pic: Did Somebody Say Defense Game?

[wilsonmw04]

How long are you going to have in a pushing match with a 6 cim drive before you start popping breakers?

[Deke]

Love it!

[JohnFogarty]

This makes me glad we chose 6CIM drive.

[cbale2000]

Originally Posted by Chris is me

I don't see this as having much higher turning scrub than a six wheel drive. In theory, this system would behave like a tread drive and this would turn even without drop. I guess we shall see!

Our wheels are set at a 32 foot arc, which allows the drive to act, more or less, like a 6 or 8 wheel drive at any given time. What we've found over the years, however, is that during a pushing match with a typical 6WD robot, the forces of the two robots can actually push 6WD robots onto their back set of wheels and lift their front and middle ones up (even just a little), thus reducing their traction and making them effectively 2WD robots. Having extra wheels on an arc helps ensure constant contact with the floor during all conditions.

Originally Posted by Garrett.d.w

...If I were holding the joysticks in this case :

That pretty much sums up what I'm expecting.

Originally Posted by Duncan Macdonald

Is this a similar design to what your team ran in 2008? If so, what reliability changes were made?
2008 robot
Context (Still one of the classiest decisions in FIRST history)

2008 was an interesting case, we had a situation where we had a custom gearbox, that had a 3rd stage consisting of an 8 tooth gear that undercut a 50+ tooth cast iron gear, which was custom made (with a 3 week lead time to get more) and we had forgotten spares (and we were in Canada). This years robot, by comparison, uses standard VEXPro 3 CIM Ball Shifters, with parts we can get easily, and if necessary pick up from The Robot Space (one of the new VEXPro partners), less than an hour away.

The drive system itself is also much closer to our 2007 bot due to the use of gears to drive the wheels, rather than chain, and a very similar gearbox configuration to the drive we had that year. With the exception of a bearing failure in the finals of West Michigan (which we went on to win anyways), there was virtually no reliability issues with the 2007 drive system.

Originally Posted by Animal Control

Yes. Can you sent a video with it running

Soon, I'm hoping we have it driving next week.

Originally Posted by magnets

Adding surface area in contact with the ground (up to a point) increases traction. It doesn't matter if it's through wider wheels, or more wheels. If friction worked just the way the coefficient of friction says it will, then neither width nor number of wheels will affect traction at all. You could have three 0.1" thick wheels, and have the same amount traction, but friction doesn't follow the super simple columbic prediction, especially on carpet.

Exactly, our wheels this year are also wider than we've used before (1.5in wide this year, compared to 1in past years), so we're hoping they will provide us with a bit more traction.

Originally Posted by wilsonmw04

How long are you going to have in a pushing match with a 6 cim drive before you start popping breakers?

We had no issues with the individual 40A breakers on our 2007 robot (the predecessor to this design), now, whether or not the 2 additional MiniCIMs cause the main breaker to pop is another question. We're using such a low low-gear that we should loose traction before stalling the motors though (at least, this has been the case on past machines).


Apologies for the wall of text, just figured I'd answer all the questions in one go.

[Christopher149]

Their 2007 'bot for reference.

[BBray_T1296]

Originally Posted by wilsonmw04

How long are you going to have in a pushing match with a 6 cim drive before you start popping breakers?

well, you should be fine operating at 39amps per motor for a while
And if you are worried about the 234 amps on the main breaker, allow me to show you the current graph as per the manufacturer



As you can see, you can draw 200% (240 amps) for 10 seconds minimum, as per the specifications

EDIT: link fixed?

[wilsonmw04]

Originally Posted by BBray_T1296

well, you should be fine operating at 39amps per motor for a while
And if you are worried about the 234 amps on the main breaker, allow me to show you the current graph as per the manufacturer



As you can see, you can draw 200% (240 amps) for 10 seconds minimum, as per the specifications

link is broken, please try again. I'm worried about the snap fuses. I've seen what a resetting snap fuse does to a robots drive train. It isn't pretty.

[nathannfm]

Any reason you chose to assemble the 2 gearboxes identically instead of mirror images (which would have made the drive symmetrical)?

[cbale2000]

Originally Posted by nathannfm

Any reason you chose to assemble the 2 gearboxes identically instead of mirror images (which would have made the drive symmetrical)?

No particular reason, its just how they were assembled. In the past we would use identically assembled gearboxes (or driving one gearbox to different wheel) for maintenance reasons, since pulling a gearbox with a long hex shaft output off is difficult when the boxes are right up against each other. This year the gearboxes are so huge though, it really doesn't make a difference.

We have to pull the panels apart when we install the idler gears anyways, so we may yet change the configuration of the one gearbox, especially since it's not hard to switch.

[BBray_T1296]

Originally Posted by wilsonmw04

link is broken, please try again. I'm worried about the snap fuses. I've seen what a resetting snap fuse does to a robots drive train. It isn't pretty.

Really? I can see it just fine.
Anyways, that pic is a screenshot from this pdf, page 34 you will find our switch.

So long as you do stay within the 40amps per motor, you should not need to worry too much about the main

[wilsonmw04]

Originally Posted by BBray_T1296

Really? I can see it just fine.
Anyways, that pic is a screenshot from this pdf, page 34 you will find our switch.

So long as you do stay within the 40amps per motor, you should not need to worry too much about the main

yep, still broken for me. the 40 amp resetting fuses are what scare me. I have never seen a main blow, but the quick resetting 40's popping = a bad day with robots.

[cbale2000]

Originally Posted by wilsonmw04

yep, still broken for me. the 40 amp resetting fuses are what scare me. I have never seen a main blow, but the quick resetting 40's popping = a bad day with robots.

Not saying it can't happen, but in my 8 years on my team I've never seen us trip a 40 amp breaker.

Correct me if I'm wrong here, but doesn't adding the 3rd motor to the gearbox take some of the load off of the other two, thus making a breaker reset less likely?

Originally Posted by cbale2000

Not saying it can't happen, but in my 8 years on my team I've never seen us trip a 40 amp breaker.

Correct me if I'm wrong here, but doesn't adding the 3rd motor to the gearbox take some of the load off of the other two, thus making a breaker reset less likely?

In normal driving, it does take a bit of load off the other motors, but in a pushing match where all motors are giving maximum output you have a larger chance of tripping the breaker with a 6 motor drive than a 4 motor drive. The solution is to just not get into heavy and sustained pushing matches.

[Chris is me]

Originally Posted by Andrew Lawrence

In normal driving, it does take a bit of load off the other motors, but in a pushing match where all motors are giving maximum output you have a larger chance of tripping the breaker with a 6 motor drive than a 4 motor drive. The solution is to just not get into heavy and sustained pushing matches.

This is not a universal, blanket statement. It's less correct than the statement you were referring to actually, which is pretty much always true. With a 6 motor drive, all other things equal, each individual motor will draw less current than each individual motor in a 4 motor system. This makes it harder to trip the 40 amp resetting breakers, which tend to trip first in a 4 motor drive. The tradeoff is that, at certain gear ratios, you are more likely to trip the 120A breaker. The most vulnerable edge case is an extended pushing match when geared above ~10 feet per second. A low gear for a 6 motor 2 speed drive can certainly be geared low enough to prevent this from basically ever happening in a 2 minute long match.

This tradeoff is something teams should carefully analyze and play with when deciding to use a 6 motor drive. Study the breaker spec sheets, both 40A and 120A. Experiment with different motors or robot cooling systems. Run a shifter and push in low gear. There are a lot of ways to try and mitigate this potential pitfall.

(By the way, consider running a low gear that is traction limited with each motor drawing, say, 30 amps instead of 40, if you are extra paranoid)

[Deke]

Originally Posted by Chris is me

(By the way, consider running a low gear that is traction limited with each motor drawing, say, 30 amps instead of 40, if you are extra paranoid)

Or at ~25-30 amps if your team 703. Loving this drive train.

They could push for the entire season and not break a sweat with the motors, breakers, or batteries.