How to find values from a bode plot in matlab

Question

Uzair Akhtar 2020-3-23

评论： Star Strider 2020-4-9

How would I find find these values from a bode plot

wcrit

kcrit

darova 2020-3-23

Can you be more specific?

Answer 1

Robert U 2020-3-24

Hi Uzair Akhtar,

According to controller theory you - probably - have to extract the following values:

w_crit: probably, the angular frequency at which angle drops below 180° (here your plant response changes sign, thus the controller turns to be unstable. Due to not controllable time delays in the signal chain, usually, there is a phase margin to be maintained)
k_crit: absolute gain value at w_crit (as long as your total gain at w_crit is below at least -3 dB the controller will not react to that parts of the signal and a change in sign is not critical. In order to stabilize the controller a gain margin is maintained).

All information needed can be extracted using bode() provided by control toolbox.

Kind regards,

Robert

Uzair Akhtar 2020-4-9

Thanks for this, is there a way to extract these values from the bode plot.

Star Strider 2020-4-9

You would likely have to calculate them.

You can get the necessary information from the bode function by requesting outputs from it:

See the documentation for details.