what is your opinion on the new mecanum wheels do you think they are worth it
i have had problems in first were turning isnt perfect and were if it isnt lined up it doesnt work well
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If your turning wasn’t too good, check out your code. There are lots of good mecanum codes flying around, use them!
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We used mecanum in FRC this past year and were tickled with it. (wheels were direct-driven from AndyMark nanotubes) If it’s “not lined up” then nothing will work well. VEX is a bit more forgiving since it’s all standardized sizes and a smaller scale - if your team is careful, it should be easier to get things perfectly lined up.
Depends on how you play the game. I’d probably at least consider them for the Isolation Zone.
I think they will allow more options for intake mechanisms while using a holonomic drive. Being able to use a typical U shaped drive base will be nice.
The first team I mentored experimented with a mecanum driver but has yet to use one in competition. It really wasn’t too hard to work with.
Jay
Mechanum has the same weakness as Holonomic with Omniwheels, Many operations require balanced reaction forces between multiple wheels. If there are friction changes, or unbalanced weight between wheels, or non-planar field, or non-planar wheelbase, they don’t work as well as you’d hope. Most Holonomic wheelbases can’t climb the platform in Elevation game, for example.
“worth it” varies greatly between situations and teams and game design.
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We are located near FRC team 357, they were one of the first FRC teams to use this drive (they call it the Jester Drive) on their robots. It’s a drive system that needs to be paired to the game and the exact gearing to make it perform across all aspects of the game.
Having written that, it’s not the perfect drive for all games. Do the engineering design process to think it through. Don’t discard it out of hand, but decide for the game you want to play and your game strategy, if it meets your needs.
Unlike with holonomic drives, Mecanum wheel drives maintain all their forward torque, provided they don’t slip.
Mecanum wheels could also make an interesting intake: horizontal wheels to pull the objects in (towards the robot), then once the objects hit a stop (behind the wheels), they are pushed straight up by the angled rollers on the wheels.
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Your idea really interests me but I’m having trouble picturing it. Do you think you could do a quick sketch/explanation?
Thanks!
Yeah no problem!
http://img510.imageshack.us/img510/6918/mechanumintake.png
If I were going to do this, I would probably put preliminary intake rollers/flap rollers to get the objects to the point where the mechanum wheels could grab them.
If you try this, I’d love to know how it turns out.
I’m not sure I will try this but it is a great idea. I will put this down a an option for sure though! 
That’s not immediately obvious to me.
Can you clarify how you came to that conclusion, either mathematically, or experimentally?
I was always under the impression that mecanum wheels only work because they do slip. Having rollers on 45 degree angles on the wheels allows for force cancelation between wheels (allowing sideways motion.) The downside to this, however, is that the rollers will also always be slipping when traveling straight forwards. All the slippage of the rollers constitutes as force which is canceled (read: lost) between wheels to allow motion in whatever direction.
An H-drive, conversly, mantains all of its forces in the forward direction because the omni wheels only slip perpendicularly (and so don’t slip when going forward.) A holonomic drive set up with omni wheels at 45 degee angles to each other functions exactly like mecanum wheels and will experience force cancelation between wheels in a similar fashion.
In regards to maintaining torque they are actually rather equal.
Mecanum wheels allow all of the drivetrains motor’s to power the robot in any direction. However, as stated earlier this only works due to force cancelation. As a result, a portion of all of the motor’s power is canceled while the robot is traveling in any direction.
An H-drive’s forward facing wheels are not slipping, so the motors are providing 100% power in the forward and reverse direction. However, during this the slide/ kicker wheel’s motor(s) are not powered at all (providing no power in the forward/ reverse direction.) The opposite is true when going straight sideways. When going in any diagional direction the kicker wheels aren’t canceling forces. Rather one provides all of the motion in the x direction while the other provides all of the motion in the y. The rollers on the wheels slip in corrdinance with the respective x and y components so the robot slides.
My conclusion is that a H-drive is a more efficient use of your motors. However, mecanum wheels require only 4 motors. (H-drive requires a motor for the slide wheel.) Because you are only allowed 10 motors mecanum is an attractive simple to make holonomic drive which still keeps 6 motors free for the other robot functions.
Regards, Bryan
As BJC said above a mecanum wheels do slip, and that drive will produce the same forward force as an omni wheel holonomic drive (a mecanum wheel drive is also holonomic, so you can’t use that term to make distinction). You can see evidence of this when you drive a mecanum wheeled robot forward. It will not go as fast as a robot with 4 regular wheels because the mecanum wheels are “throwing away” some of that torque by slipping the rollers.
H-drive can be done with 3 motors only.
Plus-shape (vs X shape) omni-wheel holonomic can be done with 4 motors.
I sometimes call this ortho-graphic drive, mostly used with X,Y independently, plus rotation.
For a congested area like the college Gateway Isolation zone, holonomic makes a lot of sense. Programming consistency from using X only, then Y only, isn’t as important as it might seem, since Gateway rules allow hand readjustment when robot is touching the starting tile.
Sure!
Here’s my reasoning:
If the wheels do not slip on the field, there will be no force lost when driving straight because the 45 degree rollers are not moving relative to the rest of the wheel (it would be like they were just the tread of a regular high-traction wheel). If the wheels do not slip, there is no energy/force lost to friction, so torque does not suffer. I only say “assuming the wheel does not slip” because the rollers do not have to slip for the wheel to turn: try just rolling it on your desk. As long as the wheel does not move sideways at all, the rollers stay still relative to the wheel. If the rollers were to turn, that would mean the wheel is moving sideways (once again, if they do not slip).
In fancier terms: If the rollers rotate, a component of the velocity vector will be parallel to the axis of the wheel’s rotation. The rollers only rotate if they slip (I explain that slipping force below).
Opposing forces from the opposite side wheels keep the robot from moving sideways:
http://img155.imageshack.us/img155/2487/mecanumphysics.png*
If the rollers were only rotated 30 degrees instead of 45 (just to illustrate my point):
http://img854.imageshack.us/img854/2651/mecanumphysics2.png
Based on these figures, it is more likely that the wheels do slip, but that does not mean they have to slip.
*I did not take into account any resistance to the robot’s motion, however this should not be a large factor and may not even affect the force vectors I drew.
oh, it didnt occur to me that they will only go as fast as a standard holo while going forward (which is .707:1)
takes things on a new spin… (pun intended)
I don’t think they lose any speed (see my above post) when the wheels are not slipping because the rollers aren’t rotating/slipping.
I’m sorry, but they do slip. Think about it this way: If the rollers were positioned perpendicular to the direction of movement, then they would slip continuously, and take away all the speed of the robot, and it wouldn’t move much no matter how much it spun its wheels (theoretically, if it were on a non foam surface). If the rollers are parallel to the direction of movement like in the omni wheels, the rollers would not slip at all, and no forward energy would be lost. Now every in every position other than parallel or perpendicular to the direction of movement, the rollers will slip a little, and waste some of the forward motion of the robot. If you still don’t believe me, take a look at this video of a mecanum drive forklift, and you can see the rollers spinning: http://www.youtube.com/watch?v=IlmKcohyXG0
If you look at this video at 1:23 you can see that the rollers are not spinning nor slipping while travelling straight:
http://www.youtube.com/watch?v=vAiwLRGsNrE&hd=1
When the robot is travelling sideways (the whole point of the mecanums), the rollers do turn, but however do not slip on the ground. The force explanation is above, but here’s the motion one:
When the roller turns, it wants to move at a 45 degree angle from the axis of the wheel. This will create the following behavior:
When driving forward, the rollers will not spin, but the wheel will be spinning forwards.
When (attempting to be) driving at a 45 degree angle (like in an x-omni drive with 2 opposite wheels powered), the rollers will spin towards the direction of motion, but the wheel will not be spinning at all (the spinning of the rollers accounting for the entire movement of the wheels). Thus, the wheels cannot transfer any power in this direction (exactly the same as an omni wheel being pushed sideways).
When driving sideways, the rollers will spin towards the direction of motion (at only 70% the speed of the previous case, obviously), and the wheels will also rotate (the direction of the rotation being dependent on whether it is a right or left Mecanum wheel).