How can a rope, which is connected to one end of a muscle, only transmit force without changing its length and without affecting the power output of my muscles?

[shansongjunjie]

Intro

Hi!

I am a graduate student at CAU, I use MuJoCo for my research on muscle-rope manipulation.

My setup

My question

I have built a muscle on my thigh, which belongs to one link, with one end fixed and the other sliding. The sliding end is also connected to a rope, and the other end of the rope is connected to the calf. The thigh and calf are also connected. Both muscles and ropes are established using the spatial tendon method, with only muscle attributes established on the actuator. The problem we are facing now is that after the muscles exert force and shorten, the rope is stretched and the calf is not pulled up as expected. In this case, how can I modify the code to ensure that the rope length remains unchanged while the calf is pulled up? If the stiffness of the rope is indicated, the corresponding muscle tension will greatly increase, which is different from the actual situation. I hope there is a way to solve this problem, or is there any other way to simulate the process of muscle rope calf pulling?

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).

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.

[Balint-H]

Hello!

When you say rope, do you mean a tendon element? Or are you trying to model cable/rope slacking/dynamics as well e.g. with the elasticity plugin?

If it is just a tendon element then you can set its length range and limited attributes to have a fixed length (if you want to only pull and prevent pushing, then set the minimum length to near zero) https://mujoco.readthedocs.io/en/latest/XMLreference.html#tendon-spatial

[shansongjunjie]

Thank you for your answer！ I have already done this. The code is as follows:




But as shown in Figure 2, the length of this rope was still stretched and not restricted, and I am also puzzled where the problem lies.If the stiffness of this rope is increased, the preset muscle force will not be able to pull up the rope or the calf.

[shansongjunjie]

[Balint-H]

You could investigate the solimp and solref parameters that make the length constraint stronger (without applying proportional stiffness throughout)