Mecanum vs. Tank Drive?

[niklas674]

Mecanums are great for an intake, past that I definitely would not recommend.

[Electronica1]

As much of a fan as I am of using mecanum and not picking tank without proper game analysis, if you use pure mecanum this year you will have a bad time. From what I understand, you are not protected while placing gears, so all the fine adjustment in the world isn't going to help when you are being clobbered whenever you get close. If you can't do swerve, octocanum, nonadrive or butterfly drive, then you should stick with skid steer and focus on compensating for the less maneuverability.

[bEdhEd]

Originally Posted by niklas674

While our best year involved us using a mecanum drive (mistake), we practically never used it in matches, the positioning was close enough that we didn't have to make large changes. Bottom line is that with plenty of drivers practice, you won't need to be able to strafe because your drivers will get to the exact spot almost every time.

__________________________________________________ ___________
TL;DR: Even in a game free of opponent pushing where mecanum was a viable option for strafing motion, it still was not our first choice in the design of the drive system. In the end it failed to work as well as we wanted.
__________________________________________________ ___________
I recall a few factors in 2015 in deciding to use mecanum. One was the fact that we were trying to be a landfill clearing robot, which required fine maneuverability due to the perpendicular orientation of the totes. We also constructed our robot to ride along the alliance wall behind the yellow totes as part of our autonomous, which required the strafing motion. But even with that in mind, we didn't even opt to use mecanum first.

Our first drive system iteration was an H drive with 4 omnis on the outside and one perpendicular omni in the middle. We wanted H drive over mecanum first, because we didn't want to bother with programming mecanum when H drive was a simpler option, and furthermore, H drive strafing is not as dependent on mass distribution as mecanum, if at all (as far as I know, correct me if I am wrong please). We knew that grabbing totes and making stacks at the forward area of the robot would drastically change the distribution, so H drive seemed to be the smarter option.

Unfortunately, the center wheel of our H drive would cause the robot to lose contact with some of the floor when driving up the scoring platform. At this point of failure in our prototyping, we settled for mecanum, and I say "settled" for a reason. Through all our days of practice and competition, the strafing abilities were hampered due to poor distribution of mass, especially with the addition of totes in a held stack. Because of this, our intial idea of a strafing wall-riding autonomous was given up for simpler options.

As Niklas mentioned, we barely strafed at all and drove the robot more like a tank, and I feel like we should have just designed for tank. After all, the only teams who were major scorers that beat us in all three of our regionals in finals did not have mecanum (118: 2 tread 2 omni tank, 254: 6 tread tank, 624: 5 omni H, 1678: 6 tread 2 omni tank) and two of them were the world champs, 1678 and 118, both of which we competed against at two separate competitions in the finals. I think I can chalk up our losses to these teams partly to a lack of proper control due to an improperly implemented mecanum drive.

The lesson I got from this was that even in a game where mecanum may seem like a good option due to the game design (no opponent pushing and lots of opportunity for fine control), it still might not be that great of a choice, so be careful with that decision. Some of the best teams use some form of tank drivein every game without fail, so I like to go their directions.

[ImMoMo]

Originally Posted by Caleb Sykes

20fps is pretty fast, unless you have a shifter, consider choosing a slower top speed in order to get higher acceleration.

I agree, we've ordered two gearing options.. 15 ft/s and 20 ft/s... I don't think we'll be seeing 20 ft/s being super useful..

I do have a question, we're using a single reduction clamp gearbox overall 3.8:1 ratio, and the torque load seems to be about 2.8... while each CIM has a stall torque of 2.22--- I believe this is an issue... but also I'm not super familiar with stall torque... is it correlated to the torque load?

Originally Posted by Electronica1

If you can't do swerve, octocanum, nonadrive or butterfly drive, then you should stick with skid steer and focus on compensating for the less maneuverability.

For the most part I agree with you.

One of the discussions we had while choosing our d-train was in regards to the precision required to place the gear.

I believe that butterfly or octanum would be ideal for this game, and it's relatively simple w/ COTS from VEX or very simple machining with a mill.

One of the counterpoints our students presented for defense was that, even if you have a tank drive and you're trying to place a gear, an opposing bot can knock you off of alignment as well. I think their reasoning to use mecanum wheels was to more easily avoid robots, and quickly align.

[ns3517]

I would say tank because the cons of mecanum out weigh the benefits

[sonichammer7476]

It all depends on what strategy you are going to go for of course, but if you are going to play any defense or fight for balls in the neutral zone, I highly recommend tank over mecanum. 99% of the time, you will get tossed around like a rag doll when facing non-mecanum robots. No traction. But as one user suggested earlier, if you are very familiar with mecanum and not familiar at all with tank, or your driver is skilled enough to handle the challenges of mecanums in this game, then go with mecanum. Otherwise, I say tank.

[chandrew]

Originally Posted by Tom Ore

We like mecanum wheels - if it's feels right for the game we'll use them. We have a total of 12 regional wins and 9 of the wins were with mecanum wheels.

+1
Our best regional year was 2014 with a mecanum

[MrForbes]

heh...our best regional year was 2009, with those hard plastic wheels.

Anecdotes aren't real helpful, some times.

[Max Boord]

Originally Posted by Kevin Sevcik

I'm curious if butterflies and octocanums collect alliance selection dings like straight mecanum. Or is it not relevant because teams willing/capable of these drives are usually pickers?

H drive and mecanum are treated worse than any other drivetrain (automatic DNP list most of the time). Buttterfly, octocanum and swerve are a little more complicated. Basically if I wasn't confident that they could fix it if it somehow broke, they would get lumped into the mecanum drive pile. If I was confident that the team could fix it if it broke, I would still put them below tank due to the inherent reliability issues but it would not completely prevent a team from being picked.

[MikePenn]

Originally Posted by MrForbes

heh...our best regional year was 2009, with those hard plastic wheels.

Anecdotes aren't real helpful, some times.

You could say last year was our best year with the plastic wheels

[chandrew]

Originally Posted by MrForbes

heh...our best regional year was 2009, with those hard plastic wheels.

Anecdotes aren't real helpful, some times.

I should clarify my post. The maneuverability of the mecanum drive allowed us significant advantages for alignment and evading defense, which allowed us to play at a higher level than we could have with a tank drive. The only issue we had with it was that the plastic rollers consistently liked to break off.

It likely is not the right drive train if you are trying to win Einstein, but it can perform very well at a regional level with good driver practice.

[BotDesigner]

Originally Posted by MrForbes

heh...our best regional year was 2009, with those hard plastic wheels.

Anecdotes aren't real helpful, some times.

2014 is considered one of the most defensive heavy games in FIRST history. If teams found a way to get them to work that year, I am sure you could find a way to get them to work well this year.

[JohnFogarty]

Originally Posted by BotDesigner

2014 is considered one of the most defensive heavy games in FIRST history. If teams found a way to get them to work that year, I am sure you could find a way to get them to work well this year.

1102 who I've returned to working with this year used mechanum in 2014, seeded 3rd at their regional that year. The thing that held them back from winning their event that year was not their drive train I assure you.

I'm going to be doing some 6" 6WD 4 motor tank vs 6" 4 motor mechanum push tests this weekend. I hope I can bring back some useful information to settle this debate in some way.

[Taylor]

Originally Posted by JohnFogarty

Ill go ahead and say it. 1102 who I've returned to working with this year used mechanum in 2014, seeded 3rd at their regional that year. The thing that held them back from winning their event that year was not their drive train I assure you.

I'm going to be doing some 6" 6WD 4 motor tank vs 6" 4 motor mechanum push tests this weekend. I hope I can bring back some useful information to settle this debate in some way.

While your initiative is appreciated, and the results sure to be interesting, I think that sort of misses the point of mecanums.

Originally Posted by chandrew

The maneuverability of the mecanum drive allowed us significant advantages for alignment and evading defense, which allowed us to play at a higher level than we could have with a tank drive.

[Kevin Sevcik]

Originally Posted by ImMoMo

I do have a question, we're using a single reduction clamp gearbox overall 3.8:1 ratio, and the torque load seems to be about 2.8... while each CIM has a stall torque of 2.22--- I believe this is an issue... but also I'm not super familiar with stall torque... is it correlated to the torque load?

Explain what you mean by torque load and how you're determining it. Are you talking about the required torque to overcome the friction of the wheels?