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Eric
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Trying to stabilise a SISO feedback loop with a Cascade PID controller - but it's not working!

Asked by Eric
on 29 Mar 2016
Latest activity Answered by Arkadiy Turevskiy on 26 Apr 2016
Hi there,
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!!

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1 Answer

Answer by Arkadiy Turevskiy on 26 Apr 2016

First off, looks like you have a typo in your Cq. As Pq already has integrating action, you don't really need an integrator in Cq. Cq=48 stabilizes the inner loop nicely. If you use that Cq, you will get outer loop plant transfer function very close to what you show. Now, a PID controller won't be able to control that, as you have 4 poles at the origin. Are you sure you have your Px right? It seems strange you have to integrate 4 times to get a position.

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