Re: plotting transfer function in octave 5.2: How to fix error: set: "dataaspectratio' must be finite

[Torsten Lilge]

On Fri, 2020-08-07 at 22:06 +0300, Sergei Steshenko via Help-octave
wrote:
> 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.
> > 
> > Stephen
> > 
> > <
> > https://octave.1599824.n4.nabble.com/file/t372348/TwoLoopControlDiagram.jpg
> > >
> > 
> > <
> > https://octave.1599824.n4.nabble.com/file/t372348/TwoLoopControlDiagramWithZOH.jpg
> > >
> > 
> > 
> > 
> > --
> > Sent from: 
> > https://octave.1599824.n4.nabble.com/Octave-General-f1599825.html
> > 
> > 
> "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.
> 
> --Sergei.
> 

One of the most common misunderstandings when doing discrete-time
control is, that ZOH aren't really "added" to the control. In general, a
D/A converter *is* a ZOH and has to be modeled as such.

You have a cascaded control with the voltage control in the outer loop,
right? Then G2 is a discrete-time controller and its output is the
setpoint for the inner, namely the current controller (also discrete-
time). This, the real, continous-time system is H1, H2, F1, and F2. Is
this correct?

When considering the complete control loop in discrete-time, you have
ZOH1 as a real D/A converter. You would have to discretize the systems
ZOH1-H1-F1 and ZOH1-H1-H2-F2 and you can represent the complete comtrol
loop in discrete-time.

Torsten