Introduces a new "strain" equality mode (flex_edgeequality = 3) for flexcomps with trilinear interpolation. This mode enforces constraints on the three invariants of the Green-Lagrange strain tensor at 8 Gauss quadrature points within each flex element, resulting in 24 equality constraints per flexcomp.
Also fixes a bug when attaching a flex with constraints. PiperOrigin-RevId: 881349765 Change-Id: I8e6df8239488bfdc87d7e5b473568975e38c737e
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Alessio Quaglino
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Copybara-Service
Copybara-Service
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@@ -3614,7 +3614,9 @@ saving the XML:
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for the entire flex, independent of the number of vertices. The positions of the vertices are updated using
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quadratic interpolation over the bounding box. While this option requires more degrees of freedom than trilinear
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flexes, it enables curved deformation modes, while the only modes achievable for trilinear flexes are
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strech/compression and shear.
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strech/compression and shear. To understand the difference between the two parametrizations, see `a trilinear cube
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<https://github.com/google-deepmind/mujoco/blob/main/model/flex/trilinear.xml>`__ and `a quadratic cube
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<https://github.com/google-deepmind/mujoco/blob/main/model/flex/quadratic.xml>`__.
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Note that a higher interpolation order generally requires a smaller time step for stability, although usually not as
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large as with the "full" option and a fine mesh.
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@@ -3820,10 +3822,12 @@ element is used to adjust the properties of all edges in the flex.
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.. _flexcomp-edge-equality:
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:at:`equality`: :at-val:`[false, true, vert], "false"`
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The type of equality constraint applied to this edge. If **false**, no equality constraint is applied. If **true**,
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then edge constraints are enforced. If **vert**, an averaged constraint is used, see
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:ref:`flexvert<equality-flexvert>`.
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:at:`equality`: :at-val:`[false, true, vert, strain], "false"`
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The type of equality constraint applied to this edge. If :at-val:`false`, no equality constraint is applied. If
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:at-val:`true`, then edge constraints are enforced. If :at-val:`vert`, an averaged constraint is used, see
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:ref:`flexvert<equality-flexvert>`. if :at-val:`strain`, then a constraint is added to enforce that the invariants of
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the strain tensor do not change; this is only equality constraint type supported for trilinear and quadratic
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:ref:`dofs<body-flexcomp-dof>` elements and :ref:`here<equality-flexstrain>`.
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.. _flexcomp-edge-solref:
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.. _flexcomp-edge-solimp:
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@@ -4254,7 +4258,8 @@ The elasticity model is a `Saint Venant-Kirchhoff
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<https://en.wikipedia.org/wiki/Hyperelastic_material#Saint_Venant%E2%80%93Kirchhoff_model>`__ model discretized with
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piecewise linear finite elements, intended to simulate the compression or elongation of hyperelastic materials subjected
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to large displacements (finite rotations) and small strains, since it uses a nonlinear strain-displacement but a linear
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stress-strain relationship.. See also :ref:`deformable <CDeformable>` objects.
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stress-strain relationship. See also :ref:`deformable <CDeformable>` objects and `this model
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<https://github.com/google-deepmind/mujoco/blob/main/model/flex/floppy.xml>`__.
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.. _flex-elasticity-young:
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@@ -4787,7 +4792,8 @@ This element constrains the length of one tendon to be a quartic polynomial of a
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This element constrains the lengths of all edges of a specified flex to their respective lengths in the initial model
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configuration. In this way the edges are used to maintain the shape of the deformable entity. Note that all other
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equality constraint types add a fixed number of scalar constraints, while this element adds as many scalar constraints
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as there are edges in the specified flex.
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as there are edges in the specified flex. See `this model
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<https://github.com/google-deepmind/mujoco/blob/main/model/flex/plate.xml>`__ for an example.
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.. _equality-flex-name:
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.. _equality-flex-class:
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@@ -4811,8 +4817,9 @@ as there are edges in the specified flex.
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This element constrains the trace and the derminant of the strain tensor to that of the identity matrix as in Chen, Kry,
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and Vouga, "Locking-free Simulation of Isometric Thin Plates", 2019. The strain tensor is computed per triangle and
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averaged over all triangles adjacent to a vertex. This reduces the number of constraints from 2T to 2V, freeing
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V degrees of freedom to avoid locking. It is only supported for dimension 2, i.e., cloth-like flexes.
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averaged over all triangles adjacent to a vertex. This reduces the number of constraints from 2T to 2V, freeing V
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degrees of freedom to avoid locking. It is only supported for dimension 2, i.e., cloth-like flexes. See `this model
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<https://github.com/google-deepmind/mujoco/blob/main/model/flex/poncho.xml>`__ for an example.
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.. _equality-flexvert-name:
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.. _equality-flexvert-class:
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@@ -4829,6 +4836,32 @@ V degrees of freedom to avoid locking. It is only supported for dimension 2, i.e
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Name of the flex whose vertices are being constrained.
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.. _equality-flexstrain:
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:el-prefix:`equality/` |-| **flexstrain** |*|
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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This element constrains the strain invariants of a trilinear or quadratic flex to their initial values. Specifically, it
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enforces that the trace and determinant of the deformation gradient remain constant, preserving volume and preventing
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excessive stretching. This constraint type is only supported for dimension 3 trilinear flexes (i.e., volumetric
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deformable bodies using trilinear interpolation). See `this model
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<https://github.com/google-deepmind/mujoco/blob/main/model/flex/strain.xml>`__ for an example.
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.. _equality-flexstrain-name:
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.. _equality-flexstrain-class:
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.. _equality-flexstrain-active:
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.. _equality-flexstrain-solref:
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.. _equality-flexstrain-solimp:
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:at:`name`, :at:`class`, :at:`active`, :at:`solref`, :at:`solimp`
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Same as in :ref:`connect <equality-connect>` element.
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.. _equality-flexstrain-flex:
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:at:`flex`: :at-val:`string, required`
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Name of the flex whose strain invariants are being constrained.
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.. _equality-distance:
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:el-prefix:`equality/` |-| **distance** |*|
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@@ -2045,6 +2045,30 @@
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:ref:`solimp<equality-flexvert-solimp>`
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.. dropdown:: :ref:`flexstrain<equality-flexstrain>` |*|
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.. grid:: 2 3 4 4
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:gutter: 0
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.. grid-item::
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:ref:`name<equality-flexstrain-name>`
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.. grid-item::
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:ref:`class<equality-flexstrain-class>`
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.. grid-item::
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:ref:`flex<equality-flexstrain-flex>`
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.. grid-item::
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:ref:`active<equality-flexstrain-active>`
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.. grid-item::
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:ref:`solref<equality-flexstrain-solref>`
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.. grid-item::
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:ref:`solimp<equality-flexstrain-solimp>`
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.. dropdown:: :ref:`tendon<tendon>` |*|
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@@ -15,6 +15,8 @@ General
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``ten_J_colind`` have been moved from :ref:`mjData` to :ref:`mjModel` and are no longer computed at run time by
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``mj_tendon`` but at compile time.
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- Added a new ``strain`` :ref:`equality constraint<flexcomp-edge-equality>` type for trilinear and quadratic
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:ref:`dofs<body-flexcomp-dof>`.
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- Flexes now support collisions with SDF geoms.
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- Improved memory requirements for ``ten_J`` and ``ten_J_colind`` by reducing the upper bound for the number
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of non-zeros ``nJten``.
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@@ -609,6 +609,7 @@ typedef enum mjtEq_ { // type of equality constraint
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mjEQ_TENDON, // couple the lengths of two tendons with cubic
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mjEQ_FLEX, // fix all edge lengths of a flex
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mjEQ_FLEXVERT, // fix all vertex lengths of a flex
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mjEQ_FLEXSTRAIN, // fix strain invariants of a trilinear flex
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mjEQ_DISTANCE // unsupported, will cause an error if used
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} mjtEq;
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typedef enum mjtWrap_ { // type of tendon wrap object
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@@ -1350,7 +1351,7 @@ struct mjModel_ {
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mjtNum* flex_damping; // Rayleigh's damping coefficient (nflex x 1)
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mjtNum* flex_edgestiffness; // edge stiffness (nflex x 1)
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mjtNum* flex_edgedamping; // edge damping (nflex x 1)
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int* flex_edgeequality; // 0: none, 1: edges, 2: vertices (nflex x 1)
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int* flex_edgeequality; // 0:none, 1:edges, 2:vertices, 3:strain (nflex x 1)
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mjtByte* flex_rigid; // are all vertices in the same body (nflex x 1)
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mjtByte* flexedge_rigid; // are both edge vertices in same body (nflexedge x 1)
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mjtByte* flex_centered; // are all vertex coordinates (0,0,0) (nflex x 1)
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