Control of Deposited Zinc in Hot Dip Gal

8.7.2 Process Model

The static model of the hot dip galvanizing process can be approached by

m KD _P _m

where m is the deposited mass per unit area, K is a constant of proportionality, D is the distance between the air knives and the strip, P is the air pressure and V is the strip speed. _m accounts for unpredictable effects and/or modelling errors. At SOLLAC Sainte Agathe, the control variable is the air pressure.

A linearized model around an operating point (P₀,D₀,V₀) can be obtained using a standard Taylor series expansion for variations of pressure (∆P), speed (∆V) and distance (∆D). It has the form

Air
366 Digital Control Systems


m KD₀ ^V0 ∆D ∆μ∆P _m 0

where P₀,D₀,V₀ are the values of the pressure, distance and speed defining the operating point and ∆P, ∆D, ∆V, are the variations of these variables. It can be seen that using the pressure as the control variable one can compensate for the disturbances created by variations of distance and speed as well as by the term _m.

Finished Steel ^product strip

Preheat oven

Measurement of _knives deposited mass

Zinc bath