1 ChE / MET 433 19 Mar 12. 2 ChE / MET 433 19 Mar 12 Ziegler-Nichols (ZN I) (QDR or QAD Tuning)...
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Transcript of 1 ChE / MET 433 19 Mar 12. 2 ChE / MET 433 19 Mar 12 Ziegler-Nichols (ZN I) (QDR or QAD Tuning)...
1
ChE / MET 433
19 Mar 12
2
ChE / MET 433
19 Mar 12
Ziegler-Nichols (ZN I)
(QDR or QAD Tuning)
(Ultimate Gain)
LG
++
sL
PGcG-
sE+ sR sC)(sM
??PG
??LG
Feedback Controller Tuning: (General Approaches)
1) Simple criteria; i.e QAD via ZN I, tr, etc• easy, simple, do on existing process• multiple solutions
2) Time integral performance criteria• ISE integral square error• IAE integral absolute value error• ITAE integral time weighted average error
3) Semi-empirical rules• FOPDT (ZN II)• Cohen-Coon
4) ATV, or Autotuning
5) Trial and error
6) Rules of thumb
3
Procedure, 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 Kc
o “bump” process with small setpoint change
o Find Kc where loop response is undamped
4
Time Time Time
Time Time Time
Dynamic Changes as Kc is Increased for a FOPDT Process
gainultimateKCU
Procedure, 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 Kc
o “bump” process with small setpoint change
o Find Kc where loop response is undamped
• Record Kc (call it Kcu – the ultimate gain)
• Measure Tu (the ultimate period)
• Use Table 7-1.1 to get tuning constants
• Adjust controller settings to calculated values
• Test to see if need to make fine adjustments
6
UT
7
Quarter-decay-ratio response (sometimes called QAD)
Response to disturbance should be close to QDR (QAD)
Ziegler Nichols I (Ultimate Gain Method)
• Don’t need to know mathematical models
• Easy to use
• Use on any process you can get to oscillate
Advantages:
• Must force loop / process to oscillate (operating close to unstable)
• Tuning constants not unique, except for P-only
Disadvantages:
8
Quarter Decay Ratio (QAD)
• Good for load disturbances
• Prevents large initial deviations w/o too much oscillations
• Gives good “Ball Park” values; leading to fast responses
for most processes
Advantages:
• For SP changes, may
overshoot too much
• Parameters for PI, PID, not
unique
• May be too aggressive for
cases where K or to
change.
Disadvantages:
9
10
PS Exercise: Tuning Two Tanks in Series
• Launch Loop Pro Trainer
• Select Case Studies
• Select Gravity Drained Tanks
• Press the pause button
• Adjust controller output to 50%
• Press run (continue) button and let run till achieve steady
state
• Click the rescale button to re center the plot
• Adjust controller output to achieve a level in tank 2 of 2
meters
• Click 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/min
• Then adjust the set point up and down 0.5 m
• Observe how the system behaves.
Loop Pro Trainer (process simulator):
11
PS Exercise: Tuning Two Tanks in Series
• Launch Loop Pro Trainer
• Select Case Studies
• Select Gravity Drained Tanks
• Now, double Kc and observe effect.
• Double it again…
• Try it at Kc = 2 %/m
Loop Pro Trainer (process simulator):
12
PS Exercise: Tuning Two Tanks in Series
• Now 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):
13
PS Exercise: Tuning Two Tanks in Series
• Now 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 let’s tune the controller.
• Use the Ziegler Nichols I method to find Kcu and Tu.
• Tune the controller for:
• P – only control
• And then for PI control.
Loop Pro Trainer (process simulator):
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Loop-Trainer
Kcu ~ 72, delta R = 4 –> 4.5
15
Kcu ~ 72, delta R = 4 –> 4.5…set Kc = 1/2Kcu = 36
16
ChE / MET 433
21 Mar 12
Feedback Controller Tuning: (General Approaches)
1) Simple criteria; i.e QAD via ZN I, tr, etc• easy, simple, do on existing process• multiple solutions
2) Time integral performance criteria• ISE integral square error• IAE integral absolute value error• ITAE integral time weighted average error
3) Semi-empirical rules• FOPDT (ZN II)• Cohen-Coon
4) ATV, or Autotuning
5) Trial and error
6) Rules of thumb
17
18
PS Exercise: Tuning Two Tanks in Series
Different opinions:
1. 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.
2. 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
tI
19
Step Change Responses:
20
Is Kc or tI too high?
Time
Lag
c
ys
Time
Lag
c
ys
Time
Lag
ys
c
Kc too
larg
e Properly tuned
controller
tI too large
COortm %)(
PVortc )(
PVortc )(
COortm %)(
COortm %)(
PVortc )(
Feedback Controller Tuning: (General Approaches)
1) Simple criteria; i.e QAD via ZN I, tr, etc• easy, simple, do on existing process• multiple solutions
2) Time integral performance criteria• ISE integral square error• IAE integral absolute value error• ITAE integral time weighted average error
3) Semi-empirical rules; FOPDT fit to Open Loop Step Test • Ziegler-Nichols Open Loop (ZN II)• Cohen-Coon
4) ATV, or Autotuning
5) Trial and error
6) Rules of thumb
21
22
Ziegler Nichols II (ZN II)
Fit response to FOPDT model
LG
++
sL
PGcG-
sE+ sR sC)(sM
??PG
??LG
1
s
eKG
st
P
o
KinallKKK PTV ,,
Procedure, usually done open loop:
Ziegler Nichols II (FOPDT fit)
• Put controller in Manual mode
• Manually make step change in controller output
• Observe (record) data and fit to FOPDT model
23
1
s
eKfit
sto
timedeadprocesseffectivet
tconstatimeprocesseffective
gainSSprocessK
o
n
Open-Loop Step Test……..FOPDT
24
25
Open-Loop Step Test……..FOPDT: Loop Pro Method
K
K
K
26
Open-Loop Step Test……..FOPDT: Loop Pro Method
27
ot
ot
ot
Open-Loop Step Test……..FOPDT: Loop Pro Method
min3.0
28
)(tm
)(tc
m
sc
m
cK s
Open-Loop Step Test……..FOPDT: Smith & Corripio Method
29
ot&Open-Loop Step Test……..FOPDT: Smith & Corripio Method
Estimation of
Fit 3 suggested for non-integrating processes:
sc
sc632.0@
sc283.0@
1t 2t
2
1223
tt
tt
o
Fit 3: 7-2.16 p 239
30
ot&Open-Loop Step Test……..FOPDT: Smith & Corripio Method
Estimation of
Fit 1 suggested for integrating processes.
inF
outF = constant
non-integrating process(self-regulating)
inF
hFout
h
)(tuAFif in
integrating process
h
What happens to h ??
Procedure in open loop:
Ziegler Nichols II (FOPDT fit)
• Put controller in Manual mode
• Manually make step change in controller output
• Observe (record) data and fit to FOPDT model
31
1
s
eKfit
sto
timedeadprocesseffectivet
tconstatimeprocesseffective
gainSSprocessK
o
n5.01.0
ot
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, and work well only on processes where the dead time is between 1/4 and 2/3 of the time 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 ½ to avoid problems later on.
32
* Jacques F. Smuts, Process Control for Practitioners, Opticontrols, Inc (2011)
33
PS Exercise: Compare “Loop Pro” and “Fit 3” FOPDT Methods
• Launch Loop Pro Trainer
• Select Case Studies
• Select Gravity Drained Tanks
• Press the pause button
• Adjust controller output to 51%
• Tune controller for operation around a tank level of 2 meters
34
PS Exercise: Use The Step Test (ZN II, or Open Loop FOPDT Fit) to Tune The PI
Controller
35
ChE / MET 433 23 Mar 12
Feedback Controller Tuning: (General Approaches)
1) Simple criteria; i.e QAD via ZN I, tr, etc• easy, simple, do on existing process• multiple solutions
2) Time integral performance criteria• ISE integral square error• IAE integral absolute value error• ITAE integral time weighted average error
3) Semi-empirical rules• FOPDT (ZN II)• Cohen-Coon
4) ATV, or Autotuning
5) Trial and error
6) Rules of thumb
36
• disturbance/load change
• setpoint change
Time Integral Performance Criteria
)(tc )(oldSP
Integrate error from old SP
)(tc SPnew
Integrate error from new SP
37
Smith/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)
0
)( dtteIAE
0
)( dttetITAE
• 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 Criteria
Eqn: 7-2.17 p 245
38
Time Integral Performance Criteria
39
Time Integral Performance Criteria
40
41
PS EX: Find PI Parameters for IAE Criteria
• Launch Loop Pro Trainer
• Select Case Studies
• Select Gravity Drained Tanks
• Put your PI tuning parameters into
the simulator controller and check
tuning.
• Do the parameters need to be
adjusted?
42
PS EX: Find PI Parameters for IAE Criteria
43
In-Class EX: Loop Pro Demo Fitting
44
45
ChE / MET 433 26 Mar 12
• Single step; can be analyzed by hand
• Pulse, doublet, pseudo-random binary sequence (PRBS) tests;
require computer tools for analysis
46
Step Testing Thoughts
Data collected should meet these criteria:
• Process at steady state before data collected
• Signal to noise ratio should be 10 or greater
• Collected data should be done when no disturbances were present
• After fitting, the model appears to fit the data visually
47
Step Testing ThoughtsSingle step
+ simple, graphical analysis can be done
- long time away from desired operating level (DLO; or SP)
- Data only on one side of DLO
Pulse (two step tests in rapid succession; 1 up and 1 back
down)
+ only need to let measured process variable show a clear response
- long time away from desired operating level (DLO; or SP)
- Data only on one side of DLO
48
Step Testing ThoughtsDoublet Test
+ two pulse tests; one up; one down; ending at beginning level
+ obtain data on both sides of DLO
+ relatively quickly return to normal operation level
+ a preferred method of some in industry for open loop tests
- since done open loop; could be concern for certain systems
49
Step Testing ThoughtsPRBS Test (pseudo-random binary sequence )
+ theoretically PV shouldn’t vary far from DLO
- need a well defined, random test
- should have some idea of process gain, time constant, and
deadtime
- might take longer than a doublet test
50
Step Testing Comparisons
PRBS
Doublet
51
PS EX: Find PI Parameters for IAE Criteria
• Can do closed loop studies, and fit to FOPDT
• Controller aggressive enough for 10 times S to N response
• Data should begin and end at steady state
• No load disturbances should occur
• Do step, pulse, doublet changes to the set point.
• Fit data to FOPDT; check tuning parameters on the process
52
Step Testing Thoughts
53
ChE / MET 433 28 Mar 12
Feedback Controller Tuning: (General Approaches)
1) Simple criteria; i.e QAD via ZN I, tr, etc• easy, simple, do on existing process• multiple solutions
2) Time integral performance criteria• ISE integral square error• IAE integral absolute value error• ITAE integral time weighted average error
3) Semi-empirical rules• FOPDT (ZN II)• Cohen-Coon
4) ATV, or Autotuning
5) Trial and error
6) Rules of thumb
54
Relay feed back test or ATV
Auto-Tune Variation (ATV)*
+ Keeps process close to normal operation+ More efficient for process with long time constant.
55
* Åström and Hägglund (1983);* Luyben & Luyben (1997)
General method:• determine reasonable h value to move FCE (3 – 10 % change)• Input the change +h• When PV starts to move, input change of –2h• When PV cross the set point, input change of +2h• When PV re-crosses the set point, input change of –2h• Repeat until constant oscillations of PV are maintained (~3-4 cycles)• Record amplitude (a) and period of oscillation (Pu)
)(tm
)(tc sp
Auto-Tune Variation (ATV)
56
• Calculate Ku from ATV results.*
• ZN settings
• TL settings** (less aggressive and recommended for more sluggish processes)
a
hKu
4
2.1/45.0 uZNIu
ZNc PKK
45.0/31.0 uTLIu
TLc PKK
* Riggs & Karim (2006)** TL = Tyreus & Luben
Relay feed back test or ATV
Auto-Tune Variation (ATV)
+ Keeps process close to normal operation+ More efficient for process with long time constant.
57
1.9
2
2.1
2.2
2.3
0 20 40 60Time (hours)
Mol
e P
erce
nt Open Loop Test
ATV Test
Riggs & Karim (2006)
58
PS EX: Find PI Parameters using the ATV Method
Auto-Tune or Self-Tuning Controllers
59
General loop auto-tuning:• On demand or on-the-fly (continuous updating)• Can be simple step test or pulse doublet• Can be sophisticated self-tuning for difficult process
Example single point industrial controllers:
http://www.watlow.com/downloads/en/manuals/945e_a.pdf
Example single point industrial controllers:
60
61
Feedback Controller Tuning: (General Approaches)
1) Simple criteria; i.e QAD via ZN I, tr, etc• easy, simple, do on existing process• multiple solutions
2) Time integral performance criteria• ISE integral square error• IAE integral absolute value error• ITAE integral time weighted average error
3) Semi-empirical rules• FOPDT (ZN II)• Cohen-Coon
4) ATV, or Autotuning
5) Trial and error
6) Rules of thumb
62
• Select the tuning criterion for the control loop.• Apply filtering to the sensor reading• Determine if the control system is fast or slow
responding.– For fast responding, field tune (trail-and-error)– For slow responding, apply ATV-based tuning
Trial and Error (field tuning)*
63
• Turn off integral and derivative action.• Make initial estimate of Kc based on process knowledge.
• Using setpoint changes, increase Kc until tuning criterion is met
Time
y s
ab
c
* J.B. Riggs, & M.N. Karim Chemical and Bio-Process Control, 3rd ed. (2006)
• Decrease Kc by 10%.
• Make initial estimate of tI (i.e., tI=5tp).
• Reduce tI until offset is eliminated
• Check that proper amount of Kc and tI are used.
Time
y s
a
b
c
64
Trial and Error (field tuning)*
* J.B. Riggs, & M.N. Karim Chemical and Bio-Process Control, 3rd ed. (2006)
Kc
tI
65
Kc and I levels good?
Feedback Controller Tuning: (General Approaches)
1) Simple criteria; i.e QAD via ZN I, tr, etc• easy, simple, do on existing process• multiple solutions
2) Time integral performance criteria• ISE integral square error• IAE integral absolute value error• ITAE integral time weighted average error
3) Semi-empirical rules• FOPDT (ZN II)• Cohen-Coon
4) ATV, or Autotuning
5) Trial and error
6) Rules of thumb
66
67
Rules of Thumb
* D.A. Coggan, ed., Fundamentals of Industrial Control, 2nd ed., ISA, NC (2005)
*
68
Higher Order Process
Feedback Controller Tuning: (General Approaches)
1) Simple criteria; i.e QAD via ZN I, tr, etc• easy, simple, do on existing process• multiple solutions
2) Time integral performance criteria• ISE integral square error• IAE integral absolute value error• ITAE integral time weighted average error
3) Semi-empirical rules• FOPDT (ZN II)• Cohen-Coon
4) ATV, or Autotuning
5) Trial and error
6) Rules of thumb
69
70
ChE / MET 433