control of multiple-input, multiple-output processes multiloop controllers modeling the interactions...
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Control of Multiple-Input, Multiple-Output Processes
Multiloop controllers• Modeling the interactions• Relative Gain Array (RGA)• Singular Value Analysis (SVA)• Decoupling strategiesC
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Control of multivariable processes
In practical control problems there typically are a number of process variables which must be controlled and a number which can be manipulated
Example: product quality and through put must usually be controlled.
• Several simple physical examples are shown in Fig. 18.1.
Note "process interactions" between controlled and manipulated variables.
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In this chapter we will be concerned with characterizing process interactions and selecting an appropriate multiloop control configuration.
If process interactions are significant, even the best multiloop control system may not provide satisfactory control.
In these situations there are incentives for considering multivariable control strategies
Definitions:
•Multiloop control: Each manipulated variable depends on only a single controlled variable, i.e., a set of conventional feedback controllers.
•Multivariable Control: Each manipulated variable can depend on two or more of the controlled variables.
Examples: decoupling control, model predictive control
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Multiloop Control Strategy•Typical industrial approach•Consists of using n standard FB controllers (e.g. PID), one for
each controlled variable.
•Control system design1. Select controlled and manipulated variables.2. Select pairing of controlled and manipulated variables.3. Specify types of FB controllers.
Example: 2 x 2 system
Two possible controller pairings:U1 with Y1, U2 with Y2 …orU1 with Y2, U2 with Y1
Note: For n x n system, n! possible pairing configurations.
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Transfer Function Model (2 x 2 system)
Two controlled variables and two manipulated variables(4 transfer functions required)
Thus, the input-output relations for the process can be written as:
)()(
)(),(
)(
)(
)()(
)(),(
)(
)(
222
221
1
2
122
111
1
1
sGsU
sYsG
sU
sY
sGsU
sYsG
sU
sY
PP
PP
)()()()()(
)()()()()(
2221212
2121111
sUsGsUsGsY
sUsGsUsGsY
PP
PP
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Or in vector-matrix notation as,
where Y(s) and U(s) are vectors,
And Gp(s) is the transfer function matrix for the process
)()()( sUsGsY P
)(
)()(,
)(
)()(
2
1
2
1
sU
sUsU
sY
sYsY
)s(G)s(G
)s(G)s(G)s(G
22P21P
12P11PP
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Control-loop interactions
• Process interactions may induce undesirable
interactions between two or more control loops.
Example: 2 x 2 system
Control loop interactions are due to the presence
of a third feedback loop.• Problems arising from control loop interactions
i) Closed -loop system may become destabilized.
ii) Controller tuning becomes more difficult
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Block Diagram Analysis
For the multiloop control configuration the transfer function between a controlled and a manipulated variable depends on whether the other feedback control loops are open or closed.
Example: 2 x 2 system, 1-1/2 -2 pairingFrom block diagram algebra we can show
Note that the last expression contains GC2 .
),()(
)(11
1
1 sGsU
sYP
222
2211211
1
1
1)(
)(
PC
CPPP GG
GGGG
sU
sY
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(second loop open)
(second loop closed)
Relative gain array
• Provides two useful types of information:
1) Measure of process interactions
2) Recommendation about best pairing of
controlled and manipulated variables.
• Requires knowledge of s.s. gains but not
process dynamics.
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Example of RGA Analysis: 2 x 2 system• Steady-state process model,
The RGA is defined as:
where the relative gain, ij, relates the ith controlled variable and the jth manipulated variable
2221212
2121111
UKUKY
UKUKY
2221
1211RGA
gain loop-closed
gain loop-openij
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Scaling Properties:
i) ij is dimensionless
ii)
For 2 x 2 system,
Recommended Controller Pairing
Corresponds to the ij which has the largest positive value.
0.1j
iji
ij
211112
2211
211211 1,
KKKK
1
1
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In general:1. Pairings which correspond to negative pairings should not be selected.2. Otherwise, choose the pairing which has ij closest
to one.
Examples:Examples: Process Gain Relative Gain Matrix, : Array, :
22
11
K0
0K
0K
K0
21
12
22
1211
K0
KK
2221
11
KK
0K
10
01
01
10
10
01
10
01
K
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211112
2211
211211 1,
KKKK
1
1
Recall, for 2X2 systems...
2221212
2121111
UKUKY
UKUKY
EXAMPLE:EXAMPLE:
29.229.1
29.129.2
25.1
5.12
KK
KKK
2221
1211
Recommended pairing is Y1 and U1, Y2 and U2.
EXAMPLE:EXAMPLE:
64.036.0
36.064.0
25.1
5.12K
Recommended pairing is Y1 with U1, Y2 with U2.
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EXAMPLE: Thermal Mixing SystemEXAMPLE: Thermal Mixing System
The RGA can be expressed in terms of the manipulated variables:
Note that each relative gain is between 0 and 1. Recommendedcontroller pairing depends on nominal values of W,T, Th, and Tc.
See Exercise 18.16
ch
hc
c
hc
h
hc
h
hc
c
WW
WWW
WWW
WWW
WWW
T
W
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EXAMPLE: Ill-conditioned Gain Matrix
y = Ku
2 x 2 process y1 = 5 u1 + 8 u2
y2 = 10 u1 + 15.8 u2
specify operating point y, solve for u
-1
Adj
det
K= =
Ku K y y
11 22 11 2211
11 22 12 21
RGA : det
K K K K
K K K K K
effect of det K → 0 ?
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RGA for Higher-Order Systems:
For and n x n system,
Each ij can be calculated from the relation
Where Kij is the (i,j) -element in the steady-state gain matrix,
And Hij is the (i,j) -element of the .
Note that,
1 2
1 11 12 1
2 21 22 2
1 1
n
n
n
n n n nn
U U UY
Y
Y
ijijij HK
UKY
T1KH
HK
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EXAMPLE: HydrocrackerEXAMPLE: Hydrocracker
The RGA for a hydrocracker has been reported as,
Recommended controller pairing?
4321
4
3
2
1
919.1900.0030.2215.0
910.1270.0314.3135.0
154.1286.0429.0011.0
164.0080.0150.0931.0UUUU
Y
Y
Y
Y
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Singular Value AnalysisK = W VT
is diagonal matrix of singular values(1, 2, …, r)
The singular values are the positive square roots of the eigenvalues of
KTK (r = rank of KTK)W,V are input and output singular vectors Columns of W and V are orthonormal. Also
WWT = IVVT = I
Calculate , W, V using MATLAB (svd = singular value decomposition)Condition number (CN) is the ratio of the largest to the smallest singular value and indicates if K is ill-conditioned.
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CN is a measure of sensitivity of the matrix properties to changes in a specific element.
Consider
(RGA) = 1.0
If K12 changes from 0 to 0.1, then K becomes a singular matrix, which corresponds to a process that is hard to control.
RGA and SVA used together can indicate whether a process is easy (or hard) to control.
K is poorly conditioned when CN is a large number (e.g., > 10). Hence small changes in the model for this process can make it very difficult to control.
1 0
10 1
K
10.1 0( ) = CN = 101
0 0.1K
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Selection of Inputs and Outputs
• Arrange the singular values in order of largest to smallest and look for any σi/σi-1 > 10; then one or more inputs (or outputs) can be deleted.
• Delete one row and one column of K at a time and evaluate the properties of the reduced gain matrix.
• Example:
0.48 0.90 0.006
0.52 0.95 0.008
0.90 0.95 0.020
K
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CN = 166.5 (σ1/σ3)
The RGA is as follows:
Preliminary pairing: y1-u2, y2-u3,y3-u1.
CN suggests only two output variables can be controlled. Eliminate one input and one output (3x3→2x2).
0.5714 0.3766 0.7292
0.6035 0.4093 0.6843
0.5561 0.8311 0.0066
W
1.618 0 0
0 1.143 0
0 0 0.0097
0.0541 0.9984 0.0151
0.9985 0.0540 0.0068
0.0060 0.0154 0.9999
V
2.4376 3.0241 0.4135
1.2211 0.7617 0.5407
2.2165 1.2623 0.0458
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Alternative Strategies for Dealing with Undesirable Control Loop Interactions
1. "Detune" one or more FB controllers.2. Select different manipulated or controlled variables. e.g., nonlinear functions of original variables3. Use a decoupling control scheme.4. Use some other type of multivariable control scheme.
Decoupling Control Systems
Basic Idea: Use additional controllers to compensate for processinteractions and thus reduce control loop interactions
Ideally, decoupling control allows setpoint changes to affect onlythe desired controlled variables.
Typically, decoupling controllers are designed using a simple process model (e.g. steady state model or transfer function model)
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Design Equations:
We want cross-controller, GC12, to cancel out the effect of U2 on Y1.Thus, we would like,
Since U2 0 (in general), then
Similarly, we want G21 to cancel the effect of M1 on C2. Thus, werequire that...
cf. with design equations for FF control based on block diagramanalysis
12 11 2 12 2 0P PT G U G U
1212
11
P
P
GT
G
21 22 1 21 1
2121
22
0P P
P
P
T G U G U
GT
G
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Process Interaction
Corrective Action (via “cross-controller” or “decoupler”).Ideal Decouplers:
1212
11
2121
22
( )( )
( )
( )( )
( )
P
P
P
P
G sT s
G s
G sT s
G s
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Alternatives to Complete Decoupling
• Static Decoupling (use SS gains)• Partial Decoupling (either GC12 or GC21 is set equal to zero)
Variations on a Theme:
•Partial Decoupling:Use only one “cross-controller.”
•Static Decoupling:Design to eliminate SS interactionsIdeal decouplers are merely gains:
•Nonlinear DecouplingAppropriate for nonlinear processes.
1212
11
2121
22
P
P
P
P
KT
K
KT
K
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