How can I design a feedforward control in a cascade feedback controlled system ?

Nowdays I try to design feedforward control in motor driven system. I applied cascade feedback control (P and PI control) and velocity
feedforward in velocity loop. From now, I would like to improve control performance. So I plan to modify the feedforward control in my system. I survay some articles related with feedforward control in cascade feedback control. In summary, I found three kind of feedforward schemes.
- velocity and acceleration feedforwards are put in the velocity loop.
- velocity and acceleration feedforwards are put in the current loop.
- velocity feedforward is applied to the velocity loop and acceleration feedforward is applied to the current loop, respectively.
I know intuitively that the velocity feedforward is applied to the velocity command and the acceleration feedforward is applied to the current command.
But in some articles, the acceleration feedforward term is applied to the velocity command better than to the current command. Why? Can you explain me?
I am confused with that. If you had some experience or knowledge on that, please reply on me. If possible, also explain the theoretical backgrounds.
Looking forward to kind reply.
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