simscape problem degenerate mass:connection revolute and rectangular joint. Stop simulation

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Elisa Panero
Elisa Panero on 22 Jun 2018
Answered: Steve Miller on 25 Jun 2021
Hi guys. I am working with simscape multibody modelling with the attempt to simulate the interaction between human body and exoskeleton. I am modeling the interface between the two of them with kinematic constraints, particularly with rectangular and revolute joint in series. I know that there is the possibility to model the connection with the contact forces library, but at the moment I am trying to work with kinematic constraints. If I try to start the simulation it gives me an error (degenerate mass). If I interpose a virtual mass between the two joints it sounds good. I've found similar past questions in the forum but I don't understand why the model doesn't work. Is it a problem of my model or is there the impossibility to connect rectangular and revolute joint in series?
Thanks in advance!

Answers (1)

Steve Miller
Steve Miller on 25 Jun 2021
Hi Elisa,
You will generally get error messages about degenerate mass distribution if you have a degree of freedom that has no inertia associated with it. For example, if you have two prismatic joints connected in series with no inertia between them and a force is applied along that axis, you will get infinite acceleration which is not physical and causes all kinds of numerical issues.
It depends how you have connected your joints and oriented your frames as to whether or not the combination you highlighted (Rectangular Joint + Revolute Joint) would lead to this issue. I wouldn't expect it there, but I would expect it with a Planar Joint - Revolute Joint, because you could have a rotational degree of freedom with no inertia associated with it.

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