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The finite element solver supports various types of constraints that you to influence and control the simulation. A common example is where the finite element simulation needs to partially follow a target animation. In addition to these target animation constraints there are constraints that may connect simulated points together, such as Fuse Constraints. Furthermore, the finite element solver supports SBD Spring constraints between points. Region constraints allow you to constrain regions inside a solid object, without requiring a matching topology.
Supported FEM Constraint Types
The following constraint types are supported only by the FEM Solver:
Built-in Target Constraints for the FEM Objects (distributed)
FEM Target Constraint (lumped)
FEM Fuse Constraint
FEM Surface Attach Constraint
FEM Surface Slide Constraint
FEM Region Constraint
In addition, the following constraint types are recognized both by FEM and the Wire Solver:
SBD Pin Constraint
SBD Spring Constraint
FEM Object Built-In Target Constraints (distributed)
Each FEM Solid Object and FEM Hybrid Object has a target constraint built in. This target constraint is optimized for smoothness: This built-in target constraint gives you the best possible result with FEM if you are trying to guide a sim with a previously existing animation that has the exact same topology. Instead of pulling exclusively on the primitive vertices, the solver distributes the pull of the target constraint evenly over all the interior points of the tetrahedra and triangles. Continuous target constraints are useful when you want to smoothly guide or influence a simulation with a previously existing animation for the same topology.
The distributed soft target constraints are completely distributed over an entire object. The Solid Object and the Hybrid Object have Target Strength and Target Damping controls. These controls the densities of the soft constraint forces (per cubed unit of length). This is very useful when you want to mix simulation with pre-existing animation, but you don’t want any visible artifacts between constrained and unconstrained regions. The solver recognizes point attributes
targetdamping that act as multipliers for the corresponding parameters on the FEM Object. These can be used to paint a smooth falloff from constrained regions to unconstrained regions.
The target animation may be specified in two ways. The easiest way is by pointing the Target Deformation of your Solid Object or Hybrid Object to the target animation. At each solve step, the finite element solver will look up the animated geometry for you at the corresponding times. In a more advanced setup, you may you may specify a
targetP point attribute on the simulation geometry instead of using the Target Deformation. However, to animate
targetP over time a SOP solver will be needed in the DOP network.
When using the built-in target constraints it is recommended that you choose Target Strength as low as possible without making the constrained simulated points diverge too much from the animation. Pick the lowest value that will give you good looking results. Picking excessively large values will do the job in theory, but this is not recommended in practice because this may adversely affect the performance of the solver.
FEM Target Constraints (lumped)
FEM Target Constraint: In contrast to the built-in continuous target constraint, this DOP constraint allows individual points to be pulled by target positions. The impact of these forces are local to the specified points only; in contrast to the smooth built-in target constraint, FEM Target Constraints only affect the points that are specified (not their neighboring points). These lumped target constraints are most useful when you want to constrain individual vertices of your simulation geometry to an animation.
Lumped target soft constraints are soft constraints where the force is concentrated in individual points instead of being distributed over the entire continuum of the Solid or Hybrid Object. Such constraints can be created by putting down an Finite Element Target Constraint and setting its Type to SOFT. When the SOFT type is used, the parameters under the Soft Controls tab can be used to control the strength of the soft constraints. The higher the Target Strength, the larger the forces on the constrained points will be an the closer the simulation will try to match the animation. The Target Damping parameter may need to be fine tuned in order to prevent the soft constraints from overshooting and oscillating around the target positions. The lumped soft constraints of the FEM Target Constraint are the preferred option when a discrete selection of points needs to be constrained (instead of a smooth region). An example use case is when you want to constrain a few corners of a box object.
Hard target constraints may be created in two ways. The easiest way is to create an Finite Element Target Constraint and set its Type to HARD. You may use the Constrain Points to Target shelf tool to create an Finite Element Target Constraint. The other, more advanced option, is to create an integer point attribute that is called
pintoanimation, all points that have the value 1 will follow the target animation exactly. All points that have the value 0 are left unaffected.
FEM Fuse Constraint
In addition to constraints between simulation and animation, there are also constraints that have other simulated points as a goal. For example, the Finite Element Fuse Constraint can be used to connect pairs of points. When this constraint is set to HARD, then each pair of constrained points effectively becomes a single point. This feature may be used to form a single connected object from multiple objects or multiple connected components within a simulated object. In SOFT mode, the fuse constraint acts more like a spring between the points, but without a directional bias.
FEM Attach Constraint
This allows points of your FEM Solid Object or FEM Hybrid Object to be constrained to the surface of another FEM Object or to a Static Object. Each point in the constrained point group is constrained to a fixed relative position on a goal object. The goal object’s geometry may consist of tetrahedra, polygons, polylines or a particle system of points. It is also possible to create attachments between connected components of the same object by specifying the same object twice in the constraint
FEM Slide Constraint
This allows a specified group of points of an FEM Solid Object or an FEM Hybrid Object to slide against the surface of another FEM Object. This may be useful in a muscle simulation to keep groups of muscles together. In a tissue/skin simulation, an FEM Slide Constraint may be used to keep the skin sliding on and sticking to underlying bones and muscles or a fascia surface. It is possible to create slide constraints between distinct components of the same FEM object by specifying this object twice in the FEM Slide Constraint.
FEM Region Constraints
FEM Region Constraint allows regions within a simulated FEM object to be constrained without requiring a matching topology. This constraint may work in both directions, because both objects involved in an FEM Region Constraint may be FEM Solid Object or FEM Hybrid Objects. This constraint currently works between tets and tets (not between triangles and tets or triangles and triangles).
Region constraints are a type of soft constraint that can be used to virtually merge multiple solid objects and hybrid objects together. For example, using region constraints objects can be put inside each other without actually merging their geometries into a single geometry. As an example, region constraints are used in the muscle system to put muscles and bones into a surrounding organic tissue, where each of these objects are simulated by the finite element solver. The Strength and Damping parameters on the Region constraints have the same units as the Target Strength and Target Damping parameters on the Solid/Hybrid Object, respectively.
SBD Spring Constraint
Springs between points of simulated objects are supported as well, by means of the SBD Spring constraint.
Using Soft Constraints for Reducing Relative Velocities
In the most common use cases of soft constraints, the constraints act on the positions of simulation objects; we want the positions of the vertices (FEM nodes) to loosely match the position of other vertices of another simulation object or positions in an existing animation. However, sometimes it may be useful for match the velocities instead of positions. For example, if an object is attached to a fast moving object, we may want to use the damping feature of the soft constraint to make the constrained object match the velocity of the other object. This approach may be used with several FEM constraints, including the FEM Attach Constraint, the FEM Slide Constraint, and the FEM Region Constraint.
Hard versus Soft Constraint Modes
The FEM solver supports two main types of constraints: hard and soft constraints. In the case of hard constraints, the constrained points are made to follow the prescribed target animation exactly. Soft constraints are a method of constraining that mixes simulation and animation - the constrained points do not follow the target animation exactly. You can control how closely the soft constraints follow the target animation by means of a Target Strength parameter. Hard constraints have the advantage that you are guaranteed that the constrained points follow the animation exactly. When soft constraints are used, it is often necessary to try multiple different values of the Target Strength before the right amount of matching is achieved. Soft constraints are generally preferred, because they lead to more forgiving setups where less can go wrong.