Measuring Reaction Forces and Moments at the Base of a Cable Composite in MuJoCo #2424

Shebani3 · 2025-02-15T02:29:11Z

Shebani3
Feb 15, 2025

Intro

Hi!

I am an undergrad student, I use MuJoCo for my research on Whisker sensors for robots.

My setup

MuJoCo version: 3.2.5
Python API
Windows 11

My question

My whisker sensor consists of 2 parts: The whisker shaft and whisker base(follicle). To model the whisker shaft I am using a cable composite with elastic plugin to model 'hair-like' behaviour. At the base of the cable composite, I have added a 3-axis force and a 3-axis torque sensor to simulate the whisker follicle where mechanoreception takes place. I would like to measure the reaction forces and moments developed at the base on bending the cable. However, I have a few questions:

I have created a site at the base with force and torque sensors attached. To test the force sensor, I swept a constant force along the length of the cable while measuring the force at the base. I expect some change in the measured force from the initial segment to the final segment however, it was constant. Have I correctly integrated the force sensor in my model?
Can the vmax parameter in the elastic plugin be used to get the mechanical stress at the base of the cable? According to Get internal mechanical stress in elasticity plugin #1322, it is not, but I wanted to make sure I understood it correctly?
Lastly, is there a better way to measure the forces and moments at the base?

Thank you for your time.

Minimal model and/or code that explain my question

Whisker xml

<mujoco model="Whisker">
<option timestep = "0.0002"></option>
<option gravity="0 0 0"/>
<extension>
   <plugin plugin="mujoco.elasticity.cable"/>
</extension>

  <statistic center="0 0 0.2" extent="1"/>

  <compiler autolimits="true"/>

  <visual>
    <global elevation="0"/>
  </visual>

  <default>
    <geom solref="0.01 1" solimp=".95 .99 .0001"/>
  </default>

  <worldbody>
   
   <light diffuse=".4 .4 .4" specular="0.1 0.1 0.1" pos="0 0 2.0" dir="0 0 -1" castshadow="false"/>
   <light directional="true" diffuse=".8 .8 .8" specular="0.2 0.2 0.2" pos="0 0 4" dir="0 0 -1"/>
   <geom name="ground" type="plane" size="0 0 100" pos="0 0 0" quat="1 0 0 0" condim="1" rgba="0.15 
    0.14 0.05 1"/>

   <body name="Composit_whisker_body"  pos="0.0 0.0 0.4" euler="0 0 0">
    
   <site name="base_site" pos="0 0 0" size="0.01" rgba="0 0.42 0 0.1" euler="0 90 0"/>
   <site name="whis_base_site" type="capsule" size="0.01 0.029" rgba="0 1 1 0.4" euler = "0 90 0" 
        pos = "0.026 0.0 0"/>
   
   <composite prefix="SegCom" type="cable" curve="s 0 0" count="21 1 1" size = "0.3"
       offset="0 0 0" initial="none">
      
   <plugin plugin="mujoco.elasticity.cable">
        <config key="twist" value="1e8"/>
        <config key="bend" value="10e8"/>
        <config key="vmax" value="1"/>
    </plugin>

  <joint kind="main" type="hinge" axis ="0 1 0"  stiffness="0.5" damping="0.1" />
  <geom type="capsule" size="0.004" rgba=".8 .2 .1 1" mass="5.27e-3"/>
      
  </composite>

    </body>

    <body name="sliderz" pos="0 0.09 0.5">
    <joint name="slidex" type="slide" axis="1 0 0" damping="5" range="-1 1"/>
    <joint name="slidey" type="slide" axis="0 1 0" damping="5" range="-1 1"/>
    <joint name="slidez" type="slide" axis="0 0 1" damping="5" range="-1 1"/>

    <geom name = "testbox" type = "box" size="0.01 0.05 0.05" rgba=".8 .9 .1 1" density= "1000"/>

    </body>

  </worldbody>

<sensor> 
  <force name="base_force_sensor" site="whis_base_site" />
  <torque name="base_torque_sensor" site="whis_base_site"/>
</sensor>

<actuator>
    <position name="move_x" joint="slidex"  gear="1 0 0 0 0 0" ctrlrange="-.25 0.5" kp="100" />
    <position name="move_y" joint="slidey"  gear="1 0 0 0 0 0" ctrlrange="-.25 0.5" kp="100" />
    <position name="move_z" joint="slidez"  gear="1 0 0 0 0 0" ctrlrange="-.25 0.5" kp="100" />
</actuator>

</mujoco>

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Measuring Reaction Forces and Moments at the Base of a Cable Composite in MuJoCo #2424

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Measuring Reaction Forces and Moments at the Base of a Cable Composite in MuJoCo #2424

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Shebani3 Feb 15, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 0 comments

Shebani3
Feb 15, 2025