Problems with Vex Pro mecanum wheels

[omsahmad]

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.

[Alan Anderson]

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

[omsahmad]

Originally Posted by Alan Anderson

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

They are loose.

[Calvin Hartley]

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)

[omsahmad]

Originally Posted by Calvin Hartley

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)

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...

[JesseK]

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).

[Calvin Hartley]

Originally Posted by omsahmad

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...

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.

[Ether]

Originally Posted by JesseK

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.

What torque are you referring to in this context?

[JesseK]

Originally Posted by Ether

What torque are you referring to in this context?

Torque available for acceleration.

[Ether]

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.

[JesseK]

Originally Posted by Ether

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.

What do you mean by 'compliant'?

[omsahmad]

Originally Posted by Ether

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.

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

Any recommendations to make it better?

[Ether]

Originally Posted by JesseK

What do you mean by 'compliant'?

Definition 3.

[Ether]

Originally Posted by omsahmad

Any recommendations to make it better

Glue the rollers to their axles and forget about strafing.

[mr.roboto2826]

Originally Posted by Ether

Glue the rollers to their axles and forget about strafing.

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