Method for controlling airborne laser radar echo signal intensity self-gain

Abstract

The invention discloses a method for controlling airborne laser radar echo signal intensity self-gain, and relates to the technical field of radars. The method comprises the following steps: setting a target value and a dynamic interval of peak data received by an FPGA (Field Programmable Gate Array) from a peak detection circuit, and calculating the current ambient light intensity through a plurality of light intensity sensors; switching different control modes according to a set illumination threshold, setting a hysteresis region, and calculating an average value and a change trend of light intensity by adopting a sliding window; peak value data of echo signals output by the photoelectric detector PMT are transmitted back to the FPGA through the peak value detection circuit. The stability of the photoelectric conversion self-gain system is improved, the working range of the self-gain system is enlarged, the detection distance of the airborne laser radar can be improved, the defense deployment conditions of the airborne laser radar are reduced, the influence of environment illumination is reduced, the detection efficiency is improved, and the method is used for stabilizing the intensity of echo signals of the airborne laser radar.

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] Among the above methods, the linearity of the control voltage of the first scheme is different for different types or kinds of photodetectors. Secondly, when the ambient light intensity is too strong, the control voltage is adjusted. Since the number of photons returning to the telescope is originally small, adjusting the control voltage when the ambient light intensity is strong cannot significantly improve the intensity of the echo signal. The second is to directly adjust the output signal of the photodetector. Compared with the first scheme, it solves the problem that the echo signal intensity 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 the excessive intensity of the laser emission, the saturated signal cannot be attenuated to a normal state by adjusting the gain of the programmable controller. This is because an operational amplifier is used, and its slew rate and bandwidth gain product are certain values, while the photoelectric converter output The signal frequency is relatively high, so the adjustment of gain and output voltage is very limited, which reduces the dynamic range, and excessive gain will also amplify noise factors such as 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 a large range of adjustment of echo signal gain; the third solution is to directly change the voltage range for sampling the echo signal, which can avoid the noise interference caused by the second solution, but if only the sampling range is increased without increasing the sampling resolution, it will cause a larger error in the time point of extracting the peak value, thereby further reducing the detection accuracy of the airborne lidar. If both are improved, the total power consumption of the system will be greatly increased (the power consumption of a 16-bit resolution ADC is generally 35 times higher than that of a 12-bit ADC), and the sampling frequency of a high-resolution ADC is usually reduced, resulting in a further decrease in detection accuracy. Summary of the invention

[0004] The echo signal intensity of the above-mentioned airborne laser radar will be affected by factors such as the emitted laser intensity, the laser flight distance and the ambient light intensity. When the control voltage of the photodetector is adjusted and the output signal is directly processed, the echo signal enhancement effect 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 the airborne laser radar.

[0005] In order 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 laser radar, characterized in that it includes the following steps: S1, setting the target value and dynamic range of the peak data of the peak detection circuit received by the FPGA, and calculating the current ambient light intensity through a number of light intensity sensors; S2, switching different control modes according to the set light 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] Furthermore, 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, the FPGA enters a low-light mode when it determines that the ambient light intensity is within the low-light range based on the light threshold, and the low-light mode includes the following steps: S211, when the FPGA receives the output value of the peak detection circuit collected by the ADC circuit and it is too large or too small, it 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, the FPGA uses the DDS technology to adjust the control voltage of the PMT.

[0008] Further, 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, and the strong light mode includes the following steps: S221, the FPGA reversely infers the original intensity of the echo signal 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, the gain value is input into the compilation of the PGA.

[0009] Further, in S2, the FPGA judges that the ambient light intensity is within the medium light range according to the light threshold and enters the medium light mode, and 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 the DDS technology to adjust 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 strong light mode and updates the strong light threshold to the value of the ambient light intensity at this time.

[0010] Further, 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 the initial value of the PMT control voltage is preloaded, the control voltage weight of the controllable liquid crystal light valve is The PMT control voltage weight varies linearly from 100% to 0%. Linear variation from 0% to 100%:

[0011] ;

[0012] ;

[0013] ;

[0014] In the formula, is the control voltage of PMT; 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 control voltage of the controllable liquid crystal light valve; is the calibration factor.

[0015] Further, 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 of the controllable liquid crystal light valve is The PMT control voltage weight varies linearly from 0% to 100%. Linear change from 100% to 0%:

[0016] ;

[0017] ;

[0018] ;

[0019] In the formula, The initial value of the voltage added to the control voltage of the pre-controllable liquid crystal light valve.

[0020] Further, 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 of the PMT is PGA gain value weights linearly from 100% to 0%. Linear setting from 0% to 100%:

[0021] ;

[0022] ;

[0023] + 1;

[0024] In the formula, The initial value of the gain added to the pre-PGA; is the current peak detection circuit output value; is the voltage strength of the set echo signal; is the actual gain value of the PGA.

[0025] Further, in S2, when the light intensity value 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 of the PMT is PGA gain value weights linearly from 0% to 100% Linear setting from 100% to 0%:

[0026] ;

[0027] ;

[0028] ;

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

[0030] Furthermore, an abnormal situation is set, 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;

[0031] When the ambient light intensity changes too quickly, exceeding the upper limit of the rate change setting When , the hysteresis interval will be temporarily shortened to improve the response speed;

[0032] ;

[0033] In the formula, is the transformation slope; For the The light intensity value collected is in Lux; It is the average light intensity of the last 10 samples; For the The time point of subsampling; It is the time average of the last 10 samples;

[0034] 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;

[0035] 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.

[0036] The beneficial effects of the present invention are as follows: the present invention improves the stability of the photoelectric conversion self-gain system and increases the working range of the self-gain system, which can further improve the detection distance of the airborne laser radar, while reducing the deployment conditions of the airborne laser radar, reducing the impact of ambient light, and improving the detection efficiency, and is used to stabilize the intensity of the airborne laser radar echo signal.

[0037] The present invention can automatically update the light intensity threshold and dynamically adjust the hysteresis interval according to the light intensity change rate to avoid frequent switching between control modes. The self-gain process is disassembled and a controllable liquid crystal light valve is used 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. The control voltage of the photodetector PMT is controlled as an auxiliary control. The peak detection circuit is used as a fast feedback method to control the self-gain. A variety of self-gains are coordinated to ensure that the airborne laser radar can operate stably around the clock. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0039] Figure 2 Shown is a control flow chart of the present invention.

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

[0041] Figure 4 Shown is a schematic diagram of the programmable gain amplifier PGA.

[0042] Figure 5 Shown is the controllable light valve gain curve.

[0043] Figure 6 Shown is the PMT control voltage gain curve.

[0044] Figure 7 Shown is the echo signal during the measurement of the night-controllable light valve amplification curve.

[0045] Figure 8 The figure shows the function image that needs to be fitted after the average value of the linear part of the control voltage of the controllable light valve is calculated.

[0046] Fig. 9 Shown is the echo signal during the measurement of the night-PMT control voltage amplification factor curve.

[0047] Fig.10 The figure shows the function image that needs to be fitted in the linear part of the PMT control voltage.

[0048] Fig.11Shown is the echo signal when measuring the control voltage amplification factor curve of the daylight-controllable light valve.

[0049] Fig.12 Shown is the echo signal during the measurement of the daytime-PMT control voltage magnification curve.

[0050] Fig.13 Shown is the light intensity variation curve from 15:40 to 19:37 on a certain day in the experimental site.

[0051] Fig.14 Shown is the light intensity variation curve from 21:22 to 10:03 on a certain day in the experimental site.

[0052] Fig.15 Shown is the echo signal measured at 10:20 am when the system was not started.

[0053] Fig.16 Shown is the echo signal measured when the system was started at 10:23 am.

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

[0055] Fig.18 Shown is the echo signal measured when the system was started at 18:44 at night. DETAILED DESCRIPTION

[0056] A method for controlling the self-gain of the echo signal intensity of an airborne laser radar is characterized in that it includes the following steps: S1, setting the target value and dynamic range of the peak data of the peak detection circuit received by the FPGA, and calculating the current ambient light intensity through a plurality of light intensity sensors; S2, switching different control modes according to the set light 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.

[0057] Furthermore, 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, the FPGA enters a low-light mode when it determines that the ambient light intensity is within the low-light range based on the light threshold, and the low-light mode includes the following steps: S211, when the FPGA receives the output value of the peak detection circuit collected by the ADC circuit and it is too large or too small, it 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, the FPGA uses the DDS technology to adjust the control voltage of the PMT.

[0059] Further, 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, and the strong light mode includes the following steps: S221, the FPGA reversely infers the original intensity of the echo signal 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, the gain value is input into the compilation of the PGA.

[0060] Further, in S2, the FPGA judges that the ambient light intensity is within the medium light range according to the light threshold and enters the medium light mode, and 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 the DDS technology to adjust 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 strong light mode and updates the strong light threshold to the value of the ambient light intensity at this time.

[0061] Further, 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 the initial value of the PMT control voltage is preloaded, the control voltage weight of the controllable liquid crystal light valve is The PMT control voltage weight varies linearly from 100% to 0%. Linear variation from 0% to 100%:

[0062] ;

[0063] ;

[0064] ;

[0065] In the formula, is the control voltage of PMT; 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 control voltage of the controllable liquid crystal light valve; is the calibration factor.

[0066] Further, 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 of the controllable liquid crystal light valve is The PMT control voltage weight varies linearly from 0% to 100%. Linear change from 100% to 0%:

[0067] ;

[0068] ;

[0069] ;

[0070] In the formula, The initial value of the voltage added to the control voltage of the pre-controllable liquid crystal light valve.

[0071] Further, 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 of the PMT is PGA gain value weights linearly from 100% to 0%. Linear setting from 0% to 100%:

[0072] ;

[0073] ;

[0074] + 1;

[0075] In the formula, The initial value of the gain added to the pre-PGA; is the current peak detection circuit output value; is the voltage strength of the set echo signal; is the actual gain value of the PGA.

[0076] Further, in S2, when the light intensity value 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 of the PMT is PGA gain value weights linearly from 0% to 100% Linear setting from 100% to 0%:

[0077] ;

[0078] ;

[0079] ;

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

[0081] Furthermore, an abnormal situation is set, 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;

[0082] When the ambient light intensity changes too quickly, exceeding the upper limit of the rate change setting When , the hysteresis interval will be temporarily shortened to improve the response speed;

[0083] ;

[0084] In the formula, is the transformation slope; For the The light intensity value collected is in Lux; It is the average light intensity of the last 10 samples; For the The time point of subsampling; It is the time average of the last 10 samples;

[0085] 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;

[0086] 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.

[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] In the formula, Not applied The initial refractive index difference at ; It is a constant related to the characteristics of liquid crystal materials and represents the characteristic value of voltage.

[0094] Phase delay With external voltage The relationship is:

[0095] ;

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

[0097] ;

[0098] ;

[0099] ;

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

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

[0102] ;

[0103] ;

[0104] Since both the o-light and the e-light will pass through the transmission axis of the polarizer 2, the two beams of light will inevitably interfere with each other during transmission. According to the principle of interference superposition of light, the final output light intensity can be obtained as:

[0105] ;

[0106] ;

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

[0108] .

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

[0110] ;

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

[0112] exist Greater than or equal to When The gain can be changed by adjusting the resistance value.

[0113] Photodetector PMT: The basic principle of PMT is the photoelectric effect. It is a vacuum component that converts weak incident light signals into larger electrical signals. 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 hit the first pole multiplier, and then pass through a series of multipliers, and finally the current output is realized at the anode.

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

[0115] ;

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

[0117] The overall process is as follows: first, perform peak calibration, set the target value and dynamic range of the FPGA receiving peak detection circuit, then all light intensity sensors start working, and input the light intensity values ​​converted by each 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 it according to the set light threshold. Among them, the weak light range is between 0Lux and 400Lux, mainly for night and cloudy weather; the medium light range is between 400Lux and 6000Lux, mainly for cloudy to sunny and cloudy conditions during the day; the strong light range is greater than 6000Lux, mainly for the strongest light moment at noon. Finally, the peak data of the echo signal output by the photodetector PMT is transmitted back to the FPGA through the peak detection circuit to complete the control of the entire loop.

[0118] Control scheme enabled at different light thresholds: When the FPGA judges that the ambient light intensity is within the low light range according to the light threshold through the data sent back by the light intensity sensor and enters the low light mode, the control of the controllable liquid crystal light valve will be given priority. When the FPGA receives the output of the peak detection circuit collected by the ADC circuit, which is too large or too small, the internal PID algorithm is used to calculate the voltage that the DAC needs to convert when using the controllable liquid crystal light valve function, and then the DAC outputs the voltage value to the control voltage of the controllable liquid crystal light valve to control the attenuation of the return light intensity until the FPGA judges that the value output by the peak detection circuit is within the normal range, and then the FPGA uses DDS technology to adjust the control voltage of the PMT with precise and small voltage steps to make detailed adjustments.

[0119] When the FPGA judges that the ambient light intensity is in the medium light range according to the light threshold, and the output of the peak detection circuit is within the set dynamic range, the control voltage of the PMT is prioritized. The FPGA uses DDS technology to 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 will directly jump to the low light mode processing program and update the low light threshold to the current ambient light value; if it is much smaller, it will directly jump to the high light mode processing program and update the high light threshold to the current ambient light value.

[0120] When the FPGA uses the data sent back by the light intensity sensor, it determines that the ambient light intensity is within the strong light range according to the light threshold and enters the strong light mode. Since the number of photons coming back is small at this time, only the programmable gain amplifier PGA can be enabled to control the echo signal intensity output by the PMT. The FPGA reversely infers the original intensity of the echo signal through 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 value set by the self-gain system, and then inputs it into the compilation of the PGA.

[0121] Smooth switching mechanism between different modes:

[0122] To prevent the ambient light intensity from fluctuating near the set mode threshold, the hysteresis zone is set to prevent the mode from switching frequently. The FPGA uses a sliding window (10 samples in size) to calculate the average value and change trend.

[0123] When the light intensity value from the low light range to the bright light range exceeds 110% of the light threshold for more than three times, the control voltage of the controllable liquid crystal light valve is kept unchanged. After preloading the initial value of the PMT control voltage, the control voltage weight of the controllable liquid crystal light valve is The PMT control voltage weight varies linearly from 100% to 0%. Linear variation from 0% to 100%:

[0124] ;

[0125] ;

[0126] ;

[0127] In the formula, is the control voltage of PMT; 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 control voltage of the controllable liquid crystal light valve; is the calibration factor.

[0128] When the light intensity from the ambient light range to the ambient weak light range is continuously lower than 90% of the light threshold for more than three times, the control voltage of the PMT is kept unchanged, and the initial value of the control voltage of the controllable liquid crystal light valve is preloaded. The PMT control voltage weight varies linearly from 0% to 100%. Linear change from 100% to 0%:

[0129] ;

[0130] ;

[0131] ;

[0132] In the formula, An initial value of the voltage is added to the control voltage of the controllable liquid crystal light valve;

[0133] 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, the control voltage of the PMT is kept unchanged. After preloading the initial value of the PGA gain, the control voltage weight of the PMT is PGA gain value weights linearly from 100% to 0%. Linear setting from 0% to 100%:

[0134] ;

[0135] ;

[0136] ;

[0137] In the formula, The initial value of the gain added to the pre-PGA; is the current peak detection circuit output value; is the voltage strength of the set echo signal; is the actual gain value of the PGA.

[0138] When the light intensity from the ambient strong light range to the ambient light range is continuously lower than 95% of the light threshold for more than three times, 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 of the PMT is PGA gain value weights linearly from 0% to 100% Linear setting from 100% to 0%:

[0139] ;

[0140] ;

[0141] ;

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

[0143] Set an abnormal situation, that is, when the ambient light intensity is detected to fluctuate continuously near the boundary of the hysteresis zone for three times, the hysteresis zone will be automatically expanded, and by default it will be expanded to 1.5 times the hysteresis zone;

[0144] When the ambient light intensity changes too quickly, exceeding the upper limit of the rate change setting When the hysteresis interval is temporarily shortened, the response speed is improved; the setting value The default setting is ±60 Lux / second, which roughly matches the rate at which ambient light intensity changes in daily life.

[0145] ;

[0146] In the formula, is the transformation slope; For the The light intensity value collected is in Lux; It is the average light intensity of the last 10 samples; For the The time point of subsampling; It is the time average of the last 10 samples;

[0147] 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;

[0148] 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.

[0149] The present invention is described below with reference to practical examples:

[0150] The gain adjustment of the echo signal in the present invention involves three key parts: programmable gain amplifier (PGA), PMT control voltage and controllable light valve. PGA has been integrated with communication protocol by chip manufacturers to achieve gain adjustment, and no additional design is required; while the voltage-gain relationship of PMT control voltage and controllable light valve needs to be calibrated 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~5V, the transmittance of the controllable light valve shows a gradient attenuation characteristic, proving that it has the ability to adjust light intensity.

[0152] Then, according to the illumination and distance, several laser points are projected on the building, and the laser points are: 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. Gain curves are measured by adjusting the controllable liquid crystal light valve and PMT control voltage with a step size of 0.1V during the day and night. When measuring at night, the system attenuation coefficient is first set to -10dB, and the controllable light valve gain curve is tested for two near-point echo intensity tests and one far-point echo intensity test, while the PMT control voltage gain curve is only tested for one near-point echo intensity test; during the day, due to the significant increase in ambient light interference, the system gain is adjusted to 3dB, and only the echo intensity test at the near-point 1 position is performed on the gain curves of both.

[0153] Combination Figure 5 , Figure 6 As shown, in Figure 5 In the figure, the red line represents the night-peripoint 1 magnification curve, the blue line represents the night-peripoint 2 magnification curve, the purple line represents the night-apogee magnification curve, and the green line represents the day-peripoint 1 magnification curve; in Figure 6 In the figure, the blue line represents the day-peripoint 1 magnification curve, and the orange line represents the night-peripoint 1 magnification curve. At night, the echo signal of the controllable light valve gain curve measurement was measured at three laser landing points with the same control voltage. The first measurement result is shown in Figure 7 As shown, in Figure 7 In the figure, symbol 1 indicates the position of the reference level of the laser echo signal channel, symbol 2 indicates the position of the reference level of the laser pulse trigger signal channel, and symbol T indicates the position of the trigger level determined by the oscilloscope with the laser pulse trigger signal channel, wherein the minimum value of the echo signal control voltage represented by symbol 1 is -457.86mV. In the second measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -418.60mV. In the third measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -93.026mV.

[0154] From the experimental results Figure 5 , Figure 6 as well as Figure 7 It can be clearly observed that the linear range of the gain curve obtained by measuring the controllable light valve in the night environment corresponds to the control voltage in the range of 2.1V to 4.4V. In order 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 control voltage of the controllable light valve is set to 2.5V and the maximum value is set to 4V.

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

[0156] ;

[0157] At night, the echo signal of the PMT control voltage gain curve was measured three times at the same location with different control voltages. The results of the first measurement are as follows: Fig. 9 As 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 with the laser pulse trigger signal channel, wherein the minimum value of the echo signal control voltage represented by symbol 1 is -1.5455V. In the second measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -772.55mV. In the third measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -2.4178V.

[0158] According to the technical specifications of the PMT (photomultiplier tube), the input range of the control voltage was set to the range specified by the device when measuring its gain curve. The experiment found that within this set voltage range, all changes in the control voltage were within the linear range of the gain curve. To ensure the stability of the system, the minimum value of the control voltage was selected to be 0.55V and the maximum value was 1V.

[0159] like Fig.10 As shown, next, Python is also used to fit the data of the linear interval to calculate the corresponding linear equation:

[0160] ;

[0161] During the day, the echo signal of the controllable light valve control voltage gain curve was measured three times at the same location with different control voltages. The first measurement result is as follows: Fig.11 As 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 with the laser pulse trigger signal channel, wherein the minimum value of the echo signal control voltage represented by symbol 1 is -461.91mV. In the second measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -350.88mV. In the third measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -478.12mV.

[0162] During the day, the echo signal of the PMT control voltage gain curve was measured three times at the same location with different control voltages. The results of the first measurement are as follows: Fig.12 As 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 with the laser pulse trigger signal channel, wherein the minimum value of the echo signal control voltage represented by symbol 1 is -380.88mV. In the second measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -376.82mV. In the third measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -368.72mV.

[0163] In actual experimental scenarios, excessive ambient light intensity during the day has a significant impact on the echo gain control mechanism. Through experimental verification, it is found that there are two verification paths for the conclusion proposed in the present invention that "the interference of ambient light during the day causes the PMT control voltage to approach zero in the echo gain regulation effect": one is directly through measuring the gain response curve of the PMT control voltage during the day, and the other is through analyzing the attenuation characteristics of the controllable light valve under strong light conditions. The gain curve experimental results of the two show a high degree of mutual verification. The gain curve phenomenon of the PMT under strong light and the light intensity control characteristics of the controllable light valve in the range of 2.1V~5V form a complementary verification.

[0164] When the control voltage of the controllable light valve is in the range of greater than 2.1V and less than 5V, the echo signal will be enhanced. This is because the controllable light valve will attenuate the intensity of all returned light, including external interference light. In the back-stage light receiving circuit, there is a filter corresponding to the laser light wave, which makes the received light positively correlated with the light returned from the laser landing point. Therefore, when the controllable light valve linearly attenuates the overall return light intensity, it actually reduces the influence of external interference light, thereby enhancing the echo signal.

[0165] Finally, after completing the construction of the linear gain function of the PMT and the controllable light valve, it was integrated into the system control algorithm, and a dual-period dynamic gain debugging experiment was carried out at 10:15 during the day and 18:30 at night. The change curve of the ambient light intensity during the debugging process is shown in Figure 13. Fig.14 As shown in Figure 1, the peak value of the echo of the target near point 1 irradiated by the laser is set to 800mV. At 10:20 am, when the system was not started, the echo signal was measured three times. The result of the first measurement is as follows: Fig.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 with the laser pulse trigger signal channel, wherein the minimum value of the echo signal control voltage represented by symbol 1 is -238.07mV. In the second measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -224.13mV. In the third measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -284.81mV.

[0166] The echo signal was measured three times when the system was started at 10:23 am. The results of the first measurement are as follows: Fig.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 with the laser pulse trigger signal channel, wherein the minimum value of the echo signal control voltage represented by symbol 1 is -796.33mV. In the second measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -813.77mV. In the third measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -854.45mV.

[0167] At 18:40 at night, when the system was not started, the echo signal was measured three times. The results of the first measurement are as follows: Fig.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 with the laser pulse trigger signal channel, wherein the minimum value of the echo signal control voltage represented by symbol 1 is -1.5461V. In the second measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -1.5064V. In the third measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -1.4647 V.

[0168] When the system was started at 18:44 at night, the echo signal was measured three times. The results of the first measurement are as follows: Fig.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 with the laser pulse trigger signal channel, wherein the minimum value of the echo signal control voltage represented by symbol 1 is -732.39mV. In the second measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -813.77mV. In the third measurement result, the minimum value of the echo signal control voltage represented by symbol 1 is -773.05mV. Using the above Fig.15 , Fig.16 , Fig.17 as well as Fig.18 The dynamic gain adjustment curves at different time periods and the output stability verification results are presented.

[0169] The final experiment shows that the present invention verifies the effectiveness of the gain adjustment system in complex lighting environments through systematic experiments. The research results show that the dual-period 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 up to 10000Lux during the day and the light intensity 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 technicians in this technical field within the essential scope 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.

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, 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.

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, 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 the PGA.

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, 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.

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 value from the low-light range to the high-light 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 the initial value of the PMT control voltage is preloaded, the control voltage weight of the controllable liquid crystal light valve is The PMT control voltage weight varies linearly from 100% to 0%. Linear variation from 0% to 100%: ； ； ； In the formula, is the control voltage of PMT; 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 control voltage of the controllable liquid crystal light valve; is the calibration factor.

7. The method for controlling the self-gain of the airborne laser radar echo signal strength according to claim 6, 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 of the controllable liquid crystal light valve is The PMT control voltage weight varies linearly from 0% to 100%. Linear change from 100% to 0%: ； ； ； In the formula, The initial value of the voltage added to the control voltage of the pre-controllable liquid crystal light valve.

8. The method for controlling the self-gain of the airborne laser radar echo signal strength according to claim 7, 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 of the PMT is PGA gain value weights linearly from 100% to 0%. Linear setting from 0% to 100%: ； ； + 1； In the formula, The initial value of the gain added to the pre-PGA; is the current peak detection circuit output value; is the voltage strength of the set echo signal; is the actual gain value of the PGA.

9. A method for controlling the self-gain of the airborne laser radar echo signal strength according to claim 8, 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 of the PMT is PGA gain value weights linearly from 0% to 100% Linear setting from 100% to 0%: ； ； ； In the formula, is the gain of PMT; is the calibration coefficient; is the actual input value of the PMT control voltage; is the actual gain value of the PGA.

10. The method for controlling the self-gain of the airborne laser radar echo signal strength according to claim 9, 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 quickly, exceeding the upper limit of the rate change setting When , the hysteresis interval will be temporarily shortened to improve the response speed; ； In the formula, is the transformation slope; For the The light intensity value collected is in Lux; It is the average light intensity of the last 10 samples; For the The time point of subsampling; 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.

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