Approximate shape analysis: when w=0, the integration does not start from the real axis 1; The second-order undamped phase angle changes at the frequency 1, so w
When w= 1+, the amplitude is infinite and the phase angle is-180-45=-225 degrees; When w is not too large, the amplitude is very small, and the final phase angle is-270.
Summary: The curve starts from 1, decreases slightly, and then turns to -45, tending to infinity. When w= 1+, -225 degrees tends to infinity, and when w is infinite, it tends to the origin and the direction is -270 degrees. This section should be close to a straight line, and the angle near the origin will change. When w= 1, the phase angle suddenly changes-180 degrees.
As shown in the figure:
Extended data:
The general method of hand-drawn N-graph is as follows:
(1) in this form of frequency characteristics:
② Calculate the mode, radiation angle, real part and imaginary part of the starting point (ω=0) and ending point (ω→∞) of Nyquist diagram;
③ Calculate the coordinates of special points (including intersecting with real axis, intersecting with imaginary axis, asymptote, etc.). );
(4) According to the obtained auxiliary lines such as points and asymptotes. Make a rough picture. Because the open-loop Nyquist diagram does not need to know the position of the asymptote accurately when it is used in system analysis, it is generally enough to take the asymptote as the coordinate axis.
Baidu Encyclopedia-Nyquist Diagram