No discussion of is complete without addressing singularities. While the "Wrist Singularity" (J4 and J6 alignment) is the most famous, the "Elbow Singularity" or "Alignment Singularity" involves J2 and J3.
The becomes critical because of gravity compensation . If J2 moves up by 10 degrees, the center of gravity of the entire upper arm (including J3, J4, J5, and the tool) shifts. The J3 motor must instantly adjust its holding torque to prevent the wrist from drooping. In FANUC controllers, this is managed via a dynamic model—not independent PID loops. fanuc j2 j3 interaction