Discrete filter transfer function (Z domain) time response not matching with analog filter transfer function (S domain) time response

Objective: To create simulink model for discrete low paas filter

Inputs: 1. Frequency response plot (Bode Plot) 2. Time domain response (Input output graphs) 3. Sampling Time = 0.01 sec

Process followed to acquire transfer function from available inputs:

           1. Used SISO tool in control system toolbox, derived following S domain transfer function(sysc)

   (23.92 s^4 + 436.3008 s^3 + 2776.82496 s^2 + 5137.08312 s + 2872.93552)
---------------------------------------------------------------------------(s^5 + 43.28 s^4 + 673.4475 s^3 + 3168.310416 s^2 + 5369.704295 s +2849.712516)

           2. Bode Plot and time domain response for above TF is as per expected results

           3. Used following function in command window to convert above S domain TF into Z domain TF    

                        sysd = c2d(sysc, 0.01, 'tustin')

           4. Above command gave below Z domain TF(sysd)

(0.1061 z^5 - 0.2994 z^4 + 0.1757 z^3 + 0.211 z^2 - 0.2187 z + 0.08839) --------------------------------------------------------------------------- (z^5 - 4.593 z^4 + 8.429 z^3 - 7.729 z^2 + 3.541 z - 0.6485)

            5. Implemented above TF in simulink model

Problem: Even though, Bode plots for Z domain TF and S domain TF are exactly matching, Time domain response of Z domain TF simulink model is not matching at all with that of S domain TF simulink model. However step response of above Z domain TF is as per expectation when plotted in command window using step command.

Support required for: Why does Z domain TF give correct (as per expected) time domain results when plotted in command window using step command, but incorrect (not as per expected) time domain results when plotted through simulink model by giving step signal input.

Answer 1

[Azzi Abdelmalek]

Maybe you did not set correctly the step time and sample time

[Rupesh]

We have set the proper step time and sample time.Please check if there is anything wrong