why draw the plot this way?

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why draw the plot this way?
workaholic
06-20-2009

workaholic posted on
June 20, 2009, 10:24 pm

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consider  a system  in its Fourier transform form:

1+j*w*T/ (1+j*w*T1)   and its nonminimum phase counterpart: 1-j*w*T/
(1+j*w*T1)  where  0<T<T1

in control textbooks, the bode plot is draw like this: the amplitude
is the same in the bode plot, while the first system's phase curve is
within 0  to  -90. Due to nonmininmum phase characteristics of the
nonminimum phase sytem, the phase curve is 0 to -180.  However the
Matlab always draw the nonminimum phase sytem's phase curve within 180
to 0. Why? we all know that nonmininmum phase system contains a large
scale of the phase lag, why does Matlab use this kind of fashion?

Tim Wescott posted on
June 21, 2009, 10:54 am
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Re: why draw the plot this way?

On Sat, 20 Jun 2009 19:24:48 -0700, workaholic wrote:

There's a bug in Matlab's Bode plotter?

Have you tried doing it manually? I.e. get the frequency response into a
vector, then plot the vector's amplitude and phase vs. frequency?

--
www.wescottdesign.com

workaholic posted on
June 23, 2009, 4:30 am
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Re: why draw the plot this way?

On 6æ9C�21æ97¥, äB8�E5�8810æ97¶54å88�, Tim Wes=

Although I haven't done it manually, I have a chart on the Modern
Control Engineering, 3rd Edition, Ogata. The illustration on it is
just what I stated in my last post. the nonminimum phase counterpart's
phase may also begin in approximately 0 deg and end at -180 deg. I do
not know why Matlab begin with 180.