Getting Started

LTI Objects

For convenience, the Control System Toolbox uses custom data structures called LTI objects to store model-related data. For example, the variable sys_dc created for the DC motor example is called an SS object. There are also TF, ZPK, and FRD objects for transfer function, zero/pole/gain, and frequency data response models respectively. The four LTI objects encapsulate the model data and enable you to manipulate linear systems as single entities rather than as collections of vectors or matrices.

To see what LTI objects contain, use the get command. This code describes the contents of sys_dc from the DC motor example.

• get(sys_dc)
               a: [2x2 double]
               b: [2x1 double]
               c: [0 1]       
               d: 0           
               e: []          
       StateName: {2x1 cell}  
              Ts: 0           
         ioDelay: 0           
      InputDelay: 0           
     OutputDelay: 0           
       InputName: {''}        
      OutputName: {''}        
      InputGroup: {0x2 cell}  
     OutputGroup: {0x2 cell}  
           Notes: {}          
        UserData: [] 
  

You can manipulate the data contained in LTI objects using the set command; see the Control System Toolbox online reference pages for descriptions of set and get.

Another convenient way to set or retrieve LTI model properties is to access them directly using dot notation. For example, if you want to access the value of the A matrix, instead of using get, you can type

• sys_dc.a
  

at the MATLAB prompt. MATLAB returns the A matrix.

• ans =
  
     -4.0000   -0.0300
      0.7500  -10.0000
  

Similarly, if you want to change the values of the A matrix, you can do so directly, as this code shows.

• A_new = [-4.5 -0.05; 0.8 -12.0];
  sys_dc.a = A_new;
  

For more information on LTI properties, type ltiprops at the MATLAB prompt. For a complete description of LTI objects, see Creating and Manipulating Models online under the Control System Toolbox.

Adding Delays to Linear Models

MIMO Models