3. Introduction
โถ PID tuning is to find the optimum Kp, Ki and Kd for the controller.
Control objective > Setpoint tracking, Disturbance rejection
Actions > Instantaneous proportional action, Reset integral action, Rate derivative
action
Optimum criteria > Depends on application and system requirements
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4. Introduction
โถ Conceptual real-world example
Driver
(PID)
Car mechanism
(Process)
Crosswind
Front wheels
angle Car position
Driverโs eyes
(Feedback)
Desired position
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6. Introduction
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โถ Many tuning methods have been proposed for PID controllers each of which
has its advantages and disadvantages. So, no one can be considered the best
for all purposes.
โถ Closed-loop methods tune the PID while it is attached to the loop while in
open-loop methods the process is estimated using a FOPDT model
โถ A comparison of the most popular methods is to be done
โถ Simulation will be implemented for 1st, 2nd and 3rd-order processes, some of
which are lag-dominant and the others are dead-time dominant.
โถ IAE as criterion (which adds up the time and amplitude weight of the error)
7. Objectives
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โถ Compare studied tuning methods for performance and robustness
โถ Develop a GUI to do the comparison automatically for a given process model
8. Closed-loop methods
โถ Ziegler-Nichols Closed-loop
โถ Tyreus-Luyben
โถ Damped Oscillation
PID Process
D
C PV
Feedback
SP
Tuning
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9. Open-loop methods
โถ Ziegler-Nichols Open-loop
โถ C-H-R
โถ Cohen-Coon
โถ Ciancone-Marlin
โถ Minimum Error Integral
PID Process
D
PV
Tuning
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10. Ziegler-Nichols Closed-loop
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โถ ยผ decay ratio as design criterion (stability condition)
โถ Trial-and-error procedure to find ๐ฒ๐ and ๐ท๐
โถ Drives the process into marginal stability
โถ Performs well when ๐๐ โฅ ๐๐๐ (lag dominant)
โถ Performs very poorly for ๐๐ > ๐๐๐ (dead-time dominant)
โถ Fast recovery from disturbance but leads to oscillatory response
โถ Not applicable to open-loop-unstable processes
โถ Some processes do not have ultimate gain
11. Ziegler-Nichols Closed-loop
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Controller ๐พ๐ ๐๐ ๐๐
P 0.5๐พ๐ข - -
PI 0.45๐พ๐ข 0.83๐๐ข -
PID 0.6๐พ๐ข 0.5๐๐ข 0.125๐๐ข
โถ Procedure:
๏ Set ๐พ๐ and ๐พ๐ to 0
๏ Increase ๐พ๐ till sustained oscillation and find ๐พ๐ข and ๐๐ข
๏ Use the correlations in the table below
12. Tyreus-Luyben
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โถ An improvement for Ziegler-Nichols closed-loop to make response less
oscillatory
โถ More robust to imprecise model
โถ Gives better disturbance response
โถ Procedure:
๏ Same procedure as Ziegler-Nichols closed-loop
Controller ๐พ๐ ๐๐ ๐๐
PI 0.45๐พ๐ข 2.2๐๐ข -
PID 0.313๐พ๐ข 2.2๐๐ข 0.16๐๐ข
13. Damped Oscillation
โถ Another improvement for Ziegler-Nichols closed-loop
โถ Solves the problem of marginal stability
โถ Can be used with open-loop-unstable processes
0.8
0.6
0.4
0.2
0
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1
1.2
0 10 20 30 40 50 60 70 80
4:1
14. Damped Oscillation
Controller ๐พ๐ ๐๐ ๐๐
PI ๐๐๐๐ข๐ ๐ก๐๐ ๐๐/6 -
PID ๐๐๐๐ข๐ ๐ก๐๐ ๐๐/6 ๐๐/1.5
โถ Procedure:[1]
๏ Set ๐พ๐ and ๐พ๐ to 0
๏ Increase ๐พ๐ till ยผ damping ratio is maintained and find ๐๐ only
๏ Use the correlations in the table below to find ๐๐ and ๐๐
๏ Adjust ๐พ๐ till ยผ damping ratio is maintained again
[1] Liptaฬk, Beฬla G., and Kriszta Venczel. Instrument Engineers' Handbook: Process Control 4thed, Volume T
w
o
.
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15. Ziegler-Nichols Open-loop
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โถ ยผ decay ratio as design criterion
โถ Performs well when ๐๐ โฅ 2๐ก๐ (lag dominant)
โถ Performs very poorly for ๐ก๐ > 2๐๐ (dead-time dominant)
โถ Fast recovery from disturbance but leads to oscillatory response
16. Ziegler-Nichols Open-loop
๐บ๐ ๐ =
โถ Procedure:
๏ The process dynamics is modeled by a first order plus dead time model
๐พ๐๐โ๐ก๐๐
๐๐๐ + 1
2.5
2
1.5
1
0.5
0
-0.5
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17. Ziegler-Nichols Open-loop
๏ PID parameters are calculated from the table below
Controller ๐พ๐ ๐๐ ๐๐
P 1 ๐๐
๐พ๐ ๐ก๐
- -
PI 0.9 ๐๐
๐พ๐ ๐ก๐
๐ก๐
0.3
-
PID 1.2 ๐๐
๐พ๐ ๐ก๐
2๐ก๐ 0.5๐ก๐
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18. C-H-R
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โถ A modification of Ziegler-Nichols Open-loop
โถ Aims to find the โquickest response with 0% overshootโ or โquickest
response with 20% overshootโ
โถ Tuning for setpoint responses differs from load disturbance responses
19. C-H-R
Setpoint
Controller ๐พ๐ ๐๐ ๐๐ ๐พ๐ ๐๐ ๐๐
0% overshoot 20% overshoot
P 0.3 ๐๐
๐พ๐ ๐ก๐
- -
0.7 ๐๐
๐พ๐ ๐ก๐
- -
PI 0.35๐๐
๐พ๐ ๐ก๐
1.2๐๐ -
0.6 ๐๐
๐พ๐ ๐ก๐
๐๐ -
PID 0.6 ๐๐
๐พ๐ ๐ก๐
๐๐ 0.5๐ก๐
0.95๐๐
๐พ๐ ๐ก๐
1.4๐๐ 0.47๐ก๐
Disturbance
P 0.3 ๐๐
๐พ๐ ๐ก๐
- -
0.7 ๐๐
๐พ๐ ๐ก๐
- -
PI 0.6 ๐๐
๐พ๐ ๐ก๐
4๐ก๐ -
0.7 ๐๐
๐พ๐ ๐ก๐
2.3๐ก๐ -
PID 0.95๐๐
๐พ๐ ๐ก๐
2.4๐ก๐ 0.42๐ก๐
1.2 ๐๐
๐พ๐ ๐ก๐
2๐ก๐ 0.42๐ก๐
โถ Procedure:
๏ Same as Ziegler-Nichols Open-loop
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20. Cohen-Coon
โถ Second in popularity after Ziegler-Nichols tuning rules
โถ ยผ decay ratio has considered as design criterion for this method
โถ More robust
๐
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๐
๐ ๐
(i.e. ๐ก > 2๐)
โถ Applicable to wider range of
โถ PD rules available
21. Cohen-Coon
โถ Procedure:[1]
๏ The process reaction curve is obtained by an open loop test and the FOPDT
model is estimated as follows:
3
๐๐ =
2
๐ก2 โ ๐ก1
๐ก๐ = ๐ก2 โ ๐๐ 1.5
2
2.5
1
0.5
0
-0.5
[1] Smith,C.A.,A.B. Copripio; โPrinciples and Practice of Automatic Process Controlโ, John Wiley & Sons,1985
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23. Ciancone-Marlin
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โถ Design criteria:
๏ Minimization of IAE
๏ Assumption of ยฑ25% change in the process model parameters
โถ A set of graphs are used for the tuning
โถ Tuning for setpoint responses differs from load disturbance responses
24. Ciancone-Marlin
โถ Procedure:
๏ Estimate the process with FOPDT as for Cohen-Coon method
๏ Calculate the ratio
๐ก๐
๐ก๐+๐๐
๐ ๐
๏ From the appropriate graph determine the values (๐พ ๐พ , ,
๐๐ ๐๐
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๐ก๐+๐๐ ๐ก๐+๐๐
)
๏ Do the calculation to find the PID parameters
27. Minimum Error Integral
โถ Considers the entire closed loop response not like the ยผ-decay tuning methods which
considers only the first two peaks
โถ Less oscillations in response than ยผ-decay
โถ Performs well when ๐๐ โฅ ๐๐๐ (lag dominant)
โถ Performs very poorly for ๐๐ > ๐๐ (dead-time dominant)
โถ Tuning for setpoint responses differs from load disturbance responses
โถ Different error integrals can be used (IAE, ISE, ITAE, ITSE)
โ
๐ผ๐ด๐ธ = โซ ๐(๐ก) ๐๐ก ,
0
โ
๐ผ๐๐ธ = โซ ๐(๐ก)2 ๐๐ก ,
0
โ
๐ผ๐๐ด๐ธ = โซ ๐ก ๐(๐ก) ๐๐ก ,
0
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โ
๐ผ๐๐๐ธ = โซ ๐ก๐(๐ก)2 ๐๐ก
0
28. Minimum Error Integral
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โถ Procedure:
๏ Estimate the process with FOPDT as for Cohen-Coon method
๏ Use the appropriate table to find the PID parameters
31. Simulation and Results
โถ Simulation performed for two purposes:
๏ Performance Assessment
๏ Robustness Assessment
โถ Simulation for two response objectives:
๏ Set point tracking
๏ Disturbance rejection
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