Control of Deposited Zinc in Hot Dip Gal

8.7.5 Open Loop Adaptation

To maintain the performance of the control system, the controller is adapted as a function of:

 Speed of the steel strip
 The distance between the air knives and the steel strip

In addition to the adaptation of the sampling frequency as a function of the speed of the steel strip, the operation range of the hot dip galvanization process has been divided into three speed regions (the speed of the strip may vary from 30 m/min to

¹¹ The choice made for the dominant and auxiliary poles is close to the choice made in the example of the internal model control of systems with time delay discussed in Chapter 3, Section 3.5.7.
370 Digital Control Systems



Hot Dip Galvanizing : Nyquist Plot

3

Modulus margin: 0.511

2

1

^a) ^{0 (-1,j0)}

Delay Margin: 10.9s Gain Margin: 2.04 Phase Margin: 60.3°

T_s = 12 s

-1

^-₂ Closed Loop Poles : ₁ = 0.2, ₂ = 0.3

-3

-2 -1 0 1 2 3 4 5 6 7 Real Axis
Hot Dip Galvanizing : Nyquist Plot

Modulus margin: 0.52 ³ Delay Margin: 25.7s
Gain Margin: 2.09 ₂ Phase Margin: 60°
T_s = 12 s 1

^b) ^{0 (-1,j0)}

-1
Closed Loop Poles : ₁ = 0.2, ₂ = 0.3, _i = 0.1 i = 3,...,9

-3

-2 -1 0 1 2 3 4 5 6 7 Real Axis
Figure 8.49a,b. Nyquist plot for hot dip galvanizing (a₁=-0.2, d=7): a Closed loop poles: 0.2; 0.3; b Closed loop poles. 0.2; 0.3; 7 0.1

Practical Aspects of Digital Control 371
180 m/min) and in three regions for the distance between the air knives and the steel strip which lead to a total of nine regions of operation, each one defined by a range of speeds and a range of distances. An identification of the plant model has been done for various operating points within each region of operation. A controller design on the basis of a model estimated for the central operating point within a region has been designed, and tested in simulation on the various identified plant models for the corresponding region. The controllers have been stored in a table. The controllers are switched when the system moves in a new region of operation. A bumpless transfer from one controller to another is implemented. In addition hysteresis is used to avoid unnecessary switching when one operates very close to the regions boundaries.
8.7.6 Results
Figure 8.50 shows one of the typical results obtained when one of the sides is under digital regulation and the other side is under computer aided manual control (the operator has on display a moving short time history of the deposited zinc and applied pressure).
The analysis of this curve points out two relevant facts when closed loop digital control is used:
A smaller dispersion of the coating thickness (the standard deviation on the side where the feedback control was applied is about 30 % smaller than on the side under manual control). This assures a better finished product quality (extremely important for the use in the automotive industry, for example).
The average quantity of deposited zinc is reduced by 3% still guaranteeing the specifications for minimum zinc deposit. This corresponds to a significant reduction of the zinc consumption and to an important economic impact.
Table 8.10 summarize the results obtained in different operation regions.