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I would like to use the method of Zigler-Nichols method for open loop. I got the step reponse from Simulink scope but How can I calculate the L and A from the scope? enter image description here enter image description here

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    \$\begingroup\$ Your top graph called "OUT" looks remarkably like "in". \$\endgroup\$ – Andy aka Dec 4 '19 at 7:59
  • \$\begingroup\$ yes it is an IN. I took from a pdf file where it explains how to tune a PID in Open loop \$\endgroup\$ – Yaakov Dec 4 '19 at 8:07
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    \$\begingroup\$ Focus in on your bottom picture near the origin and project a line to the Y axis from the constant slope part of the response. It's hit and miss of course but nobody says Zigler-Nicholsis an exact science. And fix your question with the right name for the top graph. \$\endgroup\$ – Andy aka Dec 4 '19 at 8:22
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    \$\begingroup\$ Ziegler-Nichols is not very good, generally. \$\endgroup\$ – Chu Dec 4 '19 at 8:57
  • \$\begingroup\$ Yes, I just tried both methods open loop and closed loop Ziegler and it is unstable at all. I have a system with high order, which is best ? \$\endgroup\$ – Yaakov Dec 4 '19 at 9:06

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