Patent Publication Number: US-11656340-B2

Title: LIDAR device

Description:
TECHNICAL FIELD 
     The present disclosure relates to a LIDAR device to calculate a distance to an object. 
     BACKGROUND ART 
     A LIDAR, which stands for Light Detection and Ranging, is a remote sensing method to measure distances to object in a scanning area. Such a LIDAR system has been well used in a variety of fields including in the field of autonomous driving area. LIDAR system typically uses a light emitter, a light receiver, and a mirror to reflect the emitted light toward the scanning area. The scanning mirror is usually used by rotating the mirror in one direction to scatter laser beams around a surrounding area. 
     SUMMARY 
     One aspect of the present disclosure is a LIDAR device for measuring a distance to an object in a scanning zone. The device includes a light source, a mirror, a rotatable shaft, and a motor. The light source is configured to emit a light beam having a predetermined light beam width for scanning the scanning zone. The mirror has a reflective surface and a back surface opposite to the reflective surface. The mirror is configured to reflect, with the reflective surface, the light beam emitted from the light source toward the scanning zone. The rotatable shaft has a center axis parallel to the reflective surface of the mirror. The shaft is connected to the back surface of the mirror. 
     The motor is configured to rotate the shaft to cause the mirror to swing between a first position corresponding to one end of the predetermined scanning zone and a second position corresponding to another end of the predetermined scanning zone, an angle of incidence of the light beam to the reflective surface being greater when the mirror is at the first position than when the mirror is at the second position. 
     When a light beam center is defined as a center of the light beam width of the light beam from the light source to the reflective surface, then a motion area is defined as one side of the light beam center that includes the center axis of the shaft. 
     The light source and the mirror are arranged to have a positional relationship such that, when viewed in a direction along the center axis of the shaft, a mirror center, which is defined as a point on the reflective surface closest to the center axis of the shaft, is aligned with the light beam center when the mirror is at the first position, and the mirror center shifts within the motion area when the mirror is swinging between the first position, non-inclusive, and the second position, inclusive. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above and other objectives, features and advantages of the present disclosure will become more apparent from the following detailed description made with reference to the accompanying drawings. 
         FIG.  1    is a schematic view of a LIDAR device according to a first embodiment. 
         FIG.  2    is a top view showing a positional relationship between a emitting module and a scanning mirror. 
         FIG.  3    is a side view of the LIDAR device. 
         FIG.  4    is a diagram showing the reflection of the laser beam by the scanning mirror at −60 deg. in (a), 0 deg. in (b), and +60 deg. in (c). 
         FIG.  5    is a block diagram of the LIDAR device. 
         FIG.  6    is a timing chart of detection signals, emission control signals, and return signals according to the first embodiment. 
         FIG.  7    is a flowchart performed by the LIDAR device according to the first embodiment. 
         FIG.  8    is a schematic view of a comparative example showing a positional relationship between the light emitting module and the scanning mirror. 
         FIG.  9    is a timing chart of a comparative example of detection signals, emission control signals, and return signals. 
         FIG.  10    is a timing chart of detection signals, emission control signals, and return signals according to a second embodiment. 
         FIG.  11    is a flowchart performed by the LIDAR device according to the second embodiment. 
         FIG.  12    is a timing chart of detection signals, emission control signals, and return signals according to a third embodiment. 
         FIG.  13    is a flowchart performed by the LIDAR device according to the third embodiment. 
         FIG.  14    is a timing chart of detection signals, emission control signals, and return signals according to a fourth embodiment. 
         FIG.  15    is a flowchart performed by the LIDAR device according to the fourth embodiment. 
         FIG.  16    is a block diagram of the LIDAR device according to a fifth embodiment. 
         FIG.  17    is a timing chart of detection signals, emission control signals, and return signals according to the fifth embodiment. 
         FIG.  18    is a flowchart performed by the LIDAR device according to the fifth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a plurality of embodiments of the present disclosure will be described with reference to the drawings. In the following embodiments, the same or equivalent parts are denoted by the same reference numerals as each other, and explanations will be provided to the same reference numerals for simplifying descriptions. Furthermore, in the following embodiments, a laser imaging detection and ranging (LIDAR) device is mounted in a vehicle such as an automotive, but the LIDAR device  1  may be mounted in any kind of vehicles such as motorbikes, airplanes, ships, drones, or the like. 
     First Embodiment 
       FIGS.  1  to  3    show a schematic view of the LIDAR device  10  according to the first embodiment. The LIDAR device  10  is configured to calculate a distance to an object X in a scanning zone SZ using time-of-flight (ToF) techniques. The LIDAR device  10  basically includes a light emitting module  12 , a light receiving module  14 , a scanner module  18  (see  FIG.  3   ), and a motor controller  50 . Calculation of a distance to an object X is performed by a controller  21  that is integrally disposed in the light receiving module  14 , as will described later. It should be noted that the scanner module  18  is not illustrated in  FIG.  1    for explanatory purposes. 
     The LIDAR device  10  is formed as a single component housed in a box-like case  10   a  as illustrated in  FIG.  1   . As shown in  FIG.  3   , in this embodiment, the light emitting module  12  and the light receiving module  14  are arranged in a direction along a vertical direction (an up-down direction) of a vehicle to which the LIDAR device  10  is mounted. In other words, the light emitting module  12  and the light receiving module  14  are arranged in a direction along rotatable shaft  24  of scanner module  18 . More specifically, the light emitting module  12  is disposed above the light receiving module  14 . However, the arrangement of the light emitting module  12  and the light receiving module  14  is not necessarily limited to this example. For example, the light emitting module  12  and the light receiving module  14  may be arranged in a direction along a horizontal or a slanted direction of the vehicle. 
     The light emitting module  12 , or a light source, is configured to emit a laser light toward a scanning mirror  16  of the scanner module  18 . As shown in  FIG.  2   , the light emitting module  12  includes two pairs of light emitters  20   a ,  20   b  and transmitter lenses  22   a ,  22   b . Each of the light emitters  20   a ,  20   b  is, for example, a semiconductor laser diode configured to emit a pulsed laser. Each of the light emitters  20   a ,  20   b  is electrically connected to the controller  21  and configured to emit a laser light when the light emitting module  12  receives an emission control signal from the controller  21 . Therefore, the emission timing of the light emitters  20   a ,  20   b  are controllable through the emission control signals output from the controller  21 . 
     The light emitting module  12  is further configured to output an actual emission timing to the controller  21 . The actual emission timing is a timing at which the light emitting module  12  actually emits a light beam. As will described below, the actual emission timing is used for compensating an error generated when calculating a distance by the controller  21 . 
     Each of the transmitter lens  22   a ,  22   b  is a lens configured to focus the pulsed laser emitted from the light emitter to form a vertical line (or a vertical band) extending in a direction along the vertical direction of the vehicle (see  FIG.  1   ). That is, the LIDAR device  10  adopts the  1 D line-scanning method which performs a horizontal scanning with a vertical-line laser beam. 
     In this embodiment, the two pairs of the light emitters  20   a ,  20   b  and the transmitter lenses  22   a ,  22   b  are arranged in the horizontal direction. Hereinafter, a laser beam emitted from one (the left pair  20   a ,  22   a  in  FIG.  2   ) of the two pairs of the light emitters and the transmitter lenses is referred to as a first laser beam, and a laser beam emitted from the other one (the right pair  20   b ,  22   b  in  FIG.  2   ) of the two pairs of the light emitters and the transmitter lenses is referred to as a second laser beam. Then, the first and second laser beams emitted from the two light emitters  20   a ,  20   b  and focused through the transmitter lenses  22   a ,  22   b  are collectively referred to as a light beam band LB. 
     As shown in  FIG.  2   , the light beam band LB of the two light emitters  20   a ,  20   b  has a light beam width W which is a width in a direction perpendicular to a light travelling direction when viewed from above. Then, a light beam center BC is defined as a center of the light beam width W of the light beam band LB. More specifically, the light beam center BC is a center line extending along center points of the light beam width W in a direction perpendicular to the light travelling direction when viewed above. 
     The scanner module  18  includes the scanning mirror  16 , a rotatable shaft  24 , a driving motor  26 , and an angle sensor  28 . The rotatable shaft  24  is a shaft configured to rotate about a center axis CX. In this embodiment, the center axis CX extends along a direction in parallel with the vertical direction of the vehicle. The rotatable shaft  24  has a columnar shape with a specified diameter. The side surface  24   a  of the rotatable shaft  24  is connected to the mirror  16 . 
     The scanning mirror  16  is a mirror configured to reflect the laser beam toward the scanning zone SZ directly or indirectly through another one or more mirror. Furthermore, the scanning mirror  16  in this embodiment is configured to reflect the return beam reflected by an object X toward the light receiving module  14 . That is, the scanning mirror  16  serve as both a transmitter mirror and a receiver mirror. 
     The scanning mirror  16  is a plate-like member in this embodiment and includes a reflective surface  16   a  and a back surface  16   b  that is opposite to the reflective surface  16   a . The back surface  16   b  of the scanning mirror  16  is connected to the side surface of the rotatable shaft  24 . Thus, as shown in  FIG.  2   , the reflective surface  16   a  is away from the center axis CX of the rotatable shaft  24  with a predetermined distance (i.e., the radius of the rotatable shaft  24 ), and as a result, the scanning mirror  16  is rotated around, not about, the center axis CX (i.e., swinging). 
     In this embodiment, the reflective surface  16   a  has a rectangular shape when viewed from the front (see  FIG.  5   ). The reflective surface  16   a  is elongated in a direction along the vertical direction. Referring to  FIG.  5   , two edge portions  30  of the reflective surface  16   a  are defined as portions that include elongated sides (i.e., the lengths of the reflective surface  16   a ) of the reflective surface  16   a  extending in the direction along the vertical direction. Also, a center portion  32  of the reflective surface  16   a  is defined as a portion that is an center area of the reflective surface  16   a  between the two edge portions  30 . Then, a mirror center MC is defined as a point on the reflective surface  16   a , when viewed in a direction along the center axis CX (i.e., the vertical direction), that is closest to the center axis CX of the rotatable shaft  24 , as shown in  FIG.  2   . In this embodiment, the mirror center MC is a center point between the two edge portions  30  when viewed in the direction along the center axis CX. In other words, the mirror center MC is a center point of the width of the reflective surface  16   a.    
     The driving motor  26  is an electric motor configured to rotate the rotatable shaft  24  about the center axis CX. The driving motor  26  is electrically connected to the motor controller  50  and the operation of the driving motor  26  is controlled by motor driving signals output from the motor controller  50 . The motor controller  50  is, for example, an electronic control unit (ECU) that includes at least one processor and one memory. The memory includes random access memory, read only memory, flash memory, or a combination of these. The memory has stored thereon instructions which, when executed by the processor, cause the processor to control the driving motor  26 . 
     The motor controller  50  is configured to control the driving motor  26  to operate alternately in opposite directions. As a result, the rotatable shaft  24  is rotated by the motor back and forth so that the scanning mirror  16  swings between a first position and a second position in a predetermined scanning angle range. That is, the scanning mirror  16  periodically swings between the first position and the second position. 
     As shown in  FIG.  2   , the first position of the scanning mirror  16  is a position corresponding to one end A of the scanning zone SZ and the second position of the scanning mirror  16  is the other end B of the scanning zone SZ. As shown in  FIG.  4   , an angle of incidence of the light beam to the reflective surface  16   a  is greater when the mirror is at the first position than when the mirror  16  is at the second position. In this embodiment, the scanning angle range is set to 120 degrees (i.e., −60°≤scanning angle θ≤+60°, and the scanning angle θ is −60° when the scanning mirror  16  is at the first position and the scanning angle θ is +60° when the scanning mirror  16  is at the second position (see  FIG.  4   ). 
     Referring to  FIG.  2   , a motion area is defined as one side of the light beam center BC that includes the center axis CX of the shaft  24  when viewed in a direction along the center axis CX (the hatched area in  FIG.  2   ). Then, the light emitting module  12  and the scanning mirror  16  are arranged to have a positional relationship such that, when viewed in a direction along the center axis CX of the rotatable shaft  24 , the mirror center MC is aligned with the light beam center BC when the mirror is at the first position (see  FIG.  2    and (a) of  FIG.  4   ). On the other hand, the mirror center MC shifts within the motion area when the mirror  16  is swinging between the first position, non-inclusive, and the second position, inclusive (see (b) and (c) in  FIG.  4   ). In other words, the mirror center MC of the scanning mirror  16  moves within the motion area except when the scanning mirror  16  reaches the first position. Thus, the mirror center MC and the light beam center BC are offset from each other except when the mirror  16  is at the first position. 
     As shown in (a) of  FIG.  4   , when the scanning mirror  16  is at the first position, the first laser beam emitted from the left light emitter is reflected at the left edge portion  30  of the reflective surface  16   a  and the second laser beam emitted from the right light emitters is reflected at the right edge portion  30  of the reflective surface  16   a . On the other hand, as shown in (c) of  FIG.  4   , when the scanning mirror  16  is at the second position, the first and second laser beams emitted from both the light emitters are reflected at the center portion  32  of the reflective surface  16   a.    
     The scanner module  18  further includes the angle sensor  28  that detects rotation angles of the scanning mirror  16 . The angle sensor  28  may be an optical sensor, a mechanical sensor, an ultrasonic sensor, or the like. The angle sensor  28  is configured to detect a rotation angle at a plurality of predetermined angle intervals during each rotation cycle of the scanning mirror  16  between the first position and the second position. In this embodiment, the angle sensor  28  is configured to detect each 0.1 degree of the rotation angle of the mirror  16  (i.e., the maximum angle resolution is 0.1°). However, the resolution of the angle sensor  28  is not necessarily limited to 0.1 degree, and may be 0.05 or 0.2 degree, for example. 
     The angle sensor  28  is connected to the controller  21  and is configured to output a detection signal indicative of the rotation angle of the mirror at the angle intervals (i.e., at 0.1 degree intervals). Inventors of the present disclosure have found that since the scanning mirror  16  is moved to swing between the first position and the second position, an acceleration is applied to the scanning mirror  16  during swinging. Therefore, the rotational speed of the scanning mirror  16  varies (does not maintain a constant value) during one rotation cycle of the scanning mirror  16 . As a result, the rotation angle of the scanning mirror  16  is not counted by the angle sensor  28  at same time intervals, as shown in  FIG.  6   . That is, the angle sensor  28  outputs a detection signal at the same angle intervals, but different time intervals, to the controller  21 . 
     The light receiving module  14  includes a light receiver  34  and the controller  21 . The light receiver  34  includes an integrated circuit  36  having a plurality of light sensitive devices, and the controller  21  is provided within the integrated circuit  36  of the light receiver  34 . In other words, the light receiver  34  and the controller  21  are integrally formed as a single module in this embodiment. In this embodiment, the plurality of light sensitive devices of the light receiver  34  are single photon avalanche diodes (SPADs)  38  which are formed as a 2-D SPAD array  34   a  by arranging the plurality of SPADs  38  in both columns and rows. Since the SPAD array  34   a  constitutes a digital circuit and therefore has a high angular resolution as compared with other light sensitive devices forming an analog type circuit. Hence, the light receiver  34  can detect a return beam at small rotation angle intervals such as 0.1 degree intervals. The light receiver  34  (the SPAD array  34   a ) outputs a return signal, which is a digital signal, in accordance with a return beam reflected by an object X to the controller  21  upon receiving the return beam. The light receiver  34  also includes a decoder  40  that is configured to enable diodes  38  in a column to receive a return beam. 
     The controller  21  in this embodiment is configured to control emission of laser beams by controlling the light emitting module  12 . The controller  21  is further configured to calculate a distance to an object X based on the difference between the light emission timing at which the light emitting module  12  emitted a laser beam and the light receipt timing at which the light receiving module  14  received a return beam, as will described later. Since the controller  21  is implemented on the integrated circuit  36 , which is a digital circuit, together with the light receiver  34  (the SPAD array  34   a ), the controller  21  is capable of performing the above-mentioned functions without a programmable processor. 
       FIG.  5    shows functional blocks of the controller  21 . Although  FIG.  5    shows the controller  21  having these functions, one or some of functions may be executed by one or more physically separated circuit. The controller  21  includes, as functional blocks, an emission control section  39  and a calculating section  40 . 
     The emission control section  39  is configured to control the light emitting module  12  by outputting an emission control signal to the light emitting module  12 . In this embodiment, the emission control section  39  is configured to output an emission control signal upon receiving a detection signal from the angle sensor  28  (see  FIG.  6   ). Thus, the light emitting module  12  emits a laser beam each time the angle sensor  28  detects a rotation angle of the scanning mirror  16 . In other words, the light emitting module  12  emits a laser beam at the same intervals as the angle intervals (i.e., 0.1 degree intervals). Furthermore, the emission control section  39  is configured to output a signal output timing to the calculating section  40 . The signal output timing is a timing at which the emission control section  39  output the control signal to the light emitting module  12 . 
     The calculating section  40  is configured to calculate a distance to an object X using the return signal from the light receiving module  14  and the signal output timing from the emission control section  39 . More specifically, the calculating section  40  calculates a distance to an object X from the time difference between the signal output timing and the light receipt timing (i.e., the return signal) using the time-of-flight principle. Furthermore, the calculating section  40  is configured to receive the above-described actual emission timing from the light emitting module  12  (see  FIG.  5   ). Then, the calculating section  40  is configured to correct the calculated distance using the actual emission timing. That is, there is a time lag after the controller  21  outputs the signal output timing until the light emitting module  12  actually emits a laser beam. Therefore, the controller  21  corrects the distance calculated from the signal output timing using the actual emission timing. 
       FIG.  7    shows a flowchart executed by the LIDAR device  10  to calculate a distance to an object X. When the angle sensor  28  detects a rotation angle of the scanning mirror  16  at step S 10 , the angle sensor  28  outputs a detection signal indicative of the detected rotation angle to the controller  21  (the emission control section  39 ) at step S 20 . As described above, the angle sensor  28  detects a rotation angle at the predetermined rotation angle intervals (i.e., 0.1 degree intervals) although the rotational speed of the scanning mirror  16  varies between the first position and the second position. When the controller  21  receives the detection signal from the angle sensor  28 , the controller  21  outputs an emission control signal to the light emitting module  12  at step S 30 . 
     When the light emitting module  12  receives the emission control signal, the light emitting module  12  emits a laser beam toward the scanning mirror  16  at step S 40 . The light emitting module  12  further outputs the actual emission timing to the controller  21  (the calculating section  40 ) at step S 50  as a timing at which the light emitting module  12  actually emits the laser timing. 
     The emitted laser beam is reflected at the reflective surface  16   a  of the scanning mirror  16  and travels to the scanning zone SZ. Then, if the laser beam is reflected by an object X, the return signal comes back to the LIDAR device  10  and is reflected again by the reflective surface  16   a  of the scanning mirror  16  toward the light receiving module  14 . When the return beam reaches the light receiving module  14 , the light receiving module  14  (the SPAD array) detects the return beam at step S 60 , and then the light receiving module  14  outputs a return signal to the controller  21  (the calculating section  40 ) in response to receiving the return beam at step S 70 . 
     The controller  21  (the calculating section  40 ) calculates a distance to the object X using the signal output timing and the return signal at step S 80 . Then, the controller  21  (the calculating section  40 ) corrects the calculated distance using the actual emission timing at step S 90 . 
     As described above, the LIDAR device  10  according to the first embodiment includes the light emitting module  12  and the scanning mirror  16  that are arranged to have a positional relationship such that, when viewed in a direction along the center axis CX of the rotatable shaft  24 , the mirror center MC is aligned with the light beam center BC when the mirror  16  is at the first position. The first position is defined as a position corresponding to the one end A of the scanning zone SZ, and the angle of incidence of the light beam to the reflective surface  16   a  has a maximum value in the scanning angle range when the scanning mirror  16  is at the first position. On the other hand, the mirror center MC shifts within the motion area when the mirror  16  is swinging between the first position, non-inclusive, and the second position, inclusive. The second position is defined as a position corresponding to the other end B of the scanning zone SZ, and the angle of incidence of the light beam to the reflective surface  16   a  has a minimum value in the scanning angle range when the scanning mirror  16  is at the second position. 
     Thus, since the light beam center BC is aligned with the mirror center MC when the scanning mirror  16  is at the first position, the first light beam and the second light beam are reflected at the edge portions  30  of the reflective surface  16   a  between which the mirror center MC is located. More specifically, as shown in (a) of  FIG.  4   , the first light beam and the second light beam are reflected at the left edge portion and the right edge portion of the reflective surface  16   a , respectively, when the scanning mirror  16  is at the first position. Therefore, the width of the reflective surface  16   a  (the scanning mirror  16 ) can be minimized as long as the reflective surface  16   a  can receive the first and second light beams at the both edge portions  30  when the mirror  16  is at the first position. As a result, the size of the LIDAR device  10  can be reduced because of the mirror  16  with a minimum width. 
     On the contrary, if the light emitting module  12  and the scanning mirror  16  are arranged so that the mirror center MC is aligned with the center axis CX as illustrated in  FIG.  8   , at least the left side of the reflective surface  16   a  needs to be extended so as to catch the first laser beam when the scanning mirror  16  is at the first position. As a result, the size of the LIDAR device  10  would be increased because of the mirror  16  with an extended width. 
     In this embodiment, the angle sensor  28  is disposed to detect rotation angles of the scanning mirror  16  and outputs a detection signal at a plurality of predetermined angle intervals (0.1 degree intervals in this embodiment) during each rotation cycle between the first position and the second position of the scanning mirror  16 . Then, the controller  21  outputs a control signal to the light emitting module  12  upon receiving the detection signal from the angle sensor  28 , as shown in  FIG.  6   . As a result, the light emitting module  12  can emit a laser beam at the same intervals as the plurality of predetermined angle intervals. 
     Here,  FIG.  9    shows a comparative example where the light emitting module  12  is controlled to emit a laser beam at a plurality of predetermined “time” intervals (for example, each 27.8 microseconds). Since the scanning mirror  16  swings between the first and second positions, the rotational speed irregularly varies due to an acceleration applied to the mirror  16 . Thus, although the light emitting module  12  can emit a laser beam at a predetermined “time” intervals, the light emitting module  12  cannot emit a laser beam at a plurality of predetermined “rotation angle” intervals between the first and second positions. As a result, the amount of laser beams would vary for each region of the scanning zone SZ corresponding to each of the rotational angle intervals. 
     On the contrary, since the light emitting module  12  is controlled to emit laser beams based on rotation angles, not time intervals, of the scanning mirror  16  detected by the angle sensor  28  according to this embodiment, the LIDAR device  10  can emit a laser beam evenly for each rotational angle interval. Thus, the LIDAR device  10  can scan the scanning zone SZ equally. 
     The light receiving module  14  includes the integrated circuit  36  on which the controller  21  is implemented. That is, the controller  21  is integrally formed with the light receiver  34  (the SPAD array  34   a ) in this embodiment, and the distance between the controller  21  and the light receiver  34  can be reduced as compared with a situation where the controller  21  is physically separated away from the light receiver  34 . As a result, a required time for transmitting the return signal to the controller  21  from the light receiver  34  can be reduced, and therefore accuracy of the calculated distance can be increased. 
     The light receiving module  14  includes the plurality of SPADs  38  as light sensitive devices. The SPADs  38  have a sensitivity to receive a return beam with high resolution time intervals. Therefore, the light receiving module  14  can detect a return beam even at small rotation angle intervals (i.e., 0.1 degree in this embodiment), and thus the LIDAR device  10  can finely scan the scanning zone SZ. Furthermore, the SPAD array forms a digital circuit together with the controller  21 . Thus, the distance to an object X can be calculated without a processor, which results in reduction of a manufacturing cost of the LIDAR device  10 . 
     In this embodiment, the light emitting module  12  is configured to output the actual emission timing at which the light emitting module  12  actually emits the light beam. Then, the controller  21  corrects the calculated distance using the actual emission timing. Thus, although the distance is calculated using the signal output timing at which the controller  21  output the control signal to the light emitting module  12 , and therefore the calculated distance inevitably includes an error generated from the time lag between the signal output timing and the actual emission timing, the error can be corrected, or compensated, using the actual emission timing. As a result, the LIDAR device  10  can obtain a distance to an object X with high accuracy. 
     Second Embodiment 
     Next, the second embodiment of the present disclosure is described with reference to  FIGS.  10  to  11   . In the following description, only different portions from the first embodiment will be described. 
     In the first embodiment, the controller  21  is configured to output an emission control signal upon receiving a detection signal from the angle sensor  28 . In the second embodiment, the controller  21  is configured to, upon receiving the detection signal from the angle sensor  28 , output a plurality of control signals to the light emitting module  12  prior to receiving a subsequent one of the detection signal. 
     More specifically, as shown in  FIG.  10   , the controller  21  outputs a predetermined number of control signals within each rotational angle interval (i.e., between when the controller  21  receives a detection signal and when the controller  21  receives a subsequent detection signal). In this embodiment, the number of the control signals output within each rotation angle interval is 10. Furthermore, the controller  21  continuously outputs a control signal at predetermined “time” intervals (not rotational angle intervals) ten times and then stops outputting the control signal when the tenth control signal is outputted. 
     The controller  21  is further configured to calculate a distance to an object X using a plurality of return signals corresponding to the plurality of control signals for each rotational angle interval. For example, the controller  21  calculates ten distances corresponding to the ten control signals for each rotation angle interval. Then, the controller  21  accumulates the ten distances and obtains an average distance from the ten calculated distances. 
       FIG.  11    shows a flowchart of a process performed by the LIDAR device  10  according to the second embodiment. It should be noted that steps for correcting the calculated distance using the actual emission timing (i.e., steps S 50  and S 90  in  FIG.  7   ) are eliminated in the second embodiment. 
     When the angle sensor  28  detects a rotation angle of the scanning mirror  16  at step S 100 , the angle sensor  28  outputs a detection signal indicative of the detected rotation angle to the controller  21  at step S 110 . When the controller  21  receives the detection signal from the angle sensor  28 , the controller  21  outputs an emission control signal to the light emitting module  12  at step S 120 . 
     When the light emitting module  12  receives the emission control signal, the light emitting module  12  emits a laser beam toward the scanning mirror  16  at step S 130 . The emitted laser beam is reflected at the reflected surface of the scanning mirror  16  and travels to the scanning zone SZ. Then, if the laser beam is reflected by an object X, the return beam comes back to the LDIAR device and is reflected again by the reflective surface  16   a  of the scanning mirror  16  toward the light receiving module  14 . When the return beam reaches the light receiving module  14 , the light receiving module  14  detects the return beam at step S 140 , and then the light receiving module  14  outputs a return signal to the controller  21  at step S 150  in response to receiving the return beam. 
     The controller  21  calculates a distance to the object X using the signal output timing and the return signal at step S 160 . Then, the controller  21  determines whether the number of the emission control signals emitted after receiving the detection signal is ten at step S 170 . If the number is not ten (step S 170 : No), the process proceeds to step S 180 , and the controller  21  determines whether a predetermined time interval (for example, 3 microseconds) has elapsed after outputting the emission control signal. If the time interval has not elapsed (step S 180 : No), the controller  21  repeats step S 180 . If the time interval has elapsed (step S 180 : Yes), the process proceeds to step S 120  and the controller  21  outputs again an emission control signal. Then, the process repeats steps S 120  to S 160 , and the controller  21  determines whether the number of the emitted emission control signals is ten. If Yes at step S 170 , the controller  21  stops outputting an emission control signal at step S 190 , and then calculates an average distance from the ten calculated distances at step S 200 . Then, the process returns to step S 100 . 
     As described above, the LIDAR device  10  according to the second embodiment outputs a plurality of emission control signals upon receiving a detection signal from the angle sensor  28  until receiving a subsequent detection signal. Then, the controller  21  calculates a plurality of distances to an object X based on a plurality of return signals corresponding to the plurality of emission control signals, and obtains an average distance from the plurality of calculated distances. Therefore, the LIDAR device  10  can obtain a distance to an object X with high accuracy. 
     Third Embodiment 
     Next, the third embodiment of the present disclosure is described with reference to  FIGS.  12  to  13   . In the following description, only different portions from the first and second embodiments are described. 
     In the second embodiment, the controller  21  is configured to output a predetermined number (for example, ten) of emission control signals after receiving a detection signal until receiving a subsequent detection signal. In the third embodiment, the controller  21  is configured to continuously output emission control signals at predetermined time intervals after receiving a detection signal until receiving a subsequent detection signal (see  FIG.  14   ). 
     As with the second embodiment, the controller  21  calculates a plurality of distances to an object X for each rotation angle interval based on a plurality of return signals corresponding to the plurality of emission control signals, and then obtains an average distance from the plurality of calculated distances. 
       FIG.  13    shows a flowchart of a process performed by the LIDAR device  10  according to the second embodiment. It should be noted that, as with the second embodiment, steps for correcting the calculated distance using the actual emission timing (i.e., steps S 50  and S 90  in  FIG.  7   ) are eliminated in the third embodiment. 
     When the angle sensor  28  detects a rotation angle of the scanning mirror  16  at step S 300 , the angle sensor  28  outputs a detection signal indicative of the detected rotation angle to the controller  21  at step S 310 . When the controller  21  receives the detection signal from the angle sensor  28 , the controller  21  outputs an emission control signal to the light emitting module  12  at step S 320 . When the light emitting module  12  receives the emission control signal, the light emitting module  12  emits a laser beam toward the scanning mirror  16  at step S 330 . 
     When the return beam reaches the light receiving module  14 , the light receiving module  14  detects the return beam at step S 340 , and then the light receiving module  14  outputs a return signal to the controller  21  in response to receiving the return beam at step S 350 . 
     The controller  21  calculates a distance to the object X using the signal output timing and the return signal at step S 360 . Then, the controller  21  determines whether the controller  21  receives a subsequent detection signal from the angle sensor  28  after receiving the previous detection signal at S 370 . If step S 370  is No, the process proceeds to step S 380 , and then the controller  21  determines whether a predetermined time interval (for example, 3 microseconds) has elapsed after outputting the emission control signal. If the time interval has not elapsed (step S 380 : No), the controller  21  repeats step S 380 . If the time interval has elapsed (step S 380 : Yes), the process returns to step S 320  and the controller  21  outputs again an emission control signal. Then, the process repeats steps S 320  to S 370 , and the controller  21  output a plurality of emission control signals at the predetermined time intervals until receiving the subsequent detection signal. 
     If the controller  21  determines that the controller  21  receives the subsequent detection signal from the angle sensor  28  (step S 370 : Yes), the controller  21  calculates an average distance from the plurality of calculated distances at step S 380 . Then, the process returns to step S 320 , and the controller  21  outputs again a plurality of emission control signals at the predetermined time intervals by repeating steps S 320  to S 370 . 
     As described above, the LIDAR device  10  according to the third embodiment continuously outputs a plurality of emission control signals at predetermined time intervals for each rotation angle interval. Then, the controller  21  calculates a plurality of distances to an object X for each rotation angle interval based on a plurality of return signals corresponding to the plurality of emission control signals, and obtains an average distance from the plurality of calculated distances. Therefore, as with the second embodiment, the LIDAR device  10  can obtain the distance to an object X with high accuracy. 
     Fourth Embodiment 
     Next, the fourth embodiment of the present disclosure is described with reference to  FIGS.  14  to  15   . In the following description, only different portions from the first to third embodiments are described. 
     In the first embodiment, the controller  21  is configured to automatically output an emission control signal upon receiving a detection signal from the angle sensor  28 . In the fourth embodiment, the controller  21  is configured to output an emission control signal if the detection signal (the rotation angle) matches any one of a plurality of target rotation angles (see  FIG.  14   ). In this embodiment, the target rotation angles are set to 0.0, 0.2, 0.4, 0.6, . . . , 120.00 degree, for example. 
       FIG.  15    shows a flowchart of a process performed by the LIDAR device  10  according to the fourth embodiment. It should be noted that steps for correcting the calculated distance using the actual emission timing (i.e., steps S 50  and S 90  in  FIG.  7   ) are eliminated in the fourth embodiment. 
     When the angle sensor  28  detects a rotation angle of the scanning mirror  16  at step S 400 , the angle sensor  28  outputs a detection signal indicative of the detected rotation angle to the controller  21  at step S 410 . When the controller  21  receives the detection signal from the angle sensor  28 , the controller  21  determines whether the detection signal matches any one of the target rotation angles at step S 420 . If No at step S 420 , the process returns to step S 400  and repeats steps S 410  to S 420 . If Yes at step S 420 , the controller  21  outputs an emission control signal to the light emitting module  12  at step S 430 . 
     When the light emitting module  12  receives the emission control signal, the light emitting module  12  emits a laser beam toward the scanning mirror  16  at step S 440 . Accordingly, the LI DAR device  10  can emit a laser beam when the scanning mirror  16  is at a desired rotation angle. When the return beam reaches the light receiving module  14 , the light receiving module  14  detects the return beam at step S 450 , and then the light receiving module  14  outputs a return signal to the controller  21  in response to receiving the return beam at step S 460 . 
     The controller  21  calculates a distance to the object X using the signal output timing and the return signal at step S 470 . Then, the process returns to step S 400  and repeats steps S 410  to S 470 . 
     As described above, the LIDAR device  10  according to the fourth embodiment outputs an emission control signal when the rotation angle matches any one of the plurality of target rotation angles. Thus, the LIDAR can emit a laser beam toward a desired area in the scanning zone SZ accurately. 
     In the above-described fourth embodiment, the target rotational angles are set in a regular manner (e.g., 0.0, 0.2, 0.4, etc.). However, the target rotational angles may be set in an irregular manner using, for example, a predetermined target angle table stored in at least one memory. In such a case, at least one processor may be used to determine whether the detection signal (the rotational angle) matches a target rotation angle. 
     Fifth Embodiment 
     Next, the fourth embodiment of the present disclosure is described with reference to  FIGS.  16  to  18   . In the following description, only different portions from the first to fourth embodiments are described. In the above-described embodiments, the LIDAR device  10  emits a laser light in a constant manner for each rotation angle interval (for example, one laser beam is emitted for each rotation angle in the first embodiment and a plurality of laser beams are emitted for each rotation angle in the second and third embodiments). In the fifth embodiment, the LIDAR device  10  is configured to emit a laser beam in a different manner for a specific target area within the scanning zone SZ the remaining area. 
     In this embodiment, a specific rotation angle range of the scanning mirror  16  corresponding to the target area is defined in the rotation angle range (for example, −20°≤specific rotation angle θs≤+20°). As shown in  FIG.  17   , the light emitting module  12  is configured to emit a laser beam in a different manner within the rotational angle range than the other rotational angle range (i.e., −60°≤θ&lt;−20° and +20°&lt;θ≤+60). For example, the light emitting module  12  is controlled to emit a predetermined number of laser beams (e.g., 10) for each rotational angle interval within the specific rotation angle range. Hereinafter, the rotation angle range of the scanning mirror  16  other than the specific rotation angle range is referred to as a regular range within which the light emitting module  12  is controlled to emit a laser beam upon receiving the detection signal from the angle sensor  28  as with the first embodiment. 
     In this embodiment, the controller  21  is separately provided with the light receiving module  14  as shown in  FIG.  16   . That is, the light receiving module  14  does not integrally include the controller  21  as described in the first embodiment. The controller  21  is an electronic control unit (ECU) that includes at least one processor  21   a  and at least one memory  21   b  instead of, or in combination of, at least one circuit in this embodiment. The memory  21   b  includes random access memory, read only memory, flash memory, or a combination of these. The memory  21   b  has stored thereon instructions which, when executed by the processor  21   a , cause the processor  21   a  to perform a variety of tasks as will be described later. The memory  21   b  also has stores the specific rotational angle range. 
       FIG.  18    shows a flowchart executed by the LIDAR device  10  according to this embodiment. It should be noted that steps for correcting the calculated distance using the actual emission timing (i.e., steps S 50  and S 90  in  FIG.  7   ) are eliminated in the second embodiment. 
     When the angle sensor  28  detects a rotation angle of the scanning mirror  16  at step S 500 , the angle sensor  28  outputs a detection signal at step S 510 . When the controller  21  receives the detection signal from the angle sensor  28 , the controller  21  (the processor  21   a ) determines whether the rotation angle of the scanning mirror  16  is within the specific rotation angle range at step S 520 . If No at step S 520  (the rotation angle is within the regular range), the process proceeds to step S 530 , and the controller  21  outputs an emission control signal. Accordingly, the controller  21  outputs an emission control signal upon receiving the detection signal when the rotation angle of the scanning mirror  16  is within the regular angle. 
     When the light emitting module  12  receives the emission control signal, the light emitting module  12  emits a laser beam toward the scanning mirror  16  at step S 540 . Then, when the return beam reaches the light receiving module  14 , the light receiving module  14  detects the return beam at step S 550 , and then the light receiving module  14  outputs a return signal to the controller  21  at step S 560  in response to receiving the return beam. 
     The controller  21  calculates a distance to an object X using the signal output timing and the return signal at step S 570 . Then the process returns to step S 500 . In this way, when the rotation angle is within the regular range, a distance to the object X is calculated for each rotation angle interval. 
     If the controller  21  determines that the rotation angle is within the specific rotation angle range at step S 520  (step S 520 : Yes), the process proceeds to step S 580 , and then the controller  21  outputs an emission control signal at step S 580 . When the light emitting module  12  receives the emission control signal, the light emitting module  12  emits a laser beam toward the scanning mirror  16  at step S 590 . Then, when the return beam reaches the light receiving module  14 , the light receiving module  14  detects the return beam at step S 600 , and then the light receiving module  14  outputs a return signal to the controller  21  at step S 610  in response to receiving the return beam. 
     If the controller  21  receives the return signal, the controller  21  calculates a distance to the object X using the return signal at step S 620 . Then, the controller  21  determines whether the number of emission of the emission control signal after receiving the detection signal is ten at step S 630 . If No at step S 630 , the process proceeds to step S 640 , and the controller  21  determines whether a predetermined time interval (for example, 3 microseconds) has elapsed after outputting the emission control signal. If the time interval has not elapsed (step S 640 : No), the controller  21  repeats step S 640 . If the time interval has elapsed (step S 640 : Yes), the process proceeds to step S 580  and the controller  21  outputs again an emission control signal. Then, the process repeats steps S 590  to S 620 , and the controller  21  determines whether the number of the emitted emission control signals is ten at S 630 . If Yes at step S 630 , the controller  21  stops outputting an emission control signal at step S 650 , and then calculates an average distance from the ten calculated distances at step S 660 . Then, the process returns to step S 500 . 
     Accordingly, when the rotation angle is within the specific rotation angle range, the controller  21  outputs ten emission control signals for each rotation angle interval. The controller  21  calculates ten distances based on ten return signals corresponding to the ten emission control signals. Then, the controller  21  obtains an average distance from the ten calculated distances for each rotation angle interval within the specific rotation angle range. Therefore, the LIDAR device  10  according to the fifth embodiment can finely scan a specific area of the scanning zone SZ and obtain a distance to an object X with high accuracy for such a specific area. 
     Modifications to Embodiments 
     Several modifications may be applied to the above-described embodiments. 
     For example, in the embodiments described above, the light emitting module is configured to emit two laser beams from two light emitting elements. However, the light emitting module may emit a laser beam from a single light emitting element or three or more laser beams from three or more light elements. 
     In the above-described embodiments, the light emitting module is configured to emit laser light during the entire cycle of the scanning mirror (i.e., one way from the first positon to the second position and the other way from the second position to the first position). However, the light emitting module may be configured to emit laser light during only one way of the scanning mirror. For example, the light emitting module may be controlled to emit laser lights only when the mirror is swinging from the first position to the second position. In this case, the mirror is quickly returned back to the first position immediately after the mirror moved to the second position. 
     In the above-described embodiments, the light receiver includes a plurality of SPADs. However, other light sensitive elements may be used. However, if SPADs are used as a light receiver, since the SPADs can output a digital signal, there is no need to use a processor to calculate a distance to an object as described above. 
     In this application, the terms “module” and “system” may include hardware components such as housings, fixtures, wiring, etc. In addition, in this application, the term “processor” may refer to, be part of, or include circuits or circuitry that may include processing core hardware (shared, dedicated, or group) that executes code and memory hardware (shared, dedicated, or group) that stores code executed by the processing core hardware. As such, the term “processor” may be replaced by the term “circuit”. 
     The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     Although the terms “first”, “second”, “particular”, etc. are be used to describe various elements, these terms may be only used to distinguish one element from another. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments. 
     Spatially relative terms, such as “front”, “rear,” “left”, “right”, and the like, may be used for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the sensor system in the figures is rotated, elements described as “front/rear” would then be oriented “left/right” with respect to the vehicle. Thus, the example term “front” can encompass any direction in practice. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.