Hi dear experts ,

i want to program a PID controller and S7 1200 with PID compact  , i have some questions ,

first , why should we program and insert PID_Compact Block inside Cycle Interrupt OBs ? cant we do it inside OB1 ? why is it necessary ?!!

second , if i want to program PID inside OB3x (for ex :OB30) which Cycle time i have to choose? 1ms ? 100ms? ...

what should i do if i want  to program 20* PID controller inside a project ? how to adjust Cycle times to prevent Time Interference??? 

with best Wishes

Arian_A

Hi there,

regarding calling the PID block in a cyclic interrupt OB:

We are talking about controlling here, and for this you want the call cycle of the control function to be constant. With OB1 you usually do not have this, due to OB1 having the lowest priority. With a cyclic interrupt OB you can parametrize the call cycle of this OB, and therefor then have a constant call cycle of the code you call inside this OB. This is the benefit you have. Depending on your application and code, OB1 might also be suitable enough, but using a cyclic interrupt OB just ensures this in a better way.

Regarding the cycle time to choose:

You can find information on the processing time of the PID instructions in the manual, for PID Compact it is shown here: https://support.industry.siemens.com/cs/de/en/view/108210036/99858334091

I'd go with the approach "as short as needed, as long as possible", whatever this means in detail then depends on your application only.

Regards,
daviso

Hi Dear Daviso ,

thank you for answering my question,appreciate that

can i ask you about "how many Cycle OB and How much delay time i must consider to avoid "Heavy Pressure on CPU "  ? should i consider that at all? is it important

thank you ,

with best wishes

Arian-A

Hi there,

you stated something about 20 PIDs in total. Given the processing time from the manual this would result in 20 * 0.3ms = 6ms processing time of all the PID blocks. This would also be the absolute minimum call cycle of the cyclic interrupt OB in which you call the PIDs, best a little more.

I like to see things from a practical point of view, rather than just looking at the theory behind it. Therefor as I said, think about which call cycles of the PID functions your application needs, to follow the "as fast as needed, as slow as possible" approach. Perhaps you might then need to group some of the PIDs, if some of the PIDs need to be called in other (faster / slower) cycles as others; in this case you might considering to use more than one cyclic interrupt OB with different call cycles. Only then you need to consider a delay between the cyclic interrupt OBs.

But which values to use, how to parametrize this in detail...I cannot say. This depends on the application you want to control with the PIDs, the rest of the program, how you structured the rest of the program, etc. Either just try it out, or get application assistance from Siemens.

Regards,
daviso

thank you dear DAVISO

that was my answer i was looking for

appreciate that dude.

with best regards .

Arian Ahmadi

Hi dear CaOd ,

you're right , my Problem should be Clear , i'm working on it to get to the point need to process PID Block , and when i reached that point i will tell you ,thank you for your help dude.appreciate that

With best Wishes

Arian Ahmadi

@daviso please recheck the scan time of pid blocks. I've attached a screenshot for reference, it's below 200ɱs(micro sec) for 1200 plcs and around 50ɱs(micro sec) for 1500.
 

PID block cycle time for CPU

jees_antony

@daviso please recheck the scan time of pid blocks. I've attached a screenshot for reference, it's below 200ɱs(micro sec) for 1200 plcs and around 50ɱs(micro sec) for 1500.
 

PID block cycle time for CPU

 Yes, that's the manual I linked above last year.