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diff --git a/octave_packages/control-2.3.52/@lti/sminreal.m b/octave_packages/control-2.3.52/@lti/sminreal.m
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+## Copyright (C) 2009, 2010, 2011 Lukas F. Reichlin
+##
+## This file is part of LTI Syncope.
+##
+## LTI Syncope is free software: you can redistribute it and/or modify
+## it under the terms of the GNU General Public License as published by
+## the Free Software Foundation, either version 3 of the License, or
+## (at your option) any later version.
+##
+## LTI Syncope is distributed in the hope that it will be useful,
+## but WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+## GNU General Public License for more details.
+##
+## You should have received a copy of the GNU General Public License
+## along with LTI Syncope. If not, see .
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {@var{sys} =} sminreal (@var{sys})
+## @deftypefnx {Function File} {@var{sys} =} sminreal (@var{sys}, @var{tol})
+## Perform state-space model reduction based on structure.
+## Remove states which have no influence on the input-output behaviour.
+## The physical meaning of the states is retained.
+##
+## @strong{Inputs}
+## @table @var
+## @item sys
+## State-space model.
+## @item tol
+## Optional tolerance for controllability and observability.
+## Entries of the state-space matrices whose moduli are less or equal to @var{tol}
+## are assumed to be zero. Default value is 0.
+## @end table
+##
+## @strong{Outputs}
+## @table @var
+## @item sys
+## Reduced state-space model.
+## @end table
+##
+## @seealso{minreal}
+## @end deftypefn
+
+## Author: Lukas Reichlin
+## Created: October 2009
+## Version: 0.4
+
+function sys = sminreal (sys, tol = 0)
+
+ if (nargin > 2) # sminreal () not possible (inside @lti)
+ print_usage ();
+ endif
+
+ if (! isa (sys, "ss"))
+ warning ("sminreal: system not in state-space form");
+ sys = ss (sys); # needed by __sys_prune__
+ endif
+
+ if (! (is_real_scalar (tol) && tol >= 0))
+ error ("sminreal: second argument is not a valid tolerance");
+ endif
+
+ [a, b, c, d, e] = dssdata (sys, []);
+
+ a = abs (a) > tol;
+ b = abs (b) > tol;
+ c = abs (c) > tol;
+
+ if (! isempty (e))
+ e = abs (e) > tol;
+ a = a | e;
+ endif
+
+ co_idx = __controllable_states__ (a, b);
+ ob_idx = __controllable_states__ (a.', c.');
+
+ st_idx = intersect (co_idx, ob_idx);
+
+ sys = __sys_prune__ (sys, ":", ":", st_idx);
+
+endfunction
+
+
+function c_idx = __controllable_states__ (a, b)
+
+ n = rows (a); # number of states
+ a = a & ! eye (n); # set diagonal entries to zero
+
+ c_vec = any (b, 2); # states directly controllable
+ c_idx = find (c_vec); # indices of directly controllable states
+ c_idx_new = 0; # any vector of length > 0 possible
+
+ while (all (length (c_idx) != [0, n]) && length(c_idx_new) != 0)
+
+ u_idx = find (! c_vec); # indices of uncontrollable states
+
+ #{
+ ## debug code
+ a(u_idx, :)
+ repmat (c_vec.', length (u_idx), 1)
+ a(u_idx, :) & repmat (c_vec.', length (u_idx), 1)
+ any (a(u_idx, :) & repmat (c_vec.', length (u_idx), 1), 2)
+ find (any (a(u_idx, :) & repmat (c_vec.', length (u_idx), 1), 2))
+ #}
+
+ c_idx_new = u_idx (find (any (a(u_idx, :) & repmat (c_vec.', length (u_idx), 1), 2)));
+ c_idx = union (c_idx, c_idx_new);
+ c_vec(c_idx_new) = 1;
+
+ endwhile
+
+endfunction
+
+
+## ss: sminreal
+%!shared B, C
+%!
+%! A = ss (-2, 3, 4, 5);
+%! B = A / A;
+%! C = sminreal (B); # no states should be removed
+%!
+%!assert (C.a, B.a);
+%!assert (C.b, B.b);
+%!assert (C.c, B.c);
+%!assert (C.d, B.d);
+
+%!shared A, B, D, E
+%!
+%! A = ss (-1, 1, 1, 0);
+%! B = ss (-2, 3, 4, 5);
+%! C = [A, B];
+%! D = sminreal (C(:, 1));
+%! E = sminreal (C(:, 2));
+%!
+%!assert (D.a, A.a);
+%!assert (D.b, A.b);
+%!assert (D.c, A.c);
+%!assert (D.d, A.d);
+%!assert (E.a, B.a);
+%!assert (E.b, B.b);
+%!assert (E.c, B.c);
+%!assert (E.d, B.d);