Fine Tuning TechniquesPractical controller tuning tips Abdul Aziz Ishak, Universiti Teknologi MARA Malaysia (2009)

Practical Controller Tuning Tips1. Tune coarse, not fine. The performance of a feedback controller is not sensitive to the precise adjustment of its tuning parameters significantly.Armando B. Corripio, Tuning of Industrial Control Systems, 1990, pp78-81ExamplePressure control loopCalculated optimum:PB = 58.3%I = 5.75 sPressure control loopCalculated optimum:PB = 60%I = 6 s Abdul Aziz Ishak, Universiti Teknologi MARA (2007)2. Tune with confidence. Select the integral time first, set the derivitive time to about one fourth of the integral time, and then adjust the proportional gain to obtain tight control.D = 0.25 x I3. Trial-and-error tuning: Fix one parameter while adjusting the other. Derivitive, D, follows integral, I, by 0.25 x I.Unit 5: Fine Tuning Strategies

Practical Controller Tuning Tips4. Try a longer integral time. Not to force the process reaches the set point faster than the process can respond.Armando B. Corripio, Tuning of Industrial Control Systems, 1990, pp78-81ExamplePressure control loopCalculated optimum:PB = 60%I = 6 sPressure control loopCalculated optimum:PB = 60%I = 12 s Abdul Aziz Ishak, Universiti Teknologi MARA (2007)5. Based on the characterictics of PIDs (plus experience), increase PB, I or D: not more than twice, while decrease PB, I or D: not less than half.Increase: PBnew = 2 x PBold or Inew = 2 x IoldDecrease: PBnew = 0.5 x PBold or Inew = 0.5 x IoldD = 0.25 x IUnit 5: Fine Tuning Strategies

Practical Controller Tuning Tips6. Tuning very controllable process (dead time < 0.1 x time constant): Use own judgement when calculated optimum PIDs reccommends lower PB (fast action) and lower I (fast action).Armando B. Corripio, Tuning of Industrial Control Systems, 1990, pp78-81ExampleFlow control loopCalculated optimum:PB = 15%I = 3 sFlow control loopCalculated optimum:PB = 30%I = 9 s Abdul Aziz Ishak, Universiti Teknologi MARA (2007)7. On-line tuning (depending on your controller): Keep changing the desired parameter (P, I or D) until the process becomes stable. The set of PIDs obtained may not be the best. Must perform set point change to see whether this set of PIDs will produce stabilty.Unit 5: Fine Tuning Strategies

Fine tuning techniquesUnit 5: Fine Tuning Strategies Abdul Aziz Ishak, Universiti Teknologi MARA Malaysia (2009)

Unit 5: Tutorial 5Effect of proportional band, PBThe response curve below is the result of a set point change from 50% to 60%. The PB is 30, 100 and 200% and I is 6 s. Assume that the process is of linear characteristic and a linear control valve had been installed.a. On the response curve, label the PBs accordingly. Justify your answer.You are to make a set point change from 50 to 60%. Sketch the expected responses fori) PB =15% and I = 6 sii) PB = 400% and I = 6 s Abdul Aziz Ishak, Universiti Teknologi MARA Malaysia (2009)

Chart1

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Time, s

Level, %

Automatic Response of Level ControlPB = 30, 100, 200%, I = 6 s

Sheet1

Automatic Response of Level Control

PB =see below

I =6 s

PB=15PB=30PB=50PB=100PB=200

Time, s%%%%%

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Sheet2

Sheet3

Unit 5: Tutorial 5Effect of integral time, IThe response curve below is the result of a set point change from 50 to 60%. The PB is 100 and I is 12 s. If you were to change the integral time, I froma. 12 to 20 s12 to 6 show would the process response curve be? Justify your answer.050100150200Time, sLevel, % I = 12 sSP Abdul Aziz Ishak, Universiti Teknologi MARA Malaysia (2009)

*PID Controller Tuning Based Integral Error Criteria

*Alternative approach for the closed-loop tuning Controller design based on performance indexTuning relationship are intended to minimize the integral of the error.Three popular performance indices are:Integral of the absolute value of error (IAE)

Integral of the squared error (ISE)

Integral of the time weighted absolute error (ITAE) Integral Error Criteria

*Mukasurat Definition of error integrals for disturbances and set point changes

*Minimum IAE formulas for disturbance inputsMinimum IAE formulas for set point changes

ModeKcIDP--PI-PID

ModeKcIPPI-PID

*Minimum ITAE performance index for disturbance inputs

ControllerModeABPIP0.859-0.977I0.674-0.680PIDP1.357-0.947I0.842-0.738D0.3810.995

*Minimum ITAE performance index for set point changesFor proportional and derivative modeFor integral mode

ControllerModeABPIP0.586-0.916I1.03-0.165PIDP0.965-0.85I0.796-0.1465D0.3080.929

ExampleFind the PI controller setting for a process with transfer function for load changes :

*=0.169 =1.85

ControllerModeABPIP0.859-0.977I0.674-0.680PIDP1.357-0.947I0.842-0.738D0.3810.995

*For the process model,

Determine PI controller setting based on the IAE tuning relations for set-point changes.

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