find_x0.m

The main features of the considered identification problem are that there is no an a priori separati

% Choose initial approximation for stlsidentuy
% Uses subid and Polyx

function [x0,sys] = find_x0(u,y,nx,eiv)

if ~isa(u,'ss') 
  [a0,b0,c0,d0] = subid(y,u,ceil(nx/size(y,2))+1,nx,[],[],1); 
  sys  = ss(a0,b0,c0,d0,-1);
else
  sys = u;
  [a0,b0,c0,d0] = ssdata(sys);  
end

% convert sys to x
if issiso(sys)
  [N,D] = tfdata(sys,'v');
  x0    = [fliplr(N)'; -fliplr(D(2:end))'];
else
  % Use Polyx
  [N,D] = ss2lmf(a0,b0,c0,d0);
  N     = punpck(N); 
  D     = punpck(D);
  x0    = [N -D]';
  d     = size(x0,2);
  x0    = -x0(1:end-d,:)/x0(end-d+1:end,:);
end