Re: Mecanum Drivetrains
 

Well considering you pushed a traction drive with a traction drive....

I'm also curious for any pictures/videos you have of this system in action, I've played with some locking wheel concepts, but never found anything truly elegant.

-Aren

Re: Mecanum Drivetrains
 

A BaneBots P80 on each corner is another option, if the gear ratio works well with the chosen wheel size. This provides a compact, simple, and, in my experience with this application, very reliable drive train.

Re: Mecanum Drivetrains
 

Our first mecanum drive (2010) used 9:1 Banebots p60 gear boxes direct driving four mecanum wheels (and supported on the far side with pillow blocks), and default mecanum code without sensors. The drivers had instant, intuitive control of the machine, because it handles exactly like a 3rd-person shooter with move/strafe--if you play COD or Fallout or whatnot, you've got the basics down already.

The thing to realize about uneven terrain is that mecanum drives work just like tank drive (or close enough that it makes little matter to a human driver) when going forward, backward, or turning normally...the concerns about uneven terrain are, by and large, overblown.

We've since gotten much more sophisticated, with gyros and encoders and octocanum, but the initial mecanum drive we played with was the easiest drive train we've ever built, programmed, or driven--it barely qualified as an afternoon project to get it up and running.

Re: Mecanum Drivetrains
 

When building a mecanum drive, it can be helpful to create some type of suspension so that each wheel has the same weight above it. It can be as simple as having the pair of back wheels on a pivot.

Remember, driver practice is really important.

Also, be careful of any bumps on the field. When aligning with the pyramid this year, the little 1/2" bump in the floor caught our mecanum wheels, and made it really hard to line up.

Finally, realize that there are many teams that will immediately disregard your robot in alliance selections. While I don't agree that all mecanum robots are bad, many teams have this opinion, and just won't choose a mecanum robot for eliminations.

Re: Mecanum Drivetrains
 

This opinions is based on the fact that given a robot with 6wd or an mecanum drive the 6wd offers more tactical flexibility in that they can hold their ground better should I need to stick them in the way of someone else for a couple seconds. The exception to this was in 2012 - Mecanums were practically DNP for me due to their difficulty with the bridges.

Please don't assume it's a bias AGAINST mecanum, perhaps it is merely a difference in priorities and needs.

Re: Mecanum Drivetrains
 

I would put our 2012 driver using mecanums against any other robot in terms of bridge balancing. We nick named him "bridge-dancer" that year because he could do fancy manuevers on the bridge, including driving across while turning 90 degrees to accomplish a triple balance.

Re: Mecanum Drivetrains
 

For 2012/2013 I agree that mecanum wheels were a poor choice. Our team used them, and now every member on the team hates them with a passion.

At our regional, we only picked other robots that could function as a defensive robot, so mecanum wheeled bots were off the list, even through we had mecanum wheels on our robot, which we quickly decided to remove/replace with hi-grip wheels.

However, for 2011, there were some good first picks that had mecanum.

Re: Mecanum Drivetrains
 

Yup. Our mecanum wheels had no issues whatsoever with the bridge.

Re: Mecanum Drivetrains
 

Interesting, every mecanum drive system I saw that year (excepting 357) had issues with the bridge. What wheels? Did you do anything to the rollers? What sort of feedback did you have?

Re: Mecanum Drivetrains
 

Our mecanum was also very good going up the bridge. We had a good bridge lowering device but often had to go up second to help push another mecanum up.

Single Balance
http://www.youtube.com/watch?v=fWYN5xVIyzw&t=11m1s

Here is one of us double balancing
http://www.youtube.com/watch?v=fWYN5xVIyzw&t=13m15s

We just had the 8" andy Mark wheels.

Re: Mecanum Drivetrains
 

One of the important things in 2012 was to set your speed controllers to brake mode and not coast mode if you were using mecanum drive. As much as a problem as this would have been for a 6WD, we found very early on that trying to balance with these settings was near to impossible with mecanum.

I don't know for certain, but I'd be willing to bet that a lot of the bad bridge-balancing mecanums never did this.

Re: Mecanum Drivetrains
 

Andymark wheels, standard, no feedback whatsoever. We were planning on using our traction wheels (octocanum), but found that by and large we just didn't need to. (We did use them to "downshift" to push other robots up the bridge, but we had no issues getting ourselves up/down the bridge.)

Re: Mecanum Drivetrains
 

Brace yourselves, a text wall is coming! :ahh:

After some distraction, that post really was badly typed. Red: You will notice my post did mention the recommendation of using of some sort of springy material to absorb deformities. That being said, on both of our robots, in 2011, and in 2012, we had our typical solid-as-a-rock welded aluminum box tubing frames. These were set and welded by one particular employee at our sponsor's shop, with precision as a top priority. For this reason our frames are extremely rigid and (realistically) perfectly 'square'. Like the instance I mentioned in the first post, we were subject to, and did experience raised/disabled wheels in the competition. But! because (Blue:) we controlled our drive not with motor power, but with motor RPM (using encoders), we had almost no trouble at all. When a wheel is lost (for whatever reason) and the driver attempts to do anything, the left side with only 1 wheel suddenly finds itself with half the torque of the right side with both wheels at matching PWM input. The code detects that the only left wheel is not spinning as fast as the right wheels while the robot tries to go, so it amps up the power to the lacking wheel to match RPMs, and thus balances the torques*. This operation was also the case when the robots were strafing. This whole scenario was a very rare occasion though, as most obstacles in 2011-2012 were faced head on anyways (ramps/key in 2012, minibot poles in 2011), so 2 wheels climbed the bank at the same time, making the leveling problem a non-issue. I concede the obstacle of 2013's pyramid, and who knows what 2014 will have in store, so I concur, I am making the recommendation to put springs or some other form of 'loose' configuration on future mecanum 'bots.

*while the 3 wheels are not slipping (or in mecanum's case: equally slipping), the only way they can all turn at the same speed is with the lone wheel compensating for it's disadvantage with double the torque

As for (Green:) driver experience. Like I said, That being said, a driver with hours of practice will be able to more efficiently and more effectively traverse the field, avoiding defense 'bots and evading traps. Magenta: These guys are trying mecanum for the first time, likely from some variant of tank drive last year. Any drivers-to-be on the team (who will more than likely NOT be rookies) will have probably driven this year's robot, an thus will, likely, have "preconceived notions about how a tank-style drive handles." Sometimes it can be easy to forget the strafing ability, especially in the heat of a match when you are used to driving tank from an event in the past (or simply spending hours driving a tank 'bot while waiting for the mecanum base to be finished).

One last rant: Green again: In every single last instance possibly imaginable, tons of practice is superior to a lack thereof. Commercial pilots spend thousands of hours training in flight simulators before they operate a real jet. Imagine handing me a copy of Flight Simulator X and saying "you have one hour". After that one hour, you stick me in the captains seat of a 747 and tell me to fly to Seattle. Not the best idea, I have to say. Same goes for a FRC robot. If practice is so worthless, and anyone can drive the robot as well as a trained 'pro', why not hold a school wide raffle on Thursday before your regional. One lucky student gets to drive the robot in competition, be it a band kid or a cheerleader. Maybe you could sell the tickets-what a great fundraising idea!

Re: Mecanum Drivetrains
 

Encoders on all 4 motors (rationale and details would require a separate post with lots of minutiae and some drawings) for closed loop speed control with tuned PID values. Andymark 6" regular mecanums. No special modifications to the rollers other than making sure each one spun freely on its axis.

Re: Mecanum Drivetrains
 

i love Mecanums!!

But in competitive applications like first, i believe is the words of Colin Chapman (founder of Lotus cars) -- “Simplify, then add lightness" Mecanums do neither!!

now i'm hugely biased as the gang of mentors i hang with is all about 6 or 8 WCD.

granted some 340 students are working on a Mecanum as we speak.