Imposing Constraints on Model Parameter Values
All identified linear (IDLTI) models, except idfrd, contain a Structure property.
The Structure property contains the adjustable
entities (parameters) of the model. Each parameter has attributes
such as value, minimum/maximum bounds, and free/fixed status that
allow you to constrain them to desired values or a range of values
during estimation. You use the Structure property
to impose constraints on the values of various model parameters.
The Structure property contains the essential
parameters that define the structure of a given model:
For identified transfer functions, includes the numerator, denominator, and delay parameters
For polynomial models, includes the list of active polynomials
For state-space models, includes the list of state-space matrices
For information about other model types, see the model reference pages.
For example, the following example constructs an idtf model, specifying values for the Numerator and Denominator parameters:
num = [1 2]; den = [1 2 2]; sys = idtf(num,den)
You can update the value of the Numerator and Denominator properties
after you create the object as follows:
new_den = [1 1 10]; sys.Denominator = new_den;
To fix the denominator to the value you specified (treat its
coefficients as fixed parameters), use the Structure property
of the object as follows:
sys.Structure.Denominator.Value = new_den; sys.Structure.Denominator.Free = false(1,3);
For a transfer function model, the Numerator, Denominator,
and IODelay model properties are simply pointers
to the Value attribute of the corresponding parameter
in the Structure property.
Similar relationships exist for other model structures. For
example, the A property of a state-space model
contains the double value of the state matrix. It is an alias to the A parameter
value stored in Structure.A.Value.