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PID

Created by: Sindhu at: 6/6/2009 11:25 AM (6 Replies)

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6/6/2009 11:25 AM	Rate (0)
Sindhu Posts: 163 Rating: (0)	Im using FC105(SCALING ANALOG VALUE)in OB1,the output(real value,scaled to engineering value) is stored in the DB5.i have created DB5 for storing all thescaled values(Analog inputs scaling) Lets take an example A transmitter LT-809 is controlling the valve LY-809(LV-809). SCALING IN OB1:The scaled value is stored in DB5.DBD1(SHARED DB). PID IN OB35:The value DB5.DBD1 is connected to the PV_IN of thePID block(FB41,DB41). UNSCALING IN OB1:LMN is unscaled Q1.is it necessary that scaling,unscaling must be in OB35 as I find no reason doing it in OB1. Q2.How does DB5.DBD1 gets linked with DB41.DBD10 of FB41. Should I move the value of DB5.DB1to DB41.DBD10 in some OB.Pls elaborate.
Sindhu Posts: 163 Rating: (0)
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6/6/2009 12:16 PM	Rate (1)
daga Gold Member Joined: 3/18/2009 Last visit: 12/6/2024 Posts: 306 Rating: (94)	Sindhu@KEAS-CBE Q1.is it necessary that scaling,unscaling must be in OB35 as I find no reason doing it in OB1. As a good practice, yes. You should also scale the Input and output values in the OB where you call your PID (in this case OB35) Moreoever, you should aslo define a partial image partition (PIP) for these IOs in hardware configuration associated with the same OB. (again as good practice) Sindhu@KEAS-CBE Q2.How does DB5.DBD1 gets linked with DB41.DBD10 of FB41. Should I move the value of DB5.DB1to DB41.DBD10 in some OB.Pls elaborate. Firstly, You cannot have a real value starting at odd numbered addresses (DB5.DBD1 is absurd). And to assign your scaled analog input to your PID block, you simply put it as a parameter in the call as shown in attached pic. Attachment fb41_pvin.JPG (371 Downloads)

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6/6/2009 1:21 PM	Rate (0)
Sindhu Posts: 163 Rating: (0)	Though i took DB5.DBD1 for example,it is absurd as you said. I have done exactly what you have shown but i got confused how that same vale(scaled value)willhave two refrences i.e in shared data block and instance data block of PID
Sindhu Posts: 163 Rating: (0)
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6/8/2009 9:25 AM	Rate (0)
Sindhu Posts: 163 Rating: (0)	in FB41 Block,the parameter PV_IN is set to DB5.DBD0(SHARED DB)where the scaled value of LT703 is stored.In order to connect the parameters(SP_INT,Manual On,P Sel,I Sel),should i create another DB for each PID block used.If yes then whats the use of insatnce DB of FB41 block doing.Can you pls send a program using FC105,PID(FB41),FC106 for the transmitter LT_603 for the limits 0-500mm.I chked out in the sample program,I din understand
Sindhu Posts: 163 Rating: (0)
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6/8/2009 8:35 PM	Rate (0)
daga Gold Member Joined: 3/18/2009 Last visit: 12/6/2024 Posts: 306 Rating: (94)	Sindhu@KEAS-CBE In order to connect the parameters(SP_INT,Manual On,P Sel,I Sel),should i create another DB for each PID block used. Connect it to what? I believe you want to create tags, so that you can manipulate these values from you HMI. In that case, you can directly provide the instance DB address in your tag definition. Or as you suggested you can create a shared DB (with tags in HMI) and connect it in the FB calls.(even I prefer this in most cases) In any case I would suggest that you use "multi instance" capability and create a single DB for all your PID blocks. This way you can save some DBs (which will come handy if you have a CPU with less number of DBs available) Sindhu@KEAS-CBE .......whats the use of insatnce DB of FB41 block doing. Well, PID algorithm needs to "remember" values and hence it requires an instance DB (or let's say temporary memory of an FC is not helpful) Sindhu@KEAS-CBE Can you pls send a program using FC105,PID(FB41),FC106 for the transmitter LT_603 for the limits 0-500mm.I chked out in the sample program,I din understand Sorry I cannot, may be someone else can...

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6/14/2009 7:38 AM	Rate (0)
Sindhu Posts: 163 Rating: (0)	Hello All! If the initial values are not downloaded in the CPU,then should i always give the value for high limit and low limit in data Blocks Attachment DB-INITIAL VALUE.zip (408 Downloads)
Sindhu Posts: 163 Rating: (0)
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6/15/2009 1:01 AM	Rate (0)
H-H Platinum Member Joined: 7/2/2008 Last visit: 8/7/2025 Posts: 928 Rating: (309)	Hello Sindhu, I think you are missing something here. I copied the following from this FAQ. The only value of significance to the CPU is the value in the column headed "Actual value". Even if the initial value is also loaded onto the CPU and can be monitored there in online mode, this value is not used by the CPU. What you read in your attached picture doesn't mean that you can't download initial values to PLC. Just read the FAQ carefully and you will understand how to start PLC with initial values in data blocks. For shared data block, you have to switch to data view to modify actual values whil in instance data block, the actual values can be modified in declaration view. Also check this one. you may find it useful quite soon. Best regards. H-H

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