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1 ## Copyright (C) 1996, 1998 Auburn University. All rights reserved. |
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2 ## |
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3 ## This file is part of Octave. |
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4 ## |
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5 ## Octave is free software; you can redistribute it and/or modify it |
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6 ## under the terms of the GNU General Public License as published by the |
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7 ## Free Software Foundation; either version 2, or (at your option) any |
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8 ## later version. |
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9 ## |
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10 ## Octave is distributed in the hope that it will be useful, but WITHOUT |
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11 ## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
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12 ## FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
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13 ## for more details. |
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14 ## |
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15 ## You should have received a copy of the GNU General Public License |
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16 ## along with Octave; see the file COPYING. If not, write to the Free |
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17 ## Software Foundation, 59 Temple Place, Suite 330, Boston, MA 02111 USA. |
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18 |
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19 ## -*- texinfo -*- |
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20 ## @deftypefn {Function File} {[@var{realp}, @var{imagp}, @var{w}] =} nyquist (@var{sys}, @var{w}, @var{out_idx}, @var{in_idx}, @var{atol}) |
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21 ## @deftypefnx {Function File} {} nyquist (@var{sys}, @var{w}, @var{out_idx}, @var{in_idx}, @var{atol}) |
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22 ## Produce Nyquist plots of a system; if no output arguments are given, Nyquist |
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23 ## plot is printed to the screen. |
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24 ## |
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25 ## Compute the frequency response of a system. |
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26 ## @strong{Inputs} (pass as empty to get default values) |
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27 ## @table @var |
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28 ## @item sys |
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29 ## system data structure (must be either purely continuous or discrete; |
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30 ## see is_digital) |
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31 ## @item w |
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32 ## frequency values for evaluation. |
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33 ## if sys is continuous, then bode evaluates @math{G(jw)} |
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34 ## if sys is discrete, then bode evaluates @math{G(exp(jwT))}, where |
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35 ## @math{T} is the system sampling time. |
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36 ## @item default |
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37 ## the default frequency range is selected as follows: (These |
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38 ## steps are NOT performed if @var{w} is specified) |
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39 ## @end table |
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40 ## @enumerate |
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41 ## @item via routine __bodquist__, isolate all poles and zeros away from |
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42 ## @var{w}=0 (@var{jw}=0 or @math{exp(@var{jwT})=1}) and select the frequency |
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43 ## range based on the breakpoint locations of the frequencies. |
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44 ## @item if @var{sys} is discrete time, the frequency range is limited |
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45 ## to @var{jwT} in |
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46 ## @ifinfo |
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47 ## [0,2p*pi] |
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48 ## @end ifinfo |
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49 ## @iftex |
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50 ## $[0,2p*\pi]$ |
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51 ## @end iftex |
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52 ## @item A "smoothing" routine is used to ensure that the plot phase does |
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53 ## not change excessively from point to point and that singular |
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54 ## points (e.g., crossovers from +/- 180) are accurately shown. |
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55 ## @end enumerate |
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56 ## outputs, inputs: names or indices of the output(s) and input(s) to be |
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57 ## used in the frequency response; see sysprune. |
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58 ## |
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59 ## @strong{Inputs} (pass as empty to get default values) |
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60 ## @table @var |
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61 ## @item atol |
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62 ## for interactive nyquist plots: atol is a change-in-slope tolerance |
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63 ## for the of asymptotes (default = 0; 1e-2 is a good choice). This allows |
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64 ## the user to ``zoom in'' on portions of the Nyquist plot too small to be |
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65 ## seen with large asymptotes. |
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66 ## @end table |
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67 ## @strong{Outputs} |
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68 ## @table @var |
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69 ## @item realp |
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70 ## @itemx imagp |
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71 ## the real and imaginary parts of the frequency response |
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72 ## @math{G(jw)} or @math{G(exp(jwT))} at the selected frequency values. |
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73 ## @item w |
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74 ## the vector of frequency values used |
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75 ## @end table |
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76 ## |
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77 ## If no output arguments are given, nyquist plots the results to the screen. |
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78 ## If @var{atol} != 0 and asymptotes are detected then the user is asked |
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79 ## interactively if they wish to zoom in (remove asymptotes) |
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80 ## Descriptive labels are automatically placed. |
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81 ## |
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82 ## Note: if the requested plot is for an MIMO system, a warning message is |
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83 ## presented; the returned information is of the magnitude |
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84 ## ||G(jw)|| or ||G(exp(jwT))|| only; phase information is not computed. |
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85 ## @end deftypefn |
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86 |
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87 ## Author: R. Bruce Tenison <btenison@eng.auburn.edu> |
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88 ## Created: July 13, 1994 |
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89 ## A. S. Hodel July 1995 (adaptive frequency spacing, |
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90 ## remove acura parameter, etc.) |
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91 ## Revised by John Ingram July 1996 for system format |
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92 |
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93 function [realp, imagp, w] = nyquist (sys, w, outputs, inputs, atol) |
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94 |
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95 ## Both bode and nyquist share the same introduction, so the common |
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96 ## parts are in a file called __bodquist__.m. It contains the part that |
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97 ## finds the number of arguments, determines whether or not the system |
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98 ## is SISO, andd computes the frequency response. Only the way the |
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99 ## response is plotted is different between the two functions. |
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100 |
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101 ## check number of input arguments given |
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102 if (nargin < 1 | nargin > 5) |
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103 usage("[realp,imagp,w] = nyquist(sys[,w,outputs,inputs,atol])"); |
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104 endif |
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105 if(nargin < 2) |
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106 w = []; |
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107 endif |
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108 if(nargin < 3) |
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109 outputs = []; |
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110 endif |
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111 if(nargin < 4) |
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112 inputs = []; |
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113 endif |
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114 if(nargin < 5) |
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115 atol = 0; |
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116 elseif(!(is_sample(atol) | atol == 0)) |
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117 error("atol must be a nonnegative scalar.") |
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118 endif |
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119 |
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120 ## signal to __bodquist__ who's calling |
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121 |
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122 [f, w, sys] = __bodquist__ (sys, w, outputs, inputs, "nyquist"); |
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123 |
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124 ## Get the real and imaginary part of f. |
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125 realp = real(f); |
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126 imagp = imag(f); |
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127 |
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128 ## No output arguments, then display plot, otherwise return data. |
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129 if (nargout == 0) |
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130 dnplot = 0; |
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131 while(!dnplot) |
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132 oneplot(); |
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133 gset key; |
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134 clearplot(); |
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135 grid ("on"); |
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136 gset data style lines; |
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137 |
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138 if(is_digital(sys)) |
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139 tstr = " G(e^{jw}) "; |
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140 else |
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141 tstr = " G(jw) "; |
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142 endif |
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143 xlabel(["Re(",tstr,")"]); |
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144 ylabel(["Im(",tstr,")"]); |
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145 |
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146 [stn, inn, outn] = sysgetsignals(sys); |
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147 if(is_siso(sys)) |
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148 title(sprintf("Nyquist plot from %s to %s, w (rad/s) in [%e, %e]", ... |
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149 inn{1}, outn{1}, w(1), w(length(w))) ) |
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150 endif |
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151 |
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152 gset nologscale xy; |
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153 |
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154 axis(axis2dlim([[vec(realp),vec(imagp)];[vec(realp),-vec(imagp)]])); |
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155 plot(realp,imagp,"- ;+w;",realp,-imagp,"-@ ;-w;"); |
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156 |
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157 ## check for interactive plots |
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158 dnplot = 1; # assume done; will change later if atol is satisfied |
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159 if(atol > 0 & length(f) > 2) |
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160 |
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161 ## check for asymptotes |
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162 fmax = max(abs(f)); |
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163 fi = max(find(abs(f) == fmax)); |
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164 |
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165 ## compute angles from point to point |
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166 df = diff(f); |
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167 th = atan2(real(df),imag(df))*180/pi; |
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168 |
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169 ## get angle at fmax |
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170 if(fi == length(f)) fi = fi-1; endif |
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171 thm = th(fi); |
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172 |
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173 ## now locate consecutive angles within atol of thm |
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174 ith_same = find(abs(th - thm) < atol); |
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175 ichk = union(fi,find(diff(ith_same) == 1)); |
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176 |
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177 ## locate max, min consecutive indices in ichk |
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178 loval = max(complement(ichk,1:fi)); |
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179 if(isempty(loval)) loval = fi; |
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180 else loval = loval + 1; endif |
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181 |
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182 hival = min(complement(ichk,fi:length(th))); |
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183 if(isempty(hival)) hival = fi+1; endif |
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184 |
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185 keep_idx = complement(loval:hival,1:length(w)); |
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186 |
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187 if(length(keep_idx)) |
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188 resp = input("Remove asymptotes and zoom in (y or n): ",1); |
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189 if(resp(1) == "y") |
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190 dnplot = 0; # plot again |
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191 w = w(keep_idx); |
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192 f = f(keep_idx); |
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193 realp = real(f); |
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194 imagp = imag(f); |
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195 endif |
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196 endif |
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197 |
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198 endif |
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199 endwhile |
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200 w = []; |
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201 realp=[]; |
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202 imagp=[]; |
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203 endif |
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204 |
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205 endfunction |
