Below is the function `lnyquist.m`. This function is a modified version of MATLAB's `nyquist` command, and has the same attributes as the original, with a few improvements. The function `lnyquist.m` plots

`(log2(1+abs(G(jw))),angle(G(jw))`

in polar coordinates by taking the log of the magnitude, the magnitude scale is compressed and the overall shape of the Nyquist plot is easier to see on the screen. We use log base 2 and add one to the magnitude so as to preserve the key attributes of the -1 point for the Nyquist plot.

The `lnyquist` function also takes poles on the imaginary axis into account when creating the Nyquist plot, and plots around them.

Copy the following text into a file `lnyquist.m`. Put the file in the same directory as the MATLAB software, or in a directory which is contained in MATLAB's search path.

function [reout,imt,w] = lnyquist(a,b,c,d,iu,w) %LNYQUIST Nyquist frequency response for continuous-time linear systems. % % This Version of the NYQUIST Command takes into account poles at the % jw-axis and loops around them when creating the frequency vector in order % to produce the appropriate Nyquist Diagram (The NYQUIST command does % not do this and therefore produces an incorrect plot when we have poles in the % jw axis). % % NOTE: This version of LNYQUIST does not account for pole-zero % cancellation. Therefore, the user must simplify the transfer function before using % this command. % % LNYQUIST(A,B,C,D,IU) produces a Nyquist plot from the single input % 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. % % LNYQUIST(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. % % LNYQUIST(A,B,C,D,IU,W) or LNYQUIST(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] = LNYQUIST(A,B,C,D,...) % [RE,IM,W] = LNYQUIST(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. % See also: LOGSPACE,MARGIN,BODE, and NICHOLS. % % 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 % WCM 8-30-97 % ******************************************************************** Modifications made to the nyquist - takes into account poles on jw axis. then goes around these to make up frequency vector % % if nargin==0, eval('exresp(''nyquist'')'), return, end --- Determine which syntax is being used --- nargin1 = nargin; nargout1 = nargout; if (nargin1==1),% System form without frequency vector [num,den]=tfdata(a,'v'); z = roots(num); p = roots(den); zp = [z;p]; wpos = zp(find(abs(zp)>0)); if(min(abs(p)) == 0) wstart = max(eps, 0.03*min([1;wpos])); else wstart = max(eps, 0.03*min(abs(zp))); end wstop = max([1000;30*wpos]); w = logspace(log10(wstart),log10(wstop),max(51,10*max(size(zp))+1)); %w = freqint2(num,den,30); [ny,nn] = size(num); nu = 1; %error('Wrong number of input arguments.'); elseif (nargin1==2), if(isa(a,'ss')|isa(a,'tf')|isa(a,'zpk')) % System with frequency vector [num,den]=tfdata(a,'v'); w = b; else% Transfer function form without frequency vector num = a; den = b; z = roots(num); p = roots(den); zp = [z;p]; wpos = zp(find(abs(zp)>0)); if(min(abs(p)) == 0) wstart = max(eps, 0.03*min([1;wpos])); else wstart = max(eps, 0.03*min(abs(zp))); end wstop = max([1000;30*wpos]); w = logspace(log10(wstart),log10(wstop),max(51,10*max(size(zp))+1)); %w = freqint2(num,den,30); end [ny,nn] = size(num); nu = 1; elseif (nargin1==3), % Transfer function form with frequency vector num = a; den = b; w = c; [ny,nn] = size(num); nu = 1; elseif (nargin1==4), % State space system, w/o iu or frequency vector error(abcdchk(a,b,c,d)); [num,den] = ss2tf(a,b,c,d); [z,p,k]= ss2zp(a,b,c,d); [num,den] = zp2tf(z,p,k); zp = [z;p]; wpos = zp(find(abs(zp)>0)); if(min(abs(p)) == 0) wstart = max(eps, 0.03*min([1;wpos])); else wstart = max(eps, 0.03*min(abs(zp))); end wstop = max([1000;30*wpos]); w = logspace(log10(wstart),log10(wstop),max(51,10*max(size(zp))+1)); %w = freqint2(a,b,c,d,30); nargin1 = 2;%[iu,nargin,re,im]=mulresp('nyquist',a,b,c,d,w,nargout1,0); %if ~iu, if nargout, reout = re; end, return, end [ny,nu] = size(d); elseif (nargin1==5), % State space system, with iu but w/o freq. vector error(abcdchk(a,b,c,d)); z = tzero(a,b,c,d); p = eig(a); zp = [z;p]; wpos = zp(find(abs(zp)>0)); if(min(abs(p)) == 0) wstart = max(eps, 0.03*min([1;wpos])); else wstart = max(eps, 0.03*min(abs(zp))); end wstop = max([1000;30*wpos]); w = logspace(log10(wstart),log10(wstop),max(51,10*max(size(zp))+1)); %w = freqint2(a,b,c,d,30); [ny,nu] = size(d); else error(abcdchk(a,b,c,d)); [ny,nu] = size(d); end if nu*ny==0, im=[]; w=[]; if nargout~=0, reout=[]; end, return, end ********************************************************************* depart from the regular nyquist program here now we have a frequency vector, a numerator and denominator now we create code to go around all poles and zeroes here. if (nargin1==5) | (nargin1 ==4) | (nargin1 == 6) [num,den]=ss2tf(a,b,c,d); end tol = 1e-6; %defined tolerance for finding imaginary poles z = roots(num); p = roots(den); ***** If all of the poles are at the origin, just move them a tad to the left*** if norm(p) == 0 if(isempty(z)) tad = 1e-1; else tad = min([1e-1; 0.1*abs(z)]); end length_p = length(p); p = -tad*ones(length_p,1); den = den(1,1)*[1 tad]; for ii = 2:length_p den = conv(den,[1 tad]); end zp = [z;p]; wpos = zp(find(abs(zp)>0)); if(min(abs(p)) == 0) wstart = max(eps, 0.03*min([1;wpos])); else wstart = max(eps, 0.03*min(abs(zp))); end wstop = max([1000;30*wpos]); w = logspace(log10(wstart),log10(wstop),max(51,10*max(size(zp))+1)); %w = freqint2(num,den,30); end zp = [z;p]; % combine the zeros and poles of the system nzp = length(zp); % number of zeros and poles ones_zp=ones(nzp,1); %Ipo = find((abs(real(p))<1e-6) & (imag(p)>=0)) %index poles with zero real part + non-neg imag part Ipo = find((abs(real(p)) < tol) & (imag(p)>=0)); %index poles with zero real part + non-neg imag part if ~isempty(Ipo) % **** only if we have such poles do we do the following:************************* po = p(Ipo); % poles with 0 real part and non-negative imag part check for distinct poles [y,ipo] = sort(imag(po)); % sort imaginary parts po = po(ipo); dpo = diff(po); idpo = find(abs(dpo)>tol); idpo = [1;idpo+1]; % indexes of the distinct poles po = po(idpo); % only distinct poles are used nIpo = length(idpo); % # of such poles originflag = find(imag(po)==0); % locate origin pole s = []; % s is our frequency response vector %w = sqrt(-1)*w; % create a jwo vector to evaluate all frequencies with for ii=1:nIpo % for all Ipo poles [nrows,ncolumns]=size(w); if nrows == 1 w = w.'; % if w is a row, make it a column end; if nIpo == 1 r(ii) = .1; else % check distances to other poles and zeroes pdiff = zp-po(ii)*ones_zp; % find the differences between % poles you are checking and other poles and zeros ipdiff = find(abs(pdiff)>tol); % ipdiff is all nonzero differences r(ii)=0.2*min(abs(pdiff(ipdiff))); % take half this difference r(ii)=min(r(ii),0.1); % take the minimum of this diff.and .1 end; t = linspace(-pi/2,pi/2,25); if (ii == originflag) t = linspace(0,pi/2,25); end; % gives us a vector of points around each Ipo s1 = po(ii)+r(ii)*(cos(t)+sqrt(-1)*sin(t)); % detour here s1 = s1.'; % make sure it is a column % Now here I reconstitute s - complex frequency - and % evaluate again. bottomvalue = po(ii)-sqrt(-1)*r(ii); % take magnitude of imag part if (ii == originflag) % if this is an origin point bottomvalue = 0; end; topvalue = po(ii)+sqrt(-1)*r(ii); % the top value where detour stops nbegin = find(imag(w) < imag(bottomvalue)); % nnbegin = length(nbegin); % find all the points less than encirclement if (nnbegin == 0)& (ii ~= originflag) % around jw root sbegin = 0 else sbegin = w(nbegin); end; nend = find(imag(w) > imag(topvalue)); % find all points greater than nnend = length(nend); % encirclement around jw root if (nnend == 0) send = 0 else send = w(nend); end w = [sbegin; s1; send]; % reconstituted half of jw axis end else w = sqrt(-1)*w; end %end % this ends the loop for imaginary axis poles ************************************************************* back to the regular nyquist program here Compute frequency response if (nargin1==1)|(nargin1==2)|(nargin1==3) gt = freqresp(num,den,w); else gt = freqresp(a,b,c,d,iu,w); end *********************************************************** nw = length(gt); mag = abs(gt); % scaling factor added ang = angle(gt); mag = log2(mag+1); % scale by log2(mag) throughout for n = 1:nw h(n,1) = mag(n,1)*(cos(ang(n,1))+sqrt(-1)*sin(ang(n,1))); end; % recalculate G(jw) with scaling factor gt = h; *********************************************************** ret=real(gt); imt=imag(gt); If no left hand arguments then plot graph. if nargout==0, status = ishold; plot(ret,imt,'r-',ret,-imt,'g--') plot(real(w),imag(w)) modifications added here %******************************************* % set(gca, 'YLimMode', 'auto') limits = axis; % Set axis hold on because next plot command may rescale set(gca, 'YLimMode', 'auto') set(gca, 'XLimMode', 'manual') hold on % Make arrows for k=1:size(gt,2), g = gt(:,k); re = ret(:,k); im = imt(:,k); sx = limits(2) - limits(1); [sy,sample]=max(abs(2*im)); arrow=[-1;0;-1] + 0.75*sqrt(-1)*[1;0;-1]; sample=sample+(sample==1); reim=diag(g(sample,:)); d=diag(g(sample+1,:)-g(sample-1,:)); % Rotate arrow taking into account scaling factors sx and sy d = real(d)*sy + sqrt(-1)*imag(d)*sx; rot=d./abs(d); % Use this when arrow is not horizontal arrow = ones(3,1)*rot'.*arrow; scalex = (max(real(arrow)) - min(real(arrow)))*sx/50; scaley = (max(imag(arrow)) - min(imag(arrow)))*sy/50; arrow = real(arrow)*scalex + sqrt(-1)*imag(arrow)*scaley; xy =ones(3,1)*reim' + arrow; xy2=ones(3,1)*reim' - arrow; [m,n]=size(g); hold on plot(real(xy),-imag(xy),'r-',real(xy2),imag(xy2),'g-') end xlabel('Real Axis'), ylabel('Imag Axis') limits = axis; % Make cross at s = -1 + j0, i.e the -1 point if limits(2) >= -1.5 & limits(1) <= -0.5 % Only plot if -1 point is not far out. line1 = (limits(2)-limits(1))/50; line2 = (limits(4)-limits(3))/50; plot([-1+line1, -1-line1], [0,0], 'w-', [-1, -1], [line2, -line2], 'w-') end % Axis plot([limits(1:2);0,0]',[0,0;limits(3:4)]','w:'); plot(-1,0,'+k'); if ~status, hold off, end % Return hold to previous status return % Suppress output end %reout = ret; % plot(real(p),imag(p),'x',real(z),imag(z),'o');

Published with MATLAB® 9.2