Error in LineForce handling of potential root

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Reported by HansOlsson on 6 Mar 2008 08:13 UTC
None

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Migrated-From: https://trac.modelica.org/Modelica/ticket/50

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Comment by HansOlsson on 6 Mar 2008 08:19 UTC
When examining the LineForce I noticed this was similar to PointMass – and I remembered that we had to correct PointMass since it could lead to incorrect simulation results – basically if a pointmass was selected as root and something was connected to it was as if the point mass got an infinite rotational inertia.

Thus this got me worried; and, indeed, the LineForce has the same defect; I have attached two models that should behave very similarly – but they differ considerably in the result – since the frametranslation only is allowed to rotate in the ThreeSprings2 model –but not in the other model (you can also see that the frametranslation looks very odd in this model). My understanding is that this model is partially undefined – the frametranslation can freely rotate along its axis (i.e. one unspecified rotational degree of freedom – due to small inertia it is rotating very rapidly around this axis); whereas the original library model has 3 unspecified rotational d.o.fs.

The bug has been present since the early days of Modelica 2 (at least), and furthermore I cannot find any good correction of this. Doing as for PointMass would make it impossible to simulate the library example model ThreeSprings, and the documentation of ThreeSprings says "This example demonstrates that 3-dimensional line force elements (here: Modelica.Mechanics.MultiBody.Forces.Spring elements) can be connected together in series without having a body with mass at the connection point (as usually required by multi-body programs). This is advantageous since stiff systems can be avoided, say, due to a stiff spring and a small mass at the connection point"

It might be that this is a general problem with low priority potential roots.

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Modified by dietmarw on 6 Mar 2008 14:27 UTC

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Modified by HansOlsson on 30 May 2008 07:38 UTC

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Modified by otter on 21 Jan 2009 11:53 UTC

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Modified by otter on 21 Jun 2009 10:17 UTC

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Comment by dietmarw on 25 Nov 2009 11:53 UTC
Milestone MSL3.1.1 deleted

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Comment by otter on 8 Aug 2010 21:02 UTC
Fixed in c969687.

The problem is fixed by removing the Connections.potentialRoot(..) definitions in all line force elements and instead adding a flag to explicitly set the orientation matrix at frame_a or frame_b of the line force element. By default, these flags are false and therefore direct connection of line force elements gives a translation error. If the flags are appropriately set, the connection of line force elements is possible. The details are explained in a new section of the users guide (MultiBody.UsersGuide.Tutorial.ConnectionOfLineForces).

All example and test models with series connection of line force elements have been appropriately adapted.