[Question] Decomposing joint forces into actuation forces and external forces

[kbkartik]

Question

I am trying to understand if I can retrieve joint forces into two components: actuation forces calculated by ImplicitDrive (given position or velocity targets) and external forces acting on a joint.

There seems to be a way to get actuation forces through get_dof_actuation_forces().

However, regardless of what stiffness and damping values I set in ImplicitActuatorCfg, the output of get_dof_actuation_forces is always 0.0.

Also, I am not sure how to retrieve the external forces acting on the joint.

[PeterL-NV]

The method get_dof_actuation_forces() from ArticulationView should reflect actuation forces applied by joint drives. If it always returns 0.0, there may be configuration or timing issues.

The common reasons this method might always return 0.0 are:

• Drive Disabled or Zero Gains: Ensure your joint's drive is enabled and that stiffness and damping parameters in ImplicitActuatorCfg are set to non-zero values. If these are zero, actuation force will not be computed.
• Target Not Set: No force will be applied unless you set a position or velocity target for the joint. If neither target is set, actuation forces remain zero.
• Physics Step Timing: Only after a simulation step (when physics is stepped forward) are forces calculated. Query actuation forces after the simulation step, not before.
• Type of Actuator: ImplicitDrive should support forces reflecting your settings; if you're using a custom or 'none' actuator, output will be zero.

To retrieve the external forces, you can try get_link_incoming_joint_force, this is the joint reaction forces and torques to external loads.