Add elasticity parameters to the schema.

PiperOrigin-RevId: 675115200
Change-Id: Ic6608ce61cbb857dca71b3d9cbdac057e8f1ac7a

doc/XMLreference.rst

@@ -3746,6 +3746,23 @@ element is used to adjust the properties of all edges in the flex.
    Edge stiffness and damping, passed through to the automatically generated flex.
 
 
+.. _flexcomp-elasticity:
+
+:el-prefix:`flexcomp/` |-| **elasticity** (*)
+'''''''''''''''''''''''''''''''''''''''''''''
+
+.. _flexcomp-elasticity-young:
+.. _flexcomp-elasticity-poisson:
+.. _flexcomp-elasticity-damping:
+.. _flexcomp-elasticity-thickness:
+
+.. |body/flexcomp/elasticity attrib list| replace::
+   :at:`young`, :at:`poisson`, :at:`damping`, :at:`thickness`
+
+|body/flexcomp/elasticity attrib list|
+   Same as in :ref:`flex/elasticity<flex-elasticity>`.
+   All attributes are passed through to the automatically-generated flex.
+
 .. _flexcomp-pin:
 
 :el-prefix:`flexcomp/` |-| **pin** (*)
@@ -4131,6 +4148,39 @@ these mechanisms to be combined as desired.
 :at:`damping`: :at-val:`real(1), "0"`
    Damping of all edges.
 
+
+.. _flex-elasticity:
+
+:el-prefix:`flex/` |-| **elasticity** (?)
+'''''''''''''''''''''''''''''''''''''''''
+
+.. _flex-elasticity-young:
+
+:at:`young`: :at-val:`real(1), "0"`
+   Young's elastic modulus, a measure of tensile and compressive stiffness for continuum elastic materials.
+   Units of :math:`\textrm{pressure}=\textrm{force}/\textrm{area}`.
+
+.. _flex-elasticity-poisson:
+
+:at:`poisson`: :at-val:`real(1), "0"`
+   Poisson's ratio, the ratio of transverse deformation to applied longitudinal strain.
+   This unitless quantity is in the range :math:`[0, 0.5)`.
+   Small or large values imply compressibility or incompressiblity, respectively.
+
+.. _flex-elasticity-damping:
+
+:at:`damping`: :at-val:`real(1), "0"`
+   Rayleigh's damping coefficient, units of time.
+   This quantity scales the stiffness defined by Young's modulus to produce the damping matrix.
+
+.. _flex-elasticity-thickness:
+
+:at:`thickness`: :at-val:`real(1), "-1"`
+   Shell thickness, units of length; only for used 2D flexes. Used to scale the stretching stiffness.
+   This thickness can be set equal to 2 times the :ref:`radius <deformable-flex-radius>` in order to match the geometry,
+   but is exposed seperately since the radius might be constrained by considerations related to collision detection.
+
+
 .. _flex-contact:
 
 :el-prefix:`flex/` |-| **contact** (?)

doc/XMLschema.rst

@@ -514,6 +514,13 @@
 |                                    |    |    +-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+ |
 +------------------------------------+----+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
 | |_2| flexcomp |br| |_2| |L|        |    | .. table::                                                                                                                                                                                                                                                                   |
+| :ref:`elasticity                   | ?  |    :class: mjcf-attributes                                                                                                                                                                                                                                                   |
+| <flexcomp-elasticity>`             |    |                                                                                                                                                                                                                                                                              |
+|                                    |    |    +-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+ |
+|                                    |    |    | :ref:`young<flexcomp-elasticity-young>`                         | :ref:`poisson<flexcomp-elasticity-poisson>`                     | :ref:`damping<flexcomp-elasticity-damping>`                     | :ref:`thickness<flexcomp-elasticity-thickness>`                 | |
+|                                    |    |    +-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+ |
++------------------------------------+----+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| |_2| flexcomp |br| |_2| |L|        |    | .. table::                                                                                                                                                                                                                                                                   |
 | :ref:`contact                      | ?  |    :class: mjcf-attributes                                                                                                                                                                                                                                                   |
 | <flexcomp-contact>`                |    |                                                                                                                                                                                                                                                                              |
 |                                    |    |    +-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+ |
@@ -581,6 +588,13 @@
 |                                    |    |    | :ref:`stiffness<flex-edge-stiffness>`                           | :ref:`damping<flex-edge-damping>`                               |                                                                 |                                                                 | |
 |                                    |    |    +-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+ |
 +------------------------------------+----+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| |_2| flex |br| |_2| |L|            |    | .. table::                                                                                                                                                                                                                                                                   |
+| :ref:`elasticity                   | ?  |    :class: mjcf-attributes                                                                                                                                                                                                                                                   |
+| <flex-elasticity>`                 |    |                                                                                                                                                                                                                                                                              |
+|                                    |    |    +-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+ |
+|                                    |    |    | :ref:`young<flex-elasticity-young>`                             | :ref:`poisson<flex-elasticity-poisson>`                         | :ref:`damping<flex-elasticity-damping>`                         | :ref:`thickness<flex-elasticity-thickness>`                     | |
+|                                    |    |    +-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+ |
++------------------------------------+----+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
 | |_| deformable |br| |_| |L|        |    | .. table::                                                                                                                                                                                                                                                                   |
 | :ref:`skin                         | \* |    :class: mjcf-attributes                                                                                                                                                                                                                                                   |
 | <deformable-skin>`                 |    |                                                                                                                                                                                                                                                                              |

doc/includes/references.h

@@ -1961,6 +1961,10 @@ typedef struct mjsFlex_ {          // flex specification
   double edgedamping;              // edge damping
   float rgba[4];                   // rgba when material is omitted
   mjString* material;              // name of material used for rendering
+  double young;                    // Young's modulus
+  double poisson;                  // Poisson's ratio
+  double damping;                  // Rayleigh's damping
+  double thickness;                // thickness (2D only)
 
   // mesh properties
   mjStringVec* vertbody;           // vertex body names

include/mujoco/mjspec.h

@@ -424,6 +424,10 @@ typedef struct mjsFlex_ {          // flex specification
   double edgedamping;              // edge damping
   float rgba[4];                   // rgba when material is omitted
   mjString* material;              // name of material used for rendering
+  double young;                    // Young's modulus
+  double poisson;                  // Poisson's ratio
+  double damping;                  // Rayleigh's damping
+  double thickness;                // thickness (2D only)
 
   // mesh properties
   mjStringVec* vertbody;           // vertex body names

introspect/structs.py

@@ -9631,6 +9631,26 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  ),
                  doc='name of material used for rendering',
              ),
+             StructFieldDecl(
+                 name='young',
+                 type=ValueType(name='double'),
+                 doc="Young's modulus",
+             ),
+             StructFieldDecl(
+                 name='poisson',
+                 type=ValueType(name='double'),
+                 doc="Poisson's ratio",
+             ),
+             StructFieldDecl(
+                 name='damping',
+                 type=ValueType(name='double'),
+                 doc="Rayleigh's damping",
+             ),
+             StructFieldDecl(
+                 name='thickness',
+                 type=ValueType(name='double'),
+                 doc='thickness (2D only)',
+             ),
              StructFieldDecl(
                  name='vertbody',
                  type=PointerType(

model/plugin/elasticity/floppy_flex.xml

@@ -34,12 +34,8 @@
     <flexcomp type="grid" count="24 4 4" spacing=".1 .1 .1" pos=".1 0 1.5"
               radius=".0" rgba="0 .7 .7 1" name="softbody" dim="3" mass="25">
       <contact condim="3" solref="0.01 1" solimp=".95 .99 .0001" selfcollide="none"/>
-      <plugin plugin="mujoco.elasticity.solid">
+      <elasticity young="5e4" damping="0.002" poisson="0.2"/>
-        <config key="poisson" value="0.2"/>
+      <plugin plugin="mujoco.elasticity.solid"/>
-        <!--Units are in Pa (SI)-->
-        <config key="young" value="5e4"/>
-        <config key="damping" value="0.002"/>
-      </plugin>
     </flexcomp>
 
     <body>

model/plugin/elasticity/jelly_flex.xml

@@ -46,11 +46,8 @@
               radius=".005" rgba="0 .7 .7 1" mass="5" name="softbody">
       <edge damping="1"/>
       <contact selfcollide="none"/>
-      <plugin plugin="mujoco.elasticity.solid">
+      <elasticity young="5e4"/>
-        <config key="poisson" value="0"/>
+      <plugin plugin="mujoco.elasticity.solid"/>
-        <!--Units are in Pa (SI)-->
-        <config key="young" value="5e4"/>
-      </plugin>
     </flexcomp>
   </worldbody>
 </mujoco>

model/plugin/elasticity/press_flex.xml

@@ -35,22 +35,16 @@
               radius=".005" rgba="0 .7 .7 1" dim="3" mass="5">
       <contact condim="3" solref="0.01 1" solimp=".95 .99 .0001" selfcollide="none"/>
       <edge damping="1"/>
-      <plugin plugin="mujoco.elasticity.solid">
+      <elasticity young="1e4" poisson="0.4"/>
-        <config key="poisson" value="0.4"/>
+      <plugin plugin="mujoco.elasticity.solid"/>
-        <!--Units are in Pa (SI)-->
-        <config key="young" value="1e4"/>
-      </plugin>
     </flexcomp>
 
     <flexcomp name="B" type="grid" count="4 4 4" spacing=".2 .2 .2" pos="1 0 .5"
               radius=".005" rgba="0 .7 .7 1" dim="3" mass="5">
       <contact condim="3" solref="0.01 1" solimp=".95 .99 .0001" selfcollide="none"/>
       <edge damping="1"/>
-      <plugin plugin="mujoco.elasticity.solid">
+      <elasticity young="5e4" poisson="0"/>
-        <config key="poisson" value="0"/>
+      <plugin plugin="mujoco.elasticity.solid"/>
-        <!--Units are in Pa (SI)-->
-        <config key="young" value="5e4"/>
-      </plugin>
     </flexcomp>
 
     <body>

model/plugin/elasticity/trampoline_flex.xml

@@ -44,13 +44,9 @@
               radius=".001" mass="10" name="plate" dim="2">
       <contact condim="3" solref="0.01 1" solimp=".95 .99 .0001"/>
       <edge equality="false" damping="10"/>
+      <elasticity young="3e5" poisson="0" thickness="1e-2"/>
       <pin id="0 15 240 255"/>
-      <plugin plugin="mujoco.elasticity.membrane">
+      <plugin plugin="mujoco.elasticity.membrane"/>
-        <config key="poisson" value="0"/>
-        <config key="thickness" value="1e-2"/>
-        <!--Units are in Pa (SI)-->
-        <config key="young" value="3e5"/>
-      </plugin>
     </flexcomp>
   </worldbody>
 </mujoco>

plugin/elasticity/README.md

@@ -37,11 +37,7 @@ Implemented in [membrane.cc](membrane.cc).
 
 The membrane plugin discretized an extensible 2D continuum. It is intended to simulate the stretching of membranes subjected to tensile stresses, where the bending is negligible.
 
-Parameters:
+Parameters: see [flex parameters](mujoco.readthedocs.io/en/latest/XMLreference.html#flexcomp-elasticity) in the XML Reference docs.
-
-   - `young` [Pa]: Young's modulus.
-   - `poisson` [Pa]: Poisson's ratio; if 0, then the material only opposed shear deformations; if near 0.5, then the material is nearly incompressible (rubber-like).
-   - `thickness` [m]: shell thickness, used to scale the stretching stiffness.
 
 ### Solid
 
@@ -49,8 +45,5 @@ Implemented in [solid.cc](solid.cc).
 
 The membrane plugin discretized an extensible 3D continuum. It is Saint Venant–Kirchhoff model intended to simulate the compression or elongation of hyperelastic materials subjected to large displacements (finite rotations) and small strains, since it uses a nonlinear strain-displacement but a linear stress-strain relationship.
 
-Parameters:
+Parameters: see [flex parameters](mujoco.readthedocs.io/en/latest/XMLreference.html#flexcomp-elasticity) in the XML Reference docs.
 
-   - `young` [Pa]: Young's modulus.
-   - `poisson` [Pa]: Poisson's ratio; if 0, then the material only opposed shear deformations; if near 0.5, then the material is nearly incompressible (rubber-like).
-   - `damping` [Ns/m^2]: damping coefficient for Rayleigh damping.

plugin/elasticity/shell.cc

@@ -242,7 +242,7 @@ void Shell::RegisterPlugin() {
   plugin.capabilityflags |= mjPLUGIN_PASSIVE;
 
   const char* attributes[] = {"face",    "edge",    "young",
-                              "poisson", "thickness"};
+                              "poisson", "thickness", "damping"};
   plugin.nattribute = sizeof(attributes) / sizeof(attributes[0]);
   plugin.attributes = attributes;
   plugin.nstate = +[](const mjModel* m, int instance) { return 0; };

plugin/elasticity/solid.cc

@@ -222,7 +222,7 @@ void Solid::RegisterPlugin() {
   plugin.name = "mujoco.elasticity.solid";
   plugin.capabilityflags |= mjPLUGIN_PASSIVE;
 
-  const char* attributes[] = {"face", "edge", "young", "poisson", "damping"};
+  const char* attributes[] = {"face", "edge", "young", "poisson", "damping", "thickness"};
   plugin.nattribute = sizeof(attributes) / sizeof(attributes[0]);
   plugin.attributes = attributes;
   plugin.nstate = +[](const mjModel* m, int instance) { return 0; };

src/user/user_init.c

@@ -236,6 +236,7 @@ void mjs_defaultFlex(mjsFlex* flex) {
   flex->activelayers = 1;
   flex->rgba[0] = flex->rgba[1] = flex->rgba[2] = 0.5f;
   flex->rgba[3] = 1.0f;
+  flex->thickness = -1;
 }
 
 

src/user/user_mesh.cc

@@ -2675,6 +2675,18 @@ void mjCFlex::Compile(const mjVFS* vfs) {
     if (model->Bodies()[vbodyid]->plugin.instance) {
       mjCPlugin* plugin_instance =
           static_cast<mjCPlugin*>(model->Bodies()[vbodyid]->plugin.instance);
+      if (young > 0) {
+        plugin_instance->config_attribs["young"] = std::to_string(young);
+      }
+      if (poisson > 0) {
+        plugin_instance->config_attribs["poisson"] = std::to_string(poisson);
+      }
+      if (thickness > 0) {
+        plugin_instance->config_attribs["thickness"] = std::to_string(thickness);
+      }
+      if (damping > 0) {
+        plugin_instance->config_attribs["damping"] = std::to_string(damping);
+      }
       plugin_instance->config_attribs["face"] = userface;
       plugin_instance->config_attribs["edge"] = useredge;
     }

src/xml/xml_native_reader.cc

@@ -326,6 +326,7 @@ const char* MJCF[nMJCF][mjXATTRNUM] = {
             "flatskin", "pos", "quat", "axisangle", "xyaxes", "zaxis", "euler"},
         {"<"},
             {"edge", "?", "5", "equality", "solref", "solimp", "stiffness", "damping"},
+            {"elasticity", "?", "4", "young", "poisson", "damping", "thickness"},
             {"contact", "?", "13", "contype", "conaffinity", "condim", "priority",
                 "friction", "solmix", "solref", "solimp", "margin", "gap",
                 "internal", "selfcollide", "activelayers"},
@@ -346,6 +347,7 @@ const char* MJCF[nMJCF][mjXATTRNUM] = {
                 "friction", "solmix", "solref", "solimp", "margin", "gap",
                 "internal", "selfcollide", "activelayers"},
             {"edge", "?", "2", "stiffness", "damping"},
+            {"elasticity", "?", "4", "young", "poisson", "damping", "thickness"},
         {">"},
         {"skin", "*", "9", "name", "file", "material", "rgba", "inflate",
             "vertex", "texcoord", "face", "group"},
@@ -1388,6 +1390,15 @@ void mjXReader::OneFlex(XMLElement* elem, mjsFlex* flex) {
     ReadAttr(edge, "damping", 1, &flex->edgedamping, text);
   }
 
+  // elasticity subelement
+  XMLElement* elasticity = FirstChildElement(elem, "elasticity");
+  if (elasticity) {
+    ReadAttr(elasticity, "young", 1, &flex->young, text);
+    ReadAttr(elasticity, "poisson", 1, &flex->poisson, text);
+    ReadAttr(elasticity, "thickness", 1, &flex->thickness, text);
+    ReadAttr(elasticity, "damping", 1, &flex->damping, text);
+  }
+
   // write error info
   mjs_setString(flex->info, ("line " + std::to_string(elem->GetLineNum())).c_str());
 }
@@ -2674,6 +2685,15 @@ void mjXReader::OneFlexcomp(XMLElement* elem, mjsBody* body, const mjVFS* vfs) {
     ReadAttr(edge, "damping", 1, &dflex.edgedamping, text);
   }
 
+  // elasticity
+  XMLElement* elasticity = FirstChildElement(elem, "elasticity");
+  if (elasticity) {
+    ReadAttr(elasticity, "young", 1, &dflex.young, text);
+    ReadAttr(elasticity, "poisson", 1, &dflex.poisson, text);
+    ReadAttr(elasticity, "damping", 1, &dflex.damping, text);
+    ReadAttr(elasticity, "thickness", 1, &dflex.thickness, text);
+  }
+
   // contact
   XMLElement* cont = FirstChildElement(elem, "contact");
   if (cont) {

src/xml/xml_native_reader.h

@@ -101,7 +101,7 @@ class mjXReader : public mjXBase {
 };
 
 // MJCF schema
-#define nMJCF 243
+#define nMJCF 245
 extern const char* MJCF[nMJCF][mjXATTRNUM];
 
 #endif  // MUJOCO_SRC_XML_XML_NATIVE_READER_H_

test/plugin/elasticity/elasticity_test.cc

@@ -44,10 +44,8 @@ TEST_F(ElasticityTest, FlexCompatibility) {
         <flexcomp name="soft" type="grid" count="3 3 3"
                   radius="0.01" dim="3"mass="1">
             <pin id="2"/>
-            <plugin plugin="mujoco.elasticity.solid">
+            <elasticity young="5e4" poisson="0.2"/>
-              <config key="poisson" value="0.2"/>
+            <plugin plugin="mujoco.elasticity.solid"/>
-              <config key="young" value="5e4"/>
-            </plugin>
         </flexcomp>
       </body>
     </worldbody>
@@ -128,11 +126,8 @@ TEST_F(PluginTest, ElasticEnergyMembrane) {
   <worldbody>
     <flexcomp type="grid" count="8 8 1" spacing="1 1 1"
               radius=".025" name="test" dim="2">
-      <plugin plugin="mujoco.elasticity.membrane">
+      <elasticity young="2" poisson="0" thickness="1"/>
-        <config key="poisson" value="0"/>
+      <plugin plugin="mujoco.elasticity.membrane"/>
-        <config key="young" value="2"/>
-        <config key="thickness" value="1"/>
-      </plugin>
       <edge equality="false"/>
     </flexcomp>
   </worldbody>
@@ -214,10 +209,8 @@ TEST_F(ElasticityTest, ElasticEnergySolid) {
   <worldbody>
     <flexcomp type="grid" count="8 8 8" spacing="1 1 1"
               radius=".025" name="test" dim="3">
-      <plugin plugin="mujoco.elasticity.solid">
+      <elasticity young="2" poisson="0"/>
-        <config key="poisson" value="0"/>
+      <plugin plugin="mujoco.elasticity.solid"/>
-        <config key="young" value="2"/>
-      </plugin>
       <edge equality="false"/>
     </flexcomp>
   </worldbody>