Problem plotting bode for a 2nd order system

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François Blas
François Blas 2019-5-13
编辑： Jan 2019-5-15
Hi, I'm getting started with simulink, I tried to simulate a 2nd order mass spring system but the bode plot seems odd :
K1 = 50
K=1/m1 = 1
I don't know which linear analysis point to choose
This system should have a resonance iif c1 < 1/sqrt(2)
If I choose open loop output, then bode NEVER shows a resonance no matter what c1 is.
If i choose output measurement, bode shows me a resonance if c1 < 9 (instead of 1/sqrt(2))
So, which analysis point should i use ?
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Jan
Jan 2019-5-15
编辑：Jan 2019-5-15
[MOVED from section for answers] François Blas on 13 May 2019 at 19:54
It's been 9hours, no one knows how to plot bode in simulink ?
Jan
Jan 2019-5-15
编辑：Jan 2019-5-15
@François Blas: Again: Please stop bumping your question by posting pseudo-answers. This behavior is not liked in this forum and it is possible, that you question is ignored in consequence.
Counting the time since you've asked your question looks taunting.

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Sulaymon Eshkabilov
编辑：Sulaymon Eshkabilov 2019-5-13
Hi Francois,
I have tested and simulated your example and it works as it has to w.r.t. the values of M1, K1, C1. In fact, for your data: K1 = 50, M1 =1, C1 = 2 (e.g). The resonant occurs at the frequency of sqrt(50/1) (approximately 7 Hz) and not @ sqrt(2) as you were anticipating. The selection of output measurement, is the right one. Here is my tested Simulink model. It is in fact correct that the resonant occurs at sqrt(50) => 7Hz that can be seen from the TF of SMD: 1/ (M1s^2+C1s+K1) ==> resonant freq: = sqrt(K1/M1).
Good luck.
  1 个评论
François Blas
François Blas 2019-5-15
Damn, I've been confusing friction coefficient and damping coefficient: C1 and what de call ξ (at least in France) in "2ξ/ω0" (you guys might use m or z for that)
Everything is fine ! Thanks for all the answers !

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