6.6 Frame attachments

A model illustrating how to connect frames to an FEM model is defined in

  artisynth.demos.tutorial.FrameFemAttachment

It creates an FEM beam, along with a rigid body block and a massless coordinate frame, that are then attached to the beam using nodal and element-based attachments. The build method is shown below:

Lines 3-22 create a MechModel and populate it with an FEM beam and a rigid body box. Next, a basic Frame is created, with a specified pose and an axis length of 0.3 (to allow it to be seen), and added to the MechModel (lines 25-28). It is then attached to the FEM beam using an element-based attachment (line 30). Meanwhile, the box is attached to using a nodal-based attachment, created from all the nodes associated with elements 22 and 31 (lines 33-36). Finally, all attachment nodes are set to be rendered as green spheres (lines 39-41).

To run this example in ArtiSynth, select All demos > tutorial > FrameFemAttachment from the Models menu. Running the model will cause it to settle into the state shown in Figure 6.12. Forces can interactively be applied to the attached block and frame using the pull tool, causing the FEM model to deform (see the section “Pull Manipulation” in the ArtiSynth User Interface Guide).

A FemAttachedFrame is an AttachedFrame that is connected to a FemModel3d, analogous to FrameAttachedFrame components that are connected to Frames (Section 3.10.1). The attachment is element-based: the frame’s position and orientation track a weighted combination of the surrounding FEM nodes, using natural coordinates within the enclosing (or nearest) element. The frame therefore moves and changes orientation as the FE model deforms.

FemAttachedFrame objects can be created using the constructors

where FemAttachedFrame() creates a default frame with no attachment, FemAttachedFrame(TFW) sets the initial world pose to TFW, and FemAttachedFrame(elem,TFW) also attaches the frame to the specified element. Once created, the nature of the FE attachment can be set or changed using

The setFromNodes() methods mirror those available for FemMarker (Section 6.4.8): the first two take explicit weights, while the last two determine weights automatically using inverse-distance weighting as described by equation (6.4).

Once created, the attached frame must be added to the MechModel hierarchy. One choice is to add them directly to the FE model to which they are attached. Each FemModel3d maintains an attachedFrames container, which may be managed using the following FemModel3d methods:

Alternatively, attached frames can be added to the MechModel’s frames container, which is managed using the MechModel methods

The following model demonstrates how to add FemAttachedFrames to an FEM beam:

This example can be found in artisynth.demos.tutorial.FemAttachedFrames. It extends FemBeam (Section 6.2.5), adding five evenly-spaced FemAttachedFrames along the beam’s local -axis (lines 20–27). Each frame is positioned at a world-coordinate pose computed from the beam’s length, and added to the FEM model using fem.addAttachedFrame(TFW), which automatically finds and attaches to the nearest element. The frames are rendered as solid arrow triads with an axis length of 0.12. Finally, the FEM surface is made semi-transparent (line 29) so the embedded frames are visible.

To run this example in ArtiSynth, select All demos > tutorial > FemAttachedFrames from the Models menu. The model should load and initially appear as in Figure 6.13. Running the model will cause the beam to fall under gravity, with all attached frames moving and rotating to track the deformation of the beam.

The ability to connect frames to FEM models, as described in Section 6.6, makes it possible to interconnect different FEM models directly using joints, as described in Section 3.4. This is done internally by using FrameFem3dAttachments to connect frames C and D of the joint (Figure 3.9) to their respective FEM models.

As indicated in Section 3.4.3, most joints have a constructor of the form

that creates a joint connecting bodyA to bodyB, with the initial pose of the D frame given (in world coordinates) by TDW. The same body and transform settings can be made on an existing joint using the setBodies() methods also described in Section 3.4.3. For these constructors and methods, it is possible to specify FEM models for bodyA and/or bodyB. Internally, the joint then creates a FrameFem3dAttachment to connect frame C and/or D of the joint (See Figure 3.9) to the corresponding FEM model.

However, unlike joints involving rigid bodies or frames, there are no associated or transforms (since there is no fixed frame within an FEM to define such transforms). Methods or constructors which utilize or can therefore not be used with FEM models.

Sometimes, the default FrameFem3dAttachment attachment created by the joint constructor (or the setBodies() method) is not suitable, because the attachment’s support must be distributed across a node set that is either larger, or distributed differently. In that case, one may create a custom FrameFem3dAttachment using a specified set of nodes (and weights, if needed), as described in Section 6.6. This can then be used to connect the joint to the FEM via one of the setBodies() or setBodyA/B() methods that accepts custom FrameAttachments:

Note that when being used as a joint connector, FrameFem3dAttachment does not need to be associated with an actual frame component.

A model connecting two FEM beams by a joint is defined in

  artisynth.demos.tutorial.JointedFemBeams

It creates two FEM beams and connects them via a special slotted-revolute joint. The build method is shown below:

Lines 3-16 create a MechModel and populate it with two FEM beams, fem1 and fem2, using an auxiliary method addFem() defined in the model source file. The leftmost nodes of fem1 are set fixed. A SlottedRevoluteJoint is then created to interconnect fem1 and fem2 at a location specified by TDW (lines 19-21). Lines 24-29 set some parameters for the joint, along with various render properties.

To run this example in ArtiSynth, select All demos > tutorial > JointedFemBeams from the Models menu. Running the model will cause it to drop and flex under gravity, as shown in Figure 6.14. Forces can interactively be applied to the beams using the pull tool (see the section “Pull Manipulation” in the ArtiSynth User Interface Guide).