Why this multibody system doesn't work in SystemModeler?

Questions
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1

Hi,
Sorry for the beginner’s doubt, but why the system below doesn’t work?

mass_bar1.png
mass_bar1.png1220×765 49 KB

When I try to run, it returns an error:

Error: Equation without any variables revealed after application of replacements:
    0.0 = 0.0   Equation before replacements:
    0.0 = atan2(spring1.lineForce.lineShape.widthDirection[2] * spring2.lineForce.lineShape.widthDirection[3] - spring1.lineForce.lineShape.widthDirection[3] * spring2.lineForce.lineShape.widthDirection[2], 1.0)   Replacements:
    spring2.lineForce.lineShape.widthDirection[3] -> 0.0
    spring2.lineForce.lineShape.widthDirection[2] -> 1.0
    spring1.lineForce.lineShape.widthDirection[3] -> 0.0
    spring1.lineForce.lineShape.widthDirection[2] -> 1.0                                                      Error: Equation without any variables revealed after application of replacements:
    0.0 = 0.0   Equation before replacements:
    0.0 = 1.5707963267949 * (1.0 - sign(spring1.lineForce.lineShape.widthDirection[3] * spring2.lineForce.lineShape.widthDirection[3] + spring1.lineForce.lineShape.widthDirection[1] * spring2.lineForce.lineShape.widthDirection[1] + spring1.lineForce.lineShape.widthDirection[2] * spring2.lineForce.lineShape.widthDirection[2]))   Replacements:
    spring2.lineForce.lineShape.widthDirection[3] -> 0.0
    spring2.lineForce.lineShape.widthDirection[2] -> 1.0
    spring2.lineForce.lineShape.widthDirection[1] -> 0.0
    spring1.lineForce.lineShape.widthDirection[3] -> 0.0
    spring1.lineForce.lineShape.widthDirection[2] -> 1.0
    spring1.lineForce.lineShape.widthDirection[1] -> 0.0 Error: Equation without any variables revealed after application of replacements:
    0.0 = 0.0   Equation before replacements:
    0.0 = atan2(spring1.lineForce.lineShape.widthDirection[1], 1.0)   Replacements:
    spring1.lineForce.lineShape.widthDirection[1] -> 0.0

But when I try the following system with one fixed point, it works:

mass_bar.png
mass_bar.png588×398 18.3 KB

I tried to initialize the body properly in each system, but just the second one works.
I know that there is some constraint issue but I can’t see clearly why it doesn’t work.
I can imagine the system working mechanically but bringing it to multibody approach in modelica it still hard for me.

Follow attached both models.

OBS: Gravity vector is {0,0,-1}.

mass_bar.mo (2.82 KB)
mass_bar1.mo (5.63 KB)

2

A single prismatic constraint is enough to force the whole system to remain horizontal, see the model attached. Due to the nature of the constraint, this will be the case even if the springs/dampers are set with different parameter values.

You can let the system rotate if you introduce a spherical constraint component, Modelica.Mechanics.MultiBody.Joints.Spherical, between the end of the prismatic and the horizontal translation that goes to the body. The rotation will clearly manifest if, for instance, you set the springs/dampers with different parameter values, or you play with the initial values.

mass_bar1_one_prismatic.mo (3.91 KB)