relative gain measure of interaction we have seen that interaction is important. it affects whether...
TRANSCRIPT
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RELATIVE GAIN MEASURE OF INTERACTION
We have seen that interaction is important. It affects whether feedback control is possible, and if possible, its performance.
Do we have a quantitative measure of interaction?
The answer is yes, we have several! Here, we will learn about the RELATIVE GAIN ARRAY.
Our main challenge is to understand the correct interpretations of the RGA.
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RELATIVE GAIN
We are here, and making progress all the time!
• Defining control objectives
• Controllability & Observability
• Interaction & Operating window
• The Relative Gain
• Multiloop Tuning
• Performance and the RDG
• SVD and Process directionality
• Robustness
• Integrity
• Control for profit
• Optimization-based design methods
• Process design- Series and self-regulation- Zeros (good/bad/ugly)- Recycle systems- Staged systems
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RELATIVE GAIN MEASURE OF INTERACTION
Let’s start here to build understanding
OUTLINE OF THE PRESENTATION1. DEFINITION OF THE RGA
2. EVALUATION OF THE RGA
3. INTERPRETATION OF THE RGA
4. EXTENSIONS OF RGA
5. PRELIMINARY CONTROL DESIGN IMPLICATIONS OF RGA
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RELATIVE GAIN MEASURE OF INTERACTION
+ +
+ +
G11(s)
G21(s)
G12(s)
G22(s)
Gd2(s)
Gd1(s)
D(s)
CV1(s)
CV2(s)MV2(s)
MV1(s)The relative gain between MVj and CVi is ij . It is defined in the following equation.
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k What have we assumed about the other controllers?
Explain in words.
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RELATIVE GAIN MEASURE OF INTERACTION
Now, how do we determine
the value?
OUTLINE OF THE PRESENTATION1. DEFINITION OF THE RGA
2. EVALUATION OF THE RGA
3. INTERPRETATION OF THE RGA
4. EXTENSIONS OF RGA
5. PRELIMINARY CONTROL DESIGN IMPLICATIONS OF RGA
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RELATIVE GAIN MEASURE OF INTERACTION
j
i
CVj
i
MVj
i
CVMVCVKMV
MVCVMVKCV
ij
1
ij
kI
k
The relative gain array is the element-by-element product of K with K-1. ( = product of ij elements, not normal matrix multiplication)
jiijij kIkKK T
1
1. The RGA can be calculated from open-loop gains (only).
Open-loop
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
Closed-loop
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RELATIVE GAIN MEASURE OF INTERACTION
1. The RGA can be calculated from open-loop values.
The relative gain array for a 2x2 system is given in the following equation.
2211
211211
1
1
KK
KK
What is true for the RGA to have 1’s on diagonal?
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
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RELATIVE GAIN MEASURE OF INTERACTION
2. The RGA elements are scale independent.
2
1
2
1
2
1
2
1
109
101
109
1010
MV
MV
CV
CV
MV
MV
CV
CV *
. *
Original units Modified units
What is the effect of changing the units of the CV, expressing CV as % of instrument range, or changing the capacity of the final element on ij ?
10910
91010
22
1
21
1
/or
or
CVCV
CV
MVMV
MVCan we prove that thisis general?
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RELATIVE GAIN MEASURE OF INTERACTION
3. The rows and columns of the RGA sum to 1.0.
109
910
2
1
21
CV
CV
MVMV
For a 2x2 system, how many elements are independent?
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
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RELATIVE GAIN MEASURE OF INTERACTION
3. The rows and columns of the RGA sum to 1.0.
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
Class exercise: prove this statement.
Hint: a matrix and its inverse commute, i.e.,
K K-1 = K-1 K = I
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RELATIVE GAIN MEASURE OF INTERACTION
3. The rows and columns of the RGA sum to 1.0.
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
K K-1 = I = K-1 K
n
1i
n
1m1
n
1j
n
1m1
jrow of sum ji if 1 and ji if 0
i col of sum ji if 1 and ji if 0
ijmjjm
n
kkjik
ijmiim
n
kkjik
kIkkkI
kIkkIk
From the left hand equation, the elements of I are equal to
From the right hand equation, the elements of I are equal to
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RELATIVE GAIN MEASURE OF INTERACTION
4. In some cases, the RGA is very sensitive to small errors in the gains, Kij.
2211
211211
1
1
KK
KK
When is this equation very sensitive to errors in the individual gains?
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
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RELATIVE GAIN MEASURE OF INTERACTION
4. In some cases, the RGA is very sensitive to small errors in the gains, Kij.
Change in FD used infinite difference for
derivative
11 for a positivechange in FD
11 for a negativechange in FD
Average 11 forpositive and negative
changes in FD
2% .796 .301 .5480.5% .673 .508 .5900.2% .629 .562 .5960.05% .605 .588 .597
We must perform a thorough study to ensure that numerical derivatives are sufficiently accurate!
The x must be sufficiently small (be careful about roundoff).
constant
constant
k
k
CVj
i
MVj
i
ij
MVCV
MVCV
From McAvpy, 1983
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RELATIVE GAIN MEASURE OF INTERACTION
4. In some cases, the RGA is very sensitive to small errors in the gains, Kij.
We must perform a through study to ensure that numerical derivatives are sufficiently accurate!
The convergence tolerance must be sufficiently small.
Convergencetolerance of
equations (some ofall errors squared)
11 for a positivechange in FD
11 for a negativechange in FD
Average 11 forpositive and negative
changes in FD
10-4 -4.605 8.080 -.88710-6 -.096 1.068 .50310-8 .556 .615 .58610-10 .622 .568 .59510-16 .629 .562 .596
constant
constant
k
k
CVj
i
MVj
i
ij
MVCV
MVCV
Average gains from +/-
From McAvpy, 1983
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RELATIVE GAIN MEASURE OF INTERACTION
5. The relative gain elements are independent of the control design for the “ij” inputs and outputs being considered.
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
AC
TC
Solvent
Reactant
FS >> FR
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RELATIVE GAIN MEASURE OF INTERACTION
6. A permutation in the gain matrix (changing CVs and MVs) results in the same permutation in the RG Array.
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
109
1010
2
1
2
1
MV
MV
CV
CV109
910
2
1
21
CV
CV
MVMV
Process gain RGA
1010
109
2
1
1
2
MV
MV
CV
CV ??
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RELATIVE GAIN MEASURE OF INTERACTION
How do weuse values to
evaluate behavior?
OUTLINE OF THE PRESENTATION1. DEFINITION OF THE RGA
2. EVALUATION OF THE RGA
3. INTERPRETATION OF THE RGA
4. EXTENSIONS OF RGA
5. PRELIMINARY CONTROL DESIGN IMPLICATIONS OF RGA
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RELATIVE GAIN MEASURE OF INTERACTION
MVj CVi
ij < 0 In this case, the steady-state gains have different signs depending on the status (auto/manual) of the other loops.
A
A
CA0
CA
CSTR with A B
Solvent
A
Discuss interaction and RGA in this system.
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
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RELATIVE GAIN MEASURE OF INTERACTION
MVj CVi
ij < 0 In this case, the steady-state gains have different signs depending on the status (auto/manual) of other loops
We can achieve stable multiloop feedback by using the sign of the controller gain that stabilizes the multiloop system.
Discuss what happens when the other interacting loop is placed in manual!
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
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RELATIVE GAIN MEASURE OF INTERACTION
MVj CVi
ij < 0 the steady-state gains have different signs
For ij < 0 , one of three BAD situations occurs
1. Multiloop is unstable with all in automatic.
2. Single-loop ij is unstable when others are in manual.
3. Multiloop is unstable when loop ij is manual and other loops are in automatic
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
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0 100 200 300 4000.975
0.98
0.985
0.99
0.995IAE = 0.3338 ISE = 0.0012881
XD
, D
istil
late
Lt K
ey
0 100 200 300 4000.005
0.01
0.015
0.02
0.025
0.03IAE = 0.58326 ISE = 0.0041497
XB
, B
otto
ms
Lt
Ke
y
0 100 200 300 400 50013.3
13.4
13.5
13.6
13.7
13.8
Time
Reb
oile
d V
ap
or
0 100 200 300 400 5008.5
8.6
8.7
8.8
8.9
9
Time
Ref
lux
Flo
w
FR XB
FV XD
Example of pairing on a negative RGA (-5.09). XB controller has a Kc with opposite sign from single-loop control! The system goes unstable when a constraint is encountered. But, we can achieve stable control with pairing on negative RGA!
FR
FV
XB
XD
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RELATIVE GAIN MEASURE OF INTERACTION
MVj CVi
ij < 0 the steady-state gains have different signs
For ij < 0 , one of three situations occurs
1. The process gij(s) has a RHP zero
2. The overall plant has a RHP zero
3. The system with gij(s) removed has a RHP zero
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
See Skogestad and Postlethwaite, 1996
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RELATIVE GAIN MEASURE OF INTERACTION
MVj CVi
ij = 0 In this case, the steady-state gain is zero when all other loops are open, in manual.
T
L
Could this control system work?
What would happen if one controller were in manual?
Heating tank without boiling
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
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RELATIVE GAIN MEASURE OF INTERACTION
MVj CVi
0<ij<1 In this case, the multiloop (ML) steady-state gain is larger than the single-loop (SL) gain.
What would be the effect on tuning of opening/closing the other loop?
Discuss the case of a 2x2 system paired on ij = 0.1
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
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RELATIVE GAIN MEASURE OF INTERACTION
MVj CVi
ij= 1 In this case, the steady-state gains are identical in both the ML and the SL conditions.
+ +
+ +
G11(s)
G21(s)
G12(s)
G22(s)
Gd2(s)
Gd1(s)
D(s)
CV1(s)
CV2(s)MV2(s)
MV1(s)
What is generally true when ij= 1 ?
Does ij= 1 indicate no interaction?
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
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RELATIVE GAIN MEASURE OF INTERACTION
ij= 1 In this case, the steady-state gains are identical in both the ML and the SL conditions.
AC
TC
CSTR with zero heat of reaction
Solvent
Reactant
FS >> FR
Determine the relative gain.
Discuss interaction in this system.
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MV
CV
MV
CV
MV
CV
MV
CV
k
k
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RELATIVE GAIN MEASURE OF INTERACTION
ij= 1 In this case, the steady-state gains are identical in both the ML and the SL conditions.
..
..
..
k
k
K22
11
........k
....
....
kk
k
K
n1
2221
11
0
0
0
IRGA
1
1
1
1
10
0Lower diagonal gainmatrix
Diagonal gainmatrixDiagonal gainmatrix
Both give an RGA that is diagonal!
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
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RELATIVE GAIN MEASURE OF INTERACTION
MVj CVi
1<ij In this case, the steady-state multiloop (ML) gain is smaller than the single-loop (SL) gain.
What would be the effect on tuning of opening/closing the other loop?
Discuss a the case of a 2x2 system paired on ij = 10.
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
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FR XD
FRB XB
FD XD
FRB XB
RELATIVE GAIN MEASURE OF INTERACTION
XD, XB FeedComp.
RGA RGA
.998,.02 .25 46.4 .07
.998,.02 .50 45.4 .113
.998,.02 .75 66.5 .233
.98, .02 .25 36.5 .344
.98, .02 .50 30.8 .5
.98, .02 .75 37.8 .65
.98, .002 .25 66.1 .787
.98, .002 .50 46 .887
.98, .002 .75 48.8 .939
1. Do level loops affect the composition RGA’s?2. Does the process operation affect RGA’s?
Rel. vol = 1.2, R = 1.2 Rmin
From McAvpy, 1983
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RELATIVE GAIN MEASURE OF INTERACTION
MVj CVi
ij= In this case, the gain in the ML situation is zero. We conclude that ML control is not possible.
How can we improve the situation?
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
Have we seen this result before?
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RELATIVE GAIN MEASURE OF INTERACTION
OUTLINE OF THE PRESENTATION1. DEFINITION OF THE RGA
2. EVALUATION OF THE RGA
3. INTERPRETATION OF THE RGA
4. EXTENSIONS OF RGA
5. PRELIMINARY CONTROL DESIGN IMPLICATIONS OF RGA
Let’s extend theconcept
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RELATIVE GAIN MEASURE OF INTERACTION
The relative gain between MVj and CVi is ij .
constant
constant
k
k
CVj
i
MVj
i
ij
MVCV
MVCV
The basic definition involves steady-state gain information.
• Some plants are unstable
• Control performance is influenced by dynamics
• Many plants have an unequal number of MVs and CVs
• Control design involves structures other than single-loop
• Disturbances are not considered!
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RELATIVE GAIN MEASURE OF INTERACTION
We can evaluate the RGA of a system with integrating processes, such as levels.
Redefine the output as the derivative of the level; then, calculate as normal. (Note that L is unstable, but dL/dt is stable.)
slurry of density
21
1
21
mm
mD
FmmAdt
dLA out
m2m1m
L
A
= density
D = density
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
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RELATIVE GAIN MEASURE OF INTERACTION
We can evaluate the RGA of a system with integrating processes, such as levels.
Redefine the output as the derivative of the level; then, calculate as normal.
/)/(
)/(/
DDD
DD
mm
1
121
m2m1m
L
A
= density
D = density
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
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FR XD
FRB XB
RELATIVE GAIN MEASURE OF INTERACTION
A frequency-dependent RGA can be calculated using the transfer functions in place of the steady-state gains.
+ +
+ +
G11(s)
G21(s)
G12(s)
G22(s)
Gd2(s)
Gd1(s)
D(s)
CV1(s)
CV2(s)MV2(s)
MV1(s)
We can evaluate the RGA of dynamics processes
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10-4
10-2
100
102
10-5
100
frequency, radians/min
ampl
itude
, X
D(jw
)/F
R(jw
)
10-4
10-2
100
102
10-5
100
ampl
itude
, X
D(jw
)/F
V(jw
)
10-4
10-2
100
102
10-4
10-2
100
ampl
itude
, X
B(jw
)/F
R(jw
)
10-4
10-2
100
102
10-4
10-2
100
ampl
itude
, X
B(jw
)/F
V(jw
)
Bode plots of the individual transfer functions for a distillation tower
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10
-310
-210
-110
010
110
210
-1
100
101
frequency, rad/time
ampl
itude
rat
io
frequency dependent RGA for distillation tower
Bode plot of the RGA 11 element. What frequency range is most important for feedback control?
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RELATIVE GAIN MEASURE OF INTERACTION
The basic definition involves steady-state gain information.
• Some plants are unstable
• Control performance is influenced by dynamics
• Many plants have an unequal number of MVs and CVs
• Control design involves structures other than single-loop
• Disturbances are not considered!
Apparently, there is a lot more to learn.
We better plan to address these issues in the remainder of the course
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RELATIVE GAIN MEASURE OF INTERACTION
Let’s evaluatesome design
guidelines basedon RGA
OUTLINE OF THE PRESENTATION1. DEFINITION OF THE RGA
2. EVALUATION OF THE RGA
3. INTERPRETATION OF THE RGA
4. EXTENSIONS OF RGA
5. PRELIMINARY CONTROL DESIGN IMPLICATIONS OF RGA
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RELATIVE GAIN MEASURE OF INTERACTION
Proposed Guideline #1
Select pairings that do not have any ij<0
• Review the interpretation, i.e., the effect on behavior.
• What would be the effect if the rule were violated?
• Do you agree with the Proposed Guideline?
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
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RELATIVE GAIN MEASURE OF INTERACTION
Proposed Guideline #2
Select pairings that do not have any ij=0
• Review the interpretation, i.e., the effect on behavior.
• What would be the effect if the rule were violated?
• Do you agree with the Proposed Guideline?
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
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RELATIVE GAIN MEASURE OF INTERACTION
• We conclude that the RGA provides excellent insight into the INTEGRITY of a multiloop control system.
• INTEGRITY: A multiloop control system has good integrity when after one loop is turned off, the remainder of the control system remains stable.
• “Turning off” can occur when (1) a loop is placed in manual, (2) a valve saturates, or (3) a lower level cascade controller no lower changes the valve (in manual or reached set point limit).
• Pairings with negative or zero RGA’s have poor integrity
RGA and INTEGRITY
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RELATIVE GAIN MEASURE OF INTERACTION
Proposed Guideline #3
Select a pairing that has RGA elements as close as possible to ij=1
• Review the interpretation, i.e., the effect on behavior.
• What would be the effect if the rule were violated?
• Do you agree with the Proposed Guideline?
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
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FR XD
FRB XB
FD XD
FRB XB
0 50 100 150 2000.98
0.982
0.984
0.986
0.988IAE = 0.26687 ISE = 0.00052456
XD
, lig
ht k
ey
0 50 100 150 2000.02
0.021
0.022
0.023
0.024IAE = 0.25454 ISE = 0.0004554
XB
, lig
ht k
ey
0 50 100 150 2008.5
8.6
8.7
8.8
8.9
9SAM = 0.31512 SSM = 0.011905
Time
Ref
lux
flow
0 50 100 150 20013.5
13.6
13.7
13.8
13.9
14SAM = 0.28826 SSM = 0.00064734
Time
Reb
oile
d va
por
0 50 100 150 2000.98
0.982
0.984
0.986
0.988IAE = 0.059056 ISE = 0.00017124
XD
, lig
ht k
ey
0 50 100 150 2000.019
0.02
0.021
0.022
0.023IAE = 0.045707 ISE = 8.4564e-005
XB
, lig
ht k
ey
0 50 100 150 2008.46
8.48
8.5
8.52
8.54SAM = 0.10303 SSM = 0.0093095
Time
Ref
lux
flow
0 50 100 150 20013.5
13.6
13.7
13.8
13.9
14SAM = 0.55128 SSM = 0.017408
Time
Reb
oile
d va
por
RGA = 6.09 RGA = 0.39
For set point response, RGA closer to 1.0 is better
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FR XD
FRB XBFD XD
FRB XB
0 50 100 150 200
0.975
0.98
IAE = 0.14463 ISE = 0.00051677
XD
, lig
ht k
ey
0 50 100 150 2000
0.005
0.01
0.015
0.02
0.025IAE = 0.32334 ISE = 0.0038309
XB
, lig
ht k
ey
0 50 100 150 2008.5
8.55
8.6
8.65
8.7SAM = 0.21116 SSM = 0.0020517
Time
Ref
lux
flow
0 50 100 150 20013.1
13.2
13.3
13.4
13.5
13.6SAM = 0.38988 SSM = 0.0085339
Time
Reb
oile
d va
por
RGA = 6.09 RGA = 0.39
0 50 100 150 2000.95
0.96
0.97
0.98
0.99IAE = 0.45265 ISE = 0.0070806
XD
, lig
ht k
ey
0 50 100 150 2000
0.005
0.01
0.015
0.02
0.025
0.03IAE = 0.31352 ISE = 0.0027774
XB
, lig
ht k
ey
0 50 100 150 2008
8.1
8.2
8.3
8.4
8.5
8.6SAM = 0.51504 SSM = 0.011985
Time
Ref
lux
flow
0 50 100 150 20011
11.5
12
12.5
13
13.5
14SAM = 4.0285 SSM = 0.6871
Time
Reb
oile
d va
por
For feed composition disturbance response, RGA farther from 1.0 is better
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RELATIVE GAIN MEASURE OF INTERACTION
Using guidelines #1 and #2, the control possibilities for this example process were reduced from 36 to 4.
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RELATIVE GAIN MEASURE OF INTERACTION
• Tells us about the integrity of multiloop systems and something about the differences in tuning as well.
• Uses only gains from feedback process!
• Does not use following information- Control objectives- Dynamics- Disturbances
• Lower diagonal gain matrix can have strong interaction but gives RGAs = 1
Can we design controls without this information?
The RGA gives useful conclusions from S-S information, but not enough to design process control
Powerful results from limited information!
“Interaction?”
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Workshop on Relative Gain Array
INTERACTION IN FEEDBACK SYSTEMS
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Workshop on Relative Gain Array: Problem 1
The RGA has been evaluated, but the regulatory control system (below the loops being analyzed using RGA) has been modified. Instead of adjusting a valve directly, one of the loops being evaluated will adjust a flow controller set point (which adjusts the same valve). How would you evaluate the new RGA?
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Workshop on Relative Gain Array: Problem 2
You have decided to pair on a loop that has a negative RGA element. Discuss the tuning that is appropriate for this loop.
A
A
CA0
CA
CSTR with A B
Solvent
A
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Workshop on Relative Gain Array: Problem 3
Discuss what information can be obtained from the RGA and some information that cannot.
closed loopsother
open loopsother
constant
constant
j
i
j
i
CVj
i
MVj
i
ij
MVCV
MVCV
MVCV
MVCV
k
k
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Workshop on Relative Gain Array: Problem 4
You would like to evaluate the steady-state RGA for a process, but the feedback controllers must remain in automatic status. How can you obtain the needed data?
• Explain a procedure that might yield the needed information.
• Discuss the practicality of the approach.
1
2
3
13
14
15LC-1
LC-3
dP-1
dP-2
To flare
T5
T6
TC-7
AC-1
LAHLAL
PAH
PC-1
P3
F4
F7
F8
F9
PV-3
TAL
T10
AC-2
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Workshop on Relative Gain Array: Problem 5
TC
LC
FC
VC
SP = 95% open
To the SP of the feed flow
CV to the VC controller
MV from the VC controller
MV from the TC controller
Determine the relative gain array for the TC and VC controllers. Discuss the behavior of this design and generalize the conclusions.
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RGA WORKSHOP PROBLEM 6Three CSTR's with the configuration in the figure and with the design parameters below areconsidered in this example; the common data is given below, and the case-specific data andsteady-states are given in the table.
F=1 M3 , V=1 M3 , CA0=2.0 kg-moles/M3, Cp=1 cal/(g C), =106 g/M3 , ko = 1.0x1010
min-1 , E/R = 8330.1 K-1 , (Fc)s=15 M3/min , Cpc=1 cal/(g K) , c=106 g/M3 , b=0.5Case I (Example 3.10) II III
-?Hrxn 106 cal/(kg-mole) 130 13 -30
a (cal/min)/? K 1.678x106 1.678x106 0.7746x106
T0 ? K 323 370 370
Tcin ? K 365 365 420 (heating)
Ts ? K 394 368.3 392.7
CAs kg-mole/M3 0.265 0.80 0.28
Gain matrices for the three Cases, about the appropriate steady-state
Inputvariable
Case I Case II Case III
CA T CA T CA T
CA0 - 0.161 23.8 0.3615 1.309 0.2214 -6.267
Fc 0.0158 -1.28 .0034 -0.1144 -0.0085 0.657
T0 -.0026 0.211 -.0049 0.1678 -.0032 0.243
F -.0948 26.33 0.4251 1.864 .4527 -16.30
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RG
A W
OR
KS
HO
P. P
RO
BL
EM
6
(co
nti
nu
ed)
A. Calculate the relative gain arrays for the three cases shown in the table below.
CASE I CASE II CASE III
CA0 FC CA0 FC CA0 FC
CA
T
Notes:1. CA0 is controlled by adjusting the reactant valve2. FC is achieved by adjusting the valve on the pipe to the heat exchanger coils.3. For Cases I and II, the coils provide coling, and for Case III, the coils provide heating.4. The feed total flow and temperature controllers are in operation for Cases I-III
B. Determine conclusions for control design for each case. Explain each result on physicalgrounds.
C. Discuss relationships among the cases.