[FTC]: Mecanum wheels or omni wheels?

[FRC Team CC]

Hey Chief Delphi Forum,

We are the Charging Champions, a six-member rookie (FTC) team from SoCal. Being quite unfamiliar with the TETRIX kit components, we were contemplating on whether to buy eight 4-inch omni wheels (with two wheels at each corner) or four 4-inch mecanum wheels. Can you please tell us the advantages and disadvantages of both combinations, and which one worked better in previous years? If you used another combination that worked, can you please let us know? All help is appreciated. GO FTC TEAMS!!!

Thanks,
The Charging Champions

[wgardner]

You might want to check out the Drive Train references in the middle of this page.
http://cheer4ftc.blogspot.com/p/drive-trains.html

Particularly this one.
https://docs.google.com/file/d/0B0zY...XpyN1U/preview

They're written for FRC but are very applicable for FTC too, particularly the advice about going for a simple design when you're rookies. It's very easy for rookies to underestimate how hard it is to build and program a complicated FTC robot and end up with a half finished robot that's never been tested or driven in practice before a tournament.

Good luck!

[pyroslev]

Keep the risk of static electricity in mind. Wheels in general on robots are Vann Der Graff Generators with the frames storing charges like Leyden Jars.

Omni Wheels: It is a wheel with 10 more wheels on it. This adds up to 11 VDG Generators. Add two Omni Wheels and you get 21 VDG Generators. Multiply that doubled up Omni Wheels by four, one on each corner, and you get get 84 VDG Generators. Boggling risk for static.

Mecanum Wheels: There is still a risk as there is with the Omni Wheels. There are two Mecanum styles I've seen, Stationary such as this http://cdn3.volusion.com/vyfsn.knvgw...jpg?1410248327 and non-fixed rollers http://cdn3.volusion.com/vyfsn.knvgw...jpg?1410248327 Stationary rollers are lower static risk and non-fixed are higher risk.

All of this is based on what I've seen. More wheels/rollers, greater static risk. That's my 2 Cents.

[Ether]

Originally Posted by pyroslev

There are two Mecanum styles I've seen, Stationary such as this http://cdn3.volusion.com/vyfsn.knvgw...jpg?1410248327 and non-fixed rollers http://cdn3.volusion.com/vyfsn.knvgw...jpg?1410248327 Stationary rollers are lower static risk and non-fixed are higher risk.

What do you mean by "stationary" and "non-fixed" rollers?

[pyroslev]

Originally Posted by Ether

What do you mean by "stationary" and "non-fixed" rollers?

The stationary ones have the central hub that rollers (Which don't rotate) are mounted on. I've seen this style more for FTC than other. The other style have the rollers mounted between two plates. I see this more in FRC

I never learned the proper engineering name for the two styles of Mecanums.

[Ether]

Originally Posted by pyroslev

The stationary ones have the central hub that rollers (Which don't rotate)

What do you mean "rollers (Which don't rotate)" ??

[pyroslev]

Stationary Rollers. Never really seen these rollers move except when they had come loose.
http://www.vexrobotics.com/vexpro/wh...ller-kits.html

Non-Stationary Rollers. The wheels between the plates can move rotate. It's this style of mecanum wheel that still has the same Vann Der Graff Generator potential as Omni Wheels.
http://www.andymark.com/PhotoDetails...ctCode=am-0081

[Andrew Schreiber]

Originally Posted by pyroslev

Stationary Rollers. Never really seen these rollers move except when they had come loose.
http://www.vexrobotics.com/vexpro/wh...ller-kits.html

Non-Stationary Rollers. The wheels between the plates can move rotate. It's this style of mecanum wheel that still has the same Vann Der Graff Generator potential as Omni Wheels.
http://www.andymark.com/PhotoDetails...ctCode=am-0081

The ones you've labeled "stationary" do indeed move. Source - They were on our intake last year.

[pyroslev]

I've not personally used Mecanums on a robot since FRC 2008 and they were the AndyMark ones. Thanks for the illumination on that one.

Back to the topic on hand. I still prefer either style of Mecanum. Better maneuverability until someone crafts a practical FTC swerve drive. Easy enough to code given the abundance of use. No more static risk than Omni Wheels.

If I had a penny for every robot with Omnis that shocked me as an FTA compared to ones that use Mecanums, one change barrel would certainly be far more full.

[Ether]

Originally Posted by pyroslev

Stationary Rollers. Never really seen these rollers move except when they had come loose... Non-Stationary Rollers. The wheels between the plates can move rotate.

Both designs have rollers that are free to rotate.

Even though they are free to rotate, mecanum rollers don't roll much (theoretically not at all) when driving straight forward or backward on a non-compliant surface. This is true for any mecanum design.

It's this style of mecanum wheel that still has the same Vann Der Graff Generator potential as Omni Wheels.

What reasoning (or experience) convinces you that mecanum wheels act as Van de Graaff generators?

[pyroslev]

Originally Posted by Ether

What reasoning (or experience) convinces you that mecanum wheels act as Van de Graaff generators?

A school with multiple teams all use the same core design for the robots over the years. The chassis was same with a few differences in manipulators and their wheel choice. I've noticed the team with 'standard' doubled up omni wheels discharged with greater frequency compared to their Mecanum counterpart. (Notice being a relative term for "That one shocked me more") No voltage data or hard count of how many times it happened but it happened enough I was wary of touching the Omni Wheeled Robot more than the Mecanum.

For that reason I'm thankful the use of Anti-Static sprays is allowed this year.

[FRC Team CC]

Here is the link to the four inch mecanum wheels (Tetrix - Andymark):

http://www.andymark.com/mobile/Product.aspx?id=39589

Link to omni wheel:
http://www.andymark.com/mobile/Product.aspx?id=39507

[RRLedford]

We have transitioned through several iterations of of non-tank drives over last four FTC seasons - 4-wheel omni, Kiwi omni and mechanum wheels starting last year and continuing this year.
The mechanum approach has clearly produced the best results.

2014/2015 - VEX wheels again; tested AndyMark and not as good, though easier to attach sprockets onto; gyro control now implemented & more precise autonomous; speed also doubled;
Our first competition is 12-6-15 in Peoria, IL
We are currently focused on autonomous scoring and endgame. We will have three dedicated swing arms; for each goal height; and then, based on partner capabilities, load the best two arms with our two balls. we then select which autonomous program to used, based on our target goals. The mecanum drive with gyro stabilization allows for reasonably fast and complex yet accurate paths
We plan to grab one goal, score large ball in it, then score 2nd ball in closest adjacent goal (moving laterally if needed). and possibly grabbing it too on the way to placing them both into the auto scoring zone.
At end game we will make sure all three goals along with our bot are all up on ramp.
PICs





FTC 2013/14 - Best mecanum drive in state of IL at finals but with no gyro control driving straight was still a problem. Hybrid of Tetrix bearing modules, VEX mecanum wheels on Bosch-Rexroth (or 80/20) slotted frame was super reliable and rugged; Hardened 10-32 SHCS axles (non-driven & polished going through the bronze bearings) are the best.
Chain drive ALWAYS or us.



FTC 2012/13 - Heavy Kiwi bot needed three wheels per axle. with just two, they sunk too deeply into foam surface; still drove poorly (erratically) with no gyro control; rear wheel hooked the plywood scoring platform


-Dick Ledford

[DavisDad]

We used mecanum wheels last year and were very happy with the performance:

https://www.youtube.com/watch?v=UOgjS3hmQHI

https://www.youtube.com/watch?v=QL6r...ZeTfjZhZOdMzgw