Interaction between flexcomp and composite bodies. Trouble creating flexcomp/cloth. #2014

nhatminh2h · 2024-03-01T13:47:08Z

nhatminh2h
Mar 1, 2024

Hi,

I'm a PhD student and I'm trying to use MuJoCo for soft aquatic robot simulation.

I'm looking for some help with simulating a tendon-driven aquatic fin. The ultimate goal is to simulate the entire biomimetic robot but this is a solid starting point. In the context of MuJoCo, the interaction of tendon, composite and flexcomp. I'm hoping to match this with my experiment results.

The video is what I am trying to simulate, a tendon-driven actuator embedded in a membrane:

mujoco_example.mov

I have made a model of the tendon part below, but I cant figure how to make and attach the membrane.

Tendon model, I have only manage to make cable type work. 1D elastic model seems accurate enough but is there a more elegant way to control the geom?

minimal XML

<mujoco model="flat_belt_tendon">
  <compiler angle="radian"/>
  <option timestep="0.005" integrator="implicit"/>
  <default class="main">
    <joint limited="true" stiffness="100" range="-1 1" damping="1"/>
    <geom size="0.001 0 0" type="capsule" rgba="0.9 0.7 0.1 1"/>
    <site size="0.001 0.005 0.005" rgba="0.9 0.9 0.9 1"/>
    <default class="visual_cylinder">
      <geom size="0.02 0 0" type="cylinder" rgba="0.3 0.9 0.3 0.4"/>
      <site size="0.005 0.005 0.005" rgba="0.5 0.5 0.5 1"/>
    </default>
  </default>
  <extension>
    <plugin plugin="mujoco.elasticity.cable">
      <instance name="composite">
        <config key="twist" value="1e9"/>
        <config key="bend" value="5e7"/>
        <config key="vmax" value="0.6"/>
      </instance>
    </plugin>
    <plugin plugin="mujoco.elasticity.shell"/>
  </extension>
  <custom>
    <text name="composite_" data="rope_"/>
  </custom>
  <worldbody>
    <geom name="root" pos="0 0 0.902" type="sphere" rgba="1 0 0 1"/>
    <geom name="end" pos="0.033 0 0.902" type="sphere" rgba="1 0 0 1"/>
    <site name="root" pos="0 0 0.902"/>
    <light pos="0 0 1" dir="0 0 -1"/>
    <body name="B_first" pos="0 0 0.9">
      <geom name="G0" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 3.53699e-05 1 1"/>
      <site name="S_first" pos="0 0 0" size="0.005" group="3" rgba="0.5 0.5 0.5 1"/>
      <plugin instance="composite"/>
      <body name="B_1" pos="0.00103125 0 0">
        <joint name="J_1" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
        <geom name="G1" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 3.42248e-05 1 1"/>
        <plugin instance="composite"/>
        <body name="B_2" pos="0.00103125 0 0">
          <site name="S_2" pos="0 0 0.0017" size="0.005" group="3" rgba="0.5 0.5 0.5 1"/>
          <joint name="J_2" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
          <geom name="G2" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 3.19733e-05 1 1"/>
          <plugin instance="composite"/>
          <body name="B_3" pos="0.00103125 0 0">
            <joint name="J_3" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
            <geom name="G3" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 2.97997e-05 1 1"/>
            <plugin instance="composite"/>
            <body name="B_4" pos="0.00103125 0 0">
              <joint name="J_4" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
              <geom name="G4" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 2.77047e-05 1 1"/>
              <plugin instance="composite"/>
              <body name="B_5" pos="0.00103125 0 0">
                <joint name="J_5" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                <geom name="G5" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 2.56887e-05 1 1"/>
                <plugin instance="composite"/>
                <body name="B_6" pos="0.00103125 0 0">
                  <site name="S_6" pos="0 0 0.0017" size="0.005" group="3" rgba="0.5 0.5 0.5 1"/>
                  <joint name="J_6" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                  <geom name="G6" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 2.37521e-05 1 1"/>
                  <plugin instance="composite"/>
                  <body name="B_7" pos="0.00103125 0 0">
                    <joint name="J_7" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                    <geom name="G7" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 2.18946e-05 1 1"/>
                    <plugin instance="composite"/>
                    <body name="B_8" pos="0.00103125 0 0">
                      <joint name="J_8" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                      <geom name="G8" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 2.01162e-05 1 1"/>
                      <plugin instance="composite"/>
                      <body name="B_9" pos="0.00103125 0 0">
                        <joint name="J_9" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                        <geom name="G9" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 1.84165e-05 1 1"/>
                        <plugin instance="composite"/>
                        <body name="B_10" pos="0.00103125 0 0">
                          <site name="S_10" pos="0 0 0.0017" size="0.005" group="3" rgba="0.5 0.5 0.5 1"/>
                          <joint name="J_10" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                          <geom name="G10" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 1.6795e-05 1 1"/>
                          <plugin instance="composite"/>
                          <body name="B_11" pos="0.00103125 0 0">
                            <joint name="J_11" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                            <geom name="G11" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 1.52514e-05 1 1"/>
                            <plugin instance="composite"/>
                            <body name="B_12" pos="0.00103125 0 0">
                              <joint name="J_12" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                              <geom name="G12" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 1.3785e-05 1 1"/>
                              <plugin instance="composite"/>
                              <body name="B_13" pos="0.00103125 0 0">
                                <joint name="J_13" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                                <geom name="G13" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 1.23952e-05 1 1"/>
                                <plugin instance="composite"/>
                                <body name="B_14" pos="0.00103125 0 0">
                                  <site name="S_14" pos="0 0 0.0017" size="0.005" group="3" rgba="0.5 0.5 0.5 1"/>
                                  <joint name="J_14" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                                  <geom name="G14" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 1.10817e-05 1 1"/>
                                  <plugin instance="composite"/>
                                  <body name="B_15" pos="0.00103125 0 0">
                                    <joint name="J_15" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                                    <geom name="G15" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 9.84375e-06 1 1"/>
                                    <plugin instance="composite"/>
                                    <body name="B_16" pos="0.00103125 0 0">
                                      <joint name="J_16" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                                      <geom name="G16" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 8.6809e-06 1 1"/>
                                      <plugin instance="composite"/>
                                      <body name="B_17" pos="0.00103125 0 0">
                                        <joint name="J_17" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                                        <geom name="G17" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 7.59268e-06 1 1"/>
                                        <plugin instance="composite"/>
                                        <body name="B_18" pos="0.00103125 0 0">
                                          <site name="S_18" pos="0 0 0.0017" size="0.005" group="3" rgba="0.5 0.5 0.5 1"/>
                                          <joint name="J_18" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                                          <geom name="G18" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 6.57861e-06 1 1"/>
                                          <plugin instance="composite"/>
                                          <body name="B_19" pos="0.00103125 0 0">
                                            <joint name="J_19" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                                            <geom name="G19" size="0.000515626 0.003 0.0001" pos="0.000515626 0 0" type="box" group="3" density="900" rgba="0 5.63831e-06 1 1"/>
                                            <plugin instance="composite"/>
                                            <body name="B_20" pos="0.00103125 0 0">
                                              <joint name="J_20" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                                              <geom name="G20" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 4.77141e-06 1 1"/>
                                              <plugin instance="composite"/>
                                              <body name="B_21" pos="0.00103125 0 0">
                                                <joint name="J_21" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                                                <geom name="G21" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 3.9776e-06 1 1"/>
                                                <plugin instance="composite"/>
                                                <body name="B_22" pos="0.00103125 0 0">
                                                  <site name="S_22" pos="0 0 0.0017" size="0.005" group="3" rgba="0.5 0.5 0.5 1"/>
                                                  <joint name="J_22" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                                                  <geom name="G22" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 3.25659e-06 1 1"/>
                                                  <plugin instance="composite"/>
                                                  <body name="B_23" pos="0.00103125 0 0">
                                                    <joint name="J_23" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                                                    <geom name="G23" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 2.60817e-06 1 1"/>
                                                    <plugin instance="composite"/>
                                                    <body name="B_24" pos="0.00103125 0 0">
                                                      <joint name="J_24" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                                                      <geom name="G24" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 2.03214e-06 1 1"/>
                                                      <plugin instance="composite"/>
                                                      <body name="B_25" pos="0.00103125 0 0">
                                                        <joint name="J_25" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                                                        <geom name="G25" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 1.52835e-06 1 1"/>
                                                        <plugin instance="composite"/>
                                                        <body name="B_26" pos="0.00103125 0 0">
                                                          <site name="S_26" pos="0 0 0.0017" size="0.005" group="3" rgba="0.5 0.5 0.5 1"/>
                                                          <joint name="J_26" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                                                          <geom name="G26" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 1.09668e-06 1 1"/>
                                                          <plugin instance="composite"/>
                                                          <body name="B_27" pos="0.00103125 0 0">
                                                            <joint name="J_27" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                                                            <geom name="G27" size="0.000515626 0.003 0.0001" pos="0.000515626 0 0" type="box" group="3" density="900" rgba="0 7.37049e-07 1 1"/>
                                                            <plugin instance="composite"/>
                                                            <body name="B_28" pos="0.00103125 0 0">
                                                              <joint name="J_28" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                                                              <geom name="G28" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 4.49395e-07 1 1"/>
                                                              <plugin instance="composite"/>
                                                              <body name="B_29" pos="0.00103125 0 0">
                                                                <joint name="J_29" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                                                                <geom name="G29" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 2.33678e-07 1 1"/>
                                                                <plugin instance="composite"/>
                                                                <body name="B_30" pos="0.00103125 0 0">
                                                                  <site name="S_end" pos="0.0023 0 0.0001" size="0.005" group="3" rgba="0.5 0.5 0.5 1"/>
                                                                  <joint name="J_30" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                                                                  <geom name="G30" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 8.98749e-08 1 1"/>
                                                                  <plugin instance="composite"/>
                                                                  <body name="B_last" pos="0.00103125 0 0">
                                                                    <joint name="J_last" pos="0 0 0" limited="false" type="ball" group="3" stiffness="0" range="0 0" damping="0.0005"/>
                                                                    <geom name="G31" size="0.000515625 0.003 0.0001" pos="0.000515625 0 0" type="box" group="3" density="900" rgba="0 3.59498e-08 1 1"/>
                                                                    <site name="S_last" pos="0.00103125 0 0" size="0.005" group="3" rgba="0.5 0.5 0.5 1"/>
                                                                    <plugin instance="composite"/>
                                                                  </body>
                                                                </body>
                                                              </body>
                                                            </body>
                                                          </body>
                                                        </body>
                                                      </body>
                                                    </body>
                                                  </body>
                                                </body>
                                              </body>
                                            </body>
                                          </body>
                                        </body>
                                      </body>
                                    </body>
                                  </body>
                                </body>
                              </body>
                            </body>
                          </body>
                        </body>
                      </body>
                    </body>
                  </body>
                </body>
              </body>
            </body>
          </body>
        </body>
      </body>
    </body>
  </worldbody>
  <contact>
    <exclude body1="B_first" body2="B_1"/>
    <exclude body1="B_1" body2="B_2"/>
    <exclude body1="B_2" body2="B_3"/>
    <exclude body1="B_3" body2="B_4"/>
    <exclude body1="B_4" body2="B_5"/>
    <exclude body1="B_5" body2="B_6"/>
    <exclude body1="B_6" body2="B_7"/>
    <exclude body1="B_7" body2="B_8"/>
    <exclude body1="B_8" body2="B_9"/>
    <exclude body1="B_9" body2="B_10"/>
    <exclude body1="B_10" body2="B_11"/>
    <exclude body1="B_11" body2="B_12"/>
    <exclude body1="B_12" body2="B_13"/>
    <exclude body1="B_13" body2="B_14"/>
    <exclude body1="B_14" body2="B_15"/>
    <exclude body1="B_15" body2="B_16"/>
    <exclude body1="B_16" body2="B_17"/>
    <exclude body1="B_17" body2="B_18"/>
    <exclude body1="B_18" body2="B_19"/>
    <exclude body1="B_19" body2="B_20"/>
    <exclude body1="B_20" body2="B_21"/>
    <exclude body1="B_21" body2="B_22"/>
    <exclude body1="B_22" body2="B_23"/>
    <exclude body1="B_23" body2="B_24"/>
    <exclude body1="B_24" body2="B_25"/>
    <exclude body1="B_25" body2="B_26"/>
    <exclude body1="B_26" body2="B_27"/>
    <exclude body1="B_27" body2="B_28"/>
    <exclude body1="B_28" body2="B_29"/>
    <exclude body1="B_29" body2="B_30"/>
    <exclude body1="B_30" body2="B_last"/>
  </contact>
  <deformable>
    <skin name="Skin" rgba="1 1 1 1" inflate="0.0002" vertex="0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0" face="0 2 3 0 3 1 2 4 5 2 5 3 4 6 7 4 7 5 6 8 9 6 9 7 8 10 11 8 11 9 10 12 13 10 13 11 12 14 15 12 15 13 14 16 17 14 17 15 16 18 19 16 19 17 18 20 21 18 21 19 20 22 23 20 23 21 22 24 25 22 25 23 24 26 27 24 27 25 26 28 29 26 29 27 28 30 31 28 31 29 30 32 33 30 33 31 32 34 35 32 35 33 34 36 37 34 37 35 36 38 39 36 39 37 38 40 41 38 41 39 40 42 43 40 43 41 42 44 45 42 45 43 44 46 47 44 47 45 46 48 49 46 49 47 48 50 51 48 51 49 50 52 53 50 53 51 52 54 55 52 55 53 54 56 57 54 57 55 56 58 59 56 59 57 58 60 61 58 61 59 60 62 63 60 63 61 62 64 65 62 65 63 66 69 68 66 67 69 68 71 70 68 69 71 70 73 72 70 71 73 72 75 74 72 73 75 74 77 76 74 75 77 76 79 78 76 77 79 78 81 80 78 79 81 80 83 82 80 81 83 82 85 84 82 83 85 84 87 86 84 85 87 86 89 88 86 87 89 88 91 90 88 89 91 90 93 92 90 91 93 92 95 94 92 93 95 94 97 96 94 95 97 96 99 98 96 97 99 98 101 100 98 99 101 100 103 102 100 101 103 102 105 104 102 103 105 104 107 106 104 105 107 106 109 108 106 107 109 108 111 110 108 109 111 110 113 112 110 111 113 112 115 114 112 113 115 114 117 116 114 115 117 116 119 118 116 117 119 118 121 120 118 119 121 120 123 122 120 121 123 122 125 124 122 123 125 124 127 126 124 125 127 126 129 128 126 127 129 128 131 130 128 129 131 0 68 2 0 66 68 2 70 4 2 68 70 4 72 6 4 70 72 6 74 8 6 72 74 8 76 10 8 74 76 10 78 12 10 76 78 12 80 14 12 78 80 14 82 16 14 80 82 16 84 18 16 82 84 18 86 20 18 84 86 20 88 22 20 86 88 22 90 24 22 88 90 24 92 26 24 90 92 26 94 28 26 92 94 28 96 30 28 94 96 30 98 32 30 96 98 32 100 34 32 98 100 34 102 36 34 100 102 36 104 38 36 102 104 38 106 40 38 104 106 40 108 42 40 106 108 42 110 44 42 108 110 44 112 46 44 110 112 46 114 48 46 112 114 48 116 50 48 114 116 50 118 52 50 116 118 52 120 54 52 118 120 54 122 56 54 120 122 56 124 58 56 122 124 58 126 60 58 124 126 60 128 62 60 126 128 62 130 64 62 128 130 1 3 69 1 69 67 3 5 71 3 71 69 5 7 73 5 73 71 7 9 75 7 75 73 9 11 77 9 77 75 11 13 79 11 79 77 13 15 81 13 81 79 15 17 83 15 83 81 17 19 85 17 85 83 19 21 87 19 87 85 21 23 89 21 89 87 23 25 91 23 91 89 25 27 93 25 93 91 27 29 95 27 95 93 29 31 97 29 97 95 31 33 99 31 99 97 33 35 101 33 101 99 35 37 103 35 103 101 37 39 105 37 105 103 39 41 107 39 107 105 41 43 109 41 109 107 43 45 111 43 111 109 45 47 113 45 113 111 47 49 115 47 115 113 49 51 117 49 117 115 51 53 119 51 119 117 53 55 121 53 121 119 55 57 123 55 123 121 57 59 125 57 125 123 59 61 127 59 127 125 61 63 129 61 129 127 63 65 131 63 131 129 0 1 67 0 67 66 64 131 65 64 130 131">
      <bone body="B_first" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="0 66" vertweight="1 1"/>
      <bone body="B_first" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="1 67" vertweight="1 1"/>
      <bone body="B_1" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="2 68" vertweight="1 1"/>
      <bone body="B_1" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="3 69" vertweight="1 1"/>
      <bone body="B_2" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="4 70" vertweight="1 1"/>
      <bone body="B_2" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="5 71" vertweight="1 1"/>
      <bone body="B_3" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="6 72" vertweight="1 1"/>
      <bone body="B_3" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="7 73" vertweight="1 1"/>
      <bone body="B_4" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="8 74" vertweight="1 1"/>
      <bone body="B_4" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="9 75" vertweight="1 1"/>
      <bone body="B_5" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="10 76" vertweight="1 1"/>
      <bone body="B_5" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="11 77" vertweight="1 1"/>
      <bone body="B_6" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="12 78" vertweight="1 1"/>
      <bone body="B_6" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="13 79" vertweight="1 1"/>
      <bone body="B_7" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="14 80" vertweight="1 1"/>
      <bone body="B_7" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="15 81" vertweight="1 1"/>
      <bone body="B_8" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="16 82" vertweight="1 1"/>
      <bone body="B_8" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="17 83" vertweight="1 1"/>
      <bone body="B_9" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="18 84" vertweight="1 1"/>
      <bone body="B_9" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="19 85" vertweight="1 1"/>
      <bone body="B_10" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="20 86" vertweight="1 1"/>
      <bone body="B_10" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="21 87" vertweight="1 1"/>
      <bone body="B_11" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="22 88" vertweight="1 1"/>
      <bone body="B_11" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="23 89" vertweight="1 1"/>
      <bone body="B_12" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="24 90" vertweight="1 1"/>
      <bone body="B_12" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="25 91" vertweight="1 1"/>
      <bone body="B_13" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="26 92" vertweight="1 1"/>
      <bone body="B_13" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="27 93" vertweight="1 1"/>
      <bone body="B_14" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="28 94" vertweight="1 1"/>
      <bone body="B_14" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="29 95" vertweight="1 1"/>
      <bone body="B_15" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="30 96" vertweight="1 1"/>
      <bone body="B_15" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="31 97" vertweight="1 1"/>
      <bone body="B_16" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="32 98" vertweight="1 1"/>
      <bone body="B_16" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="33 99" vertweight="1 1"/>
      <bone body="B_17" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="34 100" vertweight="1 1"/>
      <bone body="B_17" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="35 101" vertweight="1 1"/>
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      <bone body="B_18" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="37 103" vertweight="1 1"/>
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      <bone body="B_25" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="50 116" vertweight="1 1"/>
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      <bone body="B_26" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="52 118" vertweight="1 1"/>
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      <bone body="B_28" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="56 122" vertweight="1 1"/>
      <bone body="B_28" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="57 123" vertweight="1 1"/>
      <bone body="B_29" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="58 124" vertweight="1 1"/>
      <bone body="B_29" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="59 125" vertweight="1 1"/>
      <bone body="B_30" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="60 126" vertweight="1 1"/>
      <bone body="B_30" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="61 127" vertweight="1 1"/>
      <bone body="B_last" bindpos="0 -0.003 0" bindquat="1 0 0 0" vertid="62 128" vertweight="1 1"/>
      <bone body="B_last" bindpos="0 0.003 0" bindquat="1 0 0 0" vertid="63 129" vertweight="1 1"/>
      <bone body="B_last" bindpos="-0.002 -0.003 0" bindquat="1 0 0 0" vertid="64 130" vertweight="1 1"/>
      <bone body="B_last" bindpos="-0.002 0.003 0" bindquat="1 0 0 0" vertid="65 131" vertweight="1 1"/>
    </skin>
  </deformable>

  <tendon>
  <spatial name ="tendon" range="0 .1" limited="true" width=".0005" rgba=".95 .3 .3 1" stiffness="0" >
    <site site="root"/>
    <site site ="S_2"/>
    <site site ="S_6"/>
    <site site ="S_10"/>
    <site site ="S_14"/>
    <site site ="S_18"/>
    <site site ="S_22"/>
    <site site ="S_26"/>
    <site site ="S_end"/>
  </spatial>
</tendon>

<actuator>
  <cylinder timeconst="0.005" ctrllimited="true" ctrlrange="-0.0008 0.0008" name="1_1" tendon="tendon" gear="10"/>
</actuator>
</mujoco>

Membrane.

minimal XML

<mujoco model="flat_belt">

  <option gravity="0 0 -9.81" density="1000" viscosity="0.0009" integrator="implicit" timestep="0.005"/>

  <extension>
    <plugin plugin="mujoco.elasticity.shell"/>
    <plugin plugin="mujoco.elasticity.cable"/>
  </extension>


  <default>
    <joint axis="0 1 0" range="-1 1" limited="true" damping="1" stiffness="100"/>
    <geom type="capsule" size=".001" rgba=".9 .7 .1 1"/>
    <site rgba=".9 .9 .9 1" size=".001"/>
    <default class="visual_cylinder">
      <geom type="cylinder" fromto="0 .015 0  0 -.015 0" size=".02" rgba=".3 .9 .3 .4"/>
    </default>
  </default>

  <worldbody>
    <light pos="0 0 5"/>

    <site name="root" pos="0 0 .902"/>
    
    <geom type="sphere" name="root" pos="0 0 .902" rgba="1 0 0 1"/>
    <geom type="sphere" name="end" pos="0.033 0 .902" rgba="1 0 0 1"/>
    
    
    <flexcomp name="membrane" type="grid" dim="2" count="5 4 1" spacing="0.01 0.008 0.002" pos="0.02 0 .9" radius="0.001">
      <plugin plugin="mujoco.elasticity.shell">
        <config key="poisson" value="0"/>
        <config key="thickness" value="1e-5"/>
        <config key="young" value="0.125e6"/>
      </plugin>
    </flexcomp>
    

  </worldbody>
</mujoco>

yuvaltassa · 2024-03-06T17:39:05Z

yuvaltassa
Mar 6, 2024
Maintainer

You are trying some very advanced stuff... we've never tried this ourselves so it's difficult to help you 🙂

To your specific question, flexcomps, like composites, will expand to a lower-level representation when saved to XML. In that representation you should be able to attach a tendon. Since I see you did this with the composite I expect that you tried this with the flexcomp as well. Did you try and fail?

0 replies

nhatminh2h · 2024-03-11T11:32:30Z

nhatminh2h
Mar 11, 2024
Author

Thank you for getting back to me. I really enjoy the demos on your youtube channel!

I have tried playing with the composite generated XML file(attaching sites for the tendon, is there a macro to add these?). I have attached the best attempt below(elasticity.cables + trial and error with the bending stiffness). It's quite promising but I tried the exact thing with grid and it seems to crash mujoco.

Screen.Recording.2024-02-28.at.15.16.22.mov

I will go through all the approaches one more time systematically and get back to you. How would I set fluidshape to ellipsoid for flexcomp since they dont have geom?

0 replies

robotlearning123 · 2024-03-22T16:17:45Z

robotlearning123
Mar 22, 2024

The video is cool! Any update for the new model?

0 replies

niaralston · 2025-09-08T15:00:17Z

niaralston
Sep 8, 2025

Hi @nhatminh2h

I'm a masters student from Queen's University working on modeling an soft inflatable walking robot on MuJoCo, and I'm also getting stuck on this issue! Were you able to find a solution? ☺️

0 replies

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