I am trying to solve a problem in simulink of the form u' = a + bu + cu^n. ANy suggestions? I'm still new to this, but this has me stuck…
MATLAB: Transient Problem in Simulink
nonlinearodesimulink
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Two remarks here:
1. If you draw the Bode plot of the transfer function of the derivator, it looks like a line that ramps up with 20 dB/decade. In other words, for high frequencies it amplifies the signal (most likely it is a noise in this region). So, what you want is to avoid this phenomena, however keeping the derivator behavior for lower frequencies. In order to achieve this, replace your derivative block with a transfer function block:
where you have to choose tau sufficiently low. Notice that if tau = 0, then you end up with the derivator H(s) = s.
2. You say something about using integrators. This is possible if you solve your equation the other way around, namely you know x and want to calculate y. The dynamics of y is described by:
dy/dt = (1/L) (2y-x)
For this, you need to know the initial condition y(0) of your system, together with x(t). You code this equation in matlab or Simulink using an integrator block.
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
These demos may also be helpful: http://www.mathworks.com/products/simcontrol/demos.html?file=/products/demos/shipping/slcontrol/scdblockspecification.html
HTH. Arkadiy
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