No Data from data.force_matrix_w in Contact Sensor – Based on Direct Workflow RL Environment

[KnuniK]

Hi everyone,

I'm currently working with IsaacLab and trying to integrate contact sensors into my RL environment. My setup is based on the official Direct Workflow RL Environment tutorial, and I followed the instructions from the contact sensor documentation to add sensors.

However, I'm running into an issue where the contact sensors appear to exist, but I'm not receiving any data from them.

print(self.scene["contact_forces"])
print("Received force matrix of: ", self.scene["contact_forces"].data.force_matrix_w)

Which results in:

Contact sensor @ '/World/envs/env_./Robot/Walker_extend/._Ext':
view type : <class 'omni.physics.tensors.impl.api.RigidBodyView'>
update period (s) : 0.0
number of bodies : 3
body names : ['Links_Ext', 'Rechts_Ext', 'Vorne_Ext']
Received force matrix of: None

So it seems like the sensor is registered correctly in the scene and detects the bodies, but the actual data.force_matrix_w returns None.
Any help or tips would be greatly appreciated!

[RandomOakForest]

Thank you for posting this. The behavior you are observing—where data.force_matrix_w for your contact sensor returns None—has a clear cause in the current design of IsaacLab’s contact sensors:

Key Facts from the Docs

• Contact Sensor Types:
  Contact sensors in IsaacLab can be attached to multiple rigid bodies in a single definition (e.g., all feet of a robot), or you can define one sensor per rigid body.¹
• Force Matrix Output:
  • If your contact sensor is attached to a single body, and you're using filter_prim_paths_expr to specify what it should sense contacts with, data.force_matrix_w returns the filtered contact forces as a matrix.
  • If your contact sensor is attached to multiple bodies (i.e., more than one body listed in its config), data.force_matrix_w will always return None by design—regardless of filtering or contacts.¹

“When the force matrix is queried, the result is None because this is a many body sensor, and presently Isaac Lab only supports ‘many to one’ contact force filtering. Unlike the single body contact sensor, the reported force tensor has multiple entries, with each row corresponding to the contact force on a single body of the sensor (matching the ordering at construction).”¹

What This Means for Your Setup

• The contact sensor you defined is scoped over three bodies: 'Links_Ext', 'Rechts_Ext', and 'Vorne_Ext'.
• With this configuration, IsaacLab does not populate data.force_matrix_w—it remains None.¹
• However, the per-body net contact force will be available in data.net_forces_w, where each row corresponds to a body attached to the sensor.

How to Get Filtered Force Data

If you want to get filtered force data (matrix form), you must define one contact sensor per body you wish to monitor. For example:

contact_L = ContactSensorCfg(
    prim_path="{ENV_REGEX_NS}/Robot/Links_Ext",
    update_period=0.0,
    history_length=6,
    debug_vis=True,
    filter_prim_paths_expr=["{ENV_REGEX_NS}/Cube"]
)
contact_R = ContactSensorCfg(
    prim_path="{ENV_REGEX_NS}/Robot/Rechts_Ext",
    update_period=0.0,
    history_length=6,
    debug_vis=True,
    filter_prim_paths_expr=["{ENV_REGEX_NS}/Cube"]
)
# Repeat for each body.

Then access the filtered contact force matrix as:

scene["contact_L"].data.force_matrix_w
scene["contact_R"].data.force_matrix_w

Each will provide the filtered contact force for that specific body.

Summary Table

Sensor Configuration	force_matrix_w	net_forces_w
Single body, filtered	Returns matrix	Returns per-body tensor
Multi-body, any filter	None	Returns per-body tensor

Footnotes

1. https://isaac-sim.github.io/IsaacLab/main/source/overview/core-concepts/sensors/contact_sensor.html ↩ ↩² ↩³ ↩⁴