Quote:
Originally Posted by SuperNerd256
I'm not saying no mecanums (or however you spell it), but these are some points made in previous threads:
-Less traction on the key
-As already said, loss in pushing power
-Heavy
-Forces you to use maximum number of motors for drivetrain (not usually an issue)
-While they may not affect your ability to go on the bridge, the ability to stay without being pushed off or falling off of the bridge tips your direction will be tough to deal with
-You won't be driving circles around other's bots unless you have a lot of practice.
-In order to work to their fullest extent, require a lot of thorough engineering
-More complicated to code, even at the minimum level
Like I said, I'm not telling you not to use them, but when you do, be careful and engineer it through (Not meant to sound like most interesting man in the world meme, though I'm tempted.  ) |
Most of these are fairly situational arguments. We're going with mecanum this year and I'm fairly happy with the decision, though I suppose that requires some justification. A little history is in order. This is gonna be long...
We, like many other Hawaii teams, started in Overdrive. We built an 8-wheel tank drive and did fairly well, though our claw was too easily damaged and we didn't make it very far in the competition. We observed that some of the drives that were highly successful in Overdrive were omnidirectional drives that could abuse the "lap" rule and kept it in mind for later use.
Lunacy was Lunacy, of course. We didn't get a chance to try omnidirectional movement, though we actually ended up being a really solid defensive robot because of our uncommon three-wheel single-swerve drive. We misanalyzed the game and didn't build a floor collection device, so we didn't make it out of Hawaii. Our driver loved being more mobile and better at turning than all of the other robots at the competition.
In Breakaway we again misanalyzed the game and thought that crossing the bumps was going to be an important part of gameplay, which it really wasn't. But we didn't want to be able to be blocked coming off of the bump, so we figured we needed horizontal movement on top of the bump or right behind it. So we picked mecanum. We did fairly well that year, captaining a semifinalist alliance, but were knocked out by the eventual champions. However, or "ball magnet" was very substandard and we couldn't strafe while using it, so we ended up not using the strafing at all. It was pretty bad, honestly; we had no room to use mecanum's maneuvering and we couldn't even really strafe. However, our driver got some practice maneuvering outside of the competition and we got software for mecanum developed.
In Logomotion we analyzed the game perfectly well, but the design we set out to build was beyond our capabilities to achieve. We decided on mecanum because we had had experience with it the previous year, we thought that strafing in the zones would be useful, and we liked the added maneuverability. The drive was actually great, but our arm was finished too late and we were unable to use it at competition, rendering us a defensive/feeder robot. We used the newly-lightning-fast mecanum drive to great effect in feeding, but because we were a feeder and not a scorer, we once again failed to make it to Worlds. However, we improved our mecanum software, got used to how strategy plays with mecanum, and saw how useful it can be on a big field.
This year, we saw the field and immediately thought "mecanum". The wide-open spaces, no-pushing zones, and small game objects just scream for a robot that uses maneuverability rather than power to avoid opponents. We'd gotten used to the idea of the mecanum drive, we had a very good one from the previous year sitting in our shop to model off of, and our analysis showed that bump-crossing (sort of like in Breakaway) is nice but not necessary. Our driver, who had gotten very good with mecanum, has sadly graduated, but our new trainees have taken to it quickly. We're pretty happy with our decision.
So with regards to your points...
1) Precise motion on the key is fairly irrelevant if you turret your ball-scoring device.
2) The loss in pushing power actually isn't as severe as people make it out to be. We've never been pushed around by a tank drive and we've even pushed a few (sideways) ourselves.
3) It's possible to get an effective robot very light this year by consolidating mechanisms. We are estimating that we'll come in way light even with mecanums. We need the extra weight.
4) Not an issue because there are so many motors this year.
5) Refuted by our testing of the bridge balancing two mecanum robots. Sorry, we don't have video.
7) Already done in previous years.
8) Already done in previous years. (Breakaway was sort of a happy mistake...)
You may have noticed that I skipped 6. I did it because I wanted to add even more text to an already unnecessarily overlong post.
Too many people think of mecanum in ways that are fundamentally wrongheaded. Mecanum isn't going to "run circles around" a competent defensive robot. Honestly, if you're running circles around anyone, regardless of your drive, their drive is probably atrocious. Neither is pure horizontal strafing very useful. The actual value of mecanum is in the more complex maneuvers that few first-time-with-mecanum teams discover (we certainly didn't discover them in '10). The bootleg turn, circle strafing, and impact redirection are three examples of genuinely valuable things that mecanum enables you to do much more easily than tank drive does, but you hardly ever hear about them in mecanum discussion. Pro-mecanum people and anti-mecanum people alike often don't know what they're talking about.
Other assorted thoughts on mecanum drives, because apparently I don't feel like I've typed enough yet...
The pushing-power thing is blown way out of proportion. Unless your drive is stupidly weak or your opponent's is stupidly strong, you're not going to get shoved around, and mecanums can push tanks sideways (and occasionally head-to-head).
Mecanum can maneuver well on the bridge and the key alike, based on our tests. Super-precise maneuvers on the key don't seem all that useful anyways.
The fragility thing is blown way out of proportion, too. As long as you don't do anything stupid (like drive over the bump this year without wheel guards), you'll be fine.
Not everyone can drive a mecanum well. Some people who pick them up drive them like tank drives that can strafe horizontally, which is dumb, because that just makes them weaker tank drives with a bonus feature that your driver is likely to forget about. It does take practice, and it also takes a different way of thinking.
The no-mecanums-on-Einstein thing is a silly myth. FRC is fairly stratified and teams feel comfortable doing what they've done before; teams who have been around for a while (which tend to be the best teams) have been doing tank for a long time. Many teams who have the resources to get to Einstein have the resources to build a swerve instead of a mecanum. Which is better.
Even though we've used mecanums each of the last three years, I'd like to think we're not set in our ways. This year, we essentially knew going in that we wanted to use mecanum because it was the only thing that we've really refined. However, this was a good year to be forced to use mecanum. I've always had some doubts--a lot of very smart mentors from very good teams swear off mecanum, and I figure they can't all be wrong and we can't be the only ones who are right. We're planning to branch out next summer when we have the resources and the time to develop a good new drive.
In conclusion, mecanums are good when:
-there are wide-open spaces on the field
-game objects are fairly small
-your programmers have a modicum of experience
-the game is not pushing-heavy
Mecanums are bad when:
-your driver is entrenched into "tank-thinking"
-your programmers are very inexperienced
-the field contains many obstacles or small spaces
-you need to push to get access to a zone
-you have the resources to build a swerve
I hope that this post didn't bore anyone to tears and that it was informative, if only to help you see the (maybe misguided from your point of view) thought process of someone who's coached a mecanum robot for all of his time in FRC.