Applying external forces to specific rigid bodies each simulation step

[ucacaxm]

Hi,

Is there a supported/public API in ProtoMotions to apply external forces to specific rigid bodies at each simulation step?

We found apply_rigid_body_force_tensors used in test_isaacgym_friction_mode.py via _gym and _sim directly, but those are private attributes.

Questions:

1. Is there a public wrapper for applying per-body forces (similar to _apply_simulator_torques but for rigid body forces)?
2. If not, is accessing simulator._gym / simulator._sim directly safe, or will it conflict with ProtoMotions' tensor refresh cycle?
3. Is there a recommended hook point to inject forces without fighting the simulation loop?

Thanks

[tesslerc]

Thanks for the question!

1. Public wrapper for per-body forces: No, there isn't one currently. The base Simulator interface (protomotions/simulator/base_simulator/simulator.py) only exposes _apply_simulator_torques() for joint-level torques (used internally by PD control). There's no apply_rigid_body_forces() or equivalent.
2. Accessing _gym / _sim directly: It works, but it's fragile and IsaacGym-specific — it won't port to Newton, Genesis, or MuJoCo backends. You'd also need to make sure the force application happens at the right point in the step cycle (after _apply_control(), before the physics solve) to avoid being overwritten by tensor refreshes.
3. Recommended hook point: The cleanest injection point would be inside the simulator's step() method, after _apply_control() and before the physics substep. There's no built-in callback/hook for this.
4. We'd recommend a common function accessible externally. Accepts both list of body names and list of body indices. Converts to simulator-specific indices (from common). And then each simulator implements it using the simulator-specific API.

You can open a feature request and we'll try to reach this asap, or if you implement yourself we'd be very happy to accept a PR!