pic: Did Somebody Say Defense Game?
 

Here's a few specs...

The drive consists of:
12x - 4" VEXPro Traction Wheels in 1.5" wide configuration
2x - VEXPro 3-CIM Ball Shifters with 9.07:1 High Gear and 24:1 Low Gear
22x - 50-tooth VEXPro gears for the wheels and idlers
4x - CIM motors (2 on each gearbox)
2x - MiniCIM motors (1 on each gearbox)

• Each wheel is combined with a 50-tooth gear via a VEXPro Versa Hub.
• Two wheels are directly driven via Hex output from the gearboxes.
• All 6 wheels on each side are driven via connecting 50-tooth idler gears (not shown in this picture) in a 1:1 ratio.
• The wheels are aligned on a 32' arc, allowing for easy turning with minimal scrub.

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Cross your fingers that it will turn without popping the breakers.

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Actually, they're missing a couple of wheels. (2007 joke)

If it's anything like what they've done before, the wheels are in a bit of an arc, so it acts more like a 6WD drop than a 4WD "bounce turn".

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Looks like it'd be more of an 8WD, but I could be wrong.

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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!

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I agree, looks like they are set in pairs.

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

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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)

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Yes. Can you sent a video with it running

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Oh god. it's back.

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Hey! That's almost as many wheels as ours.

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Adding wheels doesn't really increase your traction/pushing power, does it? I'm sure that the extra width helps, but not the number.

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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.

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Yeah, I doubt that all of those wheels are on the ground at the same time though, unless they don't want to turn.

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

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Love it!

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This makes me glad we chose 6CIM drive.

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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.

That pretty much sums up what I'm expecting.

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.

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

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.

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. :rolleyes:

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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?

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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.

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

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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

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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.

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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.

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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? :confused:

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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.

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Correct, current = torque. So by adding a 3rd motor to each gearbox you reduce the load on the 40amp breakers while the load on the main breaker will remain the same for the same torque, regardless of motor count.

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To answer your question, yes I did.

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so adding three motors doesn't increase torque? interesting.

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Mad Crazy Drivetrain.

IMHO, this may be the year that Mecanums make it to Einstein.

A defensive Robot that can strafe in the Goalie zone or between two robots passing is more akin to defensive play in Hockey, Soccer, and Basketball.

Pushing or preventing is another issue, but that may not be the key to defense in Aerial Assist.

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Adding an extra motor per side does increase the theoretical maximum available torque. However, traction limitations and battery limitations do not allow that torque to be fully utilized.

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Correct, you would have to be able to feed the motor the extra current to increase the torque. So basically you increase the system stall torque by increasing motors, but can you feed it the extra current?

If two drivetrains exactly the same in traction, weight, gear ratios were in a push off/breaker cook race and the only difference is the motor count (lets say a 4 cim versus a 6 cim), the 4 cim drivetrain will pop its breakers faster. The 6 cims will have less current going through the individual 40a breakers (total flow divided by 6 instead of 4), and slightly less through the main breaker (because they should be running at a slightly higher efficiency).

My assumption why people pop breakers with 6 cims faster than 4 cims is because they think they can push while geared high, when really both systems are limited by their breakers. So you would need the correct gearing in either case to be a good pushing robot.

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There's a limited amount of pushing force that you can have before the wheels start slipping. So if you have two CIMs geared really slow, or 15 CIMs geared really slow, once the wheels slip, you won't see any difference. However, with more CIMs, your traction limited speed will be faster, which is useful for playing defense. If you've got a crazy high traction robot that moves really slowly, it'll be hard to catch up to somebody.

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I agree on the 40 amp breakers. Once those things heat up, they're really prone to tripping.

With 6 full size CIMs on drive, you can trip the main doing full forward to full reverse. It's scary to think it could happen in a competition. Also, once you've driven for a minute or so, you can get the battery voltage low enough to cause the radio to drop out. Those CIMs can draw 100 amps at stall. It'll take a few seconds for the 40 amps to start tripping, but the 120 amp breaker trips quickly when you try to put 600 amps through it....

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Can you explain this one, I don't fully understand.

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in order to get to traction limited, no matter how many CIMs you have, you need a certain amount of torque at the wheels. Let's just say for my robot, it's 16 times the torque I can get directly, with no gear reduction from a single CIM motor. So, I would have a 16:1 gear reduction with a single CIM to be traction limited.

If I had two CIMs, I would need to have 16 times the torque from one CIM, or because their torque adds together, I would need 8 times the torque from two CIMs. So, I would put these two CIMs in an 8:1 gear reduction.

The second, 8:1 gear reduction with 2 CIMs will have the same amount of torque at the wheels (16 times the torque directly from a CIM) as the 16:1 with a single CIM, but the 8:1 will go twice as fast.

What you said in your previous post was correct, you need proper gearing to take advantage of your motors.

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I think that a brown-out and re-set of the cRIO is a very likely drivetrain induced failure. Not just the breakers blowing. I don't know which one, 6cim or 4cim, would be more likely to trigger a brown-out, though I think a brown-out or low-voltage reset is much more likely than blowing a main breaker.

Full disclosure: I have not a lot of experience blowing 40A breakers, or main breakers for that matter. I've always geared transmissions on robots a little conservatively, prioritizing robustness over ultimate performance. One robot team 95 made had issues with that, a full 4-wheel swerve in 2003, that could drain a battery down to the point of brown-out half-way through a match.

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Excellent, thank you! A more visual explanation helps me, I miss things when I cant visualize them.

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We ran a 6 motor, 2" wide wheel 6WD last year. We had troubles popping the main breaker but not the 40A breakers. Because your motors are sharing the load the current is actually LESS per motor. Therefor it is actually HARDER to trip the 40A breakers.

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Tripping the 120 amp? Never had that happen... Anyone who has, what caused it (that wasn't a short :P)? Your drivetrain?

Popping that is quite a scary thought...

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What do you think the root cause of the high current load was from? Pushing, turning, hard accels?

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The only time we experienced the main breaker popping was when we went from full speed forward to full speed backwards. Adding in a small ramp prevented us from tripping the main again.

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Hence our use of a 24:1 Low Gear + 4in wheels on this design.

We've already got our programmers working on this exact thing, we've shredded far too many AM toughbox gears in the past to not know better. :rolleyes:



Oh, and for those of you that might not have noticed, I did update the OP with some specs.

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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)

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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.

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Having competed against their 2007 robot.... :yikes: