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PID Not Returning To Manual Value

Created by: Mike_TR at: 11/25/2009 11:01 PM (2 Replies)

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11/25/2009 11:01 PM	Rate (0)
Mike_TR Posts: 2 Rating: (0)	Hello, Using Step 7 and 317-2 CPU I seem to be having an issue with the PID controls where I have the PID set on an interrupt OB35 every 80ms. When the PID is placed in manual mode the output is reduced and finally returned to a value of a certain percentage of the original value before entering manual mode. When exiting manual mode with smooth transistion enabled the percent return is applied and for most cases it can return fine in 1 PID cycle or 80ms. For other cases the PID requires 2 PID cycles or 160ms and for one case which all preform the reduction in the same way will require 3 PID cycles of 240ms to properly return. If the proper time amount is not give the PID upon returning to automatic mode will load the value before the percent return was calculated and moved into the PID MAN. If anyone has any ideas as to why this is happening any help would be appreciated. To summarize the events Current Output Loaded into PID MAN = x PID Switched Into Manual Reductions applied to PID Manual = x - y Manual Condition have been corrected PID set to 80% of previous output value and held in for 160ms = 0.8 * x PID set to automatic PID loads value of last manual reduction before 80% value is applied, but PID MAN is still set to proper 80% value. Its almost like the PID disregarded the PID Man value of 80%, but requires some extra cycles to accept that it is not an error. I have a per scan log I can upload if anyone has had simillar issues and needs more clarification. Also the PID waits for the 80ms interrupt to return to automatic mode after the timer so the return to automatic is not happening during interrupt periods. Thanks Mike.
Mike_TR Posts: 2 Rating: (0)
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11/25/2009 11:24 PM	Rate (0)
fritz Diamond Expert Joined: 10/7/2005 Last visit: 3/22/2023 Posts: 2962 Rating: (1022)	Hello Mike_TR Can you pelaseupload your S7 project here (I understand your description but wouldn't have the faintest idea what to advise without being able tocheckthe logic implemetation of it)?
	Cheers fritz The advice and opinion given in this thread is that of the author and does not necessarily reflect the views of I S Systems Pty Limited. To find out how I S Systems can help you with your automation needs please visit www.issystems.com.au.
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12/2/2009 10:45 PM	Rate (0)
Mike_TR Posts: 2 Rating: (0)	I've made some progress on the issue. It seems that the PID Parameter MP7 calculates the manipulated value but however is lagged behind almost trending the actual PID Man value by 2 cycles. When the PID returns to automatic mode if it is not allowed 2 full cycles of the PID Man value being held constant it will return whatever value it currently has calcuated. Best I could come up with hope that helps someone in the future.
Mike_TR Posts: 2 Rating: (0)
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