What's the difference between two command?

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?

[Dyuman Joshi]

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

Answer 1

[Paul]

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')

35    % Old help
36    %warning(['This calling syntax for ' mfilename ' will not be supported in the future.'])
37    %NYQUIST Nyquist frequency response for continuous-time linear systems.
38    %   NYQUIST(A,B,C,D,IU) produces a Nyquist plot from the inputs
39    %   IU to all the outputs of the system:             
40    %               .                                    -1
41    %               x = Ax + Bu             G(s) = C(sI-A) B + D  
42    %               y = Cx + Du      RE(w) = real(G(jw)), IM(w) = imag(G(jw))
43    %
44    %   The frequency range and number of points are chosen automatically.
45    %
46    %   NYQUIST(NUM,DEN) produces the Nyquist plot for the polynomial 
47    %   transfer function G(s) = NUM(s)/DEN(s) where NUM and DEN contain
48    %   the polynomial coefficients in descending powers of s. 
49    %
50    %   NYQUIST(A,B,C,D,IU,W) or NYQUIST(NUM,DEN,W) uses the user-supplied
51    %   freq. vector W which must contain the frequencies, in radians/sec,
52    %   at which the Nyquist response is to be evaluated.  When invoked 
53    %   with left hand arguments,
54    %       [RE,IM,W] = NYQUIST(A,B,C,D,...)
55    %       [RE,IM,W] = NYQUIST(NUM,DEN,...) 
56    %   returns the frequency vector W and matrices RE and IM with as many
57    %       columns as outputs and length(W) rows.  No plot is drawn on the 
58    %   screen.

[Dyuman Joshi]

I wonder how old that syntax is. The documentation pages say that ss and tf were introduced before R2006a.

So, I guess older than that? Or maybe it was discontinued afterwards?

[Paul]

That syntax is as old as the Control System Toolbox itself, so nearly as old as the first commerical release of Matlab. If you're asking when ss and tf were implemented, that was probably around V5 would be my guess, so mid-90's.

[Dyuman Joshi]

Wow. I was not even born then haha.

[Sam Chak]

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.