Question

In 1942, Ziegler and Nichols published a classic paper that introduced the continuous cycling method and the step test method for PID controller tuning. 3. (a) What are the other two ways to tune a PID controller? What are the advantages and disadvantages for the two methods? (10 marks) (b) A step test was carried out on an electrical oven. Using the data in Table Q3, process the reaction curve in Figure Q3 and produce the tuning parameters for a PID controller (15 marks) Table Q3 WJK Coking Plant-Industrial Controls Department Date of Test 23/03/0 Input Level-50 kW Start Input Level-End 60 kW Process Unit Output Level-Start 700 °C Electrical oven Output Level-End 780 °C 800 780 760 8 740 2 720 O. 700 680 50 40 30 10 20 Time. S Figure Q3

Answer 1

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a. The other 2 methods of PID tuning are: manual and Cohen-Coon. The manual tuning does not require any math and is performed online. However, it requires an experienced personnel to perform the tuning. The Cohen-Coon method results in good process models, however it is an offline process and is good only for 1st order models.

b. The required parameters for the tuning, from the process step response are:

Gain K = 780/60 = 13

Delay = td = 7 seconds (approx)

Time constant = T = 13 seconds (approx).

Now, substitute the values according to the table below:

τp τ K #-Η Ηρ 1τ 1 + 3τ/ P Kt 30+3 t,/τ ta 9+20t/τ 1τ PI 0.9 12τ/ 4 32+6 t/τ 1τ /4 #6-θ tα PID 11 +2 t4/τ 13+8 t,/τ Kt3 4τ. t

Thus the PID variables are: Kc = 0.2097, Ti = 14.25 and Td = 2.32

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