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M91 PID problem

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I am trying to set up a level controller on a M91-2-UA2 and I'm having problems with the output of the P part of the regulator(Kp wich is shown in SI120 for loop 1).

With the proportional band set to 100% the output should be the same as the error, but in my program this only works up to 50%, if the error is 51% or more the output is 100%.

Here is a list of error value and the output form the controller:

error        output

0%          0%

25%        25%

50%        50%

51%        100%

75%        100%

100%      100%

This is with booth I and D set to 0.

Is there a parameter that can set this kind of behavior?

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  • MVP 2023

The proportional band is acting like it's centered around the setpoint, with 50% on either side.  According the Help, your logic should be valid.  But it's not acting the way it should.  


Try setting the proportional band to 200%.


What are the values of your PV Hi and Lo limits?



Joe T.

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I have tried to simulate the program with the proportional band at 200%, this is my results:

error        output

0%          0%

50%        25%

100%      50%

101%     100%


With this setup I will never get the jump in output because the error will never bee 101%, but 200% proportional band is too slow for the process and the regulation will not work.


Here is a image form the documentation showing how the output of the proportional band should work and how mine is working. The red line is how it should work, mine works like the green line, which I have added to the image.


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