Limiting Drivetrain Motors

Technical Motors
1

I don’t think that anyone can deny that FRC in general is in a bit of drive train war right now. The latitude we’ve been given in the last few years in motor choice and availability have made 6 cim drive trains increasingly common, with some few teams even moving to an 8 motor drive train.

That isn’t a bad thing from an engineering standpoint.

Drive trains area already limited by the 120 amp breaker. Assuming the new CrossTheRoad PDB and current monitoring does what we hope it will, teams should be able to ride ‘upper limit’ of power while not popping their breaker.

I have to wonder, though, if we would be better served in FRC by limiting the total power output of the drivetrain. There is an argument to be made about the increasing price of multi-cim multi-speed gearboxes. A reasonable argument can also be made that the kitbot drive train has been rendered obsolete. This year, we saw an extremely brutal game. This made partially possible by the wide open field, but the high acceleration and high top-end speed that our drivetrains delivered were also responsible. A lot of robots left the field in pieces: even those of multiple-time world champions.

I think we’ve reached the point where it’s time for FRC to consider reigning in the drive train power. I wouldn’t be adverse to a max 6 cim, or even 4 cim and 2 mini-cim ‘power’ limit. I don’t have my motor sheet in front of me to lay down the power numbers of those motors right now. What do other folks think?

2

Here’s a thread where Dr. Joe expresses a similar opinion… in 2007

I’m still not sure why the CIM limit was increased from 4 to 6.

3

Mr. Line,

I believe this would trend towards a lot of complaining about FIRST is suppose to be an engineering challenge why are they limiting us. I believe that COTS items will catch up quickly in the drivetrain race. This to me is one of the reasons why FRC introduced more motors, to reward innovation and the challenge of not having enough weight/time to integrate all the motors you have. That being said I myself do not take issue with the proposal if FIRST sets a rule like this, the teams must follow the rules on allowable drivetrain motors. Overall, I don’t think this really makes anyone less or more competitive.

4

I think that the drivetrain ‘arms race’ is fantastic. It forces teams to consider a serious engineering decision - how many CIMs should we put in our drivetrain? There are weight, power, traction, acceleration, lost-opportunity, and reliability factors to weigh out. There are also numerous successful drivetrains that teams can base their designs on, but the teams with a careful eye for detail and execution will still have an advantage in this facet of FRC that is being continually refined and improved.

Despite a field that seems to be narrowing, there are still numerous drive solutions that can be successful. Swerve, butterfly, 6wd, 8wd, 10+wd, octanum, etc. all have robots and teams that champion them with great success. I, personally, cannot wait to see what the next evolution of the incredible drivetrain race develops.

I really hope FIRST doesn’t artificially limit the most important FRC sub-system.

5

I actually like where FIRST is right now with drivetrains. I feel there is a lot of freedom to try different drive train configurations. The majority of teams only use 2 or 4 cims in their drivetrain even with the 6 cims rule. I really like this freedom because with this freedom some teams develop very pushy robots or very fast ones that make games really fun to watch.

6

Part of me likes the idea, part of me doesn’t.

In once sense, it would be nice to limit power to help preserve the robots all these teams build. This was definitely the hardest hitting year I’ve seen… and I know my team’s robot took more damage from impacts this year than we did from falling off the tower multiple times last year!

In another… going up against stronger robots with more powerful drive trains does present an engineering challenge. It’s a challenge to build robust robots. It’s a challenge to design a strong drive train that avoids tripping the main breaker. It’s a challenge to play any FRC game well.

Rather than setting a more-or-less arbitrary limit on “drive train power”, I’d rather see the rules limit overall motors to a number reasonable for the game being played. Look at this year, for example:

6 CIM - 337W each
4 Banebots - max of 273W each
4 AndyMark 9015 - 45W each
4 Denso - 18W each
4 Bag/Mini-CIM - max 229W each
2 Window - 23W each
2 Vex 2-wire - 4W each
1 snow blower - 30W each

(Per 2014 rules and 2014 motor data sheet)

That’s a lot of power - 4366W, if my math is correct. When I look back over previous years, our robots have never been anywhere close to that:
2014: 5 CIM, 1 RS-550 - total of 1939W
2013: 4 CIM, 2 mini-CIM, 3 AndyMark, 1 Window - total of 1964W
2012: 4 CIM, 2 RS-550, 2 FP, 1 AndyMark - total of about 2300W (I think the FP’s were around 200W each… hard to remember)
2011: 4 CIM, 2 RS-550, 1 FP, 1 Window - total of about 2300W

I would contend that we’re getting a lot more power in the rules than robots really need to be successful.

Another side of this, of course, is in the design of the game. Having game aspects that require a lot of power from the motors means you need to have more motors available. This year, we had a game where all game piece manipulation and scoring could fairly easily be done with a relatively small amount of motor power (in a large part thanks to stored energy devices like catapults that use springs or hammers that use gravity). I know we got ours for about 400W this year. Compared to previous years (600W+ in 2013 to climb the tower, 800W+ in 2012 to shoot the basketballs, 600W+ in 2011 to manipulate the tubes), this required fewer motors to accomplish, which would leave more for the drive train.

So for me, I would vote for fewer motors allowed overall, with game play designed to offer a tradeoff between power in the drive train and power for accomplishing other game tasks.

7

What’s crazy is the power level changes since 07 have almost doubled (estimating).

8

Personally, I could see it being cumbersome to write and enforce rules which regulate the exact mechanisms that particular motors on a robot power. Additionally, there are diminishing marginal returns to additional motors incorporate on a drive train. Adding the 3rd and 4th motors is a no-brainer. Adding the 5th and 6th motors requires proper design and a solid understanding of the drive train physics to reap significant benefits, and introduces an increased risk of popping the main breaker under certain conditions. Adding the 7th and 8th motors sees even less gains which may be inconsequential even with a proper understanding of the physics of drive trains and requires extreme caution with respect to popping the main breaker. As number of motors increases, a limit of improved functionality is approached. Where each team draws the line and what they consider “worth it” is up to them.

I would still argue that a kitbot drive train could be extremely competitive in this year’s game. From scouting in Archimedes, I am aware of at least a handful of such robots.

IMO, the brutality of Aerial Assist had much more to do with game design than anything else. While the GDC may have intended the game to operate like a rube goldberg robotics demonstration like we saw in the Arizona Regional Finals or on the Weather Channel matches, it was quite clear from the get go that it would be a highly defensive game. When at most 2 robots from an alliance, and more often only 1, can interact with the game piece at a time do we expect the other partner(s) to just sit there or attempt to interfere with the opponents’ offensive efforts?

Historically FIRST allowed additional CIMs beyond the previously typical 4 in 2010 (a climbing game, elevation of 18"; 5 CIMs allowed) and 2013 (another climbing game, elevation of up to 60"; 6 CIMs allowed). My understanding up through then was that the additional CIMs were included to encourage teams to pursue climbing without necessarily taking away from the typical drive train motor allotment or requiring cool PTO stuff. I would guess that the GDC likes to see teams pursuing a variety of game tasks, not ignoring them.

Then this year they also included 6 CIMs, which sort of broke any supposed support behind “climbing = more CIMs”. I would’ve expected something along the lines of 4 CIMs + 2 MiniCIMs, still with teams using all 6 of those in the drive train as supported by the new VP and WCP gearboxes.

9

I think it’s fine. The main breaker is good enough to limit power.
6 cim vs. 4 cim is kind of overblown IMO. If you have the room, put in 6. If you don’t, have 4. It really is not going to affect your entire robot as long as you allocate motors to other places first. There’s plenty of calculators and the like to help you decide if you will be helped by a 6 cim drive.

10

IMO the kop drive train was more competitive this year than ever before. Although pnw is not know to have strong drive trains we had our most successful season yet running a kitbot drive. The only time we regretted that choice was in Galileo finals when we were too slow to get around defense.

With proper maitnence and lots of reinforcment the kop drive was one of the best on the field in most matches.The only downside to the drive was that we needed to replace the wheels at least once every event.

11

I think this is less and less of a question. Back when we were only allowed 4 CIMs and 2 FP motors (and maybe 1 other high power motor), absolutely. There were plenty of applications where you might want to utilize a high power motor in your manipulator design. Now the only choice really comes down to whether or not you can afford a few extra pounds for your drivetrain (especially with the new PDB and Roborio helping avoid brown outs). Scarcity/lost-opportunity is no longer a concern with 6-CIMs and quite a few other high-power 500 and 700-series motors available to teams.

12

One thing to consider is the context in which the motor rules of 2012, and then later 2013 came about.

In 2011, you were allowed 4 CIMs, 1 FP, 4 BB, 4 Window, and that’s about it. This essentially forced teams to use Banebots motors. I’m pretty sure the FPs sold out that year? This was also the year with defective 775s, 2+ week lead times from Banebots, and generally poor service. People didn’t like that we were all but forced to go through one supplier.

So in 2012, more options were introduced. We were allowed 4 CIMs, 2 FPs, 4 Banebots, 4 AM motors, 4 Window, past KoP motors, and there was this weird deal with auto junkyards no one used. The intent wasn’t to increase available power to teams as much as it was to not force them to use a particular supplier for their “medium power” 200ish watt motors.

In 2013, Vex began making FRC mechanical parts, so the MiniCIM / BAG were added in a similar manner to other motors: you got up to 4 of them. I suspect climbing motivated the use of 6 CIMs. I assume the GDC saw teams using 6 CIMs without climbing, didn’t see drivetrains destroying other drivetrains much, and decided it wouldn’t be a bad thing for 2014 either.

Basically, I’m contesting the assumption that the GDC opening up the motor rules is about them wanting us to have more mechanical power. I think they just want us to have options as to how we drive mechanisms. In the spirit of that, a power limit would keep these options open but limit the overall power . Not saying I support or oppose it, but that’s one solution to the problem without backpedaling on why I feel these changes were made in the first place.

13

I’m not sure we need to limit motor power. The main breaker does an okay job but the real limiter is traction. As long as the carpet and weight limit are in place there is only so much power you can physically transfer to the ground.

We ran 8 motors this year. We never popped our main breaker and one of the main reasons that I would continue doing it is that are motors actually stayed much colder than in the past.

14

I see this as limiting innovation. I can guarentee teams are working hard right now to create 6 cim swerve and tex-coast drives wich would likley never be created if there was a rule created banning 6 cim drives.

Also, if your robot had a PTO gearbox do the motors in it count as drive train motors or whatever else the gearbox is driving?

15

We used a 6 motor drive train (4 CIMs, 2 Mini-Bike Motors) back in 2006 on a similarly open field with no bumpers on the robot. We also built the frame out of solid sheets of 3/16" aluminum, needless to say we had no durability issues with the frame.

Rules that try to limit drive motors are really just limiting mechanism options, and rules putting a cap on drive system power will make inspections more complex, and likely longer.

The game animation for as long as I’ve been in FIRST has included the line “Robots should be built robustly”, IMO teams simply need to design with robot durability in mind, otherwise the exciting contact sport that is FRC eventually becomes a dance recital.

16

I don’t think the kitbot is obsolete by the available power, but it does require a little work to tap the power. We did switch in the AndyMark 3CIM4U gearboxes ($150 for the pair, if I recall) and ran our underweight robot just fine all season. Total BOM on the change would probably be around $260 with the two extra CIMs and a pair of speed controllers (which IFI has a PDV for if you want to split hairs). If a sub-$300 parts order is causing major insomnia, I’d question whether the team was really prepared to compete in FRC.

The other notion: 6-CIM setups have fewer parts (and fewer small moving parts) compared to a shifting design. Could this be FIRST’s subtle helping hand to ensure drivetrains stay mobile?

That said, if FIRST was worried about it I say limit teams to five CIMs. Teams will either do without, shift gears, or learn a thing or two about matching motors to party like it’s 2004.

17

I don’t think this is FIRST’s plan but I do think it is an advantage. I’ve never been on a team that has used shifting gearboxes in competition. Our setup this year shifted the wheels to get speed reductions but we haven’t used something that can fail in the same way as your standard two speed gearbox.

18

I would hate to see the arms race end, fast robots and harder hit are way more fun for all parties, other then rookies.

19

And anyone trying to actually play the game. It gets old going out and spending most of the match being smashed into by teams that are incapable of playing the game.

20

Fair point, but this year we got called for hitting the defense bots back too hard. Teams that are blockading / pinning a partner should design for 13 fps hits or get out the way.

i’m sick of playing tea party games / soccer.