Motivation The uniformity of the coating weight in length and cross profile is together with the surface quality one of the most important quality parameter of hot dip galvanized material.
Coating consumption and off tolerance material on continuous hot dip galvanizing can be reduced with precise close loop coating weight thickness control. Furthermore, the plant efficiency can be increased and costs reduced.
Payback time of the Zinc Coating Control System is counted in months.
Challenge
Air knife stripping is quite stable process itself, but ever-shortening campaigns lead to frequent product changes and adjustments of the process parameters.
Temperature handling practices in oven can cause long continuous speed change over the strip length.
The air knife stripping process of the coating is strongly nonlinear and is influenced by many parameters of the production line. The most important of these parameters are the following:
Base material properties (temperature, roughness, etc.)
Structure
The Zinc Coating Control System main functions are pre-setting, feed forward and feedback in connection with a coating thickness gauge.
In operation the online auto adaptive model-based feed forward control is the main control component and the feedback control takes care of the small residual errors.
In the design of the control system special weight has been given to a calm predictable behavior for the operator and yet a very fast and accurate response in process transitions.
The following diagram presents the main structure of the close loop model-based coating weight control system (blue) and the relation to the air knife automation (green) on hot dip galvanizing line.
Operation
The control system can be operated through a single HMI dialogue with all important control values displayed. Operator can freely select the combination between the automatic control functions and the control outputs that he wants to set manually.
Control values for the next strip can be prepared in advance and they will be activated by the weld seam.
The next picture displays the control screen.
Control features
The Zinc Coating Control System has the following proven features and modes:
Sum pressure control
Differential pressure control
Position balancing
Symmetric skewing
Asymmetric skewing
Coating weight target optimization
Differential coating
Spot controls
Electromagnetic Strip Stabilizer Integration
Stabilizer overcompensation according coating weight profile
Correction roll intermesh control according coating weight crossbow and stabilizer signals