How to change the joint limits?

[mariogpascual]

Hi, I wanted to ask how to change the current joint limits of Upkie and what is the rationale behind the current values. I think I only access these values through box_[position, velocity, torque]_limits functions, so would changing the URDF in upkie_description be enough to perform this modification?

Thanks!

[stephane-caron]

Changing the URDF is the expected way update the limits. We need to take care to modify the limits both on the C++ and Python side:

• The C++ side, used by the Bullet simulation spine, loads the URDF from the @upkie_description dependency. This dependency is defined in tools/workspace/upkie_description/repository.bzl. To update it, fork the upkie_description repository, commit your modifications, then update the remote and commit fields of this file accordingly.
• The Python side, used by the UpkieServosEnv environment, actually loads the URDF description from PyPI (at this line of the BUILD file). This is not convenient to carry around some local updates.

The current values were selected based on two points:

• For knee position limits: wheels touching the hips.
• For hip position limits: no good reason. I will update them to $[-0.5, 0.5]$ revolutions 😉 (Cables are the main limiters for hip positions, as they get fully stretched at $0.5$ rev.)

@mariogpascual Would that hip-limit update be sufficient to address your use case?

[stephane-caron]

Opening an issue about this at upkie/upkie_description#7

[stephane-caron]

Joint limits are now detailed in the upkie_description wiki: Joint limits.