Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Involving measuring – analyzing – or testing
Reexamination Certificate
1999-08-25
2001-05-22
Wong, Edna (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Involving measuring, analyzing, or testing
C205S138000, C205S141000, C204S211000, C204S228100, C204S228700, C204S229200, C204S912000
Reexamination Certificate
active
06235178
ABSTRACT:
FIELD OF THE INVENTION
The invention present relates to a method and an apparatus for coating a metal strip using a coating metal, in particular for coating a steel strip using zinc or a zinc
ickel compound.
BACKGROUND INFORMATION
A conventional coating plant typically has one or more electroplating cells, in which there is an electrolyte which contains the metals for the coating of the metal strip. The metal strip is led through the electrolyte liquid. In addition, anodes are arranged in the electrolyte. The metal strip is coated by means of an electric current between the anodes and the metal strip, which acts as a cathode. In the process, the current is controlled in such a way that a layer of a desired thickness is deposited on the metal strip.
However, in the industrial coating of metal strips there are two conflicting requirements. On the one hand, a predefined coating (weight) setpoint should, as far as possible, not be exceeded, since too thick a coating consumes an unnecessarily large amount of material and leads to higher costs. However, in order to be able to guarantee the desired properties of the metal strip, it must be ensured that the coating (weight) does not fall below a specific lower limit at any point on the strip.
SUMMARY
An object of the present invention is to provide a method and an apparatus for coating a metal strip using coating metal which makes it possible to keep as accurately as possible to a predefined coating (weight) setpoint value. In this case, compliance with a specific coating (weight) lower limit in particular is to be guaranteed, without the metal strip being coated to an unnecessarily high degree.
According to the present invention, the object is achieved by a method and an apparatus in which the current through the electroplating cell is controlled as a function of the properties of the electroplating cell by a monitor controller that has a process model and a controller part, the current is controlled in such a way that a layer of a desired thickness is deposited on the metal strip, in which case, in the event of changes in the state of the coating plant, in particular when a new metal strip enters or the coating falls below a minimum intended layer thickness, time constants of the controller part are matched to the altered state of the coating plant. In this way, it is possible to suppress measurement noise, and large controller movements, which can be caused by the process model not being an exact image of the coating plant, are adequately smoothed, and at the same time it is possible to react rapidly to external events, such as a new strip entering the plant or the occurrence of critical states, such as the coating weight falling below a minimum value.
To this end, in a particularly advantageous way, the current is controlled using a dynamic low-pass filter as a function of the coating weight mean value, in particular as a function of the ratio of coating weight mean value and coating weight value ascertained by the process model, it being the case that when a new metal strip enters the coating plant, the time constant of the dynamic low-pass filter for controlling as a function of the coating weight mean value is reduced and subsequently enlarged. In the case of a simple low-pass filter, the time constant, once set, would be effective from the start. In contrast, the dynamic low-pass filter used according to the present invention operates in such a way that it initially includes direct access at each strip start. After this, a time constant is set which rises slowly to a specific value guaranteeing adequate smoothing. The result of this is that the monitor controller sets the predefined coating weight setpoint value at the strip start as rapidly as is at all possible, i.e., without any kind of smoothing. With an increase in the measured values, the dynamic low-pass filter then changes into a mode of operation which smooths large control movements, in particular those caused by measurement noise and differences between the process model and the real process.
In a further advantageous embodiment of the present invention, the current is controlled by means of a dynamic low-pass filter as a function of the coating weight minimum value, in particular as a function of the ratio of coating weight minimum value and coating weight value ascertained by the process model, it being the case that, when the coating weight falls below the minimum value, the time constant of the dynamic low-pass filter is reduced to a very small value, and above the coating weight minimum value, it is set to a large value ensuring adequate smoothing. In this case, it is particularly advantageous, when the coating weight exceeds the minimum value, initially to keep the output from the dynamic low-pass filter constant for a specific waiting time, and after this waiting time has expired to control the said output as a function of the coating weight minimum value, in particular as a function of the ratio of coating weight minimum value and coating weight ascertained by the process model, the time constant of the dynamic low-pass filter being set to a value ensuring adequate smoothing.
REFERENCES:
patent: 50-10256 (1975-04-01), None
patent: 54-101842 (1979-08-01), None
patent: 5-226541 (1993-09-01), None
patent: 7-173700 (1995-07-01), None
patent: 1 437 417 (1988-11-01), None
Kenyon & Kenyon
Siemens Aktiengesellschaft
Wong Edna
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