Simulating a Digital Filter using Z-Transform

[Pete.Dunc]

Good evening!

I have some sampled data logged in text (.lvm) format which I would like to perform off-line digital filtering to.

I have a Z-transform of the form ( 1 + a * z^-1 )/( 1 + b * z^-1 ) to describe the behaviour for the given sampling rate.

I will use the "Read from Measurement File" to import to LabVIEW as dynamic data, I can then arrange that in an array if need be.  I am just unsure of how to implement the z-transform; are there any examples performing a similar function?

I apologise for the "can you help me?" type post but I have struggled to find information on building this.

Any help/references appreciated!

Peter 

[Rodrigo_Cuenca]

I'm assuming that you don't have the Control Design and Simulation Module.

You can calculate the difference equation, and implement it with shift registers.

[Pete.Dunc]

Thank you for the quick reply.

I do have access to that module and have located the H(z) "Discrete Transfer Function" element.

I have looked at some help files / examples and it would appear that all the simulation elements must lie within the Simulation Loop.  Can I pass an array into this loop to use as the input? 

I attach the VI I am using to obtain an array of my input data.

Regards,

Peter

[Rodrigo_Cuenca]

You don't need the simulation VIs, you need the Control VIs, try something like this.

Your file already gives you an array so you don't need the For loop.

[Pete.Dunc]

Could you possibly take a screenshot and post?  Unfortunately I am running LabVIEW 8.5.

Peter

[Coq_Rouge]

By the way we are talking about the Z plan and not the S plane? Because if you have the "NI LabVIEW Digital Filter Design Toolkit" you can specify poles and Zeros in the Z plane and hence simulate your filter

[Rodrigo_Cuenca]

I don't know the versions of your toolkits, but here is the vi in Labview 8.5, and also a screenshot

[Pete.Dunc]

I used the "NI LabVIEW Digital Filter Design Toolkit" and the difference equation method to create my filter.  But I am getting some odd results when I apply my input data to the filter.

I have made some notes on the VI to what I am trying to do.

Basically trying to reduce the response time of a first-order system using filtering; the filter will cancel out the previous time constant and replace it with a new one.  I have pasted the bode plot of the system into my VI as there is no Flash on my computer to upload on here.

I am still trying to get my head around what is happening - the noise content of my real input signal could be having an adverse effect.

[ edit : attaching the VI ] 

Peter

Message Edited by PjayD on 04-14-2010 09:38 AM

[Pete.Dunc]

Ignore the above post - I realised the 'strange behaviour was the filter adjusting to the initial steady steady state.  Putting a longer steady state at the beginning of my sample data let the filter settle before trying to compensate for a change.

In real life the system would run continuously so the large error would only occur when the system first started up.

Now I just need to reduce the noise on my signal to allow for greater amplification; the more a try to speed the system up, the more the noise is amplified.

Peter