what does it mean to control coating weight? bob wilhelm objective control ltd
TRANSCRIPT
What does it mean to control coating weight?
Bob WilhelmObjective Control Ltd.
What does it mean to control coating weight?
What a strange question! What is it meant to imply? Because coating weight is a stochastic
property distributed over two sides of a two-dimensional strip, there is no such thing as “the” coating weight of the strip.
Simultaneously meeting product specifications and producing product economically has deep implications for the control strategy.
Scanning Measurement
3950 4000 4050 4100-1
0
1100
150
200
location along strip (m)
location acrossstrip (m)
co
ati
ng
we
igh
t (g
sm
)
One scan of coating weight
What do these variations represent? Cross-strip, along-strip, both, or neither?
How should this information be used for purposes of coating weight control?
0 200 400 600 800 1000 1200120
130
140
150
160
170
Cross-Direction Position (mm)
Co
atin
g W
eig
ht
(gsm
)
Weight Distribution over a Coil
Weight Distribution as a Color Map
Visualizing profile stability
Separating Coating Weight Components: CD, MD, Residual
Coating weight seen as a time series from the scanning measurement.
1000 1050 1100 1150 1200 1250 1300120
140
160
180
200
220
Co
ati
ng
We
igh
t (g
sm
)
Distance from head of coil (m)
Complete Removal of CD
0 1 2 3 4 5 6 7 80
100
200
300
400
500
600
700
Dis
cre
te P
ow
er S
pec
tru
m -
Cro
ss D
ire
ctio
n
Frequency (m-1)
0 1 2 3 4 5 6 7 8
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Sca
n A
ver
ag
e F
req
uen
cy
Res
po
nse
Power Spectrum of CD & MD
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20
1000
2000
3000
4000
Po
we
r S
pe
ctr
al
De
ns
ity
frequency (m-1)
weightweight with cross-direction removed
Removal of Estimated CD
0 200 400 600 800 1000 1200130
140
150
160
170
180
190
200
Co
atin
g W
eig
ht
(gsm
)
Strip Distance (m)
measured weightweight with average profile removed
Edge-Induced Bias Inclusion of edges
can bias skew and crossbow computations.
Skew and crossbow control control based on these results can make things worse.
Virtual “edge trim” provides a simple solution.
-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8110
120
130
140
150
160
170
180
190
200
210
Cross-Sheet Position (m)
Co
atin
g W
eig
ht
(gsm
)
Three Consecutive Coils with Coating Weight Transitions
Two-Sided Target Management
-4-
22
4
0.1
0.2
0.3
0.4
Bottom Side Weight
Top
Sid
e W
eigh
t
- 4 - 2 2 4
0.1
0.2
0.3
0.4
LowestBottom
SideWeight
Lowest Top Side Weight
Lowest Total Weight
Hig
hest
Top
/Bot
tom
Rat
io
Lowest Top/Botto
m Ratio
Nomina
l Top
/Bot
tom
Rat
io
- 4- 2
24
0.1
0.2
0.3
0.4
-4-
22
4
0.1
0.2
0.3
0.4
-4-
22
4
0.1
0.2
0.3
0.4
Bottom Side Weight
Top
Sid
e W
eigh
t
- 4 - 2 2 4
0.1
0.2
0.3
0.4
- 4 - 2 2 4
0.1
0.2
0.3
0.4
LowestBottom
SideWeight
Lowest Top Side Weight
Lowest Total Weight
Hig
hest
Top
/Bot
tom
Rat
io
Lowest Top/Botto
m Ratio
Nomina
l Top
/Bot
tom
Rat
io
- 4- 2
24
0.1
0.2
0.3
0.4
- 4- 2
24
0.1
0.2
0.3
0.4
Constraint Scenarios
a) Total weight minimized. Equal coatings maintained.
Constraint Scenarios
a) Total weight minimized. Equal coatings maintained.
b) Total weight and one side minimized with deviation from equal coating.
Constraint Scenariosa) Total weight
minimized. Equal coatings maintained.
b) Total weight and one side minimized with deviation from equal coating.
c) Both sides minimized with deviation from equal coating.
Constraint Scenariosa) Total weight
minimized. Equal coatings maintained.
b) Total weight and one side minimized with deviation from equal coating.
c) Both sides minimized with deviation from equal coating.
d) One side minimized with side-to-side ratio constrained.
80 100 120 140 160 180 200 220 240
80
100
120
140
160
180
200
220
240
Bottom Side Coating Weight
To
p S
ide
Co
atin
g W
eig
ht
Two-Sided Target Management
Nominal coating weight target
Optimized coating weight target
Total weight triple spot minimum spec
Total weight single spot minimum spec
Single side triple spotminimum spec
Target Management Dynamics Throughout a Coil
0 1000 2000 3000 4000 5000 6000 7000 8000 9000200
250
300
350
400
450
500Coil C total
Co
atin
g W
eig
ht
Location in Coil
weightnominal wt targetweight setpointscan averagesingle spot minsingle spot maxtriple spot mintriple spot maxsetpoint minsetpoint max
Target Management Results
Coil Change in Target (gsm)
% Change in Target
C -3.76 -1.32
D -7.88 -2.17
E -6.98 -2.43
F -3.68 -1.28
G -3.95 -1.38
H -6.16 -2.15
I -5.99 -2.09
Conclusions (1)
There is no such thing as “the” coating weight of a real strip.
Controlling average coating weight across the strip is a practical and appropriate technique from a control viewpoint, but is, by itself, inadequate to guarantee either product quality or zinc savings.
Conclusions (2) Coating weight control requires that
various coating characteristics be individually identified and dealt with: Cross direction Strip direction Residual
Control intended to address one problem must be carefully designed to ignore variations caused by the others.
Conclusions (3) Active automatic target management
is needed to maximize profit while guaranteeing quality. Automatic target management
addresses the all-important “residual” variation.
It is time to reexamine industry standard coating weight specifications in light of today’s production and control capabilities.
Thank you.