Abstract:
An in-situ on-line detection device and detection method for a long-distance metallurgical liquid metal component. The detection device comprises a front-end high-temperature resistant probe, a middle-end optical sensing device and a back-end control platform, wherein the head of the front-end high-temperature resistant probe is placed in a liquid metal, the tail thereof is coaxially connected to the middle-end optical sensing device, and an optical window is arranged in the connection position; and the middle-end optical sensing device is connected to the back-end control platform through a signal line. The detection device and detection method can provide a timely and valid message for quality control and a melting end, so that the detection time is greatly shortened, the detection distance can he adjusted extensively, the measurement result is accurate, and it can be achieved to measure components that are difficult to measure such as carbon, sulfur, phosphorous, etc.

Description:
TECHNICAL FIELD 
     The present invention relates to the technical field of long-distance online monitoring for high-temperature liquid component, in particular to an in-situ online detection device and method for long-distance metallurgical liquid metal component. 
     BACKGROUND 
     In the melting processes of metal and alloy, the change of chemical component is required to be detected, so as to control the product quality and judge a melting end. At present, due to the lack of an advanced and effective online measurement technology, the offline detection manner of manual sampling and sample preparation is generally adopted in the melting processes. For example, during the steel melting process, in the detection of high-temperature molten steel, the molten steel is required to be taken onto an analytical instrument for measurement and analysis after a series of processes of sampling, cooling, grinding, polishing, etc., and it needs to take 3 to 5 minutes in the entire process that will occupy more than one tenth of melting time. This time-consuming offline detection manner not only causes backward quality control, but also causes the waste of large amount of resources and energy resources. Moreover, detection equipment is expensive, bulky and difficult to adapt to modern melting production requirements. 
     A Laser Induced Breakdown Spectroscopy (LIBS) is a technology of using laser for exciting plasma and then using the emission spectra of the plasma for performing element detection and analysis. It does not need a complicated sample pretreatment process, has not strict requirements for the appearance and the size of samples, has low consumption quantity of the samples, is suitable for solid, liquid and gas simultaneously and can be used for performing simultaneous determination on multiple elements. Therefore, this technology presents an excellent application value in the aspects of in-situ, online, quick and remote analysis. 
     With the increasing enlargement, high speed and continuity of the production mode of metallurgical industry, there is an increasingly urgent demand for an in-situ online detection technology for liquid metal component, and an online detection device based on the Laser Induced Breakdown Spectroscopy (LIBS) begins to appear. 
     The literature (Reinhard Noll, Laser-Induced Breakdown Spectroscopy: Fundamentals and Applications, Springer 2012) presents the research status of detection equipment by using LIBS currently. Analytical equipment applied to metal component mainly includes short-distance laboratory equipment. The short-distance laboratory equipment is accurate in measurement and short in time required (tens of seconds), but has enormous volume and higher requirement for the environment, so it cannot better adapt to the online monitoring of complex environments, such as steel factory, etc. 
     The literature (Lin Xiaomei, Cao Jiqing, Yin Qinghui, and Liu Xiaoqing, Online Analysis of Silicone Content in AOD Furnace Based on LIBS, Magnesium Alloy, 2009 (1): 41-44) presents a device for detecting the content of Si element in an AOD furnace by using LIBS. The device uses a self-design sampler for sampling in the furnace. After that, component detection is performed in a sample pool of the device. The device has shorter detection distance and relatively complicated structural design, can only measure a single element and cannot simultaneously measure multiple elements. 
     The literature (G Mathy, B. Monfort, B. Vanderheyden, V. Tusset, Liquid steel process: advanced on line sensors under development at CRM. Metall. Anal. 30 (Suppl.1), 6-14 (2010)) presents a TeleLis system for realizing long-distance LIBS detection by using a Newtonian telescope system. The TeleLis system can realize the analysis of element component on a required measuring sample within the range of 3 to 12 meters. However, because a front optical path thereof is an open structure, an ambient environment and air will influence spectrum accuracy. Moreover, the character spectral lines of elements such as C, S, P, etc. are in an ultraviolet region. Air component in the open optical path absorbs these elements strongly, so these character elements cannot be measured. 
     The literature (Robert De Saro, Arel Weisberg, and Joe Craparo, In Situ, Real Time Measurement of Aluminum, Steel, and Glass Melt Chemistries Using Laser Induced Breakdown Spectroscopy, 2005 ACEEE Summer Study on Energy Efficiency in Industry 2005) reports a device for online measurement of elements such as Al, Cu, Fe, Mn, etc. in an aluminum alloy solution in industrial situ. A probe is inserted into a furnace body for directly measuring a metal solution surface. However, an optical structure is contained inside the probe of the device, and needs to be protected. The probe has a complicated structure. 
     The literature (Mohamed A. Khater, Laser-induced breakdown spectroscopy for light elements detection in steel: State of the art, Spectrochimica Acta Part B 81 (2013) 1-10) presents the monitoring situation of light elements by using LIBS in metal melting. Because the character spectra of the light elements mainly focus on an ultraviolet region and a deep ultraviolet region, the current major method is to generate an inert gas environment or a vacuum environment for performing measurement, while special gas environments of optical paths are relatively shorter and cannot better adapt to the severe environment of a factory. 
     In summary, there is currently no complete set of online monitoring equipment for simultaneous measurement of multiple elements (nonmetal elements such as C, S, P, etc.) based on LIBS capable of well adapting to the severe environment of a steel melting factory and having simple structure, strong applicability and lower manufacturing cost and maintenance cost. The existing equipment has different limits in the aspects of structural complexity, environmental need, measurable need, etc., and has relatively narrow application scope. 
     SUMMARY OF THE PRESENT INVENTION 
     With respect to the defects in the prior art, the present invention provides an in-situ online detection device and method for metallurgical liquid metal component to provide timely and valid information for the quality control and a melting end, so that the detection time is greatly shortened, the detection distance can be adjusted extensively, the detection device can be flexibly installed and used, the product quality can be greatly improved, the production cost is decreased and the energy consumption is reduced; moreover, a double-pulse method is applied, thereby effectively increasing the spectral quality, suppressing the noise and enhancing the accuracy and the stability of a measurement result; and the measurement on components, such as C, S, P, etc. that are difficult to measure can be achieved. 
     To realize the above-mentioned purposes, the present invention discloses the technical solution: 
     An in-situ online detection device for a long-distance metallurgical liquid metal component, wherein: 
     the head of a front-end high-temperature resistant probe ( 18 ) is placed in liquid metal ( 22 ), the tail thereof is coaxially connected to a middle-end optical sensing device ( 19 ), and an optical window ( 15 ) is arranged in the connection position; and the middle-end optical sensing device ( 19 ) is connected to a back-end control platform ( 24 ) through a signal line ( 25 ). 
     Preferably, in said middle-end optical sensing device ( 19 ), a remote beam expanding and focusing module ( 7 ) is arranged coaxially in the axis direction in which a laser generating module ( 6 ) emits laser beams. 
     A first reflecting mirror ( 9 ) is arranged on the optical axis of the emergent direction of the remote beam expanding and focusing module ( 7 ) and forms an angle of 45° with the optical axis thereof; a second reflecting mirror ( 10 ) is arranged on the optical axis of the incident direction of a remote signal collecting module ( 11 ) and forms an angle of 45° with the optical axis thereof; and a connecting line of the centers of the first reflecting mirror ( 9 ) and the second reflecting mirror ( 10 ) is perpendicular to the axes of the remote beam expanding and focusing module ( 7 ) and the remote signal collecting module ( 11 ). 
     A light splitting piece ( 20 ) and an optical fiber coupling module ( 13 ) are respectively arranged on the optical axis of the emergent direction of the remote signal collecting module ( 11 ); the optical fiber coupling module ( 13 ) is connected with an optical fiber spectrometer ( 14 ) through an optical fiber ( 21 ); and the fiber spectrometer ( 14 ) is connected with a back-end control platform ( 24 ) through a signal line ( 25 ); and a real-time imaging module ( 12 ) is vertically arranged on the optical axis of the reflecting direction of the light splitting piece ( 20 ), and is connected with the back-end control platform ( 24 ) through the signal line ( 25 ). 
     Preferably, said front-end high-temperature resistant probe ( 18 ) is an inverted cone in which inert gas is charged so that the optical paths of excitation laser beams and collected plasma signal light are in a closed inert gas environment. 
     Preferably, said back-end control platform ( 24 ) comprises a laser power supply and control unit ( 1 ), a time synchronization controller ( 2 ), an air inlet control unit ( 3 ) and a displacement platform control unit ( 4 ), wherein the above-mentioned units are connected with the middle-end optical sensing device ( 19 ) through the signal line ( 25 ). 
     Preferably, the in-situ online detection device for the long-distance metallurgical liquid metal component further comprises a laser distance measuring module ( 8 ) which is arranged in the direction of the connecting line of the centers of the first reflecting mirror ( 9 ) and the second reflecting mirror ( 10 ) and the laser distance measuring module ( 8 ) is connected with the back-end control platform ( 24 ) through the signal line ( 25 ). 
     Preferably, said middle-end optical sensing device ( 19 ) further comprises a temperature control module ( 5 ) and is connected with the back-end control platform ( 24 ) through the signal line ( 25 ); and said temperature control module ( 5 ) comprises a temperature sensor and a temperature adjusting device for real-time monitoring and adjustment of the internal temperature of the middle-end optical sensing device ( 19 ). 
     Preferably, said laser generating module ( 6 ) comprises two groups of laser heads, half-wave plates and polarizing light splitting pieces; 
     said half-wave plates are coaxially arranged on the optical axes of the laser emergent directions of the laser heads and are perpendicular to the optical axes; and 
     said polarizing light splitting pieces are coaxially arranged on the optical axes of the emergent directions of the half-wave plates and form Brewster&#39;s angles with the optical axes, and the second polarizing light splitting piece ( 31 ) can receive the laser beams reflected by the first polarizing light splitting piece ( 30 ). 
     Preferably, said remote beam expanding and focusing module ( 7 ) comprises a first movable base ( 32 ) and a diverging lens and a converging lens which are coaxially arranged in the emergent direction of the laser generating module ( 6 ); 
     said first movable base ( 32 ) can move on the optical axis of the emergent direction of the laser generating module ( 6 ); 
     said diverging lens is arranged in the first movable base ( 32 ) perpendicularly to the optical axis; and 
     said converging lens is arranged between the first movable base ( 32 ) and the first reflecting mirror ( 9 ) perpendicularly to the optical axis. 
     Preferably, said remote signal collecting module ( 11 ) consists of a second movable base ( 37 ), a hyperboloidal reflecting mirror ( 38 ) and a paraboloidal reflecting mirror ( 39 ) and is connected with the back-end control platform ( 24 ) through the signal line ( 25 ); 
     said second movable base ( 32 ) can move on the optical axis of the emergent direction of the second reflecting mirror ( 10 ); 
     said hyperboloidal reflecting mirror ( 38 ) is parallel to and coaxial with the emergent direction of the second reflecting mirror ( 10 ), and is arranged in the second movable base ( 37 ); and 
     said paraboloidal reflecting mirror ( 39 ) is arranged between the hyperboloidal reflecting mirror ( 38 ) and the light splitting piece, and a central hole is reserved in the center. 
     Preferably, said remote signal collecting module ( 11 ) consists of a second movable base ( 37 ), a spherical reflecting mirror and an aspherical reflecting mirror and is connected with the back-end control platform ( 24 ) through the signal line ( 25 ); 
     said second movable base ( 37 ) can move on the optical axis of the emergent direction of the second reflecting mirror ( 10 ); 
     said spherical reflecting mirror is parallel to and coaxial with the emergent direction of the second reflecting mirror ( 10 ), and is arranged in the second movable base ( 37 ); and 
     said aspherical reflecting mirror is arranged between the spherical reflecting mirror and the light splitting piece, and a central hole is reserved in the center. 
     Preferably, said optical fiber coupling module ( 13 ) comprises a third diverging lens ( 40 ), a third converging lens ( 41 ) and a fourth converging lens ( 42 ) which are coaxially placed in parallel in sequence, and the optical axis thereof coincides with the optical axis of the remote signal collecting module. 
     Preferably, said real-time imaging module ( 12 ) comprises an achromatic lens ( 43 ) and a CCD ( 44 ); and the achromatic lens ( 43 ) and the CCD ( 44 ) are coaxially placed; the axis direction thereof is perpendicular to the direction of the optical axis of the remote signal collecting module ( 11 ); and the axis passes through the center of the light splitting piece ( 20 ). 
     An in-situ online detection method for a long-distance metallurgical liquid metal component comprises the following steps: 
     placing the head of the front-end high-temperature resistant probe ( 18 ) in liquid metal ( 22 ); and charging inert gas through the air inlet pipe ( 16 ) of the high-temperature resistant probe to form a closed inert gas environment; 
     emitting distance measuring laser by the laser distance measuring module ( 8 ); after the distance measuring laser passes through the first reflecting mirror ( 9 ) and the second reflecting mirror ( 10 ), irradiating the distance measuring laser on a liquid metal surface ( 23 ); measuring the position information of the liquid metal surface ( 23 ); and feeding the same back to the back-end control platform ( 24 ); 
     performing focusing adjustment on the first movable base ( 32 ) and the second movable base ( 37 ) by the back-end control platform ( 24 ) through the position information of the liquid metal surface ( 23 ) fed back by the laser distance measuring module ( 8 ) so that the focal position, on which the laser focuses, for exciting the plasma and the collection position of remote signals coincide on the liquid metal surface ( 23 ); 
     receiving a laser generating signal of the back-end control platform ( 24 ) by the laser generating module ( 6 ); emitting laser beams; after the laser beams pass through the remote beam expanding and focusing module ( 7 ), irradiating the laser beams on the first reflecting mirror ( 9 ); enabling the reflected laser beams to pass through the second reflecting mirror ( 10 ); and irradiating the laser beams to the liquid metal surface ( 23 ) through the inert gas environment of the front-end high-temperature resistant probe ( 18 ) to produce plasma signal light; 
     transmitting the generated plasma signal light to the remote signal collecting module ( 11 ) through the inert gas environment of the front-end high-temperature resistant probe ( 18 ); forming parallel plasma signal light through the paraboloidal reflecting mirror ( 39 ) and the hyperboloidal reflecting mirror ( 38 ) in the remote signal collecting module ( 11 ); and transmitting the same to the optical fiber coupling module ( 13 ) through the light splitting piece ( 20 ); and 
     after the parallel plasma signal light passes through the optical fiber coupling module ( 13 ), transmitting the parallel plasma signal light to the optical fiber spectrometer ( 14 ) through the optical fiber ( 21 ) for performing light signal collection and photoelectric conversion; and feeding the same back to the back-end control platform ( 24 ) to finish the collection of the plasma signal light. 
     Preferably, the in-situ online detection method for the long-distance metallurgical liquid metal component according to claim  13  further comprises the real-time imaging module ( 12 ) which receives a part of signal light and scene background light reflected by the light splitting piece ( 20 ), feeds the same back to the back-end control platform ( 24 ), displays the appearance of the liquid metal surface ( 23 ) and the formation condition of the plasma in real time, and adjusts the focusing position and the collection position in real time. 
     The present invention has the following beneficial effects and advantages: 
     Sampling and sample preparation are not required, and high-temperature melt metal component is subject to in-situ online detection, thereby greatly shortening the detection time, providing real-time information for the metallurgical production process, greatly improving the product quality, reducing the production cost and reducing the energy consumption. 
     A plug-in high-temperature resistant probe is adopted, thereby avoiding the influence of impurities of cinder on a melt surface, etc. on measurement, enabling the measurement result to actually reflect the state of a measured substance and ensuring an accurate result. 
     The high-temperature resistant probe has simple structure and does not have other optical structures internally; the design and manufacturing cost is relatively low; and different lengths of high-temperature resistant probes can be replaced according to different needs. 
     The measurement on components, such as C, S, P, etc. that are difficult to measure can be achieved through the transmission of an optical path in an inert gas environment. 
     The measurement distance can be changed according to the length of the selected high-temperature resistant probe; detection at a distance of 1.5 to 10 meters or above can be realized; and the prevent invention can be flexibly applied to different environmental conditions. 
     Through the beam combination manner of double pulses, the plasma excitation of double-pulse laser on the samples is realized, thereby enhancing the intensity of plasma signals and reducing the noise; and relative to a single-pulse excitation manner, the detection precision and stability can be greatly enhanced. 
     The present invention has wide scope of application, and not only can be applied to the measurement of metallurgical melt metal, but also can be suitable for the measurement of other ordinary liquid as well as the measurement of gas and solid substances. 
     If the high-temperature resistant probe is disassembled, the present invention can be used as a long-distance element online analytical instrument, and can realize long-distance detection of 1.5 to 10 meters or above. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall structural diagram of the present invention; 
         FIG. 2  is a structural diagram of a laser generating module in the present invention; 
         FIG. 3  is a structural diagram of a remote beam expanding and focusing module and a remote signal collecting module in the present invention; and 
         FIG. 4  is a structural diagram of a real-time imaging module and an optical fiber coupling module in the present invention. 
     
    
    
     Wherein  1  is a laser power supply and control unit;  2  is a time synchronization controller;  3  is an air inlet control unit;  4  is a displacement platform control unit;  5  is a temperature control unit;  6  is a laser generating module;  7  is a remote beam expanding and focusing module;  8  is a laser distance measuring module;  9  is a first reflecting mirror;  10  is a second reflecting mirror;  11  is a remote signal collecting module;  12  is a real-time imaging module;  13  is an optical fiber coupling module;  14  is an optical fiber spectrometer;  15  is an optical window;  16  is an air inlet pipe of a high-temperature resistant probe;  17  is an air outlet pipe of a high-temperature resistant probe;  18  is a high-temperature resistant probe;  19  is a middle-end optical sensing device;  20  is a light splitting piece;  21  is an optical fiber;  22  is liquid metal;  23  is a liquid metal surface;  24  is a back-end control platform;  25  is a signal line;  26  is a first laser head;  27  is a second laser head;  28  is a first half-wave plate;  29  is a second half-wave plate;  30  is a first polarizing light splitting piece;  31  is a second polarizing light splitting piece;  32  is a first movable base;  33  is a first diverging lens;  34  is a second diverging lens;  35  is a first converging lens;  36  is a second converging lens;  37  is a second movable base;  38  is a hyperboloidal reflecting mirror;  39  is a paraboloidal reflecting mirror;  40  is a third diverging lens;  41  is a third converging lens;  42  is a fourth converging lens;  43  is an achromatic lens; and  44  is a CCD. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present patent will be further described below in combination with the drawings and the embodiments. 
       FIG. 1  shows an overall structural diagram of the present invention which comprises a front-end high-temperature resistant optical probe  18 , a middle-end optical sensing device  19  and a back-end control platform  24 , wherein: 
     The head of the front-end high-temperature resistant probe  18  is placed in metallurgical liquid metal  22 ; a sealed cavity body is formed internally; and inert gas is charged through the air pipe  16  to form an optical path environment of the inert gas; the middle-end optical sensing device  19  receives an operating signal of the back-end control platform  24 ; firstly, a distance from the metallurgical liquid metal surface  23  is measured through a laser distance measuring part  8 ; the first movable base  32  and the second movable base  37  in the remote beam expanding and focusing module  7  and the remote signal collecting module  11  are adjusted according to a feedback signal so that detection laser focuses on the liquid metal surface  23 ; signal collecting points are in the same position; the laser receives a triggering signal of the back-end control platform for generating the detection laser, passes through the remote beam expanding and focusing module  7  and the reflecting mirrors, realizes coaxiality with the remote signal collecting module  11 , and focuses on the liquid metal surface  23  to generate the plasma; the appearance of the position of a focus point and the condition of the generated plasma are observed in real time through the real-time imaging module  12 ; and after passing through the remote signal collecting module  11 , the generated plasma signal light passes through the optical fiber coupling module  13  and the optical fiber  21  and enters the optical fiber spectrometer  14  to finish the collection of plasma spectral signals. 
     The head of the front-end high-temperature resistant probe  18  is placed in the metallurgical liquid metal  22 ; a sealed space is formed internally; inert gas is charged to realize an optical path environment of the inert gas; and no optical element exists in the front-end high-temperature resistant probe  18  so that it is easy to replace. The front-end high-temperature resistant probe comprises an external fireproof material part and an air charging part; the head thereof has an open structure and the tail thereof is connected with the middle-end optical sensing device; when the front-end high-temperature resistant probe is inserted into the metallurgical liquid metal, a sealed cavity body is formed therein; and the inert gas is charged therein through the air charging part to form an inert gas space so that optical paths of the excitation laser and the collected plasma light are in the inert gas environment. 
     The middle-end optical sensing device  19  realizes the functions of long-distance plasma excitation and signal light collection through a telescope structure, and achieves that the excitation optical path and the collection optical path are coaxial; the detection distance can be adjusted according to needs; automatic focusing can be performed; a remote laser-induced breakdown spectral sensing device with the function of real-time observation of the surface morphology of the detected substance comprises a laser generating module  6 , a remote beam expanding and focusing module  7 , a remote signal collecting module  11 , a coaxial adjusting part (a first reflecting mirror  9  and a second reflecting mirror  10 ), a laser distance measuring module  8 , a real-time imaging module  12  and a spectrum collecting part; and the space of the excitation and the collection optical paths is the inert gas environment formed by the front-end high-temperature resistant probe. 
     The laser generating module  6  in the middle-end optical sensing device generates a single laser beam or double coaxial laser beams by receiving a laser generating signal of the back-end control platform; the emergent direction of the light beam is the axis direction of the remote beam expanding and focusing module  7 ; and after passing through the remote beam expanding and focusing module  7 , the laser is incident into a coaxial adjusting part and passes through the coaxial adjusting part to achieve that the optical axis of an excitation optical path and the optical axis of the remote signal collecting module  11  are coaxial so that the laser focuses on the measured liquid metal surface  23  through the optical path environment of the inert gas of the front-end high-temperature resistant probe  18  to form the plasma. The laser emitted by the laser distance measuring module  8  and the excitation laser which is emergent from the remote beam expanding and focusing module  7  are incident into the coaxial adjusting part in a direction of 90°, pass through the coaxial adjusting part, achieve coaxiality with the optical path of the excitation laser and the remote signal collecting module  11 , and are emitted to the liquid metal surface  23 . The generated plasma signals enter the remote signal collecting module  11  through the optical path environment of the inert gas of the front-end high-temperature resistant probe  18 . After passing through the remote signal collecting module  11 , the plasma signals pass through the light splitting piece  20 . A part of plasma signal light is incident into the real-time imaging module  12  to realize the real-time monitoring of the surface appearance of a detection point, and the rest part of light is incident into the spectrum collecting part to realize the collection of the plasma signal light. The light splitting piece  20  forms an angle of 45° with the direction of the optical axis of the remote signal collecting module  11 . The overall middle-end optical sensing device  19  is a constant-temperature sealed box body, and has an optical window  15  in the coaxial optical path direction. 
     A temperature control module  5  is connected with the back-end control platform  24  through a signal line  25 , and consists of a temperature sensor and a temperature adjusting device for real-time monitoring and adjustment of the internal temperature of the middle-end optical sensing device  19  so that the internal temperature is kept within a needed temperature range. 
     The real-time imaging module  12  receives a part of signal light and scene background light reflected by the light splitting piece  20 , feeds the same back to the back-end control platform  24 , displays the appearance of the liquid metal surface  23  and the formation condition of the plasma in real time, and adjusts the focusing and collection positions according to the practical situation in real time in more detail. 
     The spectrum collecting part comprises an optical fiber coupling module  13 , an optical fiber  21  and a spectrometer  14 . The optical fiber coupling module  13  is placed on the optical axis of the plasma signal light that has passed through the light splitting piece  20 , and the optical fiber  21  is connected with the optical fiber coupling module  13  and the optical fiber spectrometer  14 . The signal light that has passed through the light splitting piece  20  is incident on the optical axis of the optical fiber coupling module  13  and passes through the optical fiber coupling module  13  so that the signal light is coupled into the optical fiber  21 , followed by entry into the spectrometer  14  to realize the collection of the signal light. 
     The spectrum collecting part can also directly consist of an optical coupling system and a grating spectrometer; the optical coupling system is placed on the optical axis of the plasma signal light that has passed through the light splitting piece; and after passing through the optical coupling system, the signal light enters the grating spectrometer to realize the collection of the signal light. 
       FIG. 2  shows a structural diagram of a laser generating module  6  in the present invention. The laser generating module  6  comprises a laser generating and beam combining part that comprises a first laser head  26 , a second laser head  27 , a first half-wave plate  28 , a second half-wave plate  29 , a first polarizing light splitting piece  30  and a second polarizing light splitting piece  31 , wherein the first half-wave plate  28  and the first polarizing light splitting piece  30  are placed in sequence in the incident direction of the laser emitted by the first laser head  26 , and respectively form a vertical angle and a Brewster&#39;s angle with the direction of the light ray of the laser; the second half-wave plate  29  and the second polarizing light splitting piece  31  are placed in the incident direction of the laser emitted by the second laser head  27 , and respectively form a vertical angle and a Brewster&#39;s angle with the direction of the light ray of the laser; the pulse laser emitted by the first laser head  26  is reflected to the second polarizing light splitting piece  31  by the first polarizing light splitting piece  30 , and is emitted to the direction of the optical axis of the remote focusing part after the pulse laser passes through the second polarizing light splitting piece  31 ; the pulse laser emitted by the second laser head  27  is emitted to the direction of the optical axis of the remote focusing part through the second half-wave plate  29  and the second polarizing light splitting piece  31 ; and the beam combination function is realized. 
     Through the double-pulse beam combination manner, the plasma excitation of double-pulse laser on the samples is realized, thereby enhancing the intensity of plasma signals and reducing the noise; and relative to a single-pulse excitation manner, the detection precision and stability can be greatly enhanced. 
     If single-beam laser generation is required to be realized, the first laser head  26 , the first half-wave plate  28 , the second half-wave plate  29 , the first polarizing light splitting piece  30  and the second polarizing light splitting piece  31  can be removed; only the second laser head  27  is reserved; and the emergent direction of the laser is the direction of the optical axis of the remote beam expanding and focusing module  7 . 
     The beam combination function of the first polarizing light splitting piece  30  and the second polarizing light splitting piece  31  can also be realized through the combination of the reflecting mirrors and a polarizing beam splitting prism. 
       FIG. 3  shows a structural diagram of a remote beam expanding and focusing module  7  and a remote signal collecting module  11  in the present invention. The remote beam expanding and focusing module  7  comprises a first diverging lens  33 , a second diverging lens  34 , a first converging lens  35 , a second converging lens  36  and a first movable base  32 . The first diverging lens, the second diverging lens  34  and the first converging lens  35  are coaxial with the second converging lens  36 , and the first diverging lens  33  and the second diverging lens  34  are placed on the first movable base  32 . The first movable base  32  can move in the direction of the optical axis of the remote beam expanding and focusing module. The coaxial adjusting part comprises a first reflecting mirror  9  and a second reflecting mirror  10 , wherein the first reflecting mirror  9  is placed on the optical axis of the remote beam expanding and focusing module  7  and forms 45° with the direction of the optical axis thereof; the second reflecting mirror  10  is placed on the optical axis of the remote signal collecting module  11  and forms 45° with the optical axis thereof; and a connecting line of the centers of the first reflecting mirror  9  and the second reflecting mirror  10  is perpendicular to the directions of the optical axes of the remote beam expanding and focusing module  7  and the remote signal collecting module  11 . After passing through the first reflecting mirror  9  and the second reflecting mirror  10 , the excitation laser realizes coaxiality with the remote collecting module. The first reflecting mirror  9  is a reflecting mirror having a hole in the center or a laser dichroic mirror. The direction of the distance measuring laser emitted by the laser distance measuring part  8  forms 90° with the direction of the optical axis of the remote beam expanding and focusing module  7 , and the distance measuring laser is incident on the center of the first reflecting mirror  9  of the coaxial adjusting part. The remote signal collecting module consists of a hyperboloidal reflecting mirror  38 , a paraboloidal reflecting mirror  39  and a second movable base  37 . The hyperboloidal reflecting mirror  38  is coaxial with the paraboloidal reflecting mirror  39 , namely that the axial direction thereof is the axial direction of the remote signal collecting module  11 ; and the hyperboloidal reflecting mirror  38  is placed on the second movable base  37 . The second movable base  37  can move in the direction of the optical axis, and the collection of optical signals in different focal positions is realized by adjusting the position of the second movable base  37  in the direction of the optical axis. The plasma signal light passes through the inert gas environment of the front-end high-temperature probe, and passes through the paraboloidal reflecting mirror  39  and the hyperboloidal reflecting mirror  38  to form parallel beams. 
     The first movable base  32  can move in the direction of the optical axis of the remote beam expanding and focusing module  7 ; and the continuous adjustment of the focus position of the pulse laser within the scope above 1.5 to 10 meters can be realized by adjusting the position of the first movable base  32  in the direction of the optical axis and performing focusing adjustment. 
     With respect to the composition of the lenses, 3 to 10 lenses can also exist to realize a remote beam expanding and focusing function. 
     The coaxial adjusting part comprises a first reflecting mirror  9  and a second reflecting mirror  10 , wherein the first reflecting mirror  9  is placed on the optical axis of the remote beam expanding and focusing module  7  and forms 45° with the direction of the optical axis thereof; the second reflecting mirror  10  is placed on the optical axis of the remote signal collecting module  11  and forms 45° with the optical axis thereof; and a connecting line of the centers of the first reflecting mirror  9  and the second reflecting mirror  10  is perpendicular to the axes of the remote beam expanding and focusing module  7  and the remote signal collecting module  11 . After passing through the first reflecting mirror  9  and the second reflecting mirror  10 , the excitation laser realizes coaxiality with the remote collecting module. 
     The first reflecting mirror  9  is a laser dichroic mirror or a reflecting mirror having an opening in the center. If there is no laser distance measuring system  8 , the reflecting mirror can be an ordinary optical reflecting mirror. 
     The direction of the distance measuring laser emitted by the laser distance measuring part  8  forms 90° with the direction of the optical axis of the remote beam expanding and focusing module  7 , and the distance measuring laser is incident on the center of the reflecting mirror  9  of the coaxial adjusting part along the connecting line of the centers of the first reflecting mirror  9  and the second reflecting mirror  10 . After passing through the second reflecting mirror  10 , the distance measuring laser realizes coaxiality with the optical path of the plasma excitation beam and the optical path of the remote signal collecting module  11  and is emitted to the liquid metal surface. Distance measuring signal light thereof returns to a laser distance measuring instrument according to an original optical path to realize a laser distance measuring function. 
     The second movable base  37  can move in the direction of the optical axis, and the collection of optical signals in different focal positions is realized by adjusting the position of the second movable base  37  in the direction of the optical axis and performing focusing adjustment. 
     The hyperboloidal reflecting mirror  38  and the paraboloidal reflecting mirror  39  can also consist of a group of spherical reflecting mirror and aspherical reflecting mirror. 
       FIG. 4  shows a structural diagram of a real-time imaging part  12  and an optical fiber coupling module  13  in the present invention. The optical fiber coupling module  13  consists of a third diverging lens  40 , a third converging lens  41  and a fourth converging lens  42  which are coaxially placed in parallel, and the optical axis of the optical fiber coupling module  13  coincides with the optical axis of the remote signal collecting module. 
     The real-time imaging module  12  comprises an achromatic lens  43  and a CCD  44 . The achromatic lens  43  and the CCD  44  are coaxially placed; the axis direction thereof is perpendicular to the direction of the optical axis of the remote signal collecting module  11 ; and the axis passes through the center of the light splitting piece  20 . 
     The front-end high-temperature resistant probe  18  and the middle-end optical sensing device are integrated into a whole and are installed on a hydraulic platform which can move in a perpendicular direction. 
     The back-end control platform as a platform of manual control, operation and display is placed in a control room, is connected with the middle-end optical sensing device through a signal line for controlling a laser signal of the middle-end optical sensing device, controls the movement of displacement platforms and the like in the remote beam expanding and focusing module  7  and the remote signal collecting module  11  through the information of the laser distance measuring part and the real-time imaging part, receives electrical signals, which can be transmitted in long distance, transmitted by the middle-end optical sensing device for processing, and controls the gas flow of the front-end high-temperature resistant probe  18 . 
     The back-end control platform of the present invention partially controls the laser distance measuring part for performing the distance measurement of the liquid metal surface, adjusts the detection distance and the collection distance according to measured information, performs focusing adjustment, controls the generation of the laser signal and the collection of detection signals, controls air charging flow, displays the surface morphology information of the detection point and the generation condition of the plasma in real time, simultaneously processes and analyzes the detection data and provides manual operation and display functions. A time synchronization controller controls the emission time delay of two laser pulse beams and the collection time of the detection signals of the spectrometer. The signal-to-noise ratio of the spectral signals can be increased by optimizing the time delay between the two laser beams and the signal collection time of the spectrometer. The detection signals of the spectrometer are transmitted into a computer; and the processing courses of denoising, background elimination, peak searching and the like are performed on the data in the computer to obtain the strength of the characteristic spectral line of a measured component. The concentration of the measured component can be computed through the strength of the characteristic spectral line and an established strength-concentration calibration curve. 
     Focusing adjustment has the following steps: 
     setting a distance from the laser distance measuring module  8  to the center of the second reflecting mirror  10  as 1, a constant value, setting a distance from the second reflecting mirror  10  to the measured liquid metal surface  23  as s to obtain the measurement value of the laser distance measuring module  8 : a=1+s; 
     enabling the first movable base  32  to be within the movement scope itself; enabling a movement position value x1 and a position value s1 for focusing the laser to form a monotone function that corresponds to a unique determined value; and setting a corresponding relationship therebetween to be x1=f1(s1), while representing s1 as an addition form of s and a constant offset 11, i.e., s1=s+11 to obtain x1=f1 (s+11); 
     enabling the second movable base  37  to be within the movement scope itself; enabling a movement position value x2 and a position value s2 of the collection position of the signal light to form a monotone function that corresponds to a unique determined value; and setting a corresponding relationship therebetween to be x2=f2(s2), while representing s2 as an addition form of s and a constant offset 12, i.e., s2=s+12 to obtain x2=f2 (s+12); and 
     calibrating the functions x1=f1(s+11) and x2=f2(s+12) to obtain the values of x1 and x2 through the known s so as to obtain the position values x1 and x2 for the movement of the first movable base  32  the second movable base  37  through a value obtained by laser distance measurement, thereby realizing focusing adjustment. 
     Through the above-mentioned method and steps, an automatic focusing function can be realized, namely that the laser focusing position of the remote beam expanding and focusing module  7  and the signal collection position of the remote signal collecting module  11  are automatically adjusted through a measurement feedback signal of the laser distance measuring module  8  so that the laser focusing position and the signal collection position coincide on the liquid metal surface  23  to be measured. 
     The device of the present invention not only can realize in-situ detection, but also can realize online detection. The difference between in-situ and online is that in-situ detection means detection without disassembling original equipment and interfering with an original production process, and emphasizes spatiality; and online detection means instantaneous detection on a detection object on a production line and emphasizes timeliness. Through the adoption of the device of the present invention, instantaneous and quick detection without interfering with the production process can be realized. 
     By taking the detection of metallurgical liquid steel as an example, the working process of the device of the present invention is as follows: 
     Firstly, the front-end high-temperature resistant optical probe  18  is slowly extended into a melt (metallurgical liquid metal  22 ) through an elevating mechanism, and an air charging pipe  16  and an air charging pipe  17  open the gas flow of 30 L/min to form an optical path environment of the inert gas. When the front-end high-temperature resistant optical probe  18  is inserted into a level depth between 20 cm to 30 cm, the position thereof is fixed; the laser distance measuring part  18  starts to work and emits distance measuring laser; and the position to be measured of the metallurgical liquid metal surface  23  in the high-temperature resistant probe  18  is measured and fed back to the back-end control platform. 
     The back-end control platform performs position adjustment on the first movable base  32  and the second movable base  37  in the remote beam expanding and focusing module  7  and the remote signal collecting module  11  through the distance position measured by the laser distance measuring part so that the focusing position of the measurement laser and the collection position of remote signals are identical and are on the metallurgical liquid metal surface  23 . 
     After the position adjustment is finished, the first laser head  26  and the second laser head  27  receive the signals of the back-end control platform and emit the laser. The laser emitted by the first laser head  26  passes through the first half-wave plate  28 , the first polarizing light splitting piece  30  and the second polarizing light splitting piece  31 , and is incident into the remote beam expanding and focusing module  7  in the direction of the optical axis of the remote beam expanding and focusing module  7 . The laser emitted by the second laser head  27  passes through the second half-wave plate  29  and the second polarizing light splitting piece  31  to realize beam combination with the laser emitted by the first laser head  26 , and is likewise incident into the remote beam expanding and focusing module  7  in the direction of the optical axis of the remote beam expanding and focusing module  7 . 
     Two laser beams after the beam combination pass through the remote beam expanding and focusing module  7  to form a wide focusing beam with a larger diameter; and the wide focusing beam is incident on the first reflecting mirror  9  of the coaxial adjusting part. The first reflecting mirror  9  is placed in the direction of the optical axis of the remote beam expanding and focusing module  7  and forms 45° with the optical axis of the remote beam expanding and focusing module  7 . The laser that has passed through the first reflecting mirror  9  is reflected to the second reflecting mirror  10 . The second reflecting mirror  10  is placed in the direction of the optical axis of the remote signal collecting module  11  and forms 45° with the optical axis of the remote signal collecting module  11 . Moreover, a connecting line of the centers of the first reflecting mirror  9  and the second reflecting mirror  10  is perpendicular to the directions of the optical axes of the remote beam expanding and focusing module  7  and the remote signal collecting module  11 . The second reflecting mirror  10  reflects the two focusing laser beams to the direction of the optical axis of the remote signal collecting module  11  to realize coaxiality between the excitation optical path and the collection optical path. 
     The two laser beams after the beam combination are made to pass through the second reflecting mirror  10  and an optical window  15  and to focus on the metallurgical liquid metal surface  23 , so as to form the plasma. After passing through the optical window  15 , the light ray emitted by the plasma is incident into the remote signal collecting module  11 , and in the remote signal collecting module  11 , passes through the paraboloidal reflecting mirror  39  and the hyperboloidal reflecting mirror  38  to form parallel beams. 
     The light splitting piece  20  is placed in the direction of the optical axis of the remote signal collecting module  11  and forms 45° with the optical axis. After passing through the light splitting piece  20 , the parallel signal light is partially reflected to a real-time display system  12 ; the surface appearance of a sampling point and the formation condition of the plasma can be observed in real time and fed back to the back-end control platform; the focusing position can be adjusted slightly; and the rest part of the parallel signal light is incident into the optical fiber coupling module  13  that is likewise placed in the direction of the optical axis of the remote signal collecting module  11 , and enters the spectrometer  14  after passing through the optical fiber  21  to finish the collection of the plasma signal light. 
     The spectrometer performs light splitting and photoelectric conversion on the collected plasma emission light rays, and transmits converted electrical signals to a console through a cable to perform data analysis and processing. The middle-end optical sensing device  19  is a constant-temperature box body, and the temperature is controlled below 30° C.