[MBDyn-users] Hints

masarati at aero.polimi.it masarati at aero.polimi.it
Mon Mar 15 11:22:56 CET 2010


> I would like to use "Hints", so I made an example : I would like to add a
> velocity to the current velocity of the node to a node from T2 to T3 :
> "
>         driven: TEST, string, "(Time-T2)*(Time-T3) <= 0",
>                 hint, "drive{Var = model::xvelocity(NODE); ",
>         joint: TEST, linear velocity,
>                 NODE,
>                         1., 0., 0.,
>                         double step, T2, T3, Pulsion+Var, Var;
> "
> I tried severals way to write the joint, but each time, it imposes a
> velocity of "Pulsion" instead of what I would like "Pulsion + actual
> velocity". Which is the correct way to write it?

Hi.  You can't do it like that for a simple reason: the "linear velocity"
joint does not support hints (yet).  You could work things around a little
bit by using a total joint that imposes the **displacement** in that
direction.

You'd need to use hints to build a linear imposed displacement (or a ramp)
whose slope is the current velocity of the node plus the "Pulsion" value
you want to impose, starting from the current position of the node. 
Something like

     <http://www.mbdyn.org/documentation/examples/velocitypulse>

Please note that in this case you're imposing the **position** rather than
the **velocity**; so, if you look at the velocities, you'll see some
spikes related to numerical integration.  They can be smoothed out by
adding the additional velocity in a smoother manner, e.g. using a "ramp"
plus a "cosine" drive.

Cheers, p.



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