Why is the error of the close loop PID max of 1? File is provided.. please help

Question

Nitesh 2015-9-7

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回答： Arkadiy Turevskiy 2015-9-11

pid_simulink.mdl

Why is the error of the PID close loop equal to the step input?? I would like to perform iteration update using ILC method which will reduce the overall settling time and other performance index. but i need to compare that the maximum error for my system is decreasing. If i use RMS of the iteration for a set of values, the values decrease. But for max error, it is always same as input step.

Please help. I have attached the file as a sample of what i need from the error signal After simulation,using the following command, i will get a plot of the error.

figure(1)
  plot(e)
e_max=max(abs(e))

the plot for 1st iteration shows it has 1 as max error which the command above will display...

the image below is for my RMS values which shows it is decreasing for 15 updates

but since for all my iteration, the max error is 1, i get the plot below for 15 iteration.

Any suggest on how can i obtain so realistic values of the error to see a decreasing values of error?? Absolute tracking error method?? any help or suggestion??

Thank you

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Answer 1

Arkadiy Turevskiy 2015-9-11

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The max error is always one because at time 0, when you command the reference to step from 0 to 1, your plant model is initialized at 0. So, no matter how you change the controller gains, the initial error will always stay 1 - this is your initial condition.