Distance measurement program, distance measurement method, and information processing device

Abstract

The present invention improves the accuracy of distance measurement.　A housing (20) incorporates an irradiation unit (21) that emits a laser beam, a mirror (22) configured such that the angle at which the mirror reflects the laser beam can be changed, and a light-receiving unit (23) that receives a laser beam. A processing unit (12) acquires a first round-trip time of the laser beam emitted from the irradiation unit (21) to an object (30) via the mirror (22) and received by the light-receiving unit (23), and a first angle of rotation of the mirror (22). On the basis of in-housing flight time information (11a) that specifies the in-housing flight time of the laser beam for each angle of rotation of the mirror (22), the processing unit (12) performs correction for subtracting a first in-housing flight time corresponding to the first angle from the first round-trip time.

Description

Distance Measurement Program, Distance Measurement Method, and Information Processing Apparatus 【0001】 The present invention relates to a distance measurement program, a distance measurement method, and an information processing apparatus. 【0002】 LiDAR (Light Detection and Ranging) is used to measure the distance from a certain point to an object. LiDAR irradiates the object with laser light and receives the laser light scattered by the object. Based on the time from the emission to the reception of the laser light measured by LiDAR, the distance to the object can be measured. 【0003】 For example, a method has been proposed to detect an underwater object from an airborne platform such as an aircraft equipped with an altimeter using a contrast lidar (optical detection range) system. In this method, the system projects a light pulse from the airborne platform toward a backscattering medium such as water, hits an object surrounded by the backscattering medium, and detects the light pulse reflected from the object after a selected delay corresponding to the time the light pulse traveled to and from the object. The system calculates the selected delay based on input data including the altitude of the airborne platform. 【0004】 Japanese Patent Laid-Open No. 3-148088 【0005】 When irradiating an object with laser light from a housing incorporating a laser irradiator, the laser light generated from the laser irradiator may be reflected by a mirror that can rotate in two axial directions to scan the laser light two-dimensionally. In this case, the optical path length of the laser light in the housing varies depending on the rotation angle of the mirror. Therefore, for example, if the optical path length in the housing is made constant to calculate the distance to the object, the variation in the optical path length corresponding to the angle is not considered, and an error occurs in the calculated distance. 【0006】 On one aspect, the present invention aims to improve distance measurement accuracy. 【0007】In one embodiment, a distance measuring program is provided that measures the distance to an object based on the round-trip time of a laser beam. The distance measuring program causes a computer to perform the following processes: The computer obtains the first round-trip time of the laser beam that is irradiated onto the object via the mirror and received by the light-receiving unit from the irradiating unit of a housing that houses an irradiating unit that emits a laser beam, a mirror whose angle of reflection of the laser beam can be changed, and a light-receiving unit that receives the laser beam, and the first angle of rotation of the mirror. The computer performs a correction by subtracting the first time within the housing that corresponds to the first angle from the first round-trip time, based on information that defines the time of flight of the laser beam within the housing for each angle of rotation of the mirror. 【0008】 In one embodiment, a distance measurement method performed by a computer is provided. In another embodiment, an information processing device having a storage unit and a processing unit is provided. 【0009】 In one aspect, the distance measurement accuracy can be improved. The above and other objects, features and advantages of the present invention will become apparent from the following description in conjunction with the accompanying drawings illustrating preferred embodiments as examples of the present invention. 【0010】 This is a diagram illustrating the information processing device of the first embodiment. This is a diagram showing an example of the hardware of the information processing device of the second embodiment. This is a diagram showing an example of the hardware of an underwater LiDAR. This is a diagram showing an example of the rotation axis of a scanning mirror. This is a diagram showing an example of laser sensing by an underwater LiDAR. This is a diagram showing an example of round-trip time measurement. This is a diagram showing an example of the change in optical path length inside the housing due to the angle of the scanning mirror. This is a diagram showing an example of the software of the information processing device. This is a diagram showing an example of an optical path length table. This is a flowchart showing an example of distance measurement processing. This is a diagram showing an example of an optical path length table. This is a diagram showing an example of the appearance of an underwater LiDAR. This is a diagram showing a comparative example. 【0011】 Hereinafter, this embodiment will be described with reference to the drawings. [First Embodiment] The first embodiment will be described. 【0012】Figure 1 is a diagram illustrating an information processing device according to a first embodiment. The information processing device 10 has a storage unit 11 and a processing unit 12. The storage unit 11 may be a volatile semiconductor memory such as RAM (Random Access Memory), or a non-volatile storage such as an HDD (Hard Disk Drive) or flash memory. The processing unit 12 is a processor such as a CPU (Central Processing Unit), GPU (Graphics Processing Unit), or DSP (Digital Signal Processor). However, the processing unit 12 may also include application-specific electronic circuits such as an ASIC (Application Specific Integrated Circuit) or FPGA (Field Programmable Gate Array). The processor executes programs stored in memory such as RAM (which may also be the storage unit 11). A collection of multiple processors is sometimes called a "multiprocessor" or simply a "processor". 【0013】 The information processing device 10 measures the distance to the object 30 based on the round-trip time of the laser beam. The information processing device 10 has a predetermined communication interface (not shown in the figure) and connects to the housing 20 via this communication interface. 【0014】 The housing 20 houses the irradiation unit 21, the mirror 22, the light receiving unit 23, and the processing unit 24. For example, the housing 20 is an optical device that outputs laser light. The housing 20 seals the irradiation unit 21, the mirror 22, the light receiving unit 23, and the processing unit 24 to protect them from the external environment. The housing 20 has a window 20a. The window 20a is transparent. The material of the window 20a is acrylic or glass. Laser light can pass through the window 20a. The housing 20 may also house an information processing device 10. 【0015】The irradiation unit 21 generates laser light and irradiates it toward the mirror 22. The mirror 22 reflects the laser light. The mirror 22 can change the angle at which it reflects the laser light. For example, the mirror 22 has rotation axes 22a and 22b. The rotation axes 22a and 22b are axes that pass through the center of the mirror 22. Rotation axis 22a is, for example, an axis perpendicular to the direction of irradiation of the laser light from the irradiation unit 21 to the mirror 22. Rotation axis 22b is, for example, an axis parallel to the direction of irradiation of the laser light from the irradiation unit 21 to the mirror 22. The mirror 22 can rotate around rotation axes 22a and 22b, respectively. The mirror 22 changes the angle at which it reflects the laser light according to the angle of rotation around rotation axes 22a and 22b. 【0016】 The light-receiving unit 23 is a sensor that receives laser light. For example, laser light emitted from the irradiation unit 21 is reflected by the mirror 22, passes through the window 20a, reaches an object 30 located outside the housing 20, and is scattered by the object 30. The laser light scattered by the object 30 passes through the window 20a and reaches the light-receiving unit 23, where it is detected. When the light-receiving unit 23 detects the laser light, it outputs a detection signal to the processing unit 24. 【0017】 The processing unit 24 controls the irradiation unit 21, the mirror 22, and the light receiving unit 23. The processing unit 24 is, for example, a processor such as a CPU or DSP. The processing unit 24 may also include application-specific electronic circuits such as ASICs or FPGAs. 【0018】The processing unit 24 controls the generation of laser light by the irradiation unit 21. The processing unit 24 also scans the object 30 in two dimensions by controlling the rotation angle of the mirrors around the rotation axes 22a and 22b. Specifically, the laser light is scanned toward each point on a plane perpendicular to the direction from the window 20a toward the object 30. The processing unit 24 receives the laser light detection signal from the light receiving unit 23. The processing unit 24 determines the round-trip time of the laser light by calculating the difference between the laser light generation time and the laser light detection time based on the detection signal. The processing unit 24 transmits the round-trip time and the rotation angle of the mirror at the time the round-trip time was obtained to the information processing device 10. The rotation angle of the mirror 22, i.e., the mirror angle, is expressed as (a, b), for example, using angle a around rotation axis 22a and angle b around rotation axis 22b. 【0019】 The processing unit 12 obtains information on round-trip time and mirror angle from the housing 20. Based on the round-trip time and mirror angle, the processing unit 12 calculates the distance between the housing 20 and the object 30 as follows. 【0020】 Here, the storage unit 11 stores in advance the time-of-flight information 11a within the housing. The time-of-flight information 11a is information that defines the time-of-flight within the housing for each angle of the mirror 22 of the laser beam that is irradiated from the irradiation unit 21 of the housing 20 onto an object via the mirror 22 and received by the light receiving unit 23. The time-of-flight within the housing is the time the laser beam travels within the housing, out of the round-trip time of the laser beam between the housing 20 and the object. The time-of-flight within the housing is the sum of the time it takes for the laser beam to reach the window 20a from the irradiation unit 21 via the mirror 22 and the time it takes for the returned laser beam to reach the light receiving unit 23 from the window 20a. 【0021】For example, the in-cabinet flight time information 11a has a record of the in-cabinet flight time t1 corresponding to the mirror angle (a1, b1). This record indicates that the in-cabinet flight time for the mirror angle (a1, b1) is t1. The in-cabinet flight time information 11a also has a record of the in-cabinet flight time t2 corresponding to the mirror angle (a2, b2). Similarly, the in-cabinet flight time information 11a also has records of the in-cabinet flight time for other mirror angles that can be set for the mirror 22. 【0022】 Furthermore, instead of the in-housing flight time information 11a, the in-housing optical path length may be stored for each mirror angle. The in-housing optical path length is the length of the optical path inside the housing 20, which is the round-trip optical path length of the laser light between the housing 20 and the object. The in-housing optical path length is the sum of the optical path length from the irradiation unit 21 to the window 20a via the mirror 22 and the optical path length from the returning laser light from the window 20a to the light receiving unit 23. 【0023】 The optical path length within the housing is converted to the time of flight within the housing using the speed of light within the housing. Time of flight within the housing = (Optical path length within the housing) / (Speed ​​of light within the housing). Therefore, even if the time of flight information 11a retains the optical path length within the housing instead of the time of flight within the housing, the time of flight information 11a can be said to be information that defines the time of flight within the housing. Note that if a medium exists inside the housing 20, the speed of light in the medium is c / n, where n (n>1) is the refractive index of the medium and c is the speed of light in a vacuum. c = 300,000 (km / s). The "medium" here refers to the substance present in the space through which the laser light passes. For example, the medium inside the housing 20 is air. The refractive index of air is approximately 1.0003. The refractive index of air can be considered to be almost the same as the refractive index of a vacuum (=1). 【0024】 The processing unit 12 performs a correction by subtracting the in-cabinet flight time corresponding to the mirror angle obtained from the housing 20 from the round-trip time obtained from the housing 20, based on the in-cabinet flight time information 11a which defines the in-cabinet flight time of the laser beam for each angle of rotation of the mirror. 【0025】Here, the processing unit 12 is assumed to have acquired information on the round-trip time ΔT and the mirror angle (ax, bx) from the housing 20. Based on the in-housing flight time information 11a, the processing unit 12 acquires the in-housing flight time t corresponding to the mirror angle (ax, bx) acquired from the housing 20. The processing unit 12 performs a correction by subtracting the in-housing flight time t from the round-trip time ΔT to obtain the corrected round-trip time (ΔT-t). 【0026】 The processing unit 12 calculates the distance between the housing 20 and the laser beam irradiation point on the object 30 based on the corrected round-trip time (ΔT-t). Specifically, the processing unit 12 can determine this distance by the calculation (ΔT-t) × (speed of light outside the housing) × 1 / 2. If a medium exists outside the housing 20, the speed of light in the medium is c / n, where n (n>1) is the refractive index of the medium and c is the speed of light in a vacuum. For example, the medium outside the housing 20 may be water or air. 【0027】 For example, the processing unit 12 can obtain three-dimensional coordinate data of the object 30 by using the calculation results of the distance to each irradiation point on the surface when the surface of the object 30 is scanned two-dimensionally with laser light from the housing 20. 【0028】 According to the information processing device 10, the first round trip time (ΔT) of the laser light irradiated from the irradiation unit 21 of the housing 20 to the object via the mirror 22 and received by the light receiving unit 23, and the first angle of rotation of the mirror 22 are acquired. Based on the housing in-flight time information 11a which defines the in-flight time of the laser light for each angle of rotation of the mirror 22, a correction is performed by subtracting the first in-flight time (t) corresponding to the first angle from the first round trip time. 【0029】This allows the information processing device 10 to improve its distance measurement accuracy. Figure 1 illustrates a laser beam 40 reflected at one mirror angle and a laser beam 40a reflected at another mirror angle. Since the angles of the mirrors 22 are different for the laser beams 40 and 40a, the optical path length from the mirror 22 to the window 20a is different. That is, the optical path length from the irradiation unit 21 to the window 20a via the mirror 22 is different for the laser beams 40 and 40a. Also, since the angles of the mirrors 22 are different for the laser beams 40 and 40a, the optical path length from the window 20a to the light receiving unit 23 is different. For this reason, for example, if the flight time of the laser beams 40 and 40a inside the housing 20 is kept constant and the distance to each irradiation point on the object 30 is calculated, an error will occur in the calculated distance. 【0030】 Therefore, the information processing device 10 can appropriately correct the round-trip time by subtracting the flight time inside the housing, which corresponds to the difference in optical path length for each mirror angle, from the round-trip time of the laser beam acquired from the housing 20. As a result, the information processing device 10 can improve the accuracy of acquiring the round-trip time. The information processing device 10 can reduce the above error and improve the distance measurement accuracy by calculating the distance to the object 30 using the corrected round-trip time. 【0031】 [Second Embodiment] Next, a second embodiment will be described. Figure 2 is a diagram showing an example of the hardware of the information processing device according to the second embodiment. 【0032】 The information processing device 100 includes a processor 101, RAM 102, HDD 103, GPU 104, input interface 105, media reader 106, and communication interfaces 107 and 108. These units of the information processing device 100 are connected to a bus internally. The processor 101 corresponds to the processing unit 12 of the first embodiment. The RAM 102 or HDD 103 corresponds to the storage unit 11 of the first embodiment. The information processing device 100 may also be called a computer. 【0033】The processor 101 is an arithmetic unit that executes program instructions. The processor 101 is, for example, a CPU. The processor 101 loads at least a portion of the program and data stored in the HDD 103 into the RAM 102 and executes the program. The processor 101 may include multiple processor cores. The information processing device 100 may have multiple processors. The processor that executes one of the multiple processes performed by the information processing device 100 may be different from the processor that executes a different process from the multiple processes. A collection of multiple processors is sometimes called a "multiprocessor" or simply a "processor". A processor may also be called a "processor circuitry". 【0034】 RAM 102 is a volatile semiconductor memory that temporarily stores programs executed by the processor 101 and data used by the processor 101 for calculations. The information processing device 100 may also be equipped with other types of memory, and may be equipped with multiple types of memory. 【0035】 The HDD 103 is a non-volatile storage device that stores software programs such as the OS (Operating System), middleware, and application software, as well as data. The information processing device 100 may also be equipped with other types of storage devices such as flash memory or SSD (Solid State Drive), and may be equipped with multiple non-volatile storage devices. 【0036】 The GPU 104 outputs an image to the display 111 connected to the information processing device 100, according to instructions from the processor 101. Any type of display can be used as the display 111, such as a CRT (Cathode Ray Tube) display, a liquid crystal display (LCD), a plasma display, or an organic electro-luminescence (OEL) display. 【0037】The input interface 105 acquires input signals from the input device 112 connected to the information processing device 100 and outputs them to the processor 101. The input device 112 can be a pointing device such as a mouse, touch panel, touchpad, or trackball, or a keyboard, remote controller, or button switch. Furthermore, multiple types of input devices may be connected to the information processing device 100. 【0038】 The media reader 106 is a reading device that reads programs and data recorded on the recording medium 113. The recording medium 113 can be, for example, a magnetic disk, an optical disk, a magneto-optical disk (MO), or semiconductor memory. Magnetic disks include flexible disks (FD) and HDDs. Optical disks include CDs (Compact Discs) and DVDs (Digital Versatile Discs). 【0039】 The media reader 106 copies programs and data read from the recording medium 113 to other recording media such as RAM 102 or HDD 103. The read programs are executed by the processor 101, for example. The recording medium 113 may be a portable recording medium and may be used for distributing programs and data. The recording medium 113 and HDD 103 are sometimes referred to as computer-readable recording media. 【0040】 The communication interface 107 is connected to the network 114 and communicates with other information processing devices via the network 114. The communication interface 107 may be a wired communication interface connected to a wired communication device such as a switch or router, or a wireless communication interface connected to a wireless communication device such as a base station or access point. 【0041】 The communication interface 108 is connected to the underwater LiDAR 200 and communicates with the underwater LiDAR 200. The communication interface 108 is a wired communication interface. The communication interface 108 may also be a wireless communication interface. 【0042】 The underwater LiDAR 200 is a sensor device used for measuring three-dimensional data (3D data) in water. The applications of the underwater LiDAR 200 include, for example, inspection of underwater facilities in an offshore wind power generation system and calculation of the amount of algae (e.g., volume) in blue carbon, which involve obtaining 3D data in water. For example, the underwater LiDAR 200 is mounted on an underwater robot such as an autonomous underwater vehicle (AUV). Data acquisition by the underwater LiDAR 200 is performed remotely. The measurable distance range using the underwater LiDAR 200 is, for example, about 1 m to 10 m. 【0043】 FIG. 3 is a diagram showing an example of the hardware of the underwater LiDAR. The underwater LiDAR 200 includes a processor 201, a memory 202, a communication interface 203, a laser irradiator 204, a scanning mirror 205, and a photodetector 206. 【0044】 The processor 201 is, for example, a CPU. The processor 201 executes a program stored in the memory 202. The processor 201 may also be a DSP, an ASIC, an FPGA, etc. 【0045】 The memory 202 stores data used for the processing of the processor 201. The memory 202 may include a volatile storage device and a non-volatile storage device. The communication interface 203 is connected to the information processing device 100 and communicates with the information processing device 100. For example, the communication interface 203 and the communication interface 108 of the information processing device 100 are connected by a LAN (Local Area Network) cable or the like. 【0046】 The laser irradiator 204 is a light source device that generates laser light. The laser light emitted by the laser irradiator 204 is, for example, visible light. However, the wavelength range of the laser light may be other wavelength ranges such as infrared light depending on the application. The laser irradiator 204 generates laser light toward the scanning mirror 205. The laser irradiator 204 is an example of the irradiation unit 21 of the first embodiment. 【0047】The scanning mirror 205 is a mirror that reflects the laser light generated by the laser irradiator 204. The scanning mirror 205 has a rotation axis and rotates around the rotation axis to two-dimensionally scan the laser light over the region to be measured. A mirror suitable for the wavelength range of the laser light, such as a mirror for visible light, may be used. The scanning mirror 205 is, for example, a MEMS (Micro Electro Mechanical Systems) mirror. The scanning mirror 205 is an example of the mirror 22 of the first embodiment. 【0048】 The photodetector 206 is a sensor that detects the laser light scattered back by the object. The photodetector 206 is an example of the light receiving unit 23 of the first embodiment. FIG. 4 is a diagram showing an example of the rotation axis of the scanning mirror. 【0049】 The scanning mirror 205 has a horizontal axis 205a and a vertical axis 205b. For example, the vertical axis 205b is an axis along the traveling direction of the laser light from the laser irradiator 204 toward the scanning mirror 205. The horizontal axis 205a is an axis perpendicular to the vertical axis 205b. However, the horizontal axis 205a may be an axis along the traveling direction of the laser light from the laser irradiator 204 toward the scanning mirror 205. The horizontal axis 205a and the vertical axis 205b pass through the center of the scanning mirror 205. The scanning mirror 205 is rotatable about the horizontal axis 205a. The scanning mirror 205 is rotatable about the vertical axis 205b. 【0050】 A reference posture of the scanning mirror 205 where the scanning mirror 205 does not rotate about each of the horizontal axis 205a and the vertical axis 205b, that is, an angle of 0°, is predetermined. For example, the reference posture of the scanning mirror 205 is a posture in which the direction perpendicular to the direction in which the laser light travels from the laser irradiator 204 to the scanning mirror 205 coincides with the direction of the perpendicular to the reflecting surface 205c of the scanning mirror 205. The reflecting surface 205c is a surface of the scanning mirror 205 that reflects the laser light. 【0051】Figure 5 shows an example of laser sensing by an underwater LiDAR. The underwater LiDAR 200 is used in underwater environments 50 such as the sea, rivers, and lakes. In addition to the hardware illustrated in Figure 3, the underwater LiDAR 200 further includes a waterproof cylinder 207, a window 208, and a lens 209. Note that some hardware of the underwater LiDAR 200 is omitted from the illustration in Figure 5. The underwater LiDAR 200 is connected to an information processing device 100 located outside the underwater environment 50, such as on the sea surface. The underwater LiDAR 200 may have the information processing device 100 built into it. 【0052】 The waterproof cylinder 207 is a cylindrical housing that covers the main body of the underwater LiDAR 200 and prevents water from entering the interior. The waterproof cylinder 207 has a window 208. The window 208 is provided on one of the two opposing surfaces (bottom surface) of the waterproof cylinder 207. The window 208 transmits laser light. The material of the window 208 is acrylic or glass. The medium inside the waterproof cylinder 207 is air. 【0053】 Lens 209 is a light-receiving lens that guides the laser light to the photodetector 206. The underwater LiDAR 200 may also have other lenses, such as light-emitting lenses, between the laser irradiator 204 and the scanning mirror 205, or between the scanning mirror 205 and the window 208, but these are not shown in the illustration. 【0054】 The scanning mirror 205 reflects the laser light incident from the laser irradiator 204 to the scanning mirror 205 toward the object 300 to be measured in the underwater environment 50. The scanning mirror 205 rotates around the horizontal axis 205a and the vertical axis 205b, scanning the laser light irradiation point on the object 300 in two dimensions. The photodetector 206 detects the laser light scattered at the irradiation point, such as the object 300 or its vicinity. 【0055】The processor 201 measures the time difference ΔT between the laser emission time at the laser irradiator 204 and the detection time at the photodetector 206 for each rotation angle (mirror angle) around the horizontal axis 205a and vertical axis 205b of the scanning mirror 205. ΔT is called the round-trip time of the laser light. The processor 201 transmits the mirror angle and the round-trip time ΔT to the information processing device 100. 【0056】 Figure 6 shows an example of round-trip time measurement. The time chart 60 shows an example of signals received by the processor 201 from the laser irradiator 204 and the photodetector 206. The laser emission signal 61 represents a pulse of laser light (laser pulse) emitted by the laser irradiator 204. For example, when the laser irradiator 204 emits a laser pulse, it may output a laser emission signal 61 corresponding to that laser pulse to the processor 201. For example, when the processor 201 receives the laser emission signal 61, it starts the timer. 【0057】 The detection signal 62 is input from the photodetector 206 to the processor 201 when laser detection is performed by the photodetector 206. When the processor 201 receives the detection signal 62, it stops the timer. The processor 201 measures the time from the start to the stop of the timer as the round-trip time ΔT. 【0058】 The information processing device 100 can measure the distance between the underwater LiDAR 200 and the object 300 based on the round-trip time ΔT. Figure 7 shows an example of how the optical path length inside the housing changes depending on the angle of the scanning mirror. 【0059】 Figure 7(A) illustrates the case where the angle of the scanning mirror 205, i.e., the mirror angle = α. Figure 7(B) illustrates the case where the mirror angle = β. The mirror angle is represented by a set of the angle of rotation around the horizontal axis 205a and the angle of rotation around the vertical axis 205b. 【0060】In the underwater LiDAR200, the optical path length of the laser light inside the housing, i.e., the internal optical path length, changes depending on the mirror angle. The internal optical path length is the sum of the optical path length from the laser irradiator 204 to the window 208 via the scanning mirror 205, and the optical path length from the window 208 to the photodetector 206 via the lens 209. 【0061】 Therefore, the aforementioned round-trip time ΔT includes a measurement error corresponding to the change in optical path length due to the mirror angle. Accordingly, the information processing device 100 provides a function to measure the distance to the object 300 while taking this measurement error into consideration. 【0062】 Figure 8 shows an example of the software for the information processing device. The information processing device 100 includes an optical path length table storage unit 120, a data acquisition unit 130, an internal optical path length identification unit 140, an internal flight time calculation unit 150, an underwater flight time calculation unit 160, an underwater distance calculation unit 170, a light projection angle conversion unit 180, a 3D coordinate data generation unit 190, and a 3D coordinate data storage unit 195. 【0063】 The optical path length table storage unit 120 and the three-dimensional coordinate data storage unit 195 utilize the storage areas of RAM 102 and HDD 103. The data acquisition unit 130, the internal optical path length identification unit 140, the internal flight time calculation unit 150, the underwater flight time calculation unit 160, the underwater distance calculation unit 170, the light projection angle conversion unit 180, and the three-dimensional coordinate data generation unit 190 are realized by the execution of a program stored in RAM 102 by the processor 101. 【0064】The optical path length table storage unit 120 stores an optical path length table. The optical path length table contains information indicating the optical path length within the housing according to the mirror angle. The data acquisition unit 130 controls the underwater LiDAR 200 and acquires data from the underwater LiDAR 200. The data acquisition unit 130 instructs the underwater LiDAR 200 to start measurement. When instructed to start measurement, the underwater LiDAR 200 measures the round-trip time ΔT of the laser light while sequentially changing the mirror angle. The data acquisition unit 130 sequentially acquires data from the underwater LiDAR 200 indicating the round-trip time ΔT of the laser light and the mirror angle at the time of measurement of the round-trip time ΔT. The data acquisition unit 130 may further acquire other data, such as intensity data indicating the intensity of the laser light measured by the photodetector 206. 【0065】 The internal optical path length identification unit 140 identifies the internal optical path length corresponding to the mirror angle acquired by the data acquisition unit 130, based on the optical path length table stored in the optical path length table storage unit 120. 【0066】 The housing-internal time-of-flight calculation unit 150 calculates the time of flight of the laser light inside the housing, i.e., the housing-internal time-of-flight t, based on the housing-internal optical path length determined by the housing-internal optical path length determination unit 140. The housing-internal time-of-flight t is the housing-internal optical path length / (speed of light inside the housing). The speed of light inside the housing is (speed of light in a vacuum) / (refractive index of the medium inside the housing). As mentioned above, the medium inside the housing, i.e., inside the waterproof cylinder 207 and window 208, is air. 【0067】 The underwater flight time calculation unit 160 performs a correction by subtracting the in-housing flight time t from the round-trip time ΔT acquired by the data acquisition unit 130, and calculates the corrected round-trip time (ΔT-t). The underwater distance calculation unit 170 calculates the distance L from the underwater LiDAR 200 to the laser beam irradiation point in the underwater environment 50 based on the corrected round-trip time (ΔT-t). The distance L = (ΔT-t) × (speed of light in water) × 1 / 2. The speed of light in water is (speed of light in a vacuum) / (refractive index of water). 【0068】The projection angle conversion unit 180 converts the mirror angle acquired by the data acquisition unit 130 into a projection angle relative to the laser beam irradiation point in the underwater environment 50 (for example, the irradiation point on the object 300). The projection angle is, for example, the angle between the surface corresponding to the window 208 of the waterproof cylinder 207 and the direction of propagation of the projected laser beam. 【0069】 The 3D coordinate data generation unit 190 generates 3D coordinate data based on the distance to each irradiation point scanned two-dimensionally by the scanning mirror 205 and the projection angle for each irradiation point. The 3D coordinate data is, for example, point cloud data representing the surface shape of the object 300. The 3D coordinate data generation unit 190 stores the generated 3D coordinate data in the 3D coordinate data storage unit 195. 【0070】 The 3D coordinate data storage unit 195 stores the 3D coordinate data generated by the 3D coordinate data generation unit 190. Figure 9 shows an example of an optical path length table. 【0071】 The optical path length table 121 is pre-stored in the optical path length table storage unit 120. The optical path length table 121 includes items such as the horizontal angle of the mirror, the vertical angle of the mirror, the optical path length (laser irradiator → window), and the optical path length (window → photodetector). 【0072】 The "Mirror Horizontal Angle" field registers the angle of the scanning mirror 205 around the horizontal axis 205a. The "Mirror Vertical Angle" field registers the angle of the scanning mirror 205 around the vertical axis 205b. The units for the mirror horizontal angle and mirror vertical angle are degrees (°). The "Optical Path Length (Laser Irradiator → Window)" field registers the optical path length of the laser light from the laser irradiator 204 through the scanning mirror 205 to the window 208, corresponding to the pair of mirror horizontal angle and mirror vertical angle. The "Optical Path Length (Window → Photodetector)" field registers the optical path length of the laser light from the window 208 to the photodetector 206, corresponding to the pair of mirror horizontal angle and mirror vertical angle. The units for the optical path length (Laser Irradiator → Window) and optical path length (Window → Photodetector) are, for example, cm. 【0073】For example, the optical path length table 121 has records for mirror horizontal angle "-10", mirror vertical angle "-10", optical path length (laser irradiator → window) "35", and optical path length (window → photodetector) "35". This record indicates that when the mirror horizontal angle is -10° and the mirror vertical angle is -10°, the optical path length (laser irradiator → window) is 35 cm and the optical path length (window → photodetector) is 35 cm. The optical path length inside the housing is "optical path length (laser irradiator → window) + optical path length (window → photodetector)". Therefore, when the mirror horizontal angle is -10° and the mirror vertical angle is -10°, the optical path length inside the housing is 35 cm + 35 cm = 70 cm. 【0074】 The optical path length table 121 pre-stores the values ​​for the optical path length (laser irradiator → window) and the optical path length (window → photodetector) for each set of possible values ​​for the horizontal mirror angle and the vertical mirror angle. 【0075】 Next, the procedure for distance measurement using a system including the information processing device 100 and the underwater LiDAR 200 will be described. Figure 10 is a flowchart showing an example of the distance measurement process. 【0076】 (S10) The data acquisition unit 130 instructs the underwater LiDAR 200 to start measurement. Here, the object to be measured for distance is the object 300. When the processor 201 of the underwater LiDAR 200 receives the instruction to start measurement, it starts driving the scanning mirror 205. The processor 201 also instructs the laser irradiator 204 to start measurement. Steps S11 to S18 shown below are executed for each mirror angle. 【0077】 (S11) The laser irradiator 204 emits laser pulses. As a result, laser light is irradiated from the underwater LiDAR 200 toward the object 300. The processor 201 transmits data indicating the current mirror angle to the information processing device 100. Then, the process proceeds to steps S12 and S13. Steps S13 and S14 shown below may be executed in parallel with step S12. 【0078】(S12) The processor 201 receives a detection signal from the photodetector 206 of the laser light scattered at the irradiation point on the object 300. The processor 201 then measures the round-trip time ΔT and transmits data indicating the round-trip time ΔT to the information processing device 100. The data acquisition unit 130 acquires the data indicating the round-trip time ΔT. Then the process proceeds to step S15. 【0079】 (S13) The data acquisition unit 130 acquires data indicating the mirror angle from the underwater LiDAR 200. The data acquisition unit 130 may also receive data from the underwater LiDAR 200 that includes the round-trip time ΔT and the mirror angle. Alternatively, after receiving the data indicating the mirror angle, the data acquisition unit 130 may receive data indicating the round-trip time ΔT corresponding to the mirror angle. 【0080】 (S14) The internal optical path length determination unit 140 determines the internal optical path length corresponding to the acquired mirror angle based on the optical path length table 121. The internal flight time calculation unit 150 calculates the internal flight time (correction time) t based on the determined internal optical path length. t = internal optical path length / (speed of light inside the housing). 【0081】 (S15) The underwater flight time calculation unit 160 corrects the round trip time. Specifically, the underwater flight time calculation unit 160 calculates the corrected round trip time (ΔT-t) by subtracting the in-housing flight time t from the round trip time ΔT. 【0082】 (S16) The underwater distance calculation unit 170 calculates the distance L from the underwater LiDAR 200 to the object 300. L = (ΔT - t) × (speed of light in water) × 1 / 2. (S17) The projection angle conversion unit 180 converts the mirror angle to the projection angle of the laser light from the underwater LiDAR 200. The 3D coordinate data generation unit 190 generates data showing the 3D coordinates of the irradiation point in the underwater environment 50 based on the distance L and projection angle calculated by the underwater distance calculation unit 170, and outputs the generated data to the 3D coordinate data storage unit 195. 【0083】(S18) The data acquisition unit 130 determines whether the measurement is complete or not. If the measurement is complete, the measurement process ends. If the measurement is not complete, the mirror angle of the scanning mirror 205 is changed by one step, and the process proceeds to step S11. For example, the data acquisition unit 130 may determine that the measurement is complete when it has measured all the mirror angles included in the optical path length table 121, and determine that the measurement is not complete if there are unmeasured mirror angles. 【0084】 In this way, the information processing device 100 can appropriately correct the round-trip time ΔT of the laser light acquired from the underwater LiDAR 200 by subtracting the flight time t inside the housing, which corresponds to the difference in the optical path length inside the housing for each mirror angle. Therefore, the information processing device 100 can improve the accuracy of acquiring the round-trip time. By using the corrected round-trip time (ΔT-t) to calculate the distance to the object 300, the information processing device 100 can reduce the error corresponding to the optical path length inside the housing for each mirror angle shown in Figure 7, and improve the distance measurement accuracy. 【0085】 Here, the laser light projected by the underwater LiDAR 200 passes through the window 208. As mentioned above, the material of the window 208 is acrylic or glass, and has a refractive index greater than 1. For this reason, the information processing device 100 may further correct the round-trip time ΔT based on the optical path length inside the window 208 for each mirror angle. 【0086】 Figure 11 shows an example of an optical path length table. The optical path length table 122 is stored in the optical path length table storage unit 120 in place of the optical path length table 121. The optical path length table 122 includes the following items: mirror horizontal angle, mirror vertical angle, optical path length (laser irradiator → window), optical path length within the window (inside the housing → outside the housing), optical path length within the window (outside the housing → inside the housing), and optical path length (window → photodetector). The items mirror horizontal angle, mirror vertical angle, optical path length (laser irradiator → window), and optical path length (window → photodetector) are the same as the items with the same names in the optical path length table 121. 【0087】The "Optical Path Length Inside Window (Inside Housing → Outside Housing)" field registers the optical path length inside window 208 for the laser beam exiting from inside the housing to outside the housing. The "Optical Path Length Inside Window (Outside Housing → Inside Housing)" field registers the optical path length inside window 208 for the laser beam that has returned from outside the housing to inside the housing. Values ​​corresponding to the mirror horizontal angle and mirror vertical angle pairs are pre-registered in the "Optical Path Length Inside Window (Inside Housing → Outside Housing)" and "Optical Path Length Inside Window (Outside Housing → Inside Housing)" fields. The unit for "Optical Path Length Inside Window (Inside Housing → Outside Housing)" and "Optical Path Length Inside Window (Outside Housing → Inside Housing)" is, for example, cm. 【0088】 The internal optical path length identification unit 140 can identify the internal optical path length and the internal optical path length based on the optical path length table 122, corresponding to the mirror angle acquired from the underwater LiDAR 200. The internal optical path length based on the optical path length table 122 is "internal optical path length (inside housing → outside housing) + internal optical path length (outside housing → inside housing)". 【0089】 Furthermore, the in-casing time-of-flight calculation unit 150 can calculate the time-of-flight τ within the window based on the optical path length within the window. τ = optical path length within the window / (speed of light within the window). The speed of light within the window is the speed of light in a vacuum / (refractive index of the material of the window 208 (e.g., acrylic or glass)). 【0090】 The underwater flight time calculation unit 160 corrects the round-trip time ΔT based on the flight time t inside the housing and the flight time τ inside the window. Specifically, the underwater flight time calculation unit 160 calculates the corrected round-trip time (ΔT - t - τ) by subtracting the flight time t inside the housing and the flight time τ inside the window from the round-trip time ΔT. 【0091】 The underwater distance calculation unit 170 then calculates the distance L from the underwater LiDAR 200 to the object 300. L = (ΔT - t - τ) × (speed of light in water) × 1 / 2. In this way, the information processing device 100 can further improve the distance measurement accuracy by correcting the round-trip time ΔT, taking into account the difference in optical path length within the window for each mirror angle. 【0092】Figure 12 shows an example of the external appearance of an underwater LiDAR. Figure 12 schematically shows a perspective view of the underwater LiDAR 200. The internal hardware of the underwater LiDAR 200 is covered and protected by a waterproof cylinder 207 and a window 208. The underwater LiDAR 200 has an outlet inside the window 208, and emits RGB laser light 70 from this outlet. The RGB laser light 70 is laser light that contains three wavelengths corresponding to red, green, and blue in the visible light spectrum. The wavelengths of light contained in the RGB laser light 70 are, for example, 465 nm, 525 nm, and 640 nm. The RGB laser light 70 is an example of laser light emitted by the laser irradiator 204. 【0093】 The underwater LiDAR 200 may also be equipped with an R light receiving unit 210, a G light receiving unit 211, a B light receiving unit 212, and a camera 220 inside the window 208. The R light receiving unit 210 receives light of the wavelength corresponding to red in the RGB laser light 70. The G light receiving unit 211 receives light of the wavelength corresponding to green in the RGB laser light 70. The B light receiving unit 212 receives light of the wavelength corresponding to blue in the RGB laser light 70. Each of the R light receiving unit 210, G light receiving unit 211, and B light receiving unit 212 has the aforementioned photodetector 206 and lens 209. 【0094】 Camera 220 is an imaging device used to capture video and still images. The information processing device 100 can also acquire video and still image data captured by camera 220 from the underwater LiDAR 200. In this case, the information processing device 100 has an optical path length table for each wavelength. 【0095】As mentioned above, the underwater LiDAR 200 is mounted on underwater robots such as AUVs and used to measure objects in the water, such as in the ocean. In the second embodiment, the underwater environment 50 was given as an example of the environment in which the underwater LiDAR 200 is used, but the underwater LiDAR 200 may be used in other environments. For example, the underwater LiDAR 200 may be used to measure objects in the atmosphere. In that case, both the internal and external media of the underwater LiDAR 200 will be air. The refractive index of air can change depending on environmental parameters such as temperature, humidity, atmospheric pressure, and carbon dioxide concentration. Therefore, the information processing device 100 can further improve the distance measurement accuracy by using, for example, a value based on the refractive index corresponding to the values ​​of these environmental parameters as the speed of light in air. 【0096】 Next, a comparative example will be described. Figure 13 shows a comparative example. The comparative example, the underwater LiDAR 200a, has a processor 201, a laser irradiator 204, a photodetector 206, a waterproof cylinder 207, a window 208, and a lens 209, but does not have a scanning mirror 205. Therefore, the underwater LiDAR 200a differs from the underwater LiDAR 200 in that the laser beam is not scanned by the rotation of the scanning mirror 205. 【0097】 In the underwater LiDAR 200a, since a scanning mirror 205 is not used, the change in optical path length within the housing according to the mirror angle, as illustrated in Figure 7, does not occur. In other words, in the underwater LiDAR 200a, the optical path length from the laser irradiator 204 to the window 208 and the optical path length from the window 208 to the photodetector 206 can be considered constant. 【0098】 Therefore, for example, when measuring the distance to an object 300 using an underwater LiDAR 200a in an underwater environment 50, the internal optical path length of the underwater LiDAR 200a can be considered constant among the total optical path length of the laser light traveling back and forth between the underwater LiDAR 200a and the object 300. Thus, when using an underwater LiDAR 200a, it is sufficient to perform a correction by subtracting a constant time corresponding to a constant internal optical path length from the round-trip time ΔT. 【0099】On the other hand, when using the underwater LiDAR200, the optical path length inside the housing changes depending on the mirror angle, as illustrated in Figure 7. In this case, if a correction is performed by subtracting the round-trip time ΔT at a constant time regardless of the mirror angle, the change in the optical path length inside the housing according to the mirror angle is not reflected, and an error occurs in the measured distance. 【0100】 Therefore, the information processing device 100 can improve the distance measurement accuracy by performing a correction based on the optical path length table 121, which subtracts the in-housing flight time t corresponding to the difference in in-housing optical path length for each mirror angle from the round-trip time ΔT of the laser light acquired from the underwater LiDAR 200. 【0101】 As explained above, the information processing device 100, which measures the distance to an object based on the round-trip time of the laser beam, performs the following processing, for example. The processor 101 acquires the first round-trip time of the laser beam that is irradiated onto the object via the mirror and received by the light-receiving unit from the irradiating unit of a housing that houses an irradiating unit that emits laser beam, a mirror whose angle of reflection of the laser beam can be changed, and a light-receiving unit that receives the laser beam, and the first angle of rotation of the mirror. Based on information that defines the time of flight of the laser beam within the housing for each angle of rotation of the mirror, the processor 101 performs a correction by subtracting the first time of flight within the housing corresponding to the first angle from the first round-trip time. 【0102】 This allows the information processing device 100 to improve its distance measurement accuracy. The underwater LiDAR 200 or waterproof cylinder 207 is an example of a housing. The laser irradiator 204 is an example of an irradiation unit. The photodetector 206 is an example of a light receiving unit. The optical path length table 121 is an example of information that defines the time of flight of the laser light inside the housing for each mirror angle. As shown in the optical path length table 121, the mirror angle including the first angle may be represented by a set of the angle around the first rotation axis of the mirror and the angle around the second rotation axis of the mirror that is perpendicular to the first rotation axis. This allows the information processing device 100 to suitably improve its distance measurement accuracy in applications where the laser light is scanned in two dimensions by rotating the mirror using these two rotation axes. 【0103】For example, the information defining the time of flight of the laser beam within the housing for each angle of rotation of the mirror may include multiple optical path lengths within the housing for the laser beam corresponding to multiple angles of the mirror. In the above correction, the processor 101 may identify the first optical path length within the housing corresponding to the first angle from this information and calculate the first time of flight within the housing based on the first optical path length and the speed of light in the medium inside the housing. 【0104】 This allows the information processing device 100 to appropriately obtain the first time of flight within the enclosure. For example, if the medium inside the enclosure is air, the processor 101 may monitor the temperature, humidity, and pressure of the air inside the enclosure and calculate the first time of flight within the enclosure using the speed of light at a refractive index corresponding to the temperature, humidity, and pressure of the air inside the enclosure. This allows the information processing device 100 to improve the accuracy of obtaining the first time of flight within the enclosure and further enhance the distance measurement accuracy. 【0105】 For example, the enclosure has a window that transmits laser light. The information defining the time of flight of the laser light inside the enclosure for each angle of rotation of the mirror may further define the time of flight of the laser light inside the window for each angle of rotation of the mirror. In the above correction, the processor 101 may subtract the first time of flight inside the window and the first time of flight inside the enclosure corresponding to the first angle from the first round trip time based on this information. 【0106】 This allows the information processing device 100 to further improve the distance measurement accuracy. The information defining the time of flight of the laser beam within the housing for each angle of rotation of the mirror may have multiple optical path lengths within the window of the laser beam corresponding to multiple angles of rotation of the mirror. In the above correction, the processor 101 may identify the first optical path length within the window corresponding to the first angle from this information and calculate the time of flight within the first window based on the first optical path length within the window and the speed of light in the window material. 【0107】As a result, the information processing device 100 can appropriately obtain the first window flight time. The processor 101 calculates the distance between the housing and the object based on the second round trip time obtained by the above correction to the first round trip time. As a result, the information processing device 100 can improve the distance measurement accuracy. The corrected round trip time ΔT-t exemplified in the second embodiment is an example of the second round trip time. Also, the corrected round trip time ΔT-t-τ is an example of the second round trip time. 【0108】 Furthermore, the internal medium of the housing is, for example, air. The housing may also be installed underwater, such as in the sea, during use. For example, when the processor 101 determines the round-trip time of the laser beam between the housing and the object underwater, it determines the round-trip time of the laser beam underwater by subtracting the first internal flight time corresponding to the first angle from the first round-trip time. Thus, the information processing device 100 is particularly suitable for distance measurement using a housing operated underwater, such as an underwater LiDAR 200. For example, the information processing device 100 can improve the accuracy of distance measurement to an object 300 located underwater. 【0109】 Furthermore, the in-casing flight time is the sum of the flight time of the laser light from the irradiation unit through the mirror to the window of the casing (for example, the inner surface of the window) and the flight time of the laser light from the window (for example, the inner surface of the window) to the light receiving unit. This allows the information processing device 100 to appropriately define the in-casing flight time and improve the distance measurement accuracy. 【0110】 The time of flight within the enclosure may be defined by the total optical path length of the laser light from the irradiator through the mirror to the window of the enclosure (e.g., the inner surface of the window) and the optical path length of the laser light from the window (e.g., the inner surface of the window) to the light receiving unit. In this case, the processor 101 can obtain the time of flight within the enclosure by dividing the total optical path length by the speed of light in the medium inside the enclosure. 【0111】The information processing in the first embodiment can be achieved by having the processing unit 12 execute a program. The information processing in the second embodiment can be achieved by having the processor 101 execute a program. The program can be recorded on a computer-readable recording medium 113. 【0112】 For example, a program can be distributed by distributing a recording medium 113 on which the program is stored. Alternatively, the program may be stored on another computer and distributed via a network. A computer may, for example, store (install) a program stored on the recording medium 113 or a program received from another computer into a storage device such as RAM 102 or HDD 103, and then read and execute the program from that storage device. 【0113】 The above merely illustrates the principle of the present invention. Furthermore, numerous modifications and changes are possible for those skilled in the art, and the present invention is not limited to the exact configurations and applications shown and described above. All corresponding modifications and equivalents are considered to be within the scope of the present invention as defined by the appended claims and their equivalents. 【0114】 10 Information processing device 11 Storage unit 11a Time of flight information inside the enclosure 12 Processing unit 20 Enclosure 20a Window 21 Irradiation unit 22 Mirror 22a, 22b Rotation axis 23 Light receiving unit 24 Processing unit 30 Object 40, 40a Laser light

Claims

1. A distance measuring program for measuring the distance to an object based on the round-trip time of a laser beam, wherein the program causes a computer to acquire the first round-trip time of the laser beam irradiated from the irradiating unit that emits the laser beam, the first angle of rotation of the mirror, and the first angle of rotation of the mirror, of a housing that houses the irradiating unit that emits the laser beam, the mirror that can change the angle of reflection of the laser beam, and the first angle of rotation of the mirror, and to perform a correction by subtracting the first time of flight within the housing corresponding to the first angle from the first round-trip time, based on information that defines the time of flight of the laser beam within the housing for each angle of rotation of the mirror.

2. The distance measuring program according to claim 1, wherein the information comprises a plurality of internal optical path lengths of the laser beam corresponding to a plurality of angles of the mirror, and the correction involves determining a first internal optical path length corresponding to a first angle from the information, and calculating the first internal flight time based on the first internal optical path length and the speed of light in the medium inside the housing, thereby causing the computer to perform the following processing.

3. The distance measuring program according to claim 1, wherein the housing has a window through which the laser light is transmitted, the information further defines the flight time of the laser light within the window for each angle of rotation of the mirror, and the correction causes the computer to perform a process of subtracting the first flight time within the window corresponding to the first angle and the flight time within the first housing from the first round trip time based on the information.

4. The distance measuring program according to claim 3, wherein the information has a plurality of window-internal optical path lengths of the laser light corresponding to a plurality of angles of rotation of the mirror, and the correction involves determining a first window-internal optical path length corresponding to a first angle from the information, and calculating the first window-internal flight time based on the first window-internal optical path length and the speed of light in the material of the window, thereby causing the computer to perform the following process.

5. The distance measuring program according to claim 1, which causes the computer to perform a process of calculating the distance between the housing and the object based on the second round trip time obtained by the correction to the first round trip time.

6. The distance measuring program according to claim 1, which determines the round-trip time of the laser beam between the housing and the object in water by the correction.

7. The distance measuring program according to claim 1, wherein the housing has a window through which the laser light is transmitted, and the flight time inside the housing is the sum of the flight time of the laser light from the irradiation unit through the mirror to the window and the flight time of the laser light from the window to the light receiving unit.

8. A distance measuring method for measuring the distance to an object based on the round-trip time of a laser beam, wherein a computer acquires the first round-trip time of the laser beam irradiated from the irradiating unit that emits the laser beam, the first angle of rotation of the mirror, and the first angle of rotation of the mirror, of a housing that houses the irradiating unit that emits the laser beam, the mirror that can change the angle of reflection of the laser beam, and the first angle of rotation of the mirror, and performs a correction by subtracting the first in-house flight time corresponding to the first angle from the first round-trip time, based on information that defines the in-house flight time of the laser beam for each angle of rotation of the mirror.

9. An information processing device for measuring the distance to an object based on the round-trip time of a laser beam, comprising: a storage unit that stores information defining the in-house flight time of the laser beam, which is irradiated from the irradiating unit that emits the laser beam, a mirror whose angle of reflection of the laser beam can be changed, and a light receiving unit that receives the laser beam, for each angle of rotation of the mirror; and a processing unit that acquires a first round-trip time of the laser beam irradiated from the irradiating unit that emits the laser beam, a mirror whose angle of reflection of the laser beam can be changed, and a light receiving unit that receives the laser beam, for each angle of rotation of the mirror; and performs a correction by subtracting the first in-house flight time corresponding to the first angle from the first round-trip time based on the information.

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