Creating and Manipulating Models

Resizing LTI Systems

Resizing a system consists of adding or deleting inputs and/or outputs. To delete the first two inputs, simply type

• sys(:,1:2) = []
  

In deletions, at least one of the row/column indexes should be the colon (:) selector.

To perform input/output augmentation, you can proceed by concatenation or subassignment. Given a system sys with a single input, you can add a second input using

• sys = [sys,h];
  

or, equivalently, using

• sys(:,2) = h;
  

where h is any LTI model with one input, and the same number of outputs as sys. There is an important difference between these two options: while concatenation obeys the precedence rules (see page 2-5), subsystem assignment does not alter the model type. So, if sys and h are TF and SS objects, respectively, the first statement produces a state-space model, and the second statement produces a transfer function.

For state-space models, both concatenation and subsystem assignment increase the model order because they assume that sys and h have independent states. If you intend to keep the same state matrix and only update the input-to-state or state-to-output relations, use set instead and modify the corresponding state-space data directly. For example,

• sys = ss(a,b1,c,d1)
  set(sys,'b',[b1 b2],'d',[d1 d2])
  

adds a second input to the state-space model sys by appending the B and D matrices. You should simultaneously modify both matrices with a single set command. Indeed, the statements

• sys.b = [b1 b2]
  

and

• set(sys,'b',[b1 b2])
  

cause an error because they create invalid intermediate models in which the B and D matrices have inconsistent column dimensions.

Referencing Channels by Name

Arithmetic Operations