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Joshep_kerr

Discrete Transfer Function Problem

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Hi guys,

 

I'm having some problems with the discrete Transfer Function block. I'm trying to recreate the TF behavior I'm getting from Matlab, but I'm nowhere near. I have tried using both the Polynomial and the Poles and Zeros option to enter the values, but the output remains the same. 

 

 z^4 - 3.753 z^3 + 5.272 z^2 - 3.286 z + 0.7666 / 4.421e-07 z^4 - 4.333e-07 z^3

 

I've attached my Embed file, that contains the Reference and the TF blocks. I've also included a couple of screenshots from Matlab that show the behavior I'd like to obtain from Embed.

I've made sure that Embed and Matlab have the same Time Step, which you'll be able to see from the reference input (it's the same on both softwares). I don't know what else to do, I really hope you can help me out.

 

Bezier_test.vsm

 

 

 

Bezier_reference.jpg

Matlab_diagram.JPG

TF_behavior.jpg

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Hi Joshep,

 

You need to be careful when using discrete transfer functions.  The values to the right of the decimal are very important, if you truncate them you can get significantly different results. 

 

I applied the "reg" signal created from the two "Bezier" blocks to three transfer functions; 
1.  The transfer function typed into this correspondance (red)
2.  The transfer function from your .vsm model executed on a 64 bit computer (blue)
3.  Same as 2. but executed on a 32 bit computer (green)

On the traces below, you'll see how important it is not to truncate coefficients by comparing the red response with either the blue or green.  The blue response is the correct response.  If you had codegen'd the transfer function and run that code on a 32 bit microcontroller, you'd get the green response which clearly has some noise problems.  

 

I've also attached the model used to generate these responses. 

Ric

 

ComparisonPlots.vsmimage.thumb.png.119c27acd690b2d8e4b414e69e5f23f7.png

Sreeram Mohan likes this

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