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Tips#

A short collection of practical tips for getting stable, realistic behavior out of externally-driven Newton actuators.

Add armature for stability and realism#

Externally-driven joints can excite high-frequency dynamics that the implicit USD drive normally damps for you. The simplest fix — and one that also makes the simulation more physically realistic — is to add a small armature (virtual rotor inertia) to every actuated DOF via set_dof_armatures().

For a typical DC motor driving a joint through a gearbox, the effective armature seen at the joint is:

\[I_\text{armature} \;=\; I_\text{rotor} \cdot N^2\]

where \(I_\text{rotor}\) is the motor’s rotor inertia and \(N\) is the gear ratio. Real actuators with very high gear ratios contribute significant armature at the joint — often dominating all other inertias of the robot — so a non-zero armature is closer to physical reality regardless of stability concerns.

Increase the physics rate for high-gain actuators#

If your controller gains are very high (stiff PD, aggressive PID, neural policies that produce large efforts), the physics integrator may be coarse at the default 60 Hz timestep. Raising the physics rate will make the integration for accurate. Real actuators often run in the range of 1-10 kHz to achieve very low tracking errors while maintaining stability. Such high rates are not are not currently recommended in Isaac Sim, but increasing the physics rate can still significantly increase accuracy when very low tracking error is desired.

from isaacsim.core.simulation_manager import SimulationManager

SimulationManager.set_physics_dt(1.0 / 120.0)

The cost is proportionally more compute per simulated second.

Symptom: high-frequency vibration#

If your robot exhibits visible high-frequency oscillation — typically faster than the natural frequency of the joints — it almost always means one of the two tips above is being violated:

  • The actuated joints are missing armature (or have far less than the real hardware would carry through its gearbox).

  • The physics rate is too low for the gains you’re using.

The fix is one (or both) of:

  • Increase the armature on the affected DOFs (start by ~10× the value you have, or compute it from the rotor-inertia / gear-ratio formula above).

  • Increase the physics rate (e.g. from 60 Hz to 120 or 200 Hz).