S7 PLC PID Controller

S

Thread Starter

shaju stephen

Dear Sirs,
We have a Siemens 315 2DP PLC used on Sheet Extruder to control the Barrel & Die zone temperatures (250 to 300 deg C). Total of 18 zones are controlled with FB41 Function Block in the user program.

During the process, all the Temperature Control loops in the PLC stops updating both the Inputs & Outputs and the CPU seems like "hanging".

Actual Temperature reading (PV) will be same as at the time of "hanging" without any change.

Outputs like "Heater On" and "Cooling Blower On" will maintain continously ON, if it was ON at the start of 'hanging".

After may be 10 minutes, CPU will start updating the Inputs and Outputs. By the time the for those zones whose heaters were ON, will have high temperature (+ 30C). For those zones whose blowers were ON will have less temperature.

There is no error indication on the CPU.

Please suggest a suitable remedy for this problem.

Thanks & regards

S Stephen
 
G

George Buckbee

PID controls are often mis-configured in PLCs. In looking at hundreds of installations, we have seen that well over 90% of PID blocks in PLCs are improperly configured, leading to confusing or inaccurate results.

In this case, I suspect that the code for the PID blocks are not being run at a high enough priority.

According to the Siemens manual: The calculation of the values in the control blocks is only correct if the block is called at regular intervals. For this reason, you should call the control blocks in a cyclic interrupt OB (OB30 to OB38). Enter the sampling time in the CYCLE parameter.

You can find more on-line. The above text is an excerpt from page 25-1 of: System Software for S7-300/400 System and Standard Functions

A5E00261410-01

http://www.kar.elf.stuba.sk/predmety/pkom/Literatura/s7sfc__b.pdf

For more info on PID Tuning and Performance Supervision, check out ExperTune's library at:

http://www.expertune.com/r2.asp?f=Alist <http://www.expertune.com/r2.asp?f=Alist&l=articles.html> &l=articles.html

-George

George Buckbee
<http://www.expertune.com/>
(262)369-7711 (office)
Plant Performance Supervision with PlantTriage http://www.expertune.com/PlantTriage.html
 
V

Vincent Meunier

Hi,

Perhaps 18 loop is too much, is the FB41 call in an interrupt OB?

What is the scan time of the calling OB?

--
Vincent Meunier
vincentmeunier @ free. fr
 
Sounds like a comm speed problem. Are you using profi DP for remote racks? How far are your racks from each other and the processor?
 
I think 18 loops is not too much, I made 80 loops FB41 on CPU315-2DP and it works well for 3 years...
 
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