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.