CN106405527A - Airborne LiDAR device capable of adaptively compensating for elevation changes of to-be-measured terrain - Google Patents

Abstract

An airborne LiDAR device that can self-adaptively compensate for changes in the elevation of the measured terrain is characterized in that it can perform laser scanning in two directions at the same time. The point cloud bandwidth prediction scan. When the plane is flying at a straight line at a constant speed and the elevation of the measured terrain changes, the scanning point cloud bandwidth is predicted in real time by the forward oblique downward line of sight laser scanner, and the terrain elevation change adaptive regulator calculates the terrain elevation to make the line of sight directly below The scanning point cloud bandwidth and point density remain constant for the scanning field of view and pulse repetition frequency. The rotation angle value of the rotating prism is obtained by the photoelectric shaft angle encoder, and the pulse repetition frequency of the laser pulse transmitter is adjusted, and the laser pulse with the corrected frequency is output within the corrected scanning field angle range. This self-adaptive adjustment method can ensure that the laser scanning point cloud bandwidth and point density remain unchanged when the airborne LiDAR measures terrain with changing elevations.

Description

An airborne LiDAR device that can adaptively compensate the elevation change of the measured terrain

technical field

The invention relates to the problem of self-adaptive adjustment of airborne LiDAR to terrain elevation change.

Background technique

Airborne LiDAR is a new technology of terrain surveying and mapping based on the principle of laser ranging. It is an integration of a variety of latest technology measurement equipment, including airborne platforms, laser scanners, global positioning systems GPS and inertial measurement units IMUs.

The working process of the airborne LiDAR is as follows: the aircraft flies in a straight line at a constant speed on the set route, the flight altitude is planned in advance, and the ideal flight altitude remains unchanged in the local horizontal plane reference coordinate system. The track and attitude angle information of the laser scanner load platform is measured in real time by GPS and IMU, and the distance from the scanning mirror of the laser scanner to the laser foot point on the ground is calculated according to the flight time of the pulse emitted by the laser scanner. Obtain the scanning angle at the time of the laser pulse emission, and calculate the three-dimensional coordinates of the laser footpoint on the ground according to the above data. A large number of laser footpoints form a laser point cloud, and after subsequent point cloud processing, a reconstructed 3D imaging product of the measured terrain can be obtained.

The bandwidth and point density of the lidar scanning point cloud determine the quality of the 3D reconstruction product of the measured terrain. The ideal laser point cloud is the scanning point cloud obtained by the lidar when the elevation of the measured terrain remains constant, while the actual laser point cloud is the scanning point cloud obtained by the lidar when the elevation of the measured terrain changes. When the terrain elevation does not change, the obtained ideal laser point cloud bandwidth is always constant, and the point cloud distribution density is uniform. When the measured terrain elevation changes, the actual laser point cloud distribution bandwidth and point density both change significantly. When the elevation of the measured terrain increases, the relative height between the aircraft and the measured ground will decrease. At this time, the bandwidth of the point cloud will decrease, which will cause missing scans of important terrain areas. When the elevation of the measured terrain decreases, the relative height between the aircraft and the measured terrain will increase, and the bandwidth of the point cloud will increase at this time. However, since the laser pulse repetition frequency remains unchanged, the number of laser points in each scanning line is fixed, so the points The lower the density, the lower the resolution of the measurement, and the distortion of the three-dimensional imaging of the terrain reconstruction will increase. Therefore, the adaptive adjustment of terrain elevation changes for airborne LiDAR is very necessary and has important practical significance.

At present, the existing various point-scanning 3D imaging airborne LiDARs and other related types of LiDARs, such as push-broom line-scanning airborne LiDARs, do not have real-time compensation functions and devices for terrain elevation changes. There is no research, description and report on the real-time compensation technology of airborne LiDAR for terrain elevation changes.

Contents of the invention

The present invention provides an airborne LiDAR device (1) capable of adaptively compensating for measured terrain elevation changes, which is characterized in that it includes an adaptive laser LiDAR (11), an adaptive regulator for terrain elevation changes (12) and a laser scanning bandwidth Measuring instrument (13); the adaptive laser LiDAR (11), characterized in that it includes an adjustable frequency laser pulse rangefinder (111), a mirror (112), a positive 6-sided rotating prism (113), a rotating motor ( 114), the photoelectric shaft angle encoder (115), the scanning direction of the outgoing laser pulse is the line of sight directly below the airborne platform perpendicular to the flight trajectory; the laser scanning bandwidth measuring instrument (13) is characterized in that it includes a fixed frequency laser The pulse rangefinder (131), the reflecting mirror (132) and the oblique 6-faced pyramidal rotating prism (133), the scanning direction of the outgoing laser pulse is the forward oblique downward line of sight direction. The frequency-adjustable laser pulse rangefinder (111) hits the side of the 6-sided rotating prism (113) through the reflector (112), and scans the line of sight directly below the aircraft platform after reflection. Obtain the three-dimensional coordinates of the laser foot point on the ground to be measured. The laser pulse emitted by the fixed-frequency laser pulse rangefinder (131) is reflected by the reflector (132) to one side of the oblique 6-sided pyramid rotating prism (133), and hits the flying Directly in front of a terrain surface with a scanning line spacing, pre-scanning can be performed to obtain the ranging value of the scanning laser foot point, which is provided to the terrain elevation change adaptive regulator (12), combined with the GPS (2) and the flight parameter information measured by the IMU (3), the three-dimensional coordinates of the scanning laser foot point can be obtained, and then the predicted bandwidth of the scanning line can be calculated; through further calculation, the scanning point can be obtained in the next laser scanning line. The correction value of the scanning field angle and the laser pulse frequency that both cloud bandwidth and point density remain unchanged; the real-time rotation angle of the positive six-sided rotating prism (113) is obtained by the photoelectric shaft encoder (115) , feed back to the terrain elevation change adaptive regulator (12), only generate a square wave signal of a specified frequency within the corrected scanning field angle range, and provide it to the adjustable frequency laser pulse range finder (111), Make the frequency-adjustable laser pulse rangefinder (111) emit a laser pulse with a corrected frequency, so as to ensure that the bandwidth and point density of the laser scanning point cloud in the direction of the line of sight directly below remain unchanged, and are not affected by changes in terrain elevation . The frequency-adjustable laser pulse rangefinder (111) adopts an externally modulated laser with adjustable frequency, which has a dedicated signal interface, and the user inputs a square wave signal to control the emission frequency of the laser pulse, and the modulation frequency can be as high as 10MHz. The terrain elevation change adaptive regulator (12) generates a square wave signal of a required frequency, triggers the frequency-adjustable laser pulse rangefinder (111), and generates a laser pulse of a corresponding frequency.

Wherein, the airborne LiDAR device (1), the global positioning system GPS (2), and the inertial measurement unit (IMU) (3) that can adaptively compensate for the elevation change of the measured terrain are all installed on the airborne platform (4). The airborne LiDAR device (1) capable of adaptively compensating the elevation change of the measured terrain can perform laser scanning in two directions at the same time, one is the direction of the line of sight directly below the airborne platform, and the other is the direction of the line of sight obliquely below the forward direction . The laser scanning in the direction of the line of sight directly below is the main working scanner. It measures the distance of each laser foot point, and combines the flight status information measured by the GPS (2) and the IMU (3) to obtain the distance of each ground. The three-dimensional coordinates of the laser foot point. The laser scanning in the direction of the line of sight obliquely downward is a bandwidth prediction laser. There is a small angle β between the scanning direction and the line of sight directly below, and the scanning bandwidth of the terrain at a certain distance in front can be obtained. Compared with the initial measurement bandwidth of scanning, if there is a deviation, the scanning angle and laser pulse frequency of the laser scanner in the direction of the line of sight directly below can be adjusted in real time, so that the bandwidth and point density of the laser scanning point cloud in the direction of the line of sight directly below are always the same. Change.

Wherein, the rotating motor (114) is coaxially fixedly connected with the photoelectric shaft-angle encoder (115), the positive 6-sided rotating prism (113) and the oblique 6-sided pyramidal rotating prism (133). There is a small inclination angle β between the side of the oblique 6-sided pyramid rotating prism (133) and the rotation axis. The fixed-frequency laser pulse rangefinder (131) emits a fixed-frequency laser pulse, which passes through the reflector (132) and shoots to the side of the oblique 6-sided pyramid rotating prism (133), and the reflected The outgoing laser pulses point to the forward obliquely downward line of sight, and perform tangential scanning perpendicular to the flying direction as the oblique 6-sided pyramid rotating prism (133) rotates. The echoes of the laser feet on the ground to be measured are reflected back to the fixed-frequency laser pulse rangefinder (131) to obtain laser distance measurement, which is measured by combining the GPS (2) and the IMU (3) The flight parameter information can calculate the three-dimensional coordinates of each laser foot point, and then obtain the bandwidth of a scanning line. The adjustable-frequency laser pulse rangefinder (111) emits adjustable-frequency laser pulses, which pass through the reflector (112) and shoot to the side of the positive six-sided rotating prism (113). The laser pulses point to the line of sight directly below the airborne platform, and perform tangential scanning perpendicular to the flight direction as the positive six-sided rotating prism (113) rotates. The echoes of the laser feet on the ground to be measured are reflected back to the adjustable-frequency laser pulse rangefinder (111) to obtain laser distance measurement, which is measured by combining the GPS (2) and the IMU (3) The flight parameter information can be used to calculate the three-dimensional coordinates of each laser foot point. The rotating motor (114) rotates at a constant speed, and the real-time rotation angles of the positive 6-sided rotating prism (113) and the inclined 6-sided pyramidal rotating prism (133) are obtained by the photoelectric shaft encoder (115) . According to the angle measurement value of the photoelectric shaft angle encoder (115), the frequency-adjustable laser pulse rangefinder (111) can be controlled to emit laser pulses within the required rotation angle, and the frequency of the laser pulses can meet the scanning point cloud Bandwidth and point density remain constant throughout.

Wherein, O ₁ is the specular laser reflection point of the positive 6-sided rotating prism (113), O ₂ is the specular laser reflection point of the oblique 6-sided pyramidal rotating prism (133), and the distance between O ₁ O ₂ is L, the laser scanning angle is ±θº, the relative height between the aircraft and the measured ground is H, assuming that the measured ground is a plane, D ₁ and D ₂ are the scanning line of the line of sight directly below and the scanning line of the line of sight obliquely below the front, respectively _Bandwidth , L1 is the distance between _D1 and _D2 . The inclination angle between the side of the oblique 6-sided pyramid rotating prism (133) and the rotation axis is βº, so the included angle between the scanning surface reflected from the side and the vertical line is 2βº. It can be obtained by calculation, D ₁ =D ₂ *cos2β, therefore, after the scanning line bandwidth D ₂ is obtained by measuring the oblique 6-sided pyramid rotating prism (133), it can be obtained by calculation if the line of sight directly below is used The scanning line bandwidth D ₁ during laser scanning measurement is the predicted scanning line bandwidth in the direction of the line of sight directly below.

Among them, the terrain elevation position when the airborne lidar starts to measure is the ideal terrain elevation position, and the ideal scanning field angle is ±θ ₀ º. When measuring the actual terrain with elevation changes, the scanning bandwidth is required to be constant, then D ₁ *tan(θ ₀ )=D ₂ *tan(θ _B ), from which the compensated scanning field angle can be obtained o. In addition, it is assumed that the angular velocity of the rotating electrical machine (114) is º/s, if the number of scanning points of each scanning line is a constant m, then when the scanning field angle is ±θ ₀ º, the scanning time to obtain a laser scanning line is , then the laser pulse repetition frequency at the ideal terrain elevation is (Hz). When the scanning field of view is º, the laser pulse repetition frequency at this time is (Hz). It can be seen that according to the predicted laser scanning line bandwidth D ₂ , the compensated scanning field of view can be obtained º, and then the compensated pulse repetition frequency can be obtained . By adaptively adjusting the scanning field angle and laser emission frequency of the laser scanning directly below the line of sight, the adverse effects of terrain elevation changes on the laser scanning point cloud can be compensated in real time, so that the scanning bandwidth and point density of the ground laser point cloud remain constant. The resolution and quality of airborne LiDAR 3D imaging products are effectively guaranteed.

Wherein, control software is compiled in the terrain elevation change adaptive regulator (12). In the main program, interrupt 1 is mainly set, that is, when the rotation angle of the positive 6-sided rotating prism (113) is -30º, -90º, -150º, -210º, -270º, -330º counterclockwise, the interrupt subroutine is started 1 Enabled, the rotating prism has six interruptions per revolution, the same number of sides as the rotating prism. In the interrupt 1 subroutine, first collect the laser scanning ranging value, the flight status value measured by GPS and IMU, and the actual measurement value of the rotation angle of the photoelectric shaft angle encoder; calculate the three _- dimensional coordinates of the laser foot point, and obtain the ideal scanning line bandwidth D1 and Predicted scanning line bandwidth D ₂ ; then, calculate the corrected scanning field angle and square wave frequency value of the laser scanning in the line of sight directly below the next line; finally, according to the measured value of the photoelectric shaft encoder (115), During the scanning process of the positive 6-sided rotating prism (113) on each side, only output a square wave signal with a corrected frequency within the corrected scanning field angle range, triggering the adjustable frequency laser pulse rangefinder (111 ) emits a laser pulse of the corresponding frequency. In interrupt subroutine 2, when the actual rotation angle of the positive 6-sided rotating prism (113) measured by the photoelectric shaft encoder (115) is When interrupt 2 is started, the terrain elevation change adaptive regulator (12) starts to output frequency of a square wave signal, triggering the adjustable-frequency laser pulse rangefinder (111) to emit a frequency of of laser pulses. In interrupt subroutine 3, when the photoelectric shaft encoder (115) measures the actual rotation angle of the positive 6-sided rotating prism (113) as When interrupt 3 is started, the terrain elevation change adaptive regulator (12) stops outputting at a frequency of The square wave signal causes the frequency-adjustable laser pulse rangefinder (111) to stop emitting laser pulses.

Wherein, the control system of the terrain elevation change adaptive regulator (12) uses an ARM (LPC2138) embedded system as a controller. The flight state value measured by the GPS (2) and the IMU (3) is input to the ARM (LPC2138) through the serial port 1; the adjustable frequency laser pulse range finder (111) and the fixed frequency laser pulse range finder The laser distance measurement value of the distance meter (131) is input to ARM (LPC2138) through serial port 2; the rotation angle measured by the photoelectric shaft angle encoder (115) is input to ARM (LPC2138) through serial port 3. ARM (LPC2138) can generate square wave with adjustable frequency, adopt numerical calculation synthesis and D/A conversion module, and output square wave signal with frequency range of 1 Hz~50 kHz. The generation of the square wave is formed by the alternate output of two signal data of different sizes, and the length of each signal data output is determined according to the required signal frequency. When measuring the terrain at a certain elevation, the required scanning field of view is , the laser pulse frequency is , according to the scanning angle of the positive 6-sided rotating prism (113) measured by the photoelectric shaft encoder (115), the ARM (LPC2138) can control the start and stop of the square wave signal output. When the scanning angle of the positive 6-sided rotating prism (113) measured by the photoelectric shaft encoder (115) is , the ARM (LPC2138) output frequency is The square wave signal triggers the frequency-adjustable laser pulse rangefinder (111), and the frequency is of laser pulses. When the scanning angle of the positive 6-sided rotating prism (113) measured by the photoelectric shaft encoder (115) is , the ARM (LPC2138) stops outputting the square wave signal, triggering the frequency-adjustable laser pulse rangefinder (111) to stop outputting laser pulses.

Description of drawings

Figure 1 is a schematic diagram of the measurement of the lidar when the terrain has sinusoidal fluctuations.

Figure 2 is a schematic diagram of the influence of terrain elevation changes on the distribution bandwidth and point density of laser scanning point clouds.

Figure 3 is a schematic diagram of the working principle of the airborne LiDAR device with adaptive compensation function for the elevation change of the measured terrain.

Fig. 4 is a system composition block diagram of an airborne LiDAR device (1) that can adaptively compensate the elevation change of the measured terrain.

Fig. 5 is a schematic structural diagram of an airborne LiDAR device (1) capable of adaptively compensating the elevation change of the measured terrain.

Fig. 6 is a schematic diagram of a method for predicting and measuring scanning point cloud bandwidth.

Fig. 7 is a schematic diagram of an adaptive compensation method for terrain elevation changes by airborne laser scanning.

Fig. 8 is a software working flow chart of the terrain elevation change adaptive regulator (12).

Fig. 9 is a schematic diagram of hardware system connection of the terrain elevation change adaptive regulator (12).

detailed description

The patent embodiments of the present invention will be further described in detail below in conjunction with the accompanying drawings.

Figure 1 is a schematic diagram of the measurement of the lidar when the terrain has sinusoidal fluctuations. The simulated flight altitude of the airborne laser radar is 100 meters, and the flight speed is 60 meters per second with a uniform linear motion. The fluctuation of the measured terrain is a sinusoidal change with an amplitude of 20 meters and a period of 120 meters. The laser radar is installed on the airborne platform, and uses a rotating polygonal prism system to linearly scan the measured terrain in the line of sight directly below.

Figure 2 is a schematic diagram of the influence of terrain elevation changes on the distribution bandwidth and point density of laser scanning point clouds. The bandwidth and point density of the lidar scanning point cloud determine the quality of the 3D reconstruction product of the measured terrain. The ideal laser point cloud is the scanning point cloud obtained by the lidar when the elevation of the measured terrain remains constant, while the actual laser point cloud is the scanning point cloud obtained by the lidar when the measured terrain is a sinusoidal change with an amplitude of 20 meters and a period of 120 meters. Obtained scanned point cloud. Assuming that the initial height of the aircraft from the ground to be measured is 100 meters when the measurement starts, the flight speed is 60 m/s, and the scanning field of view is . When the terrain elevation does not change, the bandwidth of the ideal laser point cloud is always constant, and the distribution density of the point cloud is uniform. When the measured terrain elevation changes, the actual laser point cloud distribution bandwidth and point density both change significantly. When t=0, the scanning bandwidth is 44.3 meters, and the number of laser points per scanning line is 20, so the distance between laser points is 2.215 meters; when t=0.5 seconds, the scanning bandwidth is 35.5 meters, and the distance between laser points is 1.775 meters ; When t=1.5 seconds, the scanning bandwidth is 53.2 meters, and the distance between laser points is 2.66 meters. It can be seen that when the elevation of the measured terrain increases, the relative height between the aircraft and the ground to be measured decreases, and at this time the bandwidth of the point cloud decreases, resulting in missed scanning of important terrain areas. When the elevation of the measured terrain decreases, the relative height between the aircraft and the measured terrain increases. At this time, the bandwidth of the point cloud increases. However, since the number of points in each scanning line remains unchanged, the point density decreases, which reduces the measurement resolution and reconstructs the three-dimensional terrain. Imaging distortion will increase.

Figure 3 is a schematic diagram of the working principle of the airborne LiDAR device with adaptive compensation function for the elevation change of the measured terrain. The airborne LiDAR device (1), the global positioning system GPS (2), and the inertial measurement unit (IMU) (3) that can adaptively compensate the elevation change of the measured terrain are all installed on the airborne platform (4). The airborne LiDAR device (1) capable of adaptively compensating the elevation change of the measured terrain can perform laser scanning in two directions at the same time, one is the direction of the line of sight directly below the airborne platform, and the other is the direction of the line of sight obliquely below the forward direction . The laser scanning in the direction of the line of sight directly below is the main working scanner. It measures the distance of each laser foot point, and combines the flight status information measured by the GPS (2) and the IMU (3) to obtain the distance of each ground. The three-dimensional coordinates of the laser foot point. The laser scanning in the direction of the line of sight obliquely downward is a bandwidth prediction laser. There is a small angle β between the scanning direction and the line of sight directly below, and the scanning bandwidth of the terrain at a certain distance in front can be obtained. Compared with the initial measurement bandwidth of scanning, if there is a deviation, the scanning angle and laser pulse repetition frequency of the laser scanner in the direction of the line of sight directly below can be adjusted in real time, so that the bandwidth and point density of the laser scanning point cloud in the direction of the line of sight directly below are always the same. Change.

Figure 4 is a system block diagram of an airborne LiDAR device (1) that can adaptively compensate for changes in the elevation of the measured terrain. The present invention provides an airborne LiDAR device (1) capable of adaptively compensating for measured terrain elevation changes, which is characterized in that it includes an adaptive laser LiDAR (11), an adaptive regulator for terrain elevation changes (12) and a laser scanning bandwidth Measuring instrument (13); the adaptive laser LiDAR (11), characterized in that it includes an adjustable frequency laser pulse rangefinder (111), a mirror (112), a positive 6-sided rotating prism (113), a rotating motor ( 114), the photoelectric shaft angle encoder (115), the scanning direction of the outgoing laser pulse is the line of sight directly below the airborne platform perpendicular to the flight trajectory; the laser scanning bandwidth measuring instrument (13) is characterized in that it includes a fixed frequency laser The pulse rangefinder (131), the reflecting mirror (132) and the oblique 6-faced pyramidal rotating prism (133), the scanning direction of the outgoing laser pulse is the forward oblique downward line of sight direction. The frequency-adjustable laser pulse rangefinder (111) hits the side of the 6-sided rotating prism (113) through the reflector (112), and scans the line of sight directly below the aircraft platform after reflection. Obtain the three-dimensional coordinates of the laser foot point on the ground to be measured. The laser pulse emitted by the fixed-frequency laser pulse rangefinder (131) is reflected by the reflector (132) to one side of the oblique 6-sided pyramid rotating prism (133), and hits the flying Directly in front of a terrain surface with a scanning line spacing, pre-scanning can be performed to obtain the ranging value of the scanning laser foot point, which is provided to the terrain elevation change adaptive regulator (12), combined with the GPS (2) and the flight parameter information measured by the IMU (3), the three-dimensional coordinates of the scanning laser foot point can be obtained, and then the predicted bandwidth of the scanning line can be calculated; through further calculation, the scanning point can be obtained in the next laser scanning line. The correction value of the scanning field angle and the laser pulse frequency that both cloud bandwidth and point density remain unchanged; the real-time rotation angle of the positive six-sided rotating prism (113) is obtained by the photoelectric shaft encoder (115) , feed back to the terrain elevation change adaptive regulator (12), only generate a square wave signal of a specified frequency within the corrected scanning field angle range, and provide it to the adjustable frequency laser pulse rangefinder (111), Make the frequency-adjustable laser pulse rangefinder (111) emit a laser pulse with a corrected frequency, so as to ensure that the bandwidth and point density of the laser scanning point cloud in the direction of the line of sight directly below remain unchanged, and are not affected by changes in terrain elevation . The frequency-adjustable laser pulse rangefinder (111) adopts an externally modulated laser with adjustable frequency, which has a dedicated signal interface, and the user inputs a square wave signal to control the emission frequency of the laser pulse, and the modulation frequency can be as high as 10MHz. The terrain elevation change adaptive regulator (12) generates a square wave signal of a required frequency, triggers the frequency-adjustable laser pulse range finder (111), and generates a laser pulse of a corresponding frequency.

Fig. 5 is a schematic structural diagram of an airborne LiDAR device (1) capable of adaptively compensating the elevation change of the measured terrain. The rotating motor (114) is coaxially fixedly connected with the photoelectric shaft-angle encoder (115), the positive 6-sided rotating prism (113) and the oblique 6-sided pyramidal rotating prism (133). There is a small inclination angle β between the side of the oblique 6-sided pyramid rotating prism (133) and the rotation axis. The fixed-frequency laser pulse rangefinder (131) emits a fixed-frequency laser pulse, which passes through the reflector (132) and shoots to the side of the oblique 6-sided pyramid rotating prism (133), and the reflected The outgoing laser pulses point to the forward obliquely downward line of sight, and perform tangential scanning perpendicular to the flying direction as the oblique 6-sided pyramid rotating prism (133) rotates. The echoes of the laser feet on the ground to be measured are reflected back to the fixed-frequency laser pulse rangefinder (131) to obtain laser distance measurement, which is measured by combining the GPS (2) and the IMU (3) The flight parameter information can calculate the three-dimensional coordinates of each laser foot point, and then obtain the bandwidth of a scanning line. The adjustable-frequency laser pulse rangefinder (111) emits adjustable-frequency laser pulses, which pass through the reflector (112) and shoot to the side of the positive six-sided rotating prism (113). The laser pulses point to the line of sight directly below the airborne platform, and perform tangential scanning perpendicular to the flight direction as the positive six-sided rotating prism (113) rotates. The echoes of the laser feet on the ground to be measured are reflected back to the adjustable-frequency laser pulse rangefinder (111) to obtain laser distance measurement, which is measured by combining the GPS (2) and the IMU (3) The flight parameter information can be used to calculate the three-dimensional coordinates of each laser foot point. The rotating motor (114) rotates at a constant speed, and the real-time rotation angles of the positive 6-sided rotating prism (113) and the inclined 6-sided pyramidal rotating prism (133) are obtained by the photoelectric shaft encoder (115) . According to the angle measurement value of the photoelectric shaft angle encoder (115), the frequency-adjustable laser pulse rangefinder (111) can be controlled to emit laser pulses within the required rotation angle, and the frequency of the laser pulses can meet the scanning point cloud Bandwidth and point density remain constant throughout.

Fig. 6 is a schematic diagram of a method for predicting and measuring scanning point cloud bandwidth. O ₁ is the specular laser reflection point of the positive 6-sided rotating prism (113), O ₂ is the specular laser reflection point of the oblique 6-sided pyramidal rotating prism (133), the distance between O ₁ O ₂ is L, The laser scanning angle is ±θº, the relative height between the aircraft and the measured ground is H, assuming that the measured ground is a plane, D ₁ and D ₂ are the bandwidths of the scanning line directly below the line of sight and the scanning line of the obliquely downward line of sight ahead, respectively, _L1 is the distance between _D1 and _D2 . The inclination angle between the side of the oblique 6-sided pyramid rotating prism (133) and the rotation axis is βº, so the included angle between the scanning surface reflected from the side and the vertical line is 2βº. It can be obtained by calculation, D ₁ =D ₂ *cos2β, therefore, after the scanning line bandwidth D ₂ is obtained by measuring the oblique 6-sided pyramid rotating prism (133), it can be obtained by calculation if the line of sight directly below is used The scanning line bandwidth D ₁ during laser scanning measurement is the predicted scanning line bandwidth in the direction of the line of sight directly below.

Fig. 7 is a schematic diagram of an adaptive compensation method for terrain elevation changes by airborne laser scanning. In the figure, D ₁ is the point cloud bandwidth obtained by scanning the line of sight directly below at the ideal terrain elevation position, and D ₂ is the predicted bandwidth of the laser point cloud obtained by scanning in the direction of the line of sight obliquely below at the actual terrain elevation position. Let the terrain elevation position when the airborne lidar starts to measure be the ideal terrain elevation position, and the ideal scanning field of view is º, when measuring the actual elevation terrain position, the scanning bandwidth is required to be constant, then D ₁ *tan(θ ₀ )= D ₂ *tan(θ _B ), thus, the compensated scanning field angle can be obtained o. In addition, it is assumed that the angular velocity of the rotating electrical machine (114) is º/s, and at the same time, the number of scanning points m of each scanning line is constant, then when the scanning field angle is ±θ ₀ º, the scanning time to obtain a laser scanning line is , then the laser pulse frequency at the ideal terrain elevation is (Hz). When the scanning field of view is º, the laser scanning pulse frequency at this time is (Hz). It can be seen that according to the predicted laser scanning line bandwidth D ₂ , the compensated scanning field of view can be obtained º, and then the compensation pulse frequency can be obtained . By adaptively adjusting the scanning field angle and laser emission frequency of the laser scanning directly below the line of sight, the adverse effects of terrain elevation changes on the laser scanning point cloud can be compensated in real time, so that the scanning bandwidth and point density of the ground laser point cloud remain constant. The resolution and quality of airborne LiDAR 3D imaging products are effectively guaranteed.

Fig. 8 is a software workflow of the terrain elevation change adaptive regulator (12). In the terrain elevation change adaptive regulator (12), control software is compiled. In the main program, interrupt 1 is mainly set, that is, when the positive 6-face rotating prism (113) rotates counterclockwise at an angle of -30°, -90°, -150°, -210°, -270°, -330° When the interrupt subroutine 1 is enabled, the rotating prism has six interrupts per one revolution, which is the same as the number of sides of the rotating prism. The control flow chart of the interrupt 1 subroutine is shown in Figure 8(a). When the interrupt 1 subroutine is triggered, the laser scanning range measurement value, the flight status value measured by GPS and IMU, and the actual measurement value of the rotation angle of the photoelectric shaft encoder are collected first. ; Calculate the three-dimensional coordinates of the laser foot point to obtain the ideal scan line bandwidth D ₁ and the predicted scan line bandwidth D ₂ ; then, calculate the compensated scanning field angle and square wave frequency value of the laser scanning in the direction of the line of sight directly below the next line; Finally, according to the actual measured value of the photoelectric shaft angle encoder (115), during the scanning process of the positive 6-sided rotating prism (113) on each side, only the required frequency is output within the specified scanning field angle range. wave signal, triggering the frequency-adjustable laser pulse range finder (111) to emit a laser pulse of a corresponding frequency. The flow chart of interrupt subroutine 2 is as shown in Figure 8 (b), when the actual angle of rotation of the described positive 6-face rotating prism (113) measured by the photoelectric shaft encoder (115) is θ _r =-θ _B , start interrupt 2, and the adaptive adjuster for terrain elevation change (12) begins to output a square wave signal with a frequency of f _B , triggering the frequency-adjustable laser pulse rangefinder (111) to send a laser pulse with a frequency of f _B . The flow chart of interrupt subroutine 3 is as shown in Figure 8 (c), when the actual angle of rotation of the described positive 6-face rotating prism (113) measured by the photoelectric shaft angle encoder (115) is _θr =+ _θB , Start interrupt 3, the adaptive adjuster for terrain elevation change (12) stops outputting the square wave signal with frequency f _B , so that the frequency-adjustable laser pulse range finder (111) stops sending out laser pulses.

Fig. 9 is a schematic diagram of hardware system connection of the terrain elevation change adaptive regulator (12). The control system of the terrain elevation change self-adaptive regulator (12) uses an ARM (LPC2138) embedded system as a controller. The flight state value measured by the GPS (2) and the IMU (3) is input to the ARM (LPC2138) through the serial port 1; the adjustable frequency laser pulse range finder (111) and the fixed frequency laser pulse range finder The laser range measurement value of the distance meter (131) is input to the ARM (LPC2138) through the serial port 2; the rotation angle measured by the photoelectric shaft angle encoder (115) is input to the ARM (LPC2138) through the serial port 3. ARM (LPC2138) can generate a square wave signal with adjustable frequency, and output a square wave signal with a frequency range of 1 Hz to 50 kHz through the D/A conversion module. The generation of the square wave is formed by the alternate output of two signal data of different sizes, and the length of each signal data output is determined according to the required signal frequency. When measuring the terrain at a certain elevation, the required scanning field of view is , the laser pulse frequency is , according to the scanning field angle value of the positive 6-sided rotating prism (113) measured by the photoelectric shaft encoder (115), the ARM (LPC2138) can control the start and stop of the square wave signal output. When the scanning angle of the positive 6-sided rotating prism (113) measured by the photoelectric shaft encoder (115) is , ARM (LPC2138) output frequency a square wave signal that triggers the adjustable-frequency laser pulse rangefinder (111) to emit a frequency of laser pulses. When the scanning angle of the positive 6-sided rotating prism (113) measured by the photoelectric shaft encoder (115) is , the ARM (LPC2138) stops outputting square wave signals, and the frequency-adjustable laser pulse rangefinder (111) stops outputting laser pulses.

The above description of the present invention and its embodiments is not limited thereto, and what is shown in the drawings is only one of the embodiments of the present invention. Without departing from the inventive concept of the present invention, any uninvented design of structures or embodiments similar to the technical solution shall fall within the protection scope of the present invention.

Claims (5)

1. a kind of can adaptive equalization tested landform altitude change airborne LiDAR device（1）It is characterised in that inclusion self adaptation Laser LiDAR（11）, landform altitude change self-adaptive regulator（12）With laser scanning bandwidth measurement instrument（13）；Described self adaptation Laser LiDAR（11）It is characterised in that including adjustable frequency laser pulse ranging instrument（111）, reflecting mirror（112）, positive 6 faces rotation Prism（113）, electric rotating machine（114）, optical electric axial angle encoder（115）；Described laser scanning bandwidth measurement instrument（13）, its feature It is including fixed frequency laser pulse ranging instrument（131）, reflecting mirror（132）With oblique 6 face Vertebral rotation prisms（133）；Described Adjustable frequency laser pulse ranging instrument（111）Through described reflecting mirror（112）Beat in described positive 6 face rotary prisms（113）Side On, after reflection, its shoot laser pulse scanning direction is perpendicular to direction of visual lines immediately below the airborne platform of flight path, obtains Obtain the laser footpoint three-dimensional coordinate on tested ground；Described fixed frequency laser pulse ranging instrument（131）The laser pulse sending, warp Described reflecting mirror（132）Reflex to described oblique 6 face Vertebral rotation prisms（133）A side on, after reflection, its outgoing swash Light pulse scans direction is front oliquely downward direction of visual lines, plays the landform table of one scanning line spacing of dead ahead in heading On face, carry out prescan, the distance measurement value of scanning laser pin point can be obtained, be supplied to described landform altitude change self-adaptive regulator （12）, in conjunction with described GPS（2）With described IMU（3）The flight parameter information of measurement, can obtain the three-dimensional seat of scanning laser pin point Mark, and then calculate the prediction bandwidth of this scan line；By calculating further, can obtain in next laser scanning row, energy Scanning element cloud bar width and dot density is enough made all to remain constant scanning field of view angle correction value and laser pulse frequency correction value； By described optical electric axial angle encoder（115）Obtain described positive 6 face rotary prisms（113）Real-time rotational angle, feed back to described Landform altitude changes self-adaptive regulator（12）, in the scanning field of view angle range revised, only produce the square wave letter of frequency of amendment Number, it is supplied to described adjustable frequency laser pulse ranging instrument（111）, make described adjustable frequency laser pulse ranging instrument（111）Send out Go out the laser pulse of frequency of amendment, thus can ensure in the laser scanning point cloud bandwidth of underface direction of visual lines and dot density all the time Keep constant, not changed by landform altitude is affected；Described adjustable frequency laser pulse ranging instrument（111）Using frequency-adjustable Externally modulated laser, it leaves special signaling interface, to be controlled the tranmitting frequency of laser pulse by user input square-wave signal； Self-adaptive regulator is changed by described landform altitude（12）Produce the square-wave signal requiring frequency, trigger described adjustable frequency laser Tellurometer（111）, produce the laser pulse of corresponding frequencies.

2. according to described in claim 1 a kind of can the change of adaptive equalization tested landform altitude airborne LiDAR device, it is special Levy the laser scanning being can simultaneously to carry out both direction, one is the underface direction of visual lines of airborne platform, two is front under tiltedly Square direction of visual lines；Described a kind of can adaptive equalization tested landform altitude change airborne LiDAR device（1）, global positioning system System GPS（2）, Inertial Measurement Unit IMU（3）It is installed in airborne platform（4）On；Described one kind can the tested landform of adaptive equalization The airborne LiDAR device of elevation change（1）The laser scanning of underface direction of visual lines is main work scanning device, to each Laser footpoint is found range, in conjunction with described GPS（2）With described IMU（3）The state of flight information of measurement, can try to achieve each ground The three-dimensional coordinate of laser footpoint；The laser scanning of front oliquely downward direction of visual lines is bandwidth prediction laser instrument, its scanning direction with Immediately below have a little angle β between direction of visual lines, the sweep bandwidth of the landform at a certain distance from front can be obtained, with underface The laser scanning initial measurement bandwidth of direction of visual lines is compared, if having deviation so that it may align the laser scanner in down gaze direction Scanning field of view angle and laser pulse repetition frequency carry out real-time adjustment, finally make immediately below direction of visual lines laser scanning point cloud Bandwidth and dot density are constant all the time.

3. according to described in claim 1 a kind of can the change of adaptive equalization tested landform altitude airborne LiDAR device, it is special Levy and be described electric rotating machine（114）With described optical electric axial angle encoder（115）, described positive 6 face rotary prisms（113）With described Oblique 6 face Vertebral rotation prisms（133）Coaxially it is connected；Described oblique 6 face Vertebral rotation prisms（133）Side and rotary shaft between have One small inclination β；Described electric rotating machine（114）Uniform rotation, and by described optical electric axial angle encoder（115）Obtain described positive 6 Face rotary prism（113）And described oblique 6 face Vertebral rotation prisms（133）Real-time rotational angle.

4. according to described in claim 1 a kind of can the change of adaptive equalization tested landform altitude airborne LiDAR device, it is special Levy and be to change self-adaptive regulator in described landform altitude（12）In, work out control software, principal set up is good in mastery routine Interrupt 1, that is, when described positive 6 face rotary prisms（113）Rotated counterclockwise by angle be -30, -90, -150, -210, -270, - When 330, opens interrupters subprogram 1 enables, and rotary prism often rotates a circle six interruptions, the side number with rotary prism Identical；Triggering is interrupted in 1 subprogram, first collection laser scanning and ranging value, the state of flight value of GPS and IMU measurement, photo electric axis The corner actual measured value of angular encoder, calculates the three-dimensional coordinate of laser footpoint, obtains ideal scan line bandwidth D₁Sweep with prediction Retouch tape width D₂；And then, calculate direction of visual lines immediately below next laser scanning compensated after scanning field of view angle and square wave Frequency values；Finally, according to described optical electric axial angle encoder（115）Measured value, in the described positive 6 face rotary prisms of every one side （113）Scanning process in, only in the scanning field of view angle range of regulation, output requires the square-wave signal of frequency, and triggering is described can Frequency modulation rate laser pulse ranging instrument（111）Send the laser pulse of corresponding frequencies；When described optical electric axial angle encoder（115）Measurement Described positive 6 face rotary prisms（113）Actual rotational angle be θ r=- θ_BWhen, start interruption 2, described landform altitude changes self adaptation Actuator（12）Beginning output frequency is f_BSquare-wave signal, trigger described adjustable frequency laser pulse ranging instrument（111）Send Frequency is f_BLaser pulse；When described optical electric axial angle encoder（115）Measure described positive 6 face rotary prisms（113）Reality Corner is θ r=+ θ_BWhen, start interruption 3, described landform altitude changes self-adaptive regulator（12）Stopping output frequency is f_BSide Ripple signal, makes described adjustable frequency laser pulse ranging instrument（111）Stop sending laser pulse.

5. according to described in claim 1 a kind of can the change of adaptive equalization tested landform altitude airborne LiDAR device, it is special Levy and be described landform altitude change self-adaptive regulator（12）Control system is by ARM (LPC2138) embedded system as control Device processed, by described GPS（2）With described IMU（3）The state of flight value of measurement inputs to ARM (LPC2138) by serial ports 1, described Adjustable frequency laser pulse ranging instrument（111）With described fixed frequency laser pulse ranging instrument（131）Laser ranging value pass through Serial ports 2 inputs to ARM (LPC2138), by described optical electric axial angle encoder（115）The corner of measurement inputs to ARM by serial ports 3 (LPC2138)；ARM (LPC2138) can produce the square wave of frequency-adjustable, using numerical computations synthesis and D/A modular converter, output The square-wave signal of frequency range 1 Hz ~ 50 kHz；Measure a certain elevation landform when, if require scanning field of view angle be ± θ_B, laser pulse frequency be f_B, according to described optical electric axial angle encoder（115）The described positive 6 face rotary prisms of measurement（113）'s Scanning angle, ARM (LPC2138) can control beginning and the stopping of square-wave signal output；When described optical electric axial angle encoder（115） The described positive 6 face rotary prisms of measurement（113）Scanning angle be-θ_BWhen, ARM (LPC2138) output frequency f_BSquare wave letter Number, trigger described adjustable frequency laser pulse ranging instrument（111）, send frequency f_BLaser pulse；When described photoelectricity shaft angle is compiled Code device（115）The described positive 6 face rotary prisms of measurement（113）Scanning angle be+θ_BWhen, ARM (LPC2138) stops output side Ripple signal, triggers described adjustable frequency laser pulse ranging instrument（111）Stop output laser pulse.