che 491 / 433

Click here to load reader

Post on 23-Feb-2016

42 views

Category:

Documents

2 download

Embed Size (px)

DESCRIPTION

ChE 491 / 433. 22 Oct 12. ChE 491 / 433. 22 Oct 12 Ziegler-Nichols (ZN I) (QDR or QAD Tuning ) (Ultimate Gain). +. +. +. -. Feedback Controller Tuning: (General Approaches). Simple criteria; i.e QAD via ZN I, t r , etc e asy, simple, do on existing process multiple solutions - PowerPoint PPT Presentation

TRANSCRIPT

Slide 1

1ChE 491 / 43322 Oct 12

130 Mar 072 Apr 0827 Mar 092ChE 491 / 43322 Oct 12

Ziegler-Nichols (ZN I)(QDR or QAD Tuning)(Ultimate Gain)

++

-

+

230 Mar 072 Apr 0827 Mar 09Feedback Controller Tuning: (General Approaches)Simple criteria; i.e QAD via ZN I, tr, etceasy, simple, do on existing processmultiple solutionsTime integral performance criteriaISEintegral square errorIAEintegral absolute value errorITAEintegral time weighted average errorSemi-empirical rulesFOPDT (ZN II)Cohen-CoonATV, or AutotuningTrial and errorRules of thumb33Procedure, done closed loop (on-line):Ziegler Nichols I (Ultimate Gain Method)P-Only (switch off integral & derivative modes)Controller in Auto mode (closed loop)Adjust Kcbump process with small setpoint changeFind Kc where loop response is undamped44

Dynamic Changes as Kc is Increased for a FOPDT Process

Procedure, done closed loop (on-line):Ziegler Nichols I (Ultimate Gain Method)P-Only (switch off integral & derivative modesController in Auto mode (closed loop)Adjust Kcbump process with small setpoint changeFind Kc where loop response is undampedRecord Kc (call it Kcu the ultimate gain)Measure Tu (the ultimate period)Use Table 7-1.1 to get tuning constantsAdjust controller settings to calculated valuesTest to see if need to make fine adjustments6

67

Quarter-decay-ratio response (sometimes called QAD)

Response to disturbance should be close to QDR (QAD)Ziegler Nichols I (Ultimate Gain Method)Dont need to know mathematical modelsEasy to useUse on any process you can get to oscillateAdvantages:Must force loop / process to oscillate (operating close to unstable)Tuning constants not unique, except for P-only

Disadvantages:88Quarter Decay Ratio (QAD)Good for load disturbancesPrevents large initial deviations w/o too much oscillationsGives good Ball Park values; leading to fast responses for most processesAdvantages:For SP changes, may overshoot too muchParameters for PI, PID, not uniqueMay be too aggressive for cases where K or to change.Disadvantages:9

910PS Exercise: Tuning Two Tanks in SeriesLaunch Loop Pro TrainerSelect Case StudiesSelect Gravity Drained TanksPress the pause buttonAdjust controller output to 50%Press run (continue) button and let run till achieve steady stateClick the rescale button to re center the plotAdjust controller output to achieve a level in tank 2 of 2 metersClick the controller button and turn to PID control (P-Only)You may have to turn the Integral part off; and Kc = 4 %/m Press run button and adjust the disturbance up and down 0.5 l/minThen adjust the set point up and down 0.5 mObserve how the system behaves.

Loop Pro Trainer (process simulator):11PS Exercise: Tuning Two Tanks in SeriesLaunch Loop Pro TrainerSelect Case StudiesSelect Gravity Drained TanksNow, double Kc and observe effect.Double it againTry it at Kc = 2 %/m

Loop Pro Trainer (process simulator):12PS Exercise: Tuning Two Tanks in SeriesNow turn on the Integral term (tI should be 4.0 min) and do the same adjustments, observing the behavior of the system.You may need to adjust the History to see the full change.Change tI and observe the effect.Make sure you are back to the original settings (SP = 2m, Level at 2 m, etc) when you start and end with the PI controller.Loop Pro Trainer (process simulator):Note.. double Tau I to 8 min then work Tau I down to 0.5 minor lower and observe1213PS Exercise: Tuning Two Tanks in SeriesNow turn on the Integral term (tI should be 4.0 min) and do the same adjustments, observing the behavior of the system.You may need to adjust the History to see the full change.Change tI and observe the effect.Make sure you are back to the original settings (SP = 2m, Level at 2 m, etc) when you start and end with the PI controller.Now lets tune the controller.Use the Ziegler Nichols I method to find Kcu and Tu.Tune the controller for:P only controlAnd then for PI control.Loop Pro Trainer (process simulator):1314Loop-Trainer

Kcu ~ 72, delta R = 4 > 4.515

Kcu ~ 72, delta R = 4 > 4.5set Kc = 1/2Kcu = 3616ChE 491 / 43322 Oct 12

1630 Mar 072 Apr 0827 Mar 09Feedback Controller Tuning: (General Approaches)Simple criteria; i.e QAD via ZN I, tr, etceasy, simple, do on existing processmultiple solutionsTime integral performance criteriaISEintegral square errorIAEintegral absolute value errorITAEintegral time weighted average errorSemi-empirical rulesFOPDT (ZN II)Cohen-CoonATV, or AutotuningTrial and errorRules of thumb171718PS Exercise: Tuning Two Tanks in SeriesDifferent opinions:Different correlations will give different constants in the controller equations. D. Cooper suggests if one is uncertain, to start conservative, i.e. with the smallest controller gain and the largest integral (reset) time, thus, giving the least aggressive controller. Final controller tuning may best be performed on-line by trial and error, using experience and knowledge of the process, to obtain the desired controller performance.

To changes in the setpoint or load disturbances:if the process response is sluggish; Kc is too small and/or I is too large.if the process response is too quick and perhaps oscillating is not desired; Kc is too large and/or I is too small.Ziegler-Nichols may be too aggressive for many ChE applications. Luyben (Plantwide Dynamic Simulators in Chemical Processing and Control, Wiley, 2002) suggests for PI controller Kc = Ku / 3.2 and I = 2.2 * Tu .

Kc

tI19Step Change Responses:20Is Kc or tI too high?

Kc too largeProperly tuned controllertI too large

Feedback Controller Tuning: (General Approaches)Simple criteria; i.e QAD via ZN I, tr, etceasy, simple, do on existing processmultiple solutionsTime integral performance criteriaISEintegral square errorIAEintegral absolute value errorITAEintegral time weighted average errorSemi-empirical rules; FOPDT fit to Open Loop Step Test Ziegler-Nichols Open Loop (ZN II)Cohen-CoonATV, or AutotuningTrial and errorRules of thumb212122Ziegler Nichols II (ZN II)Fit response to FOPDT model

++

-

+

2230 Mar 072 Apr 0827 Mar 09Procedure, usually done open loop:Ziegler Nichols II (FOPDT fit)Put controller in Manual modeManually make step change in controller outputObserve (record) data and fit to FOPDT model23

23

Open-Loop Step Test..FOPDT2425

Open-Loop Step Test..FOPDT: Loop Pro Method

26

Open-Loop Step Test..FOPDT: Loop Pro Method27

Open-Loop Step Test..FOPDT: Loop Pro Method

28

Open-Loop Step Test..FOPDT: Smith & Corripio Method29

Open-Loop Step Test..FOPDT: Smith & Corripio MethodEstimation of Fit 3 suggested for non-integrating processes:

Fit 3: 7-2.16 p 23930

Open-Loop Step Test..FOPDT: Smith & Corripio MethodEstimation of Fit 1 suggested for integrating processes.

= constantnon-integrating process(self-regulating)

h

integrating processhWhat happens to h ??Procedure in open loop:Ziegler Nichols II (FOPDT fit)Put controller in Manual modeManually make step change in controller outputObserve (record) data and fit to FOPDT model31

31

Procedure same as for ZN II (open loop step test):Cohen-Coon: The Ziegler-Nichols rules are more sensitive to the ratio of dead time to time constant, andwork well only on processes where the dead time is between 1/4 and 2/3 of thetime constant.

The Cohen-Coon tuning rules work well on processes where the dead time is between 1/10 and 4 times the time constant.

Quarter-amplitude damping-type tuning also leaves the loop vulnerable to going unstable if the process gain or dead time doubles in value. Smuts suggests reducing Kc by to avoid problems later on.32* Jacques F. Smuts, Process Control for Practitioners, Opticontrols, Inc (2011)32

33PS Exercise: Compare Loop Pro and Fit 3 FOPDT MethodsFind Tau, K, and to by both methods. Compare.33Launch Loop Pro TrainerSelect Case StudiesSelect Gravity Drained TanksPress the pause buttonAdjust controller output to 51%Tune controller for operation around a tank level of 2 meters

34PS Exercise: Use The Step Test (ZN II, or Open Loop FOPDT Fit) to Tune The PI Controller35ChE 491 / 43329 Oct 12

3530 Mar 072 Apr 0827 Mar 09Feedback Controller Tuning: (General Approaches)Simple criteria; i.e QAD via ZN I, tr, etceasy, simple, do on existing processmultiple solutionsTime integral performance criteriaISEintegral square errorIAEintegral absolute value errorITAEintegral time weighted average errorSemi-empirical rulesFOPDT (ZN II)Cohen-CoonATV, or AutotuningTrial and errorRules of thumb3636disturbance/load change

setpoint changeTime Integral Performance Criteria

Integrate error from old SP

Integrate error from new SP37Smith/Murrill developed unique tuning relationships

IAE (Integral of the Absolute value of the Error)

ITAE (Integral of the Time-weighted Absolute value of the Error)

Determine type of input/forcing function (i.e. purpose of controller)maintain c(t) at setpoint (Regulator controller)c(t) track setpoint signal (servo control)Time Integral Performance CriteriaEqn: 7-2.17 p 24538

Time Integral Performance Criteria39Time Integral Performance Criteria

40

41PS EX: Find PI Parameters for IAE CriteriaFor disturbance change41Launch Loop Pro TrainerSelect Case StudiesSelect Gravity Drained TanksPut your PI tuning parameters into the simulator controller and check tuning.Do the parameters need to be adjusted?

42PS EX: Find PI Parameters for IAE Criteria

43In-Class EX: Loop Pro Demo FittingShow how Loop Pro Trainer can be used to fit FOPDT43

44Import POLYMATH run DATA for step change4445ChE 491 / 43329 Oct 12

4530 Mar 072 Apr 0827 Mar 09Single step; can be analyzed