[Opensim-dev] ODE plugin information

[Edoardo Morandi]

Hi,
I would like to develop a bit the ODE plugin, especially in relation to
torque and rotation methods. But I need a bit of help.
Everything related to movement use "PID" parameters. I really would like to
know what are they, how they works and how the default values have been
chosen.
I tried to imagine how gain and derivative properties (PID_G and PID_D
parameters) work, but there's really something I can't understand.
First: from what's written in ODEPrim.cs at lines 1626-1628 we can say [image:
%5Cnormalsize%5C%21%5Cvec%7Bv%7D_%7Btarget%7D%20%3D%20%5Cvec%7Bx%7D%20%5Ccdot%20%28G_%7BPID%7D%20-%20%5Ctau_%7BPID%7D%29%20%5Ccdot%20timestep.gif]
.
But in physics [image:
%5Cnormalsize%5C%21%5CDelta%20%5Cvec%7Bv%7D%20%3D%20%7B%20%5CDelta%20%5Cvec%7Bx%7D%20%5Cover%20%7B%5CDelta%20t%7D%20%7D.gif]...
If timestep is a time variable it would be true that [image:
%5Cnormalsize%5C%21%5BG_%7BPID%7D%5D%20%3D%20%5B%5Ctau_%7BPID%7D%5D%20%3D%20%5Bs%5D%5E%7B-2%7D%20%3D%20%5BHz%5D%5E2.gif]..
and that would be quite strange. That's because I need some help
understanding this code.
In the same file at lines 1656-1661 we will find that [image:
%5Cnormalsize%5C%21%5Cvec%7BF%7D%20%3D%20%28%20%5Cvec%7Bv%7D_%7Btarget%7D%20-%20%5Cvec%7Bv%7D%20%29%20%5Ccdot%20D_%7BPID%7D.gif].
The value will be changed again at lines 1748-1754, becoming [image:
%5Cnormalsize%5C%21%5Cvec%7BF%7D%20%3D%20%5Cvec%7BF%7D_0%20%2B%20%28%20%5Cvec%7Bv%7D_%7Btarget%7D%20-%20%5Cvec%7Bv%7D%20%29%20%5Ccdot%20D_%7BPID%7D%20%5Ccdot%20m.gif]
.
Again, we know that [image:
%5Cnormalsize%5C%21%5Cvec%7BF%7D%20%3D%20m%20%5Ccdot%20%5Cvec%7Ba%7D%20%3D%20%20m%20%5Ccdot%20%7B%20%7B%20%5CDelta%20%5Cvec%7Bv%7D%20%7D%20%5Cover%20%7B%20%5CDelta%20t%20%7D%20%7D.gif],
so it would bring to the dimensional equation [image:
%5Cnormalsize%5C%21%5BD_%7BPID%7D%5D%20%3D%20%5Bs%5D%5E%7B-1%7D%20%3D%20%5BHz%5D.gif]
.
I'm not saying that's wrong... try taking a cube and push it inside
opensim... it works!
The problem is that I can't implement anything related to rotations since I
can't understand what's behind these parameters...

Thanks for you support.
-- dodomorandi
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