User`s guide

ngrid
11-147
11ngrid
Purpose Superimpose a Nichols chart on a Nichols plot
Syntax ngrid
Description ngrid superimposes Nichols chart grid lines over the Nichols frequency
response of a SISO LTI system. The range of the Nichols grid lines is set to
encompass the entire Nichols frequency response.
The chart relates the complex number to , where is any
complex number. For SISO systems, when is a point on the open-loop
frequency response, then
is the correspondingvalue of the closed-loopfrequency responseassuming unit
negative feedback.
If the current axis is empty,
ngrid generates a new Nichols chart grid in the
region –40 dB to 40 dB in magnitude and –360 degrees to 0 degrees in phase.
If the current axis does not contain a SISO Nichols frequency response,
ngrid
returns a warning.
Example Plot the Nichols response with Nichols grid lines for the system.
Type
H = tf([–4 48 –18 250 600],[1 30 282 525 60])
MATLAB returns
Transfer function:
– 4 s^4 + 48 s^3 – 18 s^2 + 250 s + 600
---------------------------------------
s^4 + 30 s^3 + 282 s^2 + 525 s + 60
H 1 H
+()⁄
H
H
H
H
1H+
--------------
Hs()
4s
4
– 48s
3
18s
2
– 250s 600+++
s
4
30s
3
282s
2
525s 60++ ++
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