So, I have searched through and read alot about the different types of omni directional drive trains, and we have decided that mecanum fits our team best.
I have one question about the orientation of the wheels though. For the wheels to work correctly andmark describes that they need to form an x pattern.
Although, It does not state that the width and length between the center of the wheels needs to be a square. Some of the kids on our team think that it does need to be in a square.
Can anyone confirm that the wheels need to be aligned in a square fasion?
Don’t confuse mecanum with holonomic. Holonomic drives use omni wheels in an x pattern and work best in a symetric layout. Mecanum drives use wheels with rollers at 45 deg to the plane of rotation and don’t need to be x’ed. Both drive systems use one motor/gearbox per wheel.
Here: http://lib.store.yahoo.net/lib/yhst-33833170891817/MecanumWheelSpecSheet.pdf
It describes the wheels as being in an X.
What I’m curious is, does the x need to be a square x?
They can be in either a
/ \ or an X.
\ /
I can confirm a rectangle works fine; our robot is a rectangle and our mecanums work fantastically. I don’t know about trapezoidal shapes though…
Mecanum wheels should be installed the same as a tank drive system. The x pattern applies to the direction that the rollers face as far as I know.
Mecanums do not have to be in a square pattern (wheelbase), though it may help turning to have that.
The rollers may form either an X or an O; this may be judged based on contact pattern or on looking at the top view (which is the opposite of the contact pattern). The good news is that if you don’t like one pattern, change the wheels end-for-end and you get the other one in maybe 15-20 minutes of work. Just make sure that you’ve got either an X or an O, or your sliding will be off…
Eric’s got it, but I heard that if you have an “O” from a top view you won’t be able to rotate.
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Thanks! This helped us out a lot today.
Mechanums will ONLY work in ONE configuration That I have found.
The frame can be either square or rectangular… that doesn’t matter.
BUT there is only one way the wheels can be oriented and work properly
Look at the AXLES from the top they have to be in the X configuration as in
\ /
/ \
Which means if you flip it over and look at the Axles… (or if you were looking up at the bottom and the bot was on a piece of glass over you)… the AXLES must form an O as in
/
\ /
NO OTHER CONFIGURATION SEEMS TO WORK. Not sure why. Good luck at the game!
Steve
I would have to agree. I haven’t actually used mechanum before, but the theory holds up better on the “O” pattern. Basically, if you look at the roller from the bottom, it should be able to slide diagonally. The “X” is from the top. If the “X” were on the bottom, there would be absolutely no way to slide diagonally. Only forward/backward or only side to side would work, and I think rotating would feel pretty normal, but generally, holonomic drive trains are best when you’re able to use them point-and-shoot (including diagonals).
By the way, for anyone confused as to why I used “holonomic,” it’s because it’s a description of the movement of the drive train, not the way you accomplish it. Omniwheel drive (kiwi, 4-wheel omni), mechanum, and swerve drive are all types of holonomic. Basically, it means that the controllable degrees of freedom are equal to the total degrees of freedom on that plane. Since our robots drive on a two-dimensional plane (hopefully), there are 3 degrees of freedom; X, Y, and ω (omega, yaw, or rotation). Some advanced helicopters can be holonomic because they can control all 6 degrees of freedom on the 3 dimensional; X, Y, Z, roll, pitch, yaw. Fun Fysics Fact of the day.
We have Mecanum’s and they have to be in an X looking down from the top or you loose maneuverability (I think either zero turn radius or strafing). Also, keep in mind that mecanum’s have very little pushing power so if you plan to use these wheels your drivers better practice so that they don’t get pushed and pinned.
Conceptually, you can think of mecanums as having two vectors, here is a very good PDF on drive systems
http://www.firstroboticscanada.org/site/files/workshops/omnidirectional.pdf
Thank you for that observation! I believe you are right. The other configuration appears to cause a conflict in rotation. I KNEW there was some reason I only use the X on top method.
Steve
Would anyone know if there is a difference in the X vs O pattern in terms of if you hit the bump on a slight angle?
For some reason, it makes sense to me that one orientation of wheels would help the front end of the robot correct itself, whereas the other orientation would actually HELP your robot to roll off right the bump.
Do not make the assumption that an O pattern (viewed from above) will work. Only the X pattern (viewed from above) should be used.
You DON’T want an O pattern when looking down at the top of your robot unless you want to lose the ability to spin in place. Only X pattern (top down) configurations will have turn in place ability, unless you significantly tighten your rollers.
The reason is fairly simple. Wheels can only exert force in a direction they don’t slip in. If you think about a standard omni-wheel, you’ll realize this. It can only push front to back, not side to side. Which makes it easy to turn and easy to push sideways.
Mecanum wheels have axles that are angled 45 deg from the direction of the wheel. This means they slip in one diagonal direction, and push in the direction of the axle that’s touching the ground.
So you have a choice, all the axles pointing at the center of your robot, pushing into/out from the center of your robot (O-style), or all the axles pointing perpendicular to the center of your robot, pushing at right angles to your robot (X-style).
So, grab 4 students, put each on a corner of your robot and ask them to push on their corner to turn the robot in place. They’re going to push their corner sideways, at right angles to the center, like the X-style. If you ask them to turn the robot by all of them pushing towards the center of the robot (O-style) they’re probably going to give you funny looks and go find some screws to sort.
where is the cheapes place to buy mecanum wheels from?