No. of PID's which a PLC can handle

[manmeetvirdi]

Hi there
Can it be said as a rule of a thumb that:

No. of AI or AO which a particular PLC can handle = No. of PID's a PLC can handle

 

[dchartier]

Hello Manmeet;

What observations would you base this affirmation on?

Regards,
Daniel Chartier

 

[manmeetvirdi]

Hi Daniel
Its possible that all the AI are being used say for example to measure the temperature which are in turn to control the heaters through a PID. So it seems that number of AI which a PLC can handle should be equal to PID (FB41 and 43 for siemens)can handle without any problem.
Same thing can be said about AO to control the valves for example.
Its a indirect generalized question.
For me direct question would be how many PID a S7-400 can handle?
As I have an application wherein no of PID are 175.

 

[FrancisL]

I don't think there is simple answer, although some PLCs may have fixed limits, generally it is more a function of processing time - if all your 175 PID's are slow enough and are scheduled correctly then it may be no problem.
When I say slow enough I mean that they do not need to run on a fast cycle, most temperature control for example only needs to run every second or slower, it depends on the process lag.
By scheduled correctly I mean that you should design the software so that you do not try to run all the PID's in the same scan, but distribute them.

 

[Pandiani]

I personally don't see any strict correlation between n. of AI/AO and no. of PID controllers.
I've seen more advanced control loops implementation which uses a few AI (analog variables) and only one PID block. In such cases control deviation (input to controller) was pretty complex function of more analog variables.
Also, I've seen couple of implementations that use PID blocks and no AO at all when actuators are control valves. They use DOs for controlling (opening and closing) control valve.

 

[dchartier]

For me direct question would be how many PID a S7-400 can handle?
As I have an application wherein no of PID are 175.

This will depend on 2 parameters:
-work memory requiered to hold and process the 175 calls of the FB 41 and their associated IDBs;
-execution time of the 175 instances of the FB 41 in the cyclic OBs. Since the PID blocks must be called in a cyclic interrupt OB, you'l have to make sure that the execution time of your 175 calls in OB 35 does not cause a watchdog timer problem for the main cycle.

Hope this helps,
Daniel Chartier

 

[FrancisL]

Daniel is right, but one thing to add.
As I said about scheduled correctly -
Suppose you want the PID's to execute every second
Rather than call the PID FB 175 times in a 1 sec cyclic interrupt OB you should for example call the PID 18 times in a 0.1 sec cyclic interrupt OB, So, first time the 0.1 sec cyclic interrupt runs call 1 to 18, next time 19 to 36 etc.
Francis