Remote Detection Method of Laser Induced Breakdown Spectroscopy

Abstract

The invention discloses a laser induced breakdown spectrum remote detection method, which uses a multi-mirror self-focusing remote detection device including a laser rangefinder and a laser, a beam expansion lens group, a first laser reflector, a dichroic mirror, a primary mirror and a multi-mirror zoom structure arranged in sequence along the propagation direction of the laser beam. The laser rangefinder is used to measure the distance first, and a computer determines which secondary mirror is selected to work and the distance and direction of the guide rail movement. The guide rail rotates to drive the selected secondary mirror to rotate and be coaxial with the primary mirror. The guide rail then drives the secondary mirror to move in a straight line. The laser emits a pulse laser, which is reflected by the optical system to the surface of the object to be tested to excite plasma. During the cooling and expansion process, signal light is emitted outward, and the primary mirror converges and then the focusing system focuses and enters the spectrometer to analyze its material composition and content. The self-focusing detection device used in the detection method has higher zoom accuracy and can be used for the analysis and measurement of the components of the objects to be tested in some extreme environments where direct close contact is inconvenient.

Description

Technical Field

[0001] The invention belongs to the technical field of spectrum measurement and relates to a laser induced breakdown spectrum remote detection method. Background Art

[0002] Laser Induced Breakdown Spectroscopy (LIBS), also known as Laser-Induced Plasma Spectroscopy (LIPS) or Laser Spark Spectroscopy (LSS), is an element detection technology that has been developed in recent years and belongs to atomic emission spectroscopy. The principle of LIBS is to focus high-energy pulsed lasers onto the surface of the object to be tested to ablate the object and generate plasma. During the cooling and expansion of the laser-induced plasma, the characteristic spectrum of the object to be tested can be obtained through the focusing system and the spectrometer system. After data processing, the content of the elements that make up the object to be tested can be obtained.

[0003] In recent years, with the rapid development of LIBS technology, important research results have been achieved in the fields of medical treatment, industrial production, deep-sea and space exploration, nuclear environment and element detection of explosives.

[0004] The most notable feature of LIBS is that it does not require sample preparation and is easy to operate in actual applications. The working medium of LIBS is pulsed laser, so its measurement has little impact on the object being measured, which is close to non-destructive testing. And because the light signal emitted by the laser-induced plasma is strong enough, this makes remote LIBS possible.

[0005] In actual use, the area of ​​the object to be tested is large and the distance from the detection device is not fixed. The common laboratory fixed-focus LIBS cannot meet this requirement. In addition, the common laboratory LIBS is manual zoom, the zoom speed is slow, the optical path is difficult to debug, and the requirements for equipment users are high; some LIBS focusing systems and condensing systems are designed as separate structures, which are relatively large in size, have low space utilization, and are not easy to integrate, increasing the workload of equipment users.

[0006] When laser-induced breakdown spectroscopy is used in existing technologies to detect objects, most of the time it is done at close range. However, in reality, there are many scenarios where close range detection cannot be performed directly. For example, in dangerous environments with high temperatures and high radiation, on cliffs, in space and in the deep sea. Therefore, the application demand for remote LIBS is very huge. Summary of the invention

[0007] The purpose of the present invention is to provide a laser induced breakdown spectroscopy remote detection method capable of performing remote detection.

[0008] To achieve the above object, the technical solution adopted by the present invention is: a laser induced breakdown spectroscopy remote detection method, which is specifically carried out in the following steps:

[0009] 1) A self-focusing detection device is provided, which comprises a laser rangefinder and a laser, a beam expansion lens group, a first laser reflector, a dichroic mirror, a primary mirror and a multi-mirror zoom structure arranged in sequence along the propagation direction of the laser beam; the first laser reflector and the dichroic mirror are arranged in parallel up and down, the angles between the first laser reflector and the dichroic mirror and the horizontal plane are both 45 degrees, a focusing system is provided on the side of the dichroic mirror away from the primary mirror, and the focusing system is connected to a spectrometer; the laser, the multi-mirror zoom structure, the laser rangefinder and the spectrometer are all connected to a computer;

[0010] The primary mirror uses a concave spherical reflector with a circular hole in the center, and the concave spherical surface of the primary mirror faces the multi-mirror zoom structure;

[0011] The multi-mirror zoom structure includes a guide rail and a grating ruler arranged in parallel, a grating ruler slider which can reciprocate along the length direction of the grating ruler is installed on the grating ruler, and the grating ruler slider is connected to the guide rail through a first connecting rod; a plurality of rotating arms which can reciprocate around the axis of the guide rail are installed in sequence along the axial direction of the guide rail, the rotating arm adopts an electromagnetic clutch, a limit rod and a rotating arm support rod are fixed on the rotating arm, a secondary mirror is installed on the rotating arm support rod, and the secondary mirrors are all convex spherical lenses, the convex surfaces of all secondary mirrors are facing the main mirror, and the curvature radii of all secondary mirrors are different; a first motor and a second motor are installed at both ends of the guide rail, respectively, and control modules are installed on the first motor and the second motor, and all control modules are connected to computer signals; a second connecting rod is fixed on the guide rail between the first motor and the rotating arm adjacent to the first motor, the second connecting rod is located below the guide rail, and a second laser reflecting mirror whose inclination direction is opposite to that of the first laser reflecting mirror is installed at the lower end of the second connecting rod, and the angle between the second laser reflecting mirror and the horizontal plane is 45°;

[0012] All rotating arms are connected to the computer through the control circuit; the grating ruler is equipped with limit switches of the same number as the rotating arms; the first motor is a linear motor;

[0013] The beam expansion lens group includes a first biconcave lens and a plano-convex lens arranged side by side, and the convex surface of the plano-convex lens is arranged away from the first biconcave lens; the first biconcave lens is adjacent to the laser;

[0014] The focusing system includes a second biconcave lens, a first biconvex lens, a second biconvex lens and a negative meniscus lens which are arranged in sequence side by side, wherein the concave surface of the negative meniscus lens faces the second biconvex lens; the negative meniscus lens is adjacent to the spectrometer;

[0015] Place the object to be measured at the pre-designed position, then turn on the computer and laser rangefinder; at this time, all rotating arms are powered off;

[0016] 2) Sending a working instruction to the laser rangefinder through the computer, the laser rangefinder sends out a laser beam A, which is directed to the multi-mirror zoom structure, reflected by the second laser reflector in the multi-mirror zoom structure, and directed to the object to be measured; a beam of signal light A is reflected back from the surface of the object to be measured, and the signal light A is directed to the second laser reflector, and reflected by the second laser reflector to the laser rangefinder. After receiving the returned signal light A, the laser rangefinder calculates the distance from the primary mirror to the object to be measured, and sends the calculation result to the computer. The computer calculates the corresponding working parameters, which are used to determine which secondary mirror is selected for work and the distance and direction of movement of the guide rail;

[0017] 3) The computer sends a command to start the second motor, which drives the guide rail to rotate, and the guide rail drives the active rotors in all rotating arms to rotate. At the same time, the computer energizes the selected rotating arm through the control circuit according to the working parameters. The active rotor and the driven rotor in the energized rotating arm are combined, and the active rotor drives the driven rotor to rotate. The driven rotor drives the limit rod and the rotating arm support rod on it to rotate around the horizontal axis. When the limit rod and the rotating arm support rod rotate 180°, the limit rod touches a limit switch on the grating ruler, and the limit switch sends a signal. The computer sends a command to close The second motor, the selected rotating arm is still in the power-on state, at this time, the secondary mirror fixed on the rotating arm is coaxial with the main mirror, and the secondary mirror becomes the working secondary mirror; then, the computer issues an instruction to start the first motor, and the first motor drives the guide rail to drive the working secondary mirror to move along the axis of the guide rail in the direction determined by the computer. During the displacement process, the grating ruler continuously measures the position of the working secondary mirror. After the working secondary mirror meets the moving distance, the computer determines whether the fine zoom is completed by reading the data of the grating ruler. If it is determined that the fine zoom has been completed, the computer turns off the first motor, and the displacement of the working secondary mirror ends;

[0018] 4) The computer turns on the laser, and the laser emits a pulsed laser beam, which passes through the first double concave lens and the plano-convex lens in sequence to amplify the laser spot; the amplified laser spot passes through the first laser reflector and the dichroic mirror in sequence, and is emitted to the working secondary mirror through the central circular hole of the primary mirror, and is reflected by the convex surface of the working secondary mirror to the concave surface of the primary mirror, and is reflected by the concave surface of the primary mirror and converges to the object to be tested; when the energy density of the converged laser reaches the material threshold of the object to be tested, plasma is excited on the surface of the object to be tested, and the plasma emits signal light B outward during the cooling and expansion process;

[0019] 5) Signal light B is converged by the concave surface of the primary mirror to the working secondary mirror, and then reflected by the working secondary mirror and emitted to the dichroic mirror through the central circular hole of the primary mirror. A part of signal light B passes through the dichroic mirror, and then passes through the second biconcave lens, the first biconvex lens, the second biconvex lens and the negative meniscus lens in sequence, and enters the spectrometer. The spectrometer analyzes the received part of signal light B in conjunction with the computer, and the computer measures the material composition and content of the object to be tested through the LIBS quantitative analysis algorithm.

[0020] The detection method of the present invention has the following advantages:

[0021] 1) Long-distance measurement. The focusing device used in the detection method has a zoom range of 1 to 20 meters, which is highly applicable and can be used for most on-site remote measurement operations, realizing real-time online detection, saving time, and improving detection efficiency; it solves the problem of the unfixed distance between the object to be tested and the detection device in actual detection. At the same time, the focusing device uses optomechanical integration technology to achieve automatic zoom. When zooming, it avoids the system error caused by manual adjustment of the lens, improving the measurement accuracy and efficiency.

[0022] 2) The zoom accuracy of the focusing device used in the detection method is higher. The zoom structure using multiple mirrors (each mirror has a different radius of curvature) improves the zoom curve compared with the zoom using a single curvature mirror. When using a single curvature mirror for zooming, the zoom curve is very smooth at the end (i.e., the zoom distance changes greatly when the mirror moves a short distance); while the working mirrors with different curvatures have different zoom ranges, and the more ideal parts of the zoom curves of the mirrors with different curvatures are selected to form a new continuous zoom curve. That is, the distance between the main mirror and the working mirror in the Y direction of the multiple mirror zoom structure increases in the same zoom range, making the zoom more stable and accurate in actual operation. At the same time, the accuracy requirements of the mechanical system are reduced, saving costs. Moreover, the structure is compact and easy to integrate. The collection system and the focusing system are coaxial, the lens utilization rate is high, and the various parts of the structure are closely connected, so the space utilization rate is high, that is, the structure is small in size, and it is easy to integrate with other devices.

[0023] 3) It can be used for the analysis and measurement of the composition of the objects to be tested in some extreme environments where it is inconvenient to have direct close contact, for example: in industrial metallurgy, it is difficult to measure the composition of molten metal at close range due to the high temperature; or to measure elements in strong magnetic and radiation environments such as nuclear power plants; or to measure elements in explosive environments, etc. The self-focusing device of the present invention can also be mounted on some automated mechanical devices (drones, unmanned vehicles, pan-tilt platforms, etc.) to achieve long-term automated detection of the objects to be tested, for example: to carry out long-term automated real-time online detection of the elemental composition content of the sample to be tested on a drone. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1It is a schematic diagram of the self-focusing detection device used in the detection method of the present invention.

[0025] Figure 2 yes Figure 1 Schematic diagram of the multi-mirror zoom structure in the self-focusing detection device shown.

[0026] Figure 3 yes Figure 1 Schematic diagram of the beam expansion lens group in the self-focusing detection device shown.

[0027] Figure 4 yes Figure 1 Schematic diagram of the focusing system in the self-focusing detection device shown.

[0028] Figure 5 yes Figure 1 The diagram shows the usage status of the multi-mirror zoom structure in the self-focusing detection device.

[0029] Figure 6 This is a curve comparison chart of single-lens zoom and multi-lens group zoom.

[0030] In the figure: 1. computer, 2. laser, 3. beam expansion lens group, 4. first laser reflector, 5. primary mirror, 6. multi-mirror zoom structure, 7. object to be measured, 8. distance measurement system, 9. dichroic mirror, 10. focusing system, 11. spectrometer, 12. secondary mirror, 13. second laser reflector, 14. guide rail, 15. grating ruler, 16. first connecting support rod, 17. limit rod, 18. grating ruler slider, 19. first motor, 20. second connecting support rod, 21. rotating arm support rod, 22. rotating arm, 23. second motor, 24. working secondary mirror, 25. first biconcave lens, 26. plano-convex lens, 27. second biconcave lens, 28. first biconvex lens, 29. second biconvex lens, 30. negative meniscus lens. DETAILED DESCRIPTION

[0031] The present invention is further explained below in conjunction with the accompanying drawings and specific embodiments.

[0032] The present invention provides a method for remotely detecting an object to be detected 7 by laser induced breakdown spectroscopy, which is specifically performed in the following steps:

[0033] 1) Take Figure 1The self-focusing detection device shown in the figure comprises a laser rangefinder 8 and a laser 2, a beam expanding lens group 3, a first laser reflecting mirror 4, a dichroic mirror 9, a primary mirror 5 and a multi-mirror zoom structure 6 which are arranged in sequence along the propagation direction of the laser beam; the first laser reflecting mirror 4 and the dichroic mirror 9 are arranged in parallel up and down, and the angles between the first laser reflecting mirror 4 and the dichroic mirror 9 and the horizontal plane are both 45°, and a focusing system 10 is provided on the side of the dichroic mirror 9 away from the primary mirror 5, and the focusing system 10 is connected to a spectrometer 11; the laser 2, the multi-mirror zoom structure 6, the laser rangefinder 8 and the spectrometer 11 are all connected to a computer 1.

[0034] The main mirror 5 is a concave spherical reflector with a circular hole with a diameter of 4 mm at the center, and the concave spherical surface of the main mirror 5 faces the multi-mirror zoom structure 6.

[0035] The multi-mirror zoom structure 6 in the self-focusing detection device, such as Figure 2 As shown, it includes a guide rail 14 and a grating ruler 15 arranged in parallel, a grating ruler slider 18 that can reciprocate along the length direction of the grating ruler 15 is installed on the grating ruler 15, and the grating ruler slider 18 is connected to the guide rail 14 through a first connecting support rod 16; along the axial direction of the guide rail 14, a plurality of rotating arms 22 that can reciprocate around the axis of the guide rail 14 are sequentially installed on the guide rail 14, and a limit rod 17 and a rotating arm support rod 21 are fixedly connected to the rotating arm 22, one end of the limit rod 17 is fixedly connected to the rotating arm 22, and the other end of the limit rod 17 is self- From the end, the lower end of the rotating arm support rod 21 is fixedly connected to the rotating arm 22, and the upper end of the rotating arm support rod 21 is installed with a secondary mirror 12, and the secondary mirrors 12 are all convex spherical lenses. The convex surfaces of all secondary mirrors 12 face the main mirror 5, and the curvature radii of all secondary mirrors 12 are different; the first motor 19 and the second motor 23 are respectively installed at both ends of the guide rail 14, and the first motor 19 and the second motor 23 are installed with control modules, and all control modules are connected to the computer 1 signal; the second motor 23 is connected to the guide rail 14 through a spline connection. A second connecting rod 20 is fixedly connected to the guide rail 14 between the first motor 19 and the rotating arm 22 adjacent to the first motor 19. The second connecting rod 20 is located below the guide rail 14. The upper end of the second connecting rod 20 is fixedly connected to the guide rail 14. The lower end of the second connecting rod 20 is installed with a second laser reflector 13. The angle between the second laser reflector 13 and the horizontal plane is 45°, and the inclination direction of the second laser reflector 13 is opposite to the inclination direction of the first laser reflector 4.

[0036] The rotating arm 22 uses an electromagnetic clutch, preferably a DC24V electromagnetic clutch (Japan Sinfonia micro high torque clutch). The outer wall of the driven rotor of the electromagnetic clutch is fixed with a limit rod 17 and a rotating arm support rod 21, and the angle between the limit rod 17 and the rotating arm support rod 21 is 90°; the active rotor of the electromagnetic clutch is sleeved on the guide rail 14.

[0037] All the rotating arms 22 are connected to the computer 1 via a control circuit.

[0038] The grating ruler 15 is provided with limit switches whose number is the same as the number of the rotating arms 22 .

[0039] The first motor 19 is a linear motor.

[0040] All the rotating arms 22 are located between the first connecting strut 16 and the second connecting strut 20 .

[0041] The length of the inner spline is different from that of the outer spline in the spline connection. The outer spline is mounted on the guide rail 14, and the inner spline is mounted on the motor shaft of the second motor 23, so there is a gap between the end face of the outer spline facing the second motor 23 and the end face of the motor shaft of the second motor 23 extending into the inner spline.

[0042] like Figure 3 As shown, the beam expander lens group 3 in the self-focusing detection device includes a first biconcave lens 25 and a plano-convex lens 26 arranged side by side, and the convex surface of the plano-convex lens 26 is arranged away from the first biconcave lens 25. The first biconcave lens 25 is adjacent to the laser 2, and the plano-convex lens 26 is adjacent to the first laser reflector 4.

[0043] like Figure 4 As shown, the focusing system 10 in the self-focusing detection device includes a second biconcave lens 27, a first biconvex lens 28, a second biconvex lens 29 and a negative meniscus lens 30 arranged side by side in sequence, and the concave surface of the negative meniscus lens 30 faces the second biconvex lens 29; the negative meniscus lens 30 is adjacent to the spectrometer 11, and the second biconcave lens 27 is adjacent to the dichroic mirror 9.

[0044] When not in operation, all the limit rods 17 are arranged along the positive direction of the X axis, all the rotating arm support rods 21 are arranged along the positive direction of the Z axis, and the guide rails 14 are arranged along the Y axis direction;

[0045] Place the object 7 to be measured at the pre-designed position, then turn on the computer 1 and the laser rangefinder 8, at which point all the rotating arms 22 are not powered;

[0046] 2) Obtaining zoom parameters: Sending a working instruction to the laser rangefinder 8 through the computer 1, the laser rangefinder 8 emits a laser beam A, which is directed to the multi-mirror zoom structure 6, reflected by the second laser reflector 13 in the multi-mirror zoom structure 6, and directed to the object to be measured 7; a signal light A is reflected back from the surface of the object to be measured 7, and the signal light A is directed to the second laser reflector 13, and reflected by the second laser reflector 13 to the laser rangefinder 8. After receiving the returned signal light A, the laser rangefinder 8 calculates and obtains the distance from the main mirror 5 to the object to be measured 7 (the actual distance measured by the rangefinder 8 is the distance from the rangefinder 8 to the second laser reflector 13). The distance between the reflector 13 and the second reflector 13 to the object 7 to be measured, and because the working secondary mirror 24 will return to its original position after each measurement, the distance between the primary mirror 5 and the second reflector 13 is determined, and it is only necessary to use the actual distance measured by the rangefinder 8 minus the distance from the rangefinder 8 to the second reflector 13, and add the distance from the primary mirror 5 to the second reflector 13 to obtain the distance from the primary mirror 5 to the object 7 to be measured), and send the calculation result to the computer 1, and the computer 1 calculates the corresponding working parameters, that is, determines which secondary mirror 12 is selected to work and the distance and direction of movement of the guide rail 14;

[0047] 3) Computer 1 issues a command to start the second motor 23. The second motor 23 drives the guide rail 14 to rotate through the spline connection. The guide rail 14 drives the active rotors in all the rotating arms 22 to rotate. At the same time, computer 1 energizes the selected rotating arm 22 through the control circuit according to the obtained working parameters. The active rotor and the driven rotor in the energized rotating arm 22 are combined. The active rotor drives the driven rotor to rotate. The driven rotor drives the limit rod 17 and the rotating arm support rod 21 thereon to rotate around the horizontal axis. When the limit rod 17 and the rotating arm support rod 21 rotate 180°, the limit rod 17 touches a limit switch on the grating ruler 15. The limit switch sends a signal. Computer 1 issues a command to turn off the second motor 23. The selected rotating arm 22 is still in the energized state. At this time, the secondary mirror 12 fixed to the rotating arm 22 is coaxial with the main mirror 5. The secondary mirror 12 becomes the working secondary mirror 24 to complete the coarse zoom. Figure 5 As shown; then, the computer 1 issues an instruction to start the first motor 19, and the first motor 19 drives the guide rail 14 to drive the working secondary mirror 24 to move along the axis direction of the guide rail 14 in the direction determined by the computer 1. During the displacement process, the grating ruler 15 continuously measures the position of the working secondary mirror 24. After the working secondary mirror 24 meets the moving distance, the computer 1 reads the data of the grating ruler 15 to determine whether the fine zoom is completed. If it is determined that the fine zoom is completed, the computer 1 turns off the first motor 19, and the displacement of the working secondary mirror 24 ends;

[0048] 4) Computer 1 turns on laser 2, and laser 2 emits a pulsed laser beam, which passes through the first double concave lens 25 and the plano-convex lens 26 in sequence, and the laser spot is enlarged by 2.5 times; the enlarged laser spot passes through the first laser reflector 4 and the dichroic mirror 9 in sequence, and then is emitted to the working secondary mirror 24 through the central circular hole of the primary mirror 5, and is reflected by the convex surface of the working secondary mirror 24 to the concave surface of the primary mirror 5, and is reflected by the concave surface of the primary mirror 5, and converges to the object to be tested 7; when the energy density of the converged laser reaches the material threshold of the object to be tested 7, plasma can be excited on the surface of the object to be tested 7, and the plasma emits signal light B outward during the cooling and expansion process.

[0049] 5) The signal light B is converged by the concave surface of the primary mirror 5 to the working secondary mirror 24, and after being reflected by the working secondary mirror 24, it passes through the central circular hole of the primary mirror 5 to the dichroic mirror 9. After a part of the signal light B passes through the dichroic mirror 9, it passes through the second biconcave lens 27, the first biconvex lens 28, the second biconvex lens 29 and the negative meniscus lens 30 in sequence, and enters the spectrometer 11. The spectrometer 11 analyzes the received part of the signal light B in conjunction with the computer 1. The computer 1 measures the material composition and content of the object 7 to be tested through the LIBS (laser induced breakdown spectroscopy) quantitative analysis algorithm.

[0050] After completing the measurement, the computer 1 starts the first motor 19 to drive the guide rail 14 to move in the reverse direction, so that the rotating arm 22 returns to its original position before the linear movement. Then, the computer 1 starts the second motor 23 to drive the guide rail 14 to rotate in the reverse direction, so that the selected rotating arm rotates 180° in the reverse direction, and the working secondary mirror 24 returns to its initial position. After that, the computer 1 sends an electrical signal, and the control circuit cuts off the power to the selected rotating arm 22, and the rotating arm 22 returns to the separated state.

[0051] The part of the signal light B that can pass through the dichroic mirror 9 is visible light with a wavelength of 200nm to 800nm. The visible light band in the signal light B passes through the dichroic mirror 9 and is directed to the focusing system 10, which eliminates the interference of the pulse laser on the spectrometer 11.

[0052] The visible light passing through the dichroic mirror 9 is focused by the focusing system 10 onto the optical fiber connected to the spectrometer 11; the spectrometer 11 generates spectrum data according to the collected signal light, and finally obtains the composition of the object to be tested 7 through software analysis.

[0053] The focusing system 10, the primary mirror 5 and the working secondary mirror 24 are coaxial; the working secondary mirror 24 and the primary mirror 5 form a Cassegrain system.

[0054] During the detection process, since the first motor 19 is very small, when the second motor 23 drives the guide rail 14 to rotate, the first motor 19 rotates with the guide rail 14. When the first motor 19 is working, the second motor 23 stops working, and the first motor 19 drives the guide rail 14 to move linearly. Since the lengths of the inner spline and the outer spline in the spline connection are different, and the linear movement distance of the guide rail 14 is not large, it can be ensured that the inner spline and the outer spline will not be disengaged during the movement, and the gap between the end face of the outer spline and the end face of the motor shaft of the second motor 23 is sufficient to ensure the movement distance of the guide rail 14.

[0055] like Figure 5 As shown, the distance D between the primary mirror 5 and the working secondary mirror 24 is:

[0056] D=d1+d2+d3+(n-1)Δd(1)

[0057] (1) In the formula, d1 is the zero point of the grating ruler 15, that is, the distance between the end surface of the grating ruler 15 facing the primary mirror 5 and the plane of the primary mirror 5 in the Y direction;

[0058] d2 is the relative distance in the Y direction between the zero point of the grating ruler 15 (the end face of the grating ruler 15 facing the primary mirror 5) and the end of the grating ruler slider 18 facing the zero point of the grating ruler 15, and is also the reading of the grating ruler;

[0059] d3 is the distance in the Y direction between the end of the grating scale slider 18 facing the zero point of the grating scale 15 and the axis of the rotating arm support rod 21 where the secondary mirror 12 adjacent to the grating scale slider 18 is located;

[0060] Δd is the distance in the Y direction between the axes of the rotating arm support rods 21 where two adjacent secondary mirrors 12 are located;

[0061] n is the serial number of the rotating sub-mirror 12 among all sub-mirrors (the serial number is from the main mirror 5 to the first motor 19, the sub-mirror 12 closest to the main mirror 5 is 1, and so on).

[0062] The self-focusing detection device used in the detection method of the present invention performs coarse zooming through the selected secondary mirror 12, which means that by selecting a different secondary mirror 12, the value of n in formula (1) is changed. The fine zooming in the detection method of the present invention is that the computer 1 sends an instruction to the motor module of the first motor 19, and the first motor 19 drives the guide rail 14 to move linearly, that is, the value of d2 in formula (1) is changed.

[0063] The working secondary mirror 24 of the self-focusing detection device used in the long-distance detection method of the present invention diverges the light beam, and the main mirror 5 converges the light beam. The zoom can be achieved by changing the distance D in the Y direction between the main mirror 5 and the working secondary mirror 24: after obtaining the zoom parameters, the second motor 23 is used to select a suitable secondary mirror 12 from multiple secondary mirrors 12 for coarse zoom, and then the first motor 19 is used to drive the guide rail 14 to drive the working secondary mirror 24 to move for fine zoom.

[0064] The first biconcave lens 25 and the plano-convex lens 26 are arranged in parallel and perpendicular to the propagation direction of light. The beam expansion lens group 3 of the device can expand the light by 2.5 times.

[0065] The first biconvex lens 28 and the second biconcave lens 27 are used to eliminate aberrations, and the second biconvex lens 29 and the negative meniscus lens 30 are used to focus the light beam.

[0066] When the laser 2 emits a spot with a radius of 4 mm, the spot diameter after focusing is less than 1 mm, ensuring that the laser energy can be concentrated in a sufficiently small spatial range, so that the laser can excite high-temperature, high-density plasma on the surface of the object 7 to be tested.

[0067] The focusing optical path (first biconcave lens 25 and plano-convex lens 26) and the collecting optical path (second biconcave lens 27, first biconvex lens 28, second biconvex lens 29 and negative meniscus lens 30) are coaxially arranged, and the optimization of focusing and collecting at different distances can be controlled with a minimum adjustment amount, and the focusing spot diameter of the collected light coupled into the optical fiber after passing through the coaxial collection system is less than 0.6 mm, ensuring that the spectral signals of a wide range of wavelength bands can be focused into the telescopic focusing collection system in the optical fiber core diameter at different distances.

[0068] The self-focusing device in the present invention uses multiple sub-mirrors 12 to select zoom, which improves the zoom curve resolution of the self-focusing device, that is, for the same focus distance change, the change in the distance between the main mirror 5 and the working sub-mirror 24 corresponding to the multi-lens zoom system is greater than the change in the distance between the main mirror and the working sub-mirror in the Y direction of the single-lens zoom system in the prior art. This can reduce the zoom error and make the measurement result more accurate.

[0069] The curve comparison of the multi-mirror zoom of the self-focusing device in the present invention and the single-mirror zoom in the prior art is shown in FIG. Figure 6 As shown, the horizontal axis is the focus distance, and the vertical axis is the distance between the primary mirror 5 and the working secondary mirror 24 in the Y direction. It can be seen from the figure that the resolution of the multi-mirror zoom curve is greater than that of the single-mirror zoom curve, that is, for the same focus distance change, the distance change between the primary mirror 5 and the working secondary mirror 24 in the Y direction corresponding to the multi-lens zoom system is greater than the distance change between the primary mirror 5 and the working secondary mirror 24 in the Y direction of the single-lens zoom system, which can reduce the error of zooming and make the measurement result more accurate.

[0070] The self-focusing device in the present invention is a coaxial remote LIBS automatic zoom device using multiple lenses for zooming. The whole device is controlled by a computer 1.

Claims

1. A laser induced breakdown spectroscopy remote detection method, characterized in that: The detection method is specifically carried out in the following steps: 1) A self-focusing detection device, comprising a laser rangefinder (8) and a laser (2), a beam expanding lens group (3), a first laser reflector (4), a dichroic mirror (9), a primary mirror (5) and a multi-mirror zoom structure (6) arranged in sequence along the propagation direction of the laser beam; the first laser reflector (4) and the dichroic mirror (9) are arranged in parallel up and down, the angles between the first laser reflector (4) and the dichroic mirror (9) and the horizontal plane are both 45 degrees, a focusing system (10) is provided on the side of the dichroic mirror (9) away from the primary mirror (5), and the focusing system (10) is connected to a spectrometer (11); the laser (2), the multi-mirror zoom structure (6), the laser rangefinder (8) and the spectrometer (11) are all connected to a computer (1); The primary mirror (5) is a concave spherical reflector with a circular hole at the center, and the concave spherical surface of the primary mirror (5) faces the multi-mirror zoom structure (6); The multi-mirror zoom structure (6) comprises a guide rail (14) and a grating ruler (15) arranged in parallel, wherein a grating ruler slider (18) is mounted on the grating ruler (15) and can reciprocate along the length direction of the grating ruler (15), and the grating ruler slider (18) is connected to the guide rail (14) via a first connecting support rod (16); a plurality of rotating arms (22) are mounted on the guide rail (14) in sequence along the axis direction of the guide rail (14) and can reciprocate around the axis of the guide rail (14), the rotating arm (22) adopts an electromagnetic clutch, a limit rod (17) and a rotating arm support rod (21) are fixedly connected to the rotating arm (22), and a secondary mirror (12) is mounted on the rotating arm support rod (21), and the secondary mirrors (12) are all convex spherical lenses, and the convex surfaces of all the secondary mirrors (12) face the primary mirror ( 5), and the curvature radii of all the secondary mirrors (12) are different; a first motor (19) and a second motor (23) are respectively mounted at both ends of the guide rail (14), and control modules are mounted on the first motor (19) and the second motor (23), and all the control modules are connected to the computer (1) by signal; a second connecting rod (20) is fixedly connected to the guide rail (14) between the first motor (19) and a rotating arm (22) adjacent to the first motor (19), the second connecting rod (20) is located below the guide rail (14), and a second laser reflecting mirror (13) whose inclination direction is opposite to that of the first laser reflecting mirror (4) is mounted at the lower end of the second connecting rod (20), and the angle between the second laser reflecting mirror (13) and the horizontal plane is 45°; All the rotating arms (22) are connected to the computer (1) via a control circuit; the grating ruler (15) is provided with limit switches of the same number as the rotating arms (22); the first motor (19) is a linear motor; The beam expanding lens group (3) comprises a first biconcave lens (25) and a plano-convex lens (26) arranged side by side, wherein the convex surface of the plano-convex lens (26) is arranged away from the first biconcave lens (25); the first biconcave lens (25) is adjacent to the laser (2); The focusing system (10) comprises a second biconcave lens (27), a first biconvex lens (28), a second biconvex lens (29), and a negative meniscus lens (30) which are sequentially arranged side by side, and the concave surface of the negative meniscus lens (30) faces the second biconvex lens (29); the negative meniscus lens (30) is adjacent to the spectrometer (11); The object to be measured (7) is placed at a pre-designed position, and then the computer (1) and the laser rangefinder (8) are turned on; at this time, all the rotating arms (22) are not powered; 2) sending a working instruction to the laser rangefinder (8) through the computer (1), the laser rangefinder (8) emits a laser beam A, the laser beam A is directed to the multi-mirror zoom structure (6), reflected by the second laser reflector (13) in the multi-mirror zoom structure (6), and directed to the object to be measured (7); a beam of signal light A is reflected back from the surface of the object to be measured (7), the signal light A is directed to the second laser reflector (13), reflected by the second laser reflector (13) to the laser rangefinder (8), after receiving the returned signal light A, the laser rangefinder (8) calculates the distance from the primary mirror (5) to the object to be measured (7), and sends the calculation result to the computer (1), the computer (1) calculates the corresponding working parameters, the working parameters are used to determine which secondary mirror (12) is selected to work and the distance and direction of movement of the guide rail (14); 3) The computer (1) issues an instruction to start the second motor (23), the second motor (23) drives the guide rail (14) to rotate, the guide rail (14) drives the active rotors in all the rotating arms (22) to rotate, and at the same time, the computer (1) energizes the selected rotating arm (22) through the control circuit according to the working parameters, the active rotor and the driven rotor in the energized rotating arm (22) are combined, the active rotor drives the driven rotor to rotate, and the driven rotor drives the limit rod (17) and the rotating arm support rod (21) thereon to rotate around the horizontal axis, when the limit rod (17) and the rotating arm support rod (21) rotate 180 degrees, the limit rod (17) touches a limit switch on the grating ruler (15), the limit switch sends a signal, and the computer (1) issues an instruction to turn off the second motor (23), and the selected rotating arm (22) is turned off. The fixed rotating arm (22) is still in the power-on state. At this time, the secondary mirror (12) fixed to the rotating arm (22) is coaxial with the main mirror (5), and the secondary mirror (12) becomes the working secondary mirror (24); then, the computer (1) issues an instruction to start the first motor (19), and the first motor (19) drives the guide rail (14) to drive the working secondary mirror (24) to move along the axis direction of the guide rail (14) in the direction determined by the computer (1). During the displacement process, the grating ruler (15) continuously measures the position of the working secondary mirror (24). After the working secondary mirror (24) meets the moving distance, the computer (1) determines whether the fine zoom is completed by reading the data of the grating ruler (15). If it is determined that the fine zoom is completed, the computer (1) turns off the first motor (19), and the displacement of the working secondary mirror (24) ends; 4) The computer (1) turns on the laser (2), and the laser (2) emits a pulsed laser beam. The pulsed laser beam passes through the first double concave lens (25) and the plano-convex lens (26) in sequence to amplify the laser spot. The amplified laser spot passes through the first laser reflector (4) and the dichroic mirror (9) in sequence, and is emitted to the working secondary mirror (24) through the central circular hole of the primary mirror (5). The laser spot is reflected by the convex surface of the working secondary mirror (24) to the concave surface of the primary mirror (5), and is reflected by the concave surface of the primary mirror (5) to converge to the object to be tested (7). When the energy density of the converged laser reaches the material threshold of the object to be tested (7), plasma is excited on the surface of the object to be tested (7), and the plasma emits signal light B outward during the cooling and expansion process. 5) The signal light B is converged by the concave surface of the primary mirror (5) to the working secondary mirror (24), and after being reflected by the working secondary mirror (24), it passes through the central circular hole of the primary mirror (5) and is emitted to the dichroic mirror (9). After a portion of the signal light B passes through the dichroic mirror (9), it passes through the second biconcave lens (27), the first biconvex lens (28), the second biconvex lens (29) and the negative meniscus lens (30) in sequence, and enters the spectrometer (11). The spectrometer (11) analyzes the received portion of the signal light B in conjunction with the computer (1), and the computer (1) measures the material composition and content of the object to be tested (7) through the LIBS quantitative analysis algorithm.

2. The laser induced breakdown spectroscopy remote detection method according to claim 1, characterized in that: After the measurement is completed, the computer (1) starts the first motor (19) to drive the guide rail (14) to move in the reverse direction, so that the rotating arm (22) returns to the original position before the linear movement. Then, the computer (1) starts the second motor (23) to drive the guide rail (14) to rotate in the reverse direction, so that the selected rotating arm rotates 180 degrees in the reverse direction, and the working secondary mirror (24) returns to the initial position. After that, the computer (1) sends an electrical signal, and the control circuit cuts off the power to the selected rotating arm (22), and the rotating arm (22) returns to the separated state.

3. The laser induced breakdown spectroscopy remote detection method according to claim 2, characterized in that: A portion of the signal light B that passes through the dichroic mirror (9) is visible light with a wavelength of 200 nm to 800 nm.

4. The laser induced breakdown spectroscopy remote detection method according to claim 1, characterized in that: A limit rod (17) and a rotating arm support rod (21) are fixedly connected to the outer wall of the driven rotor of the electromagnetic clutch, and the angle between the limit rod (17) and the rotating arm support rod (21) is 90 degrees; the active rotor of the electromagnetic clutch is sleeved on the guide rail (14).

5. The laser induced breakdown spectroscopy remote detection method according to claim 1, characterized in that: The second motor (23) is connected to the guide rail (14) via a spline connection; the length of the inner spline and the length of the outer spline in the spline connection are different.

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