Control system for heavy metallic coating weight

Abstract

It is provided a method and device to control the thickness of a metallic coating on a steel wire. A device (101) for controlling a metallic coating thickness on a steel wire comprises: a digital proportional valve (113); a digital flow meter (111); a pressure sensor (115); a thickness sensor (105); a gas nozzle (119); a computer (107); and is characterized in that the relation between the gas flow measured by said digital flow meter (111) and the coating thickness measured by said thickness sensor (105) for said gas nozzle (119) is linear or parabolic.

Claims

1. A method to control the thickness of a metallic coating on a steel wire comprising the steps: producing and storing on a computer system 107 a first data set with a relation between a current reference for a proportional valve 113 and a coating thickness of the metallic coating on the steel wire as measured by a thickness sensor 105; producing and storing on the computer system 107 a second data set with a relation between the current reference for the proportional valve 113 and a flow rate as measured by a digital flow meter 111; entering a target coating thickness in the computer system 107; measuring the thickness of a metallic coating on a steel wire using a thickness sensor 105 and sending said measured thickness to the computer system 107; sending with the computer system 107 a feed forward current reference to the proportional valve 113, according to the relation provided by said second data set; and correcting with the computer system 107 said feed forward current reference until the flow measured by the digital flow meter 111 is equal to a flow needed to reach said target coating thickness according to the relation provided by said first data set.

2. The method according to claim 1, wherein a relation obtained by combining said first data set and said second data set between the coating thickness and a gas flow is linear or parabolic.

3. The method according to claim 1, wherein the method uses a gas nozzle 119 comprising at least two exit supplies and/or wherein an angle between the gas direction and the horizontal is different for the at least two exit supplies and/or wherein a different gas at a given pressure is displayed for each exit supply.

4. The method according to claim 1, wherein a derivative of the relation obtained by combining said first data set and said second data set between a coating thickness and a gas flow has a positive coefficient such that the coating thickness increases with an increased gas flow.

5. The method according to claim 1, wherein the production of said first data set and said second data set is automatised.

6. The method according to claim 1, whereby uniform metallic coating thicknesses selected in a range between 35 m and 115 m are obtained.

7. The method according to claim 1, wherein the method controls the thickness of the metallic coating on the steel wire, the steel wire having a diameter between 1 mm and 18 mm.

8. The method according to claim 1, wherein the metallic coating is selected from the group consisting of Zinc er and ZnAlX alloys, wherein X is Sr, La, Cu, Ti, Ce, Mg, Ni, Si, or Cr.

9. The method according to claim 1, wherein the standard deviation of coating thickness along a steel wire is less than 10% of the target coating thickness.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

(1) FIG. 1. Schematic description of the device for controlling heavy metallic coating weights on steel wire.

(2) FIG. 2. Example of flow rate vs coating weight calibration curve.

(3) FIG. 3. Measured flow rate vs target as a function of time.

MODE(S) FOR CARRYING OUT THE INVENTION

(4) FIG. 1. is a schematic representation of the device of the invention. The device 101 comprises: a digital proportional valve 113. It is used to control the gas pressure in the range 0.02-2 bar; a digital flow meter 111; a pressure sensor 115; a thickness sensor 105. In the present case an Eddy current device was used; a gas nozzle 119; a computer 107. Preferably, the computer is a programmable logic controller (PLC);

(5) In FIG. 1. An optional flow restrictor 117 is also represented.

(6) The steel wire 123 with a metallic coating 125 at its surface are also represented.

(7) The present device was tested with nitrogen (N2) as thickness controlling gas. The flow of N2 is indicated in FIG. 1. by the arrows 109. The maximum pressure of N2 at the entry of the digital flow meter 111 was 2 bars.

(8) The flow rate reaching the gas nozzle 119 was between 0 and 11.4 m.sup.3/h in normal conditions, i.e. assuming 1 bar pressure behind the proportional valve 113.

(9) The computer program stored in the computer 107 allows different controls of the device. The target coating weight and type of nozzle are first entered in the computer.

(10) For low coating weights, i.e. below 250 g/m.sup.2 the standard type of gas-knife nozzles is used with high flow rate, i.e. above 5 m.sup.3/h, and a negative linear relation between the coating weight CW and the gas flow rate V is observed of the type:
V=a CW+b.

(11) A negative coefficient means that the coating thickness decreases when the gas flow increases.

(12) To reach thicker coatings or higher coating weight, e.g. above 250 g/m.sup.2 or above 300 g/m.sup.2, the gas nozzle is modified such that the gas flows through at least 2 exit supplies, one at the bottom and one at the middle or at the top of the nozzle. As an illustration, the nozzle represented in FIG. 1 contains 2 exit supplies.

(13) The angle between the gas direction and the horizontal (perpendicular to the wire) at each exit supply is in the range 90 to +90 and may be different at each exit supply. With this modification, an increasing flow of nitrogen at one of the exit supplies, results in a higher coating weight.

(14) The relation between the gas flow and the coating thickness may thus have a positive coefficient.

(15) The correlation can also be parabolic:

(16) $V=a{\mathrm{CW}}^2+b\mathrm{CW}+c.$
An example of such a parabolic relation between the coating weight and the gas flow is shown in FIG. 2.

(17) The derivative

(18) $\left(\frac{\mathrm{dV}}{\mathrm{dCW}}=2a\mathrm{CW}+b\right)$
is calculated at the process value of the coating weight. This is a local linearization of the slope of the step which needs to be taken to go to the next point. The next step in flow is calculated as V=(2 a CW.sub.PV+b)CW. The V is than added up to the actual flow rate at that moment.

(19) FIG. 2. has been obtained with pure zinc as metallic coating. Using a ZnAl alloy, higher coating thickness up to 800 g/m.sup.2 was obtained with 9 m.sup.3/h using the same set-up.

(20) Method Implementation:

(21) The target coating weight is entered in the computer 107. The thickness sensor 105 gives a (feed forward) reference towards the proportional nitrogen valve 113. This feed forward is corrected (in positive or in negative direction) to achieve the real flow measured by the digital flow meter 111. Using a feed forward gives a more stable control. A calibration of valve/nozzle combination is needed. The calibration starts automatically pushing a button.

(22) The system needs a series of data to make the correlation between flow and coating weight when a modified nozzle for thick coatings is used. Using the coating weight, the system controls and adjusts, step by step, towards the correlated flow rate. A PID controller may be used to compensate the error between the desired flow and the real flow.

(23) Nozzle Detection and Calibration:

(24) In standard (low thickness coating) mode, the scanning makes a relation data set between analogue current (mA) reference for the proportional valve 113 and the flow (m.sup.3/h) measured by the digital flow meter 111. This relationship used as a feed forward for valve position.

(25) In thick coating mode, the scanning detects 2 relations. These relations are simultaneously made, thus having the same X-axis. The first data set is a relation between the analogue current (mA) reference for the proportional valve 113 and the coating weight (g/m.sup.2) as measured by the thickness sensor 105. The second data set is a relation between the analogue current (mA) reference for the proportional valve 113 and the flow rate (m.sup.3/h) as measured by the digital flow meter 111. By combining the first and the second data sets, the relation between the coating weight (g/m.sup.2) and the flow rate (m.sup.3/h) is retrieved. FIG. 2. is a calibration curve obtained following the steps described above. The calibration curve can be generated automatically.

(26) FIG. 3. is a comparison between the target flow (dotted line) and the measured flow (solid line) in m.sup.3/h as a function of time. The precise control of the flow (+/0.02 m.sup.3/h) ensures stable coating weight. For target coating weights in the range between 250 g/m.sup.2 and 1200 g/m.sup.2 standard deviations lower than 10% of the target coating weight were measured, e.g. less than 25 g/m.sup.2 for 250 g/m.sup.2 and less than 120 g/m.sup.2 for 1200 g/m.sup.2.