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The Ziegler-Nichols' method of PID controller tuning

Course: Process Control, NMBU. Dec 2017 - April 2018. The Ziegler-Nichols' method of PID controller tuning. By Finn Aakre Haugen, PhD, TechTeach (finnhaugen@hotmail.com). Aim of tuning: As fast control as possible, but with acceptable stability, i.e. ¼ decay ratio. 2.

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The Ziegler-Nichols' method of PID controller tuning

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  1. Course: Process Control, NMBU Dec 2017 - April 2018 The Ziegler-Nichols' method ofPID controller tuning By Finn Aakre Haugen, PhD, TechTeach (finnhaugen@hotmail.com) F. Haugen. Process Control. NMBU. 2018.

  2. Aim of tuning: As fast control as possible, but with acceptable stability, i.e. ¼ decay ratio 2 Disturbance step: A2/A1 = 1/4 Response in controlled process output: F. Haugen. Process Control. NMBU. 2018.

  3. 3 Experimental setup in Ziegler-Nichols’ method: F. Haugen. Process Control. NMBU. 2018.

  4. Tuning procedure of Ziegler-Nichols’ method: 4 With controller in manual mode: Bring the process to the specifiedoperating point by manually adjusting the control variableuntil the process variable is approx. equal to setpoint. Turn PID controller into P controller with gain Kp = 0. (Ti = very large. Td = 0.) Setthecontroller to automatic mode. Increase gain Kp (you may start with Kp = 1) until there are steady oscillationsin the loop due to a small setpoint step.This controller gain value is the ultimate gain, Kpu.Read off theperiod, Pu, oftheoscillations. Calculate controller parameters from the Z-N formulas: Recommending:"Relaxed ZN PI-tuning"(cf. article by FH og B. Lie): Kp = 0,32*Kpu og Ti = Pu. F. Haugen. Process Control. NMBU. 2018.

  5. 5 • Let’s try Z-N tuning of a PI controller on this simulator: • Level control of wood-chip tank F. Haugen. Process Control. NMBU. 2018.

  6. Repeated Ziegler-Nichols(for PI contollers) Assume that the PI settings are Kp0and Ti0, and that the control system - unfortunately - has poor stability with these settings showing poorly damped oscillations with period Pu. Improved PI setting can be obtained by applying ZN tuning assuming that these oscillations are true ZN oscillations: Kp = 0.45Kp0 Ti = Pu/1.2 F. Haugen. Process Control. NMBU. 2018.

  7. Ex.: PI controller in biogas control system Original ZN Repeated ZN F. Haugen. Process Control. NMBU. 2018.

  8. 8 Auto-tuning (automatic PID tuning) F. Haugen. Process Control. NMBU. 2018.

  9. Example of auto-tuner: The Relay or On/Off tuner (by Åström and Hägglund). Sustained oscillations come automatically in control loop. From the amplitude and the period of these oscillations proper PID controller parameters are calculated by an algorithm in the controller. Amplitude A Amplitude Y PeriodPu In Ziegler-Nichols’: Use Pu andKpu= 4*A/(pi*Y). F. Haugen. Process Control. NMBU. 2018.

  10. 10 • Example: • Relay-tuning of a PI temperature controller: • Temperature Control of Liquid Tank F. Haugen. Process Control. NMBU. 2018.

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