# bode plot stability

Discussion in 'The Projects Forum' started by suzuki, Jan 28, 2012.

1. ### suzuki Thread Starter Member

Aug 10, 2011
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0
Hi,

When designing a controller, my understanding is that to look for stability, you have to consider the gain and phase margins. However, i have a design that has both positive gain and phase margins (>10 dB and > 60 degrees) yet the step response to the system is still unstable. What other tests can do i to test the stability?

i have also heard of the root locus plot, but i think usually looks at the gain parameter and its effects on the poles and zeroes. Can this be unstable even with the above conditions? (+ GM, +PM)

TIA

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
May I ask what is the transfer function of this unusual system? As you suggest the root locus shows how the closed loop poles vary with loop gain variation.

3. ### crutschow Expert

Mar 14, 2008
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3,386
Can you post a plot showing the gain and phase margins of your system?

4. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
Did you actually test (measure) the gain/phase or is this something that came out of some simulation?

5. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
The critical point for most systems is what the gain is when the systems total phase shift reaches -180 degrees. If you still have positive gain then, the system is an oscillator. Another way to look at it is to see what the phase margin is when the system's gain crosses 0dB (unity gain). Phase margin is the difference between the measured phase shift and -180. Ideally, you would like about 30 - 60 degrees of phase margin at the unity gain crossover point, but in some cases 20 degrees is OK.

Last edited: Jan 28, 2012