First, the experimental purpose

1. Master the method of drawing Porter diagram and Nyquist diagram with matlab.

2. Learn to judge the stability of the system from Porter diagram and Nyquist diagram.

3. Learn to find the stability margin of the system from the Porter diagram.

4. Understand the influence of the change of K value on the amplitude-frequency and phase-frequency curves of Porter diagram.

5. Master the method of drawing the zero-pole distribution diagram of the system with matalab.

Second, the experimental principle

1. The principle of judging whether the system is stable from Nyquist diagram. Nyquist stability criterion: the necessary and sufficient condition for the stability of feedback control system is that the semi-closed curve IGH does not pass through the (-1, 0i) point, and the number of turns r around the critical point (-1, 0i) in the counterclockwise direction is equal to the number of poles of the positive real part of the open-loop transfer function.

Nyquist stability criterion: the necessary and sufficient condition for the stability of feedback control system is that the semi-closed curve IGH does not pass through the (-1, 0i) point, and the number of turns r around the critical point (-1, 0) counterclockwise is equal to the number of poles of the positive real part of the open-loop transfer function. The specific method is to observe the system transfer function first, and then get the value of p.

Observe the number of times the curve crosses from the right side of point (-1, 0i), where the top-down crossing is positive and the bottom-up crossing is negative. The complete first crossing is recorded as N- semi-crossing, with 0.5NR=2N=2(N+-N-) and z = p-r. Observe whether z is zero. If z is zero, the system is stable and b is not zero.

2. Based on the principle of judging whether the system is stable by Porter diagram, it can be known from Nyquist stability judgment that the number of curve crossings (-1, 0i) is needed to judge whether the system is stable, which corresponds to Porter diagram. With w-wc, a (WC) = g (JWC) (HWE) = L (WC) = 20LOGA (.

Therefore, wc is the demarcation point, corresponding to the phase-frequency curve observed at W.