FB41 and SFB41 don't work!!!!

Hi there. I have the following problem. I have a PLC station that is already utilized in an industry. I want to install a PID controller using the FB41. The problem is that whatever my efforts, the FB41 or SFB41 don't output a value in LMN slot. I run simulations on PLCSIM and it works fine. As soon as i put the necessary data in the PLC, the FB41 doesn't work. The code is the following:

OB35 Block (Just call the FC91 where the FB41 block is located):

FC91 Block:

My CPU is the 314, i use a CP343-1 and IM365 interface modules. Is there any chance i run out of memory? Because the PLC is already stressed. Thanks in advance.

Hello DrTroop;

I always find it fascinating when someone starts his/her thread with a claim that a standard and much-used function provided by a manufacturer (in this case Siemens, but I have seen such thread titles for products from many manufacturers) does not work.

At a quick glance of your code calling FB41, I can notice that your parameters Com_Rst, and Cycle (for starters) are not initiated. I can tell you that without these the closed loop cannot be initiated, or operate correctly.

The initialization routine of the FB 41 is active with the value of Com_Rst, so it is recommended to set it to 1 in OB 100, ans reset it to 0 in OB 35 (or even OB 1) so the FB 41 starts up properly.

The Cycle parameter provides the S7 controller the time limits for the closed-loop calculations, these must be the same as the cycle value of OB35.

The loop will start in Manual mode, you must force it to Auto mode if you want it to ptocess your dayta and calculations. Yep, that would be the Man_On parameter.

Here are a few documents that could help you out:

- A FAQ on FB 41/SFB 41 parameters:

What should you watch out for when calling and assigning parameters to the controller functions in function blocks (S)FB41, (S)FB42 and (S)FB43?

- A discussion on FB 41; you can search the forum for more posts from hdhosseini, Hamid has answered many questions on control loop design on the forum over the years

Help with Function FB41

- A tutorial document from Siemens on SFB 41, it has a very good example of how to launch ithe Control-loop Parametrization software from Step 7 (this application sets the basic startup parameters without your intervention).

http://w3.siemens.com/mcms/sce/de/fortbildungen/ausbildungsunterlagen/classic-module/tabcardseiten/Documents/weiterfuehrende-programmierung/EN_b03_controlengineering.pdf

Hope this helps,

Daniel Chartier