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Re: plotting transfer function in octave 5.2: How to fix error: set: "da

From: Torsten Lilge
Subject: Re: plotting transfer function in octave 5.2: How to fix error: set: "dataaspectratio' must be finite
Date: Tue, 11 Aug 2020 19:33:11 +0200

On Sun, 2020-08-09 at 15:33 -0500, shall689 wrote:
> Hello Torsten,
> I was going analyze everything the continuous domain and then convert
> to the
> discrete domain.
> Analysis would consist of doing the following:
> 1. first, find P and I gains for G1(s) that gives a good step response
> and
> also makes the inner current loop stable.
> 2. Reduce the inner and outer loop to one block.

Do mean reducing innerloop (G1, H1, F1) to one block?

> 3. Next, find P and I gains for G2(s) that gives a good step response
> and
> also makes system stable.
> 4. Convert everything to discrete time domain and compare continuous
> to
> discrete response.  Before converting the continuous time to the
> discrete
> time I would remove the continuous zero order hold from the system and
> then
> use the ZOH method to convert the final equation to discrete
> domain.  Is
> that the correct way to do it?  Or should I just convert G1(s) and
> G2(s)
> using the ZOH method and convert the others using the tustin method?

G1 and G2 actually are digital controllers, right? Therefore, they
already are discrete-time. If there is a ZOH after G1 (the DAC at the
boundary from discrete- to continuous-time), the continuous-time system
(H1, H2, F1, F2) has to be discretized using the ZOH-method.

If you design G1 and G2 in the Laplace-domain, I think it would not be
correct to use the ZOH-method for getting the algorithm you have to use
for the controllers, since the input of the controllers is not constant
during two samples.

> The whole process might take several iterations.
> I have been using,
> and several other youtube
> video
> and websites to gain an understanding of how to analyze the system.
> The actual diagram is a little bit more complicated than what is shown
> in
> diagram I posted.  There are actually two feedbacks in the inner
> current
> loop: an inductor current feedback (positive feedback) and an output
> current
> feedback (negative feedback).  Adding these two together you will get
> capacitor current (Icap = Iinductor - Ioutput).  Also, the voltage
> reference
> is fed forward and summed right after ZOH2(s) or G2(s) depending on
> which
> diagram is being used.

Why is there a second ZOH? Do you have two control inputs into the
continuous-time system?


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