What does it mean to control coating weight
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What does it mean to control coating weight?. Bob Wilhelm Objective Control Ltd. What does it mean to control coating weight?. What a strange question! What is it meant to imply?

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What does it mean to control coating weight?

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What does it mean to control coating weight

What does it mean to control coating weight?

Bob Wilhelm

Objective Control Ltd.


What does it mean to control coating weight1

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

200

150

coating weight (gsm)

100

1

0

location across

strip (m)

4100

4050

-1

4000

3950

location along strip (m)

Scanning Measurement


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?

170

160

150

Coating Weight (gsm)

140

130

120

0

200

400

600

800

1000

1200

Cross-Direction Position (mm)

One scan of coating weight


Weight distribution over a coil

Weight Distribution over a Coil


Weight distribution as a color map

Weight Distribution as a Color Map


Visualizing profile stability

Visualizing profile stability


Separating coating weight components cd md residual

220

200

Coating Weight (gsm)

180

160

140

120

1000

1050

1100

1150

1200

1250

1300

Distance from head of coil (m)

Separating Coating Weight Components: CD, MD, Residual

  • Coating weight seen as a time series from the scanning measurement.


Complete removal of cd

0

1

2

3

4

5

6

7

8

700

1

0.9

600

0.8

500

0.7

0.6

400

Scan Average Frequency Response

Discrete Power Spectrum - Cross Direction

0.5

300

0.4

0.3

200

0.2

100

0.1

0

0

8

0

1

2

3

4

5

6

7

-1

Frequency (m

)

Complete Removal of CD


Power spectrum of cd md

4000

weight

weight with cross-direction removed

3000

Power Spectral Density

2000

1000

0

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

-

1

frequency (m

)

Power Spectrum of CD & MD


Removal of estimated cd

200

measured weight

190

weight with average profile removed

180

170

Coating Weight (gsm)

160

150

140

130

0

200

400

600

800

1000

1200

Strip Distance (m)

Removal of Estimated CD


Edge induced bias

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.


Three consecutive coils with coating weight transitions

Three Consecutive Coils with Coating Weight Transitions


Two sided target management

Nominal Top/Bottom Ratio

Nominal Top/Bottom Ratio

Highest Top/Bottom Ratio

Highest Top/Bottom Ratio

Lowest

Lowest

Bottom

Bottom

Side

Side

Weight

Weight

Lowest Top/Bottom Ratio

Lowest Top/Bottom Ratio

-

-

-

4

4

4

-

-

-

2

2

2

0.1

0.1

0.1

0.2

0.2

0.2

0.3

0.3

0.3

0.4

0.4

0.4

Top Side Weight

Top Side Weight

2

2

2

Lowest Total Weight

Lowest Total Weight

Lowest Top Side Weight

Lowest Top Side Weight

4

4

4

0.4

0.4

0.4

0.3

0.3

0.3

0.2

0.2

0.2

4

4

4

0.4

0.4

0.4

0.1

0.1

0.1

2

2

2

0.3

0.3

0.3

2

2

2

-

-

-

0.2

0.2

0.2

4

4

4

-

-

-

0.1

0.1

0.1

-

-

-

4

4

4

-

-

-

2

2

2

2

2

2

4

4

4

Bottom Side Weight

Bottom Side Weight

Two-Sided Target Management


Constraint scenarios

Constraint Scenarios

  • Total weight minimized. Equal coatings maintained.


Constraint scenarios1

Constraint Scenarios

  • Total weight minimized. Equal coatings maintained.

  • Total weight and one side minimized with deviation from equal coating.


Constraint scenarios2

Constraint Scenarios

  • Total weight minimized. Equal coatings maintained.

  • Total weight and one side minimized with deviation from equal coating.

  • Both sides minimized with deviation from equal coating.


Constraint scenarios3

Constraint Scenarios

  • Total weight minimized. Equal coatings maintained.

  • Total weight and one side minimized with deviation from equal coating.

  • Both sides minimized with deviation from equal coating.

  • One side minimized with side-to-side ratio constrained.


Two sided target management1

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 spot

minimum spec


Target management dynamics throughout a coil

Target Management Dynamics Throughout a Coil

Coil C total

500

weight

nominal wt target

weight setpoint

450

scan average

single spot min

single spot max

400

triple spot min

triple spot max

setpoint min

350

setpoint max

Coating Weight

300

250

200

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

Location in Coil


Target management results

Target Management Results


Conclusions 1

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

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

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.


What does it mean to control coating weight

Thank you.


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