Mecanum Wheels - How do they work?

[Team3763 Adam]

My team and I are looking at using Mecanum wheels for this year's competition as a means of strafing. My few questions are:

• Could they be our only form of Drive wheels for forward and side movement, or would it be best to pair them with another set of wheels to allow for seperate forward and side movements?

• How do they work?

• How should they be mounted on a Robot Chassis. IE should the diagonal wheels be slanted in opposing directions - either away from the center of the robot or towards. ///-CenterOfRobot-\\\ OR \\\-COR-///

• What form of drive in LabVIEW would they be programmed with? Arcade but with different inputs on the joysticks? Or...

Any and all diagrams/pictures/screenshots of LabVIEW are welcome!

[Chris_Ely]

Mecanum wheels have rollers positioned at 45 degrees from the axis of rotation. This transfers the force from the rotation of the wheel by the motor 45 degrees from the axis of rotation. When you power each wheel independently and vary the speed and direction the wheels rotate, you can achieve omnidirectional motion. So, to answer you first question, you need 4 wheels (2 left, 2 right) to drive forward, backward, sideways, and anything in between. The rollers should be oriented such that they form an X when you look from above. If you search "mecanum" in the white papers section, you will find more in depth explanations and example code.

[Team3763 Adam]

What is the "White Papers Section," and with that answer arose another question - do each of the four wheels need to have their own motor/gearbox config.?

[Orion.DeYoe]

Originally Posted by Team3763 Adam

My team and I are looking at using Mecanum wheels for this year's competition as a means of strafing. My few questions are:

• Could they be our only form of Drive wheels for forward and side movement, or would it be best to pair them with another set of wheels to allow for seperate forward and side movements?

• How do they work?

• How should they be mounted on a Robot Chassis. IE should the diagonal wheels be slanted in opposing directions - either away from the center of the robot or towards. ///-CenterOfRobot-\\\ OR \\\-COR-///

• What form of drive in LabVIEW would they be programmed with? Arcade but with different inputs on the joysticks? Or...

Any and all diagrams/pictures/screenshots of LabVIEW are welcome!

When using mecanum wheels you need to put one on each corner with the axles parallel to the front bar of your frame (i.e. don't angle the wheels at all). Off the top of my head the rollers on the wheels should make an "x" pattern when viewing them from the top of the drivetrain. Check the documentation on www.andymark.com or www.vexrobotics.com/vexpro to be sure.
There is a good whitepaper here on Chief Delphi describing some of the physics concepts behind the wheel and clearing away some of the misconceptions about the wheels. It was written based off some of Ether's posts but I don't remember who it's by (can someone post a link to this whitepaper if you know which one I'm referring to?).
As for programming, there should be a "holonomic drive" VI in the WPI library VIs section that does the math for mecanum automatically. You have one joystick axis for forward and backward movement, one for strafing, and one for rotation (much like arcade).

[Team3763 Adam]

Originally Posted by Orion.DeYoe

As for programming, there should be a "holonomic drive" VI in the WPI library VIs section that does the math for mecanum automatically. You have one joystick axis for forward and backward movement, one for strafing, and one for rotation (much like arcade).

This shouldn't be too hard to figure out, then, and I will be looking in LabVIEW Friday.


Also to sketch a rough text-based diagram:
Where "///" represents the 45* rollers of the Mec Wheels.
And "|" "-" represent the Robot's Frame.

|---|------------|---|
|\\\|...............|///|
|....|...............|....|
|....|...............|....|
|....|...............|....|
|///|...............|\\\|
|---|------------|---|

[Chris_Ely]

What is the "White Papers Section," and with that answer arose another question - do each of the four wheels need to have their own motor/gearbox config.?

The white papers section is located in CD-media, above the orange bar at the top of the page. Because each wheel needs to rotate independently, each wheel must have its own motor and gearbox.

[Chris_Ely]

Originally Posted by Orion.DeYoe

There is a good whitepaper here on Chief Delphi describing some of the physics concepts behind the wheel and clearing away some of the misconceptions about the wheels. It was written based off some of Ether's posts but I don't remember who it's by (can someone post a link to this whitepaper if you know which one I'm referring to?).

Mecanum: we can't even agree on pronunciation?

[Team3763 Adam]

Originally Posted by luckof13

...each wheel must have its own motor and gearbox.

Ah there's the complications I was looking for! It's never that easy! Only kidding, but thank you for the help guys, with this knowledge I should be able to figure out everything I need.

[efoote868]

You'll also want some form of suspension or flex in your frame to keep each wheel in contact with the ground (at a minimum), you'll do best with an equal weight distribution on your wheels, and the wheels should be in a square.

Best of luck.

[Ether]

Originally Posted by efoote868

...and the wheels should be in a square

Why?

[Chris_Ely]

Originally Posted by efoote868

You'll also want some form of suspension or flex in your frame to keep each wheel in contact with the ground (at a minimum), you'll do best with an equal weight distribution on your wheels, and the wheels should be in a square.

You don't need suspension. My team ran mecanum in 2011, which had a flat floor like this year, just fine. Our wheels were also in a rectangle orientation.

[Ether]

Originally Posted by luckof13

You don't need suspension.

He didn't say you need compression. He said you'll want compression or frame flex.

[efoote868]

Originally Posted by Ether

Why?

In my experience, it made it easier to rotate while driving in a straight line. Rectangles will do OK; the square part isn't as critical as keeping all wheels on the ground.

FIRST may try their best to keep the carpet flat, but every year there seems to be bumps and places to get hung up on. The kinematics aren't easy to resolve when only 3 of the 4 wheels touch the ground.

[Ether]

Originally Posted by efoote868

In my experience, it made it easier to rotate while driving in a straight line.

A more likely cause is that the rectangular bot had poorer weight distribution or the frame wasn't flexible enough to keep traction on all 4 wheels or the rollers were binding. There's no kinematic reason why mec needs to be square in order to rotate properly.

[AGPapa]

Originally Posted by Team3763 Adam

What form of drive in LabVIEW would they be programmed with? Arcade but with different inputs on the joysticks? Or...[/list]
Any and all diagrams/pictures/screenshots of LabVIEW are welcome!

Programming mecanum in LabVIEW is very easy. You'll want to use Holonomic Drive, not Arcade.

Holonomic drive has two options, Polar and Cartesian. In Polar you specify a direction, magnitude and rotation. In Cartesian you specify x, y, and rotation. Personally, I like Cartesian better than Polar because it is more similar to a traditional arcade drive. Try both and see which you like better.

The code I've attached shows how our team used mecanum in 2011. The left joystick controls forward-back and strafing. The right joystick does turning.