Seeking Feedback: Cascaded Control Scheme for UR5e Manipulator using Fuzzy-PID & Inverse Dynamics

Hi everyone,

I am currently designing a cascaded control system for a UR5e robot arm. I’ve attached my current block diagram, but my supervisor mentioned that the architecture is either incomplete or has some conceptual errors. I would love to get some expert eyes on this.

My current setup:

• Trajectory: Quintic Polynomial to generate $q_d$, $\dot{q}_d$, and $\ddot{q}_d$.
• Outer Loop: Fuzzy-PID Position Controller.
• Inner Loops: Velocity Controller and Current Controller.
• Feedforward: Inverse Dynamics block to calculate $\tau_{ff}$ (Feedforward torque).

I have a few specific questions regarding the logic:

1. Position Controller Output: I currently have the Fuzzy-PID outputting a signal directly to the velocity summing junction. My supervisor suggested that the signal from the position controller needs a derivative to become velocity. In a standard cascaded loop, isn't the PID output itself treated as the Velocity Reference ($v_{ref}$)?
2. Velocity Feedforward: I noticed I’m not feeding the desired velocity ($\dot{q}_d$) from the Quintic Polynomial directly into the velocity loop. Should I add a feedforward path here to improve tracking performance?
3. Torque Injection: Is the summing point for the Feedforward Torque ($\tau_{ff}$) correctly placed after the velocity controller?
4. Overall Completeness: Are there any missing elements (like gravity compensation or friction models) that are typically required for a UR5e manipulator in this specific cascaded architecture?

I’ve spent a lot of time on this and want to make sure the fundamental control theory is sound before I move to simulation. Any advice or corrections would be greatly appreciated!

Thanks in advance!

https://preview.redd.it/bjb2pfxxw5kg1.png?width=866&format=png&auto=webp&s=a260416d7cd7be17107edf9f876edf0572ef2066