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Unformatted text preview: MAE143B Homework Set 4  Solutions Question 1 FPEN Question 625: Suppose that G ( s ) = 25( s + 1) s ( s + 2)( s 2 + 2 s + 16) , in the unity feedback system in the figure. Use matlab’s margin command to calculate the gain + _ G ( s ) r t y t margin and phase margin for G ( s ) and, on the basis of the Bode plots, conclude which margin would provide more useful information to the control designer for this system. We use the following matlab commands to draw the Bode plot with margins indicated in the figure below. >> sys=tf(25 * [1 1],conv([1 0],conv([1 2],[1 2 16]))) Transfer function: 25 s + 25 sˆ4 + 4 sˆ3 + 20 sˆ2 + 32 s >> margin(sys) − 100 − 80 − 60 − 40 − 20 20 Magnitude (dB) 10 − 1 10 10 1 10 2 − 270 − 225 − 180 − 135 − 90 − 45 Phase (deg) Bode Diagram Gm = 3.91 dB (at 4.22 rad/sec) , Pm = 101 deg (at 1.08 rad/sec) Frequency (rad/sec) From the figure, it is clear that the small gain margin is a real problem. Indeed, the bode plot indicates that the system frequency response comes and stays quite close to the 1 point. So a small modeling error, which is what the margins are meant to capture, could lead to instability of the unity feedback system. The huge phase margin is rather illusory, since the gain remains very close to 0dB well after the gain crossover point. The corresponding Nyquist diagram below shows better the proximity to the1 point and the failure of the phase margin to capture this. − 0.8 − 0.6 − 0.4 − 0.2 0.2 0.4 − 1 − 0.5 0.5 1 1.5 Nyquist Diagram Real Axis Imaginary Axis Question 2 FPEN Question 637: The Nyquist diagrams for two stable, openloop systems are sketched in the figure below. The proposed operating gain is indicated as K , and arrows indicate increasing frequency. In each case give a rough estimate of the following quantities for the closedloop (unity feedback) system. (a) Phase margin. The phase margin at the nominal gain K is the angle associated with the intersection of the Nyquist diagram and a circle of radius 1 /K centered at the origin. For System (a), this is roughly 25 ◦ . For System (b), it is roughly 60 ◦ ....
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 Fall '09
 CALLAFON
 Electronics terms, Nyquist plot

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