Method for simultaneously measuring the positions of more than one surface in metallurgic processes
CONTINUING APPLICATION DATA 
     This application is a Continuation-In-Part application of International Application No. PCT/SE94/00099, filed on Feb. 3, 1994, which claims priority from Swedish Patent Application No. 9300348-1, filed on Feb. 3, 1993. International Application No. PCT/SE94/00099 was pending as of the filing date of the present U.S. application and the U.S. was an elected state in the International Application No. PCT/SE94/00099. 
    
    
     CONTINUING APPLICATION DATA 
     This application is a Continuation-In-Part application of International Application No. PCT/SE94/00099, filed on Feb. 3, 1994, which claims priority from Swedish Patent Application No. 9300348-1, filed on Feb. 3, 1993. International Application No. PCT/SE94/00099 was pending as of the filing date of the present U.S. application and the U.S. was an elected state in the International Application No. PCT/SE94/00099. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to a method in metallurgical processes for simultaneously measuring the positions of more than one surface. 
     BACKGROUND INFORMATION 
     In converters, ladles, electric arc furnaces and other metallurgical vessels, it is desirable to know the exact position of the slag surface and the position of the interface between the slag amd the liquid metal. Although many methods have been used in the prior art, no method has been at the same time fast, reliable and accurate. It may also be desirable to be able to measure the positions of other surfaces for example in order to control the thickness of the lining of vessels. 
     Changes in the pattern of electromagnetic wave fronts represent the most sensitive probes in physics. Electromagnetic waves may penetrate media of varying physical properties, changing its amplitude and phase in a way which is specific to the content of the media. Thus, continuum radiation will be affected when penetrating a media in the sense that the amplitude will be attenuated and the propagation velocity will change, resulting in a sudden change of phase in the interface surface. The radioband is of particular interest in that here waves can penetrate deeper into dusty areas and penetrate through ceramic material. 
     It is known that the interference between a transmitted wave and a reflected wave will create a standing wave pattern at a specific frequency determined by the positions of the null in this standing wave pattern and that this so determined wavelength of the signal will tell the position of a single surface, EP-A-60 597. Only the position of a single surface can be determined with this technique, which severely limits the usefulness of the method in metallurgic process industry. Furthermore, the amplitude of the standing wave is measured rather than its phase which severely limits the resolution and the testability of the method. 
     It is also known that a distance can be measured if the transmitted signal is swept in frequency and the reflected and transmitted signal are mixed so that a low frequency (IF-)signal is created, DE-2812 871. The frequency of this IF-signal is dependent on the time-delay of the reflected signal as compared to the sweeping time of the transmitter. This particular method can detect only a single surface. 
     It is also known that the angle of polarisation of a transmitted signal will change when it is reflected at a large angle at two surfaces, WO 91/10899 and U.S. Pat. No. 4,818,930. These methods are both transmitting at a single frequency at a large angle (larger than the Brewster angle) to the surface and can detect only the thickness of the layer between the two surfaces and only modulus the transmitted wavelength. 
     None of the above patent publications illustrates or discusses the phase change across the frequency bandpass and none of the above methods can therefore detect the positions of several surfaces simultaneously with an antenna system mounted at right angle to the surfaces. None of the above patents illustrates nor discusses the extension to three dimensional imaging of several surfaces. The method presented here is therefore significantly different from the above mentioned prior art. 
     OBJECT OF THE INVENTION 
     It is an object of the invention to provide a method of this kind which is fast, reliable and accurate. 
     SUMMARY OF THE INVENTION 
     The time-delay of a signal relative another signal is in the Fourier-, or frequency space a linear shift of phase with frequency. If the object signal is transmitted towards and reflected in a surface, then the relative phase of the signals therefore will change linearly with frequency. If the signal is measured in steps over a frequency band, then a plot of phase with frequency would be a line with a slope corresponding to the delay of the reflecting signal compared to the reference signal. The distance car/thus be measured via such a frequency stepped system. If the signal is instead transmitted towards a semitransparent medium, then pan of the signal will be reflected, and part of the signal will propagate through the medium to be reflected in the next surface where the index of refraction again is changing. These doubly reflected waves will, when complex multiplied with the conjugate of the reference signal, show a more complicated curve of phase as a function of frequency. If data therefore are sampled as complex amplitudes in frequency channels over a frequency band, then the distances to both surfaces can be recovered. If then the signal is transmitted and received by an interferometer in the aperture plane, then the full three-dimensional structure of the two surfaces can be reconstructed. This is also true for a mixture where more than two surfaces are present. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be more closely described with reference to the drawings. 
     FIG. 1 shows a schematic representation of a system in accordance with the invention for measuring the positions of multiple surfaces. 
     FIG. 2 shows schematically a metallurgical vessel to which the invention can be applied. 
     FIG. 3 is a diagram showing a point-spread-function (PSF) of the frequency pass band obtained from an experiment described with reference to FIGS. 1 and 2. 
     FIG. 4 is a diagram of the reflections from the slag surface and metal bath surface obtained from the same experiment. 
     FIG. 5 is a flow diagram illustrating the sequence of steps performed by a computer program &#34;SLAGG&#34; set forth hereinbelow. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An example of the invention is shown in FIGS. 1 and 2. A signal is created at a defined frequency with a signal generator 1. This signal is transferred via a cable to a powersplitter 2 where one path is conveyed via a cable to an antenna 3. The second path is conveyed via a cable to a phase comparator unit 4 where it is used as reference signal. The antenna transmits the radio signal as a circular polarisation towards the metal metallurgical vessel in the form of a ladle 10 shown in FIG. 2. The signal is aimed at perpendicular angle to the surface of the metal bath in the vessel 10 and reflected at the surfaces of the slag and metal bath as shown in FIG. 2 and received by the same antenna 3 in the opposite circular polarisation due to the odd number of reflections. The received signal is transmitted through a cable to the phase comparator 4 and there complex multiplied with the conjugate of time reference signal. The amplitude and phase of the complex conjugate multiplication is stored in a table by a computer 5 and the signal generator is stepped in frequency and a new measurement is taken. This procedure continues until a fixed number of frequency channels have been measured separately over a frequency band. The equipment is controlled by a computer which also stores the data and does the signal analysis. 
     The reference wave received at time t o  and at frequency w may be written as: 
     
         U.sub.ref (w)=e.sup.jwt o 
    
     The signal reflected from the first surface and referred to the same receiving time t o  can be written as: 
     
         U.sub.S1 (w)=e.sup.j(w(t o.sup.-2D1 slag.sup./c)) 
    
     The signal reflected from the second surface and referred to the same receiving time t o  can be written as: 
     
         U.sub.S2 (w)=e.sup.j(w(t o.sup.-2D1 slag.sup./c-2D1 bad.sup.*n slag.sup./c)) 
    
     D1 slag  and D1 bad  are represented in FIG. 2. 
     D1 slag  is the distance to the first (slag) surface, from a reference position in the antenna represented as a level 11.D1 bad  is the distance between the two surfaces (slag and metal bath), c is the velocity of light in air, and n slag  is the refractive index of the medium between the two surfaces. The complex conjugate multiplication, or cross correlation in the time domain, of the reflected and reference signals is then: (U* is the conjugate of U) 
     
         S.sub.corr (w)=U*.sub.ref (w)·U.sub.S1 (w)+U*.sub.ref (w)·U.sub.S2 (w) 
    
     or, if the frequency is restricted to a pass band Bpass(w 1 ,w h ): 
     
         S.sub.corr (w)=Bpass(w.sub.1,w.sub.h)e.sup.j(-w2D1 slag.sup./c) + 
    
     
         Bpass(w.sub.1,w.sub.h)e.sup.j(-w(2D1 slag.sup./c+2D1 bad.sup.*n slag.sup.7c)) 
    
     The inverse Fourier-transform will transform from the frequency to the time-plane (delay or distance-plane). Bpass(w 1 ,w h ) can be approximated with Rect(w 1 ,w h ): 
     
         F.sup.-1 S.sub.corr (Dt)=sinc(Dt-2D1.sub.slag /c).sup.+ sinc(Dt-2D1.sub.slag /c-2D1.sub.bad* n.sub.slag /c) 
    
     The time-delay response of the system is usually called the Point Spread Function in optics, and is in this case the Fourier transform of the frequency pass band. This response is measured by studying the response of a metal reflector at a known distance. The distances to the surfaces are then reconstructed from the observed signal by deconvolving with the measured Point Spread Function. The distance can then be referred to a specified reference level, D1 ref , through a translation of the time co-ordinate: Dt=Dt-2D1 ref  /c. The reference level may be a previously measured metal reflector in the signal path, or the edge of the metal container. The transform contains the structure in the depth-direction. If the data are also sampled in the aperture plane by using an interferometer as transmitter and receiver antennas then a further two dimensional transform over the aperture-plane will show the structure over the remaining two dimensions. In the case of an interferometer as antenna the measurement will also have an aperture plane term for each measured point (u,v) in this plane: 
     
         F.sup.-1 S.sub.corr (Dt)=(sinc(Dt-2D1.sub.slag /c)+sinc(Dt-2D1.sub.slag /c-2D1.sub.bad* n.sub.slag /c))e.sup.j2P(Q x.sup.u+Q y.sup.v) 
    
     Here Q x , Q y  is the position in the image plane. u,v is the position in the Fourier, aperture, plane of the interferometer elements, which in this case consists of individual radio hornantennas the signals of which are cross correlated against each other as well as complexly multiplied with the conjugate of the reference signal. The transmitting interferometer will create a plane wave front parallel to the surfaces. The receiving interferometer will detect the changes of phase over the wavefront and thus measure positions of the surfaces as above, but in three dimensions over an area of the surfaces. 
     The above described technique and apparatus was used in a test experiment where a metal bath was iron. On top of the metal bath was a melted slag of known composition from a metallurgic plant. FIG. 4 shows detections of the slag surface as well as the metal bath surface. The levels refer to an arbitrarily chosen reference level (the floor). In this way, the thickness of the slag can be found in secondary metallurgy (ladle metallurgy) with great accuracy (1-2 mm). 
     FIG. 3 shows the point spread function (PSF) of the frequency band for the best experiment. 
     An actual working example of the invention has been implemented. Referring to FIG. 1, the following components have been utilized: 
     
         ______________________________________                                    
An actual working example of the invention has been                       
implemented. Referring to FIG. 1, the following components                
have been utilized:                                                       
______________________________________                                    
3   feed horn       Philips circular horn for FEBO                        
                    antenna                                               
    polarisation filter                                                   
                    Swedish Microwave SMW D-12                            
    ortho coupler   Swedish Microwave SMW OMT                             
                    11A                                                   
    converter waveguide to                                                
                    Quasar Technology for 10-15 GHz                       
    coaxial cable   band                                                  
2   power divider   Wiltron K240C                                         
1   signal generator                                                      
                    Wiltron System Signal Source                          
                    type 360SS69                                          
4   receiver        Wiltron Active Device Test Set                        
                    type 3621A                                            
5   data acquisition                                                      
                    Wiltron Vector Network Analyser                       
                    type 360B                                             
Addresses for these companies are as follows:                             
Quasar Microwave Technology Ltd                                           
Battle Road, Heathfield, Newton Abbot                                     
Devon TQ12 6XU                                                            
United Kingdom                                                            
Wiltron Company                                                           
490 Jarvis Drive                                                          
Morgan Hill, CA 95037-2809                                                
USA                                                                       
Swedish Microwave AB                                                      
P.O. Box 230                                                              
S-591 23 Motala                                                           
SWEDEN                                                                    
______________________________________                                    
 
    
     As concerns the inter-unit connections, the two output signals from the power divider 2 are connected to one port of the antenna 3 transmitting a signal of left hand circular polarisation with a heliax cable (Andrew FSJ4-50B) and the receiver 4 with a steel coaxial cable. The other radio frequency port of the receiver 4 is connected to the other port of the antenna 3 via a heliax coaxial cable (Andrew FSJ4-50B), receiving the signal in the opposite, right hand circular polarisation. The data acquisition system 5 is connected to the signal generator 1 and the power divider 2 via GPIB bus cables. 
     The data acquisition system 5, i.e., the Vector Network Analyser, was set to step in 501 frequency steps between 10 and 15 Ghz and the complex division of the received signal and the reference signal from the transmitter were stored for each frequency channel. A total integration time of 1 sec. was used for each data sampling. 
     The complex data was stored on computer discs and each spectrum was then analyzed off line with a special computer program. The computer program used is referred to internally as &#34;SLAGG&#34; and is illustrated in the form of the flow diagram shown in FIG. 5. 
     The actual computer program &#34;SLAGG&#34; is set forth immediately below. &#34;SLAGG&#34; and another computer program referred to internally as &#34;MOLTOM5&#34; make subroutine calls to four commercially available software packages. [The computer program &#34;MOLTOM5&#34; is more particularly utilized in conjunction with the invention disclosed in published International Application No. PCT/SE94/00100 (which corresponds to Swedish Patent Application No. 9300347-3).] These four commercially available software packages are: 
     PGPLOT from Dr. Tim Pearson, California Institute of Technology, Pasadena, Calif.; 
     LMSTR1 and GETERR from the Astronomical Image Processing System (AIPS) of National Radio Astronomy Observatory (NRAO), Socorro, N.M.; and 
     SVDFIT from Numerical Recipes, Press, Flannery, Teukolsky and Vetterling, Cambridge University Press, isbn 0 521 30811 9. 
     Each of the four commercial software packages referred to immediately above is hereby expressly incorporated by reference herein. 
     The &#34;SLAGG&#34; computer program used in the above-described working example is as follows: ##SPC1## 
     FIG. 5 illustrates in the form of a flow diagram the sequence of steps carried out by the above computer program &#34;SLAGG&#34;. 
     The above-described working example was conducted employing microwave frequencies in the 10 to 14 GHz range. However, it is believed that other frequency ranges can be effectively employed. 
     The published PCT application to which the present U.S. application corresponds, namely PCT/SE94/00099, the published Swedish application from which it claims priority SE 9300348-1, as well as all documents cited in the International Search Report issued thereon, including U.S. Pat. No. 4,737,791 issued to Jean et al. on Apr. 28, 1988, U.S. Pat. No. 5,115,242 issued to Nagamune et al. on May 19, 1992, German Auslegeschrift 28 12 871 (and its U.S. counterpart, U.S. Pat. No. 4,210,023, issued on Jul. 1, 1980) and European Patent 0060597 (and its U.S. counterpart, U.S. Pat. No. 4,458,530, issued on Jul. 10, 1984), are hereby expressly incorporated by reference as if set forth in their entirety herein. Additionally, published PCT application PCT/SE94/00099 is directed to similar subject matter disclosed in published PCT application PCT/SE94/00100, which PCT application, the published Swedish application from which it claims Priority, SE 9300347-3, together with all documents cited in the International Search Report issued thereon, namely U.S. Pat. No. 5,124,653 issued to Andresen et al., U.S. Pat. No. 4,693,614 issued to Hatono et al., U.S. Pat. No. 4,438,069 issued to Peterman et al. and U.K. Patent No. 2 030 414 (and its U.S. counterpart, U.S. Pat. No. 4,275,787, issued on Jun. 30, 1981), are also hereby expressly incorporated by reference into the present application. 
     Examples of phase (and phase difference detectors operating in the microwave frequency range are disclosed in U.S. Pat. No. 5,266,954 entitled &#34;Microwave Vehicle Detector&#34;, issued on Nov. 30, 1993 to Orlowski et al., and U.S. Pat. No. 5,180,985 entitled &#34;Multipactor Effect Measurement Device Using Phase Noise&#34;, issued on Jan. 19, 1993 to Zoccarato et al., both of these issued hereby expressly incorporated by reference herein. 
     One feature of the invention resides broadly in the method for simultaneously measuring the positions of more than one surfaces in metallurgic processes, characterized in that it comprises transmitting a radio signal over a frequency band, receiving the signals reflected from the surfaces, measuring the phase difference between the transmitted and reflected signals over the frequency band and making a transform from frequency domain to time domain resulting in the said positions. 
     Another feature of the invention resides broadly in the method characterized in that it comprises stepping a signal generator in discrete frequency steps over the frequency band and receiving the reflected signals for each frequency step, comparing the phase difference of the transmitted and reflected signals in each step over the frequency band and making a discrete transform from frequency domain to time domain resulting in the said positions. 
     Yet another feature of the invention resides broadly in the method characterized in that it comprises transmitting a radio signal in circular polarization from an antenna mounted so that the transmitted wave is aimed perpendicular to the surfaces and the reflected wave is received in the same antenna but in the opposite circular polarization. 
     Still another feature of the invention resides broadly in the method characterized in that the phases of the signals are compared by doing a complex conjugate multiplication in the frequency plane, i.e. a cross correlation in time domain, for each discrete frequency channel. 
     A further feature of the invention resides broadly in the method characterized in that the antenna is formed by an interferometer and the three dimensional structure of the surfaces are imaged by a three dimensional transform from frequency and aperture plane to time and image plane. 
     The components disclosed in the various publications, disclosed or incorporated by reference herein, may be used in the embodiments of the present invention, as well as, equivalents thereof. 
     The appended drawings in their entirety, including all dimensions, proportions and/or shapes in at least one embodiment of the invention, are accurate and to scale and are hereby included by reference into this specification. 
     All, or substantially all, of the components and methods of the various embodiments may be used with at least one embodiment or all of the embodiments, if more than one embodiment is described herein. 
     All of the patents, patent applications and publications recited herein, and in the Declaration attached hereto, are hereby incorporated by reference as if set forth in their entirety herein. 
     The corresponding foreign and international patent publication applications, namely, Swedish Patent Application No. 9300348-1, filed on Feb. 3, 1993, and PCT/SE94/00099, filed on Feb. 3, 1994, having inventor Lars Bååth, and SE-OS 9300348-1 and SE-PS 9300348-1 and International Application No. PCT/SE94/00099, and International Publication No. WO94/18549, as well as their published equivalents, and other equivalents or corresponding applications, if any, in corresponding cases in Sweden and elsewhere, and the references cited in any of the documents cited herein, are hereby incorporated by reference as if set forth in their entirety herein. 
     The details in the patents, patent applications and publications may be considered to be incorporable, at applicant&#39;s option, into the claims during prosecution as further limitations in the claims to patentably distinguish any amended claims from any applied prior art. 
     The invention as described hereinabove in the context of the preferred embodiments is not to be taken as limited to all of the provided details thereof, since modifications and variations thereof may be made without departing from the spirit and scope of the invention.