2015: Year of the Mecanum

[Kevin Sevcik]

Originally Posted by StephenNutt

You can set it up with any aspect ratio, square, long or short (within reason)

However, the farther you get from a square configuration, the more scrub you'll get when turning in place. Also, not square configs mean the wrong mecanum layout will make your robot turn backwards. Heh.

[Pollux]

There's nothing in the manual about not driving over the scoring platforms. In fact, it makes sense that many robots would have to drive on them in order to deposit their payloads.

[Pollux]

Originally Posted by Ether

Has anyone looked at the field drawings yet? In the game video it looks like the platforms aren't that high, and there are ramps all around. How high is the platform, and what is the grade of the ramps?

The platforms are two inches high and the ramps are at 16 degrees, giving robots seven inches horizontally to move two vertically.

[alicen]

Originally Posted by Kevin Sevcik

You see those bumps called the scoring platforms? Those are going to going to cause all sorts of headaches for mecanums. 5-wheel strafe drive with the strafe wheel on a pneumatic suspension should work well, I think.

Whenever we did mecanums, we always had suspension on each wheel so that all wheels were always in contact with the floor. They even went over the humps in Breakaway just fine!

[hrench]

Originally Posted by alicen

Whenever we did mecanums, we always had suspension on each wheel so that all wheels were always in contact with the floor. They even went over the humps in Breakaway just fine!

We did mecanums for logo motion with a flat-floor and they worked fine, but for rebound rumble, trying to climb the 'bridge' in the middle they were really problematic, even with suspension.

Remember that with mecanums, the left wheel is always pushing against the right wheel to make it go straight. So if one wheel gets off of the ground, your robot turns by itself.

Methinks that with a stack of totes on a forklift, the back-two wheels will be bumping off the ground regularly, and if you try to cross a platform at any kind of angle other than 90, you'll lift a wheel.

Last year I saw four wheel mecanums in the IAM14U frame, and maybe more often in 'butterfly' and 'octanum' drives. That that was acompletely flat floor. My recommendation is that if your heart is set on mecanums, use a butterfly or octo so you can put regular wheels on the floor when you need to. Also, note, the new frames are much more stiff than the old channel sections ones, so you'll need to do Something to keep those wheels on the ground. Something like independent suspension.

Mecanums work really well with Banebot transmissions too.

Last thing. Mecanums running over a noodle a problem? I think so . You tell me?

[holygrail]

Originally Posted by hrench

Last thing. Mecanums running over a noodle a problem? I think so . You tell me?

We have 6" VexPro mecanums on our 2013 robot and we drove it over several noodles with no problem. There is a bit of bumpiness when you go over, so you might want to either stabilize the stack or just drive around the noodles when loaded, but that is true with any wheels unless you have suspension of some sort.

[who716]

Originally Posted by teafreak45

I would agree with you on the drive. While watching the video, I was thinking that would be the most used and best drive train for this year.

not necessarily mecanum but a maneuverable chassis over a power chassie we would rather have a drop center then mecanum

[The Doctor]

Originally Posted by TheJoe

Could it be possible that mecanum drivetrains are the ideal way to go because of their maneuverability?

MY THOUGHTS EXACTLY!!

[alopex_rex]

Originally Posted by TheJoe

Could it be possible that mecanum drivetrains are the ideal way to go because of their maneuverability?

We thought the exact same thing, after spending all of our off-season saying "well we're almost certainly doing tank drive, but if the game really wants something else..."

Our only experience with mecanum is with an off-season project, which had trouble going straight, we think because of wonky weight distribution. The programming was easy though: WPILib has a straightforward mecanum-drive function in the DriveTrain class.

[Ether]

Originally Posted by alopex_rex

Our only experience with mecanum is with an off-season project, which had trouble going straight, we think because of wonky weight distribution.

Weight distribution is only one of many factors which can cause unsatisfactory mecanum performance.

Here are some others:

• too much roller axial free play
  
  
• too much roller spinning friction
  
  
• wheel alignment (toe-in¹, camber¹, and axial offset²)
  
  
• excessive gearbox friction (improper assembly and/or lubrication)
  
  
• chain (or belt) too tight or not properly lubricated
  
  
• warped frame
  
  
• frame too rigid
  
  
• motor manufacturing tolerances
  
  
• motor controller calibration

¹Toe-in and/or camber of a wheel causes the wheel sprocket (or pulley) to be non-coplanar with the driving sprocket (or pulley), and thus contributes to friction between the chain (or belt) and the sprocket (or pulley). Toe-in also causes scrubbing friction with the floor surface.

²"wheel axial offset" in this context means that the wheel sprocket (or pulley) is axially offset from the plane of the driving sprocket (or pulley), causing the chain (or belt) to be non-coplanar, thus creating additional friction between the chain (or belt) and the sprockets (or pulleys).

[alopex_rex]

Originally Posted by Ether

Weight distribution is only one of many factors which can cause unsatisfactory mecanum performance.

Here are some others:
...

Thanks! This looks very helpful...and is also pretty intimidating, although I imagine a lot of those factors apply to any drive train one might make.

[oison75]

Our team is also considering using a mecanum drive train. We have never used mecanum before, but we firgured we have lots of resources to lean on and using mecanums will be very adventageous. Any suggestions for drive train layout, gearboxes, motors, bearings, etc? Also where're the best places to purchase mecanum wheels, I've only seen the Andy Mark ones and Vex ones... are those good? I'm extremely excited to start playing with a mecanum robot, but an also very nervous since our team is new to mecanum... Any suggestions?

[mfalk]

We used mecanum on our Rebound Rumble robot and had only slight problems getting on the bridges and that angle was much greater than 16-degrees. My thought is that robot drivers may underestimate their center of gravity when carrying loads up high and attempting to drive over scoring zones.

[Robo Hamsters]

Originally Posted by TheJoe

we don't need the large, defensive powerhouse that FRC generally sees.

This is actually our dilemma as our first 2 years we built extremely strong defensive drive bases first and then concentrated on offensive capabilities.

So we are now working on our strategy to switch to an all offensive bot.

Good luck everyone!