2015: Year of the Mecanum

[Ether]

Originally Posted by Jalerre

For those talking about driving over the bumps, I do believe that is prohibited.

The game reveal video at 1:13 shows a bot driving over the platform.

[jtrv]

Originally Posted by Ether

The game reveal video at 1:13 shows a bot driving over the platform.

He might have been talking about the middle platform, but yes, the scoring platforms can be crossed.

[Ether]

Originally Posted by jtrv

He might have been talking about the middle platform

In the given context, that wouldn't make sense.

Also note that the middle is called a step, not a platform.

[theCADguy]

Originally Posted by Jalerre

For those talking about driving over the bumps, I do believe that is prohibited.

I believe that you are thinking of the step in the middle. You can traverse the scoring zones, however.

Also, quick question:
Can a mechanum drive be set up where the length of the drive train is greater than the width, or does it require a square wheel configuration?

[StephenNutt]

Originally Posted by theCADguy

Also, quick question:
Can a mechanum drive be set up where the length of the drive train is greater than the width, or does it require a square configuration?

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

[Kevin Sevcik]

Originally Posted by ratdude747

Unless you're talking offseason prototypes, I highly doubt that *cough *

I've been doing this too long and I'm starting to get confused. 2010 and 2011 is what I meant to say there.

[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

[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.

[lark95]

My team is also thinking of using mecnum drive. we even have an old robot from logamotion that used mecnum we are able to drive

[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.

[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).

[Ether]

Originally Posted by Kevin Sevcik

However, the farther you get from a square configuration, the more scrub you'll get when turning in place.

For a mec, the inverse kinematics are the same¹ as long as it's rectangular.

¹ignoring roller friction, axial free play, and carpet compliance

[Jalerre]

Originally Posted by Ether

The game reveal video at 1:13 shows a bot driving over the platform.

I didn't see it in the manual anywhere that's just the conclusion that our team had come to. (I didn't try to confirm it I just trusted them)

[craigboez]

If you have 3 rollers on the carpet and one on the hdpe, does the coefficient of friction come into play? I assume as long as you don't break traction on any of the wheels you can effectively transmit force vectors to the ground?

[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.