A method for controlling the self-gain of the echo signal intensity of an airborne lidar

Abstract

The present invention discloses a method for controlling the self-gain of the echo signal intensity of an airborne lidar, which relates to the field of radar technology. The method includes the steps of: setting a target value and a dynamic range for the peak data received by the FPGA peak detection circuit, and calculating the current ambient light intensity through a plurality of light intensity sensors; switching different control modes according to the set light intensity threshold, and setting a hysteresis zone, and calculating the average value and the change trend of the light intensity by using a sliding window; transmitting the peak data of the echo signal output by the photodetector PMT back to the FPGA through the peak detection circuit. The present invention improves the stability of the photoelectric conversion self-gain system, increases the working range of the self-gain system, can improve the detection distance of the airborne lidar, reduces the deployment conditions of the airborne lidar at the same time, reduces the influence of ambient light, improves the detection efficiency, and is used to stabilize the intensity of the echo signal of the airborne lidar.

Description

Technical Field

[0001] The present invention relates to the field of radar technology, and in particular to a method for controlling the self-gain of an airborne laser radar echo signal intensity. Background Art

[0002] Nowadays, airborne laser radars are required to operate under all-weather lighting conditions. However, the ambient light intensity is different in different time periods, weather conditions, and seasons. If only fixed gain is used, some scenes with too strong or too weak ambient light will not be able to be processed later. Therefore, the automatic gain part of the photoelectric conversion system is extremely demanding. If it is not properly handled, it will be difficult to achieve the goal of deploying airborne laser radars in all weather conditions. The existing photoelectric conversion system automatic gain technology is: first, directly change the control voltage of the photodetector, and judge whether to increase or decrease the control voltage by the intensity information of the current echo signal (using ADC sampling to determine whether it reaches the critical value); second, directly use ADC to read the intensity of the echo signal into the MCU, and use MCU to determine and control whether to change the gain value of the programmable gain amplifier (an operational amplifier used in radio frequency) to control the intensity of the echo signal; third, change the sampling range of the post-stage ADC, and use different levels of ADC sampling intervals to sample the echo signal at different levels of light intensity.

[0003] In the above method, for different types or kinds of photodetectors, the linearity of the control voltage is also different. Secondly, when adjusting the control voltage under the condition of too strong ambient light intensity, since the number of photons returning to the telescope is already small, adjusting the control voltage when the ambient light intensity is strong cannot significantly improve the intensity of the echo signal. The second method is to directly adjust the output signal of the photodetector. Compared with the first method, it solves the problem that the intensity of the echo signal cannot be adjusted by controlling the control voltage of the photodetector when the ambient light intensity is strong. However, when the ambient light intensity is weak and the saturation of the echo signal is caused by too strong laser emission intensity, adjusting the gain of the programmable controller cannot attenuate the saturated signal to the normal state. This is because an operational amplifier is used, and its slew rate and gain-bandwidth product are fixed values, and the signal frequency output by the photoelectric converter is relatively high. Therefore, the adjustment of the gain and output voltage is very limited, reducing the dynamic range. Moreover, too large a gain will also amplify noise factors such as the offset current and voltage of the operational amplifier, resulting in a further decrease in the output signal-to-noise ratio. So it is not suitable for adjusting the gain of the echo signal over a large range. The third method directly changes the voltage range for sampling the echo signal, which can avoid the noise interference caused by the second method. However, if only the sampling range is increased without increasing the sampling resolution, it will cause a greater error in the time point of extracting the peak value, further reducing the detection accuracy of the airborne lidar. If both are improved, it will greatly increase the total power consumption of the system (the power consumption of a 16-bit resolution ADC is generally 35 times higher than that of a 12-bit one), and the sampling frequency of a high-resolution ADC usually decreases, resulting in a still decreasing detection accuracy. Summary of the Invention

[0004] In view of the fact that the intensity of the echo signal of the above-mentioned airborne lidar is affected by factors such as the intensity of the emitted laser, the laser flight distance, and the ambient light intensity, when adjusting the control voltage of the photodetector and directly processing the output signal, the enhancement effect on the echo signal is not obvious and there is a problem of signal distortion. The present invention provides a method for controlling the self-gain of the echo signal intensity of an airborne lidar.

[0005] To solve the above technical problems, the present invention adopts the following technical solutions: A method for controlling the self-gain of the echo signal intensity of an airborne lidar, characterized by including the following steps: S1, setting the target value and dynamic range of the peak data received by the FPGA from the peak detection circuit, and calculating the current ambient light intensity through a plurality of light intensity sensors; S2, switching different control modes according to the set light intensity threshold and setting a hysteresis zone, and calculating the average value and change trend of the light intensity by using a sliding window; S3, transmitting the peak data of the echo signal output by the photodetector PMT back to the FPGA through the peak detection circuit.

[0006] Further, in S1, the FPGA calculates the ambient light intensity by weighting the light intensity values detected by each light intensity sensor.

[0007] Further, in S2, when the FPGA determines that the ambient light intensity is in the low light range based on the light intensity threshold and enters the low light mode, the low light mode includes the following steps: S211, when the FPGA receives that the output value of the peak detection circuit collected by the ADC circuit is too large or too small, it calculates the voltage that needs to be converted by the DAC when using the controllable liquid crystal light valve function through the internal PID algorithm; S212, the DAC outputs the voltage value to the control voltage of the controllable liquid crystal light valve to control the attenuation degree of the return light intensity until the FPGA determines that the voltage value output by the peak detection circuit is within the dynamic range; S213, the FPGA uses DDS technology to regulate the control voltage of the PMT.

[0008] Further, in S2, when the FPGA determines that the ambient light intensity is in the high light range based on the light intensity threshold and enters the high light mode, the high light mode includes the following steps: S221, the FPGA calculates the original intensity of the echo signal by backtracking through the gain value of the programmable gain amplifier PGA at this time; S222, the FPGA recalculates the gain value according to the ratio of the original intensity of the echo signal to the set value of the self-gain system; S223, inputs the gain value to the compilation of the PGA.

[0009] Further, in S2, when the FPGA determines that the ambient light intensity is in the medium light range based on the light intensity threshold and enters the medium light mode, the medium light mode includes the following steps: S231, when the FPGA detects that the output of the peak detection circuit is within the dynamic range, it uses DDS technology to regulate the control voltage of the PMT; S232, if the output value of the peak detection circuit is greater than the target value, it jumps to the low light mode and updates the low light threshold to the value of the ambient light intensity at this time; if the output value of the peak detection circuit is less than the target value, it jumps to the high light mode and updates the high light threshold to the value of the ambient light intensity at this time.

[0010] Further, in S2, when the light intensity value continuously exceeds 110% of the light intensity threshold three times or more from the ambient low light range to the ambient medium light range, keep the control voltage of the controllable liquid crystal light valve unchanged. After preloading the initial value of the PMT control voltage, the control voltage weight of the controllable liquid crystal light valve linearly changes from 100% to 0%, and the control voltage weight of the PMT linearly changes from 0% to 100%:

[0011] ;

[0012] ;

[0013] ;

[0014] In the formula, is the control voltage of the PMT; is the initial value of the voltage increased to the PMT control voltage; is the control voltage of the controllable liquid crystal light valve; is the actual input value of the PMT control voltage; is the actual input value of the controllable liquid crystal light valve control voltage; is the calibration coefficient.

[0015] Further, in S2, when the light intensity value from the ambient light range to the ambient low light range is continuously lower than 90% of the light threshold three or more times, the control voltage of the PMT remains unchanged. After preloading the initial value of the control voltage of the controllable liquid crystal light valve, the control voltage weight linearly changes from 0% to 100%, and the control voltage weight linearly changes from 100% to 0%:

[0016] ;

[0017] ;

[0018] ;

[0019] In the formula, is the initial value of the voltage increased to the controllable liquid crystal light valve control voltage.

[0020] Further, in S2, when the light intensity value from the ambient light range to the ambient high light range is continuously lower than 105% of the light threshold three or more times, the control voltage of the PMT remains unchanged. After preloading the initial value of the PGA gain, the control voltage weight linearly changes from 100% to 0%, and the PGA gain value weight is linearly set from 0% to 100%:

[0021] ;

[0022] ;

[0023] + 1;

[0024] In the formula, is the initial value of the gain increased to the PGA; is the output value of the current peak detection circuit; is the set voltage intensity of the echo signal; is the actual gain value of the PGA.

[0025] Further, in S2, when the light intensity value continuously drops below 95% of the light threshold three or more times from the environmental strong light range to the environmental medium light range, keep the control voltage of the controllable liquid crystal light valve unchanged. After preloading the initial value of the control voltage of the PMT, the control voltage weight of the PMT linearly changes from 0% to 100%, and the PGA gain value weight is linearly set from 100% to 0%:

[0026] ;

[0027] ;

[0028] ;

[0029] In the formula, is the gain of the PMT; is the calibration coefficient; is the actual input value of the PMT control voltage; is the actual gain value of the PGA.

[0030] Further, set abnormal situations, that is, when it is detected three times that the environmental light intensity continuously fluctuates near the boundary of the hysteresis region, the hysteresis interval will be automatically expanded;

[0031] When the change rate of the environmental light intensity is too fast and exceeds the set value of the rate transformation upper limit the hysteresis interval will be temporarily reduced to improve the response speed;

[0032] ;

[0033] In the formula, is the transformation slope; is the th collected light intensity value, with the unit of Lux; is the average light intensity of the recent 10 samplings; is the th sampling time point; is the average time of the recent 10 samplings;

[0034] If it means that the environmental light intensity drops rapidly, and the control parameters of the controllable liquid crystal light valve are initialized in advance, and it is ready to switch to the weak light mode when it exceeds;

[0035] If it means that the environmental light intensity rises rapidly, and the gain value of the PGA is initialized in advance, and it is ready to switch to the strong light mode when it exceeds.

[0036] The beneficial effects of the present invention are as follows: The present invention improves the stability of the photoelectric conversion self-gain system, increases the working range of the self-gain system, can further improve the detection distance of the airborne lidar, reduces the deployment conditions of the airborne lidar at the same time, reduces the influence of ambient light, improves the detection efficiency, and is used to stabilize the intensity of the echo signal of the airborne lidar.

[0037] The present invention can automatically update the light intensity threshold and dynamically adjust the hysteresis interval according to the light intensity change rate, avoid frequent switching between control modes, and by disassembling the self-gain process, uses a controllable liquid crystal light valve to attenuate the return light intensity to attenuate the output of the photodetector PMT. The programmable gain amplifier PGA enhances the output of the photodetector PMT when the ambient light is strong, controls the control voltage of the photodetector PMT as an auxiliary control, and the peak detection circuit is used as a fast feedback method to control the self-gain. It can ensure the stable operation of the airborne lidar all-weather through the cooperation of multiple self-gains. Description of the Drawings

[0038] Figure 1 The schematic diagram of the principle of an embodiment of the present invention is shown.

[0039] Figure 2 The control flow chart of the present invention is shown.

[0040] Figure 3 The schematic diagram of the principle of the controllable liquid crystal light valve is shown.

[0041] Figure 4 The schematic diagram of the principle of the programmable gain amplifier PGA is shown.

[0042] Figure 5 The gain curve of the controllable light valve is shown.

[0043] Figure 6 The gain curve of the PMT control voltage is shown.

[0044] Figure 7 The echo signal during the measurement of the magnification curve of the night-controllable light valve is shown.

[0045] Figure 8 The function image to be fitted after averaging the linear part of the control voltage of the controllable light valve is shown.

[0046] Figure 9 The echo signal during the measurement of the magnification curve of the night-PMT control voltage is shown.

[0047] Figure 10 The function image to be fitted for the linear part of the PMT control voltage is shown.

[0048] Figure 11The figure shows the echo signal during the measurement of the amplification factor curve of the daylight - controllable light valve control voltage.

[0049] Figure 12 The figure shows the echo signal during the measurement of the amplification factor curve of the daylight - PMT control voltage.

[0050] Figure 13 The figure shows the light intensity change curve from 15:40 to 19:37 on a certain day at the experimental site.

[0051] Figure 14 The figure shows the light intensity change curve from 21:22 to 10:03 on a certain day at the experimental site.

[0052] Figure 15 The figure shows the echo signal measured when the system was not started at 10:20 in the morning.

[0053] Figure 16 The figure shows the echo signal measured when the system was started at 10:23 in the morning.

[0054] Figure 17 The figure shows the echo signal measured when the system was not started at 18:40 at night.

[0055] Figure 18 The figure shows the echo signal measured when the system was started at 18:44 at night. Detailed implementation method

[0056] A method for controlling the self - gain of the echo signal intensity of an airborne lidar, characterized by comprising the following steps: S1, setting the target value and dynamic range of the peak data received by the FPGA peak detection circuit, and calculating the current ambient light intensity through a plurality of light intensity sensors; S2, switching different control modes according to the set light intensity threshold, setting a hysteresis zone, and calculating the average value and change trend of the light intensity by adopting a sliding window; S3, transmitting the peak data of the echo signal output by the photodetector PMT back to the FPGA through the peak detection circuit.

[0057] Further, in S1, the FPGA calculates the ambient light intensity by weighting the light intensity values detected by each light intensity sensor.

[0058] Further, in S2, when the FPGA determines that the ambient light intensity is within the low-light range based on the light intensity threshold and enters the low-light mode, the low-light mode includes the following steps: S211, when the FPGA receives that the output value of the peak detection circuit collected by the ADC circuit is too large or too small, it calculates the voltage that needs to be converted by the DAC when using the controllable liquid crystal light valve function through the internal PID algorithm; S212, the DAC outputs the voltage value to the control voltage of the controllable liquid crystal light valve to control the attenuation degree of the return light intensity until the FPGA determines that the voltage value output by the peak detection circuit is within the dynamic range; S213, the FPGA uses DDS technology to regulate the control voltage of the PMT.

[0059] Further, in S2, when the FPGA determines that the ambient light intensity is within the high-light range based on the light intensity threshold and enters the high-light mode, the high-light mode includes the following steps: S221, the FPGA calculates the original intensity of the echo signal by inversely deducing through the gain value of the programmable gain amplifier PGA at this time; S222, the FPGA recalculates the gain value according to the ratio of the original intensity of the echo signal to the set value of the self-gain system; S223, inputs the gain value to the compilation of the PGA.

[0060] Further, in S2, when the FPGA determines that the ambient light intensity is within the medium-light range based on the light intensity threshold and enters the medium-light mode, the medium-light mode includes the following steps: S231, when the FPGA detects that the output of the peak detection circuit is within the dynamic range, it uses DDS technology to regulate the control voltage of the PMT; S232, if the output value of the peak detection circuit is greater than the target value, it jumps to the low-light mode and updates the low-light threshold to the value of the ambient light intensity at this time; if the output value of the peak detection circuit is less than the target value, it jumps to the high-light mode and updates the high-light threshold to the value of the ambient light intensity at this time.

[0061] Further, in S2, when the light intensity value continuously exceeds 110% of the light intensity threshold three times or more from the ambient low-light range to the ambient medium-light range, keep the control voltage of the controllable liquid crystal light valve unchanged. After preloading the initial value of the PMT control voltage, the control voltage weight of the controllable liquid crystal light valve linearly changes from 100% to 0%, and the control voltage weight of the PMT linearly changes from 0% to 100%:

[0062] ;

[0063] ;

[0064] ;

[0065] where is the control voltage of the PMT; is the initial value of the voltage increased for the PMT control voltage; is the control voltage of the controllable liquid crystal light valve; is the actual input value of the PMT control voltage; is the actual input value of the control voltage of the controllable liquid crystal light valve; is the calibration coefficient.

[0066] Furthermore, in S2, when the light intensity value from the ambient light range to the ambient low light range is continuously lower than 90% of the light threshold three times or more, the control voltage of the PMT remains unchanged. After preloading the initial value of the control voltage of the controllable liquid crystal light valve, the weight of the control voltage of the controllable liquid crystal light valve varies linearly from 0% to 100%, and the weight of the control voltage of the PMT varies linearly from 100% to 0%:

[0067] ;

[0068] ;

[0069] ;

[0070] In the formula, is the initial value of the voltage increased for the control voltage of the controllable liquid crystal light valve.

[0071] Furthermore, in S2, when the light intensity value from the ambient light range to the ambient high light range is continuously lower than 105% of the light threshold three times or more, the control voltage of the PMT remains unchanged. After preloading the initial value of the PGA gain, the weight of the control voltage of the PMT varies linearly from 100% to 0%, and the weight of the PGA gain value is linearly set from 0% to 100%:

[0072] ;

[0073] ;

[0074] + 1;

[0075] In the formula, is the initial value of the gain increased for the PGA; is the output value of the current peak detection circuit; is the set voltage intensity of the echo signal; is the actual gain value of the PGA.

[0076] Further, in S2, when the light intensity value continuously drops below 95% of the light intensity threshold three or more times from the environmental high-light range to the environmental medium-light range, the control voltage of the controllable liquid crystal light valve remains unchanged. After preloading the initial value of the control voltage of the PMT, the control voltage weight of the PMT linearly changes from 0% to 100%, and the PGA gain value weight is linearly set from 100% to 0%:

[0077] ;

[0078] ;

[0079] ;

[0080] In the formula, is the gain of the PMT; is the calibration coefficient; is the actual input value of the control voltage of the PMT; is the actual gain value of the PGA.

[0081] Further, an abnormal situation is set, that is, when it is detected three times that the environmental light intensity continuously fluctuates near the boundary of the hysteresis region, the hysteresis interval will be automatically expanded;

[0082] When the change rate of the environmental light intensity is too fast and exceeds the set value of the rate conversion upper limit the hysteresis interval will be temporarily reduced to improve the response speed;

[0083] ;

[0084] In the formula, is the conversion slope; is the th collected light intensity value, with the unit of Lux; is the average light intensity of the recent 10 samplings; is the th sampling time point; is the average time of the recent 10 samplings;

[0085] If it indicates that the environmental light intensity drops rapidly, the control parameters of the controllable liquid crystal light valve are initialized in advance, and it is ready to switch to the low-light mode when it exceeds;

[0086] If it indicates that the environmental light intensity rises rapidly, the gain value of the PGA is initialized in advance, and it is ready to switch to the high-light mode when it exceeds.

[0087] The present invention discloses a method for controlling the self-gain of the echo signal intensity of an airborne laser radar. An implementation mode of the present invention is described in detail below in conjunction with the accompanying drawings.

[0088] Combination Figure 1 and Figure 2 As shown, a method for controlling the self-gain of the intensity of the airborne laser radar echo signal includes a plurality of light intensity sensors, a controllable liquid crystal light valve, a photodetector PMT, a DAC converter, an ADC converter, a field programmable gate array FPGA, a programmable gain amplifier PGA and a peak detection circuit. The present invention judges the intensity of the return light and performs independent peak detection on the voltage type signal converted by the photodetector PMT, uses FPGA as the control center, and uses DDS technology to accurately output the desired voltage value through the DAC converter.

[0089] The self-gain of the entire photoelectric conversion system is divided into three parts. When the ambient light intensity is weak, the controllable liquid crystal light valve realizes the attenuation function when the ambient light intensity is weak, that is, when the echo signal is very strong; when the ambient light intensity is moderate, the control voltage provided by the PMT is used for fine-tuning; when the ambient light intensity is strong, it is processed through the PGA.

[0090] like Figure 3 As shown in the figure, the controllable liquid crystal light valve: when the returning light is captured by the telescope, it first passes through the first polarizer, thereby converting the vibration direction of the returning light to be consistent with the direction of the transmission axis of the first polarizer. At this time, it is assumed that the output light intensity of polarizer 1 is E. Since when the light beam enters the liquid crystal, it will be decomposed into e light (extraordinary light) with a vibration direction parallel to the liquid crystal optical axis and o light (ordinary light) perpendicular to it, the refractive index of the liquid crystal for o light and e light is and , when there is no external electric field and The difference is the largest, and the refractive index difference is , when voltage is applied, the optical axis will fall in the direction of the electric field, resulting in changes, thus and There is a functional relationship that affects the phase delay of incident light passing through liquid crystal. (It can be understood that the liquid crystal can rotate the amplitude direction of the light output by polarizer 1 again). Finally, when the o-light and e-light are directed to the second polarizer, only the light in the same direction as the transmission axis of the second polarizer can be output. In this way, the effect of weakening the intensity of the returned light is achieved.

[0091] When the applied voltage When the voltage is greater than the threshold voltage of the liquid crystal, the refractive index difference There is an exponential relationship:

[0092] = ;

[0093] Wherein, is the initial refractive index difference without applying ; is a constant related to the characteristics of the liquid crystal material, representing the characteristic value of the voltage.

[0094] The phase delay amount has the following relationship with the applied voltage :

[0095] ;

[0096] Wherein, is the wavelength of the incident light; is the thickness of the liquid crystal.

[0097] ;

[0098] ;

[0099] ;

[0100] Wherein, is the output light intensity of polarizer 1; is the angle between the transmission axis of polarizer 1 and the optical axis of the liquid crystal; is the angle between the transmission axis of polarizer 2 and the optical axis of the liquid crystal; is the magnitude of the light intensity of the output light of polarizer 1 orthogonally decomposed parallel to the optical axis of the liquid crystal (e - optical axis); is the magnitude of the light intensity of the output light of polarizer 1 orthogonally decomposed perpendicular to the optical axis of the liquid crystal (o - optical axis);

[0101] The amplitudes of the o - light and e - light after passing through polarizer 2 after decomposition are decomposed onto the transmission axis of polarizer 2:

[0102] ;

[0103] ;

[0104] Since both the o - light and e - light will pass through the transmission axis of polarizer 2, interference of light will surely occur during the transmission of the two beams of light. According to the principle of interference superposition of light, the finally output light intensity is:

[0105] ;

[0106] ;

[0107] Then the final output of the light passing through the controllable liquid crystal light valve is:

[0108] 。

[0109] As Figure 4 shown, the programmable gain amplifier PGA: The programmable gain amplifier PGA is a special operational amplifier. Its function of amplifying signals is no different from that of an operational amplifier. However, the programmable gain amplifier can adjust the amplification gain by switching the size of the feedback resistor or adjusting the transconductance parameter.

[0110] ;

[0111] In the formula, is the input of the voltage of the programmable gain amplifier PGA; is the output of the programmable gain amplifier PGA; is the programmable feedback resistor; is the reference resistor for changing the gain;

[0112] At greater than or equal to , thus changing the resistance value can change the gain.

[0113] Photomultiplier tube PMT: The basic principle of PMT is the photoelectric effect, and it is a vacuum component that converts a weak incident optical signal into a larger electrical signal. The implementation process of PMT is that when light enters from the light window and hits the photocathode surface coated with cesium-antimony alloy, electrons are generated, and the electrons are gathered and then hit the first-stage dynode, and then pass through a series of dynodes, and finally the current output is realized on the anode.

[0114] The voltages of the dynodes at all levels of PMT are controlled by the control voltage of PMT. When the control voltage increases or decreases, it means that the voltages of the dynodes of PMT are increased or attenuated, so the electrons excited at each level are changed, and finally the anode current is changed. The gain of PMT can be expressed as:

[0115] ;

[0116] In the formula, is the number of internal dynodes of PMT; is a constant, the size of which is related to the collection efficiency of PMT and the secondary electron emission material; is a constant, the size of which is related to the dynode material and the loss during the transmission of secondary electrons to the next-stage dynode; is the linear coefficient between the actual working voltage and the control voltage of PMT.

[0117] The overall process is as follows: First, peak calibration is performed to set the target value and dynamic range of the peak detection circuit received by the FPGA. Then, all light intensity sensors start to work, and the converted light intensity values are input into the control center FPGA. The FPGA calculates the current ambient light intensity by weighting the values of each light intensity sensor, and then controls according to the set light intensity threshold. Among them, the low light range is between 0 Lux and 400 Lux, mainly for night and cloudy weather; the medium light range is between 400 Lux and 6000 Lux, mainly for daytime cloudy to partly sunny conditions; the high light range is greater than 6000 Lux, mainly for the moment when the sunlight is the strongest at noon. Finally, the peak data of the echo signal output by the photodetector PMT is sent back to the FPGA through the peak detection circuit to complete the control of the entire loop.

[0118] Control schemes enabled at different light intensity thresholds: When the FPGA judges from the data transmitted back by the light intensity sensor that the ambient light intensity is in the low light range and enters the low light mode according to the light intensity threshold, the control of the controllable liquid crystal light valve is prioritized. When the FPGA receives that the output of the peak detection circuit collected by the ADC circuit is too large or too small, the voltage that needs to be converted by the DAC when using the controllable liquid crystal light valve function is calculated through the internal PID algorithm. Then, the DAC outputs the voltage value to the control voltage of the controllable liquid crystal light valve to control the attenuation degree of the return light intensity until the FPGA judges that the value output by the peak detection circuit is within the normal range. Then, the FPGA uses DDS technology to finely adjust the control voltage of the PMT with precise and tiny voltage steps for detailed adjustment.

[0119] When the FPGA judges from the data transmitted back by the light intensity sensor that the ambient light intensity is in the medium light range and enters the medium light mode according to the light intensity threshold, and it is detected that the output of the peak detection circuit is within the set dynamic range, the control of the control voltage of the PMT is prioritized. The FPGA uses DDS technology to finely adjust the control voltage of the PMT with precise and tiny voltage steps. However, if the output of the peak detection circuit is much larger than the set target value, it directly jumps to the processing program of the low light mode and updates the low light threshold to the value of the current ambient light intensity; if it is much smaller, it directly jumps to the processing program of the high light mode and updates the high light threshold to the value of the current ambient light intensity.

[0120] When the FPGA determines that the environmental light intensity is within the strong light range based on the data transmitted back by the light intensity sensor according to the light intensity threshold and enters the strong light mode. Since the number of photons received at this time is small, only the programmable gain amplifier (PGA) can be enabled to control the intensity of the echo signal output by the PMT. The FPGA calculates the original intensity of the echo signal by inversely inferring from the gain value of the PGA at this time, and then recalculates the gain value based on the ratio of the original intensity of the echo signal to the set value of the self-gain system, and then inputs it to the compilation unit of the PGA.

[0121] Smoothing switching mechanism between different modes:

[0122] To avoid frequent switching of the control mode due to fluctuations in the environmental light intensity near the set mode threshold, a hysteresis zone is set to control the mode from switching frequently. The FPGA uses a sliding window (size of 10 samples) to calculate the average value and the change trend.

[0123] It is set that when the number of times the light intensity value continuously exceeds 110% of the light intensity threshold from the environmental low light range to the environmental medium light range is more than three times, the control voltage of the controllable liquid crystal light valve remains unchanged. After preloading the initial value of the PMT control voltage, the control voltage weight of the controllable liquid crystal light valve Changes linearly from 100% to 0%, and the control voltage weight of the PMT Changes linearly from 0% to 100%:

[0124] ;

[0125] ;

[0126] ;

[0127] In the formula, Is the control voltage of the PMT; Is the initial value of the voltage added to the PMT control voltage; Is the control voltage of the controllable liquid crystal light valve; Is the actual input value of the PMT control voltage; Is the actual input value of the controllable liquid crystal light valve control voltage; Is the calibration coefficient.

[0128] When the number of times the light intensity value continuously drops below 90% of the light intensity threshold from the environmental medium light range to the environmental low light range is more than three times, the control voltage of the PMT remains unchanged. After preloading the initial value of the control voltage of the controllable liquid crystal light valve, the control voltage weight of the controllable liquid crystal light valve Changes linearly from 0% to 100%, and the control voltage weight of the PMT Changes linearly from 100% to 0%:

[0129] ;

[0130] ;

[0131] ;

[0132] In the formula, is the initial value of the voltage for increasing the control voltage of the pre-oriented controllable liquid crystal light valve;

[0133] When the number of times that the light intensity value continuously drops below 105% of the light threshold from the ambient light range to the ambient strong light range is more than three times, keep the control voltage of the PMT unchanged. After preloading the initial value of the PGA gain, the weight of the control voltage of the PMT varies linearly from 100% to 0%, and the weight of the PGA gain value is linearly set from 0% to 100%:

[0134] ;

[0135] ;

[0136] ;

[0137] In the formula, is the initial value of the gain for increasing the PGA; is the output value of the current peak detection circuit; is the set voltage intensity of the echo signal; is the actual gain value of the PGA.

[0138] When the number of times that the light intensity value continuously drops below 95% of the light threshold from the ambient strong light range to the ambient light range is more than three times, keep the control voltage of the controllable liquid crystal light valve unchanged. After preloading the initial value of the control voltage of the PMT, the weight of the control voltage of the PMT varies linearly from 0% to 100%, and the weight of the PGA gain value is linearly set from 100% to 0%:

[0139] ;

[0140] ;

[0141] ;

[0142] In the formula, is the gain of the PMT; is the calibration coefficient; is the actual input value of the control voltage of the PMT; The actual gain value of the PGA.

[0143] An abnormal situation is set. That is, when the ambient light intensity is detected to fluctuate near the boundary of the hysteresis region three times continuously, the hysteresis interval will be automatically expanded, and by default, it will be expanded to 1.5 times the hysteresis region.

[0144] When the change rate of the ambient light intensity is too fast and exceeds the set value of the rate change upper limit the hysteresis interval will be temporarily reduced to improve the response speed; among them, the set value is defaulted to ±60 Lux / second, which roughly conforms to the daily change rate of the ambient light intensity.

[0145] ;

[0146] In the formula, is the change slope; is the th collected light intensity value, with the unit of Lux; is the average light intensity of the recent 10 samplings; is the th sampling time point; is the average time of the recent 10 samplings;

[0147] If it means that the ambient light intensity drops rapidly, and the control parameters of the controllable liquid crystal light valve are initialized in advance, and it is ready to switch to the weak light mode when exceeding;

[0148] If it means that the ambient light intensity rises rapidly, and the gain value of the PGA is initialized in advance, and it is ready to switch to the strong light mode when exceeding.

[0149] The present invention is described as follows with actual examples:

[0150] In the present invention, the gain adjustment of the echo signal involves three key parts: a programmable gain amplifier (PGA), a PMT control voltage, and a controllable light valve. The PGA has integrated a communication protocol by the chip manufacturer to achieve gain adjustment, and no additional design is required; while the PMT control voltage and the controllable light valve need to calibrate their voltage-gain relationship through experiments.

[0151] For the controllable light valve, it can be verified through experiments that when the control voltage changes in the range of 0V to 5V, the light transmittance of the controllable light valve shows a gradient attenuation characteristic, proving its light intensity adjustment ability.

[0152] Subsequently, a number of laser points are projected onto the building according to the illumination and distance. Each laser point is respectively: Day - Near Point 1 Laser Landing Point, Night - Near Point 1 Laser Landing Point, Night - Near Point 2 Laser Landing Point, Night - Far Point Laser Landing Point. The gain curve measurements are carried out by adjusting the control voltages of the controllable liquid crystal light valve and PMT with a step of 0.1V respectively during the day and at night. During the night measurement, first set the system attenuation coefficient to -10dB, conduct two near - point echo intensity tests and one far - point echo intensity test on the gain curve of the controllable light valve, and conduct only one near - point echo intensity test on the gain curve of the PMT control voltage; during the day, due to the significantly enhanced ambient light interference, after adjusting the system gain to 3dB, only conduct the echo intensity test at the position of Near Point 1 on the gain curves of both.

[0153] Combined with Figure 5 、 Figure 6 as shown, in Figure 5 , the red line represents the magnification curve of Night - Near Point 1, the blue line represents the magnification curve of Night - Near Point 2, the purple line represents the magnification curve of Night - Far Point, and the green line represents the magnification curve of Day - Near Point 1; in Figure 6 , the blue line represents the magnification curve of Day - Near Point 1, and the orange line represents the magnification curve of Night - Near Point 1. At night, the echo signals during the measurement of the gain curve of the controllable light valve are measured at the same control voltage at the three laser landing points. The first measurement result is as Figure 7 shown. In Figure 7 , symbol 1 represents the position of the reference level of the laser echo signal channel, symbol 2 represents the position of the reference level of the laser pulse trigger signal channel, and symbol T represents the trigger level position determined by the oscilloscope based on the laser pulse trigger signal channel. The minimum value of the control voltage of the echo signal represented by symbol 1 is -457.86mV. In the second measurement result, the minimum value of the control voltage of the echo signal represented by symbol 1 is -418.60mV. In the third measurement result, the minimum value of the control voltage of the echo signal represented by symbol 1 is -93.026mV.

[0154] From what is presented in the experiments Figure 5 、 Figure 6 and Figure 7 , it can be clearly observed that for the gain curve obtained by measuring the controllable light valve in a night environment, its linear range corresponds to the control voltage in the range of 2.1V to 4.4V. To ensure the stability and reliability of the system operation, a certain safety margin needs to be reserved in practical applications. Based on this consideration, the minimum value of the control voltage of the controllable light valve is set to 2.5V, and the maximum value is set to 4V.

[0155] As Figure 8As shown, after reasonably setting the control voltage range, the data in this linear interval is fitted using the Python programming language. Finally, in this way, the linear equation corresponding to this linear part is calculated:

[0156] ;

[0157] At night, the echo signals during the measurement of the PMT control voltage gain curve were measured three times at the same location with different control voltages. The results of the first measurement are as Figure 9 shown. Symbol 1 represents the position of the reference level of the laser echo signal channel, symbol 2 represents the position of the reference level of the laser pulse trigger signal channel, and symbol T represents the position of the trigger level determined by the oscilloscope based on the laser pulse trigger signal channel. The minimum value of the control voltage of the echo signal represented by symbol 1 is -1.5455V. In the results of the second measurement, the minimum value of the control voltage of the echo signal represented by symbol 1 is -772.55mV. In the results of the third measurement, the minimum value of the control voltage of the echo signal represented by symbol 1 is -2.4178V.

[0158] According to the technical specification requirements of the PMT (photomultiplier tube) itself, when measuring its gain curve this time, the input range of the control voltage is set to the range specified by this device. Experiments found that within this set voltage range, all changes in the control voltage are within the linear interval of the gain curve. To ensure the stability of the system, the minimum value of the control voltage is selected as 0.55V, and the maximum value is 1V.

[0159] As Figure 10 shown, next, Python is also used to fit the data in this linear interval to calculate the corresponding linear equation:

[0160] ;

[0161] During the day, the echo signals during the measurement of the controllable optical valve control voltage gain curve were measured three times at the same location with different control voltages. The results of the first measurement are as Figure 11 shown. Symbol 1 represents the position of the reference level of the laser echo signal channel, symbol 2 represents the position of the reference level of the laser pulse trigger signal channel, and symbol T represents the position of the trigger level determined by the oscilloscope based on the laser pulse trigger signal channel. The minimum value of the control voltage of the echo signal represented by symbol 1 is -461.91mV. In the results of the second measurement, the minimum value of the control voltage of the echo signal represented by symbol 1 is -350.88mV. In the results of the third measurement, the minimum value of the control voltage of the echo signal represented by symbol 1 is -478.12mV.

[0162] During the day, when measuring the PMT control voltage gain curve, the echo signals were measured three times at the same location with different control voltages. The result of the first measurement is as follows Figure 12 shown. Symbol 1 represents the position of the reference level of the laser echo signal channel, symbol 2 represents the position of the reference level of the laser pulse trigger signal channel, and symbol T represents the position of the trigger level determined by the oscilloscope based on the laser pulse trigger signal channel. The minimum value of the control voltage of the echo signal represented by symbol 1 is -380.88 mV. In the result of the second measurement, the minimum value of the control voltage of the echo signal represented by symbol 1 is -376.82 mV. In the result of the third measurement, the minimum value of the control voltage of the echo signal represented by symbol 1 is -368.72 mV.

[0163] In the actual experimental scenario, the excessive ambient light intensity during the day has a significant impact on the echo gain regulation mechanism. Through experimental verification, it is found that there are two verification paths for the conclusion that "the interference of ambient light during the day causes the regulation effect of the PMT control voltage on the echo gain to approach zero" proposed in the present invention: one is to directly measure the gain response curve of the PMT control voltage during the day, and the other is to indirectly deduce it by analyzing the attenuation characteristics of the controllable optical valve under strong light conditions. The experimental results of the gain curves of both show a highly mutually verifying relationship. The phenomenon of the gain curve of the PMT under strong light is complementary to the light intensity regulation characteristics of the controllable optical valve in the range of 2.1V to 5V.

[0164] When the control voltage of the controllable optical valve is in the range greater than 2.1V and less than 5V, the phenomenon of echo signal enhancement will occur. This is because the controllable optical valve attenuates the light intensity of all the returned light, including the external interference light. There is a filter corresponding to the laser light wave in the subsequent light receiving circuit, and this filter makes the received light positively correlated with the light returned from the laser landing point. Therefore, when the controllable optical valve linearly attenuates the overall returned light intensity, it actually reduces the influence of the external interference light, thereby enhancing the echo signal.

[0165] Finally, after completing the construction of the linear gain functions of the PMT and the controllable optical valve, they were integrated into the system control algorithm, and dual-period dynamic gain debugging experiments were carried out at 10:15 during the day and 18:30 at night. The curve of the ambient light intensity change during the debugging process is shown in Figure 13, Figure 14 shown. The echo peak value of the laser irradiating the near point 1 target is set to 800 mV. The echo signals were measured three times at 10:20 in the morning when the system was not started. The result of the first measurement is as follows Figure 15As shown in the figure, symbol 1 represents the position of the reference level of the laser echo signal channel, symbol 2 represents the position of the reference level of the laser pulse trigger signal channel, and symbol T represents the position of the trigger level determined by the oscilloscope for the laser pulse trigger signal channel. The minimum value of the echo signal control voltage represented by symbol 1 is -238.07 mV. In the second measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -224.13 mV. In the third measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -284.81 mV.

[0166] Three measurements of the echo signal were taken when the system was started at 10:23 am. The first measurement result is as Figure 16 As shown in the figure, symbol 1 represents the position of the reference level of the laser echo signal channel, symbol 2 represents the position of the reference level of the laser pulse trigger signal channel, and symbol T represents the position of the trigger level determined by the oscilloscope for the laser pulse trigger signal channel. The minimum value of the echo signal control voltage represented by symbol 1 is -796.33 mV. In the second measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -813.77 mV. In the third measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -854.45 mV.

[0167] Three measurements of the echo signal were taken when the system was not started at 18:40 at night. The first measurement result is as Figure 17 As shown in the figure, symbol 1 represents the position of the reference level of the laser echo signal channel, symbol 2 represents the position of the reference level of the laser pulse trigger signal channel, and symbol T represents the position of the trigger level determined by the oscilloscope for the laser pulse trigger signal channel. The minimum value of the echo signal control voltage represented by symbol 1 is -1.5461 V. In the second measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -1.5064 V. In the third measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -1.4647 V.

[0168] Three measurements of the echo signal were taken when the system was started at 18:44 at night. The first measurement result is as Figure 18 As shown in the figure, symbol 1 represents the position of the reference level of the laser echo signal channel, symbol 2 represents the position of the reference level of the laser pulse trigger signal channel, and symbol T represents the position of the trigger level determined by the oscilloscope for the laser pulse trigger signal channel. The minimum value of the echo signal control voltage represented by symbol 1 is -732.39 mV. In the second measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -813.77 mV. In the third measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -773.05 mV. Using the above Figure 15 、Figure 16 , Figure 17 and Figure 18 present the dynamic gain adjustment curves and output stability verification results at different time periods.

[0169] The final experiment shows that the present invention verifies the effectiveness of the gain adjustment system in a complex lighting environment through systematic experiments. The research results show that the dual-time self-gain system can still stabilize the echo peak within 800mV ± 10% (730mV - 830mV during the day and 730mV - 820mV at night) when the ambient light intensity fluctuates by up to 10000Lux during the day and is lower than 100Lux at night (18:30), verifying the stability of the adaptive gain algorithm and the feasibility of the present invention.

[0170] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions, or substitutions made by those skilled in the art within the scope of the essence of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for controlling the self-gain of the echo signal strength of an airborne laser radar, characterized in that: The following steps are involved: S1, setting the target value and dynamic range of the peak data received by the FPGA peak detection circuit, and calculating the current ambient light intensity through a number of light intensity sensors; S2, switches different control modes according to the set light threshold, sets a hysteresis zone, and calculates the average value and change trend of light intensity by using a sliding window; S3, transmitting the peak data of the echo signal output by the photodetector PMT back to the FPGA through the peak detection circuit; In S2, the FPGA determines that the ambient light intensity is within the low light range according to the light threshold and enters the low light mode, wherein the low light mode includes the following steps: S211, when the output value of the peak detection circuit collected by the ADC circuit is too large or too small, the FPGA calculates the voltage that the DAC needs to convert when using the controllable liquid crystal light valve function through the internal PID algorithm; S212, the DAC outputs the voltage value to the control voltage of the controllable liquid crystal light valve to control the attenuation degree of the return light intensity, until the FPGA determines that the voltage value output by the peak detection circuit is within the dynamic range; S213, FPGA uses DDS technology to adjust the control voltage of PMT; In S2, the FPGA determines that the ambient light intensity is within the strong light range according to the light threshold and performs a strong light mode, wherein the strong light mode includes the following steps: S221, FPGA reversely estimates the original strength of the echo signal through the gain value of the programmable gain amplifier PGA at this time; S222, FPGA recalculates the gain value according to the ratio of the original strength of the echo signal to the value set by the self-gain system; S223, input the gain value into the compilation of PGA; In S2, the FPGA determines that the ambient light intensity is within the medium light range according to the light threshold and enters the medium light mode, wherein the medium light mode includes the following steps: S231, when the FPGA detects that the output of the peak detection circuit is within the dynamic range, the DDS technology is used to adjust the control voltage of the PMT; S232, if the output value of the peak detection circuit is greater than the target value, jump to the low light mode and update the low light threshold to the value of the ambient light intensity at this time; if the output value of the peak detection circuit is less than the target value, jump to the high light mode and update the high light threshold to the value of the ambient light intensity at this time.

2. A method for controlling the self-gain of the airborne laser radar echo signal strength according to claim 1, characterized in that: In S1, the FPGA calculates the ambient light intensity by weighting the light intensity values ​​detected by each light intensity sensor.

3. The method for controlling the self-gain of the airborne laser radar echo signal strength according to claim 2, characterized in that: In S2, when the light intensity value from the low-light range to the bright range exceeds 110% of the light threshold for more than three times in a row, the control voltage of the controllable liquid crystal light valve is kept unchanged, and after preloading the initial value of the PMT control voltage, the control voltage weight W of the controllable liquid crystal light valve is P The PMT control voltage weight W changes linearly from 100% to 0%. C Linear variation from 0% to 100%: V C(OUT) =V C0 ×W C +V C(MIN) ; V P(OUT) =(V P -V P(MIN) )×W P +V P(MIN) ; Where V C is the control voltage of PMT; V C0 The initial value of the voltage added to the PMT control voltage; V P is the control voltage of the controllable liquid crystal light valve; V C(OUT) is the actual input value of the PMT control voltage; V P(OUT) is the actual input value of the control voltage of the controllable liquid crystal light valve; K is the calibration coefficient.

4. The method for controlling the self-gain of the airborne laser radar echo signal strength according to claim 3, characterized in that: In S2, when the light intensity from the ambient light range to the ambient weak light range is lower than 90% of the light threshold for more than three times in a row, the control voltage of the PMT is kept unchanged, and after the initial value of the control voltage of the controllable liquid crystal light valve is preloaded, the control voltage weight W of the controllable liquid crystal light valve is P The PMT control voltage weight W changes linearly from 0% to 100%. C Linear change from 100% to 0%: V C(OUT) =(V C -V C(MIN) )×W C +V C(MIN) ; V P(OUT) =V P0 ×W P +V P(MIN) ; Where V P0 The initial value of the voltage added to the control voltage of the pre-controllable liquid crystal light valve.

5. The method for controlling the self-gain of the airborne laser radar echo signal strength according to claim 4, characterized in that: In S2, when the light intensity from the ambient light range to the ambient strong light range is lower than 105% of the light threshold for more than three times in a row, the control voltage of the PMT is kept unchanged, and after the initial value of the PGA gain is preloaded, the control voltage weight W of the PMT is C The PGA gain value weight W changes linearly from 100% to 0%. G Linear setting from 0% to 100%: V C(OUT) =(V C -V C(MiN) )×W C +V C(MIN) ; A V(OUT) =A V0 ×W G +1; In the formula, A V0 The initial value of the gain added to the pre-PGA; V (OUT) is the current peak detection circuit output value; V (SET) is the voltage strength of the set echo signal; A V(OUT) is the actual gain value of the PGA.

6. A method for controlling the self-gain of the airborne laser radar echo signal strength according to claim 5, characterized in that: In S2, when the light intensity from the ambient strong light range to the ambient light range is lower than 95% of the light threshold for more than three times in a row, the control voltage of the controllable liquid crystal light valve is kept unchanged, and after the initial value of the control voltage of the PMT is preloaded, the control voltage weight W of the PMT is C The PGA gain value changes linearly from 0% to 100%, and the weight W G Linear setting from 100% to 0%: V C(OUT) =V C0 ×W C +V C(MIN) ; A V(OUT) =(A V -1)×W G +1; Where A1 is the gain of the PMT; M is the calibration coefficient; V C(OUT) is the actual input value of the PMT control voltage; A V(OUT) is the actual gain value of the PGA.

7. The method for controlling the self-gain of the airborne laser radar echo signal strength according to claim 6, characterized in that: Set an abnormal situation, that is, when the ambient light intensity is detected to fluctuate continuously near the boundary of the hysteresis zone three times, the hysteresis zone will be automatically extended; When the ambient light intensity changes too fast, exceeding the upper limit of the rate change setting value S SET When , the hysteresis interval will be temporarily shortened to improve the response speed; Where s is the transformation slope; E i is the light intensity value collected for the i-th time, in Lux; is the average light intensity of the last 10 samples; t i is the time point of the i-th sampling; It is the time average of the last 10 samples; like When , it indicates that the ambient light intensity is decreasing rapidly, the control parameters of the controllable liquid crystal light valve are initialized in advance, and it is ready to switch to the low light mode when it exceeds; like , indicating that the ambient light intensity rises rapidly, the PGA gain value is initialized in advance, and is ready to switch to the strong light mode when it exceeds.

Images