Patent Publication Number: US-2023161037-A1

Title: Distance measurement device, distance measurement method, and distance measurement program

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/JP2021/023576, filed on Jun. 22, 2021, which claims priority from Japanese Application No. 2020-125848, filed on Jul. 22, 2020. The entire disclosure of each of the above applications is incorporated herein by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a distance measurement device, a distance measurement method, and a distance measurement program. 
     Related Art 
     Generally, a distance measurement technology for measuring a distance to an object to be measured by scanning the object to be measured with light from a light source and measuring a time taken until reflected light reflected by the object to be measured returns has been known (refer to JP2014-194380A, JP2016-125970A, and JP2019-174297A). 
     In the distance measurement technology, it is necessary to detect a scan angle for scanning the object to be measured. However, in the technologies disclosed in JP2014-194380A, JP2016-125970A, and JP2019-174297A, detection accuracy of the scan angle may not be said to be sufficient. Thus, a technology for improving the detection accuracy of the scan angle is desired. 
     SUMMARY 
     The present disclosure is conceived in view of the above matter and provides a distance measurement device, a distance measurement method, and a distance measurement program that can improve detection accuracy of a scan angle for scanning an object to be measured with light from a light source. 
     A distance measurement device of a first aspect of the present disclosure comprises a light source, a reflective member that varies a scan angle for scanning an object to be measured with light from the light source and reflects the light from the light source for each varied scan angle, a polarization optical member that guides the light reflected by the reflective member to the object to be measured, a first light-receiving section that receives reflected light after the light guided by the polarization optical member is reflected by the object to be measured, and a second light-receiving section that receives the light reflected by the reflective member and the polarization optical member in this order in a case where the scan angle of the reflective member is a specific angle. 
     A distance measurement device of a second aspect of the present disclosure is the distance measurement device of the first aspect, in which a plurality of the second light-receiving sections are provided in a direction intersecting with a direction in which the polarization optical member guides the light to the object to be measured. 
     A distance measurement device of a third aspect of the present disclosure is the distance measurement device of the first aspect or the second aspect, in which the reflective member reflects the light a plurality of times by varying the scan angle after the light is reflected in a state where the specific angle is set as the scan angle until the light is reflected in a state where the scan angle is set as the specific angle again, the distance measurement device further comprises at least one processor, and the processor is configured to acquire the number of times of light generation of the light source by the first light-receiving section between a first time at which the second light-receiving section receives the light and a second time at which the second light-receiving section receives the light after the first time, and derive the scan angle corresponding to the reflected light received by the first light-receiving section based on the number of times of light generation. 
     A distance measurement device of a fourth aspect of the present disclosure is the distance measurement device of the third aspect, in which the processor is configured to derive the scan angle based on a value obtained by dividing an angle of a range in which the reflective member varies the scan angle by a number obtained by subtracting 1 from the number of times of light generation. 
     A distance measurement device of a fifth aspect of the present disclosure is the distance measurement device of the third aspect or the fourth aspect, in which the processor is configured to associate the scan angle with distance measurement data corresponding to the reflected light received by the first light-receiving section. 
     A distance measurement device of a sixth aspect of the present disclosure is the distance measurement device of any one aspect of the first aspect to the fifth aspect, in which the polarization optical member includes a transmitting unit through which the light heading toward the reflective member from the light source is transmitted, and a reflecting unit that reflects the light reflected by the reflective member toward the object to be measured and reflects the light reflected by the reflective member toward the second light-receiving section. 
     A distance measurement device of a seventh aspect of the present disclosure is the distance measurement device of any one aspect of the first aspect to the sixth aspect, in which the polarization optical member further guides the reflected light reflected by the object to be measured to the reflective member and guides the reflected light reflected by the reflective member in a direction of the light source, and the first light-receiving section receives the reflected light guided in the direction of the light source by the polarization optical member. 
     A distance measurement device of an eighth aspect of the present disclosure is the distance measurement device of the seventh aspect, further comprising a beam splitter that is positioned on an optical path between the light source and the reflective member and on an optical path between the reflective member and the first light-receiving section, causes the light to pass in a direction of the reflective member, and guides the reflected light reflected in the direction of the light source by the reflective member to the first light-receiving section. 
     A distance measurement device of a ninth aspect of the present disclosure is the distance measurement device of any one aspect of the first aspect to the sixth aspect, further comprising an optical component that guides the reflected light reflected by the object to be measured in a direction different from a direction of the light source, in which the first light-receiving section receives the reflected light guided in the direction different from the direction of the light source by the optical component. 
     A distance measurement device of a tenth aspect of the present disclosure is the distance measurement device of any one aspect of the first aspect to the ninth aspect, in which the reflective member is a MEMS mirror. 
     In addition, a distance measurement method of an eleventh aspect of the present disclosure is a distance measurement method by a distance measurement device including a light source, a reflective member that varies a scan angle for scanning an object to be measured with light from the light source and reflects the light from the light source for each varied scan angle, a polarization optical member that guides the light reflected by the reflective member to the object to be measured, a first light-receiving section that receives reflected light after the light guided by the polarization optical member is reflected by the object to be measured, and a second light-receiving section that receives the light reflected by the reflective member and the polarization optical member in this order in a case where the scan angle of the reflective member is a specific angle, in which the reflective member reflects the light a plurality of times by varying the scan angle after the light is reflected in a state where the specific angle is set as the scan angle until the light is reflected in a state where the scan angle is set as the specific angle again, the distance measurement method executed by a processor, comprising acquiring the number of times of light generation of the light source corresponding to the reception of the reflected light by the first light-receiving section between a first time at which the second light-receiving section receives the light and a second time at which the second light-receiving section receives the light after the first time, deriving the scan angle corresponding to the reflected light received by the first light-receiving section based on the number of times of light generation, and associating the scan angle with distance measurement data corresponding to the reflected light received by the first light-receiving section. 
     In addition, a distance measurement program of a twelfth aspect of the present disclosure is a distance measurement program causing a processor to execute a process for distance measurement by a distance measurement device including a light source, a reflective member that varies a scan angle for scanning an object to be measured with light from the light source and reflects the light from the light source for each varied scan angle, a polarization optical member that guides the light reflected by the reflective member to the object to be measured, a first light-receiving section that receives reflected light after the light guided by the polarization optical member is reflected by the object to be measured, and a second light-receiving section that receives the light reflected by the reflective member and the polarization optical member in this order in a case where the scan angle of the reflective member is a specific angle, in which the reflective member reflects the light a plurality of times by varying the scan angle after the light is reflected in a state where the specific angle is set as the scan angle until the light is reflected in a state where the scan angle is set as the specific angle again, the distance measurement program causing the processor to execute a process comprising acquiring the number of times of light generation of the light source corresponding to the reception of the reflected light by the first light-receiving section between a first time at which the second light-receiving section receives the light and a second time at which the second light-receiving section receives the light after the first time, deriving the scan angle corresponding to the reflected light received by the first light-receiving section based on the number of times of light generation, and associating the scan angle with distance measurement data corresponding to the reflected light received by the first light-receiving section. 
     According to the present disclosure, detection accuracy of a scan angle for scanning an object to be measured with light from a light source can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a configuration diagram illustrating an example of a distance measurement device of an embodiment. 
         FIG.  2 A  is a diagram for describing an optical path of light of irradiation from a light source. 
         FIG.  2 B  is a diagram for describing an optical path of reflected light reflected by an object to be measured. 
         FIG.  3    is a structure diagram illustrating an example of a structure of a MEMS mirror that is a reflective member of the embodiment. 
         FIG.  4    is a configuration diagram illustrating an example of a hardware configuration of a control unit of the embodiment. 
         FIG.  5    is a functional block diagram illustrating an example of a functional configuration of the control unit of the embodiment. 
         FIG.  6    is a flowchart representing an example of distance measurement processing executed by the control unit. 
         FIG.  7    is a configuration diagram illustrating an example of a distance measurement device of Modification Example 1. 
         FIG.  8    is a configuration diagram illustrating an example of a distance measurement device of Modification Example 2. 
     
    
    
     DESCRIPTION 
     Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. It is noted that the present embodiment does not limit the present invention. 
     An example of a configuration of a distance measurement device  1  of the present embodiment is illustrated in  FIG.  1   . As illustrated in  FIG.  1   , the distance measurement device  1  of the present embodiment comprises a light source  10 , a collimator lens  12 , a collimator lens  14 , a beam splitter  16 , a polarization optical member  18 , a reflective member  20 , a light-receiving element  22 , a light-receiving element  24 , and a control unit  30 . 
     The distance measurement device  1  of the present embodiment is a so-called light detection and ranging (LiDAR) system that measures a distance to an object to be measured  4  by scanning the object to be measured  4  with light from the light source  10  and measuring a time taken until reflected light reflected by the object to be measured  4  returns. Hereinafter, the “reflected light” will refer to light reflected by the object to be measured  4 . In addition, in  FIG.  1   , an x axis direction and a y axis direction (xy plane direction) will be referred to as a horizontal direction, and a z axis direction will be referred to as a vertical direction. 
     As illustrated in  FIG.  2 A , light L 1  of irradiation from the light source  10  is collimated by the collimator lens  12  and the collimator lens  14 , transmitted through the beam splitter  16  and the polarization optical member  18 , and reflected by the reflective member  20 . The object to be measured  4  is irradiated with light L 1   1  reflected by the reflective member  20 . In addition, light L 1   2  reflected by the reflective member  20  is incident on the light-receiving element  22 . The light-receiving element  22  of the present embodiment is an example of a second light-receiving section according to an embodiment of the present disclosure. 
     In addition, as illustrated in  FIG.  2 B , reflected light L 2  reflected by the object to be measured  4  is reflected by the polarization optical member  18  and then, is reflected by the reflective member  20 . Furthermore, the reflected light L 2  is transmitted through the polarization optical member  18 , reflected by the beam splitter  16 , and received by the light-receiving element  24 . The light-receiving element  24  of the present embodiment is an example of a first light-receiving section according to an embodiment of the present disclosure. 
     The light source  10  has a function of emitting light used for measuring the distance to the object to be measured  4 . In the present embodiment, a semiconductor laser that emits laser having a wavelength centered at 905 nm is used as an example of the light source  10 . Specifically, the light source  10  uses a pulse laser diode that releases a high peak light output. The pulse laser diode can release a high peak light output by generating light in accordance with a pulse trigger signal (details will be described later) input from the control unit  30 . The released light output is a few tens of W to a few hundreds of W. 
     As illustrated in  FIG.  1   , as an example, the polarization optical member  18  of the present embodiment uses an omnidirectional lens that has a rotationally symmetric shape about a z axis and has a polarization direction of 360° (omnidirectional). The polarization optical member  18  of the present embodiment includes a transmitting unit  18 A and a reflecting unit  18 B. The transmitting unit  18 A is a hole formed at a center of the polarization optical member  18  along the z axis and has a function of transmitting light heading toward the reflective member  20  from the light source  10  and the reflected light reflected by the reflective member  20 . The reflecting unit  18 B has a function of reflecting the light reflected by the reflective member  20  toward the object to be measured  4  and reflecting the light reflected by the reflective member  20  toward the light-receiving element  22 . As described above, since the polarization direction of the polarization optical member  18  of the present embodiment is 360°, a horizontal scan is performed by polarizing the light reflected by the reflective member  20  at 360° in the horizontal direction. 
     The reflective member  20  has a function of varying a scan angle for scanning the object to be measured  4  with the light from the light source  10  and reflecting the light from the light source  10  for each varied scan angle. 
     In the present embodiment, a micro electro mechanical system (MEMS) mirror is used as the reflective member  20 , and a trajectory of the light reflected by the reflective member  20  forms a circular shape by rotating the reflective member  20  about the z axis passing through the mirror as a rotation axis. The scan angle is varied by rotating the reflective member  20 . The scan angle in the present embodiment corresponds to a so-called angle of view in the LiDAR system. An angle of a range in which the reflective member  20  in the distance measurement device  1  of the present embodiment varies the scan angle is 360°. Thus, a horizontal angle of view is 360°. 
     In a case where a MEMS mirror is used as the reflective member  20 , a displacement amount of an angle (rotation) of a mirror  50  is relatively small and insignificant. However, a radius of the circular shape of the trajectory of light projected in the direction of 360° can be increased by using the polarization optical member  18  comprising an enlarging optical system. Thus, the scan angle can be accurately detected by installing the light-receiving element  24 , described later, on an optical path after the polarization optical member  18 . 
     An example of a structure of the MEMS mirror that is the reflective member  20  of the present embodiment is illustrated in  FIG.  3   . The reflective member  20  of the present embodiment comprises a mirror unit  40 , a torsion bar  41 , a movable frame  42 , a torsion bar  43 , and a fixing unit  44 . 
     The mirror unit  40  includes the mirror  50  for reflecting light. The mirror  50  rotates in accordance with rotation of the mirror unit  40 . The torsion bar  41  pivotally supports the mirror unit  40  in a rotatable manner about an axis A as a rotation axis. The movable frame  42  supports the mirror unit  40  and the torsion bar  41 . The torsion bar  43  pivotally supports the movable frame  42  in a rotatable manner about an axis B as a rotation axis. The fixing unit  44  supports these structures. 
     In addition, as illustrated in  FIG.  3   , a driving unit  52  is provided on the mirror unit  40 . The driving unit  52  generates a torque for rotationally moving the mirror unit  40  about the axis A as a rotation axis. In addition, a driving unit  54  is provided on the movable frame  42 . The driving unit  54  generates a torque for rotationally moving the movable frame  42  about the axis B as a rotation axis. Actuators (not illustrated) provided in the driving unit  52  and the driving unit  54  are considered to be of an electromagnetic driving type, an electrostatic driving type, a piezoelectric driving type, and the like and are not limited to any thereof. 
     For example, in a case where the actuators are of the electromagnetic driving type, a mechanism of generating electromagnetic force for swinging the mirror unit  40  and the movable frame  42  by causing a driving current to flow in a coil wiring line formed as a pattern in each of the driving unit  52  and the driving unit  54  in a state where an outside magnetic field is formed by arranging a permanent magnet (not illustrated) on an outside is used. In addition, in a case where the actuators are of the electrostatic driving type, fine-tooth comb electrodes are formed to face each other in each of the driving unit  52  and the driving unit  54 , and an electrostatic actuator in which one of the fine-tooth comb electrodes functions as a rotor and the other functions as a stator is formed. A mechanism of swinging the mirror unit  40  and the movable frame  42  based on electrostatic attraction generated by applying a voltage between the rotor and the stator is used. In addition, in a case where the actuators are of the piezoelectric driving type, a piezoelectric thin film and an upper electrode and a lower electrode formed in a state where the piezoelectric thin film is sandwiched therebetween in a film thickness direction are comprised in each of the driving unit  52  and the driving unit  54 . A mechanism of generating a torque for swinging the mirror unit  40  and the movable frame  42  by applying a voltage between the upper electrode and the lower electrode to excite contraction and expansion based on a piezoelectric effect and generate flexural vibration of a vibration plate is used. Examples of a material of the piezoelectric thin film include aluminum nitride, barium titanate, and piezoelectric zirconate titanate (PZT). 
     In any of the types, preferably, it is desirable to apply a torque of a sinusoidal wave to the driving unit  52  and the driving unit  54  to excite resonance vibration in which the mirror unit  40  and the movable frame  42  rotationally move about the axis A and the axis B, respectively. Accordingly, swinging at a large angle can be performed with lower input power. In this case, an amplitude of the driving voltage is reflected on a rotation angle of the mirror unit  40  (mirror  50 ), and a frequency of the driving voltage is reflected on an operation frequency of the mirror unit  40  (mirror  50 ). Here, a scan along the circular trajectory can be performed by setting the same rotation frequency about the axis A and the axis B and appropriately setting operation phases of both of the mirror unit  40  and the movable frame  42 . Furthermore, a spiral scan by modulating the amplitude of the driving voltage to change the diameter of the circle can be performed. A frequency with which the reflective member  20  is driven is not limited to a resonance frequency and may be a frequency other than the resonance frequency. 
     The beam splitter  16  is positioned between the light source  10  and the polarization optical member  18  on the optical path of the light source  10  and the reflective member  20 . In addition, the beam splitter  16  is positioned on the optical path between the reflective member  20  and the light-receiving element  24 . The beam splitter  16  has a function of causing the light from the light source  10  to pass in a direction of the reflective member  20  and guiding the reflected light reflected in a direction of the light source  10  by the reflective member  20  to the light-receiving element  24 . 
     In a case where the scan angle of the reflective member  20  is a specific angle, the light-receiving element  22  receives light that is emitted from the light source  10  and reflected by the reflective member  20  and the polarization optical member  18  in this order. In other words, the light-receiving element  22  receives a part of scan light for scanning the object to be measured  4 . Intensity of the light received by the light-receiving element  22  is the same as the laser output used for distance measurement and is a few tens of W to a few hundreds of W. However, the light can be attenuated to avoid saturation of the light-receiving element  22  by comprising a neutral density (ND) filter or the like on a front surface of the light-receiving element  22 . Examples of the light-receiving element  22  include an avalanche photodiode. In addition, it is desirable to eliminate an effect of light (sunlight or the like) other than the reflected light from the object by providing a wavelength filter that transmits only the laser wavelength on the front surface of the light-receiving element  22 . 
     The light-receiving element  24  receives the reflected light after the light guided by the polarization optical member  18  is reflected by the object to be measured  4 . Examples of the light-receiving element  24  include an avalanche photodiode. In addition, it is desirable to eliminate an effect of light (sunlight or the like) other than the reflected light from the object by providing a wavelength filter that transmits only the laser wavelength on the front surface of the light-receiving element  24 . 
     The control unit  30  controls the entire distance measurement device  1  and is connected to each of the light source  10 , the reflective member  20 , the light-receiving element  22 , and the light-receiving element  24  in a wired or wireless manner in order to perform a control related to the measurement of the distance to the object to be measured  4 . In  FIG.  1   , in order to avoid complication in the drawing, connection of the control unit  30  to the light source  10 , the reflective member  20 , the light-receiving element  22 , and the light-receiving element  24  is not illustrated. 
     Specifically, the control unit  30  outputs the pulse trigger signal (hereinafter, simply referred to as a “trigger signal”) for emitting light from the light source  10  to the light source  10 . In the light source  10 , the light source  10  is set to an ON state in accordance with the trigger signal, and light is emitted from the light source  10 . As described above, the light source  10  emits light by generating light in accordance with the trigger signal input from the control unit  30 . Thus, the number of times the trigger signal is input into the light source  10  is the number of times of light generation the light source  10  emits light. 
     In addition, the control unit  30  performs a control of rotating the mirror unit  40  of the MEMS mirror that is the reflective member  20  as described above. In addition, a light-receiving signal representing that light is received is input into the control unit  30  from each of the light-receiving element  22  and the light-receiving element  24 . 
     In addition, the control unit  30  includes a fine time measurement circuit (time to digital converter), not illustrated, and derives distance data representing the distance to the object to be measured  4  by obtaining a time of flight of light from the output of the trigger signal to the input of the light-receiving signal from the light-receiving element  24  by the fine time measurement circuit. 
     The control unit  30  of the present embodiment derives the scan angle based on the light-receiving signal input from the light-receiving element  22 . Three-dimensional positional information of the object to be measured  4  can be acquired by acquiring distance measurement data as a set of the distance data and the scan angle derived based on the light-receiving signal input from the light-receiving element  24  by the control unit  30 . 
     Hereinafter, distance measurement processing of deriving the scan angle and associating the scan angle with the distance data in an operation for measuring the distance to the object to be measured  4  by the control unit  30  will be described as an action of the distance measurement device  1  of the present embodiment. 
     An example of a hardware configuration of the control unit  30  of the present embodiment is illustrated in  FIG.  4   . As illustrated in  FIG.  4   , the control unit  30  of the present embodiment comprises a central processing unit (CPU)  60 , a read only memory (ROM)  62 , a random access memory (RAM)  64 , an interface (I/F) unit  66 , and a storage unit  68 . The CPU  60 , the ROM  62 , the RAM  64 , the I/F unit  66 , and the storage unit  68  are connected through a bus  69  such as a system bus or a control bus so that various information can be exchanged between each other. 
     The ROM  62  stores various programs and the like including a distance measurement program  63  executed by the CPU  60  in advance. The RAM  64  temporarily stores various data. The CPU  60  of the present embodiment is an example of a processor according to the embodiment of the present disclosure. In addition, the distance measurement program  63  of the present embodiment is an example of a distance measurement program according to the embodiment of the present disclosure. 
     The storage unit  68  stores the distance measurement data and other various information and the like. Specific examples of the storage unit  68  include a hard disk drive (HDD) and a solid state drive (SSD). 
     The I/F unit  66  communicates various information among the light source  10 , the reflective member  20 , the light-receiving element  22 , and the light-receiving element  24  based on wireless communication or wired communication. In addition, the I/F unit  66  communicates various information with a device outside the distance measurement device  1  based on wireless communication or wired communication. As an example, in the present embodiment, the distance to the object to be measured  4  is measured in accordance with an instruction from the device outside the distance measurement device  1 , and the distance measurement data is output to the outside device or the like in accordance with an instruction from the device outside the distance measurement device  1 . Unlike the present embodiment, a form of comprising an input device and a display device such as a display in the control unit  30  may be applied, and a from of performing distance measurement in accordance with an instruction directly provided to the control unit  30  by a user and displaying a distance measurement result on the display device such as the display comprised in the control unit  30  may be applied. 
     Furthermore, a functional block diagram of an example of a functional configuration of the control unit  30  of the present embodiment is illustrated in  FIG.  5   . As illustrated in  FIG.  5   , the control unit  30  comprises an acquisition unit  70 , a derivation unit  72 , and an association unit  74 . As an example, in the control unit  30  of the present embodiment, the CPU  60  functions as the acquisition unit  70 , the derivation unit  72 , and the association unit  74  by executing the distance measurement program  63  stored in the ROM  62  by the CPU  60 . 
     The acquisition unit  70  acquires the number of times the control unit  30  outputs the trigger signal to the light source  10  between a time Ts at which the light-receiving element  22  receives light and a time Te at which the light-receiving element  22  receives light after the time Ts. As described above, in the light source  10 , light is generated in accordance with the trigger signal input from the control unit  30 . Thus, the number of times the control unit  30  outputs the trigger signal to the light source  10  is considered to be equal to the number of times of light generation of the light source  10 . In a case where the object to be measured  4  is not a transparent object such as a glass object, the light-receiving element  24  receives the reflected light after the light emitted from the light source  10  is reflected by the object to be measured  4  as described above. Thus, the number of times the light-receiving element  24  receives the reflected light is considered to be equal to the number of times of light generation of the light source  10 . Specifically, a time at which the light-receiving signal is received from the light-receiving element  22  is the time Ts. Then, a time at which the light-receiving signal is received again from the light-receiving element  22  is the time Te. In addition, the acquisition unit  70  acquires the number of times the control unit  30  outputs the trigger signal to the light source  10  from the time Ts to the time Te. The time Ts of the present embodiment is an example of a first time according to the embodiment of the present disclosure. The time Te of the present embodiment is an example of a second time according to the embodiment of the present disclosure. 
     The derivation unit  72  derives the scan angle corresponding to the reflected light received by the light-receiving element  24  based on the number of times, acquired by the acquisition unit  70 , the control unit  30  outputs the trigger signal to the light source  10 . The association unit  74  associates the scan angle derived by the derivation unit  72  with the distance measurement data corresponding to the reflected light received by the light-receiving element  24 . 
     In addition, a flowchart representing an example of the distance measurement processing is illustrated in  FIG.  6   . In the present embodiment, the distance measurement processing illustrated in  FIG.  6    is executed by executing the distance measurement program  63  stored in the ROM  62  by the CPU  60 . 
     In step S 100  in  FIG.  6   , the acquisition unit  70  determines whether or not the light-receiving signal is input from the light-receiving element  22 . The determination in step S 100  results in a negative determination until the light-receiving signal is input from the light-receiving element  22 . On the other hand, in a case where the light-receiving signal is input from the light-receiving element  22 , the determination in step S 100  results in a positive determination, and a transition is made to step S 102 . 
     In step S 102 , the acquisition unit  70  associates 0° as the scan angle with the distance data derived based on a time from the output of the trigger signal to the input of the light-receiving signal from the light-receiving element  24 . In addition, the acquisition unit  70  records a time representing a distance measurement time point as the time Ts. 
     In the distance measurement device  1  of the present embodiment, a scan operation is repeated by varying the scan angle with a constant scan angle interval. Thus, in the horizontal scan, the light-receiving element  24  receives light reflected by the reflective member  20  and the polarization optical member  18  in this order at the time Ts and then, receives light reflected by the reflective member  20  and the polarization optical member  18  in this order again. 
     Therefore, the acquisition unit  70  determines whether or not the light-receiving signal is input from the light-receiving element  22  in step S 104 . The determination in step S 104  results in a negative determination until the light-receiving signal is input from the light-receiving element  22 . On the other hand, in a case where the light-receiving signal is input from the light-receiving element  22 , the determination in step  5104  results in a positive determination, and a transition is made to step S 106 . 
     In step S 106 , the acquisition unit  70  associates 360° that is the maximum angle in performance terms of the distance measurement device  1  as the scan angle with the distance data derived based on the time from the output of the trigger signal to the input of the light-receiving signal from the light-receiving element  24 . In addition, the acquisition unit  70  records the time representing the distance measurement time point as the time Te. 
     In next step S 108 , the acquisition unit  70  acquires the number of times the control unit  30  outputs the trigger signal to the light source  10 , that is, the number of scans, from the time Ts to the time Te. A method of acquiring the number of scans by the acquisition unit  70  is not particularly limited. For example, the acquisition unit  70  may acquire the number of scans using a counter, not illustrated, by setting a count at the time Ts to 1 and incrementing the count each time the trigger signal is output to the light source  10  until counting at the time Te. The number of scans acquired by the acquisition unit  70  is output to the derivation unit  72 . 
     In next step S 110 , the derivation unit  72  derives the scan angle and derives the scan angle for each scan. As an example, the derivation unit  72  of the present embodiment derives the scan angle of each scan based on a value obtained by dividing the range (in the present embodiment, 360°) of the scan angle by a number (n−1) obtained by subtracting 1 from a number of scans n acquired in step S 108 . The scan angle of each scan derived by the derivation unit  72  is output to the association unit  74 . 
     As a specific example, a case where the number of scans acquired in step S 108  is five and n=5 is established will be described. A value (n−1) obtained by subtracting 1 from the number of scans n is 5−1=4. In a case where 360° that is the range of the scan angle is divided by 4, 90°l (360÷4=90°) is obtained as the scan angle interval. Accordingly, it is perceived that the scan is performed every 90° in the distance measurement device  1 . From this point, the derivation unit  72  derives the scan angles in five scans as 0°, 90°, 180°, 270°, and 360° (0°). 
     In next step S 112 , the association unit  74  obtains the distance measurement data by associating the scan angle of each scan derived in step S 110  with the distance data in each scan. The distance measurement data obtained by doing so may be output to the storage unit  68  and stored in the storage unit  68  or may be output to the device or the like outside the control unit  30 . An output destination of the obtained distance measurement data is not particularly limited and, for example, may be predetermined in accordance with specifications of the distance measurement device  1  or may be settable by the user. In a case where the processing in step S 112  ends, the distance measurement processing illustrated in  FIG.  6    ends. 
     By doing so as illustrated in  FIG.  6   , the horizontal scan with respect to the object to be measured  4  at a specific position in the vertical direction ends. 
     The distance measurement device  1  of the present embodiment is not limited to the form illustrated in  FIG.  1    and, for example, may be configured as in Modification Example 1 and Modification Example 2 below. 
     (Modification Example 1) 
     In the distance measurement device  1  illustrated in  FIG.  1   , while a form in which only the horizontal scan of the object to be measured  4  is performed is applied, a form of further performing a vertical scan and having a vertical angle of view may also be applied. 
     For example, a scan in the z axis direction, that is, the vertical scan, can be performed by adjusting the rotation angle of the mirror  50  as an angle of a reflecting surface of the reflective member  20  to perform a control for a state where the trajectory of the light reflected by the reflective member  20  forms a spiral shape. For example, in a case where the scan angle in the horizontal scan reaches 360° from 0°, an inclination of the mirror  50  of the reflective member  20  is adjusted so that a scanning position in the vertical direction is changed, and the horizontal scan is performed again at the position in the vertical direction after change. 
     An example of a configuration of the distance measurement device  1  of the present modification example is illustrated in  FIG.  7   . As illustrated in  FIG.  7   , the distance measurement device  1  of the present modification example comprises light-receiving elements  22   1 ,  22   2 , and  22   3  as a plurality (in  FIG.  7   , three) of the light-receiving elements  22  in a direction (in  FIG.  7   , the z axis direction) intersecting with a direction in which the polarization optical member  18  guides light to the object to be measured  4 . 
     In the distance measurement processing in the distance measurement device  1  of the present embodiment, the vertical scan can be performed by repeating the distance measurement processing illustrated in  FIG.  6    for each vertical scan position. In the reflective member  20  that is a MEMS mirror, the displacement amount of the angle (rotation) of the mirror  50  is relatively small and insignificant. However, since the scan angle in the vertical direction can be enlarged by using the polarization optical member  18  comprising an enlarging optical system, detection accuracy of the scan angle can be improved. 
     (Modification Example 2) 
     While the distance measurement device  1  illustrated in  FIG.  1    is in the form of a coaxial optical system in which a light projection optical system and a light-receiving optical system have a common optical path, a form of a non-coaxial optical system in which an optical path of the light projection optical system is separated from an optical path of the light-receiving optical system may also be applied. 
     An example of a configuration of the distance measurement device  1  of the present modification example is illustrated in  FIG.  8   . As illustrated in  FIG.  8   , the distance measurement device  1  of the present modification example further comprises a polarization optical component  32  and an optical component  34  at a position on an opposite side of the reflective member  20  from a side on which the polarization optical member  18  is provided. The polarization optical component  32  of the present embodiment is an example of an optical component according to the embodiment of the present disclosure. 
     The polarization optical component  32  has a function of guiding the reflected light L 2  reflected by the object to be measured  4  in a direction different from the direction of the light source  10 . Thus, in the distance measurement device  1  of the present embodiment, a position at which the light-receiving element  24  is provided is different from in a case of the distance measurement device  1  of the coaxial optical system illustrated in  FIG.  1   . In addition, as illustrated in  FIG.  8   , the beam splitter  16  is not comprised in the distance measurement device  1  of the non-coaxial optical system. 
     As illustrated in  FIG.  8   , in the present embodiment, the reflected light L 2  is reflected by the polarization optical component  32  and is incident on the light-receiving element  24  after an incidence angle is adjusted by transmitting the reflected light L 2  through the optical component  34 . 
     Intensity of the reflected light L 2  is relatively weak. Thus, examples of a method for securing the intensity include enlarging an aperture of the light-receiving optical system. However, in order to use this method in the coaxial optical system, it is necessary to increase a size of the mirror  50  of the reflective member  20  that polarizes a light projection direction, thereby imposing limitations. On the other hand, by using the non-coaxial optical system in the distance measurement device  1 , the aperture of the light-receiving optical system can be enlarged without increasing the size of the mirror  50  of the reflective member  20 . Thus, in the distance measurement device  1  of the non-coaxial optical system, restrictions in the above method for securing the intensity of the reflected light L 2  are alleviated. 
     As described so far, the distance measurement device  1  of each embodiment comprises the light source  10 , the polarization optical member  18 , the reflective member  20 , the light-receiving element  22 , and the light-receiving element  24 . The reflective member  20  varies the scan angle for scanning the object to be measured  4  with the light from the light source  10  and reflects the light from the light source  10  for each varied scan angle. The polarization optical member  18  guides the light reflected by the reflective member  20  to the object to be measured  4 . The light-receiving element  24  receives the reflected light after the light guided by the polarization optical member  18  is reflected by the object to be measured  4 . In a case where the scan angle of the reflective member  20  is the specific angle, the light-receiving element  22  receives the light reflected by the reflective member  20  and the polarization optical member  18  in this order. 
     According to the above configuration, the scan angle can be derived based on a light-receiving result of the reflected light by the light-receiving element  22  that receives the light reflected by the reflective member  20  and the polarization optical member  18  in this order. Thus, according to the distance measurement device  1 , the detection accuracy of the scan angle for scanning the object to be measured  4  with light from the light source  10  can be improved. 
     Particularly, in a case where the reflective member  20  is a MEMS mirror, there is a tendency that size reduction and weight reduction are achieved and power consumption can be suppressed, compared to a case of mechanically rotating a prism or a mirror. Thus, such a case is preferable. However, there is a tendency that behavior of the MEMS mirror is susceptible to effects of an environmental change, an individual difference, a temporal change, and the like. On the other hand, the behavior of the MEMS mirror is almost constant within a time required for one horizontal scan or vertical scan. In addition, since the scan angle is not derived based on the behavior of the reflective member  20  for each scan performed a plurality of times in one horizontal scan or vertical scan, the detection accuracy of the scan angle can be further improved. 
     In the present embodiment, while the wavelength of the light source  10  is 905 nm, other wavelengths such as 1500 nm providing high safety for eyes may also be used. 
     In addition, in the present embodiment, while a form in which the reflective member  20  is a MEMS mirror (refer to  FIG.  3   ) has been described, the present invention is not limited to a MEMS mirror and may also be applied to a form of mechanically rotating a prism or a mirror. 
     In addition, in the present embodiment, while a form in which the control unit  30  controls the light source  10 , the reflective member  20 , the light-receiving element  22 , and the light-receiving element  24  and performs the distance measurement processing (refer to  FIG.  6   ) has been described, a control unit that controls the reflective member  20  and the like and a control unit that performs the distance measurement processing may be separately configured. 
     In addition, in the embodiment, for example, the following various processors can be used as a hardware structure of a processing unit that executes various processing of the acquisition unit  70 , the derivation unit  72 , and the association unit  74 . The various processors include, in addition to a CPU that is a general-purpose processor functioning as various processing units by executing software (program) as described above, a programmable logic device (PLD) that is a processor such as a field programmable gate array (FPGA) having a circuit configuration changeable after manufacturing, a dedicated electric circuit that is a processor such as an application specific integrated circuit (ASIC) having a circuit configuration dedicatedly designed to execute specific processing, and the like. 
     One processing unit may be composed of one of the various processors or may be composed of a combination of two or more processors of the same type or different types (for example, a combination of a plurality of FPGAs or a combination of a CPU and an FPGA). In addition, a plurality of processing units may be composed of one processor. 
     As an example in which the plurality of processing units are composed of one processor, first, as represented by a computer such as a client and a server, a form in which one processor is composed of a combination of one or more CPUs and software and the processor functions as the plurality of processing units is possible. Second, as represented by a system on chip (SoC) and the like, a form of using a processor that implements functions of the entire system including the plurality of processing units in one integrated circuit (IC) chip is possible. Accordingly, the various processing units are configured using one or more of the various processors as a hardware structure. 
     Furthermore, more specifically, an electric circuit (circuitry) in which circuit elements such as semiconductor elements are combined can be used as the hardware structure of the various processors. 
     In addition, in the embodiment, while an aspect in which the distance measurement program  63  is stored (installed) in advance in the ROM  62  has been described, the present invention is not limited thereto. The distance measurement program  63  may be provided in the form of a recording on a recording medium such as a compact disc read only memory (CD-ROM), a digital versatile disc read only memory (DVD-ROM), and a universal serial bus (USB) memory. In addition, the distance measurement program  63  may be in the form of a download from an outside device through a network. 
     The disclosure of Japanese Patent Application No. 2020-125848 filed on Jul. 22, 2020 is incorporated in the present specification by reference in its entirety. 
     All documents, patent applications, and technical standards disclosed in the present specification are incorporated in the present specification by reference to the same extent as in a case where each of the documents, patent applications, technical standards are specifically and individually indicated to be incorporated by reference. 
     EXPLANATION OF REFERENCES 
       1 : distance measurement device
           4 : object to be measured     10 : light source     12 ,  14 : collimator lens     16 : beam splitter     18 : polarization optical member     20 : reflective member     22 ,  22   1 ,  22   2 ,  22   3 ,  24 : light-receiving element     30 : control unit     32 : polarization optical component     34 : optical component     40 : mirror unit     41 ,  43 : torsion bar     42 : movable frame     44 : fixing unit     50 : mirror     52 ,  54 : driving unit     60 : CPU     62 : ROM     63 : distance measurement program     64 : RAM  66 : I/F unit     68 : storage unit     69 : bus     70 : acquisition unit     72 : derivation unit     74 : association unit   A, B, x, y, z: axis   L 1 , L 1   1 , L 1   2 : light   L 2 : reflected light