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[Chareyre <question159394@xxxxxxxxxxxxxxxxxxxxx>]

Question #159394 on Yade changed:
https://answers.launchpad.net/yade/+question/159394

    Status: Open => Answered

Chareyre proposed the following answer:

> This equation is very similar to the servo-control we use with rigid
> boundaries; with the difference that there we also have a gain parameter
> (let me call it that way, for me it does not matter if it is split into two
> parameters other than one). So, why here the formula is correct but with
> rigid walls we need the gain parameter?
For rigid walls (or periodic && !dynCell), it reads
displacement=stress_error/stiffness in both cases (a bit more
sophisticated in fact, but this is the basic idea).
For dynCell, it reads acceleration=stress_error/mass (idem).
I don't see what you mean by "we need a gain". Since a gain is any
factor that enter a servo-control, there is basically a gain in all
three cases.

>
> It is not good to have two methods if one of them does not work (at least to
> my experience, but I would be happy to hear from others that I am wrong). Am
> referring here to what you call "static" method (in PeriIso file). If you
> ever try to use it, you will see that it does not properly work. This is why
> my question about gain parameter.

I think Vaclav and Jan used it successfully, with a different
constitutive models than yours.
I know Jan also tried to adjust the stiffness during iterations, instead
of just predicting. So in principle it could work for many constitutive
laws.
I can't comment more personally, I never tried.
> Mass is not mass, given its current unit of measure which can be derived
> from the equation. As I see it, it is really a gain parameter (and I still
> do not see the advantage to use 1/mass when this could simply be called
> gain, unless you have a method which estimates the mass parameter from a
> physical point of view, do you?).
No, I don't have a clear derivation yet. I'm only guessing it needs to
include lengths somewhere. It's an opened question for me (and if I read
this book correctly, it's not explained either
http://www.iste.co.uk/index.php?f=a&ACTION=View&id=384). I suggest you
keep this question in mind for Radjaï in Alert school. ;-)

Still, it is an "inertia" (if you don't like mass...) because it acts on
acceleration, while a stiffness would act on position. Stiffness and
mass are all "gains" if they appear in a servo-control loop.
In the sense you use it here, gain is a very vague concept. Strictly
speaking, the unit of gain is dB and it is a function of frequency...

Bruno

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