Thread Subject: Frequency Response between SS coded by Matlab and Result in Time

Hi All,

I try to validate the frequency response of my state-space model with the result from time domain simulation. Let me tell you what I have done:

1. My state-space model (named ss_Mo) is coded by Matlab and it is MIMO (4 inputs, 4 outputs). To get the frequency response I use command >>bode(ss_Mo). Then I will get the 32 graphs (16 for gain in decibel, 16 for phase). Each input provides 4-gain and 4-phase curves.

Q1.If I want to choose only one gain and one phase, how can I use command to select only the interesting curves and replot them? Gain should be in decibel and the range of frequency 10^0 to 10^3.
           ...................................

2. The other model is done by time-domain simulator. I select the result of input and output signal to plot the frequency response by using command

>>data=iddata(Output,Input,Ts);
>>Idloop=spafdr(data);
>>bode(Idloop)

Q2.The gain of this plot is not in decibel unit. How can I change to decible unit? It seems like I have to select the gain and the phase and replot again.
              ...................................

3. The last step, I want to comapare the results of item1 and item2 in the same gain and phase curve to check the accuracy of my model in item1. Which command should I use?

Please help me for this, I have tried to solve for 2 weeks but still cannot get the result.
I would like to say thank you very much in advance for your help.

Regards
Pakorn

Hi Pakorn

Q1: To plot the response of a particular I/O transfer, say, from input 1 to
output 2, use

bode(ss_Mo(2,1))

Q2: Not sure how to change it on this particular plot, but you can always do

bode(frd(Idloop)) % convert to FRD to use the BODE command in Control
System Toolbox

Q3: Try

bode(ss_Mo(2,1),frd(Idloop))

or with custom colors:

bode(ss_Mo(2,1),'b',frd(Idloop),'r--')

HTH
- pascal

"Pakorn" <pakorn.thepparat@siemens.com> wrote in message
news:8687282.1221134309125.JavaMail.jakarta@nitrogen.mathforum.org...
> Hi All,
>
> I try to validate the frequency response of my state-space model with the
> result from time domain simulation. Let me tell you what I have done:
>
> 1. My state-space model (named ss_Mo) is coded by Matlab and it is MIMO (4
> inputs, 4 outputs). To get the frequency response I use command
> >>bode(ss_Mo). Then I will get the 32 graphs (16 for gain in decibel, 16
> for phase). Each input provides 4-gain and 4-phase curves.
>
> Q1.If I want to choose only one gain and one phase, how can I use command
> to select only the interesting curves and replot them? Gain should be in
> decibel and the range of frequency 10^0 to 10^3.
> ...................................
>
> 2. The other model is done by time-domain simulator. I select the result
> of input and output signal to plot the frequency response by using command
>
>>>data=iddata(Output,Input,Ts);
>>>Idloop=spafdr(data);
>>>bode(Idloop)
>
> Q2.The gain of this plot is not in decibel unit. How can I change to
> decible unit? It seems like I have to select the gain and the phase and
> replot again.
> ...................................
>
> 3. The last step, I want to comapare the results of item1 and item2 in the
> same gain and phase curve to check the accuracy of my model in item1.
> Which command should I use?
>
> Please help me for this, I have tried to solve for 2 weeks but still
> cannot get the result.
> I would like to say thank you very much in advance for your help.
>
> Regards
> Pakorn

Your answer are very helpful. Thank you very much, Pascal.

Hive a nice weekend.

Pakorn