fitgain (Robust Control Toolbox)

State space realization for a given magnitude Bode plot.

[ad,bd,cd,dd,logfit] = fitgain(logd,w) 
[ad,bd,cd,dd,logfit] = fitgain(logd,w,n) 
[ad,bd,cd,dd,logfit] = fitgain(logd,w,n,wt) 
[ad,bd,cd,dd,logfit] = fitgain(logd,w,n,wt,flag) 
[ssd,logfit] = fitgain()

Description

fitgain produces a stable and minimum-phase state-space realization of the given magnitude Bode plot logd.

The input variable logd is a vector containing the logarithmic magnitude Bode plot evaluated at frequency vector w.

Optional input variables:

n -- the size of the desired state-space approximant (default = 0)

wt -- a weighting vector contains the weight at each frequency point

flag -- set to "1" to display a Bode plot (default = 1).

fitgain uses the routine invfreqs in Signal Processing Toolbox to fit the continuous magnitude Bode plot. Three steps are involved:

1: Take the power spectrum density of the given magnitude data, i.e., .

2: Use the error correction method (routine invfreqs) to fit the PSD data with a rational transfer function.

3: Extract the stable and minimum phase part of the realization.

This method is not as numerically stable as fitd.