Timeline for Why can’t one see for 2nd order system that it is at its stability limit neither in the Nyquist plot nor Bode plot?
Current License: CC BY-SA 3.0
11 events
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| Nov 29, 2016 at 20:29 | comment | added | Suba Thomas | Bingo. Yes, when you are doing stability analysis, computing stability margins, and so on, you analyze the frequency response of the open-loop system. Thus, the open-loop transfer function needs to be used for Nyquist and Bode plots. The results will tell you something about the closed-loop system. | |
| Nov 29, 2016 at 20:23 | vote | accept | solid | ||
| Nov 29, 2016 at 20:23 | comment | added | solid | hang on, does this mean the matlab function nyquist() and bode() expect the open-loop system. i was thinking i had to give it the closed-loop system. so my problem was been new to matlab nyquist- and bode-functions. sorry for wasting your time and thank you. | |
| Nov 29, 2016 at 20:00 | comment | added | Suba Thomas | Please get the basics right w.r.t closed- and open-loop systems, or we will be repeating the same thing over and over. In the matlab plot (as I explained in the answer) the open loop is $\frac{1}{s^2+1}$ which gives a stable closed-loop. With the open-loop of $\frac{1}{s^2+s-1}$ the closed-loop is marginally stable. This is what the plots confirm. | |
| Nov 29, 2016 at 19:40 | comment | added | solid | but not in the matlab plot above | |
| Nov 29, 2016 at 19:29 | comment | added | Suba Thomas | For $\frac{1}{s^2+s-1}$, when the magnitude is 0 db, the phase is -180 degrees. The phase margin is 0. | |
| Nov 29, 2016 at 18:58 | comment | added | solid | I don't get your point, so let the open-loop system be $\frac{1}{s^2+s-1}$ so the closed on is $\frac{1}{s^2+s}$ now the eigenvalues of the closed one are still {0, -1} but my bode plot still shows a phase reserve of 51.8 deg when the magnitude hits 0 | |
| Nov 29, 2016 at 18:55 | vote | accept | solid | ||
| Nov 29, 2016 at 18:55 | |||||
| Nov 29, 2016 at 18:17 | comment | added | Suba Thomas | The Nyquist stability criterion and things like gain and phase margin that are seen on the Bode plot are for the closed-loop system. The utility of these plots is that you can get insights into the closed-loop behavior using the open-loop transfer function. | |
| Nov 29, 2016 at 18:15 | comment | added | solid | Am i only allowed to view the plots for closed-loops? | |
| Nov 29, 2016 at 18:10 | history | answered | Suba Thomas | CC BY-SA 3.0 |