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From: Glenn on 29 Apr 2010 07:14
Hi Denis,

This isn't a dumb question, it is a toal pain that you cant manipulate the Numerator & Denominator in a Transfer Function Block.

I had the same problem and posted a message on here (subject: "Making A Transfer Function Denominator Variable With Time") but no one could help me out. I took it as if what I was asking was impossible, and as such had to come up with a different solution.

So, basically all Transfer Functions can be built using an Integrator (1/sTn) and +,-,* & / blocks.
If you arrange the blocks correctly you can create a 1st Order Lag ("PT1") (1/(1+sTc), a Derviative ("DT1") (sTv/(1+sT1) and obvioulsy an Integrator ("INT") (1/sTn).

PT1 Block Arrangement:
* (Input - Integrator Output) / Tc = Integrator Input
* Integrator Output = PT1 response.
* So you now have a PT1 Transfer Function (1/(1+sTc)) with a Time Constant (Tc) that you are able to vary.

DT1 Block Arrangement:
* ({[Tv / T1] * Input} - Integrator Output) / T1 = Integrator Input
* ({[Tv / T1] * Input} - Integrator Output) = DT1 Response.
* So you now have a DT1 Transfer Function (sTv/(1+sT1)) with a Derivative Time Constant (Tv) and 1st Order Time Constant (T1) that you are able to vary.

INT Block Arrangement:
* Input / Tn = Integrator Input
* Integrator Output = INT Response.
* So you now have an INT Transfer Function (1/(sTn)) with an Integration Time Constant (Tn) that you are able to vary.

All I did was to arrange the blocks and then make a Subsystem out of them.

Hope that this has helped you,
Glenn (xavexstein(a)yahoo.com.au)
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