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[SOLVED] Problem with PID loop

Created by: FBettosini at: 8/15/2006 3:23 PM (8 Replies)

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8/15/2006 3:23 PM	Rate (0)
FBettosini Posts: 24 Rating: (0)	I am trying toset upa PID loop in microwin ver. 4.03 with a CPU 224 XP. I will use the PID loop to manipulate a three way valve (0-10V)for the cooling of an air conditioning system. I only want this valve to work if thecurrent temperature ishigher than the set point. Here are the values I have entered to begin: Loop Setpoint Scaling Low: 0 High: 50 Loop Parameters Gain: 1 Sample Time: 1 Integral Time: 9999999 Derivative Time: 0 Loop Input Options Scaling: Unipolar Low: 0 High: 50 Loop Output Options Output Type: Analog Scaling: Unipolar Low: 0 High: 32000 In the programm I call the subroutine generated by the wizard when temperature is higher than set point with the following parameters: PV_I - Temperature measured Set point - Set point temperature Output - Analog output to the valve As outcome the PID loop is giving 0. What am I doing wrongly? I will thank suggestions and examples. Thanks in advance Fernando
FBettosini Posts: 24 Rating: (0)	Last edited by: FBettosini at: 8/21/2006 3:10 PM
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8/15/2006 4:14 PM	Rate (0)
Wizard Diamond Member Joined: 4/24/2006 Last visit: 11/6/2024 Posts: 8768 Rating: (1170)	Hi Fernando, Have you tried the following: Gain : -1 Best regards, Wizard

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8/15/2006 5:18 PM	Rate (0)
FBettosini Posts: 24 Rating: (0)	Yes, you are right it is wrong, but there has to be another problem because the result is the same, the PID loop is giving 0. How and whenshould I call the PID loop ? Regards Fernando.
FBettosini Posts: 24 Rating: (0)
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8/16/2006 9:23 AM	Rate (0)
Wizard Diamond Member Joined: 4/24/2006 Last visit: 11/6/2024 Posts: 8768 Rating: (1170)	Hello Fernando, If you have used the instruction Wizard, there are two blocks generated. (PID0_INIT and PID_EXE) In the MAIN program block you place a call to the subroutine PID0_INIT. Use SM0.0 to call this subroutine every scan cycle. This subroutine enables the Interrupt Routine PID_EXE, which will cyclically execute the PID function. Hope this helps. Best regards, Wizard

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8/16/2006 9:25 AM	Rate (0)
Helmut_13 Posts: 2930 Rating: (432)	Hi Fernando, if you use the wizard for PID-Loops then the POU named PIDx_INIT must be called in each scan. If you want to control it "lateer" or event triggered (as you explained) then you can't use the wizard. Sollution: 1. Program the PID-Loop by "hand" (See Tip no. 54 on the second CD of MW) 2. Maybe the manual/automatic mode can be used. Enable the automatic mode only if you temperature is higher. This is just an idea but maybe it helps. Regards Helmut_13
Helmut_13 Posts: 2930 Rating: (432)
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8/16/2006 2:22 PM	Rate (0)
FBettosini Posts: 24 Rating: (0)	Hi, the problem is that the PLC is controlling an air conditioning system that has a functioning schedule and besides controls the temperature based on a wanted temperature. And that is the problem, because in one moment it can heaten and in the other itcan cool. Therefore I have to execute the PID loop manually and not with SM0.0 I have no experience in programming a PID loop manually and Tip 54 is not clear for me. Could you have any another suggestion for me? Thanks
FBettosini Posts: 24 Rating: (0)
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8/16/2006 3:25 PM	Rate (0)
Wizard Diamond Member Joined: 4/24/2006 Last visit: 11/6/2024 Posts: 8768 Rating: (1170)	Hello Fernando, I think you should try to use the manual option (like Helmut_13 also mentioned). You'll have toselect the option in the Wizard. When your temperature is below a certain setpoint, you can put the PID in manual mode. In that case the PID outputs a manual value, given at the input. This can be 0. And when your temperature gets higher, you put the PID in automatic. Best regards, Wizard
	Last edited by: Wizard at: 8/16/2006 3:27 PM
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8/21/2006 3:10 PM	Rate (0)
FBettosini Posts: 24 Rating: (0)	Hi, Thanks, in this way it is working properly Regards Fernando
FBettosini Posts: 24 Rating: (0)
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8/21/2006 3:16 PM	Rate (0)
Wizard Diamond Member Joined: 4/24/2006 Last visit: 11/6/2024 Posts: 8768 Rating: (1170)	Hi Fernando, I'm glad it's working Thank You for the feed back Best regards, Wizard

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