A method for measuring the particle size of air particles using time-of-flight method

Abstract

The invention discloses a method for measuring the particle size of air particles by using a time-of-flight method. A polarized laser beam emitted by a fiber polarized laser passes through a plano-convex cylindrical mirror after being split by a Wollaston prism. The entrance pupil of the plano-convex cylindrical mirror is overlapped with the exit surface of the Wollaston prism to generate two light curtains of the same thickness parallel to the main light. Aspheric reflectors and avalanche diode photodetectors are respectively arranged on both sides of the light curtain. The particles move in a single direction and continuously pass through the two light curtains to generate scattering. The aspheric reflectors collect the scattered light and collect the scattered light to the avalanche diode photodetector. The advantages of the invention are: the invention has a simple structure, uses a small number of optical devices, has a simple later mechanical structure design, is easy to adjust the optical path, and has high measurement accuracy. The most important thing is that there is no optical element behind the condenser, the focusing aberration is small, and a light curtain with a thickness reaching the diffraction limit can be formed.

Description

Technical Field

[0001] The invention patent relates to the field of air particle testing, specifically an optical system for air particle testing. Background Art

[0002] At present, the methods for measuring air particulate matter are divided into: weighing method, light obstruction method, angle scattering method, beta ray method, oscillating balance method and time of flight method, among which the light obstruction method and angle scattering method are optical principles, which are simple in principle and easy to implement. At present, the measurement range of the scattering method of similar instruments at home and abroad is about 0.5-20μm, but the scattering method is interfered by the refractive index and Mie scattering, and the measurement results have a certain deviation from the actual results. The time of flight method can be used to test the aerodynamic diameter of particles. The aerodynamic diameter refers to the unit density of particles with the same sedimentation velocity in the air (1g / cm 3 ) is the diameter of a spherical particle assumed when measuring the particle size, shape, and density. It is independent of the particle's geometric size, shape, and density. Because the aerodynamic diameter is most closely related to the ability of a particle to penetrate the respiratory tract and to settle, it is used to represent the size of the particle. The sedimentation velocity of a particle that penetrates the alveoli must be less than 0.003 m / s, which is equivalent to the sedimentation velocity of a unit density particle with a diameter of 7 μm. Particles with an aerodynamic diameter of more than 10 μm generally cannot pass through the nasopharynx. Although particles larger than 50 μm can reach the mouth and nose with breathing, they cannot be inhaled. In addition, such large particles will not stay in the air for too long. Most suspended dust in the air is irregular in shape and can aggregate. The process of aggregating particles depends on aerodynamic properties, not on particle size. A coal dust particle may be more than 15 μm in diameter, but it may have the same sedimentation velocity as a spherical particle with an aerodynamic diameter of 7 μm. Therefore, the aerodynamic diameter of a particle is the most important in its evaluation. In the past, the optical system structure of the time-of-flight method for testing air particles was complex; many optical devices were used and the cost was high; there were calcite and concave cylindrical mirrors behind the spherical condenser, which caused spherical aberration and made it impossible to obtain a light curtain with a thickness that reached the diffraction limit. Summary of the invention

[0003] The purpose of the present invention is to provide a method for measuring the particle size of air particles by time-of-flight method. The polarized laser beam emitted by the optical fiber polarization laser is split by a Wollaston prism and then passes through a plano-convex cylindrical mirror. The entrance pupil of the plano-convex cylindrical mirror is overlapped with the exit surface of the Wollaston prism to generate two light curtains of the same thickness parallel to the main light. Aspheric reflectors and avalanche diode photodetectors are respectively arranged on both sides of the light curtains. The particles move in a single direction and continuously pass through the two light curtains, thereby causing scattering. The aspheric reflector collects the scattered light and gathers the scattered light to the avalanche diode photodetector. The avalanche diode photodetector detects two consecutive scattered light pulses. The height of the scattered light pulse is proportional to the particle scattering diameter. The interval between two consecutive scattered light pulses is the movement time of the particle between the two light curtains. The avalanche diode photodetector collects the peak-to-peak value of the scattered light pulse height and converts it through an AD (analog-to-digital converter). The microcontroller connected to the AD (analog-to-digital converter) converts the processed scattered light pulse height signal into a voltage value and the flight time of the particle between the two light curtains and uploads it to the PC. The PC compares it with the calibration curve to obtain the scattering and kinetic particle size values ​​of the particles.

[0004] It also includes a 45-degree reflector directly facing the light curtain, and a light trap is arranged on the reflective surface of the 45-degree reflector.

[0005] The line between the focus of the aspheric mirror and the avalanche diode photodetector is perpendicular to the light curtain.

[0006] The height between the two light curtains is H = + / -f*tg(θ); where f is the focal length of the plano-convex cylindrical mirror, and θ is the beam separation half angle of the Wollaston prism.

[0007] The advantages of the present invention are: the present invention has a simple structure, uses a small number of optical devices, has a simple mechanical structure design, is easy to adjust the optical path, and has high measurement accuracy. Most importantly, there is no optical element behind the condenser, the focusing aberration is small, and a light curtain with a thickness reaching the diffraction limit can be formed. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 It is a schematic diagram of the three-dimensional structure of the present invention;

[0009] Figure 2 It is a schematic diagram of the XZ plane of the present invention;

[0010] Figure 3 It is a YZ plane schematic diagram of the present invention;

[0011] Figure 4 for Figure 1 A top view of

[0012] In the figure, 1. Fiber polarization laser, 2. Wollaston prism, 3. Plano-convex cylindrical mirror, 4. Light trap, 5. 45-degree reflector, 6. Aspheric reflector, 7. Avalanche diode photodetector, 8. Particles, 9. Light curtain. DETAILED DESCRIPTION

[0013] The present invention is described in detail below in conjunction with the accompanying drawings. As shown in the figure, the polarized laser beam emitted by the optical fiber polarization laser 1 is split by the Wollaston prism 2 and then passes through the plano-convex cylindrical mirror 3, so that the entrance pupil of the plano-convex cylindrical mirror coincides with the exit surface of the Wollaston prism to generate two light curtains 9 parallel to the main light; an aspheric reflector 6 and an avalanche diode photodetector 7 are respectively arranged on both sides of the light curtain, and particles 8 move in a single direction and continuously pass through the two light curtains 9 to cause scattering; the aspheric reflector 6 collects the scattered light and gathers the scattered light to the avalanche diode photodetector 7. The avalanche diode photodetector 7 converts the light pulse into a voltage value and uploads it to the PC.

[0014] In order to avoid affecting the detection of scattered light, a 45-degree reflector 5 is also included facing the light curtain, and a light trap 4 is arranged on the reflective surface of the 45-degree reflector.

[0015] Preferably, the line between the focus of the aspherical reflector and the avalanche diode photodetector is perpendicular to the light curtain.

[0016] The height between the two light curtains is H = + / -f*tg(θ); where f is the focal length of the plano-convex cylindrical mirror, and θ is the beam separation half angle of the Wollaston prism.

[0017] The principle of the present invention is that particles move in a single direction and continuously pass through two light curtains to cause scattering, an aspheric reflector collects scattered light, and gathers the scattered light to an avalanche diode photodetector, which detects two consecutive scattered light pulses, the scattered light pulse height is proportional to the particle scattering particle size, and the interval between the two consecutive scattered light pulses is the movement time of the particle between the two light curtains; the avalanche diode photodetector collects the peak-to-peak value of the scattered light pulse height and converts it through an AD (analog-to-digital converter), and a single-chip microcomputer connected to the AD (analog-to-digital converter) converts the processed scattered light pulse height signal into a voltage value and the flight time of the particle between the two light curtains and uploads it to a PC, and the PC compares it with a calibration curve to obtain the scattering and kinetic particle size values ​​of the particles.

[0018] The main beam passes through a 45-degree reflector and enters the light trap, thus avoiding affecting the detection of scattered light.

[0019] The components in the present invention: optical fiber polarization laser, Wollaston prism, plano-convex cylindrical mirror, light trap, 45-degree reflector, aspheric reflector, avalanche diode photodetector, etc., can be commercially available products.

Claims

1. A method for measuring the particle size of air particles using a time-of-flight method, characterized in that: The polarized laser beam emitted by the fiber polarization laser is split by a Wollaston prism and then passes through a plano-convex cylindrical mirror. The entrance pupil of the plano-convex cylindrical mirror is overlapped with the exit surface of the Wollaston prism to generate two light curtains of the same thickness parallel to the main light. Aspheric reflectors and avalanche diode photodetectors are respectively arranged on both sides of the light curtains. The particles move in a single direction and continuously pass through the two light curtains, thereby causing scattering. The aspheric reflector collects the scattered light and gathers the scattered light to the avalanche diode photodetector. The avalanche diode photodetector detects two consecutive scattered light pulses. The height of the scattered light pulse is proportional to the particle scattering particle size. The two consecutive scattered light pulses are directly proportional to the particle scattering particle size. The interval between scattered light pulses is the movement time of the particles between the two light curtains; the avalanche diode photodetector collects the peak-to-peak value of the scattered light pulse height and converts it through AD, and the single-chip microcomputer connected to AD converts the processed scattered light pulse height signal into a voltage value and the flight time of the particles between the two light curtains and uploads it to the PC end, and the PC compares it with the calibration curve to obtain the scattering and dynamic particle size values ​​of the particles; it also includes a 45-degree reflector facing the light curtain, and a light trap is set on the reflective surface of the 45-degree reflector; the focus of the aspheric reflector and the connection line between the avalanche diode photodetector are perpendicular to the light curtain.

2. The method for measuring the particle size of air particles using the time-of-flight method according to claim 1, characterized in that: The height between the two light curtains is H = + / -f*tg (θ); where f is the focal length of the plano-convex cylindrical mirror and θ is the beam separation half angle of the Wollaston prism.

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