The discrete-time identified transfer function fits well with the data (Fit to estimation data: 97.9% , see also y_ym.jpg produced by System identification Apps). However, when I tried to test it with lsim as follows, I found y and yd are quite different (see y_yd.jpg), and yd is near zeros, why??
-------------- MATLAB code ------------------
data=iddata(y, u, Ts)
np = 5;
nz = 6;
sysd= tfest(data,np,nz,'Ts', data.Ts) % H(Z^-1)
[yd, ~]= lsim(sysd, u, t);
plot(t, y, t yd) % see y_yd.jpg
--------------outputs for sysd------------------
sysd =
0.05867 z^-1 - 0.07004 z^-2 - 0.3296 z^-3 + 0.8198 z^-4 - 0.664 z^-5 + 0.1851 z^-6
----------------------------------------------------------------------------------
1 - 4.302 z^-1 + 7.507 z^-2 - 6.645 z^-3 + 2.984 z^-4 - 0.5435 z^-5
Fit to estimation data: 97.9% (simulation focus)
FPE: 0.0001175, MSE: 0.0001155
Dear Changtong,
I suggest you to see links and attached file in subject.
-Transfer function estimation - MATLAB tfest - MathWorks
https://www.mathworks.com › ... › Transfer Function Models
-Estimating Models Using Frequency-Domain Data - MATLAB & Simulink
https://www.mathworks.com/.../estimate-models-using-frequency-d...
-Version 8 of the Matlab System Identification Toolbox - Science Direct
www.sciencedirect.com/science/article/pii/.../pdf?md5...pid...
-System Identification and Parameter Estimation - Universiteit Twente
https://www.utwente.nl/en/et/wa/web_dev/old/.../exercises.pdf
-LTI Models (Creating and Manipulating Models)
radio.feld.cvut.cz/matlab/toolbox/control/manipmod/ltisys14.html
Best regards
This example has several problems in terms of identification:
- the input signal is a step which is not recommended to carry out an identification.
- the choice of np and nz is arbitrary
In fact, you have to change the data and redo the simulations
Thanks, Kamel.
In fact, I want to identify a system H(z^-1) (or H(s)) with a step signal, and then use the inverse of the identified system (H^-1) and another output signal (experimental measurement) to estimate the input signal (unknown).
In this example, I tried to switch the input and output to avoid finding the inverse of identified system. Any better idea?
- Badges
- Science topic
- Similar topics
- Mathematics