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2017 Drive Train
There's obviously a lot of debate surrounding which drive trains can serve what purposes. This thread is created to centralize this discussion to provide easier access to all teams.
One of the more uncommon drive systems which still proves useful is the h-drive. This system has omnis on all corners, with a central omni to provide lateral movement. Our team has thrown around the idea of using h-drive as a less complicated mechanum system. The movement is near-identical to mechanum and debatably easier to build. This of course being in contrast to using tank drive (reliable and simple). Can we get some insight? |
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I'm not sure how a h-drive is simpler to build. With mecanums, you simply just mount wheels on each corner. The stock AM14U3 chassis even has support for it with Toughbox Micros. Meanwhile, with a h-drive you have to add a cross member and have a whole different drive wheel and gearbox in the middle. With H-drive you can have three gearboxes (you can tie the two side wheels together), but you need at least 5 motors. Mecanum has four gearboxes but four motors needed. Both offer challenges, but a mecanum drive is a lot simpler to mount to some versa chassis quickly (or c-channel as my team uses) and get started.
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This year might be more of a swerve drive game than previous years. More control of the drivetrain can help place gears, and there's a wide open field for swerve to dance around defense in. It combines the advantages of omni or mecanums (and more) without giving up resistance towards defensive robots.
Too bad I'm not confident in my team's ability to do swerve properly.:( |
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Mecanum vs. Tank?
Mecanum has an obvious mobility advantage, but how much does it really give up in traction/strength? Is it completely outclassed in strength to a tank drive system? |
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If there was ever a game to not overthink your drivetrain and just build a really simple tank drive, this would probably be it.
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Everyone should read this before building anything but a tank drive:
http://www.simbotics.org/resources/m...rain-selection |
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Point to consider:
6-WD tank (preferably with a COTS shifter) is the fastest, easiest drivetrain to implement and program. Is a mecanum, H-drive, or swerve so much easier to line up that it outweighs the extra practice time you should have with the tank drive? Or the extra practice time with your practice bot because the tank drive is so much cheaper? Also, a defensive tank bot is going to shove a mecanum or h-drive all over the place. Good luck scoring a gear while that's happening. |
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Yeah, don't overthink the drivetrain. You shouldn't need a vertical adjustment for anything. And, if you are off horizontally, build it into a mechanism rather than moving the whole robot. If you are placing a gear, have a way to rotate it or move it side-to-side. If you are a shooter, have a way to rotate your shooter to keep it on target. Put your code into things like that, not into complex drive. |
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In 3946's experience, H-drive is harder than mecanum.
In 2013 Aerial Assault we decided to do mecanum (foolishly expecting a less defensive game), and it worked as well as expected, pretty much the first time around (OK, after fixing some wheel order issues), but we had it running two weeks after we made the decision on our prototype "Woody" (which meant ordering wheels and extra gearboxes before we could finish the build) and running like a champ on the competition robot "Buzz" before bag day. We used the WPI mecanumDrive class straight up, without encoders. In 2015 Recycle Rush, we decided to do H-drive, and never got it working correctly all the way through CMP. It was only after the 2016 season that we got it working. If you do do H-drive, do not depend on a fixed-height strafe wheel - there is no height which provides the correct amount of load. The central wheel(s) must somehow have a controlled force against the carpet, whether passively through springs or actively through pneumatics or torque actuation, or perhaps some other ways you can come up with. Another caveat which may or may not apply to you: H-drive requires a wheel near the CoG of your chassis (or one forward and one behind). This will interfere with the "chassis opening and combine" style gatherer that is likely to be commonly used by fuelbots this year. |
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we are actually gonna use 610s DT from Ultimate Ascent but wooden mobility for us was not a big thing for us this year we want to just be a role player and focus on the gears but still win a pushing a match
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6 CIM, 6 Wheel Drive, single speed. I believe it was designed around acceleration, not a very high full speed. |
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So just so some newer teams don't read this thread and think that mecanum wheels are completely useless at holding their own in a game like this, check out this video from 2014. https://www.thebluealliance.com/match/2014arc_qm29
The bright red and yellow robot (us) were running the new vexpro mecanums (before the metal casters oh how we wish they had the metal insert like they do now) and we easily could hold our own in a pushing contest. The key with mecanums is that the center of gravity needs to be as low and as perfectly centered as possible. So, do a lot of mecanum drives get pushed around, yes. BUT, if they are optimized, can they hold their own against the most pushy of robots, absolutely! Just something to think about before teams blindly throw it out because they are under the assumption they can be pushed around with minimal effort. |
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Just Sayin. |
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In my view, if you haven't built a swerve drive or H drive or mecanuum drive and had your coders code it and your drivers drive it, you really need to stick to tank drive this year.
The practice time you will lose with your robot is just not going to be able to be overcome by the extra features you are imagining you will have. Finish early. Give your coders coding time and your drivers practice time and your whole team time to iterate and improve your robot. THAT is the way to be playing after lunch on the last day of the tournament*. Dr. Joe J. *said the guy who designed his first Swerve Drive for Ladder Logic (1998 season). Chief Delphi won 3 blue banners with CD3. I KNOW that drive trains matter. But driver practice matters more. A LOT more. |
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Disclaimer: Build a tank drive. Please. Just do that. If you are the kind of person who listens to advice on CD to make drivetrain decisions, do that.
-- That said, there's a pretty reasonable argument for swerve or butterfly drive this year if it's compact. The bumper rules limit intakes since six inches of room on each side of the drive must be backed by bumper - leaving more space for various omnidirectional drives. Lining up at the peg, goals, or human loader could theoretically be faster using a strafing drivetrain. And the full field sprints lend themselves well to butterfly style drivetrains. But I mean, if you haven't considered these drives and built prototypes long before reading this post, it's too late, and the possible small advantage they could give your team is more than offset by the development difficulty. We have a bunch of other tough tasks to do in this game, don't make it harder than it has to be. |
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It seems to me that people are far overestimating how hard it will be to deliver the gear. The (horizontal) margin of error is 6 inches. That's (approximately) the width of the gear. Because the hook is spring loaded, you have even more leeway, not to mention that the place you have to deliver to is directly in front of your driver station. I'd argue it was harder to line up with the chute in 2015. If your driver really can't do it, just put a camera on the robot and use that to align, but don't build your drivetrain around this particular challenge.
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In my experience, a field-centric holonomic drive is the easiest robot to learn to drive. Mecanum provides a good trade-off between complexity (2 extra gearboxes, a bit of learning to program) for increased maneuverability. It's also the closest you'll get to a COTS solution. Any other drive really needs to be developed in the off-season. |
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There's this myth that mecanum is hard to program. It just isn't. (It's also incredibly easy to build. Just direct-drive four wheels off of appropriate gearboxes, and voila.) That said, straight-up mecanum will be a mistake this year. The parallels to Ultimate Ascent say defense will be *huge*, and mecanum drives don't do a good job powering through defense. |
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Based on the current stock Andymark has for Toughbox Micros, I think that a lot of teams are going for mecanum.:yikes:
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We used h-drive in 2015 and got it working great. Mechanum is something we have never done because a) we haven't needed it and b) its more complicated than a tank or h-drive. For this year's game tank drive is probably best. For lining things up, just practice with the tank drive and build some mechanisms in for maneuvering game pieces within the bot.
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Basic question but couldnt find it anywhere else:
How fast does the andymark KOP chassis go? |
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In fact, Ultimate ascent was easier than this game when it came to driving, because you could put yourself in safe zones when you shot. This year? Prepared to be pushed while putting the gear on or shooting into the goal. Build a tank drive. |
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On a slightly different note, what motor / gearing / wheel size setup are you all using? Ideal feet per second I would imagine is relatively high. Shifting or no shifting? We are leaning towards 6 cim ~10-12 fps no shifting but I don't have great experience to tell if we got it in the right range or not for this game.
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Personally I'm not a big shifting guy, I think it takes a very skilled driver to use it effectively. |
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Here on 1640 we plan to use our swerve drive again (big suprise). This year we are also going to introduce our CVT swerve to the field.
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I personally think mechanum or Omni with tread-wheel hybrid. That's just an idea though. Problem is the amperage for mechanum.
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Motors: my go-to combination is 4 CIMs & 2 MiniCIMs. Gearing: I am usually an advocate for single speed gearboxes, and I still think you can't go wrong with them this year. My issues with shifters is that most teams are not able to fully realize the benefits of their shifting gearbox; whether that's because of poor design, a game that doesn't need two-speeds, insufficient driver practice or just mistakes in the engineering process, they're adding weight, cost and complexity and not getting much in return. However, this year is a good year for shifting gearboxes if you go that route. you have long open stretches of field you need to get across quickly and you have tight spaces where you need to push defenders out of the way. I have not done any of the math yet to really figure out sprint distances and distance to reach full speed, so I'm just guessing on the numbers themselves. But for this game, I'd say: 12 ft/s for a single speed gearbox 8 & 14 ft/s for a two-speed gearbox. |
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You're right, 45 lbs. is much more reasonable and your chassis looks both agile and powerful. I expect you'll see a lot of success on that that chassis. But the reason I always will recommend a simple 6 or 8 wheel drop center chassis over the more complicated drive systems is that a well designed, well built, and well practiced tank drive will perform as well as something more complicated for most cases. Yes, there are situations in which it'd be really convenient to just move 6" to the left in a match. But those scenarios are not worth the time and effort that those more complicated drive systems require. And the robustness of a tank drive cannot be beaten. In short, I think it's more important for teams to field the best robot and not the best drivetrain. Any time spent on a complicated drive system is time not spent developing other subsystems. |
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Speaking of drive train, we finally came up with the drive train we are going to use for this year. We were deciding between skid steer and mechanum and I was for skid steer. However, the main mentor already ordered the parts for mechanum. We're still deciding on our robot design, but at least we have a drive train. :)
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Our team is building both an omni and the KOP drivetrain this year and making them the same dimensions with the same opening location for ball retrieval. The plan is to use the omni, but with the backup option of the KOP DT.
Are we allowed to switch out drive trains at the competition if the need arises? If so, would both drivetrains need to be included in our 120 lbs? Do we need to come to the competition with the second drive train un-assembled? |
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Bumping the thread to ask, "Our team was driving and finding alignment time issues with the feeder station and gear deployment station, we are using the AM14U2 chassis in the 6 wheel configuration. Is H-drive worth is to overcome the drive practice required to become efficient at loading?" Our team has not built it before, and I am curious as to how difficult it is with the standard chassis.
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On another note: I'm very happy that 4607 built a Butterslide drive with 4" omnis and 2" custom traction wheels. We're essentially duplicating that drivetrain with some useful improvements. Being able to strafe while also having the ability to resist pushing while scoring is going to be so useful. If we hadn't built that drivetrain in the offseason there's no way we could've done it during the build season! |
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Our team decided to go with a 6 wheel tank drive this year, with a sort of modified west coast style. We have a frame CAD model attached and would love to hear what you think.https://drive.google.com/file/d/0BzJ...ew?usp=sharing
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Otherwise, great work. My team is also doing 6wd DS WCD. So serviceable and utilitarian. |
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Please correct me if I have a misunderstanding of this. |
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I know that thread has been open for awhile now, but I'll just throw in my two sense. Our team has officially decided to go with a custom-build mecanum drivetrain. I know most teams have decided by this point, but our reasoning was as follows:
1. Agility/maneuverability will be key for auto/gear placement 2. Defensive situations can be avoided by "running away" - it is harder to trap a mecanum robot 3. Possibility of a "ground-engage" mechanism for staying in one spot (perhaps for shooting) 4. Sacrifice torc for speed - i.e. with a mecanum drivetrain we have little reason for torc, so can therefore throw torc to the wind, and go for as much speed as possible in our drivetrain. I know some have said that mecanum is not the way to go this year, but we think there's plenty of use to be had there. |
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I'm not expecting to change your design; you made an engineering decision and you should follow through that. I just want to set the record straight for other teams who may be looking to this thread for ideas. My team, to my chagrin, has run mecanum drives 4 of the 5 years I was a member (not including this year). I was a driver for 2 of those 4 years. So I have a lot of experience with the benefits and drawbacks of mecanum drives. In general, I think there are very few cases (practically none) where a decent mecanum drive is better than a good WCD. Even in 2015, I still would have preferred that my team would have stuck with a WCD. |
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Better to ask than to assume. |
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When people say "Tank Drive" I get all warm and fuzzy thinking back to my first FIRST experience.
FRC 971's 2004 Half Track ![]() COTS Snowblower tracks, mounted at an angle, with the previous year robot's spare wheels |
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Are the CIM bag motor (217-3351) and the vex mini CIM (217-3371) considered CIM motors as described in rule R32?
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https://frc-qa.firstinspires.org/manual/R32 |
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