Relationship between band and rise time in motor control

Thread Starter

alpheratz

Joined Nov 10, 2019
8
Hello everyone, I'm not sure about the category I post this thread in but it's something that has to do with theory and I hope you don't ignore it because of that.
My problem is that I study DC motor control and I find from my lecture the case that you see in the image.

xczsd.jpg

(it is transfer function of speed control), I have this general relation that can be applied to this system \(t_r \cdot f_B = 0.35 \). It's a type 0 system and you see the rise time and the crossover frequency indicated in the formula. It surprises me because in the past I studied that for this formula the cutoff frequency should be used. If this formula is right than it means that using a proportional controller I can reduce my rise time and it's again something new I've always thought I can control the regulation error only like that.

So which frequency should be used? Don't take the body diagram too seriously I put there some simple values only to make you get an idea
 

Thread Starter

alpheratz

Joined Nov 10, 2019
8
I'm honestly confused and I don't have an opinion here. I believe it's right to use the crossover frequency in this case but when we should use cutoff frequency then? I mean if you look in wikipedia for example (https://en.wikipedia.org/wiki/Rise_time) you will find that frequency indicated as cutoff. Also take a book of control theory and you will most likely find there that frequency as cutoff.
 
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