The problem of the elongation of the tendon around a fixed pulley #2579

tttj12 · 2025-04-16T06:22:32Z

tttj12
Apr 16, 2025

Intro

Hi!

I am a student . I use MuJoCo for my research on Rope-driven manipulator.

My setup

windows mujoco3.2.5

My question

We attempted to use tendons to drive the manipulator in order to simulate the rope-driven manipulator in reality. Therefore, the elongation of the tendon is of great importance to us. We found that when the tendon goes around a cylinder, the length of the tendon, whether it is the visual length or the simulated length, is calculated as a straight line. Could you please tell us what methods are available to improve the calculation accuracy so that the tendon will not pass through the geom?

Minimal model and/or code that explain my question

If you encountered the issue in a complex model, please simplify it as much as possible (while still reproducing the issue).

<site size="0.001 0.005 0.005"/>

</default>

</body>


<body name="pulley" pos="0 0 0">

  
  
  <geom name="pulley" type="cylinder" pos="0 0 0.5" quat="0.74 0.74 0 0"  solimp="0.9 0.95 0.001" solref="0.0001 1" size="0.05 0.02"  rgba="0.8 0.8 0.8 0.2" />

  <site name="site1" type="sphere" size="0.005" pos="0.05 0 0.5" rgba="1 0 0 1"/>



   <site name="site2" type="sphere" size="0.005" pos="-0.05 0 0.5" rgba="0 1 0 1"/>



  <site name="pulley1"  size="0.005" pos="0 0 .6"/>  
</body>

-0.2 0 0.3

-0.05 0 0

<body name="left_rope" pos="-0.2 0 0.4">

  <site name="left_rope" size="0.02"   pos="0 0 0" rgba="1 0 0 1" />


</body>





<body name="right_weight" pos="0.2 0 0.6">
  <joint type="free" axis="0 0 1" />
 
 <geom type="box" size="0.05 0.05 0.05"    mass="0.01" rgba="0 0 1 1"/>
   <site name="box" size="0.01"  pos="0 0 .05" rgba="1 0 0 1" />
</body>

  <site site="left_rope"/>



  <geom geom="pulley" sidesite="pulley1"/>


  <site site="box"/>
</spatial>

Model:

minimal XML

<mujoco>
  <worldbody>
    <light pos="0 0 1"/>
    <geom type="sphere" size="1" rgba="1 0 0 1"/>
  </worldbody>
</mujoco>

Code:

import mujoco
import mediapy as media
model = mujoco.MjModel.from_xml_string(xml)
data = mujoco.MjData(model)

with mujoco.Renderer(model) as renderer:
  mujoco.mj_forward(model, data)
  renderer.update_scene(data)

  media.show_image(renderer.render())

Confirmations

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

tttj12 · 2025-04-16T07:32:26Z

tttj12
Apr 16, 2025
Author

Sorry. Later, we recalculated the length of the tendon, and it is in line with the length of the arc that includes the part around the cylinder. The tendon passing through the cylinder is a visual effect. Since we are beginners, we would like to ask you something. Is it feasible to use the tendon to drive the manipulator and simulate the elongation of the tendon to be the same as the elongation of the rope driven by the motor in reality? Do we need to make any changes to the tendon statements?

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The problem of the elongation of the tendon around a fixed pulley #2579

tttj12 Apr 16, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 1 comment

tttj12 Apr 16, 2025 Author

tttj12
Apr 16, 2025

tttj12
Apr 16, 2025
Author