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07-21-2016 02:10 AM

What is the DASSL error in 3D mechanical model?

some time, 3D mechanical model is not working, because DASSL error,

here is simple 3D model

this model also can not work bacuase DASSL error

so, what is meaning of "Error detected in DASSL"

and, How can I fix **this model**?

Solved! Go to Solution.

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4 REPLIES

Solution

Solution

Accepted by topic author Kuro

07-22-2016
08:44 AM

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07-21-2016 04:30 AM - edited 07-21-2016 04:32 AM

Hi Kuro,

DASSL is the solver which is used when there are implicits variables in your system.

Without the model it's tricky to say why the initializations fails.

What you can try is to remove the implicits variables from your system.

With the 3D mechanical library these implicit variables are generated by the default junction submodels.

To do so, go to Submodel mode, select the junctions and change their submodels to the ones using contact forces:

This will remove the implict variables:

If this doesn't solve your issue I suggest you contact your hotline so they can have a look at your model.

I hope this helps

Emmanuel

Solution

Solution

Accepted by topic author Kuro

07-22-2016
08:44 AM

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07-21-2016 11:42 AM

Hi Kuro,

Your system is over constrained. The single body has 6 degrees of freedom. Each 3D ball joint (M6DOFBALL00) constraints 3 translations and allows 3 rotations. So with two of these joints you total number of degrees of freedom is 6-3-3=0

The best way to model your system is to use one 3D annular joint, submodel M6DOFTRBA00, which constraints only 2 translational degrees of freedom, and allow the one translational degree of freedom in y-direction for one joint. You will then have 6-3-2=1 degree of freedom and the DASSL solver has no issues.

This joint will look as follow (in 'Parameter Mode'):

Alternatively you could change the submodel of one 3D ball joint to M6DOFBALL01 which has all 6 degrees of freedom unconstrained, but limits the 3 translations by means of explicit states from the contact force (you have to provide a contact stiffness and damping for the translational directions). These submodels with contact forces are useful to eliminate impicit states, which require the DASSL solver (solver of differential algebraic equations or DAE solver).

Bob

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07-22-2016 08:51 AM

Now I know why DASSL error has occurred at times and how can I handle it

Thanks for your answer

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07-22-2016 10:13 AM

Thank you for your reply

Now, the meaning of redundant constraints has been crystal clear...

Single body has 6 DOF so...

two pivot joint (5 constraints + 5 constraints) = 4 redundant constraints

one pivot joint and one annular linear joint (5 constraints + 2 constraints) = 1 redundant constraints

one pivot joint and one ball joint (5 constraints + 3 constraints) = 2 redundant constraints

I've got it!

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