How To Make Something Rotate Gradually?

[phredrek-phredrek]

Hey guys, I have this problem and it's kind of fustrating.

basically, all I want to do is have this wind turbine spin from a stopped position to a certain speed, let it run at that speed for a few seconds then increase again.

My aproach is that I setup a locator as a contoller with an attribute I called speed. I just want to be able to key the speed and it'll speed up to that speed over it's keyframes.

I'm trying to use this expression but having a major problem:

turbine.rotateY=time*(controller.speed*50);

My problem is that let's say I key speed at 0 at frame 0, then at frame 30 I key the speed at 3. what will happen is that it'll slowly start to spin then at about frame 20 it'll suddenly get superfast then drop down at frame 30 and stay that that constant speed. why is it that it speeds up just before reaching the key? oh and by the way, in the graph editor, it's linear keys.

I'm open to any other suggestions on how to go about this.
thanks! majorly apreciated!

Frederic

[milodc]

QUOTE(Phredrek @ 02/07/07, 08:25 AM) [snapback]259211[/snapback]
My problem is that let's say I key speed at 0 at frame 0, then at frame 30 I key the speed at 3. what will happen is that it'll slowly start to spin then at about frame 20 it'll suddenly get superfast then drop down at frame 30 and stay that that constant speed. why is it that it speeds up just before reaching the key? oh and by the way, in the graph editor, it's linear keys.

If you bake the rotation on the turbine, you'll see that its animation curve does this:

The bend of the curve from 0 to 30 happens because, as you increase controller.speed, the right side of your equation turbine.rotateY = time * (controller.speed * 50) no longer changes at a constant rate.

You'll need to modify your script such that the rate of change in rotation remains what it was when the turbine's speed stopped increasing.

[joojaa]

Well, because its a bit tricky to solve analytically inside maya. Its a integration of the speed curve, and no such utility exsists (or well i have one but i cant give it out, its not hard to do but a bit stupid work). However its relatively easy to compute by sampling, however you loose the ability to move back and forth in the animation freely unless you cache the results and manage the cache on each change which is more then a bit tedious. (basically you have made your animation a sim that only works forward!)

basicalyy the idea is to:

CODE
$fps=25; //pal
global $lastPos=0; //note globals get put in once
if (frame==o){
   $lastPos=0;
   turbine.rotateY=0; //or whatever start value
}
else{
  turbine.rotateY =$lastPos+controller.speed/$fps;
  $lastPos=turbine.rotateY;
}

if yoyu need to cache look into frame cache node.

But i do admit that once you understand speed is slant of the pos curve you can as easily key this directly that way. (and acceleration is a slant of slant wich is a exponent function. incidenttaly this is forts thing we were taught in physics)

PS, actually integration is truly nly valid for radians but...

[phredrek-phredrek]

oh thanks alot guys!

I'm going to try out that script that you wrote. and if it doesn't work then I'll just do my original one line mel script but then bake the keys and clean it up in the graph editor.

thanks for the ideas and help! very much apreciated!!
wooooo!!!

Frederic.

[joojaa]

Not a script expression theres a wast difference.