I'm in the middle of designing a Cascade Loop PID controller for a quadcopter autopilot simulation, to allow it to move to a designated position by altering its thrust vector by pitching up or down.
However, I'm having some issues producing a viable controller. I've successfully controlled the inner loop (pitch rate) with a PI controller, but the outer loop (position) is proving very difficult to stabilize with a PID: please see the diagram for reference:
I've been using SISOtool and pidTuner to try and get the relevant PID controller and thus PID gain values that I can input into my programmed controller in MatLab - this worked for the inner loop (pitch rate), but it is not working for the outer loop.
My advisor told me that it should work with a PID just fine, so I don't really understand why it isn't.
Any advice would be tremendously appreciated, thanks!!