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Chen CL 1
A Process Heater with Feedback Control
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Chen CL 2
A Simple Pure Feedforward ControlConsidering Inlet Temperature As Main Disturbance
EB: Q Cp (To Ti) = FHv EffTi = MAIN disturbance
To = desired outlet temp
F = QCpHv
Eff(To Ti)
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Chen CL 3
Pure FF Control for Multiple DisturbancesConsidering Variation of Inlet Temperature,
Process Flow Rate, and Fuel Heating Value
Disturbance 1: variations of inlet temperature F = QCpHvEffTo Ti
Disturbance 2: variations of process flow rate
Disturbance 3: variations in fuel heating value
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Chen CL 4
Pure FF Control for Multiple DisturbancesConsidering Variation of Inlet Temperature,
Process Flow Rate, and Fuel Heating Value
Disturbance 1: variations of inlet temperature F = QCpHvEffTo Ti
Disturbance 2: variations of process flow rate
Disturbance 3: variations in fuel heating value
C C
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Chen CL 5
Pure FF Control for Multiple DisturbancesConsidering Variation of Inlet Temperature,
Process Flow Rate, and Fuel Heating Value
Disturbance 1: variations of inlet temperature F = QCpHvEffTo Ti
Disturbance 2: variations of process flow rate
Disturbance 3: variations in fuel heating value
Ch CL 6
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Chen CL 6
Dynamic Adjustment for FF Control Action
Fo(s)Fi(s)
= ds + 1gs + 1
eds
=
dg
+1 d/ggs + 1
eds
Ch CL 7
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Chen CL 7
FF Control withAdditive/Multiplicative FB Trim
Ch CL 8
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Chen CL 8
FF Control with Additive FB Trim
Most important disturbances are compensated by FF
Why feedback trim ?
Error in process model Un-measurable disturbances
FB signal should be scaled so that when it is in the center of its
range it represent zero correction to the FF signal
Process gain of the FB loop may vary inversely withprocess flow rate
Kp = measurement
controller output =
outlet temperature
fuel
To = Ti+Hv EffQ Cp
F
Kp = To
F = Hv
Eff
Q Cp 1
Fp
Ch CL 9
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Chen CL 9
FF Control with Multiplicative FB Trim
FB signal: a multiplying factor, K
If FF controller is exact
no correction is necessary K= 1
Temperature controller output should be scaled so that 0 100%of signal range represents a limited range of correction
Example: K= 0.75 1.25 FB can correct FF results by a factor25%
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Chen CL 12
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Chen CL 12
Experimental Approach for FF Control
Dynamic Effect of Manipulated Variable to Controlled Variable
TpdTo(t)
dt + To(t) = KpF(t dp)
Gp(s) = To(s)
F(s) =
Kpedps
Tps + 1
Chen CL 13
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Chen CL 13
Experimental Approach for FF Control
Desired: No Response of Controlled Variable to Load Change
without FF: To(s) = G(s)Q(s)use of FF: To(s) = G(s)Q(s) + Gp(s)F(s)
= G(s)Q(s) + Gp(s)GF(s)Q(s)
= [G(s) + Gp(s)GF(s)]
=0Q(s) = 0
Chen CL 14
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Chen CL 14
Experimental Approach for FF Control
Desired: No Response of Controlled Variable to Load Change
= GF(s) = G(s)
Gp(s) =
Keds
Ts+1
Kpedps
Tps+1
=
KKp
Tps + 1
Ts + 1
e(ddp)s
Chen CL 15
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Chen CL 15
Dynamic Compensation
Possible barriers to implementing perfect FF control: Other disturbances might exist
Process model may be incorrect
There had been no consideration of process dynamics
Abstract view of the
process:
two external influences
Load (disturbance)
Control effort
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Chen CL 18
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Chen CL 18
Implementation of Feedforward Control
Test each of the process paths:loadand processdynamics by FOPDT models
Take the ratio of the two TFs
Add feedback to compensate for un-measured disturbances or errorsin process models
Relax FB controller from the tuning if FB were used alone
(lower gain, longer reset, no D) No D action in FB (load upset is taken by FF)
Primary purpose of FB is to correct for steady-state errors in FF
controller
Responsibility of FB controller is not as great as if it werecontrolling the process alone
Chen CL 19
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Chen CL 19
Step Response of Lead-lag(with/without Dead Time) Function
Lead/Lag Only with Dead Time
Chen CL 20
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Chen CL 20
Fine Tuning The FF Controller
A FF control system by itself will rarely provide perfectcompensation for the measured disturbance
combined with FB
FB control acts only after the fact
(it must see an error to make a correction) the closer to perfect compensation the FF makes
the less correction required by FB controller
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Chen CL 23
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Incremental Effect of Various Adjustments:Summary
A change in only dead time will result in an incremental processresponse relatively soon after load change
A change in only lead-lag ratio will result in an incremental process
response that is somewhat father out in time from the time of load
change
A change in lag time (constant lead-lag ratio) will result in an
incremental process response that is the fastest away in time
Chen CL 24
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Effect of Composite Adjustments
Fine Tuning:
Observe initial response to a load change of the FF control
system
Determine the direction and relative time scale of the required
incremental corrective process response Adjust dynamic compensating terms accordingly
Example: Composite Adjustments
Decrease dead time to start the increased fuel response sooner Decrease lead-lag ratio to give less fuel increment once its
response is begun
Decrease lag time (maintaining a constant lead/lag ratio) to
cause a faster approach of the fuel to final equilibrium
Chen CL 25
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Chen CL 26
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Feed-forward: Balance Equation ApproachEX: A Mixing Process with Simple FB Control
Chen CL 27
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Chen CL 28
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Steady-State Feed-forward Scheme
Steady-state overall mass balance
0 = q5 + q1(t) + q2(t) + q7 q6(t)0 = q5+ q1(t) + q2(t) + q7 q6(t) (= const.)
q1(t) = q6(t) q2(t) 1000
Steady-state mass balance on component Aand the feed-forward relation:
0 = q5x5+ q2(t)x2+ q7x7 q6(t)x6(t)
q6(t) = 1
x6(t)[q5x5+ q7x7+ q2(t)x2]
= 1
x6(t)[850 + 0.99q2(t)]
q1(t) = 1xset6
[850 + 0.99q2(t)]
FY11Aq2(t) 1000
FY11B
Chen CL 29
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Steady-State Feed-forward Scheme
q1(t) =
1
xset6[850 + 0.99q
2(t)]
FY11A
q
2(t)
1000
FY11B
Chen CL 30
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FF Control with Dynamic Compensation
q1(t) =
1
xset6[850 + 0.99q
2(t)]
FY11A
q
2(t)
1000
FY11B
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Chen CL 32
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FF Control: Block Diagram Approach
Chen CL 33
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Pure Feed-forward Control
X6Q1
=G1X6Q2
=G2Q1M
=GFCM
TO=GF
TO
Q2=H2
X6=G2 Q2+ G1 Q1GFC M
GF TO
H2 Q2
Chen CL 34
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= [G2+ G1GFCGFH2] Q2 = 0 (desired)
GF(s) =
G2G1GFCH2
let G1GFC = KP1e
d1s
1s+1 (mf/%CO)
G2 = KP2ed
2s
2s+1 (mf/gpm)
H2(s) = KT2 (%TO/gpm)
GF =KP2KP1KT2
%CO/%TO
1s + 1
2s + 1
lead/lag
e(d2d1)s dead time
Chen CL 35
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Feed-forward Control with Feedback Trim
Chen CL 36
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Summer FY-11C: OUT = KxX+ KyY + KzZ+ B0FB signal: X; FF signal: Y; bias: B0 Kx= 1;Ky = 1;B0= (KP2/KT2KP1)40%
Steady-state value ofq2(t) 1000 gpm
Range of transmitter for q2 0 2500gpmSteady-state value of the flow 40%
Steady-state output from FF (KP2/KT2KP1)40%Bias to cancel SS FF signal (KP2/KT2KP1)40%Steady-state value ofq1(t) 1900 gpm
Range of transmitter for q1
0
3800gpm
Output from summer must be 50%
Output from FB is forced to be 50%
Ky = 0: FF is turned off
Chen CL 37
C
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FF Control for Boiler DrumSingle-Element Control
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Chen CL 40
FF f Di ill i C l
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FF for Distillation ColumnSimple Feedback Control
Chen CL 41
FF f Di ill i C l
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FF for Distillation ColumnFeed-forward Control Scheme
m
: lbm/hr;
: Btu/lbm
q
s = q
t q
1= q
t m
= q
t Kh
= f1(P)
= f2(P)
signalcharacterizers
Chen CL 42
FF C l A Di ill i C l
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FF Control on A Distillation Column
Chen CL 43
FF C t l f Di till ti F d
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FF Control for Distillation FeedFF Control Handles Two Upsets Simultaneously
Variables:
feed composition and flow-rate to a distillation column
excessive impurities to appear in bottoms product
Temperature of the two-component mixture in lower-bottoms
sump has a direct relationship to impurity concentration (sufficient
correlation often exists even for multi-component mixtures)
F, z, Fz: feed, fraction of light, flow of lighter component
Steam flow-rate should be nearly proportional to Fz temperature controller adjusts the ratio to compensate
for possible errors in measurement, model
Chen CL 44
M t diffi lt t d i ti
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Most difficult part: dynamic compensation
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Chen CL 47
FF C t l E t
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FF Control on Evaporator
Mass and Energy Balances:
(W: mass rate, kg/h; x: weight fraction;E: economy, kg vapor/kg steam)
1st effect: W0x0 = W1x1
W0 = W1+ V12st effect: W1x1 = W2x2
W1 = W2+ V2
W2 = (x0/x2)W0
W0 = W2+ V1+ V2
W00F0
1 x0x2
= V1+ V2
EWs
Ws =
1
E
0F0 1x0
x
2
Chen CL 48
Feed density and feed solids weight fraction is related
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Feed density and feed solids weight fraction is related
signal characterizer f(0) :x0 vs. 0
Feed flow signal (F0) is dynamically compensated with lead/lag
function
Product density does not respond with equal speed to changes
in feed-rate and steam flow
Changes in steam flow produce a slower response because of
thermal time lags associated with heat transfer surfaces apredominant lead function
Density normally does not vary as fast as feed flow
feed-density dynamic compensation is not included
FB trim is provided by density controller to provide desired setpoint,
x2, in feed-forward model
Chen CL 49
FF Control on Evaporator
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FF Control on Evaporator