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STEP 7 FB41 continuous PID block-sample time issue

Created by: Pandiani at: 10/29/2006 2:51 PM (4 Replies)

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10/29/2006 2:51 PM	Rate (0)
Pandiani Bronze Member Joined: 10/31/2005 Last visit: 9/4/2024 Posts: 366 Rating: (31)	Hello to all, I have used FB41 PID block several times and never really had any major problems. However, some of its parameters I just copied from one application to another without really knowing why. I always placed FB41 in OB35 cyclic interrupt block and always I entered 100 ms as sampling time as input parameter to FB41. Somebody told me that it must be entered this value, because FB41 is called in OB35 and OB35 is called every 100 ms, so if different time is entered, then, time constants for integral and derivative part won't make any sense. I wonder if it is true. What would happen if I place PID block in OB35 which is called every 100 ms, but I enter T#1s, for sample time as input parameter to FB41? Can you explain me with little more details why is needed this parameter at all since it is continuous block? I know that PLC is digital device and that it is discrete in nature because of cyclic calls, but why it is up to me to enter sample time? Thank you very much, I'm looking forward to your replies.

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10/30/2006 4:35 AM	Rate (0)
Dec Gold Member Joined: 5/19/2006 Last visit: 10/23/2024 Posts: 549 Rating: (107)	You call PID FBin a constant cycle OB task (100ms OB35 or 1s OB32 etc). When you specify TI, TD, TM_LAG as TIME values the PID block must know the sampling time in order to calculate the manipulated value considering integrator / derivative coefficients. Dec

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10/30/2006 7:53 AM	Rate (0)
Pandiani Bronze Member Joined: 10/31/2005 Last visit: 9/4/2024 Posts: 366 Rating: (31)	Okay, thanks. So, the best way would be to set sample time parameter of PID block to OB35_EXC_FREQ variable, right?

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10/30/2006 8:47 AM	Rate (0)
Dec Gold Member Joined: 5/19/2006 Last visit: 10/23/2024 Posts: 549 Rating: (107)	Even better will be to call SFC6 RD_SINFO and get OB number and/or ZI3 TOP_SI(OB3x_EXC_FREQ) [:P] Dec P.S. This is how most of PCS7Library FBs figure out in what OBs are running
	Last edited by: Dec at: 10/30/2006 8:51 AM
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10/30/2006 9:47 AM	Rate (0)
Pandiani Bronze Member Joined: 10/31/2005 Last visit: 9/4/2024 Posts: 366 Rating: (31)	Hmm, interesting info Dec. I'm not really sure how to call SFC. For example, I placed function (SFC 6) in ladder network in OB35. I see that SFC function has three parameters: RET_VAL as integer, TOP_SI ans STARTUP_SI. Last two paramters are struct. Question is: should I first define Struct variable in symbols table and then use it to store values that is returned at TOP_SI output? I can create one temp variable with data type struct, then declare 8 variables (first 6 is Byte, one Word and one Dword), then I use this variable to store TOP_SI to... It works, but I wonder if this is the right way.
	Last edited by: Pandiani at: 10/30/2006 12:30 PM
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