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PID bumpless switchover

Created by: redis at: 1/6/2010 8:47 AM (7 Replies)

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1/6/2010 8:47 AM	Rate (0)
redis Regular Member Joined: 3/13/2009 Last visit: 8/22/2024 Posts: 4 Rating: (0)	If I switch PID mode from manual to auto then output bump is made (is dependent how big manual output is). How can I make bumpless switchover. I tried it by the help (written to the loop table entry for Mn) and bump was made.

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1/6/2010 8:39 PM	Rate (0)
dchartier Grandmaster Joined: 9/27/2006 Last visit: 1/2/2026 Posts: 12479 Rating: (2722)	Hello redis; The following information is an extract from the S7-200 System Manual, in the S7-200 Instruction Set chapter, under PID loop instructions. I strongly suggest you keep it close to you when programming the S7-200 PLC, it is the best reference you could have: http://support.automation.siemens.com/WW/view/en/1109582 There is no built-in mode control for S7-200 PID loops. The PID calculation is performed only when power flows to the PID box. Therefore, “automatic” or “auto” mode exists when the PID calculation is performed cyclically. “Manual” mode exists when the PID calculation is not performed.The PID instruction has a power-flow history bit, similar to a counter instruction. The instruction uses this history bit to detect a 0-to-1 power-flow transition. When the power-flow transition is detected, it will cause the instruction to perform a series of actions to provide a bumpless change from manual control to auto control. In order for change to auto mode control to be bumpless, the value of the output as set by the manual control must be supplied as an input to the PID instruction (written to the loop table entry for Mn) before switching to auto control. The PID instruction performs the following actions to values in the loop table to ensure a bumpless change from manual to auto control when a 0-to-1 power-flow transition is detected:- Sets setpoint (SPn) = process variable (PVn)- Sets old process variable (PVn--1) = process variable (PVn)- Sets bias (MX) = output value (Mn) The default state of the PID history bits is “set” and that state is established at startup and on every STOP-to-RUN mode transition of the controller. If power flows to the PID box the first time that it is executed after entering RUN mode, then no power-flow transition is detected and the bumpless mode change actions are not performed. Hope this helps, Daniel Chartier

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1/7/2010 7:35 AM	Rate (0)
redis Regular Member Joined: 3/13/2009 Last visit: 8/22/2024 Posts: 4 Rating: (0)	dchartier, thanks for reply. Of course I read S7-200 system manual (PID loop description) several times and tried it. But I maybe don't understand the definition which was underlined by you. Could you describe how can I do it. I don't know how to write the value of the output as set by the manual control as input to the PID (to the loop table entry for Mn).

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12/1/2010 3:53 PM	Rate (0)
Ganes10 Platinum Member Joined: 8/31/2010 Last visit: 12/6/2025 Posts: 1525 Rating: (231)	Mr. dchartier thanks for the info. Can you or even someone else pl explain it more in detail. Waiting for reply
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12/1/2010 8:40 PM	Rate (0)
dchartier Grandmaster Joined: 9/27/2006 Last visit: 1/2/2026 Posts: 12479 Rating: (2722)	Hello ganes; Please explain what you do not understand in the quote above. Let's review: 1) There is no internal mode control in the S7-200 PID loop instruction. If the instruction EN signal is active, you are in auto mode. When it is not active, you are in manual mode. 2) When you are in manual mode, you (the program) control the loop output directly through the same specified analog output that you use in your loop instruction. You are writing a value to that address. 3) At the moment when you decide to activate the PID loop instruction, in order to have a bumpless transfer, you are required to write this value (scaled to a REAL valuebetween 0.0 to 1.0) to the Mnentry in the Table used for the PID loop instruction (offset 8 from the start of the table address). Please read the chapter on the PID loop instruction from the top and clarify these details before you implement them. Hope this helps, Daniel Chartier

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12/3/2010 6:28 AM	Rate (0)
Ganes10 Platinum Member Joined: 8/31/2010 Last visit: 12/6/2025 Posts: 1525 Rating: (231)	dchartier 1) There is no internal mode control in the S7-200 PID loop instruction. If the instruction EN signal is active, you are in auto mode. When it is not active, you are in manual mode. There is a Auto_Manual (BOOL) Control Bit available in PID_INIT subroutine. See attachmentFig. 01 dchartier 3) At the moment when you decide to activate the PID loop instruction, in order to have a bumpless transfer, you are required to write this value (scaled to a REAL valuebetween 0.0 to 1.0) to the Mnentry in the Table used for the PID loop instruction (offset 8 from the start of the table address). In attachment Fig. 02 can you please tell this point (3) in terms of the parameters in the PID0_INIT subroutine call. I am getting the point which you are telling, but not sure how to implement it in the programming. Is it that any additional netowrk should be used to write the value to Mn ? Attachment shot.zip (258 Downloads)
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12/3/2010 7:18 AM	Rate (0)
SelvakumaaR Bronze Member Joined: 6/4/2010 Last visit: 10/14/2025 Posts: 72 Rating: (0)	Always use SM0.0 to call that subroutine from Main.
	Regards, Selvakumar Don't want to see any Software like dotNetframework and TIA Portal in future.
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12/3/2010 7:42 AM	Rate (0)
Ganes10 Platinum Member Joined: 8/31/2010 Last visit: 12/6/2025 Posts: 1525 Rating: (231)	selvkumaar Always use SM0.0 to call that subroutine from Main. SM 0.0 is OK, how to proceed further to achieve PID bumpless switchover
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