che / met 433
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ChE / MET 433. Advanced control schemes. 18 Apr 12 Cascade Control: Ch 09 Ratio Control: Ch 10. Tuning a Cascade System. Both controllers in manual Secondary controller set as P-only (could be PI, but this might slow sys) Tune secondary controller for set point tracking - PowerPoint PPT PresentationTRANSCRIPT
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ChE / MET 433
18 Apr 12Cascade Control: Ch
09Ratio Control: Ch 10
Advanced control schemes
Tuning a Cascade System
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• Both controllers in manual• Secondary controller set as P-only (could be PI, but this might slow
sys)• Tune secondary controller for set point tracking• Check secondary loop for satisfactory set point tracking
performance• Leave secondary controller in Auto• Tune primary controller for disturbance rejection (PI or PID)• Both controllers in Auto now• Verify acceptable performance
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In-Class Exercise: Tuning Cascade Controllers
• Select Jacketed Reactor• Set T cooling inlet at 46 oC (normal operation temperature; sometimes it drops to 40 oC)• Set output of controller at 50%.• Desired Tout set point is 86 oC (this is steady state temperature)
• Tune the single loop PI control• Criteria: IMC aggressive tuning• Use doublet test with +/- 5 %CO• Test your tuning with disturbance from 46 oC to 40 oC
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In-Class Exercise: Tuning Cascade Controllers• Select Cascade Jacketed Reactor• Set T cooling inlet at 46 oC (again)• Set output of controller (secondary) at 50%.• Desired Tout set point is 86 oC (as before)
• Note the secondary outlet temperature (69 oC) is the SP of the secondary controller
• Tune the secondary loop; use 5 %CO doublet open loop• Criteria: ITAE for set point tracking (P only)• Use doublet test with +/- 5 %CO• Test your tuning with 3 oC setpoint changes• Tune the primary loop for PI control; make 3 oC set point changes (2nd-dary controller)• Note: MV = sp signal; and PV = T out of reactor• Criteria: IAE for aggressive tuning (PI)• Implement and with both controllers in Auto… change disturbance from 46 to 40 oC.• How does response compare to single PI feedback loop?
Ratio Control• Special type of feed forward control
•Blending/Reaction/Flocculation
•A and B must be in certain ratio to each other
A B
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Ratio ControlPossible control system:
•What if one stream could not be controlled?
• i.e., suppose stream A was “wild”; or it came from an upstream process and couldn’t be controlled.
A B
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FT
FC
sp
FY
FT
FC
sp
FY
Ratio ControlPossible cascade control systems:
“wild” stream
A
B
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FT
FT
FY FC
sp
A
B
AB
Desired Ratio
A
BFT
FT
FY
FCBsp
A
B
AB
Desired RatioThis unit multiplies A by the desired ratio; so output = A
BA
“wild” stream
AB
Ratio Control Uses:
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• Constant ratio between feed flowrate and steam in reboiler of distillation column
• Constant reflux ratio
• Ratio of reactants entering reactor
• Ratio for blending two streams
• Flocculent addition dependent on feed stream
• Purge stream ratio
• Fuel/air ratio in burner
• Neutralization/pH
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In-Class Exercise: Furnace Air/Fuel Ratio• Furnace Air/Fuel Ratio model• disturbance: liquid flowrate• “wild” stream: air flowrate• ratioed stream: fuel flowrate
• Minimum Air/Fuel Ratio 10/1• Fuel-rich undesired (enviro, econ, safety)• If air fails; fuel is shut down
Independent MV
PV
Ratio set point
Dependent MV
Disturbance var.
TC
TC output
Desired 2 – 5% excess O2
Check TC tuning to disturbance & SP changes.
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ChE / MET 433
18 Apr 12Feed Forward Control: Ch
11
Advanced control schemes
Feed Forward ControlSuppose qi is primary disturbance
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Heat Exchanger
TC
TT)(tqi
)(tTi
? What is a drawback to this feedback control loop?? Is there a potentially better way?
Heat ExchangerTTFT
FF
)(tTi
)(tqi
? What if Ti changes?
FF must be done with FB control!
steam
steam
Feed Forward and Feedback Control
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Heat ExchangerTTFT
TY
)(tTi)(tqi
steamTC
FF?
TYP
I)(tM FF )(tM
)(tM
FFFF MtMtMtM )()()(
Block diagram:
TPGCG
sE sT++
FFG
TTK
VG
DTKLG
sQi
++
M
FFM
M-
+ sR
FFCGFF
Feed Forward Control
No change; perfect compensation!
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PGCG-
sE+ sR sT++
FFG
TTK
VG
DTKLG
sQi
++
M
FFM
M
t0
DT
PT
tT
PT
MFF
DT
tqi
Response to MFF
Feed Forward Control
Examine FFC T.F.
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MGCG-
sE+ sR sC++
FFC
DTKDG
sQi
++FFM
M
MG sC
FFC
DTKDG
sQi
++
FFM
gpm
TO%
DTO%
FFCO%
)()( sQKFFCGsQGsC iTMiD D
For “perfect” FF control: 0sC
)()(0 sQKFFCGsQG iTMiD D
MT
D
GKGFFCD
TO%
TO%
Feed Forward Control: FFC Identification
Set by traditional means:
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DTKMT
D
GKGFFCD
Model fit to FOPDT equation: MD GG &
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seKG
D
stD
D
Do
1
seKG
M
stM
M
Mo
gpmTO%
COTO
%%
gpmTOD%
stt
D
M
MT
D oMDo
D
ess
KKKFFC
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FF Gain
Lead/lag unit
Dead time compensator
{ FFC ss }steady state FF control
{ FFC dyn }dynamic FF control
Accounts for time differences in 2 legs
Often ignored; if set term to 1
oMo ttD
Eqn: 11-2.5 p 379
Feed Forward Control: FFC IdentificationHow to determine FOPDT models :
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MG sC
FFC
DTKDG
sQi
++
FFM
gpm
TO%
DTO%
FBCO%
MT
D
GKGFFCD
With Gc disconnected:• Step change COFB, say 5%• Fit C(s) response to FOPDT
MD GG &
1
seKG
D
stD
D
Do
1
seKG
M
stM
M
Mo
gpmTO%
COTO
%%
Still in open loop:• Step change Q, say 5 gpm• Fit C(s) response to FOPDT
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ss
KKKFFC
Lg
Ld
MT
D
D
Ldm lead timeLgD lag time
Lead/Lag or Dynamic CompensatorLook at effect of these two to step change in input
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Final Change from:• Magnitude of step change,• Initial response by the lead/lag, • Exponential decay from lag,
Lg
Ld
Time
c ff
ld/ lg = ½
ld/ lg = 1
ld/ lg = 2
Output or response )(tc
Lg
Ld
Lg
Feed Forward ControlRule of Thumb: if lead-lag won’t help much; use FFCss
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3.165.0 Lg
Ld
(p 389)
In text: pp 393-395, useful comments if implementing FFC
+ -1. Compensates for disturbances
before they affect the process1. Requires measurement or
estimation of the disturbance
2. Can improve the reliability of the feedback controller by reducing the deviation from set point
2. Does not compensate for unmeasured disturbances
3. Offers advantages for slow processes or processes with large deadtime.
3. Linear based correction; only as good as the models; performance decreases with nonlinear processes.
No improvement using FFC with set point changes.
In-Class PS Exercise: Feed Forward Control
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What is the Gm, and what is the GD?Determine FCCTune PI controller to aggressive IMC
• Test PI Controller• Test PI + FFCss only• Test PI + FFC full
For disturbance: Tjacket in
50oC – 60oC – 50oC
In-Class PS Exercise: Feed Forward Control
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PI only PI + FFCss only PI + full FFC
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ChE / MET 433