The advantage is in reduced computational load on the calculating machine. Not so much for a small number of DoF, but for multi-jointed arms or humanoid robots, or hexapods and similar. If you use inverse kinematics, the computational load grows exponentially with the number of DoF. For cascaded loops, it should be smaller, since computation is local to the loops.
Second level loops need to be slower than first level loops, so that they don't cause conflicting setpoints. It works pretty good in simulations, so I'm thinking the vibrations are probably caused by mechanical deficiencies. Also, they come with increasing the gain of second level loops. Most vibrations die out after reducing the gain.
As for the local minima and being stuck, I haven encountered that problem with 4 or 5 degrees of freedom, or in the 14 dof arm.
Second level loops need to be slower than first level loops, so that they don't cause conflicting setpoints. It works pretty good in simulations, so I'm thinking the vibrations are probably caused by mechanical deficiencies. Also, they come with increasing the gain of second level loops. Most vibrations die out after reducing the gain.
As for the local minima and being stuck, I haven encountered that problem with 4 or 5 degrees of freedom, or in the 14 dof arm.