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Abdul Aziz Ishak, Dept. of Industrial Chemistry, Institut Teknologi MARA, MALAYSIA
Tel: 6-03-556-4881 / Fax: 6-03-559-7681 / E-mail: / Webpage:

based on Zieglar-Nichols formulation

1. The Fundamental

1. The white line represents a response curve from a process.
2. The red line represent a tangent line drawn at the steepest point.
3. The broken yellow line with arrow shows the deadtime.
4. The broken yellow line, making a triangle with the red line, shows how to calculate the process rate at the steepest point.
5. Calculate the reaction rate (RR) i.e. the process rate per change in controller input as shown above.
6. Choose the desired mode i.e. P or PI or PID and calculate the PIDs.

2. Example using data captured from a paperless recorder

1. Td = 1.8 s
2. RR = 0.2368 / s
3. Mode = PI : P = 47.4% and I = 6 s

3. Example using data captured from a chart recorder

1. Td = 1.8 s (Notice that Td is unmeasureable. Since I and D require Td, then estimate Td.)
2. RR = 0.231 / s (Notice that the chart speed is in mm/hr. Convert the speed into s/mm. Then, the time is
    simply the length in mm x the chart speed in s/mm.)
3. Mode = PI: Therefore, P = 111.1 x 1.8 x 0.231 = 46.2% ,  I = 3.33 x 1.8 = 6 s   (Notice that the time unit
    for Td must be similar with RR. Else, the time unit does not cancel with each other.)

TEST YOURSELF: Can you calculate the optimum PI for this process.

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