System Identification Toolbox

Periodic Inputs

It is often an advantage to use a periodic input for identification, whenever possible. See Section 13.3 in Ljung(1999). If you import or create a periodic input, as in

• u = idinput([300 2 5]) % Period 300, 2 inputs, 5 periods
  

you should set the corresponding period in the iddata object.

• u = iddata([],u,'Period',[300; 300]);
  

Normally, an even number of periods should be represented in the data. That will allow the estimation routines to do the right things. For example, etfe when called with data with periodic inputs, will honor the period and compute the frequency response on a suitably chosen frequency grid. Try this.

• m0 =idpoly([1 -1.5 0.7],[0 1 0.5]);
  u = idinput([10 1 150],'rbs');
  u = iddata([],u,'Period',10);
  e = iddata([],randn(1500,1));
  y = sim(m0, [u e])
  g = etfe([y u])
  bode(g,'x',m0) % Good fit at the 5 excited frequencies
  

Some Special Topics

Connections Between the Control System Toolbox and the System Identification Toolbox