Support XELA Fingertips in ergoCub URDF and MuJoCo Models

[carlo-alessi]

Background

I am currently working on tactile manipulation and sim-to-real transfer with ergoCub using XELA tactile fingertips. Some time ago, I converted the ergoCub URDF into a MuJoCo model using the mujoco-urdf-loader repository (https://github.com/gbionics/mujoco-urdf-loader ).

The current URDF, and consequently the derived MuJoCo model, includes only mockup fingertips. To perform tactile manipulation experiments in simulation, I manually modified the generated MuJoCo XML to replace the mockup fingertips with XELA tactile fingertips. In the MuJoCo model, I also had to tune other parameters to make it more realistic. The video shows my current setup in MuJoCo.

2026-06-05_15-00_policy_inference_ergocub_xela-step-0.mp4

The limitation

While this approach works, it creates a maintenance issue: whenever the URDF is updated upstream, I cannot easily benefit from those updates because my modifications are applied directly to the generated MuJoCo model.

Potential request

It would be helpful to provide official model variants that support XELA tactile fingertips. For example:

• Provide URDF variants that integrate XELA tactile fingertips out of the box. There are two types of XELA fingertips: one with 5 taxels, which I am currently using, and one with 7 taxels.
• Generate and expose the corresponding MuJoCo models within the ergocub-software model assets (so that any conversion issue is detected early).

I am relatively new to this model-generation workflow, so I may be missing an existing recommended approach. If there is already a better way to handle this, I would be happy to follow it.

[martinaxgloria]

Hi @carlo-alessi, thanks for the detailed issue!

To clarify our current approach: we don't include any fingertips in the URDF because we can't simulate them in gz-sim using gz-sim-yarp-plugins. If I understand your question, would you have a URDF variant that includes the fingertip geometry (visual + collision meshes) along with only the reference frames for the fingertip? Is this approach aligned with what you need, or would something different?

[carlo-alessi]

Hi @martinaxgloria, Yes!

[martinaxgloria]

@carlo-alessi do you have the CAD of those fingertips with the proper reference frames or do you only have the meshes and you manually compute the pose with respect to the parent link?

[carlo-alessi]

@martinaxgloria: Here are the CAD files: https://github.com/hsp-iit/xela-tactile-sensors-ergocub/tree/main/cad
I think you should have access to the repository.

[martinaxgloria]

Unfortunately, we don't have access @carlo-alessi

[salvi-mattia]

As agreed with @martinaxgloria , I am providing the transformations from the CSYS of the distal phalanx to each of the fingertip taxels.
Note: in the CAD model we received, the taxels are mounted in a non-repeatable way. Therefore, while the Z axis always points outward from the fingertip surface, the orientation of the other two axes (X and Y) should be considered arbitrary.
There are two different fingertip part numbers:

One for thumb and little finger
One for the index, middle, and ring fingers

Below are the transformations for the 5-taxel fingertip:

index, middle, ring

1	0	0	2.30001
0	0.344915	-0.93863	0.059874
0	0.938634	0.344915	-32.9219

1	0	0	-2.3
0	0.344915	-0.93863	0.059874
0	0.938634	0.344915	-32.9219

1	0	0	2.30001
0	-0.06668	-0.99777	-0.58007
0	0.997774	-0.06668	-37.3756

.

0	-1	0	-2.3
-0.06668	0	-0.99777	-0.58007
0.997774	0	-0.06668	-37.3756

0	-1	0	0
-0.8072	0	-0.59028	1.42986
0.590276	0	-0.8072	-41.0281

[salvi-mattia]

thumb, little

1	7.42E-05	2.73E-05	2.29899
-1.2E-08	0.344985	-0.93861	0.05891
-7.9E-05	0.938608	0.344985	-27.9222

1	7.42E-05	2.73E-05	-2.30102
-1.2E-08	0.344985	-0.93861	0.05891
-7.9E-05	0.938608	0.344985	-27.9218

1	7.89E-05	-5.3E-06	2.29864
-1.2E-08	-0.06661	-0.99778	-0.58136
-7.9E-05	0.997779	-0.06661	-32.3759

7.89E-05	-1	-5.3E-06	-2.30137
-0.06661	1.15E-08	-0.99778	-0.58136
0.997779	7.91E-05	-0.06661	-32.3755

4.67E-05	-1	-6.4E-05	-0.00166
-0.80716	1.15E-08	-0.59034	1.42829
0.590337	7.91E-05	-0.80716	-36.0284

[martinaxgloria]

Hi @carlo-alessi, we generated the first urdf with XELA fingertips with 5 taxels. In the picture below, you can see the naming convention of the links associated to each fingertip:

It is of type r/l_hand_<finger>_skin_xela5_<n>, so, for example, for the right thumb, it will be r_hand_thumb_skin_xela5_<n>, with 0 ≤ n ≤ 4

Is it feasible for you to test the following generated urdf model and give us feedback, please?

• 📦 ergoCubGazeboSN001_xela5.zip