# pic: Omni Design Trial

Nevermind, beat to it.

Hey, if my kid says so, it’s gotta be true! he designed the mecanum drivetrain for the promobot

edit–he deleted his testimony! oops

You can get an applied force for free, but you can’t get that force applied through a distance for free…then it would be work. Sideways force with no sideways velocity = no energy expended, and no work done, in that direction.

At least that’s my understanding of physics. I could be wrong.

edit–think about walking…where’s the force? where’s the work getting done? where’s the velocity?

Some of the motors force is used by the wheel trying to move sideways, but the rollers aren’t actually turning, so all of the wheel’s velocity is transferred to the ground.
The difference between mecanum and omni-wheel based holonomic is that on an omni system the wheels actually do move sideways when driving forward, while on a mecanum setup they don’t.

Getting back to the model and your questions/concerns about the motor mounts, could you add a closer picture from a better angle of that area? Perhaps you could hide/blank the omni wheel and wheel shroud…

…and by the way, that’s a really cool looking chassis design!

The difference you’re observing may simply be a function of how much you’ve tightened the screws on the AndyMark mecanum wheels. We have had a functioning mecanum drive train for more than a year and I swear on my mother that the rollers turn while we’re driving forward or backward or sideways or upways or downways or anyways at all.

You cannot transfer velocity to the ground. You can impart a force on the ground and the reaction to that force will make your robot move, but the Earth is a bit heavier than your robot and you’re probably not having a significant effect at making it go faster or slower.

If the rollers are not turning, as you suggest, then the wheel is not trying to move sideways. If the wheel is not trying to move sideways (as it cannot possibly discern your intention or what the rest of the wheels are doing) it will not, under any circumstances, strafe. A roller that’s not turning cannot magically impart a force sideways.

And if you are moving straight ahead, the rollers can’t roll. Even if they’re loose.

It works both ways.

I’ll paint half of each of the rollers and make a movie…

Again beat to it. Dang you, Nr. Forbes!!!

I do like this chassis, it looks very open, nice to work on.

What’s the difference between a wheel rolling forward for the sake of going forward and a wheel rolling forward for the sake of strafing and how did you get your rollers to know the difference?

The only difference is that the rollers are rolling relative to the body of the wheel under the strafing condition, so there is “lost motion”. In the straight ahead condition there is none, the drive system is quite efficient.

A wheel turning forward is going along the straightaway of the race track…a wheel strafing is going around a curve…usually we try to go as fast as we can on the straightaway, and we have to slow down for the curves. So, a mecanum drive is naturally suited to this game, while an omni drive is not, because it is wasting a lot of motor movement spinning the rollers on the wheels.

The rollers don’t care at all. But you might care if you’re trying to decide which drivetrain to use, and you are still considering both omni and mecanum holonomic drives.

Don’t make assumptions about other teams without out making at least an attempt at fact checking. If you had even looked at 488 in the gallery, you’d see they had a fully functional (as M Krass already said) Mecanum drive well over a year ago. On top of that, M Krass has proven over the years to be a very knowledgeable member of CD; I have never seen her post any opinions as fact, she all ways has something to back up her statements.

Now, for the thread… I would stay away from any sort of holomonic drivetrain this year. The manueverability will be nice, but all of the pushing and jostling that will happen in a match will make you wish you never did it…

Yes, I now know that she has mecanum experience…but it seems that she overlooked a highly relevant fact about the wheels. I did not think that she would have done this if she had a mecanum wheel at hand to play with.

I think we are all starting to agree on the subject now…

Under no circumstances do I agree. Your understanding of the system, wheel in hand or otherwise, is fundamentally flawed.

I design things for a living – mecanum wheels included – and am quite familiar with their operation.

The difference is in whether or not they’re constrained and allowed to move forward by the motion of the bot. In a mecanum if it is moving forward the rollers won’t turn because if they were the wheel would have a lateral velocity (different from force, there is a lateral force). On an omni when moving in the forward direction there is a lateral velocity so the rollers do turn.

However this means that in an ideal situation, when you have no friction in the bearings you will not see ANY difference between the two. You may see a slight difference in top speed because of the angle on the wheels but you will see absolutly no difference in power since no power is used up in heating the bearings.

In the real world you will see that in the forward direction since the mecanum rollers aren’t turning there will be a slightly better efficiency and more power will get to the floor, but I would be willing to bet that you couldn’t measure the difference cause when your talking about a 321 watt motor, the amount lost to the bearings is inconsiquecial.

*Though incredibly versatile, the standard Mecanum wheel has an unfortunate side effect which reduces its effeciency considerably. Its wide range of mobility is due to the fact that the peripheral rollers translate a portion of the motor force into a force perpendicular or at an angle to that produced by the motor. This means that are large portion of the force in one direction is lost through the translation into a resulting force by the rollers.

Losses of effeciency when traveling in a straight line are due to energy lost in a direction normal to that of travel through the peripheral rollers which bleed off available energy as they rotate.
*

Thanks for the explanation, Alex.

In what way?

I’d like to know…really…

Mecanum wheels are not that hard to find or that expecive. andymark makes an 8inch and 6inch wheel. last year my team used the andymarks and might be doing it again this yea its a possiblility.

For a small team the \$300 for a complete set of mecanums can be a real hardship where as you can get a complete set of omins for \$80 that are more than good enough… I don’t know how these omnis compare to the andymark but I do know that they work very well and are more that durable enough for the game.

Thinking that mecanums perform the same as a solid wheel when traveling forwards and backwards is flawed. Motors can not be applying a force and simultaneously perform as if they were not applying that force.

If you draw a vector diagram for a mecanum wheel, you get a line going at an angle to the axis of rotation. The forward pointing vector is a component vector and is of less magnitude than the diagonal vector. Summing these forward vectors give you a lower magnitude then if you had standard wheels in which the entire vector is pointing in the forward direction.