Add quadratic stiffness to flex.

PiperOrigin-RevId: 827409919
Change-Id: I3dff8ea49fb1726fec4acf5b91138d6d52c4bfba

model/flex/bunny_quadratic.xml

@@ -0,0 +1,42 @@
+<!-- Copyright 2024 DeepMind Technologies Limited
+
+     Licensed under the Apache License, Version 2.0 (the "License");
+     you may not use this file except in compliance with the License.
+     You may obtain a copy of the License at
+
+         http://www.apache.org/licenses/LICENSE-2.0
+
+     Unless required by applicable law or agreed to in writing, software
+     distributed under the License is distributed on an "AS IS" BASIS,
+     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+     See the License for the specific language governing permissions and
+     limitations under the License.
+-->
+
+<mujoco model="Trilinear">
+  <include file="scene.xml"/>
+
+  <option solver="CG" tolerance="1e-6" timestep=".0005" integrator="implicitfast"/>
+
+  <size memory="100M"/>
+
+  <visual>
+    <map stiffness="100"/>
+  </visual>
+
+  <worldbody>
+    <body>
+      <joint name="press" type="slide" axis="0 0 1" damping="500"/>
+      <geom type="box" size=".02 .2 .2" pos="0 0 .5"/>
+    </body>
+    <flexcomp type="mesh" file="bunny.obj" pos="0 0 -.01" dim="2" euler="90 0 0"
+              radius=".002" rgba="0 .7 .7 1" mass=".05" name="softbody" dof="quadratic">
+      <elasticity young="1e3" poisson="0.1" damping="0.0001" elastic2d="stretch"/>
+      <contact selfcollide="none" internal="false"/>
+    </flexcomp>
+  </worldbody>
+
+  <actuator>
+    <position name="press" joint="press" gear="-1 0 0 0 0 0" ctrlrange="-1 1" kp="1000"/>
+  </actuator>
+</mujoco>

model/flex/quadratic.xml

@@ -13,12 +13,10 @@
      limitations under the License.
 -->
 
-<mujoco model="Trilinear">
+<mujoco model="Quadratic">
   <include file="scene.xml"/>
 
-  <option solver="CG" tolerance="1e-6" timestep=".001" integrator="implicitfast">
-    <flag gravity="disable"/>
-  </option>
+  <option solver="CG" tolerance="1e-6" timestep=".001" integrator="implicitfast"/>
 
   <size memory="10M"/>
 
@@ -33,6 +31,7 @@
     </body>
     <flexcomp type="grid" count="8 8 8" spacing=".07 .07 .07" pos="0 0 1" dim="3"
               radius=".001" rgba="0 .7 .7 1" mass="5" name="softbody" dof="quadratic">
+      <elasticity young="1e4" poisson="0.1" damping="0.0001"/>
       <contact selfcollide="none" internal="false"/>
     </flexcomp>
   </worldbody>