Using System Identification Toolbox transfer function with Simulink

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I believe I am doing something fundamentally wrong when trying to import a transfer function created within the System Identification Toolbox (SIT) into Simulink.
To give a simple example of what I am doing. I have an input which is a sinusoidal wave from 12 seconds to 25 seconds with an amplitude of 1 and a frequency of 1.5rad/s which gives a measured output as seen in the following figure:
I have used SIT to create a simple 2 pole 1 zero transfer function which gives the following agreement:
I have then tried to import this transfer function into Simulink for investigation in the following configuration which has a sinusoidal input of frequency 1.5rad/s and a starting t=12. The LTI system block refers to the transfer function variable within the workspace:
When I run this simulation for 13 seconds the input to the block is as expected:
But the post transfer function signal shows little agreement with what would be expected and is an order of magnitude out:
Could someone give any insight in to where I am going wrong. I have a basic grasp of control theory but am struggling to make sense of this.
Much appreciated,
Jim

Answers (1)

Arkadiy Turevskiy
Arkadiy Turevskiy on 14 Mar 2014
The issue is the initial conditions. When you estimate the transfer function, the tool automatically estimates initial conditions too and uses them to create comparison plots you see in System Identification Toolbox.
When you then bring identified transfer function into Simulink, Simulink model uses zero initial conditions (by default, as you did not specify anything else). So what you need to do is specify initial conditions. As you can do it for a state space system, but not for a transfer function, you need to convert your system into state space form first.
Please refer to this answer and follow instruction in the response provided to that answer.

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