[MBDyn-users] reference frames

Hoff, Stefan van 't Stefan.van.t.Hoff at nlr.nl
Tue Jul 6 17:05:56 CEST 2010


Hi Patrick,

I am still investigating which visualisation tool to use. I have experimented with Blender a bit already (as post-processing tool only), but it has similar issues w.r.t. file size because the time history of all the nodes is loaded directly into the Blender file (which can take a considerable amount of time and results in a Blender file even larger than the already large input file). Perhaps there it is possible to have Blender read the data from text or binary files online, but I have not pursued this as of yet. I hope to have a student to aid me with this issue soon. For now I will stick to setting low output frequencies (binary format would also be interesting).

Concerning the reference frames; my model also makes intensive use of reference frames in the definition of the (initial) geometry, but only for displacements (translational and rotational). What is the advantage of using a reference frame to introduce the rotor speed, when joints can achieve the same thing? The velocities are not required to define the geometry and, If I understood your previous email correctly, the awk script relies on a reference node, not a frame, so for visualisation purposes it also does not provide a benefit. Or am I still missing something?

Regards,

Stefan

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Today's Topics:

   1. Re: MBDyn-users Digest, Vol 14, Issue 4 (Hoff, Stefan van 't)
   2. error message (new user) (florent combes)
   3. Re: error message (new user) (Mattia Mattaboni)
   4. Re: reference frames (Rix, Patrick)


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Message: 1
Date: Tue, 6 Jul 2010 13:17:29 +0200
From: "Hoff, Stefan van 't" <Stefan.van.t.Hoff at nlr.nl>
Subject: Re: [MBDyn-users] MBDyn-users Digest, Vol 14, Issue 4
To: "'mbdyn-users at mbdyn.org'" <mbdyn-users at mbdyn.org>
Message-ID:
        <8731121C1035A94F8B5FA822B01DF3C9019D03FC3990 at EXCHANGE-ASD.nlr.nl>
Content-Type: text/plain;       charset="us-ascii"

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?

Regards,

Stefan

-----Original Message-----
From: mbdyn-users-bounces at mbdyn.org [mailto:mbdyn-users-bounces at mbdyn.org] On Behalf Of mbdyn-users-request at mbdyn.org
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Subject: MBDyn-users Digest, Vol 14, Issue 4

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Today's Topics:

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


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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>
Message-ID:
        <8149A8736D1A6E40ADE6328890200A2CE034A4F2F2 at EXCH-MBX-E.ulaval.ca>
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Hello,

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!

-Louis

------------------------------

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>
Message-ID:
        <D3A121E7C203B64DBE468B89CB95DDA2073CD45542 at SC1050EX0002.inter.rsag.site>

Content-Type: text/plain; charset="iso-8859-1"

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.

Regards,
Patrick.
________________________________
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

LS,

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.

Regards,

Stefan van 't Hoff



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Message: 2
Date: Tue, 6 Jul 2010 15:07:01 +0200
From: florent combes <combes.florent at gmail.com>
Subject: [MBDyn-users] error message (new user)
To: mbdyn-users at mbdyn.org
Message-ID:
        <AANLkTilNScSgog98-qNkZyHbf6N8wkbatTRLpuCT5rOA at mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"

Hello,

I am a new user of MBDyn and it seems that I have some .... difficulties
trying to make a simple test.
What I try to do is to make a cantilever with a beam (50 mm large), fixed at
its left corner, with a mass (100g) at its right corner,  a spring pushing
it down on the middle (I only implemented a force on the middle node) and a
force on the mass on the right to see what force I have to put to make the
mass rise.
So I implement all the elements I mentionned, like this:

___________________________________________________________________________
*    #Test du bras de levier avec masse attach?e et ressort

       begin: data;
           integrator: multistep; # par d?faut
       end: data;

       begin: multistep;
           initial time: 0.;
           final time: 11.;
           time step: 1.e-1;

           max iterations: 10;
           tolerance: 1.e-6;

           derivatives coefficient: 1.e-6;
       end: multistep;

       begin: control data;
           structural nodes: 3;
           rigid bodies: 2;
           forces: 2;
       joints: 1;
       beams: 1;
       end: control data;

       begin: nodes;
           # node 1, ? l'origine, fixe
           structural: 1, static, null, eye, 0.,0.,0., null;
           # node 2, en bout de bras, pas de vitesse de d?part
           structural: 2, dynamic, 50.,0.,0., eye, 0.,0.,0., null;
           # node 3, en milieu de bras, pas de vitesse de d?part, point de
liaison avec le ressort
           structural: 3, dynamic, 25.,0.,0., eye, 0.,0.,0., null;
       end: nodes;

       begin: elements;
       joint: 1, clamp, 1, node, node;
           body:  1, 2, 0.1, 50.,0.,0., eye;
       body:  2, 3, 0., null, eye;
       beam:  1, 1, null, 2, null, 3, null, eye, linear elastic generic,
diag, 1.e9, 1.e9, 1.e9, 1.e4, 1.e4, 1.e5, same, same;
           force: 1, absolute, 2, 0.,0.,1., null, const, 16;
       force: 2, absolute, 3, 0.,0.,-1., null, const, 50;

       end: elements;*
_____________________________________________________________________________________________________________

But t make an error at compilation:
_______________________________________________________________
*reading from file "bras"
Creating scalar solver with Y12 linear solver
Reading Structural(1)
Reading Structural(2)
Reading Structural(3)
Reading joint 1
Reading rigid body 1
Reading rigid body 2
Reading beam 1
Reading force 1
Reading force 2
An error occurred during the execution of MBDyn; aborting ...
_______________________________________________________________
*
I am totaly lost with an error without a clue to find where it is hiding....
It is why I request your help please.

Thanks for having a look at this
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Message: 3
Date: Tue, 06 Jul 2010 15:59:34 +0200
From: Mattia Mattaboni <mattaboni at aero.polimi.it>
Subject: Re: [MBDyn-users] error message (new user)
To: florent combes <combes.florent at gmail.com>
Cc: mbdyn-users at mbdyn.org
Message-ID: <4C3336C6.40802 at aero.polimi.it>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

florent combes wrote:
> Hello,
>
> I am a new user of MBDyn and it seems that I have some ....
> difficulties trying to make a simple test.
> What I try to do is to make a cantilever with a beam (50 mm large),
> fixed at its left corner, with a mass (100g) at its right corner,  a
> spring pushing it down on the middle (I only implemented a force on
> the middle node) and a force on the mass on the right to see what
> force I have to put to make the mass rise.
> So I implement all the elements I mentionned, like this:
> ___________________________________________________________________________
> /    #Test du bras de levier avec masse attach?e et ressort
>
>        begin: data;
>            integrator: multistep; # par d?faut
>        end: data;
>
>        begin: multistep;
>            initial time: 0.;
>            final time: 11.;
>            time step: 1.e-1;
>
>            max iterations: 10;
>            tolerance: 1.e-6;
>
>            derivatives coefficient: 1.e-6;
>        end: multistep;
>
>        begin: control data;
>            structural nodes: 3;
>            rigid bodies: 2;
>            forces: 2;
>        joints: 1;
>        beams: 1;
>        end: control data;
>
>        begin: nodes;
>            # node 1, ? l'origine, fixe
>            structural: 1, static, null, eye, 0.,0.,0., null;
>            # node 2, en bout de bras, pas de vitesse de d?part
>            structural: 2, dynamic, 50.,0.,0., eye, 0.,0.,0., null;
>            # node 3, en milieu de bras, pas de vitesse de d?part,
> point de liaison avec le ressort
>            structural: 3, dynamic, 25.,0.,0., eye, 0.,0.,0., null;
>        end: nodes;
>
>        begin: elements;
>        joint: 1, clamp, 1, node, node;
>            body:  1, 2, 0.1, 50.,0.,0., eye;
>        body:  2, 3, 0., null, eye;
>        beam:  1, 1, null, 2, null, 3, null, eye, linear elastic
> generic, diag, 1.e9, 1.e9, 1.e9, 1.e4, 1.e4, 1.e5, same, same;
>            force: 1, absolute, 2, 0.,0.,1., null, const, 16;
>        force: 2, absolute, 3, 0.,0.,-1., null, const, 50;
>
>        end: elements;/
> _____________________________________________________________________________________________________________
>
> But t make an error at compilation:
> _______________________________________________________________
> /reading from file "bras"
> Creating scalar solver with Y12 linear solver
> Reading Structural(1)
> Reading Structural(2)
> Reading Structural(3)
> Reading joint 1
> Reading rigid body 1
> Reading rigid body 2
> Reading beam 1
> Reading force 1
> Reading force 2
> An error occurred during the execution of MBDyn; aborting ...
> _______________________________________________________________
> /
> I am totaly lost with an error without a clue to find where it is
> hiding.... It is why I request your help please.

The problem should be that the body 2 has a null mass, you cannot define
a rigid body without mass.

And maybe there is also a mistake in the beam element because the second
node should be the node in the middle of the beam, whereas you are using
the node 2 that is placed on the tip of the beam. Similarly the third
node should be the tip node (node 3) but you are using the node in the
middle of the beam.

Cheers,
Mattia

>
> Thanks for having a look at this
> ------------------------------------------------------------------------
>
> _______________________________________________
> MBDyn-users mailing list
> MBDyn-users at mbdyn.org
> https://mail.mbdyn.org/cgi-bin/mailman/listinfo/mbdyn-users



------------------------------

Message: 4
Date: Tue, 6 Jul 2010 16:17:35 +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>
Message-ID:
        <D3A121E7C203B64DBE468B89CB95DDA2073CD456C8 at SC1050EX0002.inter.rsag.site>

Content-Type: text/plain; charset="iso-8859-1"

Stefan,

I agree: the awk post-processing of text files is a little - let's say sub-optimal. Especially when the model and the simulation time increases the Ascii result files become incredibly large (..we had text files up to >3GB..) what is more or less normal but makes the post-proc. very time consuming.
By the way..which tool are you using for visualisation / animation ? Blender, EasyAnim, something else ?

Maybe MBDyn could additonally spawn a call to awk running the abs2rel script by passing the current buffer content each time when it writes to the *.mov file anyway. Afterwards the new block of abs2rel'ed time steps has to be appended to a *.movR result file with the complete time series.
But what remains is that always all nodes will be abs2rel'ed w.r.t. the reference node, i.e. if you have e.g. a helicopter rotor with 4 blades and you want to have the deformation of each blade relative to its root node you would have to run abs2rel 4x over the complete *.mov file and thus having always 3 blades with useless data.
The process could be optimized (and evtl. speed up a little bit) using grep to extract the nodes belonging to a specific blade into a separate file, e.g. if blade 1 has 21 nodes with labels ranging from 1000 to 1020 (and blade 2,3,4 labels 2000..,3000..,4000..) then

  grep -E "    10" > blade_1.mov;  # note that the leading spaces have to match, too.

should extract all blade_1 nodes and finally you can run abs2rel over it.
But nevertheless: ascii post-processing remains relatively slow when it comes to large and many files.

I feel, it would be more advantegeous to have multiple but smaller files and of binary (NetCDF) format (using float values the data size is over 3 times smaller compared to the text format). I'm experimenting with that.
Having many but small units is also a good environment to easily bring the post-proc on a network cluster like CONDOR where you can have many machines working in parallel on the same task with each node machine working on small units.

Concerning the reference frames: our model intensively uses the inheritance among ref.frames and we use them a lot in the definiton of nodes - and also with non-zero velocities (e.g. to inherit the rotor speed from the hub ref. to the blade refs.).
So far it was quite useful for our needs.

Regards,
Patrick.



-----Urspr?ngliche Nachricht-----
Von: mbdyn-users-bounces at mbdyn.org [mailto:mbdyn-users-bounces at mbdyn.org] Im Auftrag von Hoff, Stefan van 't
Gesendet: Dienstag, 6. Juli 2010 13:17
An: 'mbdyn-users at mbdyn.org'
Betreff: Re: [MBDyn-users] MBDyn-users Digest, Vol 14, Issue 4

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?

Regards,

Stefan

Diese E-Mail enth?lt vertrauliche und/oder rechtlich gesch?tzte Informationen. Wenn Sie nicht der richtige Adressat sind oder diese E-Mail irrt?mlich erhalten haben, informieren Sie bitte umgehend den Absender und l?schen Sie diese E-Mail. Das unerlaubte Kopieren sowie die unbefugte Weitergabe der in dieser E-Mail enthaltenen Daten ist nicht gestattet. Wie Sie wissen, kann die Sicherheit von ?bermittlungen per E-Mail nicht gew?hrleistet werden, E-Mails k?nnen missbr?uchlich unter fremdem Namen erstellt oder ver?ndert werden. Aus diesem Grund bitten wir um Verst?ndnis daf?r, dass wir zu Ihrem und unserem Schutz die rechtliche Verbindlichkeit der vorstehenden Erkl?rungen ausschlie?en m?ssen. Diese Regelung gilt nur dann nicht, wenn wir mit Ihnen eine anderweitige schriftliche Vereinbarung ?ber die Einhaltung von Sicherheits- und Verschl?sselungsstandards getroffen haben.
This e-mail contains confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and delete this e-mail. Any unauthorised copying, disclosure or distribution of the material in this e-mail is strictly forbidden. As you know, the security of e-mail transmissions can not be guaranteed. E-mails can be misused to be written or modified under false names. For that reason, we ask you to understand the necessity for us to rule out the legal obligation of the above statement, for your protection and ours. This regulation is only invalid if we have concluded a special written agreement with you about the compliance with security and encryption standards.
REpower Systems AG Sitz: Hamburg Vorstand: Andreas Nauen (Vorsitz), Per Hornung Pedersen, Lars Rytter Kristensen, Derrick Noe, Matthias Schubert
Aufsichtsratsvorsitzender: Tulsi Tanti Registergericht: AG Hamburg (Mitte) HRB Nr.: 75543



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