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A little help - i am stuck!

Created by: thedodgy1 at: 1/8/2020 6:32 PM (6 Replies)

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1/8/2020 6:32 PM	Rate (0)
thedodgy1 Bronze Member Joined: 11/11/2017 Last visit: 12/10/2025 Posts: 192 Rating: (4)	i have a PI controller that is controlling the temperature very nicely but in some instances i require the output of the PI controller to give no more than 500 (5v) and this can be adjustable if need be but i need it to still control the PI but just limit the analog output. i know i can give it a manual figure in the Mq parameter but this just sets the output to that and does not still control the PI, i have tried to use the max/min block but i am at a loss on how to achieve this. any ideas please. Thanks

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1/8/2020 6:40 PM	Rate (0)
ixo65 Grandmaster Joined: 12/18/2014 Last visit: 1/20/2026 Posts: 36494 Rating: (4552)	If you want to limit the value that the PI controller calculates to 500, you bring the PI controller out of cycle. He calculates e.g. the output value of 1000. If this is not passed on, the controller tries to readjust. I just wanted to note that. If you want, I can give you an example.

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1/8/2020 7:09 PM	Rate (0)
thedodgy1 Bronze Member Joined: 11/11/2017 Last visit: 12/10/2025 Posts: 192 Rating: (4)	but the controller (PI) would still do its thing? i am experiementing with burner control but i want to have what i call "ECO" mode so it certain circumstance the air within a cabin is just recirculated and if i have the PI control the temperature can sometimes creep 10c above the setpoint due to the air just been recirculated so i have an idea if i can limit the output to say 500 (again this can be adjustable if need be) then i can try to keep a stable temperature within reason and when the "eco" mode is off then everything back to normal.

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1/8/2020 7:16 PM	Rate (0)
ixo65 Grandmaster Joined: 12/18/2014 Last visit: 1/20/2026 Posts: 36494 Rating: (4552)	The PI controller reaches 501 in the output. Now you limit this value for the analog output to 500. Consequence: The actual value does not follow the value calculated by the PI controller and the PI controller increases its value even more because the actual value does not follow what the PI controller expects. Ultimately, the PI controller will increase the value to 1000 and will never be able to output a value below 500 .... The dog bites its tail.

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1/8/2020 7:24 PM	Rate (0)
thedodgy1 Bronze Member Joined: 11/11/2017 Last visit: 12/10/2025 Posts: 192 Rating: (4)	i understand what you are saying but surley if i can just limit the output then the controller need not no, it just thinks its outputing 100% the actual temperature will not (hopefully) not devait that much from the setpoint.

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1/8/2020 7:36 PM	Rate (0)
ixo65 Grandmaster Joined: 12/18/2014 Last visit: 1/20/2026 Posts: 36494 Rating: (4552)	Then why don't you lset ower the setpoint in this case? I still gave you an example with the output limitation. Attachment PI_controller_limited_0BA8.Standard.zip (345 Downloads)

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1/8/2020 7:38 PM	Rate (0)
thedodgy1 Bronze Member Joined: 11/11/2017 Last visit: 12/10/2025 Posts: 192 Rating: (4)	thanks i will give it a go and to answer your queston the user would forget to put it back to the correct setpoint. if this does not work so well i will go back to the drawing board and think of another way of doing it. but thanks for the input and much appreciated.

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This contribution was helpful to 1 thankful Users ixo65

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