pic: Four Speed Three CIM Gearbox

[Chris is me]

I think you went quite overboard on the weight reduction on the outer plate. A pocketed 1/8" plate with no flanges, especially with the reduction being supported off of the end of the plate past the standoffs like that, will have some issues with flex and rigidity. With gearbox designs posted to CD in general there's often an overemphasis on lightening the plates as much as one possibly can, when it's really not that substantial of a weight savings over a more conservative lightening pattern or even unlightened plate.

[AdamHeard]

Originally Posted by GeeTwo

The reason for stopping at 6 CIMS is that they are capable of efficiently turning every amp you can get through the main breaker into mechanical energy. Once you do that, any additional motors are meaningless.

And I did mean the first team to successfully execute it, not the first to attempt it.

The CIMs aren't very efficient actually, there are more efficient motors in the kit by a good margin (lower power though).

It'd be nice to get a high power, high efficiency motor on FRC legal list.


Also, it's simpler for teams just to add more motors and gearing independently than it is to centralize PTO it all. It just doesn't make sense for FRC.

[waialua359]

Originally Posted by Kevin Leonard

I mean to be fair, there was no reason to have 6 CIMs in the drivetrain this year either. I know of a team on Einstein that was running two motors. I heard it was two Mini's at one point, but I don't recall.

We are one of those teams.
Due to weight constraints, we went from a robot that had 6 motors similar to that of 2014 to a robot with only 1 CIM on each side. We started with a 2 CIM 1 Banebot 775-18 on each side, then got rid of the Banebot for several regionals, then finally 1 CIM on each side at CMPS. We did this in order to make weight as we added 1, then 2 ramps and also to add our upper claw to hold the trash can in place while stacking.
This game was perfect for having just 1 motor on each drive, considering all we did eventually was make stacks from the feeder station.

[hectorcastillo]

Originally Posted by Chris is me

I think you went quite overboard on the weight reduction on the outer plate. A pocketed 1/8" plate with no flanges, especially with the reduction being supported off of the end of the plate past the standoffs like that, will have some issues with flex and rigidity. With gearbox designs posted to CD in general there's often an overemphasis on lightening the plates as much as one possibly can, when it's really not that substantial of a weight savings over a more conservative lightening pattern or even unlightened plate.

Thanks for the feedback. Would you suggest that I cut back on the lightening holes, add some flanges, and/or thicken the plate?

[GeeTwo]

Originally Posted by AdamHeard

The CIMs aren't very efficient actually, there are more efficient motors in the kit by a good margin (lower power though).

All of the motors that are significantly more efficient than the CIM have their peak efficiency at a much higher speed (11-17k rpm vs 4500 rpm). This means that another level of gearing will be required to reduce these high-speed inputs down to a mechanically useful speed, offsetting most of the increase in motor efficiency. And you'd need more motor controllers, too. At least you could use a single encoder to know the speed of your power train.

Originally Posted by AdamHeard

Also, it's simpler for teams just to add more motors and gearing independently than it is to centralize PTO it all. It just doesn't make sense for FRC.

No argument. Everything that is currently an independent motor would have to be replaced with a clutch, CVT, or similar mechanism, which brings us back to Kevin's original suggestion to use the shifters on a PTO rather than to make a 4 speed single function transmission.

I often find that at least thinking about (and sometimes even building) the extreme cases leads to ideas that prove useful later. IIRC, the OP tossed this up as a bit of a "blue sky" thread, and I've been treating it as such. Checking back, OP included "probably weighs a thousand pounds" and "I do not actually intend to build this thing." As a result of this thread, I now know to keep an eye out for high power jobs that don't require simultaneous wheel drive. 2013 and 2015 both provided this sort of challenge (pyramid climb and can burglar respectively), so it can't be that rare. It wasn't in my mental toolbox six months ago.

Another thing that makes me really like the PTO concept is the addition of a flywheel. It is possible to tap more power from a PTO with a flywheel for a short period than from motors, especially given the new motor brownout situation. I like having a defined brownout over an undefined one, but it will probably be several years before the FRC community learns to take the best advantage of the new opportunity/challenge. At which time we'll switch to yet another control system ;-P>

Thinking a bit more, 2014 (Aerial Assault) also provided a PTO possibility with the ball thrower. We had six CIMs on our 2014 robot, two for a "kicker" and four for drive. (We used pneumatics and an AM gearmotor for the ball pickup.) We nearly always threw the ball while stationary, so sharing all six CIMs between drive and a more elegant and powerful launcher might have been an improvement on both fronts, especially as most of our "accelerator" designs would have thrown the ball from higher off the carpet than the kicker did.

[The other Gabe]

Originally Posted by Knufire

https://www.youtube.com/watch?v=h3l6Xqts4to

Why did they do that? was it just like, one of the kids said in a meeting, as a joke, "you know what would be cool?" and everyone just kinda went along with it.

also on the subject of small motors on drivetrains, I know that 2471 was running their Swerve on a mini-CIM for each wheel by DCMP in order to have more weight for a better can claw & some can burglars, with no discernible difference in performance from when the had a CIM per wheel.

[EricH]

Originally Posted by The other Gabe

Why did they do that? was it just like, one of the kids said in a meeting, as a joke, "you know what would be cool?" and everyone just kinda went along with it.

Nope. 2007 was PHYSICAL. Not only was there ramp climbing (up all random sizes and slopes of ramp), but there were 5 other robots on the field, all trying to score or block scoring in the same general area. This "area", the Rack, was a structure that could be slid and spun around, took up a fairly large chunk of real estate in the center of an otherwise flat field...

... and needed some degree of precision to place scoring objects (inner tubes) onto its scoring pegs.

All I'll say as far as it being physical is: Aerial Assist was rough. Now extend and beef up the appendages another couple of feet or so. And "meet in the middle" autonomously...at speed...or under driver control...while trying to score...or block scoring...


118 had a pretty decent swerve the previous year, but for this one they added a pair of F-P motors to a 4-CIM, ran all the wheels off the same gearbox and the same turning motor, and had a turret on top to allow for scoring in any orientation they felt like being in.

Basically, lots of push and lots of maneuverability were judged "pretty important" that year. (Though... IIRC, Einstein was a bunch of 6WD skid-steers that year, with maybe a couple of non-tank drives around.)

[GeeTwo]

I'm still wondering what was up with that central gearbox that was later distributed. It appeared to be an essentially 1:1 gearbox which brought four CIMs together onto a single shaft, which was then redistributed around on four separate swerve shafts. It seems to me that it would have been easier and more efficient to put a CIM on each swerve shaft, but to include a sprocket which tied the four shafts to the same speed to allow redistribution of energy as needed (e.g. as loading increased on two wheels due to acceleration away from them).

[AdamHeard]

Originally Posted by GeeTwo

I'm still wondering what was up with that central gearbox that was later distributed. It appeared to be an essentially 1:1 gearbox which brought four CIMs together onto a single shaft, which was then redistributed around on four separate swerve shafts. It seems to me that it would have been easier and more efficient to put a CIM on each swerve shaft, but to include a sprocket which tied the four shafts to the same speed to allow redistribution of energy as needed (e.g. as loading increased on two wheels due to acceleration away from them).

They combined it all into a single shifting gearbox (reusing a Dewalt drill planetary). They also added 2 FP motors, so distributing it would've been harder.

[ratdude747]

Originally Posted by GeeTwo

I'm still wondering what was up with that central gearbox that was later distributed. It appeared to be an essentially 1:1 gearbox which brought four CIMs together onto a single shaft, which was then redistributed around on four separate swerve shafts. It seems to me that it would have been easier and more efficient to put a CIM on each swerve shaft, but to include a sprocket which tied the four shafts to the same speed to allow redistribution of energy as needed (e.g. as loading increased on two wheels due to acceleration away from them).

Traction. By combining the motors onto one shaft, in addition to allowing for a single dewalt shifter, it also allowed any one wheel to have all the motors power available. Thus, even if a wheel lost traction, the full pushing power would still make it to the ground through the others.

Another good example is 148 in 2008... they did 2 FP plus 4 Cim, going to three swerve pods, which were steered with a van door motor. Since they had no "front" (it was an equilateral nonagon lap runner), they had no need to "steer". As a result, they could both run laps well AND provide very good defense (and this was one of many reasons why their CMP alliance were champions).

[GeeTwo]

Originally Posted by AdamHeard

They combined it all into a single shifting gearbox (reusing a Dewalt drill planetary). They also added 2 FP motors, so distributing it would've been harder.

Originally Posted by ratdude747

Traction. By combining the motors onto one shaft, in addition to allowing for a single dewalt shifter, it also allowed any one wheel to have all the motors power available. Thus, even if a wheel lost traction, the full pushing power would still make it to the ground through the others.

I didn't notice the shifter. With that bit of info, the central gear box makes perfect sense. While separate CIMs which are chained to keep the same speed would have also allowed the power generated at one wheel to be used at another, four (or six) shifters would be more complex (and costly and heavy) than the "gear manifold" and a single shifter.