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tf2ss.m
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1999-03-05
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# Copyright (C) 1996,1998 A. Scottedward Hodel
#
# This file is part of Octave.
#
# Octave 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 2, or (at your option) any
# later version.
#
# Octave 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 Octave; see the file COPYING. If not, write to the Free
# Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
function [a,b,c,d] = tf2ss(num,den)
# Conversion from tranfer function to state-space.
# The state space system
# .
# x = Ax + Bu
# y = Cx + Du
#
# is obtained from a transfer function
#
# num(s)
# G(s)=-------
# den(s)
#
# via the function call [a,b,c,d] = tf2ss(num,den).
# The vector 'den' must contain only one row, whereas the vector 'num'
# may contain as many rows as there are outputs of the system 'y'.
# The state space system matrices obtained from this function will be
# in controllable canonical form as described in "Modern Control Theory",
# [Brogan, 1991].
# Written by R. Bruce Tenison (June 22, 1994) btenison@eng.auburn.edu
# mod A S Hodel July, Aug 1995
if(nargin != 2) error("tf2ss: wrong number of input arguments")
elseif(isempty(num)) error("tf2ss: empty numerator");
elseif(isempty(den)) error("tf2ss: empy denominator");
elseif(!is_vector(num))
error(sprintf("num(%dx%d) must be a vector",rows(num),columns(num)));
elseif(!is_vector(den))
error(sprintf("den(%dx%d) must be a vector",rows(den),columns(den)));
endif
# strip leading zeros from num, den
nz = find(num != 0);
if(isempty(nz)) num = 0;
else num = num(nz(1):length(num)); endif
nz = find(den != 0);
if(isempty(nz)) error("denominator is 0.");
else den = den(nz(1):length(den)); endif
# force num, den to be row vectors
num = vec(num)'; den = vec(den)';
nn = length(num); nd = length(den);
if(nn > nd) error(sprintf("deg(num)=%d > deg(den)= %d",nn,nd)); endif
# Check sizes
if (nd == 1) a = []; b = []; c = []; d = num(:,1) / den(1);
else
# Pad num so that length(num) = length(den)
if (nd-nn > 0) num = [zeros(1,nd-nn), num]; endif
# Normalize the numerator and denominator vector w.r.t. the leading
# coefficient
d1 = den(1); num = num / d1; den = den(2:nd)/d1;
sw = nd-1:-1:1;
# Form the A matrix
if(nd > 2) a = [zeros(nd-2,1),eye(nd-2,nd-2);-den(sw)];
else a = -den(sw); endif
# Form the B matrix
b = zeros(nd-1,1); b(nd-1,1) = 1;
# Form the C matrix
c = num(:,2:nd)-num(:,1)*den; c = c(:,sw);
# Form the D matrix
d = num(:,1);
endif
endfunction