Superimpose Nichols chart on Nichols plot




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 H/(1 + H) to H, where H is any complex number. For SISO systems, when H is a point on the open-loop frequency response, then


is the corresponding value of the closed-loop frequency response assuming 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.


collapse all

Nichols Response with Nichols Grid Lines

Plot the Nichols response with Nichols grid lines for the following system:

$$H(s) = \frac{{ - 4{s^4} + 48{s^3} - 18{s^2} + 250s + 600}}{{{s^4} +
30{s^3} + 282{s^2} + 525s + 60}}.$$

H = tf([-4 48 -18 250 600],[1 30 282 525 60]);

The right-click menu for Nichols charts includes the Tight option under Zoom. You can use this to clip unbounded branches of the Nichols chart.

See Also

Was this topic helpful?