Help with Mujoco Sensors

[JakeAnd91]

Intro

Hi!

I am a researcher at the University of Utah, I use MuJoCo to model a robotic hand.

My setup

Mujoco witha python script

My question

I’m trying to measure how much force or grip strength my robotic hand applies to an object, but I want to avoid putting sensors on the object itself. My setup uses sensors mounted on the fingers, and I wrote a Python script that moves the hand around an object while plotting the sensor readings.

The issue is that the force graph looks almost identical every time, no matter what object I use. The reading rises as the finger moves, then the moment the finger touches the object, the “force” value drops sharply, levels out, and then continues to decline. Here’s an example of the graph I get:

What I expected was a clear difference between gripping soft vs. hard objects, but the readings don’t seem to reflect actual contact force at all.

Am I using force sensor correctly or am I just measuring actuator load instead of fingertip force?

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.