Re: Getting the process tranfer function

More easy way fo TF calculation should be a software that could do by
itself without too many problems

PiControl Solutions www.picontrolsolutions.com offer such solutions
for TF identification PITOPS-TFI . Just connect the OPC with the
existing data historian or the Database where the PID tags are stored.
and the software will do the rest. It will give you the TF as well as
full graphics. Data can be alos taken in Excel format as Plant data
input file

PITOPS TFI offfer special new release that can calculate the very
precise TF by new methods,
The Geometric method is the old method provided in the old Pitops
TFI. Use this first and if this does not give good results, then try
the Gradient method for first and second order transfer functions or
the Gravity method for zero-order
(ramp) transfer functions.

Click on the 3G Plots to see the new screens.

When using Gradient method, Pitops TFI calculates first order
differencing
on the data to eliminate drift and disturbances. The identifier tries
to
fit the gradient CV with the gradient model shown on the 3G plot
screen.

When using Gravity method, Pitops TFI calculates second order
differencing
on the data to further eliminate drift and disturbances. The
identifier
tries to fit the gravity CV with the gravity model shown on the 3G
plot
screen.

For ramps, Gravity method should be used, not Gradient. For first and
second order, Gravity method could also be used in addition to
Gradient
method. When drift and disturbance level is high, Gravity method will
work
better since it rejects disturbances more that Gradient or Geometric
methods.

The new Pitops TFI automatically adds filtering on the raw CV (plant
data).
You can see the Filter value options - None, Weak... etc. Pitops TFI
automatically sets filtering but you can manually adjust the filtering
also
by looking at the Gradient and Gravity trends.

If using Gradient method, and if Gradient CV still looks noisy, then
increase filtering more manually to get better results.

If using Gravity method, and if Gravity CV still looks noisy, then
increase
filtering more manually to get better results.


Regards

Manmeet manmeet@xxxxxxxxxxxxxxxxxxx



On Sep 7, 2:17 am, pnacht...@xxxxxxxxx wrote:
On Sep 6, 2:41 am, "JCH" <ja...@xxxxxxxxxxxxxxxxxxx> wrote:> For calculating/simulating acontrolloop one needs acontroltransfer
function. This can be generally found by evaluating mathematically a step
response as SIEMENS does, too:

https://publications.european-patent-office.org/PublicationServer/doc...

SIEMENS uses a semi-analytical method (obviously patent pending) whereas I
use a 100% approximation method that is much more accurate.

The basis is just using 2 'representative' points for finding the
differential equation. These 2 points can be found best via least-square
polynomial approximation with appropriate number of measured points.

I don't know how you get a transfer function or coefficients for
differential equations with just two point either. Usually I use 500
to 2000 points. The link below shows the results of system
identification of my hydralic system back in the shop

ftp://ftp.deltamotion.com/public/NG/Mathcad%20-%20Sysid2A2BV70%20T02.pdf

One can see the time intervales are in milliseconds so there are about
1600 points used to calculate the gain, damping, factor and null
offset C.



Are there other methods known as far as more accuracy is concerned?

Yes, the optim and lsqrsolve in Scilab work pretty well depending on
the problem.

Peter Nachtwey


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