Monocycle Wheel Problem. Attempting to get translate to follow rotate axis.

[YourDaftPunk]

Ok, I've attached a scene with two wheels. One is driven by expressions, the other by utility nodes and is attached to an animation path.


Notice that even with the animation path setup (my preferred version) you can adjust the motionPath1's uValue and get real time feedback on the rotation. Try grabbing a cv on the animation path and dragging it up to make a hill; the wheel will follow nicely with the correct amount of rotation.

It's setup so that the animation path curve feeds into an arcLengthDimension node. arcLengthDimension can calculate the length of a curve (or a portion of a curve). The motionPath node is also connected to the arcLengthDimension- the wheel's current uValue (which is really its position on the curve) is plugged into the arcLengthDimension.uParamValue. This means that arcLengthDim is calculating the distance from the start of the curve to the current wheel position on the curve.

The distance is then passed into a couple of multiply divide nodes which are using the formula:
distanced_travel / (pi * 2 * wheel_radius) * degrees

Graph the network to see what I'm talking about.


The expression version is calculating the distance traveled by finding the magnitude of a vector between the last position and the current position. It then uses the same formula to add rotation to the wheel. Notice that at the end of the expression, I'm storing the current position to custom attributes I created on the wheel- this means that next frame I'll have access to both the current position and the last position to find the vector. Another approach is to make the wheel a passive rigid body- the rigid body node will let you access things like velocity and lastPosition.


I hope this gives you some ideas; there are many ways to do it. There are also scripts on highend3d if you need more resources.

-shawn

edit: There was a problem with the arclen node, I just recreated the node and uploaded the file.

[MaxT2K]

Hi Shawn,

Thank you so much, you're an absolute life saver!

Both the curve path and the expression method are going to be extremely useful in my animation. I had attempted to write something similar to the expression version but I couldn't get it to work in all directions. There are a couple other scenes that I think I could use these for so now I'm looking forward to learn how they work.

Cheers,

Max

[vvkonline]

hi Shawn,

it looks like arc length is always at zero that you feed in to MD "div_distance_by_circumference"

so wheel is not spinning in the example with the nodes.

[YourDaftPunk]

Thanks Vadim, I fixed it and re-uploaded the file to my original post. All I did was remake the arclength node, and it was fine. I must have made a change that broke the node and resaved without testing, so thanks for the heads up.

-shawn

[vvkonline]

cool, thanks for sharing by the way, was looking for a way to do this without expressions.

edit:
also i was wondering how U is not from 0 to 1

[YourDaftPunk]

When you build a curve, the U value is equal to the number of spans. You can always run Rebuild Curve to rebuild it as a uniform curve from 0-1.

[vvkonline]

ah I see
thanks

[MaxT2K]

Ah, I see. I noticed as well that curve wheel wasn't spinning but I thought that it could be because I'm using an earlier version of Maya. I solved this by using a simple expression but thanks for fixing it Shawn.

And thanks vvkonline for helping out.

Max

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