Hi
I have a two-loop control system (cascade control system). The outer loop aims at generating the inner loop reference input so that the global system output tracks its reference input. Each loop has a controller which needs to be designed. First, I used two PID controllers and I got a good response for ramp and sine for both loops, but when applied a step as the desired trajectory, the inner loop did not result in a good response.
What to do in designing controllers for two loop controllers?
Dear Armin,
The easiest way to design a two-loop control system is to use the PI as a controller, and there you have many examples in the literature. However if you want to complicate your control structure and get more performance there is also a know-how for this. I propose you to see the links and the references for the one and the other method.
-Designing Cascade Control System with PI Controllers - MATLAB ...
https://www.mathworks.com › ... › PID Controller Tuning
-Double-loop PI controller design of the... - ResearchGate
https://www.researchgate.net/.../321148061_Double-loop_PI_contr...
-Two Feedback Loops are Better Than One | Machine Design
www.machinedesign.com/.../two-feedback-loops-are-better-one
-Two-loop Design for Dual-rate Cascade System - ScienceDirect
https://www.sciencedirect.com/science/article/.../S24058963183045...
-Decoupling Double-Loop Control System... (PDF Download Available)
https://www.researchgate.net/.../224326920_Decoupling_Double-L...
-Robust design of cascade control - IEEE Control Systems Society
ieeecss.org/CSM/library/1990/jan1990/w03-CascadeControl.pdf
Best regards
Do you really need two loops? Having an inner and an outer loop may make it easier to visualize what your controller is doing but I think it complicates the design. I think you will be able to achieve the same thing with just one loop. I am fairly sure that you should be able to reduce a two-loop structure to an equivalent one-loop structure. This will then be easier to design and analyze using standard techniques.
If you have a model of your process you could do open-loop loopshaping to design the gains of your controller. You can do that easiyl easily for example in Matlab with sisotools. Start with the inner loop, when designed adequately, close the loop and do the same for the new system on the outer loop.
If you do not have a model, you need to look into heuristic methods to choose the gains of your PID controller. Again start with the inner loop and make your way forward.
Also note that often you do not need all the elements of your PID controller. This depends again on the dynamics of your plant.
Zeashan H. Khan
Thank You so much sir. That was so great. It really helped me.
Do you have an idea how to implement this method for an Inertially Stabilized System(Gimbal)?
Next to all great remarks above perhaps a few basic tips:
- when possible try to use feedforward for more performance instead of increasing the feedback gains
- try to filter the steering based on the bandwidth of the resulting process to prevent overshoots and actuator saturation.
- check stability and robustness (Nyquist plot of the open-loop responses)
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