multiple-loop speed help - can we have two loops both fast?

[alex.ang1983]

is multiple loops or one loop faster

Hello everyone,

I have a question that come up to me everytime I read some papers about multiple loop devices.
Everytime for example a system has 2 loops the writer always starts to take care about which of them is slower and which is faster like if they can't be both faster.
I don't manage to understand what probelm could be if I have two loops both fast. Maybe there is a problem for the stability but I don't manage to figure out what problem could be.

Please help me on understanding this concept.
Many thanks in advance.
Best regards

 

[LvW]

Re: multiple-loop speed help

I completely agree with you since in many books and papers the relevance of multiple loops is not explained in detail.
I would answer your question as follows:

1.) If there are two loops there must be a reason for that. And in most (if not all) cases the inner (local) loop is the fastest loop with the aim to make the whole system respond faster and more reliable. If it is as slow as the outer loop it makes not much sense. Therefore, I claim you will not find any system with two loops which react with the same time constant. (May be there are counter examples, but at the moment I cannot find any).

2.) Then the question arises, which loop determines stability properties at most. Which means: Is one of both loops a dominant loop ? This is important if one is going to measure/simulate the stabilty margin. The problem is, when there are two loops, you will have two different loop gains and two different stability margins. Therefore the question: Which one is the dominant loop ?
My (preliminary) answer is: The slowest loop (which is the outer loop in most cases). Up to now I did not find any final and general answer to this question.
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Comments are welcome.
Regards