Mecanum wheels

[iceBird10]

Originally Posted by heisenburger

Hi, I was just wondering what is the purpose of the mecanum wheels, how do they work and are they worth the $400 for the set of 4, also if the sell them somewhere else cheaper than that could you give me the link?

My team has used Mecanum since before I joined FIRST, and they are undoubtedly (I think) worth the money. They offer superior maneuverability, specifically in their ability to "track" side-to-side. Their only downside is lack of traction on smooth surfaces (i.e. bridge from 2012) and less stability when pushed. Given this year's game, they would be a great option if you can afford them.

[JesseK]

Originally Posted by Ether

What driver interface do you have in mind for this? Perhaps divide this 2nd mode into 2 modes?

Mode 2a: Standard 2-joystick Y-axis-only tank drive (X-axes disabled)

Mode 2b: Right (or left) X-axis commands strafe (disable Y-axes)

Unsure, but I think you're on the right track. We're fairly certain which students will drive the robot, so it'll be whatever they're comfortable with. Our most likely candidate is very comfortable with traditional tank controls (and is very good with them), so we'd want to stick with that and create a natural strafe input. Notionally we could implement the x-axis on one or both joysticks with a very large (> 0.33) deadband for strafe detection. When strafing it would ignore the y-axis input.

[bEdhEd]

Originally Posted by Ether

There is no "h" in mecanum.

Haha my bad, there are days when I spell it mecanum, and days when I spell it mechanum. I wrote that post at like 5 in the morning in my time zone. Gimmie a break with the proofreading!!

I'll be sure to edit that, though. You won't see a single extra "h" in my post when I'm done. Thanks for the correction!

[Ether]

Originally Posted by JesseK

Notionally we could implement the x-axis on one or both joysticks with a very large (> 0.33) deadband for strafe detection. When strafing it would ignore the y-axis input.

Yeah, I thought of that, but then figured the large deadband would make it quite awkward for the driver to do fine-trim left/right strafing when, for example, trying to position the bot for a catch.

With the 2a/2b mode, just hold a button and do your strafing with minimal deadband.

[lcoreyl]

Originally Posted by thefro526

There's a bit of mis-information in this thread

yes... yes there is...

Originally Posted by thefro526

Mecanum Drive will almost always have less traction than the traditional drive train. This is due in part to a couple of factors, the wheels always being in a constant state of slip (in that they're applying force in a non-fore/aft direction) being one of them

I've linked a paper in my signature. In the paper is a link to a video showing the worms eye view of the contact patch of a mecanum wheel while being moved as it would in normal forward motion. The contact point doesn't slip. The rollers don't spin.

[lcoreyl]

Originally Posted by bEdhEd

With mecanum wheels you can lose up to 50% of your power

Originally Posted by JesseK

the default field-centric (or robot-centric with no deadband) control algorithm robs the drive train of power just by the nature of the code.

While as quoted these statements can both be true, I think it can be mis-interpreted as mecanum has less power than 4WD or 6WD. Driving straight forward or backward there is no power loss in mecanum. In any other situation where mecanum has power loss we are talking about motion that 4WD or 6WD can't do at all, so power loss does not say that 4WD or 6WD is therefore a better choice. (alternatively, I could say 6WD has 100% power loss while moving at a 45 degree angle while mecanum only has a 50% power loss) As long as your driver realizes driving all the way down the field with the robot at a 45 degree angle relative to the field would not be a wise use of mecanum, then power loss is not a real concern when deciding mecanum vs. 4WD/6WD.

[Ideal_Nerd]

last year our team loved our mecanum wheels. sadly we where borrowing them. this year we are buying them from vex. they are a lot cheaper with similar performance.
http://www.vexrobotics.com/mecanum-wheels.html

[Donut]

Originally Posted by BEN35678

What are the advantages of using the vex pro mecanums then why not just go with the AndyMark then?

The VexPro mecanums are cheaper, come pre-assembled (rollers already attached, wheel already together), and are lighter. The last factor is the most significant for me, depending on the size of the wheel you use there can be a 2-4 lb weight difference by going with a set of VexPro over Andymark mecanum wheels. The Andymark mecanum wheels are good but outside of multiple-competition durability I don't know how they are better than the VexPro ones.

Originally Posted by JesseK

Can anyone speak to the wear of VEXPro 4" Mecanum rollers compared to the 6" wheel rollers over the course of multiple competitions?

In an average year we compete in 2-3 official and 2 off-season competitions, perform in several demos & run a lot of driver practice for the next year's team. So the wheels really need to last. Yet it'd be nice to know what spare parts to get ahead of time (i.e. 1 set of spare rollers per 1 event or 2 events?) when comparing wheels. If we have to replace the 4" wheel's rollers every 1 event, we may go the 6" route so we don't have rollers that are wearing thin at the end of an event.

The 4" VexPro Mecanum wheels are a new offering this season, last year they only made 6" and 8" wheels.

[cmrnpizzo14]

Originally Posted by BEN35678

What are the advantages of using the vex pro mecanums then why not just go with the AndyMark then?

Vexpro wheels are less than half the price of the AndyMark ones.

http://www.andymark.com/product-p/am-0732.htm

http://www.vexrobotics.com/mecanum-wheels.html

[Qcom]

Originally Posted by cmrnpizzo14

Vexpro wheels are less than half the price of the AndyMark ones.

http://www.andymark.com/product-p/am-0732.htm

http://www.vexrobotics.com/mecanum-wheels.html

Question: You mentioned the Vex Mechanums cracking in an earlier post. Where did they break, and was the break catestrophic enough to warrant immediate disabling of robot movement (i.e. could the robot finish a match if this occured)?

[bEdhEd]

Originally Posted by lcoreyl

While as quoted these statements can both be true, I think it can be mis-interpreted as mecanum has less power than 4WD or 6WD. Driving straight forward or backward there is no power loss in mecanum. In any other situation where mecanum has power loss we are talking about motion that 4WD or 6WD can't do at all, so power loss does not say that 4WD or 6WD is therefore a better choice. (alternatively, I could say 6WD has 100% power loss while moving at a 45 degree angle while mecanum only has a 50% power loss) As long as your driver realizes driving all the way down the field with the robot at a 45 degree angle relative to the field would not be a wise use of mecanum, then power loss is not a real concern when deciding mecanum vs. 4WD/6WD.

I said you can lose up to 50% of power with moves like strafing. I didn't say that you can lose 50% from forward or backwards driving, and you can lose somewhere between >0% to 50% power when strafing diagonally, which is a combination of the forward/back movement and sideways strafing. There is no significant difference in power between forward/back mecanum driving and forward/back tread driving when using the same power source and transmission.

It may be important to be aware that drivers may not have success moving others robots out of the way by turning or strafing, but instead can have success by pushing straight on.

[JesseK]

Originally Posted by lcoreyl

While as quoted these statements can both be true...

Please don't mis-quote me and then poorly attempt to make a counter-point which actually says something completely tangential to what the entire quote said.

[cmrnpizzo14]

Originally Posted by Qcom

Question: You mentioned the Vex Mechanums cracking in an earlier post. Where did they break, and was the break catestrophic enough to warrant immediate disabling of robot movement (i.e. could the robot finish a match if this occured)?

I personally did not mention it but our team has used mecanum enough for me to answer. Usually it is the roller that cracks I believe, we never have had serious issues with this. Barring a catastrophe of some sort, you can still finish the match driving almost normally. Depending on the severity of the crack you might notice it in your robots driving.

You can purchase additional rollers online to replace old/cracked ones. We did this to all of our rollers to retrofit old mecanum wheels and make them sort of new again before last season. If you check the wheels regularly and make sure you have spares on hand you will be fine.

[thefro526]

Originally Posted by lcoreyl

yes... yes there is...



I've linked a paper in my signature. In the paper is a link to a video showing the worms eye view of the contact patch of a mecanum wheel while being moved as it would in normal forward motion. The contact point doesn't slip. The rollers don't spin.

I don't want to debate this too much since it's going to derail the thread, but I'm still not convinced based on the two videos (Glass table, green backlighting) that the rollers do not spin under FRC conditions. Maybe I'm nit picking, but there seemed to be some sort of displacement of the roller (Looks like a few degrees at most) even on the table top test. At FRC speeds, something tells me that this has to add up into the rollers spinning while the robot is moving in standard fore and aft directions. It's been a few years since I've played with Mecanum wheels first hand, but if I remember correctly, rollered seemed to slip while under load.... But I might be wrong.

[Ether]

Originally Posted by thefro526

I'm still not convinced based on the two videos (Glass table, green backlighting) that the rollers do not spin under FRC conditions.

They do spin somewhat under FRC conditions. The roller axial free play in the affordable COTS mec wheels typically used for FRC contributes to spinning. A compliant surface (like carpet) contributes to spinning. This is mentioned on page5 of his paper.