

I certainly disagree with that. I find shifting to a low gear (<7fps) to be a very valuable asset, in more situations than just precise movement.

While our 5fps was definitely too slow to play the whole match in, there were numerous times (especially in the defense heavy game) that you just needed to knuckle down and bulldoze your way through a pileup. Our robot used 3" Colson wheels, had the bumpers at the lowest point, and had a center of gravity roughly 5" from the floor. We could push nearly everyone, and many tank robots saw themselves skidding sideways by us. We did not have a traction problem at that torque either, and our used wheels had little measurable diameter difference to new ones, to the naked eye.

Also, on the topic of generic shifting; Being able to change gears gives you the ability to have a high upper stage (like 16+). Many people had 18fps upper stages or more. If you had a single speed gearbox, I think most people would agree (game dependent, but for 2014 in this) that having only a 18fps gear would not have been a good idea. But certainly most people who had 18fps high gear shifters would agree that it was a great gear knowing you could summon the torque when you needed it.

[Juan Martinez]

This past year we had a 4 cim ,2 minicim vexpro ballshifter west coast drive and we were geared 18 high and 12 low fps we had absolutely no problems with popping breakers. We also had no problems with pushing in high gear although our engineers warned against. Our center of gravity was within 5 inches of the floor . The only thing that reached above 18 inches on our robot was the superlight floor intake and catapult in a nonshot position.

[Caleb Sykes]

Originally Posted by Juan Martinez

This past year we had a 4 cim ,2 minicim vexpro ballshifter west coast drive and we were geared 18 high and 12 low fps

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

[Joe G.]

1687 ran 6 CIMs single speed last year, at 14.5 fps, riding on colsons with a linear tread pattern, playing some real tough defense all year long. At our first event, we tripped the breaker once in our final match, at our second, a couple more times. After that, we looked for ways to eliminate breaker trips for our offseason events. Retrofit shifters were designed, but for various reasons, couldn't be manufactured in time. Instead, we made several minor mechanical, electrical, and software tweaks to reduce current draw:

• Doubled the number of air tanks on the robot, reducing required compressor runtime
• Systematically tested every pressure switch we own to find the one with the best combination of high cutoff pressure and low startup pressure, again reducing required compresser runtime
• Switched our intake extension, our largest source of air use, to single acting pistons
• Upped wire gauge in several places in the robot, reducing losses due to wire resistance
• 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
• Added a trigger for our driver to shut off the compressor regardless of pressure during pushing matches
• Added ramping to drivetrain which limited sudden accelerations and abrupt direction changes slightly

Did not trip the breaker after this. However, we do plan to use shifters in the future, or gear lower if the game does not call for the speeds we geared for this season.

[Oblarg]

Originally Posted by Joe G.

1687 ran 6 CIMs single speed last year, at 14.5 fps, riding on colsons with a linear tread pattern, playing some real tough defense all year long. At our first event, we tripped the breaker once in our final match, at our second, a couple more times. After that, we looked for ways to eliminate breaker trips for our offseason events. Retrofit shifters were designed, but for various reasons, couldn't be manufactured in time. Instead, we made several minor mechanical, electrical, and software tweaks to reduce current draw:

• Doubled the number of air tanks on the robot, reducing required compressor runtime
• Systematically tested every pressure switch we own to find the one with the best combination of high cutoff pressure and low startup pressure, again reducing required compresser runtime
• Switched our intake extension, our largest source of air use, to single acting pistons
• Upped wire gauge in several places in the robot, reducing losses due to wire resistance
• 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
• Added a trigger for our driver to shut off the compressor regardless of pressure during pushing matches
• Added ramping to drivetrain which limited sudden accelerations and abrupt direction changes slightly

Did not trip the breaker after this. However, we do plan to use shifters in the future, or gear lower if the game does not call for the speeds we geared for this season.

Had essentially this exact same experience with a 6CIM SS geared for that speed. Instead of the measures described here, though, we simply changed our gearing to ~11fps. No more breaker problems after that.

[Joe G.]

Originally Posted by Oblarg

Had essentially this exact same experience with a 6CIM SS geared for that speed. Instead of the measures described here, though, we simply changed our gearing to ~11fps. No more breaker problems after that.

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!

[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?

[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.

[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

[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.

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.