Transcript Slide 1
ChE / MET 433
Advanced control schemes
18 Apr 12
Cascade Control: Ch 09
Ratio Control: Ch 10
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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
• 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
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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)
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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
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Select Cascade Jacketed Reactor
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Set T cooling inlet at 46 oC (again)
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Set output of controller (secondary) at 50%.
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Desired Tout set point is 86 oC (as before)
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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?
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Ratio Control
•Special type of feed forward control
A
B
• Blending/Reaction/Flocculation
• A and B must be in certain
ratio to each other
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Ratio Control
Possible control system:
sp
sp
FC
FY
FC
FT
A
FY
FT
B
• 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.
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Ratio Control
Possible cascade control systems:
“wild” stream
A
sp Desired Ratio B A
FT
A
FY
B
A
FC
B
FT
B
A
“wild” stream
FT
This unit multiplies A by
the desired ratio; so
output = A B A
A
FY
Desired Ratio
BA
sp B
FC
B
FT
B
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Ratio Control Uses:
• 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
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Furnace Air/Fuel Ratio model
disturbance: liquid flowrate
“wild” stream: air flowrate
ratioed stream: fuel flowrate
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Minimum Air/Fuel Ratio 10/1
Fuel-rich undesired (enviro, econ, safety)
If air fails; fuel is shut down
Check TC tuning to disturbance & SP changes.
Desired 2 – 5% excess O2
PV
Disturbance var.
TC
Dependent MV
Ratio set point
TC output
Independent MV
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ChE / MET 433
Advanced control schemes
18 Apr 12
Feed Forward Control: Ch 11
10
Feed Forward Control
steam
Suppose qi is primary disturbance
TC
TT
qi (t )
Ti (t )
Heat Exchanger
? What is a drawback to this feedback control loop?
? Is there a potentially better way?
?
What if Ti changes?
FF
FT
qi (t )
steam
TT
Heat Exchanger
Ti (t )
FF must be done with FB control!
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Feed Forward and Feedback Control
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FF
M FF (t )
FT
I
M (t )
TY
TC
steam
P
TY
TT
qi (t )
Ti (t )
M (t )
Heat Exchanger
M (t ) M (t ) M FF (t ) M FF
Qi s
GL
KTD
Block diagram:
GFF FFC
GFF
Rs +
E s
-
GC
+
M
+
M FF
M
GV
GPT
+
+
T s
K TT
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Qi s
Feed Forward
Control
GL
TD
KTD
GFF
Rs +
E s
-
GC
+
+
M
M FF
M
GV
GP
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+
T s
TP
K TT
qi t
TD
MFF
Response to MFF
TP
No change; perfect compensation!
T t
0
t
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Qi s
Feed Forward
Control
GD
KTD
FFC
Rs +
E s
GC
+
Qi s
+
C s
For “perfect” FF control: C s 0
GD
KTD
%TO
0 GD Qi ( s ) GM FFC KTD Qi ( s )
%TOD
FFC
%COFF
GM
+
C s GD Qi ( s ) GM FFC KTD Qi ( s )
gpm
M FF
M FF
M
-
Examine FFC T.F.
+
GM
+
%TO
+
C s
%TO
FFC
GD
K TD GM
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Feed Forward Control: FFC Identification
Set by traditional means:
FFC
%TOD
gpm
KTD
GD
K TD GM
Model fit GD & GM to FOPDT equation:
t
s
K D e oD
GD
Ds 1
t
%TO
gpm
KD
FFC
KT K M
D
FF Gain
{ FFC ss }
steady state FF
control
s
K M e oM
GM
M s 1
%TO
%CO
M s 1 to toM s
D
e
D s 1
Lead/lag
unit
Eqn: 11-2.5 p 379
Dead time
compensator
Accounts for time
differences in 2 legs
Often ignored; if
set term to 1
t
oD
toM
{ FFC dyn }
dynamic FF control
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Feed Forward Control: FFC Identification
Qi s
How to determine FOPDT
models GD & GM :
With Gc disconnected:
• Step change COFB, say 5%
• Fit C(s) response to FOPDT
t oM s
K e
GM M
M s 1
%TO
%CO
Still in open loop:
• Step change Q, say 5 gpm
• Fit C(s) response to FOPDT
t
gpm
KTD
s
K D e oD
GD
Ds 1
%TO
gpm
GD
%TOD
FFC
M FF
GM
%COFB
FFC
+
+
C s
%TO
GD
K TD GM
KD
FFC
K TD K M
Ld s 1
s 1
Lg
m Ld lead time
D Lg lag time
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Lead/Lag or Dynamic Compensator
Look at effect of these two to step change in input
Ld
Lg
ld/ lg = 2
cff
Output or
response
ld/ lg = 1
c(t )
ld/ lg = ½
Time
Final Change from:
• Magnitude of step change,
• Initial response by the lead/lag,
• Exponential decay from lag, Lg
Ld
Lg
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Feed Forward Control
Rule of Thumb: if 0.65
Ld
1.3 lead-lag won’t help much; use FFCss
Lg
(p 389)
In text: pp 393-395, useful comments if implementing FFC
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1. Compensates for disturbances
before they affect the process
1. 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.
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In-Class PS Exercise: Feed Forward Control
What is the Gm, and what is
the GD?
Determine FCC
Tune PI controller to
aggressive IMC
For disturbance: Tjacket in
50oC – 60oC – 50oC
• Test PI Controller
• Test PI + FFCss only
• Test PI + FFC full
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In-Class PS Exercise: Feed Forward Control
PI only
PI + FFCss only
PI + full FFC
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ChE / MET 433
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