Problems with Vex Pro mecanum wheels
 

This year we chose to use the vex pro mecanum wheels (http://www.vexrobotics.com/vexpro/wh...m-wheels.html), but are disappointed with their performance. They travel forward backwards and rotate alright, but they strife VERY slowly. I understand moving sideways is slower, but this is moving maybe 40% of forwards speed. They are mounted properly ("X" on top) and are run by one CIM motor each, with a 9.87:1 gear ratio.

Does anyone know what might be the problem? On a side note, our practice robot has "homemade" mecanum wheels from a previous year, and it works well, so I assume the code is fine.

Re: Problems with Vex Pro mecanum wheels
 

Are the rollers completely free to rotate, or are they a little "tight"? The roller friction is the big variable in mecanum wheel performance.

Re: Problems with Vex Pro mecanum wheels
 

They are loose.

Re: Problems with Vex Pro mecanum wheels
 

We used them this year and were very satisfied. Of course it will be slower sideways, but I don't recall exactly how slow it is for us. (We also used 1 cim to each wheel, but I don't remember the gear ratio)

Re: Problems with Vex Pro mecanum wheels
 

The robot with our homemade wheels is also certainly noticeably slower sideways vs. forwards/backwards, but as slow as the vex wheels. I can't remember how the andymark wheels performed exactly, but I do recall they were better than these vex wheels.

http://www.youtube.com/watch?v=W0XN6iSvCXo

^This is the kind of speed I am hoping for...

Re: Problems with Vex Pro mecanum wheels
 

While moving forward/back, your torque is the same as a normal wheel.

While strafing, your sideways torque is 1/SQRT(2) (~70%) of your normal torque since the wheels are driving at each other BEFORE the extra losses of friction in the rollers.

While on a diagonal, your acceleration is somewhere between 35-50% of your normal forwards acceleration since you're only spinning half of the motors and also half of your your robot is (essentially) on caster wheels.

Translating torque into acceleration has a lot to do with roller friction and robot weight. Additionally, unless there are specific dead zones for sideways & forward, the code is robbing the robot of torque needed to strafe at reasonable speeds (since no joystick is ever perfectly sideways for long).

Re: Problems with Vex Pro mecanum wheels
 

Our robot did not strafe as fast as the robot in that video, however wheel speeds will be different as well. We used the AndyMark wheels for a while before switching to the VexPro wheels. (Funny story, we changed them out Friday night at a competition, some of our other team members thought we were crazy... rightly so.) We like the VexPro wheels more than the AndyMark wheels because they seem to run smoother and are more solid. (Note: the AM wheels we have had already seen a fair amount of use, new wheels may act different.) As for speed I think the VexPro ones seemed a bit faster, but I'm only saying this from recollection, not fact.

Re: Problems with Vex Pro mecanum wheels
 

What torque are you referring to in this context?

Re: Problems with Vex Pro mecanum wheels
 

Torque available for acceleration.

Re: Problems with Vex Pro mecanum wheels
 

It's the reaction force of the floor on the wheel which causes acceleration of the vehicle.

In the ideal case of no roller friction or axial free play, and a non-compliant floor, the acceleration force in the strafing direction is the same as the accelerating force in the forward direction.

The affordable mecanum wheels used for FRC do not have thrust ball bearings on the rollers. The rollers have axial free play. The rollers have spinning friction (especially when loaded). The carpet is compliant. That's what makes the vehicle go slower in the strafing direction.

Re: Problems with Vex Pro mecanum wheels
 

What do you mean by 'compliant'?

Re: Problems with Vex Pro mecanum wheels
 

Ok that makes sense. We did dedue it had something to do with the spinning wheels.

Any recommendations to make it better?

Re: Problems with Vex Pro mecanum wheels
 

Definition 3.

Re: Problems with Vex Pro mecanum wheels
 

Glue the rollers to their axles and forget about strafing.

Re: Problems with Vex Pro mecanum wheels
 

Or this? http://www.chiefdelphi.com/media/photos/36998

Re: Problems with Vex Pro mecanum wheels
 

LOL:D... Maybe we will try switching the wheels on Wednesday back to the Andymark one? We have to take the transmissions off to swap gears anyways.

Re: Problems with Vex Pro mecanum wheels
 

I don't think he's joking around. I see no reason why a standard traction tank drive wouldn't be more advantageous for you to use instead of mecanums.

Re: Problems with Vex Pro mecanum wheels
 

Let the battle begin!

Re: Problems with Vex Pro mecanum wheels
 

Make sure to check every roller, last year we had a strafing issue and it was because one roller was stuck. Our issue looked more like an intermittent speed issue is strafing.

I would also check if each wheel can rotate forward and backwards at full speed with the robot off the ground. Hopefully you have the encoders hooked up to each gear box to see the counts per second. At least this can sort out programing versus mechanical issues.

Just to be clear how loose our rollers are on our 2012 robot, the rollers will spin a couple time after flicking them.

Re: Problems with Vex Pro mecanum wheels
 

That's fine, but we could just put real traction wheels on if we are taking the transmissions of anyways.

Re: Problems with Vex Pro mecanum wheels
 

Ok thanks, I'll test all of this out Wednesday. I would prefer to keep mecanum for this year.

Re: Problems with Vex Pro mecanum wheels
 

Problem is, the rollers can appear to be free-spinning when not under load, and then bind up when they are loaded.

There is a lot of axial force on the roller under real-world operating conditions. It is prohibitively expensive to put high-quality thrust ball bearings on FRC mecanum wheel rollers. I've never seen the VEX mecanum wheels. What provision is in the design to react the high axial forces without creating excessive friction? IOW, what two materials are scrubbing against each other when the roller is axially loaded?

Re: Problems with Vex Pro mecanum wheels
 

Another potential problem with staffing is if your frame isn't square - ie, loads on the wheels have too much difference. We had that problem this year - we shimmed one gearbox and the staffing was fine after that.

Re: Problems with Vex Pro mecanum wheels
 

Yeah I was going to caution that a wheel unloaded will be different from a loaded wheel. My team never came up with a good test method for this.

I am sure you know this Ether, but so others can follow, I will be a bit more detailed. On the andy mark wheels, there is a bushing in the wheel. This rotates on a bolt. Between the wheel and hub plates are washers. If one is too zealous, one can tighten the bolts too much compressing the washers and wheels until the wont move. We had to play with this a bit because once the wheels are on the ground, the deflection of parts can create binding too. I think if its too loose, the hub plates could touch the roller. I forget how much it took to cause the failure mode, but gut tells me that situation required the bolts of be barely tighten.

I am guessing the Vex wheels are similar with a shoulder bolt or screw that supports the rollers. Thus i may be possible to over tighten the screw until the rollers bind. The roller would be compressed by the head of the screw and the hub ( plus a few washers in there).

Re: Problems with Vex Pro mecanum wheels
 

We switched to VEX mecanum wheels for our octocanum drive this year, and had no issues whatsoever with their performance. (And the added maneuverability was very useful in lining up to the feeder slot!)

Re: Problems with Vex Pro mecanum wheels
 

So I was thinking along the same lines as Ether as well,
But we have a 28"x28" robot with a center of mass right in the middle... So my thought was maybe the wheels just aren't as smooth, but apparently other are having good luck with them.

Re: Problems with Vex Pro mecanum wheels
 

For a quick check, pick up on each corner of the robot and see if you can detect a difference. If the frame isn't square you should be able to tell.

Re: Problems with Vex Pro mecanum wheels
 

Oh sweet! I'll give that shot too. Thanks

Re: Problems with Vex Pro mecanum wheels
 

Also, your robot being 28"x28" doesn't necessarily mean your wheelbase is a square (but it may very well be or "close enough").

Re: Problems with Vex Pro mecanum wheels
 

We are also using these wheels. We used them to make us always be pointed at the target when we have vision solutions if we choose to be. I have noticed that strafeing is rather slow too...

Re: Problems with Vex Pro mecanum wheels
 

In logomotion, we also used machanuum wheels, and they strafed much better. An issue we are occuring is our gyro drifting this year too though.

Re: Problems with Vex Pro mecanum wheels
 

Mecanum does not require that wheelbase and trackwidth be equal.

Re: Problems with Vex Pro mecanum wheels
 

This is the first year ours was even close, and from what we could tell it made no difference whatsoever from a driver standpoint.

Re: Problems with Vex Pro mecanum wheels
 

A drop of oil on each roller's axle can help a lot.

Also, during use the rollers can develop sharp edges on either side and sanding those down will keep the rollers from digging into the carpet when strafing.

FYI: I speak from experience with AM mecs, not vexpro.