How to implement the Transfer Function in Real Time VIs?

Question

Hi all,
I'm relatively new to Labview Real Time modoule and want to implement one Controller(not PID one) in Deterministic Loop! I have already designed discrete Transfer Function and searching for the way to build one Controller with it! Is it right to use the Simulation Loop instead of Timed Loop (in this case Deterministic loop) and implement the controller in it?! If it's correct so, should I use the same clock and Priod as Timed Loop?!
It would be very helpful, if there is an Example about using transfer function in Real Time Loops!
thanks for your help,
Mohsen 

Answer 1

Hello mhmdi,
Sorry I can't open your VI (looks like it is a new version of LV than I have installed).
You're right - you don't need to convert to a difference equation if you have the CD&SIM Module which can take discrete time transfer functions directly. You don't need shift registers with this function, as it is effectively done internally. If you don't have CD&SIM discrete TF's can be implemented easily just in a timed (or while) loop with feedback nodes or shift registers to replicate each z-1 you need.
Some more ideas:
There are many configuration parameters and options with the Discrete TF VI - which you need to understand for your application and make sure are correct. Sometimes implementing in a basic form (timed loop and shift registers) allows you to see what is happening without any confusing options you might not need.
Are you using the Discrete TF VI in a Simulation Loop ? You might need to think if the loop being used is appropriate for a Real Time application. Maybe the timing of the TF, the loop and the DAQmx data are not all suitable for each other.
 I'm not familar with DAQmx, so not sure about any specific real-time aspetcs of that.
"if the sample frequency of the discrete transfer function in the timed-loop is at multiple integers of the sampling rate (e.g. 12000 Hz for tranfer function and 4000 Hz for sampling rate, 12000=3*4000), would it somehow improve the resolution of the controller command?"
This could actually make things worse - but depends on your transfer function if it is a problem. Think about it this way - in the scenario you state the input signal going into the Discrete TF will only change once in every three samples, that means if your transfer function includes a 1-z^(-1) factor (i.e. a derivative) - that will be zero for two samples then jump up for the next sample. You'll observe a very jittery/noisy signal, but the noise is due to samples not being correct. This will also occur if the sample times are very similar but go in and out of sync where you may get the occassional sample that is the same and hence the occassional zero in the TF.
In an application I had we had problems trying to get the data coming into a timed loop and the timed loop itself synchronise, and before it was fixed the control signal was very jittery.
Consultant Control Engineer
www-isc-ltd.com