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## Copyright (C) 1996, 2000, 2004, 2005, 2007
## Auburn University. All rights reserved.
##
##
## This program 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.
##
## This program 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 this program; see the file COPYING. If not, see
## <http://www.gnu.org/licenses/>.
## -*- texinfo -*-
## @deftypefn {Function File} {[@var{num}, @var{den}] =} ss2tf (@var{a}, @var{b}, @var{c}, @var{d})
## Conversion from transfer function to state-space.
## The state space system:
## @iftex
## @tex
## $$ \dot x = Ax + Bu $$
## $$ y = Cx + Du $$
## @end tex
## @end iftex
## @ifinfo
## @example
## .
## x = Ax + Bu
## y = Cx + Du
## @end example
## @end ifinfo
##
## is converted to a transfer function:
## @iftex
## @tex
## $$ G(s) = { { \rm num }(s) \over { \rm den }(s) } $$
## @end tex
## @end iftex
## @ifinfo
## @example
##
## num(s)
## G(s)=-------
## den(s)
## @end example
## @end ifinfo
##
## used internally in system data structure format manipulations.
## @end deftypefn
## Author: R. Bruce Tenison <btenison@eng.auburn.edu>
## Created: June 24, 1994
## a s hodel: modified to allow for pure gain blocks Aug 1996
function [num, den] = ss2tf (a, b, c, d)
if (nargin != 4)
print_usage ();
endif
## Check args
[n, m, p] = abcddim (a, b, c, d);
if (n == -1)
num = [];
den = [];
error("ss2tf: Non compatible matrix arguments");
elseif (m != 1 || p != 1)
num = [];
den = [];
error ("ss2tf: not SISO system: m=%d, p=%d", m, p);
endif
if(n == 0)
## gain block only
num = d;
den = 1;
else
## First, get the denominator coefficients
den = poly (a);
## Get the zeros of the system
[zz, g] = tzero (a, b, c, d);
## Form the Numerator (and include the gain)
if (! isempty (zz))
num = g * poly (zz);
else
num = g;
endif
## the coefficients must be real
den = real (den);
num = real (num);
endif
endfunction
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