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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: Mon, 03 Aug 2020 19:34:25 +0200

On Mon, 2020-08-03 at 01:42 -0500, shall689 wrote:
> Thanks Doug and Torsten.
> I have been working on several things and I am not even sure how I got
> a 8th
> order polynomial equation.  Currently, I am only getting a 3rd order
> polynomial transfer function and I am able to plot its step
> response.  
> Do either of you know of a good Digital Controls book?  I have a
> Inverter
> Control diagram with both an inner current and outer voltage control
> loop
> and I am stuck on whether or not to add zero order (ZOH) hold to
> both. 
> Currently I am only adding the ZOH transfer function right after the
> inner
> current loop PI.  

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. 

If you consider your control loop in continuous time (the controller has
to be represented in continuous time), the challenge is to compute the
laplace transfer function of your (discrete-time) controllers and of the
hold device you are using in your D/A converter. For the zero-order hold
this is

Gc(s) = Ghold(s) * G(z=exp(s*T)) 
      = (1 - exp(-s*T))/(s*T) * G(z=exp(s*T))

where G(z) is the z transfer function of the controller an T is the
sampling time.


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