[MBDyn-users] Beginners advice on blocking movement and modifying the model during a run

Yngve Lööf yngve_l at hotmail.com
Thu Jun 30 12:14:08 CEST 2016

I recently downloaded MBDyn. I am very impressed by the software and I am slowly trying to understand the user interface.
I will try to limit my questions to two, for the time being :)
1. Which is the simplest way to introduce a "block"?=======================================
This double pendulum is driven by a force/torque on the inner (blue) link. There is no gravity.
The first thing that happens is that the outer (red) link will DECREASE its angle to the inner link. (The center of mass of the red link has also been moved outwards.) But I wish to constrain the angle between the two links so it never goes below a preset value, maybe around 90 degrees. (I wish the angle between the links starts to increase when the blue link drives the rotation. The angle between the links should never decrease.)
One way to work around this would be to push a little with a force on the red link until centrifugal force took over and threw the red link out. But is there a simpler way to constrain this hinge so it never goes below a certain value (about 90 degrees)?
I think there is joint constraint in MBDyn, but I haven't really understood how to use it.
I would like to introduce the same kind of "block" (restriction of movement beyond a certain angle) for pin joints and also for just any moving (translation) object. An object suddenly "hits the wall". (I just want to stop the movement and see what happens to the rest of the objects.) (It is naturally good if the stopping could be done a little "softly".)

2. How do I change characteristics of my model?=====================================Maybe this question has a similar answer as the first question. Let's say that the joint between the two links in the pendulum above suddenly broke. The red link would be thrown out in a translation/rotation movement.
In other models I would like to suddenly connect things. Assume two blocks floating towards each other in space. They suddenly connect and start to rotate. How can introduce this connection (or remove it). I imagine I could maybe stop the simulation, record the position and velocity data of the .mov file, make a new model and start it with these data. In a simulation my MATLB script would have to read both these .mov files consecutively.
But is there a simpler way, just within one ".mbd" file/model, to introduce and remove connections?
Sorry for writing long email with beginner's questions. Thank you for any pointers on this.
I am a retired medical doctor. I have no solid engineering background, so I apologize for not keeping up in some areas. I have often come in contact with signal processing and programming, but not mechanics, though. In due course I hope to use multi body dynamics in bio-mechanics.
Since I am completely new here, I appreciate any advice. (In particular instruction/examples on how to use the user interface, the .mbd file.) 
Thanks for putting such excellent software in the public domain.
Best regards,
John Lof
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