chenlin/Mujoco_WASM
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

Scale flex warp constraint by thickness^3.

Browse Source 
Warp is not a true stretch mode but a parasitic shear mode so it should be scaled like bending (~thickness^3) rather than stretching (~thickness). The thickness parameter can therefore be used to set the stretch to warp stiffness ratio of the strain constraints.

PiperOrigin-RevId: 916900311
Change-Id: I52e54210fbb2a6d309abc1f084934830adce3423
This commit is contained in:
Alessio Quaglino
2026-05-17 14:32:43 -07:00
committed by Copybara-Service
parent 8a20ce24d3
commit 66d764a116
2 changed files with 16 additions and 11 deletions
Download Patch File Download Diff File Expand all files Collapse all files
model/flex/hollow_vs_solid.xml
+1 -1
View File
@@ -50,7 +50,7 @@
origin="0 0 0" count="8 2 12" cellcount="6 1 6"
type="grid" name="soft_mesh_2" dim="3" spacing=".025 .05 .025"
mass="0.43" radius="0.005" dof="trilinear" rgba="0.9 0.7 0.7 1">
<elasticity young="1e4" poisson="0.3" damping="0" elastic2d="bend" thickness="0.03"/>
<elasticity young="1e4" poisson="0.3" damping="0" elastic2d="bend" thickness="0.02"/>
<edge equality="strain" solref="0.001 1" solimp="0.99 0.999 0.001"/>
<contact selfcollide="none"
internal="false"
src/user/user_mesh.cc
+15 -10
View File
@@ -4071,8 +4071,9 @@ static void ComputeWarpMode(double* warp, const double* pos,
}
// compute the warp stiffness for a 2D face element, matching the transverse
// shear energy of the old 3D-on-2D formulation: λ_warp = μ t (d1/d0 + d0/d1)/6
// compute the warp bending stiffness for a 2D face element
// uses plate bending theory: the warp mode is a pure twist (κ_xy),
// with bending stiffness proportional to t³ (no shear locking)
// pos: node positions (3*npe doubles)
// npe: nodes per element
// normal_axis: axis perpendicular to the face
@@ -4082,13 +4083,17 @@ static double ComputeWarpStiffness(const double* pos, int npe, int normal_axis,
double E, double nu, double thickness) {
int axis0 = (normal_axis + 1) % 3;
int axis1 = (normal_axis + 2) % 3;
double d0 = pos[3*(npe-1) + axis0] - pos[axis0];
double d1 = pos[3*(npe-1) + axis1] - pos[axis1];
double mu = E / (2.0 * (1.0 + nu));
double d0 = std::abs(pos[3*(npe-1) + axis0] - pos[axis0]);
double d1 = std::abs(pos[3*(npe-1) + axis1] - pos[axis1]);
// warp stiffness from transverse shear Rayleigh quotient:
// w^T K_phys w / |w|^2 = μ t (|d1/d0| + |d0/d1|) / 6
return mu * thickness * (std::abs(d1/d0) + std::abs(d0/d1)) / 6.0;
if (d0 < 1e-30 || d1 < 1e-30) return 0;
// plate bending rigidity: D = E*t³ / (12*(1-ν²))
double D = E * thickness * thickness * thickness / (12.0 * (1.0 - nu * nu));
// warp stiffness from twist curvature Rayleigh quotient:
// w^T K_bend w / |w|^2 = D*(1-ν)*4 / (d0*d1)
return D * (1.0 - nu) * 4.0 / (d0 * d1);
}
@@ -5069,10 +5074,10 @@ void mjCFlex::Compile(const mjVFS* vfs) {
if (shell_mode) {
// pure membrane K: eigendecompose gives 5 membrane modes (Q1),
// then we add 1 explicit warp mode
// then we add 1 explicit warp mode with bending stiffness (∝ t³)
int neig = EigendecomposeStiffness(K_elem.data(), out, ndof_elem);
// add explicit warp mode with stiffness matching old transverse shear
// add explicit warp mode with plate bending stiffness
double warp_stiffness = ComputeWarpStiffness(
elem_pos.data(), npe, normal_axis, K_young, K_poisson, thickness);
if (warp_stiffness > 0) {