Initial condition of output with simulink

Hello,

I want to model this system as if it was running at steady state and then there is a step change. The value CB has a steady state value that it is running at, but I don't know how to get this. Right now when I run it, CB starts at zero and then responds to the step change, but I want it to start at 1.117. I tried using an IC block but I can't figure out where the two arrows would go if this is the correct block to use. Any ideas? Thanks

Answer 1

[Star Strider]

That may only be an option with the Control Systems Toolbox, and for state-space systems. From the documentation on LTI Systems:

• Initial states (state-space only)
• If your model is in state-space format, you can specify the initial states in vector format. The default is zero for all states.

So it’s not an option for a transfer function.

[Rick]

Okay, then I guess I should just assume it starts at zero.

[Star Strider]

That’s generally the situation (or assumption) in control systems. If you want to transform it to state space, you can specify non-zero initial conditions.

[Rick]

Thanks, I thought about making a constant block set equal to the steady state value. What would be wrong with doing that?

[Star Strider]

My pleasure.

With a transfer function, you’re dealing with Laplace transformed ODEs, and initial conditions of ODEs work differently with them. So I’m not sure that would work.

If you have the Signal Processing Toolbox, use the tf2ss function to convert it to state space. (Or you can do the calculation on paper and with MATLAB, since it’s not a difficult problem, if you have access to a good linear control textbook such as Chen, ISBN 0-19-511777-8.)