Non-zero effort reported for frictionless joint

[mindThomas]

Description

Why do I get a non-zero effort reported by the Isaac Read Effort Node for a frictionless revolute joint (Joint Friction set to 0) and where both rigid bodies have Angular Damping set to 0?
The torque seems proportional to the angular velocity and with the same sign as the angular velocity.

USD file can be found here: torque_test.zip

Isaac Sim version

5.1.0

Operating System (OS)

Ubuntu 24.04

GPU Name

RTX 5090

GPU Driver and CUDA versions

Driver Version: 580.105.08, CUDA Version: 13.0

Logs

No response

Additional information

No response

[PeterL-NV]

Hi, The non-zero effort you're seeing from the Isaac Read Effort Node on a frictionless revolute joint is expected behavior - it's reporting the constraint forces that PhysX computes to maintain the joint, not friction forces.

The effort sensor (which reads get_measured_joint_efforts()) returns the generalized forces computed by the physics solver to satisfy joint constraints. These include:

• Joint constraint forces: Forces maintaining the joint connection between bodies
• Drive forces: If you have joint drives (position/velocity targets) enabled
• External forces: Gravity, collisions, or applied forces acting through the joint
• Solver correction forces: PhysX solver iterations to resolve penetrations/instabilities

Even with zero friction and zero angular damping, the joint still experiences forces from:

• Gravity acting on the connected bodies
• Momentum transfer through the kinematic chain
• Solver stabilization

In summary, a frictionless joint under gravity or motion will still report non-zero efforts because PhysX must apply forces to maintain the joint connection and resolve the simulation.

[mindThomas]

So how would I go about getting just the effort that a particular drive is applying to a joint?
In this example the joint is freely spinning. So imagine a joint with a drive with 0 stiffness and 0 dampening. In this case I am looking for an applied torque reading of 0 even though the pendulum is swinging.