Touch Sensors not always registering contact #2483

kuds · 2025-03-07T21:54:11Z

kuds
Mar 7, 2025

Intro

Hi!

I am an independent researcher interested in using reinforcement learning to solve various problems and to build experiments in MujoCo.

My setup

Google Colab T4
Python Version: 3.11.11
Torch Version: 2.5.1+cu124
Is Cuda Available: True
Cuda Version: 12.4
Gymnasium Version: 1.0.0
Numpy Version: 1.26.4
Mujoco Version: 3.3.0
Stable-Baselines3 Version: 2.5.0
Matplotlib Version: 3.10.0

My question

I am trying to build a reinforcement learning system that learns to bounce a ball using Gymnasium, Stable Baselines3, and MuJoCo. In reviewing the simulation recording and contact data, there appears to be a misalignment between what the video shows and how many times the touch sensor registers a force. The embedded video shows at least six (6) bounces, but the sensor data only lists three (3) (see attached CSV).

ball_bounce_200000.csv

Relevant Issues:

The above issues stem from the touch sensor area being too small. So, I increased the site size to include the entire paddle, but the touch sensor does not always appear to register contact.

Relevant Documentation:

What is the reason why MuJoCo is not registering all contact instances via the touch sensor? Is it because the timestep is too big, or how the solref is defined for the paddle and ball geom?

Any help would be greatly appreciated!

Minimal model and/or code that explain my question

Link to Google Colab Notebook

[Ball Bounce] Soft Actor-Critic (SAC)

MuJoCo World Model

<mujoco>
  <option timestep=".002" />
  <asset>
    <texture name="grid" type="2d" builtin="checker" rgb1=".1 .2 .3"
      rgb2=".2 .3 .4" width="300" height="300" mark="none" />
    <material name="grid" texture="grid" texrepeat="1 1"
      texuniform="true" reflectance=".2" />
  </asset>
  <worldbody>
    <light name="light" pos="0 0 1" />
    <geom name="floor" type="plane" pos="0 0 -0.5" size="2 2 .1" material="grid" />
    <body name="paddle_body" pos="0 0 0">
      <site name="bounce_sensor" pos="0 0 0.025" size="0.21 0.21 0.075" type="box" rgba="1 0 0 .05" />
      <joint name="rotate_x" pos="0 0 0" axis="1 0 0" range="-.3 .3" damping="10" />
      <joint name="rotate_y" pos="0 0 0" axis="0 1 0" range="-.3 .3" damping="10" />
      <joint name="rotate_z" pos="0 0 0" axis="0 0 1" range="-.3 .3" damping="10" />
      <joint axis="0 0 1" limited="true" name="slider_z" range="0 0.5" type="slide" />
      <joint axis="0 1 0" limited="true" name="slider_y" range="-.2 .2" type="slide" />
      <joint axis="1 0 0" limited="true" name="slider_x" range="-.2 .2" type="slide" />
      <geom name="paddle_geom" type="box" size=".2 .2 .02" rgba="0.2 0.2 1 1" solref="0.02 0.07" />
    </body>
    <body name="ball_body" pos="0 0 0.3">
      <freejoint name="ball_freejoint" />
      <geom name="ball_geom" type="sphere" size=".04" rgba="0 1 0 1" solref="0.02 0.07" />
    </body>
  </worldbody>
  <actuator>
    <general name="act_x" joint="rotate_x" ctrlrange="-1 1" gainprm="500 0 0" />
    <general name="act_y" joint="rotate_y" ctrlrange="-1 1" gainprm="500 0 0" />
    <general name="act_z" joint="rotate_z" ctrlrange="-1 1" gainprm="500 0 0" />
    <motor ctrllimited="true" ctrlrange="-1 1" gear="50" joint="slider_z" name="slider_z_motor" />
    <motor ctrllimited="true" ctrlrange="-1 1" gear="50" joint="slider_y" name="slider_y_motor" />
    <motor ctrllimited="true" ctrlrange="-1 1" gear="50" joint="slider_x" name="slider_x_motor" />
  </actuator>
  <sensor>
    <touch name="touch_sensor" site="bounce_sensor" />
  </sensor>
</mujoco>

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 · 2025-03-08T05:41:02Z

yuvaltassa
Mar 8, 2025
Maintainer

It's a bit hard to see in your video, but I think the touch sensor is not properly aligned, it seems to be above the box rather than enclosing it. If you have a deep penetration then the position of the contact could easily be quite deep in the paddle, which would not be caught by the sensor.

0 replies

yuvaltassa · 2025-03-08T05:42:32Z

yuvaltassa
Mar 8, 2025
Maintainer

PS, you might want to consider using a fromto sensor, and registering a collision when the fromto vector flips direction

0 replies

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Touch Sensors not always registering contact #2483

kuds Mar 7, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Link to Google Colab Notebook

MuJoCo World Model

Confirmations

Replies: 2 comments

yuvaltassa Mar 8, 2025 Maintainer

yuvaltassa Mar 8, 2025 Maintainer

kuds
Mar 7, 2025

yuvaltassa
Mar 8, 2025
Maintainer

yuvaltassa
Mar 8, 2025
Maintainer