Discrepancy when plotting step response of two identical transfer functions.

I generated an idtf model from time domain data using the System Identification Toolbox (SIT). The model is "tf2" seen below.

When I plot the step function of the model inside of MATLAB everything seems fine, the response matches what is seen in the SIT:

But if I use the coefficients from tf2 and create a transfer function using tf(num,den,Ts), then I get an entirely different response.

(No, it isn't the z^-1s, keep reading!)

Somehow, if I grab the numerator and denominator from tf2 then it works. Even though it is the EXACT same transfet function as " hz" plotted above.

As my last check I tried building an idtf model using the coefficients. But it doesn't even match the original tf2 step response, even though they are the EXACT SAME THING.

This makes no sense and I'd appreciate any help I can get on the matter.

[Torsten]

Maybe you don't use enough digits for the coefficients ?

Answer 1

[Star Strider]

We do not have the original data.

I suspect that you are seeing the loss of precision due to using only 4 significant digits of a 15-significant digit result.

.

[Benjamin]

That was correct, each display of the transfer function only had 4 significant digits when the underlying numbers being processed were much longer. Thank you!

[Star Strider]

As always, my pleasure!

In 'hz2', you did the experiment I would have done.