Re: bump-free mecanum wheels
 

Yes, you need overlap between your rollers.
I'll use the term "tread width" here to mean the distance (perpendicular to the wheel axis) that the rollers contact the ground when the 'bot is moving forward. On many two-plate designs, this is also the distance between the plates.
If your rollers are at a 45 degree angle, then your roller length is the tread width multiplied by √(2).
I believe determining what angle of the wheel that will cover is an inverse sine function of (tread width)/(wheel radius).
So take 360 divided by that angle, and round up. That's how many rollers you need on your wheel.

Re: bump-free mecanum wheels
 

I think you meant "parallel".

Yes, you can get a good-enough answer just by dividing the tread_width by the wheel_radius and that gives you the angle in radians that is subtended by each roller. Pedantically, the calculation should be 2*asin(tread_width/2/wheel_radius)



If there is overlap, that has to be subtract from the angle before dividing.

Re: bump-free mecanum wheels
 

thanks, using that i updated my mechanum wheels. i don't think overlap is good, i think that what is ideal is an infinitesimal amount of overlap. just as when you slide to pieces of tile together with no overlap for a smooth surface, a smooth ride must have no overlap, but no gap either.

Re: bump-free mecanum wheels
 

If you're changing the rollers and looking for a quick and easy way to shape your rollers so that they form a perfect circular outline when mounted at 45 degrees, there's a simple calculator program at this link:

http://www.chiefdelphi.com/media/papers/2390

Just enter the desired wheel radius and the radius of the roller at its midpoint, and you'll get an output file (in CSV format) of X,Y data pairs that you can load into your CAD program and rotate around the x-axis to form the surface of the roller. CSV files can be opened directly in Excel, or you can open with any text editor.


If you don't want the rollers to overlap that's fine, but the analogy is a little off. If two pieces of tile overlap that creates a bump. If the rollers "overlap" there is no bump. Just a seamless transition period during which two rollers are touching the floor. If the rollers are contoured properly, this handoff from one roller to the next is smooth.

Re: bump-free mecanum wheels
 

it is correct that they do not create a bump, however all of the rollers are not at 45 degrees relative to the ground, due how one roller is at the begging of its for lack of a better word contact zone, and the other is at the end, they are at a slightly different angle relative to the ground. while going forward or backward, or while strafing, this should not be very noticeable, but while going close to diagonal it causes the robot to move in more of a very small zig zag pattern, which causes accelerometer white noise. this caused problems for our attempt at a robot that could stay parallel to the field at all times. it did not work while going diagonally.

Re: bump-free mecanum wheels
 

Even if you reduce the overlap to zero, a 2.5" long roller on a 4" wheel goes from ~44 degrees to 45 degrees and back to 44 degrees as it goes from end-to-end. This is greater than the change in angle during overlap.

Could you please share with us how you determined that this very small change in angle is actually the root cause of your problem, and not, say, friction or misalignment or manufacturing tolerances of the rollers/wheels?

By the way, when going forward and backward the rollers do not turn, but when strafing they do. So if roller angle were indeed the source of the problem, one might expect to see the problem when strafing.

Re: bump-free mecanum wheels
 

i suspect that when strafing the zig zagging becomes more rotational rather than side to side because of the way that all 4 wheels interact with each other. however when there are only two wheels being powered while going diagonally, it stops rotating and starts to move back and forth. i came to this conclusion because the non powered wheels oscillate as well according to our encoders. i cant tell what happens when the motors are powered because the motors interfere with the way the wheels spin freely.

Re: bump-free mecanum wheels
 

are the oscillations of the unpowered wheels in phase?

are you using the encoders to do closed-loop speed control of the wheels?



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Re: bump-free mecanum wheels
 

they are synchronous while going diagonally, however i believe they are not when strafing resulting in the rotational noise, we are not doing close loop control during tele-op, but i do believe that we were during this test which was in autonomous, this was all resulted from trying to go diagonally during autonomous to get to the second and third balls this year.

Re: bump-free mecanum wheels
 

was the frequency of the oscillations of the unpowered wheels equal to the speed of the powered wheels (in revolutions per second) times the number of rollers?

what mecanum wheels were you using when this test was done? were they COTS or custom made?



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Re: bump-free mecanum wheels
 

this was with Andymark 8 inch mechanums with the new rollers. although we are doing custom mechanums this coming year and i dont want them to have the same problems as these. i am not sure if the rate of the oscillations matched that, i will have to look into it.