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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.

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Also fixes a bug when attaching a flex with constraints.

PiperOrigin-RevId: 881349765
Change-Id: I8e6df8239488bfdc87d7e5b473568975e38c737e
This commit is contained in:
Alessio Quaglino
2026-03-10 04:25:19 -07:00
committed by Copybara-Service
parent fe6c81ed93
commit 713b5524bc
23 changed files with 899 additions and 48 deletions
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src/xml/xml_native_reader.cc
+11 -3
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@@ -366,6 +366,8 @@ std::vector<const char*> MJCF[nMJCF] = {
"active", "solref", "solimp"},
{"flexvert", "*", "name", "class", "flex",
"active", "solref", "solimp"},
{"flexstrain", "*", "name", "class", "flex",
"active", "solref", "solimp"},
{">"},
{"tendon", "*"},
@@ -669,7 +671,7 @@ const mjMap solver_map[solver_sz] = {
// constraint type
const int equality_sz = 7;
const int equality_sz = 8;
const mjMap equality_map[equality_sz] = {
{"connect", mjEQ_CONNECT},
{"weld", mjEQ_WELD},
@@ -677,6 +679,7 @@ const mjMap equality_map[equality_sz] = {
{"tendon", mjEQ_TENDON},
{"flex", mjEQ_FLEX},
{"flexvert", mjEQ_FLEXVERT},
{"flexstrain", mjEQ_FLEXSTRAIN},
{"distance", mjEQ_DISTANCE}
};
@@ -930,10 +933,11 @@ const mjMap elastic2d_map[5] = {
// flex equality type
const mjMap flexeq_map[3] = {
const mjMap flexeq_map[4] = {
{"false", 0},
{"true", 1},
{"vert", 2},
{"strain", 3},
};
@@ -2207,6 +2211,7 @@ void mjXReader::OneEquality(XMLElement* elem, mjsEquality* equality) {
case mjEQ_FLEX:
case mjEQ_FLEXVERT:
case mjEQ_FLEXSTRAIN:
ReadAttrTxt(elem, "flex", name1, true);
break;
@@ -2768,7 +2773,7 @@ void mjXReader::OneFlexcomp(XMLElement* elem, mjsBody* body, const mjVFS* vfs) {
// edge
XMLElement* edge = FirstChildElement(elem, "edge");
if (edge) {
MapValue(edge, "equality", &fcomp.equality, flexeq_map, 3);
MapValue(edge, "equality", &fcomp.equality, flexeq_map, 4);
ReadAttr(edge, "solref", mjNREF, fcomp.def.spec.equality->solref, text, false, false);
ReadAttr(edge, "solimp", mjNIMP, fcomp.def.spec.equality->solimp, text, false, false);
ReadAttr(edge, "stiffness", 1, &dflex.edgestiffness, text);
@@ -2789,6 +2794,9 @@ void mjXReader::OneFlexcomp(XMLElement* elem, mjsBody* body, const mjVFS* vfs) {
if (dflex.elastic2d >= 2 && fcomp.equality) {
throw mjXError(elem, "elasticity and edge constraints cannot both be present");
}
if (fcomp.equality == 3 && dflex.young > 0) {
throw mjXError(elem, "strain constraint and elasticity (young) cannot both be present");
}
// contact
XMLElement* cont = FirstChildElement(elem, "contact");