Spinning the Robot with Mecanum Wheels
 

We are having major problems getting the robot to spin with mecanum wheels. I beleive the code is correct as we consulted a power point from here that verifies our program matches the work of others.

The robot goes forward, backward. side to side, and at an angle just fine. But, when we try to spin it the robot just vibrates a lot. We did try putting wieght on the wheels. Has anyone else had this problem? Any fixes the might work?

Thanks in advance

PS if there is an old thread that addresses this issue I didnt find it and would be happy to go there if this was covered

Re: Spinning the Robot with Mecanum Wheels
 

could it be that your COG is off by an uneven distribution of weight?

Re: Spinning the Robot with Mecanum Wheels
 

The problem is almost certainly the setup of the wheels. There is one setup of mecanum wheels that works right and one setup that behaves as described. I don't remember what the appropriate way to swap the wheels is to get them into the right setup, but I'm sure someone will be by shortly who does know.

Re: Spinning the Robot with Mecanum Wheels
 

If you were to draw an imaginary line with the roller axes as they look from a top-down view, the lines should make an 'X'. More than likely, yours make an 'O'.

You want the 'O' on the bottom, not the top.

Another symptom of this is that it may be difficult to maintain a straight line when going diagonally.

Re: Spinning the Robot with Mecanum Wheels
 

There was some discussion about this X vs. O but I am going to check our alingment now with what you said. Thanks a lot guys

Re: Spinning the Robot with Mecanum Wheels
 

http://www.chiefdelphi.com/forums/sh...ad.php?t=80962

http://www.chiefdelphi.com/forums/sh...ad.php?t=80490

http://forums.usfirst.org/showthread.php?t=14135




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Re: Spinning the Robot with Mecanum Wheels
 

We are defintely in the wrong alignment and I have guys changing to the X alignment now. You guys have really saved us a lot of time because the next few 'tests' we were going to try would have taken a lot longer to do.

Re: Spinning the Robot with Mecanum Wheels
 

End for end OR side for side. Or just flip the frame over, if it's low enough...

Re: Spinning the Robot with Mecanum Wheels
 

Thanks, I'll try and remember that. That was what I thought, but I didn't really want to guess and have them switch them and it still be wrong.

Re: Spinning the Robot with Mecanum Wheels
 

Here's a question: if you mount your wheels in a square configuration (ie, your wheelbase = your track), is it possible to rotate your bot about center?

Re: Spinning the Robot with Mecanum Wheels
 

Yes. It'll actually work best that way.

(...unless you have your wheels swapped, in which case it won't be able to rotate itself at all.)

Re: Spinning the Robot with Mecanum Wheels
 

For wheels mounted in the correct configuration, I don't think that it's possible to rotate a bot about center if they are mounted in a square configuration. When summing moments about center they all cancel out when length = width. Only when you have a rectangular bot, where length != width, can you create one pair of moments not offset by the other pair.

This seems to be unconventional thinking, and I can't seem to find anywhere on CD that this has ever been stated. Am I missing something? Would anyone be willing to provide a vector analysis of a square 'bot rotating about center?

(It can be done using four identical wheels, but thats not what I'm after. Two "RH" and two "LH" wheels, mounted in an "O" configuration, cannot seem to create a moment about the center of a square bot).

Re: Spinning the Robot with Mecanum Wheels
 

Consider a bot with 4 mecanum wheels mounted so that they have the "X" configuration when viewed from the top. Now drive both wheels on the left side forward, and both wheels on the right side backward, all at the same speed.

The front left wheel will be pulling forward and to the right. The rear left wheel will be pulling forward and to the left. The front right wheel will be pulling backward and to the right. The rear right wheel will be pulling backward and to the left. So, all 8 force vectors are pulling CLOCKWISE. The vectors do not cancel, they add. So the bot spins clockwise.


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Re: Spinning the Robot with Mecanum Wheels
 

Specifically, I believe that when you're summing moments (R x F) you are forgetting to include make R negative in one direction.

Re: Spinning the Robot with Mecanum Wheels
 

Hi Matt,

the pronoun "you" is ambiguous.

which post were you responding to? craigboez or me?



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Re: Spinning the Robot with Mecanum Wheels
 

I was responding to craigboez's post.
Sorry about the ambiguity.

Re: Spinning the Robot with Mecanum Wheels
 

Hi Craig,

Take a look at this PDF:

http://lib.store.yahoo.net/lib/yhst-...lSpecSheet.pdf

It has a force-vector diagram and complete explanation for all three degrees of freedom: fwd/rev, slide left/right, spin CW/CCW

To spin in place clockwise, wheels 1 and 3 should be driven forward while wheels 2 and 4 are being driven backward. If you do this, the force vectors on each of the 4 wheels will produce torque moments about the center of the wheel pattern all in the clockwise direction. They do not cancel, they add.



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Re: Spinning the Robot with Mecanum Wheels
 

The correct configuration is a "X" configuration when viewed from the top of the bot, which is the same as an "O" configuration when viewed from the bottom of the bot. In other words, if you have a bot with the wheels mounted in an "X" configuration when viewed from the top, and you tip the bot on its side and view it from the bottom, it will appear as an "O" configuration.

If your wheel configuration looks like an "O" when viewed from the TOP, it is not the correct configuration, and the bot will not spin properly.


For those of you new to this discussion, here's what we mean by "X" and "O" configurations:

"X" configuration:

\..../
......
......
/....\


"O" configuration:

/....\
.......
.......
\..../




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Re: Spinning the Robot with Mecanum Wheels
 

Thanks to all for your input. The fact that the wheels, when viewed from above are different from the wheels when viewed from below was the big aha! moment here. Thanks especially to Ether for the in depth explanation - I have now seen the light.

Re: Spinning the Robot with Mecanum Wheels
 

It is kind of funny because now that I understand the problem I see the answer posted in many places.

The O is the way to go...
 

First off, nice ascii art. Good description of the problem as well.

I have been lurking on this thread for a while, but I kept quiet because I didn't really have anything worth adding... ...until now.

When this issue was first introduced in my mind as a potential problem, I had to think about which way I would orient the wheels if nobody told me the answer and I hadn't yet done the experiment. Sadly, I got it wrong. My first thought was to say that I wanted the wheels on the ground in the X position.

My reasoning went like this: I wanted the small wheels oriented such that it would make it easy to spin the robot about its center. An X seems to provide this feature. And it is true as far as it goes -- if you have the wheels in an X configuration and you lock the main axle of the wheels (i.e. turn the motors off), you should be able to spin the robot about its center fairly easily.

BUT... ...then I thought, "What is going to provide that spinning force?"

Well... ...it is SUPPOSED to be the motors driving the main axles of the wheels, of course.

BUT, the way Mecanums work is that they only apply force to the ground in the direction of the axles of those little wheels (the same can be said for holonomic wheels, by the way).

Now that X configuration is a disaster because all the wheel force vectors go right through the center (or very near the center) of the robot!

Torque is force times moment arm: Small moment arms => small torques.

Even if the wheels are putting out largish traction forces, that force is multiplied by as small moment arm. The net result is a robot that has issues spinning about its center.

So... when it comes to Mecanums the O is the way to go.

Cheers,
Joe J.

Re: Spinning the Robot with Mecanum Wheels
 

What powerpoint did you use. could you post a link?