- Added passive forces plugins

- Added new `cable` composite type:
  * The `initial` parameter specifies the joint at the starting boundary: `free`, `ball`, or `none`.
  * The boundary bodies are exposed with the names:`B_left` and `B_right`.
  * The vertex initial positions can be specified directly in the XML with the parameter `vertex`.
  * The orientation of the body frame **is** the orientation of the material frame of the curve.

- Added new `cable` passive force plugin:
  * Twist and bending stiffness can be set separately with the parameters `twist` and `bend`.
  * The stress-free configuration can be set to be the initial one or flat with the flag `flat`.
  * New cable example showing the formation of plectoneme.
  * New coil example.
  * New belt example showing interaction between twist and anisotropy.
  * Added test using cantilever exact solution.

PiperOrigin-RevId: 480033694
Change-Id: I491271bce8fccb185961477e903e5a72d172c8a3

model/plugin/coil.xml

@@ -0,0 +1,104 @@
+<!-- Copyright 2021 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="Coil">
+  <include file="scene.xml"/>
+
+  <extension>
+    <required plugin="mujoco.elasticity.cable"/>
+  </extension>
+
+  <statistic center="0 0 .2" extent="1"/>
+
+  <compiler autolimits="true"/>
+
+  <visual>
+    <global elevation="-30"/>
+  </visual>
+
+  <default>
+    <geom solref="0.01 1" solimp=".95 .99 .0001"/>
+  </default>
+
+  <worldbody>
+    <composite prefix="free" type="cable" offset="-.25 0 .3" initial="free"
+      vertex="
+        0.1         0.          0.
+        0.0809017   0.05877853  0.001256637
+        0.0309017   0.09510565  0.002513274
+       -0.0309017   0.09510565  0.003769911
+       -0.0809017   0.05877853  0.005026548
+       -0.1         0.          0.006283185
+       -0.0809017  -0.05877853  0.007539822
+       -0.0309017  -0.09510565  0.008796459
+        0.0309017  -0.09510565  0.010053096
+        0.0809017  -0.05877853  0.011309734
+        0.1        -0.          0.012566371
+        0.0809017   0.05877853  0.01256637
+        0.0309017   0.09510565  0.02513274
+       -0.0309017   0.09510565  0.03769911
+       -0.0809017   0.05877853  0.05026548
+       -0.1         0.          0.06283185
+       -0.0809017  -0.05877853  0.07539822
+       -0.0309017  -0.09510565  0.08796459
+        0.0309017  -0.09510565  0.10053096
+        0.0809017  -0.05877853  0.11309734
+        0.1        -0.          0.12566371
+        0.0809017   0.05877853  0.13823008
+        0.0309017   0.09510565  0.15079645
+       -0.0309017   0.09510565  0.16336282
+       -0.0809017   0.05877853  0.17592919
+       -0.1         0.          0.18849556
+       -0.0809017  -0.05877853  0.20106193
+       -0.0309017  -0.09510565  0.2136283
+        0.0309017  -0.09510565  0.22619467
+        0.0809017  -0.05877853  0.23876104
+        0.1        -0.          0.25132741">
+      <plugin plugin="mujoco.elasticity.cable">
+        <!--Units are in Pa (SI)-->
+        <config key="twist" value="5e8"/>
+        <config key="bend" value="15e8"/>
+      </plugin>
+      <joint kind="main" damping="0.12" armature="0.01"/>
+      <geom type="capsule" size=".005" rgba=".8 .2 .1 1"/>
+    </composite>
+    <composite prefix="actuated" type="cable" curve="cos(s) sin(s) s" count="41 1 1"
+      size="0.251327412 .1 4" offset="0.25 0 .05" initial="fixed">
+      <plugin plugin="mujoco.elasticity.cable">
+        <!--Units are in Pa (SI)-->
+        <config key="twist" value="5e8"/>
+        <config key="bend" value="15e8"/>
+      </plugin>
+      <joint kind="main" damping="0.1" armature="0.01"/>
+      <geom type="capsule" size=".005" rgba=".8 .2 .1 1"/>
+    </composite>
+    <body name="slider" pos="0.35 0 0.301327412">
+      <joint name="slide" type="slide" axis="0 0 1" damping="5"/>
+      <geom size=".01"/>
+    </body>
+  </worldbody>
+
+  <equality>
+    <connect name="right_boundary" body1="actuatedB_last" body2="slider" anchor="0.03 0 0" solref="0.0002 1"/>
+  </equality>
+
+  <contact>
+    <exclude body1="actuatedB_last" body2="slider"/>
+  </contact>
+
+  <actuator>
+    <position joint="slide" name="extend" gear="1 0 0 0 0 0" ctrlrange="-.25 1" kp="100"/>
+  </actuator>
+</mujoco>