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DYNAMIC MATRIX CONTROL
Presented by
Chinta Manohar
D Surya Suvidha
DYNAMIC MATRIX CONTROL - INTRODUCTION
Developed at Shell in the mid 1970’s
Evolved from representing process dynamics with a set of numerical coefficients
Uses a least square formulation to minimize the integral of the error/time curve
DYNAMIC MATRIX CONTROL - INTRODUCTION
DMC algorithm incorporates feedforward and multivariable control
Incorporation of the process dynamics makes it possible to consider deadtime and unusual dynamic behavior
Using the least square formulation made it possible to solve complex multivariable control calculations quickly
DMC BLOCK DIAGRAM
• At each time step control is computed by solving an openloop optimization problem for the prediction horizon
• First value of the computed control sequence is applied• At the next time step, new system is re-computed
DYNAMIC MATRIX CONTROL - EXAMPLE Consider the following furnace example
(Cutlet & Ramaker) MV (manipulated variable)
Fuel flow FIC DV(dynamic variable)
Inlet temperature TI CV(control variable)
Outlet temperature TIC
TICFIC
FpTi
Fuel
ProcessHeater
Process Flow
6
CONT… The furnace DMC
model is defined by its dynamic coefficients a and b
Response to step change in fuel, a
Response to step change in inlet temperature, b
658.0
653.0
640.0
622.0
590.0
540.0
465.0
340.0
240.0
0.0
,
986.0
949.0
904.0
836.0
736.0
600.0
414.0
214.0
086.0
014.0
ba
7
CONT… DMC Dynamic
coefficents Response to step
change in fuel, a Response to step
change in inlet temperature, b
Fuel Coeff icients ai
0
0.5
1
1.5
0 5 10
Fuel Coeff icients ai
Inlet Temperature Coefficients bi
0
0.5
1
0 5 10
Inlet TemperatureCoefficients bi
8
DYNAMIC MATRIX CONTROL – MATRIX COMPUTATION The DMC prediction may be calculated from those
coefficients and the independent variable changes
3
2
1
3
2
1
2121
123123
1212
11
3
2
1
00
0000
DV
DV
DV
MV
MV
MV
bbbaaa
bbbaaa
bbaa
ba
CV
CV
CV
CV
iiiiiii
9
DYNAMIC MATRIX CONTROL – FEED FORWARD PREDICTION Feedforward prediction is enabled by moving the
DV to the left hand side
3
2
1
21
123
12
1
13
2
1
3
2
1
0
00
MV
MV
MV
aaa
aaa
aa
a
DV
b
b
b
b
CV
CV
CV
CV
iiiii
10
CONT… Controller definition
Prediction horizon = 30 time steps
Control horizon = 10 time steps
InitializationSet CV prediction
vector to current outlet temperature
Calculate error vector
0
0
30
1
CV
CV
CV
CV
30
1
30
1
30
1
CV
CV
SP
SP
e
e
11
LEAST SQUARE APPROACH Least squares formulation including move
suppression
0
0
000
0
00
0
000
0
0
0
000
10
2
1
30
2
1
10,10
9,9
2,2
1,1
21282930
1
123
12
1
e
e
e
MV
MV
MV
aaaa
a
aaa
aa
a
Move suppression
12
DYNAMIC MATRIX CONTROLLER CYCLE
DMC Controller stepsCalculate moves using least square solutionUse predicted fuel moves to calculate changes to outlet
temperature and update predictionsShift prediction forward one unit in timeCompare current predicted with actual and adjust all 30
predictions (accounts for unmeasured disturbances)Calculate feedforward effect using inlet temperatureSolve for another 10 moves and add to previously
calculated moves
13
FURNACE RESPONSE Furnace Example Temperature
Disturbance DT=15 at t=0
Three Fuel Moves Calculated
Furnace Temperature Response
-15
-10
-5
0
5
10
15
0 5 10 15
Time
Te
mp
era
ture
575
600
625
Fuel_0Temp_0Fuel_1Temp_1Fuel_2Prediction
14
DYNAMIC MATRIX CONTROL-RULES Constrain max MV movements during each time
interval Constrain min/max MV values at all times Constrain min/max CV values at all times Drive to economic optimum Allow for feedforward disturbances
15
CONT… Restrict computed MV move sizes (move
suppression) Relative weighting of MV moves Relative weighting of CV errors (equal concern
errors) Minimize control effort
16
CONT… For linear differential equations the process output
can be given by the convolution theorem
k
j
kjjk
k dMVaCVCV0
101
001
01 dMVaCVCV 11
10
202 dMVaMVaCVCV
221
12
03
03 dMVaMVaMVaCVCV
17
CONT… Breaking up the summation terms into past and
future contributions
1Nk
kj
jjlk
lklk MVaCVCV
kkk MVaCVCV
1
11
112
22 kkkk MVaMVaCVCV2
31
23
33 kkkkk MVaMVaMVaCVCV
18
CONT… Let N=number future moves, M=time horizon to
reach steady state, then in matrix form
1
1
11
11
12
1
2
1
2
1 0
00
Nk
k
k
MMM
NMMM
NN
Mk
k
k
Mk
k
k
MV
MV
MV
aaa
aaa
aaa
aa
a
CV
CV
CV
CV
CV
CV
19
CONT… Setting the predicted CV value to its setpoint and
subtracting the past contributions, the “simple” DMC equation results
ΔMVAe
MkMkS
kkS
kkS
CVCV
CVCV
CVCV
22
11
20
CONT… Predicted response is determined from current
outlet temperature, predicted changes and past history of the MVs and DVs
Desired response is determined by subtracting the predicted response from the setpoint
Solve for future MVs -- Overdetermined systemLeast square criteria (L2 norm)Very large changes in MVs not physically
realizableSolved by introduction of move suppression
21
SCALING OF RESIDUALS To scale the residuals, a weighted least squares problem is
posed
• For example, the relative weights with two CVs
2
2
1
2
1eΔMVAW Min
2
2
2
1
1
1
2
1
0
0
w
w
w
w
w
w
W
22
DMC-RESTRICTION To restrict the size of
calculated moves a relative weight for each of the MVs is imposed
2
2
2
1
1
1
0
0
r
r
r
r
r
r
R
2
2
1
2
1
0
eΔMV
R
AWMin
23
CONT… Subject to linear constraints
The change in each MV is within a “step” bound
HI
HI
HI
Nk
k
k
LO
LO
LO
MV
MV
MV
MV
MV
MV
MV
MV
MV
1
1
I
24
CONT… Subject to linear constraints
Size of each MV step for each time interval
1
1
1
1
111
0
11
001
Nk
k
k
Nk
k
k
MV
MV
MV
MV
MV
MV
MV
MV
MV
MV
LI
25
CONT… Subject to linear constraints
MV calculated for each time interval is between high and low limits
MV
MV
MV
MV
MV
MV
MV
MV
MV
MV
MV
MV
MV
MV
MV
HI
HI
HI
Nk
k
k
LO
LO
LO
1
1
LI
26
CONT… Subject to linear constraints
CV calculated for each time interval is between high and low limits
HI
HI
HI
Nk
k
k
Mk
k
k
LO
LO
LO
CV
CV
CV
MV
MV
MV
CV
CV
CV
CV
CV
CV
1
12
1
A
27
LINEAR PROGRAM SOLUTION
The following LP subproblem is solved where the economic weights are know a priori
HILO
HILO
CVMVgMVgMVgCV
CVMVgMVgMVgCV
2*
323*
222*
1212
1*
313*
212*
1111
*33
*22
*11 MVMVMV Minimize
Subject to:
28
MODIFIED DMC The original dynamic matrix is modified
Aij is the dynamic matrix of the ith CV with respect to the jth MV,
3*
3
2*
2
1*
1,
100
010
001
MVMV
MVMV
MVMV
t
t
t
2
1
232221
131211
e
e
e
AAA
AAA
A
29
DMC TUNING RULES Approximate the process dynamics of all
controller output to measured process variables
30
CONT… Select the sampling time as close as possible to
Compute the prediction horizon, P and model horizon, N
31
CONT... Compute the control horizon , M
Select the controlled variable weights to scale process variable units to be the same
Compute the move suppression coefficients
32
CONT…
• Implement DMC using the traditional step response of the actual process and the initial values of parameters
33
RESPONSE OF DMC Performance of DMC using tuning rules T=100, P=N=198, λ=3.53
34
REFERENCES
• Tuning guidelines for DMC- Danielle Dougherty and Douglas J. Cooper-Ind. Eng. Chem. Res. 2003, 42, 1739-1752
• http://web.stanford.edu/class/archive/ee/ee392m/ee392m.1056/Lecture14_MPC.pdf
• www.che.utexas.edu/course/che360/lecture_notes
• tx.technion.ac.il/~dlewin/054414/LECTURE_12.pdf
35
THANK YOU