Obstacles for Tendons acting as Cables #2554

Mohamed-Abduljawad · 2025-04-03T09:43:21Z

Mohamed-Abduljawad
Apr 3, 2025

Intro

Hi!

I am a researcher) and I use MuJoCo for my research on Cable driven robots.

My setup

Mujoco 3.3.0
Ubuntu 22.04

My question

my four-cables cable driven robot seen below, has the anchor points actuated. Now, I always want the cables to pass thorough the corners of the square the four anchors form.

However, when I pull the cables, in so many cases the cables will not go though the desired corners (denoted in red circles) as shown below.

I tried to add a site and let the cables pass through that, but doing so, stops the tendon from acting as a cable and look like always in tension. And also, this setup tend to oscillate more.

What is the best way to have a pull-only tendons, that also go through a specific point?

Minimal model and/or code that explain my question

Model without a site (cables deviate from desired point)

<mujoco>

  <asset>
    <texture name="grid" type="2d" builtin="checker" rgb1=".1 .2 .3"
     rgb2=".2 .3 .4" width="300" height="300"/>
    <material name="grid" texture="grid" texrepeat="8 8" reflectance=".2"/>
  </asset>

  <option density="50"/>

      <default>
        <tendon limited="true" range="0 1.5" />
    </default>

  <worldbody>
      <geom size="2 2 .1" type="plane" material="grid" pos="0 0 -2"/>

    <light pos="0 -.2 2" dir="0 1 -4" diffuse="1 1 1"/>

      <body name="anchor1_body" pos="0 -1 -1" euler="135 0 0">
        <joint name="anchor1_joint" type="slide" axis="0 0 1" damping="100" range="0 1"/>
        <geom name="anchor1_geom" type="sphere" size="0.02" rgba="1 0 0 1" mass="0.01" />
        <site name="anchor1" pos="0 0 0" size="0.02" />
      </body>


      <body name="anchor2_body" pos="0 -1 1" euler="45 0 0">
        <joint name="anchor2_joint" type="slide" axis="0 0 1" damping="100" range="0 1"/>
        <geom name="anchor2_geom" type="sphere" size="0.02" rgba="1 0 0 1" mass="0.01" />
        <site name="anchor2" pos="0 0 0" size="0.02" />
      </body>


      <body name="anchor3_body" pos="0 1 1" euler="-45 0 0">
        <joint name="anchor3_joint" type="slide" axis="0 0 1" damping="100" range="0 1"/>
        <geom name="anchor3_geom" type="sphere" size="0.02" rgba="1 0 0 1" mass="0.01" />
        <site name="anchor3" pos="0 0 0" size="0.02" />
      </body>

    <body name="anchor4_body" pos="0 1 -1" euler="-135 0 0">
        <joint name="anchor4_joint" type="slide" axis="0 0 1" damping="100" range="0 1"/>
        <geom name="anchor4_geom" type="sphere" size="0.02" rgba="1 0 0 1" mass="0.01" />
        <site name="anchor4" pos="0 0 0" size="0.02" />
      </body>

      <body name="anchor4h" pos="0 1 -1" euler="-135 0 0">
        <site name="anchor4h" pos="0 0 0" size="0.02" />
      </body>
    
    <body name="frame" pos="0.1 0 0.0" euler="0 90 0">
        <geom name="frame_board" type="box" size="1 1 0.01" rgba="1 1 1 0.5" />
      </body>

    <!-- Moving EE -->
    <body name="EE" pos="0 0 0">
      <joint type="free"/>
      <geom type="box" size="0.1 0.1 0.01" rgba="0 0.5 0 1" mass="1"/>
      <!-- Cable attachment points -->
      <site name="ee1" pos="-0.1 -0.1 0" size="0.01"/>
      <site name="ee2" pos="0.1 -0.1 0" size="0.01"/>
      <site name="ee3" pos="0.1 0.1 0" size="0.01"/>
      <site name="ee4" pos="-0.1 0.1 0" size="0.01"/>
    </body>

  </worldbody>
  
  <tendon>
    <spatial name="cable1" >
      <site site="anchor1"/>
      <site site="ee1"/>
    </spatial>
    <spatial name="cable2">
      <site site="anchor2"/>
      <site site="ee2"/>
    </spatial>
    <spatial name="cable3">
      <site site="anchor3"/>
      <site site="ee3"/>
    </spatial>
    <spatial name="cable4">
      <site site="anchor4"/>
      <site site="ee4"/>
    </spatial>
  </tendon> 

  <sensor>
    <tendonlimitfrc name="cable_tension1" tendon="cable1"/>
    <tendonlimitfrc name="cable_tension2" tendon="cable2"/>
    <tendonlimitfrc name="cable_tension3" tendon="cable3"/>
    <tendonlimitfrc name="cable_tension4" tendon="cable4"/>
    <force site="ee4"/>
  </sensor>

  <actuator>
    <motor name="anchor1_motor" joint="anchor1_joint" forcerange="-100 100" gear="100" ctrlrange="-1 1"/>
    <motor name="anchor2_motor" joint="anchor2_joint" forcerange="-100 100" gear="100" ctrlrange="-1 1"/>
    <motor name="anchor3_motor" joint="anchor3_joint" forcerange="-100 100" gear="100" ctrlrange="-1 1"/>
    <motor name="anchor4_motor" joint="anchor4_joint" forcerange="-100 100" gear="100" ctrlrange="-1 1"/>
    
  </actuator>
</mujoco>

Model with a site for cable 4 (stops acting like cables)

<mujoco>

  <asset>
    <texture name="grid" type="2d" builtin="checker" rgb1=".1 .2 .3"
     rgb2=".2 .3 .4" width="300" height="300"/>
    <material name="grid" texture="grid" texrepeat="8 8" reflectance=".2"/>
  </asset>

  <option density="50"/>

      <default>
        <tendon limited="true" range="0 1.5" />
    </default>

  <worldbody>
      <geom size="2 2 .1" type="plane" material="grid" pos="0 0 -2"/>

    <light pos="0 -.2 2" dir="0 1 -4" diffuse="1 1 1"/>

      <body name="anchor1_body" pos="0 -1 -1" euler="135 0 0">
        <joint name="anchor1_joint" type="slide" axis="0 0 1" damping="100" range="0 1"/>
        <geom name="anchor1_geom" type="sphere" size="0.02" rgba="1 0 0 1" mass="0.01" />
        <site name="anchor1" pos="0 0 0" size="0.02" />
      </body>


      <body name="anchor2_body" pos="0 -1 1" euler="45 0 0">
        <joint name="anchor2_joint" type="slide" axis="0 0 1" damping="100" range="0 1"/>
        <geom name="anchor2_geom" type="sphere" size="0.02" rgba="1 0 0 1" mass="0.01" />
        <site name="anchor2" pos="0 0 0" size="0.02" />
      </body>


      <body name="anchor3_body" pos="0 1 1" euler="-45 0 0">
        <joint name="anchor3_joint" type="slide" axis="0 0 1" damping="100" range="0 1"/>
        <geom name="anchor3_geom" type="sphere" size="0.02" rgba="1 0 0 1" mass="0.01" />
        <site name="anchor3" pos="0 0 0" size="0.02" />
      </body>

    <body name="anchor4_body" pos="0 1 -1" euler="-135 0 0">
        <joint name="anchor4_joint" type="slide" axis="0 0 1" damping="100" range="0 1"/>
        <geom name="anchor4_geom" type="sphere" size="0.02" rgba="1 0 0 1" mass="0.01" />
        <site name="anchor4" pos="0 0 0" size="0.02" />
      </body>

      <body name="anchor4h" pos="0 1 -1" euler="-135 0 0">
        <site name="anchor4h" pos="0 0 0" size="0.02" />
      </body>
    
    <body name="frame" pos="0.1 0 0.0" euler="0 90 0">
        <geom name="frame_board" type="box" size="1 1 0.01" rgba="1 1 1 0.5" />
      </body>

    <!-- Moving EE -->
    <body name="EE" pos="0 0 0">
      <joint type="free"/>
      <geom type="box" size="0.1 0.1 0.01" rgba="0 0.5 0 1" mass="1"/>
      <!-- Cable attachment points -->
      <site name="ee1" pos="-0.1 -0.1 0" size="0.01"/>
      <site name="ee2" pos="0.1 -0.1 0" size="0.01"/>
      <site name="ee3" pos="0.1 0.1 0" size="0.01"/>
      <site name="ee4" pos="-0.1 0.1 0" size="0.01"/>
    </body>

  </worldbody>
  
  <tendon>
    <spatial name="cable1" >
      <site site="anchor1"/>
      <site site="ee1"/>
    </spatial>
    <spatial name="cable2">
      <site site="anchor2"/>
      <site site="ee2"/>
    </spatial>
    <spatial name="cable3">
      <site site="anchor3"/>
      <site site="ee3"/>
    </spatial>
    <spatial name="cable4">
      <site site="anchor4"/>
          <!-- stops acting like a tendon -->
      <site site="anchor4h"/>
      <site site="ee4"/>
    </spatial>
  </tendon> 

  <sensor>
    <tendonlimitfrc name="cable_tension1" tendon="cable1"/>
    <tendonlimitfrc name="cable_tension2" tendon="cable2"/>
    <tendonlimitfrc name="cable_tension3" tendon="cable3"/>
    <tendonlimitfrc name="cable_tension4" tendon="cable4"/>
    <force site="ee4"/>
  </sensor>

  <actuator>
    <motor name="anchor1_motor" joint="anchor1_joint" forcerange="-100 100" gear="100" ctrlrange="-1 1"/>
    <motor name="anchor2_motor" joint="anchor2_joint" forcerange="-100 100" gear="100" ctrlrange="-1 1"/>
    <motor name="anchor3_motor" joint="anchor3_joint" forcerange="-100 100" gear="100" ctrlrange="-1 1"/>
    <motor name="anchor4_motor" joint="anchor4_joint" forcerange="-100 100" gear="100" ctrlrange="-1 1"/>
    
  </actuator>
</mujoco>

Confirmations

I searched the latest documentation thoroughly before posting.
I searched previous Issues and Discussions, I am certain this has not been raised before.

Answered by yuvaltassa

Apr 3, 2025

The hanging-down is just a cute visualization, it doesn't affect the dynamics (basically just a way of showing that there is no tension).

Your approach with the site via-point is the correct one. The "oscillates more" issue needs to be understood and debugged separately.

View full answer

yuvaltassa · 2025-04-03T16:30:43Z

yuvaltassa
Apr 3, 2025
Maintainer

The hanging-down is just a cute visualization, it doesn't affect the dynamics (basically just a way of showing that there is no tension).

Your approach with the site via-point is the correct one. The "oscillates more" issue needs to be understood and debugged separately.

4 replies

Mohamed-Abduljawad Apr 4, 2025
Author

Thanks a lot for confirming that, Yuval. If that is the case, will the tendons still behave the same if I actuate them directly like

    <motor name="anchor4_motor" tendon="cable4" forcerange="-100 100" gear="100" ctrlrange="-1 1"/>

instead of actuating the joint:

    <motor name="anchor4_motor" joint="anchor4_joint" forcerange="-100 100" gear="100" ctrlrange="-1 1"/>

For me they seem to be able to "push" in the first case, right?. Because if I can directly assign the force on the cable that would make my simulation more accurate.

yuvaltassa Apr 7, 2025
Maintainer

Yup

yuvaltassa Apr 7, 2025
Maintainer

Of course you want <motor tendon="..." ctrlrange="-X 0"/> if you only want pulling and not pushing

Mohamed-Abduljawad Apr 10, 2025
Author

Yeah that makes sense. And I saw your recent update which looks awesome specially the tendon actuator force sensor. Because the tendonlimitfrc are failing once I actuate the tendons directly, becuase . So I think this new sensor would solve that, right?

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Obstacles for Tendons acting as Cables #2554

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Mohamed-Abduljawad Apr 3, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 1 comment · 4 replies

Uh oh!

yuvaltassa Apr 3, 2025 Maintainer

Uh oh!

Mohamed-Abduljawad Apr 4, 2025 Author

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yuvaltassa Apr 7, 2025 Maintainer

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yuvaltassa Apr 7, 2025 Maintainer

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Mohamed-Abduljawad Apr 10, 2025 Author

Mohamed-Abduljawad
Apr 3, 2025

Replies: 1 comment 4 replies

yuvaltassa
Apr 3, 2025
Maintainer

Mohamed-Abduljawad Apr 4, 2025
Author

yuvaltassa Apr 7, 2025
Maintainer

yuvaltassa Apr 7, 2025
Maintainer

Mohamed-Abduljawad Apr 10, 2025
Author