What's the difference between two command?

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준호 2024-1-27

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评论： Sam Chak 2024-1-27

Hello,

In general, the command of nyquist plot is nyquist(sys,omega), but an example of textbook the command of nyquist is nyquist(A,B,K,0,1,w), where (A,B : matrices from state space equation, K : control gain, w : frequency range).

What's the difference between the two commands?

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Dyuman Joshi 2024-1-27

Could you please share the example where that particular syntax is used?

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Answer 1

Paul 2024-1-27

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在 MATLAB Online 中打开

That's the old, obsolete form of nyquist() that was used for state space models before the Control System Toolbox introduced lti objects like ss and tf.

f = which('nyquist','-all')
f = 2×1 cell array
    {'/MATLAB/toolbox/control/ctrlobsolete/nyquist.m'                   }
    {'/MATLAB/toolbox/shared/controllib/engine/@DynamicSystem/nyquist.m'}
dbtype(f{1},'35:58')
  % Old help
  %warning(['This calling syntax for ' mfilename ' will not be supported in the future.'])
  %NYQUIST Nyquist frequency response for continuous-time linear systems.
  %   NYQUIST(A,B,C,D,IU) produces a Nyquist plot from the inputs
  %   IU to all the outputs of the system:             
  %               .                                    -1
  %               x = Ax + Bu             G(s) = C(sI-A) B + D  
  %               y = Cx + Du      RE(w) = real(G(jw)), IM(w) = imag(G(jw))
  %
  %   The frequency range and number of points are chosen automatically.
  %
  %   NYQUIST(NUM,DEN) produces the Nyquist plot for the polynomial 
  %   transfer function G(s) = NUM(s)/DEN(s) where NUM and DEN contain
  %   the polynomial coefficients in descending powers of s. 
  %
  %   NYQUIST(A,B,C,D,IU,W) or NYQUIST(NUM,DEN,W) uses the user-supplied
  %   freq. vector W which must contain the frequencies, in radians/sec,
  %   at which the Nyquist response is to be evaluated.  When invoked 
  %   with left hand arguments,
  %       [RE,IM,W] = NYQUIST(A,B,C,D,...)
  %       [RE,IM,W] = NYQUIST(NUM,DEN,...) 
  %   returns the frequency vector W and matrices RE and IM with as many
  %       columns as outputs and length(W) rows.  No plot is drawn on the 
  %   screen.

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Dyuman Joshi 2024-1-27

Wow. I was not even born then haha.

Sam Chak 2024-1-27

在 MATLAB Online 中打开

The original Nyquist code was written in 1985.

%   J.N. Little 10-11-85
%   Revised ACWG 8-15-89, CMT 7-9-90, ACWG 2-12-91, 6-21-92, 
%               AFP 2-23-93, 10-1-94
%   Copyright 1986-2003 The MathWorks, Inc.

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