Experience With 6 Cim Drive train

[jam2014]

I am looking for some real world feedback on teams experiences using a 6 cim drive train with a 2 speed transmission designed for both low speed (4-5 ft/sec) and high speed (say 14ft/sec).

If your robot got in an extended pushing / blocking match with other 6 cim robots for a good part of a match...

- Would you be at risk of blowing the breaker ? If so, how often did you blow your breaker last year. Did you replace the breaker one or more times last year. Have you implemented any software protection (reduce power after X seconds) or hardware (fan) to defend against that ?

- how warm or hot would the following be by the end of the match
- the battery
- the terminals on the battery
- the contacts of the battery connector
- the breaker, its terminals etc

- do you guys do anything to cool the cims (fans, aerosol can cooling for example)

I have heard of a team that replaced all the 14 gauge cim wires with 10g wire. Did you guys do anything like that ?

If the cim motors are stalled in a pushing match (and apparently drawing close to 100amps each), does anyone know for certain if the 40amp breakers trip at all (and reset seconds later) reducing some of the potentially 600 amp+ current off the main breaker. Maybe not if the 40 amp CIM breaker is overloaded 2.5 times (100amps / 40amps) versus 4.8 times for the main breaker (600/125).

What was your teams experience ? Thanks

[Arpan]

Hiya. 3061 used a 6-cim last year with A high gear of 21 fps, and a low of 7 (This is nuts don't do it)

We had no problems whatsoever with the battery or connecting wires getting hot. That said, we killed the main breaker whenever we hit anything in high gear for too long.

We used compressed gasduster to cool cims during eliminations.

[BBray_T1296]

Our robot used 6 Cims this year. We used VexPro Ball shifters with total reduction being 5.5fps and 15.5fps. (<- not sure if accounting for inefficiencies). Our Drivetrain setup was a 6wd tank drive with 3" Colson wheels and non-cantilevered axles. It is worth noting our center of gravity was within 5" of the floor.

We played extensive defense all year. We tripped the breaker one single time all year, and that was playing at Texas Robot Roundup after 2 regionals, and a prior off-season event. During the season, we had absolutely no breaker problems.

I will try to dig up some video, if you would like.

For your questions.
-The battery was a few degrees above room temperature, the plastic casing was no hotter that a battery just off the charger. (When swapping robot batteries there was no thermal way of telling them apart from the casing)
-Our battery terminals are heat-shrinked and taped up so the metal itself is untouchable. I will say the whole lead assembly from the battery to the PDB was significantly warm, but you could comfortably leave your hand on them.
-We have never had any issues related to the battery connector, and, like I just said, these were also warm but not concerning.

We did not have any form of cooling; no sinks, no fans. After a match the back of the motor (where the brushes are) was too hot to touch for more than a second, but the rest was ok. (By the next match the heat had mostly dissipated). I think we will seriously consider some form of thermal dissipation in the future. We may put one of the large computer fans on the bunch of 3, or make our own heat sink. We do not find the heat sink the KOP had particularly useful, because like I said, it is the back of the motor that needs cooling, not the main casing. While those would help, the heat does not propagate that far to make them useful in my experienced opinion.

We did practice our robot for about 2 hours straight, with nothing but some 30 second battery changes every 5 minutes. By the end, you could easily boil water with those things, they were roasting.

[cbale2000]

We used 6-CIM ball shifters (with 2 CIMS and 1 MiniCIM each) on our robot this year. We had no problems with breakers when driving in low gear. There was one occasion that the main breaker was tripped during a hard collision in high gear that could have been attributed to a current spike (or just the force of the collision, we're not sure), but beyond that we had no issues that I'm aware of.

I'm hoping the current monitoring capabilities of the new PD board next year will make it really easy to manage current draw and maybe even allow us to program automatic down-shifting when current draw gets too high.

[Deke]

We had 6 cim single speed at 11 fps and were traction limited with versa wheels. Never tripped any breaker.

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 (if the goal is preventing a breaker from tripping). However, if that slow speed is part of the design strategy of the game, that is a different story.

Those breakers can take some heat, let them get a little toasty, especially since FRC is held during the winter.

[Kevin Leonard]

20 used a shifting 4-CIM, 2-MiniCIM drive geared for about 6 and 16 ft/s in 2014.
We loved it's performance for most of the season.
The only issues we had were when our driver forgot to switch into low gear when we were playing defense in our second qualification match at our first regional. We ended up tripping our breaker that match.
The gearboxes themselves took a lot of damage as well throughout the season, but that was because our driver drove the robot like he stole it.

[Michael Hill]

I'm really excited for the new current sensing ability we'll have next year. It will let us automatically shift of we see too high of current.

[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.

[Deke]

Originally Posted by BBray_T1296

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.

So then the other positive is pushing correct?

There are a lot of factors involved but just to be simple, the system doesn't need to be designed to 40 amps per motor to push someone. That's why I said the low gears are too low. They are over designed for the 150 second matches to push people around, and too slow to be useful in normal driving.

I would say you can comfortably design them around 60-65 amps per motor to push, based on what we saw our drivetrain do this past year. I would like to try 70-75 amps per motor this year and see what the breakers can take, should be fun.

[z_beeblebrox]

Readers of this thread might be interested in this one from 2013.

[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!