09-01-2008 06:24

Jeff K.

Re: pic: Omni Design Trial

Looks nice. One problem though, currently won't be going anywhere, with just a cim coupled straight to a wheel. Looks nice and light. How much does it all weigh?

Do you have a waterjet or lasercutter, because all those font are going to take a nice while otherwise.

09-01-2008 07:57

AJ R

Re: pic: Omni Design Trial

It looks really nice. How fast will this go after you get the gear boxes in and how would you account for the wheels slipping?

09-01-2008 07:59

MrForbes

Re: pic: Omni Design Trial

It looks like a neat design, I wish I could use Inventor so well!

One comment on the drive choice, it seems to me that omni drive like this has a lot of "wasted motion" since either all 4 wheels are always turning at an angle to the direction you're going, or two are going in the direction of movement and the other are not turning at all. I suggest you look into changing to a mecanum drive setup, the only substantial change to your design is that you would rotate all 4 wheel/motors 45 degrees so that the wheels are parallel to the long dimension of the frame.

Mecanum has an advantage with this type of game, where you will probably spend a lot of time driving straight ahead, and in this situation the rollers on all 4 mecanum wheels do not rotate relative to the wheel body. With mecanum, you only have "wasted motion" when strafing (sideways) or rotating the robot.

(if you've already ordered the omni wheels it might be expensive to change now though!)

Also mecanum programming is something your team should be able to do, there are several code examples on the web that you can use to make it work.

09-01-2008 08:52

JamesBrown

Re: pic: Omni Design Trial

Is this true? I was under the impression that a Mecanum wheel with 45* rollers was essentially the same as using 4 omni wheels located at 45* angles.

If the rollers are at 45* angles how is there any difference in what the part of the wheel contacting the floor any different in the forward direction than it is side ways.

09-01-2008 08:58

MrForbes

Re: pic: Omni Design Trial

This is hard to visualize! but easy to demonstrate if you have a mecanum wheel to play with. Unfortunately they're hard to find....and hard to make....and expensive....

The rollers act as "tread" when the weel is going straight ahead. To be able to roll relative to the wheel, there must be sideways motion. If there is no sideways motion, the roller in contact with the floor cannot roll, it can only "go along for the ride" as the wheel turns.

(I am assuming the rollers get good traction on the floor)

09-01-2008 09:04

JamesBrown

Re: pic: Omni Design Trial

I was under the impression that the rollers were moving sideways but since the forces for the left side were equal in magnitude but opposite in direction they resulted in no net force (except forward).

I guess I need to get my hands on some Mecanums.

09-01-2008 09:06

MrForbes

Re: pic: Omni Design Trial

Another issue with both omni and mecanum drive, is that the robot would work better with some type of suspension, especially since there are "lumps" in the arena floor surface near the walls and overpass supports.

Without suspension, the robot will drive erratically when the weight on any wheel drops significantly, such as when driving over any bump that would lift a wheel off the floor. Both of these drivetrains work because all the wheels work "against" each other, the result is that it goes the way you want it to. If you lose traction on a wheel, that wheel won't be pushing the robot the direction it is supposed to, so the robot will go wonky...

09-01-2008 09:07

MrForbes

Re: pic: Omni Design Trial

There is an opposing sideways force on the rollers on both sides of the robot, but since they are constrained by the forward direction of the robot, they cannot roll sideways, so they can't roll at all.

09-01-2008 09:42

George1902

Re: pic: Omni Design Trial

If this were correct a mecanum drivetrain couldn't produce diagonal motion. It's done by driving the corner wheels, but if what you say is correct, the robot would just drive straight anyway.

In ideal conditions a mecanum wheel will behave exactly like an omni wheel at 45*. Each system will be approximately 70% (sqrt(2)/2) efficient in forward-backward and side-to-side motion.

The confusion comes in that real life is never "ideal." 70% efficiency assumes that the rollers will have zero friction with their shafts. In an omni bot the inefficiency is universal and generally cancels itself out. In a mecanum bot however, the inefficiency causes the mecanum wheel to behave more like a regular wheel, so you end up with a little more than 70% power in forward-backward motion and a little less than 70% in side-to-side motion.

If you completely eliminate the roller-shaft friction, you get a perfect 70% efficiency, and likewise if you completely lock the rollers to their shafts you get a regular wheel. Reality lies somewhere in between.

09-01-2008 10:04

T3_1565

Re: pic: Omni Design Trial

well I for one like omnis over mecanums!! Really solid looking design, except for the lack of gearboxes, we are using omnis too!! We find the mecanum bots we have seen in person drifting side to side too much, as they are always balancing forces. good luck!! nice colours too!

09-01-2008 10:22

MrForbes

Re: pic: Omni Design Trial

I was not talking about the case where the robot is moving sideways as well as forward (diagonal motion). In this case, the rollers do roll, as you say.

I disagree with this. I have a mecanum wheel in my hand, and it has loose rollers, they turn very easily. When I hold the wheel so that one roller is contacting the desk, and then move the wheel (without turning it) so that the roller rolls without slipping on the desk, the wheel body moves in a diagonal direction.

Now I'll turn the wheel body, pressing down towards the desk, but keeping the wheel moving only in a forward direction, in line with the direction the wheel is pointing, and perpendicular to the axle. This would be the same as just rolling a normal wheel across the desk. When I do this, the rollers on the wheel do NOT roll relative to the wheel, they act as a (segmented) wheel tread.

Any time that there is good traction and the rollers are rolling relative to the wheel body, then the wheel is moving sideways. The geometry of the wheel makes this a fact.

I guess I need to make a movie.....

Anyways, the way mecanum wheels work, they are NOT like omnis, and they do NOT have the same efficiency going forward and sideways/diagonal.

This is an advantage of mecanum wheels.

09-01-2008 11:24

George1902

Re: pic: Omni Design Trial

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

Rolling a wheel and powering a wheel are totally different. 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*.

This is correct, and I said as much in my previous post.

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.

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.

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.

09-01-2008 11:25

Cjmovie

Re: pic: Omni Design Trial

We do have access to a water jet, luckily. Classically all the designs coming from our team have consisted of large amounts of sheet metal. Which is pretty cool, because although it makes them all a bit homogeneous, they give a certain "1293" style to our robots with the artwork we cut into them.

Mostly what I'm worried about with this design is whether the motor mountings are strong enough. Like I said, this is the first full design I've done myself (Actually, first design I've really ever done except changing someone else's) and I'm not an engineer... I'm actually the team programmer.

My point in having Omni's vs. Mecanum is twofold. First, the one my team will probably be most happy with, is that they're a bit cheaper, and it makes every single motor assembly interchangeable for repair/replacement. But from a design standpoint, it's because it then becomes just as easy to herd a ball on the long end (therefore less change of it shifting left/right outside of the robot's pushing front) as it is to drive around robots on the short end.

As for the gearboxes, those will most likely be the same ones Banebot supplied from last year (We had ordered a couple extra, but they didn't arrive until after the season, so we can play around with those... We're thinking about doing what we did in 2006 and building a duplicate to keep at the shop).

The main idea behind this design was to make the entire thing modular. Any part can be removed from the base without more than, worst case, 6 bolts. And those on opposite sides of the robot are interchangeable, so if something gets bent up, we only need one extra for 4 shapes and we'll have a spare for anything we'll need.

Also, any idea on whether the strength of UHMW is up to the challenge of a full chassis? I'd rather not have a $2K piece of UHMW turn into two cracked-in-half robots. (Which is partially why I added some of the extra sheet metal, for reinforcement).

As for the current weigh in, it's at 40 pounds based on Inventor. But I believe the estimate of weight in Inventor for the motors is a bit low (I think it says 2 pounds), so it's probably much more realistically around 65 including bolts and bearings. I'll also need to add in the gearboxes, so my best guess is that it will end up around 85 pounds. Forty pounds should be plenty for the electronics and arm, I think. Although we do have a nasty habit of always weighing in at 120.1 pounds.... or worse. (I think one year we dropped 8 pounds by cheese-holing everything possible for our robot, with a quite humorous effect on its looks).

Thanks!

09-01-2008 11:28

Madison

Re: pic: Omni Design Trial

All other things being equal -- wheelbase, track width, coefficient of friction, etc. -- mecanum wheels and omniwheels function in exactly the same manner. There are no differences. They are mathematically identical.

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

09-01-2008 11:31

Billfred

Re: pic: Omni Design Trial

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.

09-01-2008 11:39

Nate Laverdure

Re: pic: Omni Design Trial

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

09-01-2008 11:43

MrForbes

Re: pic: Omni Design Trial

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

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!


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

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.


....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.

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.

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!

09-01-2008 11:44

Fred Sayre

Re: pic: Omni Design Trial

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

09-01-2008 11:48

Fred Sayre

Re: pic: Omni Design Trial

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.

09-01-2008 11:52

GUI

Re: pic: Omni Design Trial

Nevermind, beat to it.

09-01-2008 11:54

MrForbes

Re: pic: Omni Design Trial

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

09-01-2008 11:58

MrForbes

Re: pic: Omni Design Trial

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?

09-01-2008 12:03

GUI

Re: pic: Omni Design Trial

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.

09-01-2008 12:07

M. Mellott

Re: pic: Omni Design Trial

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!

09-01-2008 12:11

Madison

Re: pic: Omni Design Trial

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.

09-01-2008 12:13

MrForbes

Re: pic: Omni Design Trial

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....

09-01-2008 12:14

GUI

Re: pic: Omni Design Trial

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

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

09-01-2008 12:16

Madison

Re: pic: Omni Design Trial

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?

09-01-2008 12:23

MrForbes

Re: pic: Omni Design Trial

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.

09-01-2008 12:25

AdamHeard

Re: pic: Omni Design Trial

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....

09-01-2008 12:27

MrForbes

Re: pic: Omni Design Trial

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....

09-01-2008 12:30

Madison

Re: pic: Omni Design Trial

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.

09-01-2008 12:31

Alex.Norton

Re: pic: Omni Design Trial

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.

09-01-2008 12:34

Fred Sayre

Re: pic: Omni Design Trial

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.



From http://www.araa.asn.au/acra/acra2002...eter-Tlale.pdf

09-01-2008 12:43

MrForbes

Re: pic: Omni Design Trial

Thanks for the explanation, Alex.

09-01-2008 12:46

MrForbes

Re: pic: Omni Design Trial

In what way?

I'd like to know...really....

09-01-2008 12:56

Rupnick

Re: pic: Omni Design Trial

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.

09-01-2008 13:01

Alex.Norton

Re: pic: Omni Design Trial

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.

09-01-2008 13:12

Fred Sayre

Re: pic: Omni Design Trial

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.

09-01-2008 13:27

George1902

Re: pic: Omni Design Trial

Thank you, Fred. I don't know why I didn't think of quoting that paper earlier.

For anyone who's really interested in finding out all the science behind how a mecanum works, that paper is an excellent read.

I remember when I first came across that paper. I went to all of my friends saying, "See!? This is what I've been talking about!"

09-01-2008 13:40

MrForbes

Re: pic: Omni Design Trial

Thanks for the explanation, Fred. We now know that the maximum force that can be transmitted by a mecanum wheel in the forward direction is not as high as with a regular wheel. How does this affect the power transmission? Is there more frictional loss because of the side pointing vector? If so, how could I calculate that?

Also, what about the speed? If the rollers are not rotating relative to the wheel body as in straight ahead driving, is the full speed of the motor applied by the wheels?



The paper is interesting, but doesn't offer any explanation...it seems to be mostly selling the new design.....it would be nice if it had even a little bit of actual theory, and really nice if it had some empirical data to back up the claims.

09-01-2008 13:59

Fred Sayre

Re: pic: Omni Design Trial

The friction comes into play of course, but that is a very hard number to get at short of actual experiments, and of course varies greatly with the type of mecanums being used.

Speed however will change. It is our experience that the rollers do in fact spin while going straight. On the ground the force is encouraging them to spin, and "perfect" traction would prevent that, but this is usually not the case. I suspect that there is some movement on the carpet, but additionally as soon as the wheel rotates the rollers will spin more as they pick up off the floor.

If at all possible, it would be a great test to get the same diameter wheels as the mecanum and compare speeds on the official carpet. We did not go into that much detail with our prototype analysis, but we made a rough estimate with vector calculations to adjust the gear ratios accordingly for our ideal fps.

09-01-2008 14:07

MrForbes

Re: pic: Omni Design Trial

So, if you were to set up the wheels to be optimized for straight driving by adjusting the friction to be high enough that the rollers would not be able to roll at normal driving loads, would that make the sideways force vector on the rollers negligible under these conditions? I'm thinking that the inability to overcome static friction would keep the rollers acting like regular tread, and the wheels would indeed act almost like regular wheels under those conditions.

I'm thinking about this as it applies to this game only.

09-01-2008 14:47

Bob Steele

Re: pic: Omni Design Trial

I just wanted to point out a small error in the discussion regarding Force and Motion.

NET force will result in Acceleration ... constant velocity requires no force whatsoever.

When we accelerate robots we see the result of all the forces involved. The net force.

The other forces on the robot are varied and the one we should discuss here primarily is friction (which is a result of the weight of the robot and the coefficnet of friction between the wheels and the carpet). This frictional force is what accelerates the robot. This frictional force is a result of the wheels applying a force to the carpet and carpet pushing back.

The net force on the robot, when it is going forward is in that direction.
If the angled wheels on a set of mecanums are not turning it is simply indicating that the sideways forces are balanced. It does NOT indicate there are NO sideways forces. I would imagine that the pin loading on the bearings in this angled state is much different than when it is moving parallel to the wheel's orientation.

I you could measure the temperature of these wheels over the course of the match I would be very surprised if they do not get quite warm.

This loss of efficiency that we all discuss with holonomic or mecanum systems has to result in energy being spent somewhere else...

Anyway these are my suggestions on this topic. Specifically they would indicate why that holonomic drives and mecanum drives have to work in the same way. Force applied does not net acceleration. Net force does.

Good luck to everyone no matter what drive system they use!!!

Have a great season!!!

May the NET FORCE be with you!!!

09-01-2008 15:00

Fred Sayre

Re: pic: Omni Design Trial

Right, at the cost of even less efficiency going sideways. You could also adjust the roller angle to be closer to an omni wheel to increase the straight driving performance and decrease the side to side performance.

In that paper they talk about adding the feature of being able to lock or vary the angle of the rollers on a mecanum wheel to increase performance. That would be an awesome, award worthy accomplishment by a FIRST team that I hope someone will at least attempt in this or future seasons.

09-01-2008 15:09

MrForbes

Re: pic: Omni Design Trial

Or you could angle omni wheels at, say, 30 degrees to the long axis of the robot instead of 45 to achieve the same effect, right?

10-01-2008 01:18

M4 Sherman

Re: pic: Omni Design Trial

I'm kinda new with plastics, but why would you use UHMW for a chassis?

It would be virtually unbreakable, but isn't it flexible? And I though UHMW was used for more high friction situations(like chain tensioners)?

10-01-2008 11:00

MrForbes

Re: pic: Omni Design Trial

A flexible chassis is a design feature of a holonomic drive robot...helps keep all the wheels on the ground, so it will be more likely to drive the direction you intend it to.