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For a cascade control system, would it have made any difference if we tune the primary controller first and then the secondary controller?? If yes, then why???(the usual order is to tune the secondary controller first then the first)
No... Inner loop need not be faster.. Study more...Yes - the secondary has to be tuned first!
The primary controller will see the response of the secondary controller and it uses this as part of its tuning. So, if the primary controller is tuned first, it will 'see' the reaction of the secondary controller. This affects the tuning parameters of the primary controller.
If the secondary controller is tuned after the primary, the secondary controller's reaction has changed, and now the feedback response that the primary controller 'sees' has changed....and the previous tuning constants may not work.
So, the secondary must always be tuned first.
One more note: another requirement of cascade control is that the secondary loop should be faster than the primary loop. A good rule of thumb is at least 3-10 times faster.
Also, the secondary controller must respond faster to adverse conditions involving the valve, such as a sticking valve, mis-sized valve or wrong valve characteristics. This will confine the problem to the secondary controller and will have a much less detrimental effect on the primary loop.
Hello, ! I have a question. If the inner loop control time is less than three times that of the out loop, can I regard the cascade control as unnecessary? thank youYes - the secondary has to be tuned first!
The primary controller will see the response of the secondary controller and it uses this as part of its tuning. So, if the primary controller is tuned first, it will 'see' the reaction of the secondary controller. This affects the tuning parameters of the primary controller.
If the secondary controller is tuned after the primary, the secondary controller's reaction has changed, and now the feedback response that the primary controller 'sees' has changed....and the previous tuning constants may not work.
So, the secondary must always be tuned first.
One more note: another requirement of cascade control is that the secondary loop should be faster than the primary loop. A good rule of thumb is at least 3-10 times faster.
Also, the secondary controller must respond faster to adverse conditions involving the valve, such as a sticking valve, mis-sized valve or wrong valve characteristics. This will confine the problem to the secondary controller and will have a much less detrimental effect on the primary loop.
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