AU663849B2

AU663849B2 - Strip position sensor

Info

Publication number: AU663849B2
Application number: AU82844/91A
Authority: AU
Inventors: Bruce Robert Morrison
Original assignee: BHP Steel JLA Pty Ltd
Current assignee: BHP Steel JLA Pty Ltd
Priority date: 1990-07-20
Filing date: 1991-07-18
Publication date: 1995-10-26
Anticipated expiration: 2011-07-18
Also published as: AU8284491A

Description

TITLE

STRIP POSITION SENSOR

TECHNICAL FIELD

There are numbers of industrial and production processes which require the continuous passage of a metal strip along a predetermined path. In such processes it is sometimes necessary to monitor the position of the centre plane, the so called pass line, of the strip, either to enable the pass line to be maintained in a desired line or to enable the position of equipment operating on the strip to be adjusted from time to time to suit the pass line pertaining at the time. The present invention is directed to sensors for detecting the position of a metal strip and to apparatus incorporating such sensors for monitoring the position of the pass line of the strip.

BACKGROUND ART

Conventional strip position monitors use optical or electronic sensors to produce a signal indicative of the strip's position relative to the sensor, and outputting means responsive to that signal which display, record or control the position of the strip.

DISCLOSURE OF THE INVENTION

Sometimes the ambient conditions preclude the use of conventional sensors. For example, optical sensors may be unusable in extremely dusty conditions, and electronic sensors in high temperature conditions. Therefore it is an object of the present invention to provide a strip position sensor able to function satisfactorily in harsh environments such as those instanced above.

The invention was devised primarily to monitor the pass line of a steel strip in a continuous hot dip coating process, that is to say a process wherein the strip is passed through a pool of molten coating metal, typically zinc or a mixture of aluminium and zinc, to acquire a liquid coating, which thereafter solidifies on the strip. More specifically, the invention was devised to monitor the pass line as the strip leaves the pool, to enable adjustment of a conventional, gas stripping nozzle assembly, whereby gas jets are directed against each side of the strip to remove surplus liquid coating metal from the strip before it solidifies.

In certain circumstances the nozzle assembly and the strip in its vicinity are enclosed in a hood, so that a stripping gas other than air may be used. Such a hood prevents the operator from observing whether the strip is in the centre of the gap between the nozzles of the assembly, as needed to equalise the coating thickness on each side of the strip. There is no point in providing a window in the hood as it soon becomes opaque due to the deposition of a dust of solidified droplets of the coating material. Likewise the dust precludes the use of optical sensors within the hood, and the temperature militates against the use of sensors containing electronic components close to the strip, be it within or without a hood.

Thus, sensors according to the invention are able to operate in the hostile environment within such a hood, where more conventional known sensors are precluded, but it will be appreciated that they may be used with advantage in other, maybe less hostile, environments. The invention consists in a metal strip position sensor comprising a pair of substantially co-axial, spaced apart, primary coils connected in series, a pair of substantially co-axial, spaced apart secondary coils respectively associated with the primary coils and disposed with each secondary coil in field linkable relationship with its associated primary coil, and mounting means supporting said coils, with one primary coil and its associated secondary coil on one side of a desired centre plane of a metal strip to be monitored and the other primary coil and its associated secondary coil on the other side of said centre plane, and enabling such a strip, to pass freely between the coils on each side thereof.

For preference the secondary coils are between the primary coils rather than vice versa. In either instance, each secondary coil is preferably closely adjacent its associated primary coil.

The sensor of the invention may be utilised as the sensor in a strip position monitor further comprising energising means to apply a substantially constant alternating voltage of predetermined frequency across said primary coils to produce an alternating primary magnetic field, and voltage responsive means responsive to the voltages generated in the secondary coils to indicate and/or control the deviation of such a metal strip from said plane in the vicinity of said secondary coils.

For preference those voltage responsive means comprise comparator means generating a signal indicative of the relationship between the voltages generated in the respective secondary coils and outputting means responsive to that signal for displaying or controlling the strip deviation. In preferred embodiments there may be at least one further pair of co-axial secondary coils associated with the primary coils as aforesaid, wherein the pairs of co-axial secondary coils are spaced apart laterally, so that, in use, each pair is linked by a discrete portion of the total primary field. That is to say there may be a plurality of pairs of secondary coils associated with a single pair of primary coils. In that event there may be a like plurality of voltage responsive means, one for each pair of secondary coils. Alternatively there may be a single voltage responsive means and switch means to connect it selectively to each pair of secondary coils.

Still again, where the voltage responsive means comprise comparator means and outputting means there may be a comparator means for each pair of secondary coils and a single outputting means able to be switched from one comparator means to another as desired.

The only part of a monitor according to the invention, which requires to be close to the monitored strip is the characterising sensor. As this comprises simple passive coils it may be readily designed to be capable of withstanding hostile environments.

The principle of operation of the sensor is that the primary coils create a magnetic field pattern which is disturbed by changes in the position of the strip. Notwithstanding such disturbances the total magnetic flux linking the primary coils is constant, because they are in series and the voltage across them is constant, thus any change in the field pattern necessarily alters the proportion of the total flux linking them individually, and thus the fluxes linking their respectively associated secondary coils. Therefore the voltages generated in the secondaries are not only dependent on the position of the strip but also will display a unique relationship for each and every position of the strip.

More specifically stated, if the strip is ferrous and the predetermined frequency is less than about 4 kHz, the strip acts as a magnetic shunt attracting magnetic flux, thereby increasing the flux through the secondary coil it is closer to and decreasing the flux through the secondary coil it is further from. If the strip is ferrous and the frequency is more than about 6 kHz, eddy currents induced in the strip repel the field and the strip acts as a flux barrier, thereby decreasing the flux through the secondary coil it is nearer to and increasing the flux through the secondary coil it is further from. If the strip is non-ferrous and the frequency very much higher still, say 50 kHz or more, depending largely on the resistivity of the strip material, the effect is similar to the last mentioned effect for a ferrous strip.

In preferred embodiments the magnetic axes of all the coils are substantially perpendicular to the desired centre plane, the secondary coils are fairly tightly coupled to the primary coils, the coil arrangement is symmetrical about the centre plane (that is to say the coils on one side of the centre plane are a mirror image of those on the other), the primary coils are mutually aiding, and the secondary coils are mutually opposing. In these circumstances the induced secondary voltages are substantially equal when the strip is centred between the coils, but become unbalanced in one or the other direction as the strip moves away from centre. Thus the comparator means may respond to the relative magnitudes of the individual secondary voltages.

Alternatively, and preferably, the secondary coils are connected in series, to produce a differential output signal, that is to say a signal equal to the difference between their induced voltages, and that signal is fed to comparator means preferably comprising a phase sensitive rectifier deriving its reference voltage directly from the energising means. As the individual induced secondary voltages are respectively substantially in phase and 180° out of phase with the reference voltage, the output of the rectifier is a DC signal with amplitude proportional to the deviation of the strip from the central position. Deviations in one direction give positive DC signals, and deviations in the other direction give negative DC signals. The output signal of the rectifier has a very low noise level, and may be fed directly to a DC voltmeter calibrated to indicate the position of the strip to an accuracy of 0.5 mm or better.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example, an embodiment of the above-described invention is described in more detail hereinafter with reference to the accompanying drawings.

Figure 1 is a schematic plan view of a sensor according to the invention in place in relation to a strip to be monitored.

Figure 2 is a schematic sectional view taken on line 2-2 of figure 1 drawn to a larger scale.

Figure 3 is a schematic diagram of the sensor of figure 1.

Figure 4 is a block diagram of a strip position monitor incorporating the sensor of figures 1 , 2 and 3. BEST MODE OF CARRYING OUT THE INVENTION

The sensor illustrated by figures 1 , 2 and 3 comprises a pair of elongated primary coils 8 and seven pairs of independently operating secondary coils 1 to 7. The primary coils 8 are wound in the same direction when viewed along the axis of the pair and are connected in series. They are supported by non-metallic formers 9 able to withstand temperatures of the order of 250°C and held by mounting means (not shown in detail but including the formers 9 and spacers 10) on opposite sides of a steel strip 11 , which may be emerging from a galvanising bath. Those mounting means are themselves then mounted on the gas stripping nozzle assembly whereby the thickness of the zinc coating is controlled. The arrangement is such that when the strip 11 is centrally positioned between the coils 8 then it is likewise centrally positioned between the nozzles of that assembly.

It will be noticed that the formers 9, and thus the primary coils 8, are appreciably longer than the width of the strip 11. This is to eliminate edge effects and ensure when the primary coils are energised that each pair of secondary coils 1 to 7 respectively is operating under substantially identical conditions insofar as the total primary field linkage with them is concerned.

The individual secondary coils of each pair are wound on paired formers 12. The pairs of secondary coil formers are spaced apart laterally so that each pair of secondary coils links a discrete part of the primary field, not linked by any other pair. In this embodiment, the secondary coils of each pair are wound in opposite directions, when viewed along the axis of the pair, and are also connected in series, so that the output from each pair is the difference between the voltages induced in the individual coils of that pair. .

In the strip position monitor illustrated by figure 4 the primary coils 8 of a sensor 13 according to figures 1 to 3 are energised by a power amplifier 14 fed by a function generator 15. The function generator may be a commercially available unit which nominally produces a sine wave signal of 0.5 V rms and an in phase square wave signal of +/- 2V. The sine wave signal is fed to the power amplifier 14 and the square wave signal is fed, as a reference signal, to each of seven phase sensitive rectifiers 16, only one of which appears in the diagram, although in practice there is preferably one for each of the seven pairs of secondary coils. In other embodiments a single rectifier may be switched from pair to pair of secondary coils as required.

Most commercially available function generators are adjustable as to the frequency of the output signal. Preferably, a frequency within the range of from 6 to say 60 kHz, and more preferably between 6 to 30 kHz, is selected. Such a frequency is effective to generate eddy currents in the steel strip, but has little effect on zinc or other non- ferrous accretions which would otherwise produce spurious results. For preference a frequency is chosen which is also sufficiently far from the frequencies used in induction heaters to permit ready suppression of any untoward interference that might otherwise arise if such heaters should be operating nearby, also care should be taken to avoid the harmonics and sub-harmonics of magnetic influences of same. Thus, a frequency of 9.7 kHz was found appropriate in the circumstances pertaining in a continuous galvanising plant where experiments leading to the present invention were conducted. The amplifier 14 may be a commercially available unit with a nominal output of 25 V 0-pk. In this instance the primary coils 8 may be spaced apart by about 150 mm. They may each have 10 turns and the secondary coils 1 to 7 may each have 100 turns.

The combined, in this instance differential, output of each pair of secondary coils 1 to 7, which in this instance may amount to a maximum of about 1 V 0-pk, is fed to an associated phase sensitive rectifier 16, which also receives a reference voltage from the generator 15.

The positive or negative DC signals from the phase sensitive rectifier which may range from -5 V to +5 V dc may be displayed by outputting means such as a cathode ray oscilloscope or a simple voltmeter. In some instances they may constitute the error signal of a servo-mechanism effecting automatic positional adjustment of a related component or the like.

Each pair of secondary coils 1 to 7 provides an output related to the position of the strip in the locality of the pair. Thus if each pair feeds its own related comparator means the several indications may indicate lack of flatness in the strip, for example "gutter" and other related phenomena.

Claims

1. A metal strip position sensor comprising a pair of substantially co-axial, spaced apart, primary coils connected in series, a pair of substantially co-axial, spaced apart secondary coils respectively associated with the primary coils and disposed with each secondary coil in field linkable relationship with its associated primary coil, and mounting means supporting said coils, with one primary coil and its associated secondary coil on one side of a desired centre plane of a metal strip to be monitored and the other primary coil and its associated secondary coil on the other side of said centre plane, and enabling such a strip to pass freely between the coils on each side thereof.

2. A sensor according to claim 1 comprising at least one further pair of co-axial secondary coils associated with said primary coils as aforesaid, wherein the pairs of co-axial secondary coils are spaced apart laterally.

3. A sensor according to either claim 1 or claim 2 wherein the coils of the, or each, pair of secondary coils are connected in series and wound in opposite directions to produce a differential output from the pair.

4. A sensor substantially as described herein with reference to figures 1 , 2 and 3 of the accompanying drawings.

5. A metal strip position monitor comprising a sensor according to claim 1 , energising means to apply a substantially constant alternating voltage of predetermined frequency across said primary coils to produce an alternating primary magnetic field, and voltage responsive means responsive to the voltages generated in the secondary coils to indicate and/or control the deviation of such a metal strip from said plane in the vicinity of said secondary coils.

6. A monitor according to claim 5 wherein said voltage responsive means comprise comparator means generating a signal indicative of the relationship between the voltages generated in the respective secondary coils and outputting means responsive to that signal for displaying or controlling the strip deviation.

7. A monitor according to claim 6 wherein said comparator means comprise a phase sensitive rectifier fed with a reference voltage directly from said energising means and the differential output of said secondary coils.

8. A metal strip position monitor comprising a sensor according to any one of claim 2, claim 3 in so far as it depends on claim 2, or claim 3, energising means to apply a substantially constant alternating voltage of predetermined frequency across said primary coils to produce an alternating primary magnetic field, and voltage responsive means responsive to the voltages generated by each coil of each pair of secondary coils to indicate the deviation of such a metal strip from said plane in the vicinity of said each pair of secondary coils.

9. A monitor according to claim 8 wherein the coils of each pair of secondary coils are connected in series in opposition to produce a differential output, and wherein said voltage responsive means comprise a plurality of phase sensitive rectifiers respectively connected to the pairs of secondary coils and also fed with a reference voltage from said energising means, and outputting means responsive to signals from said rectifiers.

10. A metal strip position monitor substantially as described herein with reference to figure 4 of the accompanying drawings.