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Abdul Aziz Ishak, Dept. of Chemical Engineering, Universiti Teknologi MARA, MALAYSIA
E-mail: aabi@kkps.fk.um.edu.my / Webpage: http://aabi.tripod.com

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Process plants

OPTIMUM PID CALCULATION
Examples based on Zieglar-Nichols's tuning rule

1. Example using data captured using a paperless recorder of a level increasing process

1. Td = 2.6 s
2. Process rate = (PV2 - PV1) / (T2 - T1) = (80% - 60%)/(32.4 s - 12.8 s) = 1.0204 % / s
3. Controller output difference = MV2 - MV1 = 64.9% - 40.5% = 24.4%
4. RR = process rate / controller output = 1.0204 / 24.4 = 0.04182 / s
5. For level process, use PI mode. Hence, based on Z-N tuning rule:
    P = 111.1 x 0.04182 x 2.6 = 12.1%
    I = 3.33 x 2.6 = 9 s

3. Example using data captured using a chart recorder of a level increasing process

1. Chart speed = 1000 mm / hr = 3.6 s / mm
2. x-axis (time's scale) = 10 mm = 36 s ,   y-axis (measurement's scale) = 10 mm thick mark = 10 %
3. Td = 1 mm = 3.6 s
4. Time difference between point A and B = 6 mm = 21.6 s
5. Process rate = (68% - 52%) / 21.6 s = 0.7407 % / s
6. Controller output change = MV2 - MV1 = 44.3% - 25.9% = 18.4%
7. RR = process rate / step input = 0.7407 / 18.4 = 0.04026 / s
8. Level process ==> Mode = PI
    P = 111.1 x 0.04026 x 3.6 = 16.1%
    I = 3.33 x 3.6 = 12 s
 

TEST YOUR SELF: Can you calculate the optimum PI for this level decreasing process


17 October 1999