TIPS for PID (FB PID_Compact)

Here are some tips about PID (FB PID_Compact):

1General Tips:
The instances DB of the PID_Compact are located in the path Technological Objects (not in Program blocks).

2If you want that the PID runs in automatic mode without automatic optimization, you need:
a. Set the checkbox “Use manual PID parameter setting“, in the Configuration, tab “Advanced settings”.
b. Set “3” to the variable “sRet.i_Mode” of the respective instance DB
c. Extra tips:
a. if you intend to Set the checkbox “Use manual PID parameter setting“, do it, but after that change the mouse focus before to close the Configuration windows. Else your check will be loose.
b. Follow I attached a minimal sample program
To work:
1 - Adapt it to your hardware.
2 - Download it
3 - Open the Watch Table PID and go online. Set the alread selectioned values.
d. Avoid overrange values to Process Values. Use Limit instruction for float point PV. Take a look at FAQ "How can you prevent your S7-1200 PID Compact controller from going inactive in the case of a PLC power cycle and invalid process values?" (http://support.automation.siemens.com/WW/view/en/46163827)

Sometimes, however, it is not possible connect the periphery directly to the PID. For the case of use the variable “Input” it is also necessary that the incoming values are float point valid values and inside of the input monitoring limits.In the follow posts there is a library witha version of the FAQ decrypted library to be used in this case.


3 Reverse operation:
It could be obtained with a check box (Invert PID controller output) or, like S7-200/300/400, with a negative value of Proportional Gain

4 PID goes to inative mode (e.g. after power on). Two possible causes:
4a PV or SP are invalid or out of range: Take a look at the FAQ:"How can you prevent your S7-1200 PIDCompactcontroller from going inactive in the case of a PLC power cycle and invalid process values?" (http://support.automation.siemens.com/WW/view/en/46163827)
4b Monitoring time fault:
How can you restart a PID controller stopped by sample time failure or disable the sample time monitoring?
See FAQ: http://support.automation.siemens.com/WW/view/en/42321471
4c There is a propriety in the PID, " Enable last mode after CPU restart ". If it was unchecked and the PID is active and and PLC is powered down , then the PID will stay inactive. For an automatic startup after power on, it should to be checked.

5 To access directly to PID parameter look at the instace DB.sRet:
P = r_Crtl_Gain
I = r_Ctrl_Ti
D = r_Ctrl_Td
etc.

6 When using PWM control, the PWM cycle is defined as a multiple of the PID runtime cycle. This value is defined in the intern (Instance DB) variable "sRet.r_Ctrl_Cycle"

7 The offitial documentation about the FB1130 (PID Compact) stay on the Online Help of STEP 7 Basic,and onthe product manuals (http://support.automation.siemens.com/BR/view/en/40808553 pg172andhttp://support.automation.siemens.com/BR/view/en/36932465 pg 649).

Other reference is the the getting start manual of STEP 7 Basic http://support.automation.siemens.com/BR/view/en/40263542). The chapter 4 has a step by step. Click on the link "info" to get the sample programs.

8 Sample program with simulation:
I the past I was post a sample programit here, but I delete it once now we have different versions of it (10.5 and 11). Now you can found then in the Tip /tf/WW/en/Posts/52322#253011it is possible to download a sample program that simulate a field reaction. For version 11, there is also two simple logics for tests propouses: set mode = 3 and PID automatic reset (i.e. automatic set o i=3), like discripted on the post of4/6/2011, 12:40 PM

9 Error codes that doesn't are in the online help:
If several errors are pending, the values of the error codes are displayed by means of binary addition. The display of error code 0000 0003, for example, indicates that the errors 0000 0001 and 0000 0002 are pending.

10 Reset:
If the PID Compact detects some fault in the controlled process (e.g. an invalid process value supplied), at security reasons, it goes to inactive mode. In this state the output of the PID stay not active. To bring back the PID to automatic operating mode it is necessary:
1 – Eliminate the fault cause.
2 – Make a change of the internal variable sRet.i_Mode from 0 to 3.
In the follows posts there is a library that performs an automatic change of sRet.iMode to bring back the PID to the automatic mode as soon as possible. Additionally, the block saves the error code and the last operating mode before the fault.

Naturally, a automatic reset of a PID could be dangerous and should be used with knowhow of the application (!!!).

Together with the libary, there is a picture that shows how both block should be interconnected.

11 Operation with standard digital outputs.
The TIP “Sample program: PID Lean PWM generator for standard IO” (/tf/WW/en/Posts/42960#top) has a sample program that emulate a PWM with standards timers and could be used with standard outputs (naturally with low speed).

12 Open source PID
For the case to adjust the PID to particular cases (for example, execution from OB1), there is a open source PID (that has no relation with this one) in the FAQ “
PID Lean”
/tf/WW/en/Posts/45886#top

13Tuning that let very long times or aborts:
Assimetrical processes and processes withvery longreaction times couldmake the tuning processtakes a very long time or abort with fault. A detailedexplanation about this and the respectives works arounds could be explained at FAQ http://support.automation.siemens.com/WW/view/en/66995021?Datakey=47071380

to perform a PID_Compatct pretuning, it should necessary to follow some conditions like operations mode and differencesbetween setpoint change and minimal/maximal limits. Ablock (as library)should be used to monitore this condition. The inputs comments indicates where these inputs should be connected. It be found at TIP https://www.automation.siemens.com/BR/tf/WW/en/Posts/91953#453791



14 Execution Time
One PID call takes approx. 2ms for be processed (2ms +/-1ms) (CPU 6ES7 21xxx30 0AB0)

gunther_ViskoTeepak

Hi thanks for the tips but it isn't real clear for me.

I wan't to run the PID in automatic mode without automatic optimization.
Therefor I have to set 3 to the variable "sReti_mode.
Must I do this with a move instruction?
The tag of my PID compact (%DB17) is PID_FC2159, so should the move instruction move number 3 to tag PID_FC2159.sReti_mode?

And what do you mean with "change the mouse focus before to close the configuration window".
I set the checkbox "use manual PID parameter setting" and then I open another configuration screen in the advanced settings (i.e. Output limit).
Now I close the configuration window.
When I reopen the configuration window the checkbox "use manual PID parameter setting" isn't set anymore andI doubt of these PID paramterers are downloaded to the PLC processor.

Thanks in advance.

The follow library performs an automatic change of sRet.iMode to bring back the PID to the automatic mode as soon as possible. Additionally, the block saves the error code and the last operating mode before the fault.

Naturally, a automatic reset of a pID could be dangerous and should be used with knowhow of the application.

Together there is a picture that shows how both block should be interconnected.