Laser gas telemetering system capable of eliminating influence of distance change

Abstract

The invention provides a laser gas telemetering system capable of eliminating the influence of distance change, and aims to solve the problem of inaccurate concentration detection caused by different intensity and receiving quantity of optical signals received by a detector when the telemetering system is in different telemetering distances. Optical signals of different telemetering distances are converted into electric signals meeting detection requirements by automatically regulating and controlling the photoelectric conversion gain multiple, and the core is a pre-amplification circuit capable of regulating and controlling the gain multiple and an automatic gain control algorithm. A photoelectric conversion chip OPA380 and a multiplexer MAX4638 are selected to form the pre-amplification circuit capable of regulating and controlling the gain multiple, and the pre-amplification circuit is provided with eight different gain regulation and control gears. According to the automatic gain control algorithm, detection signals are identified and judged according to the voltage amplitude after photoelectric conversion, and the gain multiple of the pre-amplification circuit is regulated and controlled in real time. According to the invention, the detection accuracy can be improved, and guidance is provided for field application of the mobile telemetering system.

Description

Technical Field

[0001] The invention relates to a laser gas telemetry system for eliminating the influence of distance variation, belonging to the technical field of laser gas telemetry. Background Art

[0002] The working principle of the laser gas telemetry system is based on the open optical path tunable semiconductor laser absorption spectroscopy technology (OP-TDLAS), which uses a semiconductor laser to scan the wavelength of the characteristic absorption peak of the gas to be measured. The laser emits a laser beam of the corresponding wavelength, which is incident into the atmosphere after being collimated. When it encounters the target object, the diffusely reflected light is received by the telemetry lens, and is collected by the detector after being focused at the focal plane by the condenser. The electrical signal of the detection light signal after photoelectric conversion is the harmonic signal of the gas to be measured. By analyzing the harmonic signal, the concentration of the gas to be measured can be inverted. The laser gas telemetry system does not require on-site sampling and has the advantages of non-contact measurement, long telemetry distance, and fast response speed. It is often used to detect air pollution, especially in natural gas stations and gas pipelines where gas leaks need to be detected. It has a wide range of application needs.

[0003] However, in the actual gas detection process, external environmental factors, telemetry distance, and noise generated by the system itself will affect the strength of the echo light signal and the detector's reception of the echo light signal. Among them, the impact of the telemetry distance is the most direct and obvious, which is mainly manifested in the following two aspects:

[0004] (1) The existing laser gas telemetry system generally fixes the detector at the focal plane of the condenser. When the telemetry distance is far, the diffuse reflected light can be approximately regarded as incident on the condenser in the form of parallel light, and can be converged at its focal plane after passing through the condenser. At this time, the detector can receive the maximum light signal, ensuring that the laser gas telemetry system works normally. However, when the telemetry distance is short, the diffuse reflected light is incident on the condenser in a divergent form, and converges at a point behind its focal plane after passing through the condenser. At this time, the detector can only receive a small part of the light signal, making the amplitude of the harmonic signal after photoelectric conversion very low, and sometimes even unable to detect the harmonic signal. Similarly, if the detector is fixed at a point behind the focal plane of the condenser, when the telemetry distance is short, the detector can receive the maximum light signal; when the telemetry distance is long, the detector can only receive a small part of the light signal. Therefore, when the detector position is fixed, the telemetry distance will directly affect the detector's reception of the echo light signal, affecting the accuracy of the laser gas telemetry system in detecting gas.

[0005] (2) Different background targets have different diffuse reflectance capabilities. For example, cement roads, soil, window glass, metal building materials, etc. have different diffuse reflectances for light signals of different wavelengths. The higher the diffuse reflectance, the longer the telemetry distance. When the telemetry distance is long and encounters a target with a low diffuse reflectance, the light signal received by the detector is very weak, and sometimes the real gas concentration information will be submerged by noise or exceed the detection capability of the detector; when the telemetry distance is short and encounters a target with a strong diffuse reflectance, the light signal received by the detector is very strong, resulting in saturation distortion of the converted electrical signal and abnormal operation of the laser gas telemetry system. Therefore, when the target is different, the distance of the telemetry will affect the strength of the diffuse reflection light signal and the working stability of the laser gas telemetry system.

[0006] In order to solve the above problems, people have been seeking an ideal technical solution. Summary of the invention

[0007] The technical problem solved by the present invention is: to overcome the shortcomings of the prior art, to provide a laser gas telemetry system that eliminates the influence of distance changes, to eliminate the influence of distance changes on the laser gas telemetry system, and to solve the problem of inaccurate concentration detection caused by different strengths and amounts of light signals received by the detector at different telemetry distances.

[0008] The technical solution of the present invention is:

[0009] A laser gas telemetry system for eliminating the influence of distance change, comprising: a main control unit, a laser drive module, a programmable gain detection module, a laser, a detector, a laser sight, a laser collimator and a condenser;

[0010] The laser driving module emits a laser beam of a specified wavelength under the control of the main control unit. After passing through the laser collimator, the laser beam is emitted in parallel into the air, passes through the gas plume to be measured and irradiates the target object. The laser beam reflected by the target object passes through the gas plume to be measured again and then passes through the condenser to converge the diffuse reflected light signal onto the focal plane of the detector; the programmable gain detection module converts the optical signal output by the detector into an electrical signal. The main control unit collects the electrical signal after photoelectric conversion and controls the programmable gain detection module according to the automatic gain control algorithm to realize real-time regulation of the photoelectric conversion gain multiple.

[0011] Furthermore, the programmable gain detection mode is implemented by a photoelectric conversion preamplifier circuit capable of adjusting the gain multiple, and the photoelectric conversion preamplifier circuit includes an operational amplifier, a multiplexer and a voltage follower;

[0012] The operational amplifier realizes the conversion of optical signals into electrical signals and realizes the amplification function of electrical signals at the same time; the detector is directly connected to the negative input terminal of the operational amplifier, and the optical signal received by the detector is converted into a voltage signal that is phase-locked and demodulated through the operational amplifier; the output terminal of the operational amplifier is connected to the negative input terminal through a capacitor and a high load resistor to form a negative feedback amplifier circuit to realize the amplification of the voltage signal;

[0013] The multiplexer selects and sets the capacitor and high load resistor on the negative feedback branch of the operational amplifier, thereby realizing the regulation of the photoelectric conversion gain multiple; the input end and the output end of the multiplexer are respectively connected to the output end and the negative input end of the operational amplifier, and the capacitor and the high load resistor with a specific gain multiple are connected to the negative feedback branch of the operational amplifier according to the selection of the main control unit; the selection address input end of the multiplexer is connected to the main control unit, and the specific gain multiple is selected by the main control unit;

[0014] The voltage follower converts the high-impedance voltage signal after photoelectric conversion into a low-impedance output voltage signal to be collected by the main control unit, and its negative feedback function is used to improve the stability of the voltage signal; the positive input terminal of the voltage follower is connected to the output terminal of the operational amplifier, and at the same time, a voltage divider is performed at the positive input terminal of the voltage follower to ensure that the converted voltage signal is effectively collected by the main control unit.

[0015] Furthermore, the model of the operational amplifier is OPA380, the maximum output voltage of the operational amplifier is 4.4V, and the input bias current is 3pA.

[0016] Furthermore, the multiplexer model is MAX4638, which has 8 analog selection channels, a single channel on-resistance is 3.5Ω, and a switching time is 25ns; the 8 analog selection channels correspond to eight different gain control gears, and the gain transresistance values ​​are 2MΩ, 1MΩ, 500kΩ, 100kΩ, 50kΩ, 10kΩ, 5kΩ, and 1kΩ respectively.

[0017] Furthermore, the model of the voltage follower is AD8031.

[0018] Furthermore, the gain transimpedance value and its corresponding output characteristics are shown in Table 1.

[0019] Table 1 Gain transimpedance values ​​and corresponding output characteristics

[0020]

[0021] Furthermore, the main control unit collects the electrical signal after photoelectric conversion, and controls the programmable gain detection module according to the automatic gain control algorithm to achieve real-time regulation of the photoelectric conversion gain multiple, specifically:

[0022] (1) Parameters are set on the host computer. The main control unit of the laser gas telemetry system receives and interprets the gain control instructions of the host computer and collects the voltage amplitude of the harmonic signal after photoelectric conversion;

[0023] (2) According to the gain control instruction of the host computer, the converted voltage amplitude is judged in real time. When the voltage amplitude is higher than the upper limit of the effective detection range, the gain multiple needs to be reduced by one level; when the voltage amplitude is lower than the lower limit of the effective detection range, the gain multiple needs to be increased by one level;

[0024] When the gain multiple is adjusted to the highest or lowest level, the voltage amplitude still cannot reach the effective detection range, which means that the gain multiple setting does not conform to the actual telemetry situation, or the diffuse reflection signal received by the laser gas telemetry system is too little and the light intensity is too weak. It is necessary to adjust the telemetry direction, telemetry distance and target reflector of the telemetry system in time.

[0025] (3) When the voltage amplitude is within the effective detection range, it indicates that the optical signal received by the detector is in the optimal state. The main control unit performs phase-locked demodulation on the converted harmonic signal to obtain the second harmonic signal, and then performs low-pass filtering on the second harmonic signal and solves the concentration value of the gas to be measured through the concentration inversion algorithm.

[0026] Furthermore, the voltage amplitude of the harmonic signal after the photoelectric conversion is collected by an ADC analog-to-digital conversion circuit inside the ARM chip in the main control unit.

[0027] Further, the gain control instruction is divided into a manual control instruction and an automatic control instruction, wherein the manual control instruction is to manually select the gain multiple in the host computer, which is divided into eight different gain control gears, namely 8 gears, 7 gears, 6 gears, 5 gears, 4 gears, 3 gears, 2 gears, and 1 gear; the corresponding gain transimpedance values ​​are 2MΩ, 1MΩ, 500kΩ, 100kΩ, 50kΩ, 10kΩ, 5kΩ, and 1kΩ respectively;

[0028] The automatic control command automatically controls the gain multiple for the main control unit. After receiving the command, the laser gas telemetry system runs the automatic gain control algorithm to adjust the gain.

[0029] The beneficial effects of the present invention compared with the prior art are:

[0030] (1) The present invention designs a photoelectric conversion preamplifier circuit capable of adjusting the gain multiple, which eliminates the influence of distance change on the laser gas telemetry system to a certain extent, provides convenience for the adjustment and test of the telemetry system, increases the telemetry distance range, and improves the detection accuracy of the telemetry system;

[0031] (2) The automatic gain control algorithm proposed in the present invention realizes real-time regulation of the gain multiple of the photoelectric conversion preamplifier circuit, provides guidance for the on-site application of mobile telemetry systems such as airborne telemetry systems and handheld telemetry systems, and improves the detection stability of the telemetry system. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 The schematic diagram of the preamplifier circuit with adjustable gain multiple in the present invention;

[0033] Figure 2 This is a principle block diagram of the adaptive laser gas telemetry system of the present invention;

[0034] Figure 3 is a flow chart of the automatic gain control algorithm in the present invention;

[0035] Figure 4 It is the second harmonic signal obtained by phase-locked demodulation of the laser gas telemetry system in the present invention. DETAILED DESCRIPTION

[0036] The specific embodiments of the present invention are further described in detail below with reference to the accompanying drawings.

[0037] According to the working principle of the laser gas telemetry system, the optical signal received by the detector is usually weak, and the signal needs to be amplified when it is converted into an electrical signal. However, the strength and amount of the received optical signal at different telemetry distances are different, resulting in a large difference in the gain multiple required for optical signal conversion. Therefore, in order to eliminate the impact of distance changes on the laser gas telemetry system, it is necessary to adjust the gain multiple of the optical signal conversion in real time according to the working conditions of the telemetry system.

[0038] The present invention provides a laser gas telemetry system that eliminates the influence of distance changes. It mainly performs preliminary identification and judgment based on the reception amount of the detector and the strength of the light signal under different telemetry distances, and then converts the light signal into an electrical signal that meets the detection requirements by adjusting the photoelectric conversion gain multiple. Finally, the concentration of the converted electrical signal is inverted to obtain the concentration value to be measured.

[0039] In this case, if Figure 2 As shown, the present invention proposes a laser gas telemetry system for eliminating the influence of distance change, which is characterized by comprising: a main control unit, a laser driving module, a programmable gain detection module, a laser, a detector, a laser sight, a laser collimator and a condenser;

[0040] The laser driver module emits a laser beam of a specified wavelength under the control of the main control unit. After passing through the laser collimator, the laser beam is emitted parallel to the air, passes through the gas plume to be measured, and irradiates the target object. The laser beam reflected by the target object passes through the gas plume to be measured again, and then passes through the condenser to converge the diffuse reflection light signal to the focal plane of the detector. The programmable gain detection module converts the optical signal output by the detector into an electrical signal. The main control unit collects the electrical signal after photoelectric conversion, and controls the programmable gain detection module according to the automatic gain control algorithm to achieve real-time regulation of the photoelectric conversion gain multiple.

[0041] The invention provides a laser gas telemetry system that eliminates the influence of distance changes. The core is to realize automatic regulation of the photoelectric conversion gain multiple. Therefore, the invention designs the following Figure 1 The adjustable gain preamplifier circuit shown in FIG. Figure 3 The automatic gain control algorithm shown is applied to the existing laser gas telemetry system.

[0042] The programmable gain detection mode is implemented by a photoelectric conversion preamplifier circuit that can adjust the gain multiple. The schematic diagram of the preamplifier circuit is as follows: Figure 1 The photoelectric conversion preamplifier circuit includes an operational amplifier, a multiplexer and a voltage follower.

[0043] The present invention adopts an operational amplifier of model OPA380 to realize the conversion of optical signal to electrical signal, and realizes the amplification function of electrical signal. According to the chip manual, the maximum output voltage of the operational amplifier is 4.4V, and the input bias current is 3pA, which is 60 times smaller than the minimum input detection current of the commonly used detector, and has the advantages of small distortion, low noise, high precision, etc., which can meet the detection requirements. The detector is directly connected to the negative input terminal (FA) of the operational amplifier, and the optical signal received by it can be converted into a voltage signal that can be phase-locked demodulated by the operational amplifier. The output terminal (FB) of the operational amplifier is connected to the negative input terminal (FA) through the capacitor and high load resistor in the multiplexer to form a negative feedback amplifier circuit, so as to realize the amplification of the voltage signal.

[0044] The present invention adopts a multiplexer of model MAX4638 to set the capacitance and high load resistance on the feedback branch of OPA380 transport amplifier, so as to realize the regulation of the gain multiple of photoelectric conversion. According to the chip manual, the multiplexer has 8 analog gating channels, the on-resistance of a single channel is 3.5Ω, and the switching time is 25ns, which can meet the detection requirements. The input end (FB) and the output end (FA) of the multiplexer are respectively connected to the output end (FB) and the input negative end (FA) of the operational amplifier, and the capacitance and high load resistance of the specific gain multiple can be connected to the negative feedback branch of the operational amplifier according to the gating of the main control unit. The gating address input end (A0, A1, A2) of the multiplexer is connected to the main control unit, and the specific gain multiple is selected by the gating of the main control unit. The present invention sets eight different gain control gears of 2MΩ, 1MΩ, 500kΩ, 100kΩ, 50kΩ, 10kΩ, 5kΩ, and 1kΩ. The gain transimpedance value and its corresponding output characteristics are shown in Table 1. The gain is reasonably selected according to the detected optical signal to obtain a harmonic signal that meets the detection requirements.

[0045] Table 1 Gain transimpedance values ​​and corresponding output characteristics

[0046]

[0047]

[0048] The present invention adopts a voltage follower of model AD8031 to realize the conversion of the high impedance voltage signal after photoelectric conversion into a low impedance output voltage signal to be collected by the main control unit, and its negative feedback effect can improve the stability of the voltage signal. The positive input terminal (PD_IN) of the voltage follower is connected to the output terminal (PD_IN) of the operational amplifier, and a voltage division process is performed at its positive input terminal to ensure that the converted voltage signal can be effectively collected by the main control unit.

[0049] In order to meet the real-time control of gain in the first aspect, the present invention proposes an automatic gain control algorithm, and at the same time adds a gain control instruction in the host computer. The automatic gain control algorithm flow chart is as follows: Figure 3 shown.

[0050] The main control unit collects the electrical signal after photoelectric conversion, and controls the programmable gain detection module according to the automatic gain control algorithm to achieve real-time regulation of the photoelectric conversion gain multiple, specifically:

[0051] (1) First, parameters need to be set on the host computer. The main control unit of the laser gas telemetry system receives and interprets the gain control instructions of the host computer and collects the voltage amplitude of the harmonic signal after photoelectric conversion. The voltage amplitude is collected by the ADC analog-to-digital conversion circuit inside the ARM chip in the main control unit.

[0052] (2) Then, the converted voltage amplitude is judged in real time according to the gain control instruction of the host computer. When the voltage amplitude is higher than the upper limit of the effective detection range, the gain multiple needs to be reduced by one level;

[0053] When the voltage amplitude is lower than the lower limit of the effective detection range, the gain multiple needs to be increased by one level.

[0054] When the gain multiple is adjusted to the highest or lowest level, the voltage amplitude still cannot reach the effective detection range, which means that the gain multiple setting does not conform to the actual telemetry situation, or the laser gas telemetry system receives too few diffuse reflection signals or the light intensity is too weak. It is necessary to adjust the telemetry direction, telemetry distance, target reflector, etc. of the telemetry system in time.

[0055] (3) Finally, when the voltage amplitude is within the effective detection range, it indicates that the optical signal received by the detector is in the optimal state. The main control unit will perform phase-locked demodulation on the converted harmonic signal to obtain the second harmonic signal, and then perform low-pass filtering on the second harmonic signal and solve the concentration value of the gas to be measured through the concentration inversion algorithm.

[0056] The gain control instructions in the host computer are mainly divided into manual control instructions and automatic control instructions. The manual control instructions are to manually select the gain multiples in the host computer, which are divided into eight different gain control gears: 8 gears (2MΩ), 7 gears (1MΩ), 6 gears (500kΩ), 5 gears (100kΩ), 4 gears (50kΩ), 3 gears (10kΩ), 2 gears (5kΩ), and 1 gear (1kΩ); the automatic control instructions are for the main control unit to automatically control the gain multiples. After receiving the instructions, the laser gas telemetry system will run the automatic gain control algorithm to perform gain regulation.

[0057] Example:

[0058] Based on the above detailed introduction to the preamplifier circuit with adjustable gain multiples and the automatic gain control algorithm, the present invention designs a programmable gain detection module and uses the detection module to build an adaptive laser gas telemetry system to verify the effectiveness of a method provided by the present invention for eliminating the influence of distance changes on the laser gas telemetry system.

[0059] like Figure 2The figure shows the principle block diagram of the adaptive laser gas telemetry system, wherein ① is a power supply module, which is used to provide the telemetry system with three voltages of +12V, analog ±5V, and digital +5V; ② is a programmable gain detection module designed by the present invention, the hardware part of which is a preamplifier circuit with adjustable gain multiples, and the software part is an automatic gain control algorithm; ③ is a detector, which is used to receive diffuse reflection light signals; ④ is an aperture diaphragm, which is used to control the intensity of the light signal entering the detector; ⑤ is a condenser, which is used to converge the diffuse reflection light signal onto the focal plane; ⑥ is a green light indicating laser sight, which is used to assist in determining the irradiation range of the laser; ⑦ is a laser collimator, which is used to emit the laser beam emitted by the laser into the air in parallel; ⑧ is a telemetry lens, in which all the optical path designs of the telemetry system are concentrated; ⑨ is a gas plume to be measured, and the laser beam emitted by the telemetry system needs to pass through the gas plume to be measured in order to detect the concentration of the gas to be measured; ⑩ is a target object, which can reflect the laser beam irradiated on its surface; is the leak point, which expresses the origin of the gas plume to be measured; The laser beam is emitted by the laser and then passes through the laser collimator and then is emitted in parallel into the air to detect the concentration of the gas to be measured. Diffuse reflection beam, the laser beam is diffusely reflected back after encountering the target object. This beam carries the concentration information of the gas to be measured and can be collected by the detector after being converged by the condenser. A laser driving module, used to drive the laser to emit an optical signal of a specified wavelength; As the main control unit, it has the functions of controlling the laser drive module, controlling the programmable gain detection module, phase-locked demodulation, low-pass filtering, concentration inversion, human-computer interaction, etc. It is the control core of the entire telemetry system.

[0060] The adaptive laser gas telemetry system constructed by the present invention is used to detect methane gas. The methane absorption peak selected in the present invention is 1653.7nm. The specific operation steps are as follows:

[0061] (1) Set the laser drive parameters to ensure that the central wavelength of the laser output is the absorption peak of methane gas. The laser drive module can control the central wavelength of the laser output by controlling the TEC temperature and operating current. Adjust the module parameters to make the laser output wavelength 1653.7nm, and use a spectrum analyzer to detect the laser output wavelength.

[0062] (2) Adjust the optical path to ensure that the diffuse light signal can be received by the detector. First, fix the adaptive laser gas telemetry system, adjust the aperture diaphragm to the maximum, and place the target object 1m away from the telemetry lens. The target object is a cement board. Then, connect visible light to the input end of the laser collimator and use the visible light to adjust the direction of the diffuse light signal to ensure that there is still light signal entering the detector when the aperture diaphragm is adjusted to the minimum. At the same time, adjust the green light indicating the direction of the laser and lock it. Finally, reconnect the input end of the laser collimator to the telemetry system.

[0063] (3) Set the program-controlled gain multiple to ensure that the detection signal meets the requirements of phase-locked demodulation. First, adjust the aperture diaphragm to the maximum and use an oscilloscope to check the amplitude of the harmonic signal after photoelectric conversion; then manually adjust the gain multiple in the host computer and determine whether the gain setting meets the detection requirements based on the amplitude of the harmonic signal; finally, set the program-controlled gain to automatic control.

[0064] (4) Methane gas concentration detection. Place a gas bag filled with 500ppm methane standard gas on the telemetry path to replace the gas plume to be tested. By adjusting the aperture size, the amount of light signal received by the detector can be controlled to simulate the change of telemetry distance. Figure 4 The second harmonic signal shown in the figure, where the black solid line is the second harmonic signal when the gain multiple is 1k and the aperture is 2mm, the black dotted line is the second harmonic signal when the gain multiple is 1k and the aperture is 1mm, and the gray dotted line is the second harmonic signal when the gain multiple is 5k and the aperture is 1mm. By comparing the amplitude of the second harmonic signal, it can be seen that when the concentration of the gas to be measured is constant, the smaller the aperture of the aperture, the less light signal the detector module receives, the lower the amplitude of the second harmonic signal, the lower the inverted concentration value of the gas to be measured, and the inaccurate test result. By adjusting the gain multiple, the second harmonic signal can be adjusted to the appropriate concentration inversion requirement range, thereby improving the detection accuracy of the laser gas telemetry system.

[0065] The present invention has been described in detail above in conjunction with specific implementation methods and exemplary examples, but these descriptions cannot be understood as limiting the present invention. Those skilled in the art may make various equivalent substitutions, modifications or improvements to the technical solutions and implementation methods of the present invention without departing from the spirit and scope of the present invention, and these all fall within the scope of the present invention.

[0066] The contents not described in detail in the specification of the present invention belong to the common knowledge of those skilled in the art.

Claims

1. A laser gas telemetry system that eliminates the influence of distance changes, characterized in that include: Main control unit, laser drive module, programmable gain detection module, laser, detector, laser aimer, laser collimator and condenser; The laser driving module emits a laser beam of a specified wavelength under the control of the main control unit. After passing through the laser collimator, the laser beam is emitted in parallel into the air, passes through the gas plume to be measured and irradiates the target object. The laser beam reflected by the target object passes through the gas plume to be measured again and then passes through the condenser to converge the diffuse reflected light signal onto the focal plane of the detector; the programmable gain detection module converts the optical signal output by the detector into an electrical signal. The main control unit collects the electrical signal after photoelectric conversion and controls the programmable gain detection module according to the automatic gain control algorithm to realize real-time regulation of the photoelectric conversion gain multiple.

2. A laser gas telemetry system for eliminating the influence of distance variation according to claim 1, characterized in that: The programmable gain detection mode is implemented by a photoelectric conversion preamplifier circuit capable of adjusting the gain multiple, and the photoelectric conversion preamplifier circuit includes an operational amplifier, a multiplexer and a voltage follower; The operational amplifier realizes the conversion of optical signals into electrical signals and realizes the amplification function of electrical signals at the same time; the detector is directly connected to the negative input terminal of the operational amplifier, and the optical signal received by the detector is converted into a voltage signal that is phase-locked and demodulated through the operational amplifier; the output terminal of the operational amplifier is connected to the negative input terminal through a capacitor and a high load resistor to form a negative feedback amplifier circuit to realize the amplification of the voltage signal; The multiplexer selects and sets the capacitor and high load resistor on the negative feedback branch of the operational amplifier, thereby realizing the regulation of the photoelectric conversion gain multiple; the input end and the output end of the multiplexer are respectively connected to the output end and the negative input end of the operational amplifier, and the capacitor and the high load resistor with a specific gain multiple are connected to the negative feedback branch of the operational amplifier according to the selection of the main control unit; the selection address input end of the multiplexer is connected to the main control unit, and the specific gain multiple is selected by the main control unit; The voltage follower converts the high-impedance voltage signal after photoelectric conversion into a low-impedance output voltage signal to be collected by the main control unit, and its negative feedback function is used to improve the stability of the voltage signal; the positive input terminal of the voltage follower is connected to the output terminal of the operational amplifier, and at the same time, a voltage divider is performed at the positive input terminal of the voltage follower to ensure that the converted voltage signal is effectively collected by the main control unit.

3. A laser gas telemetry system for eliminating the influence of distance variation according to claim 2, characterized in that: The model of the operational amplifier is OPA380, which has a maximum output voltage of 4.4V and an input bias current of 3pA.

4. A laser gas telemetry system for eliminating the influence of distance variation according to claim 2, characterized in that: The multiplexer model is MAX4638, which has 8 analog selection channels. The on-resistance of a single channel is 3.5Ω and the switching time is 25ns. The 8 analog selection channels correspond to eight different gain control gears, and the gain transresistance values ​​are 2MΩ, 1MΩ, 500kΩ, 100kΩ, 50kΩ, 10kΩ, 5kΩ, and 1kΩ respectively.

5. A laser gas telemetry system for eliminating the influence of distance variation according to claim 2, characterized in that: The model of the voltage follower is AD8031.

6. A laser gas telemetry system for eliminating the influence of distance variation according to claim 4, characterized in that: The gain transimpedance value and its corresponding output characteristics are shown in Table 1. Table 1 Gain transimpedance values ​​and corresponding output characteristics 7. A laser gas telemetry system for eliminating the influence of distance variation according to claim 1, characterized in that: The main control unit collects the electrical signal after photoelectric conversion, and controls the programmable gain detection module according to the automatic gain control algorithm to achieve real-time regulation of the photoelectric conversion gain multiple, specifically: (1) Parameters are set on the host computer. The main control unit of the laser gas telemetry system receives and interprets the gain control instructions of the host computer and collects the voltage amplitude of the harmonic signal after photoelectric conversion; (2) According to the gain control instruction of the host computer, the converted voltage amplitude is judged in real time. When the voltage amplitude is higher than the upper limit of the effective detection range, the gain multiple needs to be reduced by one level; when the voltage amplitude is lower than the lower limit of the effective detection range, the gain multiple needs to be increased by one level; When the gain multiple is adjusted to the highest or lowest level, the voltage amplitude still cannot reach the effective detection range, which means that the gain multiple setting does not conform to the actual telemetry situation, or the diffuse reflection signal received by the laser gas telemetry system is too little and the light intensity is too weak. It is necessary to adjust the telemetry direction, telemetry distance and target reflector of the telemetry system in time. (3) When the voltage amplitude is within the effective detection range, it indicates that the optical signal received by the detector is in the optimal state. The main control unit performs phase-locked demodulation on the converted harmonic signal to obtain the second harmonic signal, and then performs low-pass filtering on the second harmonic signal and solves the concentration value of the gas to be measured through the concentration inversion algorithm.

8. A laser gas telemetry system for eliminating the influence of distance variation according to claim 7, characterized in that: The voltage amplitude of the harmonic signal after the photoelectric conversion is collected by the ADC analog-to-digital conversion circuit inside the ARM chip in the main control unit.

9. A laser gas telemetry system for eliminating the influence of distance variation according to claim 7, characterized in that: The gain control instructions are divided into manual control instructions and automatic control instructions, wherein the manual control instructions are to manually select the gain multiple in the host computer, which are divided into eight different gain control gears, namely 8 gears, 7 gears, 6 gears, 5 gears, 4 gears, 3 gears, 2 gears, and 1 gear; The corresponding gain transimpedance values ​​are 2MΩ, 1MΩ, 500kΩ, 100kΩ, 50kΩ, 10kΩ, 5kΩ, and 1kΩ respectively; The automatic control command automatically controls the gain multiple for the main control unit. After receiving the command, the laser gas telemetry system runs the automatic gain control algorithm to adjust the gain.

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