Water depth measurement light transmitting and receiving system based on non-coaxial and non-parallel shaft structure

Abstract

The invention discloses a water depth measurement light transmitting and receiving system based on a non-coaxial and non-parallel-axis structure. The optical axis of a light transmitting assembly and the optical axis of a light receiving assembly of the light transmitting and receiving system are of a crossed structure. The transmitting optical axis is bent downwards through the light scanning reflector and then forms a scanning angle theta with a geometric vertical axis of the laser radar, the receiving optical axis is bent downwards through the light scanning reflector and then forms a space intersection angle alpha with the transmitting optical axis, and the receiving optical axis is located at any position of 360 degrees around the transmitting optical axis at the angle alpha; a receiving view field of the light receiving assembly covers the water surface near the outer side of the water surface reflection point but does not cover the water surface reflection point, and only receives underwater light echo signals and a small amount of water surface light echo signals. Only one light receiving channel is needed, full-view-field receiving of underwater light echo signals is achieved, the optical structure is simplified, the data processing efficiency is improved, the size, weight and cost of the system are reduced, and the system is suitable for low-altitude unmanned aerial vehicle-mounted or manned vehicle-mounted water depth measurement scenes in the airspace below 100 m.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of low-altitude unmanned aerial water depth measurement laser radar, and particularly relates to a water depth measurement optical transceiver system based on a non-coaxial and non-parallel shaft structure. BACKGROUND

[0002] In the technical field of low-altitude unmanned aerial water depth measurement laser radar, the optical transceiver systems of related products on the market generally adopt a traditional coaxial or parallel shaft structure. The coaxial or parallel shaft structure adjusts laser light emitting components and light receiving components, which are installed in parallel with each other or perpendicularly to each other, into a coaxial or parallel shaft structure through one or several mirrors after light bending, and then reflects pulsed laser downward to the measured water area through the same light scanning plane mirror, while reflecting the light echo signals from the water surface and the water bottom upward to the light receiving components.

[0003] However, the absorption and scattering of light beams by rivers, lakes, oceans and other water bodies in the natural environment are relatively strong, which makes the water bottom light echo signal 10 5 times or even much smaller than the scattered light echo caused by water body scattering. In order to avoid the water bottom light echo signal being submerged or difficult to collect due to the over-strong water surface light echo signal, the coaxial light receiving system usually adopts a structure of "light receiving main mirror + split field of view light mirror + light receiving components with two field of view channels". The small field of view light receiving component only receives the water surface light echo signal and the shallow water area water bottom light echo signal, and the large field of view light receiving component only receives the deep water area water bottom light echo signal.

[0004] However, in actual application, the large field of view light receiving component does not receive the relatively strong deep water area water bottom light echo signal in the 0-5 mrad field of view, and the deep water area water bottom light echo signal that can be received in the 5-35 mrad field of view is only a part of the deep water area water bottom light echo signal in the full field of view. This structural imperfection is reflected in the water depth measurement result, that is, the maximum water depth value that can be measured by the laser radar is much less than the design expectation, which seriously affects the accuracy and effectiveness of the water depth measurement. SUMMARY

[0005] In view of the above problems in the prior art, the purpose of the present application is to provide a water depth measurement optical transceiver system based on a non-coaxial and non-parallel shaft structure, so as to solve the problems of the traditional coaxial or parallel shaft structure, such as the over-strong water surface light echo signal interfering with the water bottom light echo signal reception, the incomplete deep water area water bottom light echo signal reception resulting in the maximum measured water depth being much lower than the design expectation, and the like.

[0006] The technical scheme of the water depth measurement optical transceiver system based on a non-coaxial and non-parallel shaft structure of the present application is as follows:

[0007] A water depth measurement optical transceiver system based on a non-coaxial, non-parallel axis structure, the optical axis of the optical transmitting component and the optical axis of the optical receiving component of the optical transceiver system are cross structure; the transmitting optical axis is bent downward by the optical scanning mirror and forms a scanning angle θ with the geometric vertical axis of the laser radar, the receiving optical axis is bent downward by the optical scanning mirror and forms a spatial intersection angle α with the transmitting optical axis, the receiving optical axis is at any position around the transmitting optical axis by 360° at the angle α; the receiving field of view of the optical receiving component covers the water surface near the outside of the water surface reflection point but does not cover the water surface reflection point, only receives the water bottom light echo signal and a small amount of water surface light echo signal.

[0008] As a further improvement of the above technical scheme, the spatial intersection angle α is in the range of 0°<α<30°.

[0009] As a further improvement of the above technical scheme, the optical scanning mirror includes a first mirror and a second mirror, the aperture of the first mirror is any value between one-third and one-sixth of the aperture of the receiving primary mirror, and the first mirror is located at the center of the second mirror and has a spatial angle with the second mirror.

[0010] As a further improvement of the above technical scheme, the optical transceiver system further includes a center perforated mirror, and the center hole aperture of the center perforated mirror is smaller than the aperture of the first mirror.

[0011] As a further improvement of the above technical scheme, the optical transceiver system adopts an elliptical optical scanning mode, the optical scanning device drives the optical scanning mirror by a servo motor, the scanning light beam forms an elliptical trajectory on the water surface, and the incident angle of the scanning light beam on the water surface changes cyclically.

[0012] As a further improvement of the above technical scheme, the optical transceiver system adopts a circular optical scanning mode, the optical scanning device drives a transmission optical wedge component by a hollow motor, the transmission optical wedge component includes a center small optical wedge and an outer ring-shaped large optical wedge, the optical wedge inclination angle β2 of the outer ring-shaped large optical wedge is greater than the optical wedge inclination angle β1 of the center small optical wedge, the scanning light beam forms a circular trajectory on the water surface, and the incident angle of the scanning light beam on the water surface is fixed.

[0013] As a further improvement of the above technical scheme, the optical transceiver system adopts a Z-shaped optical scanning mode, the optical scanning device drives the optical scanning mirror by a galvanometer motor, the scanning light beam forms a Z-shaped trajectory on the water surface, and the incident angle of the scanning light beam on the water surface changes cyclically.

[0014] As a further improvement of the above technical solution, the optical transceiver system adopts a one-word optical scanning mode, the optical scanning device drives the optical scanning mirror by a servo motor, and the 360° rotating scanning light beam scans the water surface in the effective range of -20°~+20° with 0° position as the reference.

[0015] As a further improvement of the above technical solution, the water surface light echo signal amplitude received by the optical receiving assembly is any value between one-twentieth and one-fiftieth of the traditional coaxial or parallel axis structure.

[0016] The present application provides a water depth measurement optical transceiver system based on a non-coaxial and non-parallel axis structure, compared with the prior art, its beneficial effects are that:

[0017] 1. The traditional coaxial or parallel axis structure needs two field of view channels of light receiving assembly, the structure is complex, and the installation and adjustment are difficult. The water depth measurement optical transceiver system based on a non-coaxial and non-parallel axis structure has only one light receiving channel, the optical structure is simplified, and the corresponding optical transceiver installation and adjustment process is relatively simple and the difficulty is reduced, which greatly simplifies the system structure and the installation and adjustment process. The optimization of the water depth measurement optical transceiver system structure saves half of the weight, half of the volume, and half of the cost of optical components, which is conducive to the lightweight, miniaturization and commercialization of the laser radar, and is also conducive to the carrying and application of the laser radar on unmanned vehicles and the like.

[0018] 2. The water depth measurement optical transceiver system only needs one light receiving channel, and the optical receiving assembly can simultaneously receive the light echo signals of the water surface and the water bottom of the beach and the global water depth in actual measurement application, without needing to receive echo signals of different regions through two channels respectively as in the traditional structure, thereby improving the universality and integration of the system.

[0019] 3. Traditional large-field-of-view optical receiving components with coaxial or parallel-axis optical transceiver structures are characterized by the following: the optical scanning beam is emitted, and the surface and bottom light echo signals return along the original trajectory of the emitted light before entering the optical receiving system. This limits the reception of bottom light echo signals from the deep water region (5–35 mrad field of view). The water depth measurement optical transceiver system of this invention, through adjusted optical parameters, ensures that the optical receiving component only receives surface light echo signals of appropriate intensity. Bottom light echo signals do not coincide with the emitted light trajectory and enter the optical receiving system. Simultaneously, it can receive as many bottom light echo signals as possible across the entire field of view, maximizing the maximum water depth measurement value. The cross-structure optical receiving component of this invention can receive bottom light echo signals from the deep water region (0–35 mrad field of view), increasing the amount of bottom light echo signals received and thus improving the maximum water depth measurement value. In other words, the ratio of light received by the traditional coaxial and the cross-axis light receiving components of this application is (30 / 35):(35 / 35)≈0.86:1. In actual field applications, this ratio is usually (0.7~0.8):1.

[0020] 4. Traditional optical transceiver coaxial or parallel axis structures require three channels for data acquisition: a main wave (reference) channel, a large field of view channel, and a small field of view channel. Simultaneously storing data from all three channels makes the processing quite complex, with data volume exceeding 100G in half an hour. Calculating water depth requires simultaneously accessing data from the main wave (reference), small field of view (surface and shallow bottom), and large field of view (deep bottom). The solution modes for shallow water and deep water are not entirely the same, which can introduce additional errors due to different solution modes.

[0021] The optical axis cross structure data acquisition of this application requires two channels: the main wave (reference) channel and the full field of view channel. It stores data from both channels simultaneously, reducing the data storage capacity by half. This reduces the number of high-speed acquisition channels, shortens the storage capacity and reading time of the acquired data. To calculate the water depth, it is necessary to call the main wave (reference) and full field of view (bottom) data simultaneously. Only one solution mode is needed, reducing the time required for data solution by about half and greatly improving the efficiency of subsequent data processing.

[0022] For example, the AD module of the traditional coaxial structure uses 3 channels to collect and store the data generated by one light pulse transmission, and the data amount is 3*792 (including useful water surface and water bottom light echo signals and other stray light signals). The solving mode is to directly calculate the 3*792 data by waveform decomposition. The AD module of the cross structure of the application uses 2 channels to collect and store the data generated by one light pulse transmission, and the data amount is 2*792 (including useful water surface and water bottom light echo signals and other stray light signals). The solving mode is to accurately locate the solving interval containing only water surface and water bottom light echo signals through rising edge and falling edge, and the solving data amount is reduced from 792 to more than 100, and the required time for solving data is reduced by about half. BRIEF DESCRIPTION OF DRAWINGS

[0023] Fig. 1 is a structural schematic diagram of the relationship between the receiving optical axis and the transmitting optical axis scanning angle inside and outside; Fig. (a) is a schematic diagram of the receiving optical axis outside the transmitting optical axis scanning angle, and Fig. (b) is a schematic diagram of the receiving optical axis inside the transmitting optical axis scanning angle;

[0024] Fig. 2 is a schematic diagram of the light path of light transmission and reception;

[0025] Fig. 3 is a schematic diagram of the optical axis cross structure of the elliptical light scanning mode;

[0026] Fig. 4 is a schematic diagram of the optical axis cross structure of the circular light scanning mode;

[0027] Fig. 5 is a schematic diagram of the optical axis cross structure of the Z-shaped light scanning mode;

[0028] Fig. 6 is a schematic diagram of the optical axis cross structure of the I-shaped light scanning mode;

[0029] In the figure: 1, laser radar; 2, geometric vertical axis; 3, transmitting optical axis; 4, receiving optical axis; 5, spatial intersection angle a of the transmitting optical axis and the receiving optical axis; 6, scanning angle θ; 7, water surface; 8, water bottom; 9, light receiving field angle; 10, light receiving component; 11, large-diameter mirror; 12, scanning motor; 13, laser; 14, small-diameter mirror; 15, center perforated mirror; 16, A point reflected echo light beam; 17, B point reflected echo light beam; 18, third mirror; 19, hollow scanning motor; 20, combined optical wedge; 21, fifth mirror; 22, galvanometer motor; 23, fourth mirror. DETAILED DESCRIPTION

[0030] The application will be further described in detail below in combination with the drawings and specific embodiments.

[0031] The application provides a water depth measurement optical transceiver system based on a non-coaxial (or non-parallel axis) structure, and the main feature is that the optical axis of the optical transmitting component and the optical axis of the optical receiving component of the optical transceiver system are in a cross structure (non-coaxial, non-parallel axis). As shown in Figure 1 the transmitting optical axis 3 and the receiving optical axis 4 are first adjusted to be coaxial, then the transmitting optical axis 3 is bent downward by the optical scanning mirror to form a scanning angle θ6 with the geometric vertical axis (of the laser radar), and the receiving optical axis is bent downward by the optical scanning mirror to form a spatial intersection angle α with the transmitting optical axis, and the position of the receiving optical axis can be any position around the transmitting optical axis at the spatial intersection angle α, that is, according to the design needs, the receiving optical axis can be located on the outside or inside of the scanning angle of the transmitting optical axis, or can be located at other spatial positions.

[0032] A specific embodiment is described as follows.

[0033] This embodiment adopts an elliptical light scanning mode, and through fine optical parameter design and structure optimization, the water depth measurement system is realized to be efficient and light.

[0034] As shown in Figure 3 the light scanning device is composed of a scanning motor 12 and a scanning mirror (a combination of a large-diameter mirror 11 and a small-diameter mirror 14), wherein the small-diameter mirror 14 is located at the center of the large-diameter mirror 11 and has a spatial intersection angle with the large-diameter mirror 11. The transmitting light (pulsed laser beam) emitted by the laser radar 1 is coaxial with the optical receiving component through the center perforated mirror 15 (the center aperture is smaller than the aperture of the small-diameter mirror 14), and is bent downward by the small-diameter mirror 14 of the light scanning device to scan the water surface; the optical axis of the optical receiving component is bent downward by the large-diameter mirror 11 of the light scanning device to be located at any position on the side of the transmitting optical axis, and the position on the outside of the transmitting optical axis 3 (the left side shown in FIG. 3) is the optimal structure in this type of application example. When the transmitting light performs 360° elliptical light scanning around the geometric vertical axis 2, the optical axis 4 of the optical receiving component 10 is always on the outside of the scanning angle.

[0035] The light scanning device of the system core adopts a high-precision servo motor to drive the light scanning mirror, and the scanning angle control precision can reach ±0.01°, which ensures the stability and repeatability of the scanning track. The small-diameter mirror is selected to be a high-damage-threshold fused quartz laser wavelength medium film mirror, and the aperture is set to be one-fifth of the aperture of the optical receiving main mirror. This proportional design can not only ensure the effective convergence of the transmitting light, but also can reduce the shielding interference to the receiving channel to the maximum extent. The center perforated mirror 15 adopts a quartz glass substrate, and the center aperture is smaller than the aperture of the small-diameter mirror 14. Through accurate hole calibration, the transmitting light is prevented from scattering into the optical receiving system.

[0036] In the optical axis system design, the spatial intersection angle a5 between the emitting light axis 3 and the receiving light axis 4 is set to 3.5°, and the scanning angle 6 between the emitting light axis 3 and the geometric vertical axis 2 is set to 14°. This angle parameter is verified by a large number of simulations, which can ensure the water bottom echo receiving efficiency while significantly reducing the interference of water surface stray light.

[0037] In the actual measurement process, after the emitted light is bent downward by the first reflecting mirror, it performs 360° elliptical light scanning with the geometric vertical axis as the center, and the scanning frequency is set to 20Hz. When the flight height is 40 meters, the long axis of the elliptical trajectory formed by the scanning light beam on the water surface can cover a range of 20 meters, and the short axis can cover a range of 14 meters.

[0038] As shown in Figure 2 The field of view of the receiving light axis 4 strictly covers the water surface near the outside of point A, but does not include point A itself. The core advantage of this design is that although a small amount of water surface light echo signal reflected by the water surface A point can enter the light receiving assembly through the large-diameter mirror 11 and the center perforated mirror 15, its amplitude is only one tenth of that of the traditional coaxial structure, which is much lower than the saturation threshold of the detector. After the light echo signal reflected by the water bottom B point 17 is refracted out of the water surface through the outside of the water surface A point 16, it can be completely received by the 0-35mrad full field of view of the light receiving assembly, and the receiving efficiency of the refracted light is improved by more than 40% compared with the traditional structure.

[0039] By optimizing the focal length and field of angle parameters of the receiving lens, the light receiving assembly can receive the water surface and water bottom light echo signals of the beach and the global water depth in the actual measurement together with the narrowband filter. For example, in the shallow beach area, the system can realize accurate depth measurement by analyzing the double echo signals of water surface reflection and water bottom reflection; in the deep water area, it can realize stable measurement through the enhanced water bottom echo signal, without the need to switch channels or adjust parameters like the traditional structure, greatly improving the versatility of the system.

[0040] The traditional structure needs to cross-call the echo data of two channels at the same time, which not only needs to handle time synchronization error and field splicing deviation, but also needs to store double data amount. The single-channel design of the embodiment reduces the number of high-speed acquisition channels, the single-frame data amount is reduced to 10MB, the data storage capacity is reduced by 50%, and the reading time is shortened from 0.5 seconds / frame in the traditional structure to 0.1 seconds / frame. More importantly, the single-channel data does not need to be processed by complex channel alignment, and the running efficiency of the core algorithms such as peak detection and time flight calculation of the echo signal is improved by more than 3 times, which significantly reduces the complexity and error rate of the later data processing.

[0041] And, by omitting the optical components of one receiving channel, the overall number of optical components is reduced from 12 to 6, the weight is reduced from 1.5 kg of the traditional structure to 0.7 kg, and the volume is compressed from 300 cm³ to 150 cm³, fully adapting to the carrying needs of small unmanned aerial vehicles. In terms of cost, the number of expensive photodetectors and high-precision lenses is halved, the total cost of optical components is reduced by 50%, and the overall cost of the system is reduced by more than 45% compared with the traditional scheme.

[0042] In addition, the embodiment also performs excellently in environmental adaptability. For the problem of background light interference in strong light environment, through the combination of accurate control of the receiving field of view and narrowband filtering technology, the background light suppression ratio is improved by 20 dB; in turbid water, due to the increase in water bottom echo reception, the system can still maintain a depth measurement accuracy of 0.3 meters, which is 50% higher than the traditional structure. These optimizations make the system more suitable for being carried on a rotary-wing unmanned aerial vehicle (below 7 kg), and have stronger practicality in complex water environment such as estuary delta and shoal wetland, laying a solid foundation for the commercialization of laser radar water depth measurement technology.

[0043] Specific embodiment two is described as follows.

[0044] The embodiment adopts a circular light scanning mode, and through the transmission type optical structure and accurate parameter matching, efficient water depth measurement of the measured water area is realized.

[0045] As shown in FIG. 4, the circular light scanning mode is composed of a hollow scanning motor 19 and a circular special (transmission) combined optical wedge 20. The center of the circular special combined optical wedge is a small optical wedge with a diameter fa and an optical wedge angle β1, and the outer annular part is an annular large optical wedge with an outer diameter fb, an inner diameter fa and an optical wedge angle β2, β2>β1. The transmitted light (pulsed laser beam) is coaxial with the light receiving assembly through the third mirror 18 (the aperture of the third mirror 18 is smaller than the aperture of the small optical wedge), and is transmitted downward and bent by the small optical wedge of the light scanning device; the optical axis 4 of the light receiving assembly is transmitted downward and bent to any position on the side of the transmitted light axis 3 through the outer annular optical wedge of the light scanning device, and is located on the outer side of the transmitted light axis (the left side shown in FIG. 4) is the optimal structure in this type of application example. When the transmitted light rotates with the hollow scanning motor 19 to perform 360° circular light scanning cycle, the optical axis 4 of the light receiving assembly is always on the outside of the scanning angle.

[0046] The circular light scanning device adopts a hollow scanning motor to drive a transmission combined optical wedge to rotate around the motor center axis (also the optical wedge center axis). The combined optical wedge adopts high-transmittance K9 optical glass substrate, which is precisely ground and coated. The center small optical wedge has a diameter of 12.5 mm, and the optical wedge inclination angle β1 is set to 20°, which is responsible for the initial deflection of the emitted light. The outer annular large optical wedge adopts a stepped structure design, with an outer diameter of 60 mm and an inner diameter matching the small optical wedge diameter. The optical wedge inclination angle β2 is set to 25°, and the precise regulation of the scanning angle is realized through the synergistic effect of the double wedge angles.

[0047] The hollow scanning motor has a rotation speed stability of ±1 rpm and an angular displacement accuracy of ≤0.02°, ensuring that the circularity deviation of the scanning track is <1%. When the motor drives the combined optical wedge to rotate, the emitted light is transmitted through the small optical wedge and then bent downward, forming a 360° circular scanning track centered on the geometric vertical axis. The scanning frequency is set to 5 Hz. At a flight height of 40 meters, the circular track formed by the scanning beam on the water surface has a diameter of about 13 meters, and the spot spacing on the track is uniformly controlled at 0.3 meters. Compared with the elliptical scanning mode, more balanced spatial coverage density can be achieved in the measured water area.

[0048] In the design of the optical shaft system parameters, the spatial intersection angle α5 between the emitted light axis and the received light axis is set to 2.5°, and the scanning angle θ6 between the emitted light axis and the geometric vertical axis is set to 9.5°. The field of view corresponding to the received light axis (0~40mrad) does not cover the reflection point A of the emitted light beam on the water surface, which not only avoids the strong reflection interference of point A16, but also ensures the efficient reception of the water bottom reflected light. The measured data shows that the amplitude of the water surface light echo signal received by the system is only about one thirty of that of the traditional coaxial structure, which is far below the saturation threshold of the photoelectric detector. The light echo signal reflected by the water bottom point B17 is transmitted and refracted through the outer area of the large optical wedge and is captured by the full field of view of the receiving assembly, with a signal strength about 20% higher than that of the traditional structure.

[0049] Compared with the elliptical scanning embodiment and the traditional structure, the present embodiment has the following advantages: 1) The slant range of the scanning beam from the scanning device to the water surface and the incident angle of the scanning beam to the water surface are fixed, which, combined with the light receiving single-channel design, makes the polar coordinate data conversion algorithm of the circular track more concise, the single-frame data solving time is shortened to 0.08 seconds, the data processing efficiency is improved by 25% compared with the elliptical track data, and the data storage capacity is further reduced to 8MB / frame; 2) In the measured water area such as lakes and reservoirs, the deviation of the track coverage rate of the circular scanning track is <5%, which improves the measurement accuracy in the same scene by 10% compared with the elliptical scanning; 3) Compared with the elliptical embodiment, the assembly, adjustment, debugging, detection, and calibration of the circular light scanning device are reduced in the process, and the assembly and adjustment period is shortened by about 30%.

[0050] The circular embodiment also has disadvantages: the same light receiving aperture is 60 mm, the scanning servo motor of the elliptical embodiment weighs only 160 g, and the total weight of the light scanning device is about 300 g; while the hollow scanning motor of the circular embodiment weighs about 1000 g, and the total weight of the light scanning device is about 1500 g. Therefore, the circular embodiment is more suitable for being carried on large rotary wings (load 10-15 kg) unmanned aerial vehicles, fixed-wing unmanned aerial vehicles (load 10-15 kg), and provides a low-cost solution for water depth mapping in near-shore shallow water, inland lakes and other scenes.

[0051] A specific embodiment is described as follows.

[0052] The embodiment innovatively adopts a Z-shaped light scanning mode, and linear area efficient coverage of water area is achieved through single-mirror cooperative control, which is particularly suitable for water depth mapping of narrow water areas such as rivers and channels.

[0053] As shown in Figure 5 , the light scanning device is composed of a galvanometer motor 22 and a galvanometer (a combination of a fourth mirror 23 and a fifth mirror 21), wherein the smaller fourth mirror 23 is located at the center of the larger fifth mirror 21 and has a spatial included angle with the fifth mirror 21. The transmitted light (pulsed laser beam) passes through the center perforated mirror 15 (the aperture of the center hole is smaller than that of the fourth mirror 23) coaxially with the light receiving assembly, is bent downward by the fourth mirror 23 perpendicular to the water surface (0° position of the galvanometer swing center), and the light receiving assembly light axis is bent downward by the fifth mirror 21 at any position on the side of the transmitted light axis, which is located in front of (left side of FIG. 5) or behind the transmitted light axis. When the laser radar 1 moves forward, the galvanometer swings back and forth around the transmitted light axis 3 at 0° position to form a Z-shaped light scanning, and the light receiving light axis 4 is always in front of or behind the scanning light beam.

[0054] The light scanning device of the system core adopts a high-precision galvanometer motor to drive the composite reflection module composed of the fourth mirror 23 and the fifth mirror 21, wherein the galvanometer motor 22 selects a voice coil type driving structure, and the angular displacement response frequency reaches 5 kHz, which ensures the rapid switching and accurate positioning of the scanning track. The fourth mirror 23 adopts a 12 mm diameter microcrystalline glass mirror, which is located at the geometric center position of the fifth mirror 21, and the two are fixed by a precision metal bracket to form a spatial included angle of 2.5°. This angle parameter is optimized through optical tracing simulation, and efficient separation and cooperative scanning of transmitted light and received light can be realized.

[0055] In the optical path transmission design, the transmitted light is first transmitted to the fourth mirror 23 through the center perforated mirror 15, and the high-reflective coating of the mirror realizes the turning of the optical path; the receiving light axis is accurately positioned at a 5° visual angle in front of the transmitted light axis after being bent by the fifth mirror 21, forming a unique "front-view receiving" layout.

[0056] The spatial intersection angle a of the transmitting optical axis and the receiving optical axis is set to 2.5°, the galvanometer motor performs ±20° pendulum motion with the transmitting optical axis as the center, and the swing frequency is set to 10 Hz. Through the superposition of the periodic swing of the galvanometer and the linear motion of the carrier, a regular Z-shaped scanning track is formed on the water surface. When the flight height is 40 meters, the track transverse coverage width can reach 29 meters. The effective coverage rate of the relatively circular scanning mode in the long and narrow water area is increased by 30%. The incident angle of the scanning beam can be cyclically changed between 0° and the maximum field of view angle. With the dynamic angle compensation algorithm of the galvanometer, the angle change accuracy is controlled within ±0.05°.

[0057] In the actual measurement process, the transmitted light can realize perpendicular water surface incidence after being bent by the fourth reflecting mirror. When the galvanometer swings from -20° to 20° to the left, the light beam incident angle is deflected from -20° to 20° simultaneously, forming a leftward light scanning line. When the galvanometer swings from 20° to -20° to the right, the light beam incident angle is deflected from 20° to -20° simultaneously, forming a rightward light scanning line. When the aerial vehicle platform moves forward, the leftward light scanning line and the rightward light scanning line are folded back at 20° and -20° respectively, forming a continuous Z-shaped connection, ensuring the continuity of the measurement data.

[0058] This Z-shaped track can closely cover the area in front of the flight path, and is particularly suitable for continuous mapping operations when the unmanned aerial vehicle flies along the river channel. Usually, Z-shaped combined with circular light scanning mode is used for large water depth measurement laser radar carried on fixed-wing manned aircraft.

[0059] A specific embodiment is described as follows.

[0060] As shown in Figure 6 , this embodiment adopts a one-shaped light scanning mode, which realizes strip-shaped efficient coverage of water area through the spatial angle design of concentric double reflecting mirrors, and is particularly suitable for rapid mapping and channel monitoring of open water area. The system core light scanning device is composed of high-precision scanning motor and composite reflecting mirror group. The scanning motor adopts servo motor driving mode, which is stable and reliable in speed, and the control accuracy of the rotation angle ensures that the linearity deviation of the scanning track is less than 0.5%.

[0061] The composite reflecting mirror group adopts an integrated bracket design, and the small reflecting mirror is accurately embedded in the geometric center of the large reflecting mirror. The two form a spatial included angle of 2.5° through precise grinding process. This angle parameter is optimized through optical simulation, which can realize efficient separation of transmitting light and receiving light. The reflecting mirrors are made of ultra-precision optical glass substrate, the surface is coated with high-hardness aluminum film and covered with silicon dioxide protective film, the wear resistance is increased by 3 times, and the service life in humid environment can reach more than 5000 hours.

[0062] The emitted light first passes through the central perforated mirror, and forms an initial coaxial arrangement with the light receiving assembly; after being bent downward by the mirror, vertical water surface incidence is achieved when the scanning motor is at the 0° center position. The light receiving assembly optical axis is bent downward by the large mirror 7, and finally positioned at the side angle of 5° of the emitted light axis. Through a large number of application verifications, the area of 5° in front of or behind the emitted light axis is the optimal structure, which can improve the water bottom echo receiving efficiency by 20%.

[0063] When the laser radar moves forward at a speed of 15 m / s along the Y axis direction, the scanning mirror rotates continuously around the emitted light axis by 360°, and cooperates with the 2.5° spatial included angle of the mirror group, so that the scanning beam forms a linear scanning band in the effective range of -20°~+20° with the 0° position as the reference. The scanning band width linearly expands with the increase of the distance, and at the distance of 40 meters, the scanning band width can reach 29 meters. The spot spacing on the scanning line is uniformly controlled at 0.2 meters. Since the scanning motor speed can be adjusted at 120 rpm~1200 rpm, the linear scanning track spacing is correspondingly adjusted to be reduced or increased. Compared with the Z-shaped scanning mode, the strip coverage efficiency is improved by 30%. This scanning characteristic is particularly suitable for rapid general survey of large area water area, and a single unmanned aerial vehicle can complete the measurement operation of 10 square kilometers of water area.

[0064] In the actual measurement process, the system exhibits a unique dynamic receiving advantage: the light receiving optical axis is always stably positioned in the 5° area in front of or behind the scanning beam. Through this layout, the strong echo interference of the water surface reflection point A can be effectively avoided. The measured data shows that only a small amount of water surface reflection light enters the receiving assembly through the mirror 6 and the central perforated mirror, and the signal amplitude is only 1 / 35 of the traditional coaxial structure, which is much lower than the saturation threshold of the light detector; while the water bottom reflection light can be completely captured by the 0~40 mrad field of view of the receiving assembly after water surface refraction, and the signal integrity reaches 99%, and the signal-to-noise ratio is still ≥20 dB under the condition of 30 meters of water depth.

[0065] This linear scanning mode is particularly suitable for large area water area application scenarios such as reservoir capacity monitoring and lake depth surveying. Through the combined use of the circular scanning mode, a multi-level water depth measurement solution of "rapid general survey + fine measurement" can be constructed, which significantly improves the efficiency and economy of water area surveying.

[0066] The above is only a preferred embodiment of the present application, and it should be pointed out that for ordinary skilled persons in the technical field, some improvements and replacements can be made without departing from the technical principles of the present application, and these improvements and replacements should also be regarded as the protection scope of the present application.

Claims

1. A non-coaxial, non-parallel axis structure based optical transceiver system for water depth measurement, characterized in that, The optical transmitting assembly and the optical receiving assembly of the optical transceiver system are in a cross structure, the transmitting optical axis is bent downward by the optical scanning mirror and forms a scanning angle θ with the geometric vertical axis of the laser radar, the receiving optical axis is bent downward by the optical scanning mirror and forms a spatial intersection angle α with the transmitting optical axis, the azimuth of the receiving optical axis is any position around the transmitting optical axis at an angle α of 360°, and the receiving field of view of the optical receiving assembly covers the water surface near the water surface reflection point but does not cover the water surface reflection point, and only receives the water bottom light echo signal and a small amount of water surface light echo signal.

2. The system of claim 1, wherein, The spatial intersection angle α is in a range of 0°<α<30°.

3. The system of claim 1, wherein, The optical scanning mirror comprises a first mirror and a second mirror, the aperture of the first mirror is any value between one-third and one-sixth of the aperture of the main receiving mirror, the first mirror is located at the center of the second mirror and has a spatial angle with the second mirror.

4. The system of claim 3, wherein, The optical transceiver system further comprises a central perforated mirror, and the central aperture of the central perforated mirror is smaller than the aperture of the first mirror.

5. The system of claim 1, wherein, The optical transceiver system adopts an elliptical light scanning mode, the optical scanning device drives the optical scanning mirror by a servo motor, the scanning light beam forms an elliptical track on the water surface, and the incident angle of the scanning light beam on the water surface changes cyclically.

6. The system of claim 1, wherein, The optical transceiver system adopts a circular light scanning mode, the optical scanning device drives a transmission type optical wedge assembly by a hollow motor, the transmission type optical wedge assembly comprises a central small optical wedge and an outer annular large optical wedge, the wedge angle β2 of the outer annular large optical wedge is greater than the wedge angle β1 of the central small optical wedge, the scanning light beam forms a circular track on the water surface, and the incident angle of the scanning light beam on the water surface is fixed.

7. The system of claim 1, wherein, The optical transceiver system adopts a Z-shaped light scanning mode, the optical scanning device drives the optical scanning mirror by a galvanometer motor, the scanning light beam forms a Z-shaped track on the water surface, and the incident angle of the scanning light beam on the water surface changes cyclically.

8. The system of claim 1, wherein, The optical transceiver system adopts a linear light scanning mode, the optical scanning device drives the optical scanning mirror by a servo motor, and the scanning light beam rotates by 360° with the 0° position as the reference to scan the water surface in an effective range of -20° to +20°.

9. The system of claim 1, wherein, The amplitude of the water surface light echo signal received by the optical receiving assembly is any value between one-twentieth and one-fiftieth of the amplitude of the water surface light echo signal received by a traditional coaxial or parallel axis structure.

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