Changes between Initial Version and Version 1 of Ticket #5083


Ignore:
Timestamp:
2018-08-27T12:25:04Z (6 years ago)
Author:
Francesco Casella
Comment:

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  • Ticket #5083 – Description

    initial v1  
    1313  (-({-bodyBox1.frame_a.t[1], -0.0, -bodyBox1.frame_a.t[3]} * revolute1.e + revolute1.constantTorque.tau_constant)) / ({-0.0, -1.0, -0.0} * revolute1.e)
    1414}}}
    15 which seems to me plain wrong, and causes a division by zero, as {{{revolute1.e={0,0,1} }}}.
     15that would be OK if {{{{0,-1,0}*e <> 0}}}, which is unfortunately not the case here.
    1616
    1717The old FE instead expands the scalar product in the equation, passing it to the back-end as
    … …  
    2121which allows the back-end to process the equations correctly and eventually find the [https://libraries.openmodelica.org/branches/master/Modelica_trunk/files/Modelica_trunk_Modelica.Mechanics.MultiBody.Examples.Elementary.LineForceWithTwoMasses.sim correct result]. This issue currently prevents a lot of MultiBody models to simulate when using the NF.
    2222
    23 I see two problems here. The first is that the NF does not expand {{{revolute1.e}}} into its components and does not compute the scalar product explicitly, as the OF did. If this could be fixed, we should hopefully get most of the MultiBody models to work with the NF.
     23I see two problems here. The first is that the NF does not expand {{{revolute1.e}}} into its components and subsequently compute the scalar product explicitly, as the OF did. If this could be fixed, we should hopefully get most of the MultiBody models to work with the NF.
    2424
    25 The second is that the back-end solves an equation in a way that I cannot explain and seems to me plain wrong. I'll open a separate ticket on this topic.
     25The second is that the back-end solves an equation in a dubious way, which turns out to be singular. I'm not sure whether it is possible to avoid this behaviour without introducing unnecessary restrictions that could harm the successful symbolic solution of equations in other cases.
     26
     27In any case, I guess it will be best to make sure that the NF expands the scalar product as the NF did.