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? 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

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? Cross-Direction Position (mm) Coating Weight (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 Coating Weight (gsm) Distance from head of coil (m)

Complete Removal of CD

Power Spectrum of CD & MD Power Spectral Density frequency (m -1 ) weight weight with cross-direction removed

Removal of Estimated CD Coating Weight (gsm) Strip Distance (m) measured weight weight 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.

Three Consecutive Coils with Coating Weight Transitions

Two-Sided Target Management

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 Scenarios a) 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 Scenarios a) 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.

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 Coil C total Coating Weight Location in Coil weight nominal wt target weight setpoint scan average single spot min single spot max triple spot min triple spot max setpoint min setpoint max

Target Management Results CoilChange in Target (gsm) % Change in Target C D E F G H I

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.