Suppose we have a linear system with the following relation :
S(t)=E(t)+1;
S(t): is the output of the system
E(t) : is the input of the system
if we want to extract the transfer function from the previous relation, we would get :
S/E=1+1/(s*E) with s is the laplace variable.
as you see, the transfer funtion depend on the input !!!
All i know is that for the design of a regulator we must have a known transfer function (plant) which is independent of the input
if we suppose we want to put this system in a closed loop, how can we design a controller with a variable transfer function ???