[MBDyn-users] MBDyn-users Digest, Vol 14, Issue 4

Hoff, Stefan van 't Stefan.van.t.Hoff at nlr.nl
Tue Jul 6 13:17:29 CEST 2010

Hi Patrick,

Thanks for your input. It would seem then that the awk script will do what I require. The rotating reference frame would be for visualisation only (it is immaterial for the model definition), so this method would be acceptable. The downside is that it requires an additional post-processing step instead of generating the output in the rotating frame from the start (could this script not be called internal to MBDyn when triggered by a flag?).

That being said, I still fail to see the functionality of defining a reference frame with non-zero velocities. Perhaps to aid in the initial assembly of an impulsively loaded model?



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Today's Topics:

   1. Using MBDyn to model a tractor-trailer assembly (Louis Gagnon)
   2. Re: reference frames (Rix, Patrick)


Message: 1
Date: Mon, 5 Jul 2010 15:02:35 -0400
From: Louis Gagnon <louis.gagnon.10 at ulaval.ca>
Subject: [MBDyn-users] Using MBDyn to model a tractor-trailer assembly
To: "mbdyn-users at mbdyn.org" <mbdyn-users at mbdyn.org>
	<8149A8736D1A6E40ADE6328890200A2CE034A4F2F2 at EXCH-MBX-E.ulaval.ca>
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I have started looking into MBDyn recently and I am considering to use it as part of my phd work to model a tractor-trailer assembly (and an automobile). The model would have a fairly important number of DOFs and some "home-made" models for the tires and flexible components.

Is MBDyn capable of such simulation? Any one has experience modeling complicated systems with MBDYN? I did see an helicopter in the examples...

Also, would you recommend using the blender python scripts?

Thank you and have a good day!



Message: 2
Date: Tue, 6 Jul 2010 10:05:45 +0200
From: "Rix, Patrick" <Patrick.Rix at repower.de>
Subject: Re: [MBDyn-users] reference frames
To: "mbdyn-users at mbdyn.org" <mbdyn-users at mbdyn.org>
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Hi Stefan,

as far as I understood reference frames are just intended as a sort of construction helping device during the initial model assembly phase.
As you mentioned nodes that use reference frames in their pos.+orient.definition inherit the pos.+orient. & vel.+rot.vel. from the reference frames - but not the other way round, so: later during simulation when the node starts to move the ref.frame does not move with it as it is not really attached to the node. So what you desire, that a ref.frame follows a paritcular node or elem seems not to be possible (I'd like to have this option too).
I/we are modelling wind turbines (3 bladed, horizontal axis) which is somewhat similar to a helicopter rotor and we use (by now) a "revolute hinge" (as axial roation joint) and an additional "angular velocity joint" to impose a rotational vel. - as an alternative to applying a torque.

For the deformation in the rotating ref.frame there is a small awk-script called  "abs2rel.awk" in the /var subdir of the mbdyn-package.
This performs a transformation in the (rotating) frame of a rotating reference node.
e.g. :

gawk -f ./abs2rel.awk -v RefNode=1000 -v InputMode=euler123 -v OutputMode=euler123 ./your_model.mov  >  ./your_model.movR

would transform the motion data (given in global coordinates) of all nodes in file ?your_model.mov? into a moving (e.g. rotating) ref.frame of a specified reference node which is node 1000 in the example. The result would be redirected into the output file ?your_model.movR? .
(it's assumed the angles in the *.mov file are Euler angles which should be MBDyn's default - for other options look into the awk-script)

Hope this helps.

Von: mbdyn-users-bounces at mbdyn.org [mailto:mbdyn-users-bounces at mbdyn.org] Im Auftrag von Hoff, Stefan van 't
Gesendet: Sonntag, 4. Juli 2010 22:03
An: mbdyn-users at mbdyn.org
Betreff: [MBDyn-users] reference frames


I seem to be somewhat confused on the application of reference frames in MBDyn. It appears the reference components impart, say, their angular velocities to all the nodes that are refered to it (either initially or over the entire simulation, I'm not sure). What if I want the reference frame to simply follow a particular node or element instead? In my case I want to model a helicopter rotor where the rotor speed is input through, say, an axial rotation joint or perhaps an imposed (driven) torque. Secondly, I want the deformation of the blade to be (optionally) displayed in the rotating reference frame. However, since the rotor speed is (or may be) the product of the force equilibrium of the system, I do not know what the angular velocity of the reference frame should be a priori, nor is it always constant.

Is what I am looking to do achievable in MBDyn? If not, and I have to define a reference frame with the expected steady rotor speed instead, will this not interfere with the rotor speed as input through the axial rotation joint (causing problems with e.g. the initial assembly)?

Thanks in advance for your help.


Stefan van 't Hoff

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