Swerve versus Mech/Holonomic is a discussion that seems to come up every year.
My team did a crab drive last year, and swerve drive the 2 years before that. I can say that they're great for somethings (maneuverability), but at a huge cost.
In 2007/2008, we made the full 8-motor 4-wheel independent swerve drive. All 4 CIMs, 2 FP, and 2 Globes went to this application. What did that leave us elsewhere? Not all that much. The 6 most powerful motors in the it all went to the drive. Worse yet, in terms of pushing power, many (most?) teams w/ a simple 4/6WD with 4 CIMs were stronger. Why? Using high traction wheels (IFI/Pneumatic) would mean that it would be necessary to gear down the steering motors further. If they steer too slow, there's a noticeable delay between going straight and translating. You get one or the other, power of maneuverability.
Another thing that is commonly overlooked is HOW you control the robot. If you have "full swerve", you can control it rotate/translate (the same way you play a FPS game, for example). Difference is, in a FPS (and most other games where you use this control system), the system responds instantly and repeatably. In a robot system, it does not. The end result? It becomes difficult for the driver to fully utilize this system. Now you have a heavy, resource expensive (as far as time, (maybe money), and motors) system that's difficult to program and more difficult to control.
Last year, we downgraded a bit to a left/right side linked crab drive system. With the slick surface, it was unnecessary to put 4 CIMs to drive, since 2 would (in our assessment) always be sufficient to break the wheels loose. This system was really no less complicated than the independent swerve system. When you link sides, you need some way of passing power around. The benefit, though, is that you now can use less motors (minimum of 4 for left/right linked, I suppose you could get away with 2 for all linked). I suppose another advantage is controlability/programming. The control scheme we used last year was simple, left side of drive points in the direction of your left joystick, and the right side of the drive follows the right joystick. In my opinion, it was a little more controlable, but still very difficult to fully utilize (most of the time it was enough to just drive like a tank-drive robot, forward w/ minor adjustments to get balls).
Now, I have never built a mech/holonomic drive system before. I am also quite afraid to go in such a direction. It seems, to me, as if a mechanum drive would, at least, have trouble going over the ramp. I don't think a holonomic drive can do it at all, but that's just my gut feeling. Now, consider the alternative to the ramp, the tunnel. First of all, its narrow. Second, a mechanum/holonomic system would, almost undoubtedly, be unable to (or have great difficulty to) push a conventional 4/6/8/10/12... wheel drive system. If you are really the biggest scoring threat on your alliance, just about any robot would be capable of largely eliminating your ability to maneuver across the field. How big of a deal is that? I can't say for sure. Maybe crossing zones won't be necessary at all, but even then, your inability to stand your ground would prove to be quite detrimental, I suspect.
Just my 2 cents.
EDIT: Oh, as far as the comments about swerve drives being less robust goes, I wholeheartedly disagree. The only year we had issues with the swerve drive was in 2007, mostly due to calibration and chains. If you build a swerve drive, avoid chains if possible, and make a system that, once calibrated, will not lose calibration. If I recall correctly, we recalibrated 0 times during the season and never lost functionality in either side of the drive during competitions. A properly engineered swerve drive is, in my opinion, as robust as a well engineered conventional drive.
REEDIT:
Quote:
Originally Posted by JesseK
Mecanums are the 'easy' way to upgrade during the actual season, because if all else fails with the code you can easily just tank steer like usual.
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While that is true, you can also do the same with a swerve/crab system. I would imagine that swerve/crab is better than mechanums in tank mode because the wheels would have more traction.