Possible to create a spatial tendon between two pulleys? #3149

dnmiller · 2026-03-03T23:11:13Z

dnmiller
Mar 3, 2026

Intro

Hi!

I'm looking into cable-driven robots and had a general question about spatial tendons between two pulleys.

My setup

Using Mujoco 3.5.0 viewer on Mac (Apple Silicon) to model a simple robot.

My question

I'm trying to get a spatial tendon to drive a system of pulleys. My motivation for going with spatial over fixed is the dependency of the tendon on the geometry. I can get it to work if I add sliders in between the pulleys, but they need to have a significant nonzero mass for the simulation to behave. Is it possible to get a site element on a joint without a body or to have two geometries without a site in between? Would mocap elements work for this? Am I thinking of this all wrong and should just use fixed tendons? Any feedback is appreciated.

Minimal model and/or code that explain my question

I modified one of the tendon examples to demonstrate the idea. If the mass of the sliders is set too low, the simulation misbehaves.

<mujoco>
  <compiler autolimits="true"/>

  <default>
    <site type="sphere" rgba="1 0 0 1" size="0.005"/>
    <tendon rgba="0 1 0 1"/>
  </default>

  <visual>
    <headlight diffuse=".7 .7 .7"/>
  </visual>

  <worldbody>
    <body name="arm">
      <joint name="arm" axis="0 1 0" damping=".04"/>
      <geom name="shoulder" type="cylinder" size=".05 .01" zaxis="0 1 0"/>
      <geom name="arm" type="box" size=".01" fromto="0 0 0 0 0 0.3"/>
      <site name="arm" pos="0 0 .051"/>
    </body>

    <site name="right_hi" pos=".1 0 0"/>
    <site name="right_lo" pos=".1 0 -.4"/>
    <site name="left_hi" pos="-.1 0 0"/>
    <site name="left_lo" pos="-.1 0 -.4"/>

    <body name="slider_r" pos=".05 0 -.2">
      <joint name="slider_r" type="slide" damping="1"/>
      <geom name="slider_r" type="box" size=".01 .01 .01"/>
      <site name="slider_r" pos="0 0 .01"/>
    </body>

    <body name="slider_l" pos="-.05 0 -.2">
      <joint name="slider_l" type="slide" damping="1"/>
      <geom name="slider_l" type="box" size=".01 .01 .01"/>
      <site name="slider_l" pos="0 0 .01"/>
    </body>

    <body name="pulley" pos="0 0 -0.4">
      <geom name="pulley/pulley" type="cylinder" size=".05 .01" zaxis="0 1 0"/>
      <geom name="pulley/arm" type="box" size=".01" fromto="0 0 0 .3 0 0"/>
      <site name="pulley/site" pos="0 0 -.051"/>
      <joint name="pulley/joint" axis="0 1 0" damping=".04"/>
    </body>
  </worldbody>

  <tendon>
    <spatial name="rope_r0" range="0 .1">
      <site site="arm"/>
      <geom geom="shoulder" sidesite="right_hi"/>
      <site site="slider_r"/>
    </spatial>

    <spatial name="rope_r1" range="0 .1">
      <site site="slider_r"/>
      <geom geom="pulley/pulley" sidesite="right_lo"/>
      <site site="pulley/site"/>
    </spatial>

    <spatial name="rope_l0" range="0 .1">
      <site site="arm"/>
      <geom geom="shoulder" sidesite="left_hi"/>
      <site site="slider_l"/>
    </spatial>

    <spatial name="rope_l1" range="0 .1">
      <site site="slider_l"/>
      <geom geom="pulley/pulley" sidesite="left_lo"/>
      <site site="pulley/site"/>
    </spatial>
  </tendon>

  <actuator>
    <position name="pulley" joint="pulley/joint" ctrlrange="-1 1" kp="100"/>
  </actuator>
</mujoco>

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Possible to create a spatial tendon between two pulleys? #3149

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dnmiller Mar 3, 2026

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 0 comments

dnmiller
Mar 3, 2026