the force sensor between body and worldbody

[Lu-Weiyan]

Intro

Hi!

I am a student , I use MuJoCo for my research.I want to measure the external force for my robotic system,for example the wind drag.

My setup

Linux Ubuntu 20.04
Python 3.8
mujoco 3.2.2

My question

I want to build a external force estimator for my robotic system.And Let the data from force sensor be the ground truth.Hovever,I can figure how to measure it.Can I use the force sensor between body(child) and worldbody(parent) to measure it?When my body has actuators or bodies as childen,shall I subtract the forces from them?After my test,I figure out the data from the force sensor between body(child) and worldbody(parent) includes the force of actuators.What about the force from the next bodies as childen?What’s more, If I want to use the world frame to indicate the direction of the external force, shall I use the xquat of the child to transform the frame to the world frame?
myscene.md

Minimal model and/or code that explain my question

<sensor>
<gyro site="imu_drone" name="body_gyro_drone"/>
<accelerometer site="imu_drone" name="body_linacc_drone"/>
<framequat objtype="site" objname="imu_drone" name="body_quat_drone"/>
<gyro site="imu_load" name="body_gyro_load"/>
<framequat objtype="site" objname="imu_load" name="body_quat_load"/>
<accelerometer site="imu_load" name="body_linacc_load"/>
<force site="force_quad" name="force_quad"/>
<force site="force_cable" name="force_cable"/>
<force site="force_load" name="force_load"/>

Confirmations

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

[yuvaltassa]

I feel like all of your questions are clearly answered here, are they not?

[Lu-Weiyan]

Finally,I figure out qfrc_passive meets my request.Thank you