Hi Ondrej
The ability to replace joint behaviour with external torques depends on the complexity of the motion and the degrees of freedom involved.
In case of single degree of freedom, (e.g. a revolute joint) which only allows rotation around one fixed axis, it’s behaviour can be easily replicated with an external torque applied around that axis. Since the motion is constrained to a single plane, it’s easier to calculate and apply the necessary torque to achieve the same effect as the joint's inherent motion.
Therefore, when you simulated the pendulum such that it rotates only around one axis, it worked perfectly without any error between upper and lower model.
But, as gimbal joint allows rotation around three perpendicular axes, providing full 3D rotational freedom, replicating this behaviour with external torques is more complex because of nonlinear dynamics and axis coupling. The rotation around one axis can affect the other axes due to the coupling inherent in a gimbal joint, which cannot be replicated using external torque.
Therefore, for accurate simulation especially in case of gimbal joints, replicating the model with external torques is not recommended.
I hope this resolves the issue.
