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

From: Sergei Steshenko
Subject: Re: plotting transfer function in octave 5.2: How to fix error: set: "dataaspectratio' must be finite
Date: Fri, 7 Aug 2020 22:06:10 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.1.0

On 07/08/2020 18:31, shall689 wrote:
Hello Torsten,

If you consider the complete control loop in discrete time (the system
has to be discretized), then you do not need any transfer function for
your hold device. Instead, it is considered in the calculation of the
discrete time system representation. If, e.g., the used D/A has a zero-
order hold, than the discretization method makes use of the fact that
the input u is constant during a sampling interval.
Yes, if I use the ZOH method when discretizing, the ZOH will be included.

What if there are two feedbacks, i.e a voltage and a current feedback (see
diagram below)?  Should the ZOH  be placed in both loops as shown in the
second diagram?  My system is sampling all feedbacks at the same time, so
there is essentially only one delay; however, I think two zero order holds
would add two delays.




Sent from:

"I think two zero order holds would add two delays" - before deciding on adding delays think about the following. One delay stage adds 2 * pi * delay / frequency phase shift, where 'frequency' is the frequency at which you want to know phase shift.

In a control system magnitude and phase plots the most important area is the area in which magnitude response is about 0db - this is where one looks at phase margin. Let's call this frequency frequency_0db. So, if frequency_0db is much less than sample rate, then one sample delay introduces negligible phase shift.


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