8.6 Overconstrained contact

When contact occurs between collidable bodies, the system creates a set of contact constraints to handle the collision and passes these to the physics solver (Section 1.2). When one or both of the contacting collidables are FEM models, the number of constraints is usually less than the DOF count of the collidables. However, if the both collidables are rigid bodies, then the number of constraints often exceeds the DOF count, a situation which is referred to as redundant contact. Redundant contact can also occur in other collision situations, such those involving embedded meshes (Section 6.3.2) when the mesh has a finer resolution than the embedding FEM, or skinned meshes (Chapter 11) when the number of contacts exceeds the DOF count of the underlying master bodies.

Redundant contact is not usually a problem in collisions between rigid bodies, because by default ArtiSynth handles these using CONTOUR_REGION contact (Section 8.4.1), for which contacts are supplied to the physics solver as unilateral constraints which are solved using complementarity techniques that can handle redundant constraints. However, as mentioned in Section 8.4.1.2, the default implementation for vertex penetration contact is to present the contacts to the physics solver as bilateral constraints (Section 1.2), which are removed between simulation steps. This results in much faster simulation times, and usually works well in the case of FEM models, where the DOF count almost always exceeds the number of constraints. However, when vertex penetration contact is employed between rigid bodies, and sometimes in other situations as described above, redundant contact can arise, and then the use of bilateral constraints results in overconstrained contact for which the solver has difficulty finding a proper solution.

A common indicator of overconstrained contact is for an error message to appear in ArtiSynth’s console output, which typically looks like this:

  Pardiso: num perturbed pivots=12

The simulation may also go unstable.

At present there are three general ways to manage overconstrained contact:

1. 1.

   Requiring the system to use unilateral constraints for vertex penetration contact. This can be done by setting the property bilateralVertexContact (Section 8.2.1) to false in either the collision manager or behavior, but the resulting hit to computational performance may be large.

2. 2.

   Constraint reduction, described in Section 8.6.1.

3. 3.

   Regularizing the contact, using one of the methods of Section 8.7.

When using the new implicit friction integration feature (Section 8.9.4), if redundant contacts arise, it is necessary to regularize both the contact constraints, as per Section 8.7, as well as the friction constraints, by setting the stictionCreep property of either the collision manager or behavior to a non-zero value, as described in Section 8.2.1.