Transfer Function Structure Specification

You can use a priori knowledge of the expected transfer function model structure to initialize the estimation. The Structure property of an idtf model contains parameters that allow you to specify the values and constraints for the numerator, denominator and transport delays.

For example, specify a third-order transfer function model that contains an integrator and has a transport delay of at most 1.5 seconds:

init_sys = idtf([nan nan],[1 2 1 0]);
init_sys.Structure.IODelay.Maximum = 1.5;
init_sys.Structure.Denominator.Free(end) = false;

int_sys is an idtf model with three poles and one zero. The denominator coefficient for the s^0 term is zero and implies that one of the poles is an integrator.

init_sys.Structure.IODelay.Maximum = 1.5 constrains the transport delay to a maximum of 1.5 seconds. The last element of the denominator coefficients (associated with the s^0 term) is not a free estimation variable. This constraint forces one of the estimated poles to be at s = 0.

For more information regarding configuring the initial parameterization of an estimated transfer function, see Structure in idtf.