What is the meaning of phi_nominal in Mechanics.Rotational.PartialCompliantWithRelativeStates

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Modified by dietmarw on 31 Oct 2011 08:39 UTC
The partial model Mechanics.Rotational.PartialCompliantWithRelativeStates has a parameter, phi_nominal, which is used to set phi_rel.nominal. The comment states that this is the "Nominal value of phi_rel (used for scaling)".

But phi_rel is an angle of rotation, which can range anywhere from -infinity to infinity. What is the purpose of scaling it at all? Even if the angles were kept within a single rotation, then the default nominal value of 1e-4 makes no sense, since the angle would typically range from -pi to pi.

This came up because a user set phi_nominal to zero. This makes sense from a physical point of view, but of course makes no sense for scaling purposes.

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Reported by stefanv on 22 Sep 2011 15:31 UTC
The partial model Mechanics.Rotational.PartialCompliantWithRelativeStates has a parameter, phi_nominal, which is used to set phi_rel.nominal. The comment states that this is the "Nominal value of phi_rel (used for scaling)".

But phi_rel is an angle of rotation, which can range anywhere from -infinity to infinity. What is the purpose of scaling it at all? Even if the angles were kept within a single rotation, then the default nominal value of 1e-4 makes no sense, since the angle would typically range from -pi to pi.

This came up because a user set {{phi_nominal}}} to zero. This makes sense from a physical point of view, but of course makes no sense for scaling purposes.

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Migrated-From: https://trac.modelica.org/Modelica/ticket/615

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Modified by dietmarw on 31 Oct 2011 08:39 UTC

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Comment by otter on 10 Dec 2012 15:11 UTC
Fixed in b8a1114:

A new chapter Modelica.Mechanics.Rotational.UsersGuide.StateSelection has been added to explain state selection in the Rotational library and the meaning of the phi_nominal parameter. Additionally, a guard was introduced to use a nominal value of 1, if phi_nominal < Modelica.Constants.eps to cope with a wrong user input of 0.

Please read the new chapter for a longer explanation. In short:

• The default value of phi_nominal=1e-4 is defined for an elastic deformation of a drive train component that is in this order (like the SpringDamper element). A phi_nominal = 1 would most of the time switch off error control for this variable.
• For other components, like the Clutch, a phi_nominal = 1 should be set. However, the default value of 1e-4 does not hurt in this case. The only effect is that the simulation might become a bit slower, since the required error tolerance is more stringent.

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Changelog removed by otter on 10 Dec 2012 15:11 UTC