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Re: Not able to add delay to transfer function

From: Sergei Steshenko
Subject: Re: Not able to add delay to transfer function
Date: Thu, 30 Jul 2020 01:14:41 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 29/07/2020 22:53, shall689 wrote:
Hello Torsten,

That seems to work.

I also tried (1+s*tau)/(1-s*tau), but the step response gave me the error:
"open_gl_renderer: data values greater than float capacity error"

(1+s*tau)/(1-s*tau) was mentioned on page 2 of this document:


Sent from:

I do not understand why you are trying to add something to your transfer function (delay in this case). IIRC, in the beginning you stated you had frequency response of your system. I asked whether the response was complex (as in complex numbers) or just magnitude - I don't remember getting a reply; I asked the question to better understand the task at hand.

If you have complex frequency response, you probably don't even need the corresponding transfer function. This is because having the response and the PID parameters you can have the resulting complex frequency response which can be converted by 'ifft' into the resulting impulse response, which also means it can be converted into step response (probably the step response of most interest).

Anyway, having complex frequency response you can try to obtain rational polynomial (i.e. either B(s)/A(s) or B(z)/A(z)) using 'invfreqs' or 'invfreqz' functions respectively. I believe what I'm writing is methodologically correct, though from practice I know that for "funky" frequency responses obtaining the resulting rational polynomial is very tricky, i.e. small polynomial orders are not sufficient, and high polynomial orders give convergence problems - do not fit the input complex frequency response well.

If you only have magnitude response, you still need to obtain complex frequency response - otherwise you can't build the corrected systems. Is this the case and are you trying to add delay to the magnitude response ? If that's the case, I do not think your methodology is correct. Not that you don't need delay - quite the opposite. But for minimum phase systems magnitude response and phase response are tightly related - see , . So, you really need to know complete true phase response of your system.

Anyway, I'm just a curious observer ...


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