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PID FB41 resetting

Created by: Tom66 at: 10/12/2011 1:33 PM (1 Replies)

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10/12/2011 1:33 PM	Rate (0)
Tom66 Posts: 11 Rating: (0)	Hi, I am noticing that when I try to use COm_RST to reset the PID output to zero, it looks like it resets but when you remove the reset the PID picks up from the value it was at previously. How do I reset the PID block so that when it starts again it starts with a zero output. This is a very important function that the PID block must do. I tried disabling and enabling the PID block but that also just writes back in the previous output to the PID block I also notice that even though the PID block has a small gain and integral, the output sometimes jumps from 0-100% in a matter of a second. Thanks, T
Tom66 Posts: 11 Rating: (0)
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10/12/2011 7:02 PM	Rate (0)
hdhosseini Diamond Expert Joined: 1/28/2009 Last visit: 8/31/2025 Posts: 6874 Rating: (1368)	Dear Tom66 , FB41 has several inputs that effect the behaviour of the function. Back up of your project may help us to undrestand the whole situation better.The analysis provided is based on assumptions and need more details for clarification. Tom66 I am noticing that when I try to use COm_RST to reset the PID output to zero, it looks like it resets but when you remove the reset the PID picks up from the value it was at previously. This is a case you enable "MAN_ON" and then activate "COM_RST". Algorithm sets the MAN to "LMN" because of manual mode selection. When you reset by "COM_RST",LMN is changed to zero and if return to normal by putting "COM_RST" to FALSE, you have a same value in LMN. If the algorithm is in auto mode (i.e "MAN_ON"=FALSE),LMN may take different values before and after the rest.This maily depends on current error signal and history of loop(prior to reset the algorithm). Tom66 How do I reset the PID block so that when it starts again it starts with a zero output. This is a very important function that the PID block must do. I tried disabling and enabling the PID block but that also just writes back in the previous output to the PID block To do so, you can put the algorithm to manual and set manual value to zero.But please NOTE,when you are back to auto mode a sudden jump is inevitable if you have considerable error signal.This is normal due to P section in PID .To reduce the bad effect , you may use rate limiter function here or you can try simple trial function developed by me here(FB67). Tom66 . I also notice that even though the PID block has a small gain and integral, the output sometimes jumps from 0-100% in a matter of a second. As error signal defined as SP_INT - PV_IN (PV_PER=FALSE), physically speaking, they (set point and process value) should have same dimension and if possible same low and high limit.Check if your error signal changes suddenly and check what is the max value? Please also note,as we encountered here desabling P part (P_SEL=FALSE) and enable it again may cause jumps in LMN(see "5.disable and enable P part here") I hope this would be helpful.Ask in case of mismatch. Best regards hdhosseini
	Last edited by: hdhosseini at: 10/12/2011 7:55 PM Demo Channel on Youtube
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