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Old July 12th, 2008, 08:49 AM   #1
finskhan
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Fb41 Cont_c

Hi to all,anybody can tell me about the Dead band width input in FB41.What will efftect if i put 50.0 Dead Band Width.I am so confuse about Dead Band Width.
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Old July 12th, 2008, 09:02 AM   #2
finskhan
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and also tell me its necessory to call FB41 in OB1 or can i call this in any FC?
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Old July 12th, 2008, 10:25 AM   #3
dchartier
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Quote:
Originally Posted by finskhan
and also tell me its necessory to call FB41 in OB1 or can i call this in any FC?
Hello finskhan;
Who told you this? They must have had trouble with the PID loop they are trying to control.
To ensure stable and exact mathematical operations of the block, it has to be called regularly, and the block parameters have to know about the intervals. So you must call it in a cyclic OB, such as OB35, and the call interval of OB35 (see CPU properties) must be inserted in the parameter CYCLE of FB41.
Be careful what you do with the Deadband; in any control application the deadband is the area where you do not allow the controller to operate. A deadband of 1 or 2% is standard.
Say a temperature setpoint is at 20 degrees, so that your heater comes on at 20.1 degrees; say the temperature reading oscillates and suddenly you are reading 19.9 degrees; do you want to shutdown the heater immediately? and start immediately again, once the reading oscillations reach 20.1 degrees? This could lead to breakdown of the heating element and controllers. This is when you use a deadband, say of 1 degree. So once the heater is on (above 20 degrees) it will not turn off until temperature goes below 19 degrees. Less impact on the start/stop action of the heater.
In any case, the following link will take you to documentation for FB41:
http://support.automation.siemens.co...ew/en/16532187
Hope this helps,
Daniel Chartier

Last edited by dchartier; July 12th, 2008 at 10:42 AM.
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Old July 12th, 2008, 11:34 PM   #4
finskhan
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Hi,thanks for replying.I am sending my Used FB41 in which you can see what parameters are using?You can see here CYCLE input is Blank.and also programer is calling this FB in FC2.Actually this PID is using to control the pressure. Operator put the setpoint -50 then according to actual pressure and setpoint pressure Actuator open or Close.according to operator it is not working properly.
So please tell me about some paramert
TI is 30s whats is efect of this?
Dead Band Width is 50 effect oh this?
and why he put blank SYCLE and why he is using in FC2 instead of OB?
for these things is it not working properly?
Attached Files
File Type: zip FB41.zip (131.6 KB, 47 views)
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Old July 13th, 2008, 09:30 AM   #5
manmeetvirdi
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Hi there
As dchartier stated you must call FB41 from OB35.
Default time of OB35 is 100ms so set the cycle time 100ms.
What the previous person has done is wrong, he probably left the cycle time blank because FC is always called at diffrent time interval so he might have decided to leave it blank ;-)
Values for Gain , TI, TD depends upon your process, change there values and see if things are improving, you won't bust anything unless the process is very critical like controlling radiation pressure in Nuclear chamber ;-)
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Old July 13th, 2008, 11:46 AM   #6
dchartier
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Hello finskhan;

Some of your answers would require lnowledge of control theory to give you adequate answers. But hey, we're on PLCs.net, and we have great ressources available.
One special participant on this forum is Ron Beaufort; he is an excellent trainer on PLCs (the comments of his students are quite expansive, as you can see on his website) and he has devoted some time to explain in simple terms the different parameters constituting a PID loop.
You should have a look at some of his sample lessons on PID control, on his website:
http://www.ronbeaufort.com/sample_lessons.htm

This will give you a good understanding of the impact the Gain (P parameter in PID)and the TI (I parameter in PID) have in your program. You will see that your present Gain value is probably too small to control anything (unless you have a very sensitive process) and your long I value would probably make response sluggish. But do read these articles, they are simple and clear.
Then it would be worth your while to do a Search on thois forum for more threasds where he has participated, you would learn a lot.
Hope this helps,
Daniel Chartier
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Old July 13th, 2008, 11:55 PM   #7
finskhan
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Can i call FB41 in OB1?Because the purpose of calling a FB41 in OB's is that to execute the block in sequence of time.OB35 execute the block after each 100ms and OB1 will execute the block in 1sec.I think i can call it in OB1 or OB35 or any OBs that have a time to execute but the most importan thing is that We must put the CYCLE parameter that we are using OBs default time?And the 2nd parameter is TI that is a time of Reset the Integrator and i am using this 30sec i think that is also other mistake bc 30sec is too much.I think the TI is the time to update the result?
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Old July 14th, 2008, 07:20 AM   #8
PeterW
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NO NO NO...

What part of 'it has to be in OB35' can you not understand?

OB1 does not have a cycle time of 1 second, its cycle time is variable, depending on the time of the last scan, if its anywhere near 1 second then you have serious problems anyway. Depending on what you are doing it could be anywhere between 10mS and 50mS as normal.

FB41 NEEDS a consistent update sample time, that os why it MUST BE in OB35!! If 100mS is not correct for you, then you can change that.
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