Having trouble with closed loop Bode plots

2011 6 3
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I have two different Simulink models of a dc-dc converter with a closed loop. I go throught the steps of:
  • Placing the linearization input and output points
  • Computing operating points
  • Running the linearization with a Bode plot as an output
The analysis runs and the Bode plot window appears, but there is no Bode plot.
I tried generating the Bode plot of parts of the loop (like the compensator) and that was sucessful.
I suspect that I might being something wrong in placing the linearization points.
Any suggestions or pointers to specific help documentation would appreciated (I have gone through quite a bit of the help already).
Thanks in advance, Bob White

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Just a guess, but probably what happens is that part of the model (plant most likely) linearizes to 0, therefore resulting in "empty" bode plot. In the linearization tool (Control and Estimation Tools Manager), click on the "Model" node and examine your model (you can choose what form to use - state space, transfer function, zero-pole-gain).
If the linearized model is indeed zero, this might be happening if you have discontinuities in your model that do not linearize well when block by block linearization is used. This can happen, for example, if you have PWM in your model.
There are several ways to deal with this:
1) Determine which block or subsystem linearizes to zero. Then specify custom linerization behavior for that block - right-click on it, select "Linear Analysis - Specify Linearization".
2) Use frestimate
Here are some links that might be useful: http://www.mathworks.com/help/toolbox/slcontrol/ug/bspvslo.html http://www.mathworks.com/help/toolbox/slcontrol/ug/bso7mos.html http://www.mathworks.com/help/toolbox/slcontrol/ug/br5s3tf.html
http://www.mathworks.com/discovery/linearization.html
These demos may also be helpful: http://www.mathworks.com/products/simcontrol/demos.html?file=/products/demos/shipping/slcontrol/scdblockspecification.html
http://www.mathworks.com/products/simcontrol/demos.html?file=/products/demos/shipping/slcontrol/scdenginepad.html
HTH. Arkadiy

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Bob White
Bob White 2011-6-3

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Thanks, this was very helpful. There is a PWM block (compares the input signal to a sawtooth waveform) that linearizes to zero. I will look into some of the ways you suggest to correct this.

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