Industrial Control Systems - Special Structures
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Industrial Control
Behzad Samadi
Department of Electrical EngineeringAmirkabir University of Technology
Winter 2009Tehran, Iran
Behzad Samadi (Amirkabir University) Industrial Control 1 / 59
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Special Control Structures
Outline:
Set-point Weighting
Feedforward Action
Ratio Control
Cascade Control
Override Control
Selective Control
Split Range Control
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 2 / 59
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Special Control Structures
Outline:
Set-point Weighting
Feedforward Action
Ratio Control
Cascade Control
Override Control
Selective Control
Split Range Control
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 2 / 59
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Special Control Structures
Outline:
Set-point Weighting
Feedforward Action
Ratio Control
Cascade Control
Override Control
Selective Control
Split Range Control
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 2 / 59
![Page 5: Industrial Control Systems - Special Structures](https://reader034.fdocuments.us/reader034/viewer/2022042623/54bc52f34a795906048b457b/html5/thumbnails/5.jpg)
Special Control Structures
Outline:
Set-point Weighting
Feedforward Action
Ratio Control
Cascade Control
Override Control
Selective Control
Split Range Control
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 2 / 59
![Page 6: Industrial Control Systems - Special Structures](https://reader034.fdocuments.us/reader034/viewer/2022042623/54bc52f34a795906048b457b/html5/thumbnails/6.jpg)
Special Control Structures
Outline:
Set-point Weighting
Feedforward Action
Ratio Control
Cascade Control
Override Control
Selective Control
Split Range Control
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 2 / 59
![Page 7: Industrial Control Systems - Special Structures](https://reader034.fdocuments.us/reader034/viewer/2022042623/54bc52f34a795906048b457b/html5/thumbnails/7.jpg)
Special Control Structures
Outline:
Set-point Weighting
Feedforward Action
Ratio Control
Cascade Control
Override Control
Selective Control
Split Range Control
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 2 / 59
![Page 8: Industrial Control Systems - Special Structures](https://reader034.fdocuments.us/reader034/viewer/2022042623/54bc52f34a795906048b457b/html5/thumbnails/8.jpg)
Special Control Structures
Outline:
Set-point Weighting
Feedforward Action
Ratio Control
Cascade Control
Override Control
Selective Control
Split Range Control
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 2 / 59
![Page 9: Industrial Control Systems - Special Structures](https://reader034.fdocuments.us/reader034/viewer/2022042623/54bc52f34a795906048b457b/html5/thumbnails/9.jpg)
Special Control Structures
Set-point Weighting :
High load disturbance rejection → too oscillatory set-point stepresponse.
This is actually true especially when the apparent dead time of theprocess is small (with respect to the dominant time constant).
Set-point weighting for PID controllers:
u(t) = Kp(βr(t)− y(t) +1
Ti
∫ t
0e(τ)dτ − Td
dy(t)
dt)
u(t): control signal, r(t): reference input,y(t): process output, e(t): tracking error
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 3 / 59
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Special Control Structures
Set-point Weighting :
High load disturbance rejection → too oscillatory set-point stepresponse.
This is actually true especially when the apparent dead time of theprocess is small (with respect to the dominant time constant).
Set-point weighting for PID controllers:
u(t) = Kp(βr(t)− y(t) +1
Ti
∫ t
0e(τ)dτ − Td
dy(t)
dt)
u(t): control signal, r(t): reference input,y(t): process output, e(t): tracking error
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 3 / 59
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Special Control Structures
Set-point Weighting :
High load disturbance rejection → too oscillatory set-point stepresponse.
This is actually true especially when the apparent dead time of theprocess is small (with respect to the dominant time constant).
Set-point weighting for PID controllers:
u(t) = Kp(βr(t)− y(t) +1
Ti
∫ t
0e(τ)dτ − Td
dy(t)
dt)
u(t): control signal, r(t): reference input,y(t): process output, e(t): tracking error
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 3 / 59
![Page 12: Industrial Control Systems - Special Structures](https://reader034.fdocuments.us/reader034/viewer/2022042623/54bc52f34a795906048b457b/html5/thumbnails/12.jpg)
Special Control Structures
Set-point Weighting:
u(t) = Kp(βr(t)− y(t) +1
Ti
∫ t
0e(τ)dτ − Td
dy(t)
dt)
Intuitively, given a set of parameters Kp, Ti and Td , the adoption ofa set-point weight β < 1 allows the reduction of the overshoot in theset-point response, since the effect of the proportional action isreduced.
Note that this is achieved without affecting the load disturbancerejection performance.
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 4 / 59
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Special Control Structures
Set-point Weighting:
u(t) = Kp(βr(t)− y(t) +1
Ti
∫ t
0e(τ)dτ − Td
dy(t)
dt)
Intuitively, given a set of parameters Kp, Ti and Td , the adoption ofa set-point weight β < 1 allows the reduction of the overshoot in theset-point response, since the effect of the proportional action isreduced.
Note that this is achieved without affecting the load disturbancerejection performance.
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 4 / 59
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Special Control Structures
Set-point Weighting:
U = Kp(βR − Y +1
Ti sE − TdsY )
U = Kp(β +1
Ti s)R − Kp(1 +
1
Ti s+ Tds)Y
U = Kp(1 +1
Ti s+ Tds)
βs + 1
1 + Ti s + TiTds2R − Kp(1 +
1
Ti s+ Tds)Y
U = Kp(1 +1
Ti s+ Tds)(FR − Y )
F =βs + 1
1 + Ti s + TiTds2
Behzad Samadi (Amirkabir University) Industrial Control 5 / 59
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Special Control Structures
Set-point Weighting:
U = Kp(βR − Y +1
Ti sE − TdsY )
U = Kp(β +1
Ti s)R − Kp(1 +
1
Ti s+ Tds)Y
U = Kp(1 +1
Ti s+ Tds)
βs + 1
1 + Ti s + TiTds2R − Kp(1 +
1
Ti s+ Tds)Y
U = Kp(1 +1
Ti s+ Tds)(FR − Y )
F =βs + 1
1 + Ti s + TiTds2
Behzad Samadi (Amirkabir University) Industrial Control 5 / 59
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Special Control Structures
Set-point Weighting:
U = Kp(βR − Y +1
Ti sE − TdsY )
U = Kp(β +1
Ti s)R − Kp(1 +
1
Ti s+ Tds)Y
U = Kp(1 +1
Ti s+ Tds)
βs + 1
1 + Ti s + TiTds2R − Kp(1 +
1
Ti s+ Tds)Y
U = Kp(1 +1
Ti s+ Tds)(FR − Y )
F =βs + 1
1 + Ti s + TiTds2
Behzad Samadi (Amirkabir University) Industrial Control 5 / 59
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Special Control Structures
Set-point Weighting:
U = Kp(βR − Y +1
Ti sE − TdsY )
U = Kp(β +1
Ti s)R − Kp(1 +
1
Ti s+ Tds)Y
U = Kp(1 +1
Ti s+ Tds)
βs + 1
1 + Ti s + TiTds2R − Kp(1 +
1
Ti s+ Tds)Y
U = Kp(1 +1
Ti s+ Tds)(FR − Y )
F =βs + 1
1 + Ti s + TiTds2
Behzad Samadi (Amirkabir University) Industrial Control 5 / 59
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Special Control Structures
Set-point Weighting:
u(t) = Kp(βr(t)− y(t) +1
Ti
∫ t
0e(τ)dτ − Td
dy(t)
dt)
C (s) = Kp(1 +1
Ti s+ Tds)
F (s) =1 + βTi s
1 + Ti s + TiTds2
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 6 / 59
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Special Control Structures
Set-point Weighting:
F (s) =1 + βTi s
1 + Ti s + TiTds2
From another point of view, the overshoot is reduced by smoothingthe set-point signal by means of the filter F .
Example:
P(s) =1
10s + 1e−4s
Ziegler-Nichols: Kp = 3, Ti = 8 and Td = 2
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 7 / 59
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Special Control Structures
Set-point Weighting:
F (s) =1 + βTi s
1 + Ti s + TiTds2
From another point of view, the overshoot is reduced by smoothingthe set-point signal by means of the filter F .
Example:
P(s) =1
10s + 1e−4s
Ziegler-Nichols: Kp = 3, Ti = 8 and Td = 2
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 7 / 59
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Special Control Structures
Set-point Weighting:
[Visioli, 2006]Behzad Samadi (Amirkabir University) Industrial Control 8 / 59
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Special Control Structures
Feedforward Action:
Set-point tracking:
M(s) represents the desired performance.
G (s) = M(s)
P̃(s)where tildeP(s) is the minimum phase part of P(s).
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 9 / 59
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Special Control Structures
Feedforward Action:
Set-point tracking:
M(s) represents the desired performance.
G (s) = M(s)
P̃(s)where tildeP(s) is the minimum phase part of P(s).
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 9 / 59
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Special Control Structures
Feedforward Action:
Set-point tracking:
M(s) represents the desired performance.
G (s) = M(s)
P̃(s)where tildeP(s) is the minimum phase part of P(s).
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 9 / 59
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Special Control Structures
Feedforward Action:
Set-point tracking:
Y (s)
R(s)=
(MC + G )P
1 + PC
Therefore if P̃(s) = P(s) and C (s) = 0 then
Y (s)
R(s)= G (s)P(s) = M(s)
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 10 / 59
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Special Control Structures
Feedforward Action:
Set-point tracking:
Y (s)
R(s)=
(MC + G )P
1 + PC
Therefore if P̃(s) = P(s) and C (s) = 0 then
Y (s)
R(s)= G (s)P(s) = M(s)
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 10 / 59
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Special Control Structures
Feedforward Action:
Set-point tracking:
Y (s)
R(s)=
(MC + G )P
1 + PC
Therefore if P̃(s) = P(s) and C (s) = 0 then
Y (s)
R(s)= G (s)P(s) = M(s)
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 10 / 59
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Special Control Structures
Feedforward Action:
Set-point tracking:
Example:
P(s) =1
10s + 1e−5s
M(s) =1
2s + 1e−5s
G (s) =10s + 1
2s + 1[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 11 / 59
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Special Control Structures
Feedforward Action:
Disturbance rejection:
Y = PCE (s) + (H − PG )D(s)
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 12 / 59
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Special Control Structures
Feedforward Action (disturbance rejection):
H − PG = 0⇒ G = HP
Example:
P(s) =K
Ts + 1e−Ls
H(s) =KH
THs + 1e−LH s
If LH = L:
G (s) =H
P=
KH
K
Ts + 1
THs + 1
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 13 / 59
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Special Control Structures
Feedforward Action (disturbance rejection):
H − PG = 0⇒ G = HP
Example:
P(s) =K
Ts + 1e−Ls
H(s) =KH
THs + 1e−LH s
If LH = L:
G (s) =H
P=
KH
K
Ts + 1
THs + 1
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 13 / 59
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Special Control Structures
Feedforward Action (disturbance rejection):
H − PG = 0⇒ G = HP
Example:
P(s) =K
Ts + 1e−Ls
H(s) =KH
THs + 1e−LH s
If LH = L:
G (s) =H
P=
KH
K
Ts + 1
THs + 1
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 13 / 59
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Special Control Structures
Feedforward Action (disturbance rejection):
Example:
P(s) =K
Ts + 1e−Ls
H(s) =KH
THs + 1e−LH s
If LH 6= L:
G (s) =H
P≈ KH
K
(T − LH + L)s + 1
THs + 1[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 14 / 59
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Special Control Structures
Feedforward Action (disturbance rejection):
Example:
P(s) =K
(T1s + 1)(T2s + 1)
H(s) =KH
THs + 1
G (s) =H
P≈ KH
K
(T1 + T2)s + 1
THs + 1
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 15 / 59
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Special Control Structures
Feedforward Action (disturbance rejection):
Example:
P(s) =K
(T1s + 1)(T2s + 1)
H(s) =KH
THs + 1
G (s) =H
P≈ KH
K
(T1 + T2)s + 1
THs + 1
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 15 / 59
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Special Control Structures
Feedforward Action (disturbance rejection):
Example:
P(s) =K
(T1s + 1)(T2s + 1)
H(s) =KH
THs + 1
G (s) =H
P≈ KH
K
(T1 + T2)s + 1
THs + 1
[Visioli, 2006]
Behzad Samadi (Amirkabir University) Industrial Control 15 / 59
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Special Control Structures
Heat Exchanger Process:
heatexdemo - Mathworks.com
Behzad Samadi (Amirkabir University) Industrial Control 16 / 59
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Special Control Structures
Heat Exchanger Process:
t1 = 21.8 (for 28.3% of the final value)
t2 = 36.0 (for 63.2% of the final value)
τ = 32(t2− t1)
θ = t2− τ
heatexdemo - Mathworks.com
Behzad Samadi (Amirkabir University) Industrial Control 17 / 59
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Special Control Structures
Heat Exchanger Process:
t1 = 21.8 (for 28.3% of the final value)
t2 = 36.0 (for 63.2% of the final value)
τ = 32(t2− t1)
θ = t2− τ
heatexdemo - Mathworks.com
Behzad Samadi (Amirkabir University) Industrial Control 17 / 59
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Special Control Structures
Heat Exchanger Process:
t1 = 21.8 (for 28.3% of the final value)
t2 = 36.0 (for 63.2% of the final value)
τ = 32(t2− t1)
θ = t2− τ
heatexdemo - Mathworks.com
Behzad Samadi (Amirkabir University) Industrial Control 17 / 59
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Special Control Structures
Heat Exchanger Process:
t1 = 21.8 (for 28.3% of the final value)
t2 = 36.0 (for 63.2% of the final value)
τ = 32(t2− t1)
θ = t2− τheatexdemo - Mathworks.com
Behzad Samadi (Amirkabir University) Industrial Control 17 / 59
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Special Control Structures
Heat Exchanger Process:
Gp(s) =1
23.3s + 1e−14.7s
heatexdemo - Mathworks.com
Behzad Samadi (Amirkabir University) Industrial Control 18 / 59
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Special Control Structures
Heat Exchanger Process:
Gd(s) =1
25s + 1e−35s
heatexdemo - Mathworks.com
Behzad Samadi (Amirkabir University) Industrial Control 19 / 59
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Special Control Structures
Heat Exchanger Process:
PI controller setting using ITAE
Kc = 0.859(θ
τ)−0.997, Ti = (
θ
τ)0.680
heatexdemo - Mathworks.com
Behzad Samadi (Amirkabir University) Industrial Control 20 / 59
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Special Control Structures
Heat Exchanger Process:
Kc = 1.23 (blue) and Kc = 0.9 (red)
Ti = 24.56
heatexdemo - Mathworks.com
Behzad Samadi (Amirkabir University) Industrial Control 21 / 59
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Special Control Structures
Heat Exchanger Process:
F (s) = −Gd
Gp= −21.3s + 1
25s + 1e−20.3s
heatexdemo - Mathworks.com
Behzad Samadi (Amirkabir University) Industrial Control 22 / 59
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Special Control Structures
Heat Exchanger Process:
heatexdemo - Mathworks.com
Behzad Samadi (Amirkabir University) Industrial Control 23 / 59
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Special Control Structures
Heat Exchanger Process:
heatexdemo - Mathworks.com
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Special Control Structures
Ratio Control:
Keep a constant ratio between two (or more) process variables,irrespective of possible set-point changes and load disturbances.
Chemical dosing, water treatment, chlorination, mixing vessels andwaste incinerators
Air-to-fuel ratio for combustion systems
[Chau, 2002]
[Visioli, 2006]
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Special Control Structures
Ratio Control:
Keep a constant ratio between two (or more) process variables,irrespective of possible set-point changes and load disturbances.
Chemical dosing, water treatment, chlorination, mixing vessels andwaste incinerators
Air-to-fuel ratio for combustion systems
[Chau, 2002]
[Visioli, 2006]
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Special Control Structures
Ratio Control:
Keep a constant ratio between two (or more) process variables,irrespective of possible set-point changes and load disturbances.
Chemical dosing, water treatment, chlorination, mixing vessels andwaste incinerators
Air-to-fuel ratio for combustion systems
[Chau, 2002]
[Visioli, 2006]
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Special Control Structures
Ratio Control:Series metered control:
The output y2 is necessarily delayed with respect to y1, due to theclosed-loop dynamics of the second loop.
[Visioli, 2006]
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Special Control Structures
Ratio Control:Parallel metered control:
A disturbance acting on the first process can cause a large error in theratio value.
[Visioli, 2006]
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Special Control Structures
Ratio Control:Cross limited control scheme:
The two loops are interlocked by using a low and a high selectors thatforce the fuel to follow the air flow when the set-point increases andthat force the air to follow the fuel when the set-point decreases.
[Visioli, 2006]
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Special Control Structures
Ratio Control:Blend Station:
r2(t) = a(γr1(t) + (1− γ)y1(t))
Its use is suggested when no disturbances are likely to occur in theprocesses and when the two processes exhibit a different dynamics.
[Visioli, 2006]
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Special Control Structures
Ratio Control:Modified Blend Station:
P1(s) =K1
T1s + 1e−L1s , P2(s) =
K2
T2s + 1e−L2s
u1(t) = Kp1
(βr1(t)− y1(t) +
1
Ti1
∫ t
0(r1(τ)− y1(τ))dτ
)u2(t) = Kp2
(βr2(t)− y2(t) +
1
Ti2
∫ t
0(r2(τ)− y2(τ))dτ
)[Visioli, 2006]
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Special Control Structures
Ratio Control:Modified Blend Station:
P1(s) =K1
T1s + 1e−L1s , P2(s) =
K2
T2s + 1e−L2s
u1(t) = Kp1
(βr1(t)− y1(t) +
1
Ti1
∫ t
0(r1(τ)− y1(τ))dτ
)u2(t) = Kp2
(βr2(t)− y2(t) +
1
Ti2
∫ t
0(r2(τ)− y2(τ))dτ
)[Visioli, 2006]
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Special Control Structures
Ratio Control:Modified Blend Station:
γ =
{0 if L1 > L2 and t < t0 + L1 − L2
γ? + Kp(er (t) + 1Ti
∫ t0 er (τ)dτ) otherwise
er (t) = ay1(t)− y2(t)
Kp1 Ti1 β1 Kp2 Ti2 β2 γ? Kp Tp0.9T1K1L1
3L1 0 0.9T2K2L2
3L2 0 Ti2Ti1
0.5 L2T2
T1L1
T1L1
[Visioli, 2006]
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Special Control Structures
Ratio Control:Modified Blend Station:
γ =
{0 if L1 > L2 and t < t0 + L1 − L2
γ? + Kp(er (t) + 1Ti
∫ t0 er (τ)dτ) otherwise
er (t) = ay1(t)− y2(t)
Kp1 Ti1 β1 Kp2 Ti2 β2 γ? Kp Tp0.9T1K1L1
3L1 0 0.9T2K2L2
3L2 0 Ti2Ti1
0.5 L2T2
T1L1
T1L1
[Visioli, 2006]
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Special Control Structures
Ratio Control:Modified Blend Station:
γ =
{0 if L1 > L2 and t < t0 + L1 − L2
γ? + Kp(er (t) + 1Ti
∫ t0 er (τ)dτ) otherwise
er (t) = ay1(t)− y2(t)
Kp1 Ti1 β1 Kp2 Ti2 β2 γ? Kp Tp0.9T1K1L1
3L1 0 0.9T2K2L2
3L2 0 Ti2Ti1
0.5 L2T2
T1L1
T1L1
[Visioli, 2006]
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Special Control Structures
Ratio Control:Adaptive Blend Station (Hagglund, 2001):
dγ
dt=
S
Ta(ay1 − y2)
Ta = 10 max{T1,T2}
If r1 > max{y1, y2/a}+ eps then S = 1Else if r1 < min{y1, y2/a} − eps then S = −1Else S = 0[Visioli, 2006]
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Special Control Structures
Ratio Control:Adaptive Blend Station (Hagglund, 2001):
dγ
dt=
S
Ta(ay1 − y2)
Ta = 10 max{T1,T2}
If r1 > max{y1, y2/a}+ eps then S = 1Else if r1 < min{y1, y2/a} − eps then S = −1Else S = 0[Visioli, 2006]
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Special Control Structures
Ratio Control:Adaptive Blend Station (Hagglund, 2001):
dγ
dt=
S
Ta(ay1 − y2)
Ta = 10 max{T1,T2}
If r1 > max{y1, y2/a}+ eps then S = 1Else if r1 < min{y1, y2/a} − eps then S = −1Else S = 0[Visioli, 2006]
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Special Control Structures
Cascade Control:
[Chau, 2002]
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Special Control Structures
Cascade Control:
[Chau, 2002]
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Special Control Structures
Cascade Control:
Primary (master) loop: outer loop
Secondary (slave) loop: inner loop
P1: slow dynamics
P2: fast dynamics
The approach can be generalized to more than two loops.
[Visioli, 2006]
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Special Control Structures
Cascade Control:
Primary (master) loop: outer loop
Secondary (slave) loop: inner loop
P1: slow dynamics
P2: fast dynamics
The approach can be generalized to more than two loops.
[Visioli, 2006]
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Special Control Structures
Cascade Control:
Primary (master) loop: outer loop
Secondary (slave) loop: inner loop
P1: slow dynamics
P2: fast dynamics
The approach can be generalized to more than two loops.
[Visioli, 2006]
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Special Control Structures
Cascade Control:
Primary (master) loop: outer loop
Secondary (slave) loop: inner loop
P1: slow dynamics
P2: fast dynamics
The approach can be generalized to more than two loops.
[Visioli, 2006]
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Special Control Structures
Cascade Control:
Primary (master) loop: outer loop
Secondary (slave) loop: inner loop
P1: slow dynamics
P2: fast dynamics
The approach can be generalized to more than two loops.
[Visioli, 2006]
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Special Control Structures
Cascade Control:
It appears that the improvement in the cascade control performanceis more significant when disturbances act in the inner loop and whenthe secondary sensor is placed in order to separate as far as possiblethe fast dynamics of the process from the slow dynamics(Krishnaswami et al., 1990).
Additional advantage: The nonlinearities of the process in the innerloop are handled by that loop and therefore they are removed fromthe more important outer loop.
When the secondary process exhibits a significant dead time or thereis an unstable (positive) zero, the use of cascade control is not usefulin general.
[Visioli, 2006]
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Special Control Structures
Cascade Control:
It appears that the improvement in the cascade control performanceis more significant when disturbances act in the inner loop and whenthe secondary sensor is placed in order to separate as far as possiblethe fast dynamics of the process from the slow dynamics(Krishnaswami et al., 1990).
Additional advantage: The nonlinearities of the process in the innerloop are handled by that loop and therefore they are removed fromthe more important outer loop.
When the secondary process exhibits a significant dead time or thereis an unstable (positive) zero, the use of cascade control is not usefulin general.
[Visioli, 2006]
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Special Control Structures
Cascade Control:
It appears that the improvement in the cascade control performanceis more significant when disturbances act in the inner loop and whenthe secondary sensor is placed in order to separate as far as possiblethe fast dynamics of the process from the slow dynamics(Krishnaswami et al., 1990).
Additional advantage: The nonlinearities of the process in the innerloop are handled by that loop and therefore they are removed fromthe more important outer loop.
When the secondary process exhibits a significant dead time or thereis an unstable (positive) zero, the use of cascade control is not usefulin general.
[Visioli, 2006]
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Special Control Structures
Cascade Control:Design procedure:
First: Design the controller for the secondary loop
Second: Design the controller for the primary loop
[Visioli, 2006]
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Special Control Structures
Cascade Control:Design procedure:
First: Design the controller for the secondary loop
Second: Design the controller for the primary loop
[Visioli, 2006]
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Special Control Structures
Cascade Control:Example:
Gp =0.8
2s + 1, Gv =
0.5
s + 1, GL =
0.75
s + 1
[Chau, 2002]
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Special Control Structures
Cascade Control:Example:
Let us consider a proportional controller Gc2(s) = Kc2 and make thetime constant of the secondary loop equal to 0.1 second.
1 + 0.5Kc2 = 10⇒ Kc2 = 18
G ?v = 9
s+10
G ?v Gp = 0.9
0.1s+10.8
2s+1
G ?v Gp ≈ 0.72
2.05s+1e−0.05s
SIMC tuning rule:
Kc =1
k
τ1τc + θ
=1
0.72
2.05
0.15= 18.98
τi = min{τ1, 4(τc + θ)} = min{2.05, 4(0.15)} = 0.6
[Chau, 2002]
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Special Control Structures
Cascade Control:Example:
Let us consider a proportional controller Gc2(s) = Kc2 and make thetime constant of the secondary loop equal to 0.1 second.
1 + 0.5Kc2 = 10⇒ Kc2 = 18
G ?v = 9
s+10
G ?v Gp = 0.9
0.1s+10.8
2s+1
G ?v Gp ≈ 0.72
2.05s+1e−0.05s
SIMC tuning rule:
Kc =1
k
τ1τc + θ
=1
0.72
2.05
0.15= 18.98
τi = min{τ1, 4(τc + θ)} = min{2.05, 4(0.15)} = 0.6
[Chau, 2002]
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Special Control Structures
Cascade Control:Example:
Let us consider a proportional controller Gc2(s) = Kc2 and make thetime constant of the secondary loop equal to 0.1 second.
1 + 0.5Kc2 = 10⇒ Kc2 = 18
G ?v = 9
s+10
G ?v Gp = 0.9
0.1s+10.8
2s+1
G ?v Gp ≈ 0.72
2.05s+1e−0.05s
SIMC tuning rule:
Kc =1
k
τ1τc + θ
=1
0.72
2.05
0.15= 18.98
τi = min{τ1, 4(τc + θ)} = min{2.05, 4(0.15)} = 0.6
[Chau, 2002]
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Special Control Structures
Cascade Control:Example:
Let us consider a proportional controller Gc2(s) = Kc2 and make thetime constant of the secondary loop equal to 0.1 second.
1 + 0.5Kc2 = 10⇒ Kc2 = 18
G ?v = 9
s+10
G ?v Gp = 0.9
0.1s+10.8
2s+1
G ?v Gp ≈ 0.72
2.05s+1e−0.05s
SIMC tuning rule:
Kc =1
k
τ1τc + θ
=1
0.72
2.05
0.15= 18.98
τi = min{τ1, 4(τc + θ)} = min{2.05, 4(0.15)} = 0.6
[Chau, 2002]
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Special Control Structures
Cascade Control:Example:
Let us consider a proportional controller Gc2(s) = Kc2 and make thetime constant of the secondary loop equal to 0.1 second.
1 + 0.5Kc2 = 10⇒ Kc2 = 18
G ?v = 9
s+10
G ?v Gp = 0.9
0.1s+10.8
2s+1
G ?v Gp ≈ 0.72
2.05s+1e−0.05s
SIMC tuning rule:
Kc =1
k
τ1τc + θ
=1
0.72
2.05
0.15= 18.98
τi = min{τ1, 4(τc + θ)} = min{2.05, 4(0.15)} = 0.6
[Chau, 2002]
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Special Control Structures
Cascade Control:Example:
Let us consider a proportional controller Gc2(s) = Kc2 and make thetime constant of the secondary loop equal to 0.1 second.
1 + 0.5Kc2 = 10⇒ Kc2 = 18
G ?v = 9
s+10
G ?v Gp = 0.9
0.1s+10.8
2s+1
G ?v Gp ≈ 0.72
2.05s+1e−0.05s
SIMC tuning rule:
Kc =1
k
τ1τc + θ
=1
0.72
2.05
0.15= 18.98
τi = min{τ1, 4(τc + θ)} = min{2.05, 4(0.15)} = 0.6
[Chau, 2002]
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Special Control Structures
Cascade Control:Simultaneous tuning of the controllers:
Assume:P2(s) = P2m(s)P2a(s)
P2a(s) is the all-pass portion of the transfer function containing allthe nonminimum phase dynamics (P2a(0) = 1)
[Visioli, 2006]
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Special Control Structures
Cascade Control:Simultaneous tuning of the controllers:Desired inner loop transfer function:
T̄r2y2(s) =P2a(s)
(λ2s + 1)n2
λ2 and n2 are design parameters.
C2(s) =P−1
2m(s)
(λ2s + 1)n2 − P2a(s)
To approximate the controller with a PID controller:
C2(s) =1
sk(s) ≈ 1
s
[k(0) + k̇(0)s +
k̈(0)
2
][Visioli, 2006]
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Special Control Structures
Cascade Control:Simultaneous tuning of the controllers:Assume:
P21(s) = P1(s)P2a(s)
(λ2s + 1)n2= P12m(s)P12a(s)
P12a(s) is the nonminimum phase part in all-pass form.
Desired outer loop transfer function
T̄r1y1(s) =P12a(s)
(λ1s + 1)n1
Primary controller transfer function
C1(s) =P−1
12m(s)(λ2s + 1)n2
P2a(s)((λ1s + 1)n1 − P12a(s))
[Visioli, 2006]
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Special Control Structures
Cascade Control:Example:
P1(s) =K1
T1s + 1e−L1s
P2(s) =K2
T2s + 1e−L2s
T̄r2y2(s) =e−L2s
λ2s + 1
T̄r1y1(s) =e−(L1+L2)s
λ1s + 1[Visioli, 2006]
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Special Control Structures
Cascade Control:Example:
Kp2 =T2 +
L22
2(λ2+L2)
K2(λ2 + L2)
Ti2 = T2 +L2
2
2(λ2 + L2)
Td2 =L2
2
6(λ2 + L2)
3− L2
T2 +L2
22(λ2+L2)
[Visioli, 2006]
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Special Control Structures
Cascade Control:Example:
Kp1 =T1 + λ2 + (L1+L2)2
2(λ1+L1+L2)
K1(λ1 + L1 + L2)
Ti1 = T1 + λ2 +(L1 + L2)2
2(λ2 + L1 + L2)
Td1 =λ2T1 − (L1+L2)3
6(λ1+L1+L2)
T1 + λ2 + (L1+L2)2
2(λ1+L1+L2)
+(L1 + L2)2
2(λ1 + L1 + L2)
The suggestion is to set:
λ2 = 0.5L2
λ1 = 0.5(L1 + L2)
[Visioli, 2006]
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Special Control Structures
Override Control: There are two modes of operation
Normal operation: One process variable is the controlling variable.
Abnormal operation: Some other process variable becomes thecontrolling variable to prevent it from exceeding a process orequipment limit.
The limiting controller is said to override the normal processcontroller.
http://pse.che.ntu.edu.tw/chencl/Process_Control
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Special Control Structures
Override Control:
[Smith, 2002]
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Special Control Structures
Override Control:
The set point to LC50 is somewhat above h2, as shown in the figure.
The FC50 is a reverse-acting controller, while the LC50 is adirect-acting controller.
The low selector (LS50) selects the lower signal to manipulate thepump speed.
[Smith, 2002]
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Special Control Structures
Override Control:
The set point to LC50 is somewhat above h2, as shown in the figure.
The FC50 is a reverse-acting controller, while the LC50 is adirect-acting controller.
The low selector (LS50) selects the lower signal to manipulate thepump speed.
[Smith, 2002]
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Special Control Structures
Override Control:
The set point to LC50 is somewhat above h2, as shown in the figure.
The FC50 is a reverse-acting controller, while the LC50 is adirect-acting controller.
The low selector (LS50) selects the lower signal to manipulate thepump speed.
[Smith, 2002]
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Special Control Structures
Override Control:Reset Feedback (RFB) or external reset feedback:
This capability allows the controller not selected to override thecontroller selected at the very moment it is necessary.
When FC50 is being selected, its integration is working, but not thatof LC50 (its integration is being forced equal to the output of LS50).
When LC50 is being selected, its integration is working but not thatof FC50 (its integration is being forced equal to the output of LS50).
[Smith, 2002]
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Special Control Structures
Selective Control:
The high selector in this scheme selects the transmitter with thehighest output, and in so doing the controlled variable is always thehighest, or closest to the highest, temperature.
[Smith, 2002]
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Special Control Structures
Split range:A very common control scheme is split range control in which the outputof a controller is split to two or more control valves. For example:
Controller output 0% Valve A is fully open and Valve B fully closed.
Controller output 25% Valve A is 75% open and Valve B 25% open.
Controller output 50% Both valves are 50% open.
Controller output 75% Valve A is 25% open and Valve B 75% open.
Controller output 100% Valve A is fully closed and Valve B fully open.
www.contek-systems.co.uk
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Special Control Structures
Split range:
X: Steam Valve, air-to-open
Y: Cooling Water Valve, air-to-close
TC47: Temperature Controller
TY47: I/P converter
[Love, 2007]
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Special Control Structures
Split range:
ZC47A,B: Valve positioners
TV47A,B: Valves
[Love, 2007]
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Special Control Structures
Split range:Different arrangements are possible. For example, the split can beconfigured as follows:
Controller output 0% Both valves are closed.
Controller output 25% Valve A is 50% open and Valve B still closed.
Controller output 50% Valve A is fully open and Valve B closed.
Controller output 75% Valve A is fully open and Valve B 50% open.
Controller output 100% Both valves are fully open.
www.contek-systems.co.uk
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Special Control Structures
Split range:Example:
www.contek-systems.co.uk
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Special Control Structures
Split range:Example:
www.contek-systems.co.uk
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Special Control Structures
Split range:
In this application, the flare valve will need to open quickly inresponse to high pressures, but the compressor suction valve will needto move much more slowly to prevent instability in the compressors.The main problem with split range control is that the controller onlyhas one set of tuning parameters.
The solution is to replace the split range controller with twoindependent controllers, both reading the same pressure transmitter,but one controlling the flare valve and the other the suction valve.
www.contek-systems.co.uk
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Special Control Structures
Split range:
In this application, the flare valve will need to open quickly inresponse to high pressures, but the compressor suction valve will needto move much more slowly to prevent instability in the compressors.The main problem with split range control is that the controller onlyhas one set of tuning parameters.
The solution is to replace the split range controller with twoindependent controllers, both reading the same pressure transmitter,but one controlling the flare valve and the other the suction valve.
www.contek-systems.co.uk
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Special Control Structures
Two controller implementation:
www.contek-systems.co.uk
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Special Control Structures
Marriage analogy:
Split - range control is like a good marriage. One partner may bedoing 90of the work, but both partners are occasionally going to sharethe work.
Override control is like a bad marriage. One partner plays a potentiallydominating role, even though the other partner is doing all the work.
Cascade control is more like my marriage. I do the best I can, but mywife Liz constantly and lovingly recalibrates my efforts. She dampensdown the extremes in my behavior so as to promote a stablerelationship and home life.
[Lieberman, 2008]
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Special Control Structures
Marriage analogy:
Split - range control is like a good marriage. One partner may bedoing 90of the work, but both partners are occasionally going to sharethe work.
Override control is like a bad marriage. One partner plays a potentiallydominating role, even though the other partner is doing all the work.
Cascade control is more like my marriage. I do the best I can, but mywife Liz constantly and lovingly recalibrates my efforts. She dampensdown the extremes in my behavior so as to promote a stablerelationship and home life.
[Lieberman, 2008]
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Special Control Structures
Marriage analogy:
Split - range control is like a good marriage. One partner may bedoing 90of the work, but both partners are occasionally going to sharethe work.
Override control is like a bad marriage. One partner plays a potentiallydominating role, even though the other partner is doing all the work.
Cascade control is more like my marriage. I do the best I can, but mywife Liz constantly and lovingly recalibrates my efforts. She dampensdown the extremes in my behavior so as to promote a stablerelationship and home life.
[Lieberman, 2008]
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Special Control Structures
Smith predictor:
To handle systems with a large dead time
[Chau, 2002]
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Special Control Structures
Smith predictor:
To handle systems with a large dead time
[Chau, 2002]
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Special Control Structures
Smith predictor:
To handle systems with a large dead time
[Chau, 2002]
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Chau, P. C. (2002).Process Control: A First Course with MATLAB (Cambridge Series inChemical Engineering).Cambridge University Press, 1 edition.
Lieberman, N. (2008).Troubleshooting Process Plant Control.Wiley.
Love, J. (2007).Process Automation Handbook: A Guide to Theory and Practice.Springer, 1 edition.
Smith, C. A. (2002).Automated Continuous Process Control.Wiley-Interscience, 1 edition.
Visioli, A. (2006).Practical PID Control (Advances in Industrial Control).Springer, 1 edition.
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