estructuras de control - uvaprada/controlstructuresuk.pdf(advanced control) prof. cesar de prada ....
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Prof. Cesar de Prada. ISA. UVA 1
Control Structures (Advanced Control)
Prof. Cesar de Prada Dpt. Systems Engineering and Automatic Control
University of Valladolid, Spain [email protected]
Prof. Cesar de Prada. ISA. UVA 2
Control Structures
Changes made in conventional control loops in order to improve: – Disturbance rejection – Ratio between variables – Operation with several competing variables – Operation with several controllers – Operation with several actuators – Etc.
Prof. Cesar de Prada. ISA. UVA 3
Control structures
Cascade loops Feedforward compensators Ratio controllers Selective control Override control Split range Inferential control Examples of control of several process units Design methodology
Prof. Cesar de Prada. ISA. UVA 4
Standard control loop
TT
u TC
w
q T
Condensate
pa
Fv
If pa changes, T will change and the disturbance will be corrected by the controller using the signal u to the valve
No se puede mostrar la imagen en este momento.
Prof. Cesar de Prada. ISA. UVA 5
Block Diagram
u W T
pa
Reg Steam Heat exch.
q
Fv
TT
TC
T Condensate
Prof. Cesar de Prada. ISA. UVA 6
Cascade of controllers
TT
TC
w
q T
Condensate
pa
Fv
FC
FT
The external regulator (TC) changes the set point of the internal one (FC), which corrects the effect of the pressure change in pa over Fv before the disturbance affects the heat exchanger in a significant way
Prof. Cesar de Prada. ISA. UVA 7
Cascade control
W T
pa
TC Heat Exch.
q
Fv FC
The external regulator (TC) changes the set point of the internal one (FC), which corrects the effect of the pressure change in pa over Fv before the disturbance affects the heat exchanger in a significant way
Steam
Prof. Cesar de Prada. ISA. UVA 8
Cascade Control
W T
pa
TC Steam Heat Exch.
q
Fv FC
Main process (TC-Heat Exchanger) slow Secondary process (FC-Steam) fast Disturbances on the secondary part of the process that can be corrected More instrumentation
Prof. Cesar de Prada. ISA. UVA 9
Tuning/Operation
W R1 G2 G1 R2
y
Internal loops must be tuned first, then the external ones Generally speaking, cascade control is faster than single loop If a controller is in manual, all loops external to it must be in manual
Prof. Cesar de Prada. ISA. UVA 10
Closed loop TF W
R1 G2 G1 R2 y u1 y2
)s(G)s(R1)s(G)s(R
22
22
+
W R1 G1
y u1 y2
)s(WRGGR)RG1(
RGGR)s(W
RG1RGGR1
RG1RGGR
)s(Y 1221122
22111
22
2211
22
2211
1 ++=
++
+=
Prof. Cesar de Prada. ISA. UVA 11
Cascade Temp-Pressure
TT
TC
w
q T
Condensate
pa
Fv
PC
PT
An internal pressure controller (PC) can perform a more efficient operation
Prof. Cesar de Prada. ISA. UVA 12
Cascade Control
W T
pa
TC Steam Cambiador
q
ps PC
The external controller (TC) commands the SP of the internal one (PC) which corrects the effect of changes in pa over ps before they reach the heat exchanger
Prof. Cesar de Prada. ISA. UVA 13
Level control
q
LC
w
u LT
qi
h
Changes in pressure ps at the end of the line modify the flow q and the level h. The controller changes u to restore level
ps
Prof. Cesar de Prada. ISA. UVA 14
Cascade Control
q
FC
w
u LT
qi
h
LC
FT
The external regulator (LC) modifies the internal one (FC) SP, which corrects the disturbances on q before they affect in a significant way the tank level h
ps
Prof. Cesar de Prada. ISA. UVA 15
Cascade Level-Flow
W h
ps
LC Flow Tank
qi
q FC
The external regulator (LC) modifies the external one (FC) SP, which corrects the disturbances on q before they affect in a significant way the tank level h
Prof. Cesar de Prada. ISA. UVA 16
Temperature- Reactor
Reactor
TT
T
Coolant Product
TC
If the input refrigerant temperature Ti changes: Tr and T will change too and the TC controller will correct it by changing the control signal u
u
Ti
Reactant
Tr
Jacket
Coolant
Prof. Cesar de Prada. ISA. UVA 17
Cascade Temp-Temp
Reactor
TT
T Coolant
TC
Ti TT TC
Tr
The external regulator (TC1) modifies the internal one (TC2) SP, which corrects the effect of the disturbance Ti on Tr before they affect T in a significant way
Reactant
Jacket Coolant
Prof. Cesar de Prada. ISA. UVA 18
Cascade Temp-Temp
W T
Ti
TC1 Refrig Reactor Tr
TC2
The external regulator (TC1) modifies the internal one (TC2) SP, which corrects the effect of the disturbance Ti on Tr before they affect T in a significant way
Prof. Cesar de Prada. ISA. UVA 19
Temperature reactor control
50 45
1.5 ºC
10 min.
Prof. Cesar de Prada. ISA. UVA 20
Jacket temperature reactor control
Prof. Cesar de Prada. ISA. UVA 21
Cascade temp reactor control
50 45
0.3 ºC
4 min.
Prof. Cesar de Prada. ISA. UVA 22
Feedforward control
TT
u TC
w
q T
Condensate
pa
Fv
If there are changes in the flow q or in Ti: The controller only reacts when T has changed
Ti
Prof. Cesar de Prada. ISA. UVA 23
TT
u TC
w
q T
Condensado
pa
Fv
Feedforward
Changes in flow q : Feedforward compensator will modify the signal to the valve according to the flow changes as soon as they appear
FT
FY
Prof. Cesar de Prada. ISA. UVA 24
Feedforward
U(s) Y(s)
P(s) GF
G
Gp
The compensator will implement a change on Y(s) through GF and G, equal in magnitude and of opposite sign to the one produced by the disturbance P(s) through GP, in order to compensate it
Prof. Cesar de Prada. ISA. UVA 25
Feedforward
Measurable disturbances that cannot be controlled directly
Additional instrumentation and computation is needed
GP should be slower than G It is an open loop compensation. It must be
used in addition to closed loop control
Prof. Cesar de Prada. ISA. UVA 26
Block Diagram
u W Y R
P
G
GP
+ -
GF
[ ][ ] [ ]
)s(P)s(R)s(G1G)s(G)s(G)s(W
)s(R)s(G1)s(R)s(G)s(Y
)s(P)s(G)s(G)s(G)s(Y)s(W)s(R)s(G)s(P)s(G)s(P)s(G)s(U)s(G)s(Y
PF
PF
PF
++
++
=
++−==++=Closed loop
dynamics does not change
Prof. Cesar de Prada. ISA. UVA 27
How to compute GF?
U(s) Y(s)
P(s) GF
G
Gp
[ ][ ]
)s(G)s(G)s(G0)s(P)s(G)s(G)s(G)s(U)s(G)s(P)s(G)s(P)s(G)s(U)s(G)s(Y
PF
PF
PF
+=++=
=++=
)s(G)s(GG P
F −=
Prof. Cesar de Prada. ISA. UVA 28
Practical GF
)s(G)s(GG P
F −=
Realizability not assured Can be high order Linear approach: range of validity ( GP and G)
Practical GF :
)1as()1bs(KG F
F ++
−=KKK P
F =
Prof. Cesar de Prada. ISA. UVA 29
Lead/Lag
0 20 40 60 80 100 1200
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 20 40 60 80 100 1200
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
)1as()1bs(KG F
F ++
−=
b > a b < a
Prof. Cesar de Prada. ISA. UVA 30
Heat exchanger- disturbance
10 15
2 ºC
7 min.
Prof. Cesar de Prada. ISA. UVA 31
Model Temp - u
39 49
Open loop test
Prof. Cesar de Prada. ISA. UVA 32
Model Temp - u
)1s(ke)s(G
sd
+τ=
−
1s96.0e46.0 s87.0
+−
=−
Prof. Cesar de Prada. ISA. UVA 33
Model temp-warm flow
10 12
Open loop test
Prof. Cesar de Prada. ISA. UVA 34
Model Temp- warm flow
)1s)(1s(e)1s(k)s(G
21
sdL
p +τ+τ+τ
=−
2
s87.0
)1s82.0(e)1s07.4(17.0
++−− −
Prof. Cesar de Prada. ISA. UVA 35
Heat exchanger Feedforward compensator
2s87.0
2
s87.0
PF )1s82.0(
)1s4)(1s96.0(34.0
1s96.0e46.0
)1s82.0(e)1s4(17.0
)s(G)s(GG
++−+
=
+−
++−−
=−= −
−
Prof. Cesar de Prada. ISA. UVA 36
Heat exchanger with feedforward
0.2 ºC
? min. 10 15
10 15
Prof. Cesar de Prada. ISA. UVA 37
Static Compensator / model
TT
TC
w
q T
Condensado
pa
Fv
FC
FT
TT FT
v
ieH
)Tw(qcvF ρ∆
−=
Process dynamics must be included Static model provides KF
1s1as
++
τ
Prof. Cesar de Prada. ISA. UVA 38
TT
u TC
w
q T
Condensate
pa
Fv
Cascade+Feedforward
FT
FY PC
PT
Prof. Cesar de Prada. ISA. UVA 39
Control of proportions
Product A Product B
Aim: Keep the proportion (r) between B and A in the mixture
RC FT FT FY
FA FB/FA
FB
r
Prof. Cesar de Prada. ISA. UVA 40
Ratio Control
Product A Product B
r
FC FT FT FF
F rF
Best dynamic characteristics
Aim: Keep the proportion (r) between B and A in the mixture
Prof. Cesar de Prada. ISA. UVA 41
Ratio Control
FC FT FT FF RC FT FT FY
AB
2B
A
A
B
F1
Fr
FF
Fr
FFr
A
=∂∂
−=∂∂
=
1FF
rFF
rFF
B
B
A
B
AB
=∂∂
=∂∂=
A B
Controlled Variable
Gain disturbance
Gain Manipulated Var. Gain changes Gain is cte
Prof. Cesar de Prada. ISA. UVA 42
Block Diagram
rFA FB Reg Flow r
FA + -
The set point of flow FB is adjusted continuously as a function of the measured flow FA
Prof. Cesar de Prada. ISA. UVA 43
Selective Control
TT TT TT
TC
Tubular Reactor
Coolant
Raw products
Which temperature should be controlled?
T
x
Prof. Cesar de Prada. ISA. UVA 44
Selective Control
TT TT TT
TC
HS
Tubular Reactor
Coolant
Raw products
The highest temperature is selected as the one to be controlled at every time instant
Prof. Cesar de Prada. ISA. UVA 45
Selective Control
FT FC
FT FC
FT FC
SC
PT
ST
PC
Air
As the demand of every user changes with time, w is selected in order to cope with the highest one. This is not the best policy
w
Compressor Motor
Prof. Cesar de Prada. ISA. UVA 46
Selective Control / VPC
FT FC
FT FC
FT FC
SC
PT
ST
PC
Gas
w
Compressor Motor
HS VPC
90%
Pressure is adjusted continuously, so that the most wide opened valve is at ,let’s say, 90% opening VPC: Valve position control
Prof. Cesar de Prada. ISA. UVA 47
Selective Control / Safety
Reactor AT AT
HS AC
Catalyst
Reactant
If a transmitter fails (signal to zero) the selector maintains the reading of the correct one. (but if the failure is signal to 100%, the controller stops the plant)
Prof. Cesar de Prada. ISA. UVA 48
Selective Control / Safety
Reactor AT AT
MS AC
Catalyst
Reactant
Another option is a two against one policy or a intermedium value selector
AT
< <
>
<
Selects the signal in the middle
Prof. Cesar de Prada. ISA. UVA 49
Override Control
q
LC
wL
u
LT
qi
h FT FC
wF LS
wL
< Aims : Flow q as constant as possible
Minimum level above wL (Pump protection)
Prof. Cesar de Prada. ISA. UVA 50
Override Control
FC
FT
TC
TT
LS
TC TT
wT
Aims: Keep T as constant as possible
Smoke temperature below wT
T
Gas
Disturbance: oil input temperature decreases
Prof. Cesar de Prada. ISA. UVA 51
Override Control
PT
PC
FT
FC
LS
Pmin
Surge (low input pressure)
Usually at 100%. If any problem appears it will decrease until the controller overrides the FC
Protection against cavitation in the pump
Override control
Prof. Cesar de Prada. ISA. UVA 52
q
FC w u
M
∼
LC
LT
Lmin
FT
LS
well Protection against running out of water in the well
Prof. Cesar de Prada. ISA. UVA 53
Override Control
FT
FC
SC
PT
ST
PC
Compressor Motor
LS
wP
Aims:
Flow as constant as possible,
Maximum pressure on the line below wP in spite of changing demands
Prof. Cesar de Prada. ISA. UVA 54
Override Control
FT
FC
SC
PT
ST
PC
Compressor Motor
LS
wP
p
F
wP
Prof. Cesar de Prada. ISA. UVA 55
Safety
FT FC
PC
PT
Pmax To the atmosphere
Limits the maximum pressure in the supply line
Prof. Cesar de Prada. ISA. UVA 56
Split-range Control
q
u
FT
FC wF
v1 v2
u
v1 v2
v1
v2
Table
Prof. Cesar de Prada. ISA. UVA 57
Split-range Control
Gas Consumer
PT
PC
UY
v1 v2
v1 v2
u
u
Split range
v1
v2
Under normal operating conditions, pressure regulation is performed with V2, but if it reaches its minimum, then V1 is used as a release valve
Prof. Cesar de Prada. ISA. UVA 58
Split range Control
u TT TC
TT TC UY Reactor
Water
Coolant
v1 v2
u
v1 v2
v2
v1
cooling heating
Prof. Cesar de Prada. ISA. UVA 59
Inferential Control
u XC
Inferred value of x XY PT
TT
Very often, we faced variables that are expensive or difficult to measure, or which measurements are unreliable or slow. In these cases, it is possible to substitute the transmitter signal by an estimation made from physical laws, models like NN, inferences, etc.
Prof. Cesar de Prada. ISA. UVA 60
Inferential Control
q
u FT FC
TT PT
FY
Compute or estimate a non-measured controlled variable. Compute mass flow from volumetric flow, pressure and temperature
Mass flow
Prof. Cesar de Prada. ISA. UVA 61
Physical laws
Saturator TT
TC
PT PY
Overheated steam
Saturation temperature is computed from steam pressure
Tsat PC
Agua