pic: Omni Design Trial

[Billfred]

Since you brought up cost of UHMW, I checked McMaster. A 48" square sheet of 1.25" UHMW (item 8619K97, the smallest size that would let you get 28"x38") is $348.38. A 36"x24" piece (item 8619K95) is $164.58.

Just something to marinate on.

[Nate Laverdure]

Originally Posted by M. Krass

George is entirely correct in his explanations. End of story.

Obama got Oprah, Huckabee got Chuck Norris, and George1902 got M. Krass.

[MrForbes]

No, it's not the end of the story....because I have a mecanum wheel sitting here, and you don't

Originally Posted by George1902

There are no "cases" involved. The wheels behave the same always.

No, they don't. Mecanum wheels behave differently when they move only in the direction of the wheel, than they do when the move any other direction. In the straight ahead case, the rollers do NOT roll relative to the body of the wheel. This is because in this case they CAN NOT roll relative to the body of the wheel, as long as they have full traction. This is a fact, try it and see!

Rolling a wheel and powering a wheel are totally different.

Yes they are...as far as force. No, they are not....as far as velocity.

If you placed a keyed or hex shaft through the wheel and tried to turn it while pressing into the ground, you'd see the wheel would want to walk down the shaft. This is because the wheel produces a vector that isn't straight ahead, but at approximately 45*.

Exactly!!!

But if you constrain the wheel so it cannot move sideways, it still produces the sideways force vector, but the sideways velocity vector is zero.

Ok, let's try changing tack. Assume the mecanum wheels do put 100% of the power forward. We know the gearboxes can't output more than 100%, so there can't be any force sideways. Therefore, the robot could not strafe sideways.

....while the wheels are only rolling straight ahead. There are two different cases, which you seem to be ignoring. The straight ahead movement case is the only one where the motors can transmit all of their motion to the robot. In the strafing case, the motors are not transmitting all of their motion to moving the robot, and the robot goes slower when strafing than it does when going forward, assuming the same average motor speed among the 4 motors in both cases.

However, we know that the robot can strafe sideways. Therefore our assumption is false and the wheels must be producing a sideways force all the time.

No, the wheels are producing a sideways force all the time, but are only producing a sideways motion when strafing! Please try to understand that force does not equal motion.

The mecanum wheels do not put 100% of the power forward. They behave just like regular omnis at 45* angles. You get about 70% power by moving forward, reverse, left, or right.

Nope, with mecanum you get 100% going forward, and less when going sideways or at an angle.

Also, consider the case of omnis vs. mecanum, with all motors turning in such a way that none of the rollers are rolling relative to the wheel bodies. In the case of the mecanum, the robot goes straight along the axis of the wheels. The omni sits and spins in a circle. So, the omni is most efficient when it is not going anywhere!

[Fred Sayre]

Originally Posted by Nate Laverdure

Obama got Oprah, Huckabee got Chuck Norris, and George1902 got M. Krass.

So it is official, George gets the 08 nomination, congrats

[Fred Sayre]

Wait, so when you are rolling forward and there is a sideways force, where is that force coming from? It is the motors. You don't get the applied force for free.

[GUI]

Nevermind, beat to it.

[MrForbes]

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

http://www.chiefdelphi.com/forums/sh...ad.php?t=59027

edit--he deleted his testimony! oops

[MrForbes]

Originally Posted by Fred Sayre

Wait, so when you are rolling forward and there is a sideways force, where is that force coming from? It is the motors. You don't get the applied force for free.

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?

[GUI]

Originally Posted by Fred Sayre

Wait, so when you are rolling forward and there is a sideways force, where is that force coming from? It is the motors. You don't get the applied force for free.

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.

[M. Mellott]

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!

[Madison]

Originally Posted by GUI

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.

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.

[MrForbes]

Originally Posted by M. Krass

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....

[GUI]

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

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

[Madison]

Originally Posted by squirrel

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....

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?

[MrForbes]

Originally Posted by M. Krass

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.