Function to convert a symbolic expression to a transfer function.

sym2tf.m

function sys = sym2tf(symf,ts)
%TF2SYM Converts symbolic function to transfer function
%   sys = SYM2TF(symf) converts the symbolic function symf of class 'sym' 
%   to transfer function.
%
%   sys = SYM2TF(...,ts) converts the symbolic function s of class 'sym' 
%   to discrete transfer function.
%
%   See also TF, SYM.

    if length(symvar(symf)) > 1
        error("only one symbolic variable ('s' or 'z') is allowed")
    end

    [N,D] = numden(symf);

    N = sym2poly(N);
    D = sym2poly(D);

    if symvar(symf) == 's'
        sys = tf(N,D);
    elseif symvar(symf) == 'z'
        if nargin < 2
            error('You must specify the sampling time for a discrete-time transfer function')
        end

        sys = tf(N,D,ts);
    end   
end