Patent Publication Number: US-2020301128-A1

Title: Moving Object Imaging Device and Moving Object Imaging Method

Description:
TECHNICAL FIELD 
     The present invention relates to a moving object imaging device and a moving object imaging method, and more particularly, to a moving object imaging device and a moving object imaging method for imaging a flying object such as a multi-copter, and the like freely moving in space, and a traveling object such as a vehicle, and the like traveling on a road. 
     BACKGROUND ART 
     In a related art, a device for imaging a moving object such as a flying object, and the like moving in a target area has been known. In order to track and image the moving object in motion, it is required to control an optical axis of a camera so as to capture the moving object in an imaging range of the camera. As a control method for directing the optical axis of the camera toward the moving object, known is a method in which the optical axis of the camera tracks the moving object by driving a plurality of rotatably movable mirrors by using motors of respectively different rotary shafts. For example, this technology is disclosed in JP-A-10-136234 (PTL 1), and in the abstract of JP-A-10-136234 (PTL 1), the technology is described as follows: a light transmissive window W1 is provided in a light-impermeable casing B1, and an imaging device C1, an azimuth angle rotary reflection mirror M1, a tilt angle rotary reflection mirror M2, and motors m1 and m2 for rotating the mirrors M1 and M2 are disposed in the casing B1. After passing through the window W1, a light beam I from an object visual field is regularly reflected by the mirror M1 and is further reflected by the mirror M2, whereby an object image returns to an erect image and the erect image of the object is incident on the imaging device C1. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP-A-10-136234 
     SUMMARY OF INVENTION 
     Technical Problem 
     The performance required for the moving object imaging device is to acquire a clearer image. It is effective to increase the number of pixels of the camera to improve image quality. For example, when imaging is performed at 12K resolution (horizontal 1920 pixels×vertical 1080 pixels) and 4K resolution (horizontal 3840 pixels×vertical 2160 pixels), since the resolution in the vertical and horizontal directions is respectively improved by two times at the 4K resolution with respect to the 2K resolution, the same subject can be imaged with four times the number of pixels of the 2K resolution at the 4K resolution. 
     Here, when both sizes of one pixel of imaging elements of the 4K resolution and the 2K resolution are 10 μm, a size of the imaging element for the 2K resolution is 19.2 mm in height×10.8 mm in width, and the imaging element becomes two times larger by 38.4 mm in height×21.6 mm in width at the 4K resolution. Therefore, the angles of view become equalized by doubling a focal length of a lens mounted on the camera, thereby suppressing occurrence of vignetting. 
     However, when the focal length is set to be doubled while maintaining an aperture diameter of the lens, an F value indicating a degree of taking in the light by the camera becomes quadrupled, and brightness of an obtained image becomes ¼. Further, the depth of field also becomes shallow, and for example, when tracking and imaging a moving object moving at a high speed in a depth direction, the focus becomes easy to be unsharp. Further, brightness is alleviated by extending exposure time, however, extending the exposure time causes motion blur (blur) in the case of the moving object moving at a high speed. Due to the aforementioned causes, when realizing image improvement by increasing the number of pixels, since it is required to increase the aperture diameter of the lens, as disclosed in JP-A-10-136234 (PTL 1), it is required to enlarge a reflection area of a movable mirror in the moving object imaging device which images the moving object via the movable mirror. 
     However, enlargement of the movable mirror leads to an increase in load mass of a motor, such that a larger motor is required to obtain the same response performance. The large motor is required to flow more current, such that a temperature of the motor rises due to copper loss generated by a coil. Since the temperature rise of the motor leads to deterioration in torque generated by the motor, a thermal deformation of peripheral optical components, and the like, a device for actively cooling the motor is newly required, whereby the device becomes enlarged and complicated. The moving object imaging device is frequently used as a monitoring device, such that the enlargement and complexity of the device are not desirable. 
     The present invention has been made in an effort not only to solve the above-mentioned problems, but also to provide a moving object imaging device, in which an optical axis of a camera is changed by a plurality of movable mirrors having different sizes, that not only improves image quality but also maintains tracking performance while suppressing a heat generation amount of a motor driving the movable mirrors 
     Solution to Problem 
     In order to solve the above-mentioned problems, a moving object imaging device according to the present invention for tracking and imaging a moving object crossing an approximately horizontal direction may include a camera configured to capture an image of the moving object sequentially reflected by a plurality of movable mirrors; a mirror movable in a gravity direction configured to define a gravity direction of the captured image of the camera as a scanning direction; a first motor configured to change an angle of the mirror movable in the gravity direction; a mirror movable in a left-and-right direction configured to define a left-and-right direction of the captured image of the camera as a scanning direction; a second motor configured to change an angle of the mirror movable in the left-and-right direction; and a controller configured to control the camera, the first motor, and the second motor, wherein the camera captures the image of the moving object that is sequentially reflected by the mirror movable in the gravity direction and the mirror movable in the left-and-right direction. 
     Further, the moving object imaging device for tracking and imaging a moving object approaching from an approximately horizontal direction may include a camera configured to capture an image of the moving object sequentially reflected by a plurality of movable mirrors; a mirror movable in a gravity direction configured to define a gravity direction of the captured image of the camera as a scanning direction; a first motor configured to change an angle of the mirror movable in the gravity direction; a mirror movable in a left-and-right direction configured to define a left-and-right direction of the captured image of the camera as a scanning direction; a second motor configured to change an angle of the mirror movable in the left-and-right direction; and a controller configured to control the camera, the first motor, and the second motor, wherein the camera captures the image of the moving object that is sequentially reflected by the mirror movable in the gravity direction and the mirror movable in the left-and-right direction. 
     ADVANTAGEOUS EFFECTS OF INVENTION 
     According to a moving object imaging device and a moving object imaging method, since a heat generation amount of a motor can be reduced even though a large movable mirror is used to improve image quality, it is possible not only to improve the image quality but also to maintain tracking performance. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram of a moving object imaging device  1  and a flying object  2   a  in a first embodiment. 
         FIG. 2  is a top plan view of movable mirrors  12   a  and  12   b  in the first embodiment. 
         FIG. 3  is a cross-sectional diagram of a moving object imaging device when a direction of a movable mirror  12   a  is viewed from a camera mounting position in the moving object imaging device of the first embodiment. 
         FIG. 4  is a flow chart of processing which is executed by the moving object imaging device of the first embodiment. 
         FIG. 5  is a functional block diagram of a controller  14  in the first embodiment. 
         FIG. 6  illustrates a captured image which is processed to a gray scale by an image processing part  27  in the first embodiment. 
         FIG. 7A  is a diagram illustrating a current flowing through a motor  13  in the first embodiment. 
         FIG. 7B  is a diagram illustrating a current flowing through a motor  13   b  in the first embodiment. 
         FIG. 8A  is a diagram when the moving object imaging device  1  and the flying object  2   a  in the first embodiment are viewed from the sky above. 
         FIG. 8B  is a diagram when the moving object imaging device  1  and the flying object  2   a  in the first embodiment are viewed from a lateral direction. 
         FIG. 9A  is a diagram illustrating a maximum angular speed of a motor  13   a  of the moving object imaging device  1  when each flight is performed in the first embodiment. 
         FIG. 9B  is a diagram illustrating a maximum angular speed of a motor  13   ba  of the moving object imaging device  1  when each flight is performed in the first embodiment. 
         FIG. 10  is a block diagram of the moving object imaging device  1  and a traveling object  2   b  in a second embodiment. 
         FIG. 11A  is a diagram when the moving object imaging device  1  and the traveling object  2   b  in the second embodiment are viewed from the sky above. 
         FIG. 11B  is a diagram when the moving object imaging device  1  and the traveling object  2   b  in the second embodiment are viewed from a lateral direction. 
         FIG. 12A  is a diagram illustrating a maximum angular speed of a motor  13   a  of the moving object imaging device  1  when each flight is performed in the second embodiment. 
         FIG. 12B  is a diagram illustrating a maximum angular speed of a motor  13   b  of the moving object imaging device  1  when each flight is performed in the second embodiment. 
         FIG. 13  is a cross-sectional diagram of a moving object imaging device  1  when a direction of a movable mirror  12   a  is viewed from a camera mounting position in the moving object imaging device of a third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, each embodiment of the present invention will be described with reference to the drawings. Further, the present invention will be hereinafter described by being divided into a plurality of embodiments for convenience. Unless otherwise specified, the plurality of embodiments are not unrelated to each other, and one embodiment has a relationship with a part or whole parts of the other embodiment with respect to modifications, details, supplementary descriptions, and the like. Further, in all of the drawings for describing the following embodiments, those having the same functions will be denoted by the same reference sings in principle, and any redundant descriptions will omitted. 
     First Embodiment FIRST EMBODIMENT 
     Described herein are a moving object imaging device  1  according to a first embodiment of the present invention that tracks and images a flying object crossing an approximately horizontal direction, and a moving object imaging method used for the same with reference to  FIGS. 1 to 9B . 
       FIG. 1  is a block diagram including a moving object imaging device  1  of an embodiment and a flying object  2   a  which is a moving object. The flying object  2   a  shown in  FIG. 1  is a flying object (quadcopter), which is viewed from a side-surface side, and which has four propellers and is capable of freely performing a horizontal movement, a direction change, and ascent and descent by changing the number of rotation of each propeller. 
     The moving object imaging device  1  is mainly aimed at tracking and imaging the flying object  2   a  crossing the approximately horizontal direction, and is provided with a camera  11 , two movable mirrors  12   a  and  12   b  having different sizes, motors  13   a  and  13   b  for changing angles of the respective movable mirrors, and a controller  14  for controlling the camera  11  and the motors  13   a  and  13   b . Here, the meaning of “crossing the approximately horizontal direction” is a motion including a lateral movement on a captured image  107  of the camera  11 , and may include a relatively small longitudinal movement. 
     The movable mirror  12   a  is a mirror movable in a left-and-right direction in which a left-and-right direction of the captured image  107  of the camera  11  is defined as a scanning direction. The movable mirror  12   b  is a mirror movable in a gravity direction in which a gravity direction of the captured image  107  of the camera  11  is defined as a scanning direction. Further, it is characterized in that the camera  11  captures an image of the flying object  2   a  sequentially reflected by the movable mirror  12   a  and the movable mirror  12   b , and the scanning direction of the movable mirror  12   b  positioned farthest from the camera  11  is the gravity direction. Further, it is characterized in that a reflection surface of the movable mirror  12   b , a scanning direction of which is the gravity direction, is mounted so as to face a ground surface. The motors  13   a  and  13   b  have angle detectors (not shown) for detecting a rotational angle, and output the detected rotational angles to the controller  14  as detection angles  102   a  and  102   b . Further, a display device for showing the captured image  107  to an operator, a command input device  20  to which an operator inputs a command, and a storage device for recording the captured image, all of which are not illustrated in the drawings, are connected to the moving object imaging device  1 . 
     Here, a top plan view seen from the reflection surfaces of the movable mirrors  12   a  and  12   b  will be described with reference to  FIG. 2 . As shown here, the movable mirror  12   a  is provided with a reflection mirror part  121   a  and a mounting part  122   a  connecting the motor  13   a  and the reflection mirror part  121   a . The movable mirror  12   b  is provided with a reflection mirror part  121   b  and a mounting part  122   b  connecting the motor  13   b  and the reflection mirror part  121   b . In the embodiment, a length of the reflection mirror part  121   a  close to the camera  11  is set to 40 mm, and a length of the reflection mirror part  121   b  far from the camera  11  is set to 80 mm. Since the movable mirror  12   b  far from the camera  11  copes with a change of an optical axis in all of the movable areas of the movable mirror  12   a  close to the camera  11 , the movable mirror  12   b  is set to be larger than the movable mirror  12   a . As a movable area of the movable mirror  12   a  close to the camera  11  becomes larger, it is required to extend the movable mirror  12   b  far from the camera  11  in a rotational axis direction of the motor. According to the reasons described above, as a result of setting the sizes of both movable mirrors different, as shown in  FIG. 2 , moment of inertia when the small movable mirror  12   a  rotates around a motor shaft is 30.0 g·cm 2 , and moment of inertia of the large movable mirror  12   b  is 45.0 g·cm 2 . 
       FIG. 3  is a cross-sectional diagram of the moving object imaging device  1  when a direction of the movable mirror  12   a  is viewed from a mounting position of the camera  11 . Here, a distance A 1  between a rotary shaft of the motor  13   a  and a rotary shaft of the motor  13   b  is set to 42.5 mm, and a movable range of the movable mirror is set to ±20°. Further, a circle C indicates an area which is provided to prevent the movable mirror  12   b  with interfering with the motor  13   a , and a fixed distance thereof is set around the rotary shaft of the movable mirror  12   b.    
     Next, imaging operation of the moving object imaging device according to the first embodiment will be described by using a flow chart shown in  FIG. 4 . The imaging operation of the moving object imaging device  1  is roughly classified into movable mirror rotation operation for driving the movable mirrors  13   a  and  13   b  to a target deflection angle; and image acquisition operation for acquiring the captured image  107  by starting exposure of the camera  11  in a state where an optical axis  3  is fixed, such that the movable mirror rotation operation and the image acquisition operation are alternately repeated in time series. In the embodiment, since the image is captured in a state where the movable mirror is fixed, a camera having a slow imaging period can be used, and further, there exists an advantage that an exposure time can be extended under an environmental condition where a quantity of light is insufficient, thereby coping with the environmental condition. 
     First, when starting the imaging operation, the controller  14  determines whether or not the flying object  2   a  which is a tracking target is included in the captured image  107  of the camera  11  at step S 1 . Next, when the flying object  2   a  is not included in the captured image  107 , the controller  14  executes an external command mode at step S 2 , whereas when the flying object  2   a  is included in the captured image  107 , an internal command mode is executed at step S 5 . 
     The external command mode at step S 2  is a mode for an operator of the moving object imaging device  1  to operate the rotation of each movable mirror and to capture the flying object  2   a  of the tracking target in order for the flying object  2   a  thereof to be imaged by the camera  11 . Further, the operator provides a target deflection angle command of each movable mirror to the controller  14  from the outside by using a command input device  20  such as a game pad, and the like while looking at the display device at step S 3 , and when the flying object  2   a  is captured, an angle of the movable mirror is fixed at step S 4 . 
     Meanwhile, the internal command mode at step S 5  is a mode for the controller  14  to operate the rotation of each movable mirror and for tracking the flying object  2   a  of the tracking target in order for the camera  11  to image the flying object  2   a  thereof. Further, the target deflection angle command of each movable mirror is generated inside the controller  14  at step S 6 , and the movable mirror is fixed to the flying object  2   a  at a tracked angle at step S 7 . 
     At the step S 3  or the step S 6 , the controller  14  adjusts and outputs an applied voltage so that driving currents  101   a  and  101   b  corresponding to a set target deflection angle flow through the respective motors  13   a  and  13   b . As a result, the optical axis  3  of the camera  11  is controlled to face the flying object  2   a . At the step S 4  or the step S 7 , the completion of the movable mirror rotation operation at steps S 3  and S 6  by the detection angles  102   a  and  102   b  of the motors  13   a  and  13   b  is confirmed, the controller  14  outputs an imaging trigger signal  103  (refer to  FIG. 1 ) to the camera  11 , and the camera  11  starts exposure at step S 8 . When acquisition of the captured image  107  ends, the camera  11  outputs an imaging end signal  104  (refer to  FIG. 1 ) to the controller  14 , and the controller  14  confirms the presence or absence of an input of an imaging end command. When the imaging end command is not inputted, the controller  14  starts the next movable mirror rotation operation. The consecutively captured images  107  are acquired by repeating a series of above-mentioned operation, and when the imaging period is sufficiently short (for example, 30 images/sec which is the same as that of a general television), the images  107  acquired by the display device are consecutively displayed, thereby making it possible to provide a state of the flying object  2   a  crossing the approximately horizontal direction of the moving object imaging device  1  as a moving image. 
     Next, details of the external command mode and the internal command mode will be described while referring to the functional block diagram of the controller  14  shown in  FIG. 5 . 
     As shown in  FIG. 5 , the command input device  20 , the motors  13   a  and  13   b , and the camera  11  are connected to the controller  14 . Further, switches  21   a  and  21   b , storage parts  22   a  and  22   b , adders  23   a ,  23   b ,  24   a , and  24   b , compensators  25   a  and  25   b , amplifiers  26   a  and  26   b , and an image processing part  27  are provided inside the controller  14 . Further, the controller  14  may be configured with hardware such as ASIC or FPGA, or may be configured with software that executes a program loaded into a memory by a CPU, or may be configured with a combination of the hardware and the software. 
     First, a method for controlling a deflection angle of the motor  13   a  in the external command mode will be described. Further, here, while the method for controlling the motor  13   a  is described, redundant descriptions of the motor  13   b  using the same control method will be omitted. In the external command mode, a changeover switch  21   a  is on the lower side, and a deviation angle between a target angle command  105   a  given from the external commend input device  20  and the detection angle  102   a  obtained by an angle detector of the motor  13   a  is added by the adder  24   a  by inverting the detection angle  102   a  positively and negatively. The compensator  25   a  adjusts a magnitude of the driving current  101   a  flowing through the amplifier  26   a  to the motor  13   a  so as to make the deviation zero. Further, the compensator  25   a  performs PID control. 
     Then, a method for controlling the deflection angle of the motor  13   a  in the internal command mode will be described. In the internal command mode, the changeover switch  21   a  is on the upper side, and an operation amount  106   a  before one control period is recorded in the storage part  22   a . First, the image processing part  27  calculates an optical axis deviation amount  108   a  of the camera  11  based upon the captured image  107  acquired before the camera  11  performs one operation (a computation method will be described later). The optical axis deviation amount  108   a  and the operation amount  106   a  before one control period stored in the storage part  22   a  are added by the adder  23   a , which is defined as the deviation amount  108   a  which is a new target change angle command. Since a flow after the above-mentioned processing is the same as that of the case of the external command mode, description thereof will be omitted. 
     Next, a method for calculating the optical axis deviation amount of the camera will be described. The image processing part  27  has a storage part (not shown), and the storage part stores the captured image  107  before one imaging period. Then, the stored captured image  107  and a current image are converted into luminance information of 0-255 (gray scale), and a difference between respective pixel values of the two captured images  107  is obtained. A pixel, a difference value of which exceeds a predetermined value, is considered as a moving part  1  (white), and when a pixel, a difference value of which is lower than a predetermined value is set as  0  (black) (binarization processing). The aforementioned method is referred to as a frame difference method which is one type of background difference method. 
       FIG. 6  illustrates a result of the binarization processing with respect to the captured image  107 . Further, a scanning direction of the motor  13   a  is a direction in which a right side is defined as positive on right and left sides of a paper surface (hereinafter, referred to as an x-axis direction), and a scanning direction of the motor  13   b  is a direction in which an upper side is defined as positive on upper and lower sides of the paper surface (hereinafter, referred to as a y-axis direction). When an area of a moving pixel group has a predetermined size or shape in the captured image  107 , the pixel group is determined to be the flying object. At this time, a gravity center position of the moving pixel group is defined as a center position Q of the flying object in the captured image  107 , and a difference (x-axis direction is q a , y-axis direction is q b ) between coordinate values of an image center O and the center position Q of the flying object is defined as the optical axis deviation amount of the camera  11 . The next movable mirror rotation operation is performed based upon the optical axis deviation amount of each axis. 
     The moving object imaging device  1  according to the embodiment defines the flying object freely flying around space as an object for imaging (tracking). The scanning direction of the larger movable mirror  12   b  far from the camera is defined as the gravity direction. What is mentioned above is arranged in consideration of response characteristics of a deflection mechanism formed with the movable mirror and the motor, and moving characteristics of the flying object, thereby implementing tracking performance of the moving object imaging device to the maximum. 
     First, the response characteristics of the deflection mechanism formed with the movable mirror and the motor will be described. In the embodiment, since the movable mirror is stationary while the camera  11  is capturing an image, the motor repeatedly rotates and stops for each imaging period. The aforementioned operation is regarded as a reciprocating operation between two points, and power consumption of the motor is estimated, and a relationship between the moving distance and the power consumption is contemplated. Further, the motor has a plurality of mechanism resonance modes, however, the motor herein is treated as a rigid object to improve visibility, and a current flowing through the motor is also treated as a single sine wave. When a coil part of the motor is set as an inductor Lc and a resistor Rc, an equation of motion when a rotor rotates at a frequency f and a vibration amplitude θ 0 , an equation 1 is represented as follows: 
     
       
         
           
             
               
                 
                   
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     According to the equation 3, the power consumption is proportional to the fourth power of the frequency f, and is proportional to the square of the moment of inertia of the whole movable elements and the rotational angle. 
       FIGS. 7A and 7B  illustrates driving currents  101   a  and  101   b  flowing through the respective motors when the motors  13   a  and  13   b , on which the movable mirrors  12   a  and  12   b  having different sizes are mounted, are moved only by the same rotational angle, and a vertical axis represents a magnitude of the current and a horizontal axis represents time. Further, since the motor shape is the same and the resistance Rc is the same, the power consumption is proportional to the square of the current. As apparent from comparison between two drawings, the motor  13   b  on which the movable mirror  12   b  having the large moment of inertia is mounted requires a larger current than the motor  13   a  on which the movable mirror  12   a  having the small moment of inertia is mounted. Therefore, an amount of heat generation caused by copper loss of a coil increases. 
     Since the power consumption is proportional to the square of the current as described above, when a peak value of the current of the motor  13   a  is  2 A, and a peak value of the current of the motor  13   b  is  3 A, the power consumption of the motor  13   b  becomes 2.25 times (=32/22 times)at the maximum in comparison with the power consumption of the motor  13   a.    
     A heat removal amount caused by natural heat radiation of the motor is determined from a structure, and a general motor has rated power consumption to be prevented from becoming more than an allowable temperature as a specification. When the motor structure and the rotational angle cannot be changed, an only way to lower the power consumption is to lower the frequency f. That is, the deflection mechanism on which the large movable mirror is mounted is inferior in response performance in comparison with the deflection mechanism on which the small movable mirror is mounted. Further, lowering the frequency f means extending the imaging period, and when tracking of the moving object is performed by the captured image  107  as in the embodiment, the tracking performance of the motor in the scanning direction deteriorates. 
     Next, movement characteristics of the moving object  2   a  are considered.  FIG. 8A  illustrates a drawing when looking down a positional relationship between the moving object imaging device  1  and the flying object  2   a  from the sky above.  FIG. 8B  illustrates a drawing when both the moving object imaging device  1  and the flying object  2   a  are viewed from a certain point on the ground from a lateral direction. 
     The multi-copter which is an object to be imaged in the embodiment has a high moving speed in the horizontal direction, but has a low moving speed in the gravity direction. For example, while a catalog specification of Phantom 4 manufactured by DJI has a maximum horizontal speed of 20 m/s (72 km/h), an ascending speed is 6 m/s and a descending speed is 4 m/s. 
     Here, a scanning range of the movable mirror  12   b  scanning in the gravity direction is set from 0° (horizontal) to an elevation angle of 40°, and a scanning range of the movable mirror  12   a  scanning in the horizontal direction is set to 20° to the left and right. As shown in  FIG. 8B , when the flying object  2   a  exists at a point 200 m away from the moving object imaging device  1  and exits above the altitude of 53 m (the rotational angle of the motor  13   b  is) 15°, movements in respective directions of (i) ascent, (ii) descent, (iii) horizontal to left and right, and (iv) approach of the flying object  2   a  can be tracked by controlling the rotational angle of each motor as follows:
     (i) ascent (the rotational angle of the motor  13   a  is fixed at 0°, and tracking is performed by scanning of the motor  13   b ).   (ii) Descent (same as that of (i))   (iii) Horizontal directions to left and right (the rotational angle of the motor  13   b  is fixed at 15°, and tracking is performed by scanning of the motor  13   a ).   (iv) Approach direction (same as that of (i))   

     Further, the maximum angular speed of each motor and the rotational angle for each imaging period when moving from a position of the flying object  2   a  in  FIG. 8B  to the respective directions of (i) to (iv) at the maximum speed are illustrated in  FIGS. 9A and 9B . 
     As shown in  FIG. 9A , (i) the maximum angular speed of the motor  13   a  at the time of the ascent is 1.62°/sec, and (ii) the maximum angular speed at the time of the descent is 1.15°/sec. Further, as shown in  FIG. 9B , (iii) the maximum angular speed of the motor  13   b  at the time of the movement in the horizontal direction to left and right is 5.73°/sec. As can be seen from these drawings, in the movements of (i) to (iii), the maximum angular speed is approximately the same even though a distance and an altitude are different. Further, the maximum angular speed of the motor  13   a  in (iii) is about 3.3 to 5.7 times larger than the maximum angular speed of the motor  13   b  in (i) or (ii). 
     Meanwhile, as shown in  FIG. 9B , (iv) the angular speed of the motor  13   b  at the time of moving in an approach direction increases as a distance from the flying object  2   a  becomes shorter, particularly, when a distance from the moving object imaging device  1  is 80 to 65 m, (iv) the angular speed of the motor  13   b  at the time of moving in an approach direction becomes larger than the maximum angular speed 5.73°/sec of (iii). 
     When the distance to the flying object  2   a  is less than 65 m, a center of the captured image  107  acquired from a restriction of a motor movable area can not be grasped, thereby becoming difficult to perform the tracking. As described above, when a flying object freely flying around space is set as an object to be imaged (tracking), it can be seen that a severe scanning direction in the tracking performance required for the moving object imaging device is the left-and-right direction with respect to the acquired screen, except in a case where the flying object is within 85 meters of the moving object imaging device and approaches further the moving object imaging device. 
     Further, when the flying object  2   a , the maximum speed in the horizontal direction of which is 20 m/sec (72 km/h) is used, the time required for passing the distance between 85 m and 65 m in the approach direction operation (iv) is only one second, whereby it is a significantly extreme example as a situation in which the flying object  2   a  freely flying around space is tracked. Further, when an importance level of tracking the flying object approaching in the approach direction is high, it is desirable to cope with the situation by adopting the same configuration as that of a second embodiment which will be described later. 
     Based upon the above-mentioned considerations, in the moving object imaging device  1  of the embodiment that images (tracks) the flying object  2  freely flying around space, the scanning direction of the large movable mirror far from the camera  11  is set to coincide with the gravity direction where the maximum angular speed required for the movable mirror is small, thereby suppressing the power consumption required for driving the movable mirror. Therefore, the larger movable mirror can be used in comparison with a case where the scanning direction of the movable mirror far from the camera  11  is defined as the left-and-right direction of the captured image  107 , thereby making it possible to maintain both improvement of imaging quality and tracking performance. 
     Further, in the moving object imaging device  1  of the embodiment, as shown in  FIG. 3 , the reflection surface of the movable mirror  12   b , the scanning direction of which is the gravity direction, faces the ground surface. In the moving object imaging device  1  in which movable mirrors  12   a ,  12   b , and the like are stored in a casing as shown in  FIG. 3 , an opening part of the casing, that is, a direction in which the flying object  2   a  is observed becomes a left direction of a paper surface. Accordingly, for example, even when the sun is present at a point B diagonally above the left of the opening part, the reflection surface of the movable mirror  12   b  faces an opposite side of the sun, thereby having an effect of reducing inflow of reflected light caused by the movable mirror  12   b  in the casing. Further, the movable mirror  12   a  faces the point B, however, since the mirror  12   a  exists at a position deeper than the mirror  12   b , there exist few cases in which the sunlight directly hits the reflection surface, and a reflection area is smaller than the mirror  12   b , the movable mirror  12   a  has a slighter influence in comparison with an influence of the sunlight caused by the movable mirror  12   b.    
     In the embodiment, as shown in  FIG. 6 , a frame difference method is used for detecting the flying object  2   a.    
     For example, another method such as a code book method for learning a plurality of background models, and the like may be used. Further, it may be considered to improve the image quality accompanied by an increase in the number of pixels by setting a focal length of the lens the same. In this case, since an angle of view is widened, and the reflection area of the movable mirror is enlarged, the embodiment still remains effective. In the embodiment, a multi-copter is assumed as the flying object, however, since it is extremely difficult to freely fly in a vertical direction in the case of a winged aircraft which is one example of another flying object, a result in consideration of the winged aircraft is the same as a result in consideration of the multi-copter. 
     According to the configuration of the embodiment described above, even though a large movable mirror is used to improve the image quality, since the heat generation amount of the motor can be suppressed, it is possible not only to improve the image quality, but also to maintain the tracking performance. 
     Second Embodiment 
     Next, the moving object imaging device  1  of the second embodiment will be described with reference to  FIGS. 10 to 12 . The moving object imaging device  1  of the embodiment uses a traveling object  2   b  such as a vehicle approaching while traveling on a road as a tracking object. For example, the moving object imaging device  1  may be a device for automatically reading an automobile number (N system), and the like. Further, redundant descriptions of common points between the first and second embodiments will be omitted. 
       FIG. 10  is a block diagram including the moving object imaging device  1  of the embodiment and the traveling object  2   b  viewed from a side-surface side. In the first embodiment, the scanning direction of the movable mirror  12   b  positioned farthest from the camera  11  is defined as the gravity direction. Meanwhile, in this embodiment, the scanning direction of the movable mirror  12   b  positioned farthest from the camera  11  is defined as a screen horizontal direction. 
     Since the imaging operation and the movement of each part, and the like are the same as those of the first embodiment, here, only moving characteristics of the traveling object  2   b  are paid attention to.  FIG. 11A  is a diagram illustrating a positional relationship between the moving object imaging device  1  and the traveling object  2   b  from the sky above, and  FIG. 11B  is a diagram when both the moving object imaging device  1  and the traveling object  2   b  are viewed form a certain point on the ground from a lateral direction. 
     In the traveling object  2   b  linearly approaching the moving object imaging device  1 , there exists a case in which a traveling speed in an approach direction exceeds 100 km/h, and even at the time of a lane change, since a lane width is only about 3.5 m, there exists a traveling characteristic in that a traveling speed in the left-and-right direction is slow. 
     Here, a scanning range of the movable mirror  12   a  scanning in the approach direction is set to 0° (horizontal) to an elevation angle of 40°, and an investigation range of the movable mirror  12   b  scanning in the horizontal direction is set to 20°. 
     As shown in  FIG. 11P , a movement (v) in which the traveling object  2   b  approaches the moving object imaging device  1  from a point away from 40 m; and a movement (vi) in which the traveling object  2   b  approaches closer than the point away from 40 m, starts operation to change to a lane deviated in a 3.5 m horizontal direction from a point away from 30 m, and completes the lane change at a point away from 10 m and passes under the moving object imaging device  1  can be tracked by controlling the rotational angle of each motor as follows:
     (v) The rotational angle of the motor  13   b  is fixed at 0°, and tracking is performed by scanning of the motor  13   a      (vi) Tracking is performed by appropriate scanning of the motors  13   a  and  13   b      

     Further, the maximum angular speed of each motor and the rotational angle for each imaging period when the movement (v) or (vi) is performed from the position of the traveling object  2   b  in  FIG. 11A  are illustrated in  FIGS. 12A and 12B . Here, an installation position of the moving object imaging device  1  is set to 4 m above the ground surface, and a traveling object speed is set to 13.9 m/sec (50 km/h). Additionally, when the traveling object  2   b  approaches the moving object imaging device  1  at 4.8 min the movement of (v) and approaches the moving object imaging device  1  at 9.74 m in the movement of (vi), the traveling object  2   b  becomes out of the imaging range. 
     According to the comparison between  FIGS. 12A and 12B , it is found out that the maximum angular speed occurs when the traveling object is closest (84.55°/sec)in the motor  13   a  of a direction in which the traveling object approaches, on the other hand, the maximum angular speed of the motor  13   b  is relatively small. 
     Therefore, in the moving object imaging device  1 , the generated power consumption is suppressed by matching the scanning direction of the large movable mirror far from the camera  11  with the left-and-right direction of the screen in which the maximum angular speed required for the movable mirror is small. 
     Further, in the embodiment, the tracking object is described as the traveling object  2   b . However, the object to which the embodiment is applied is not limited to the traveling object, and the flying object  2   a  approaching toward the moving object imaging device  1  may be the tracking object. 
     Third Embodiment 
     In the second and third embodiments, the movable mirror  12   b  can be made small by narrowing a distance between the two motors, however, since the movable mirror, the motor, and the like physically interferes with each other, a movable area of each movable mirror is narrowed. This improvement method therefor will be described in the third embodiment. 
       FIG. 13  is a cross-sectional diagram of the moving object imaging device  1  when the direction of the movable mirror  12   a  is viewed from the camera mounting position in the third embodiment. The moving object imaging device  1  of the embodiment is characterized in that the rotary shaft of the motor  13   a  is arranged to be rotated clockwise with respect to the rotary shaft of the motor  13   b  in comparison with the cross sectional view of  FIG. 3 . 
     In  FIG. 3  of the first embodiment, the distance A 1  between the motor  13   a  and the rotary shaft of the motor  13   b  is set to 42.5 mm, and the movable range of each movable mirror is set to ±20°. Further, as an area that is provided so that the movable mirror  12   b  does not interfere with the motor  13   a , the circle C is set around the rotary shaft of the movable mirror  12   b.    
     On the other hand, also in the embodiment, the motor  13   a  is installed while avoiding the circle C that is provided in order that the movable mirror  12   b  does not interfere with the motor  13   a , and it is possible to set a distance A 2  (41.0 mm) of the rotary shaft between the motor  13   a  and the motor  13   b  smaller than the distance A 1  (42.5 mm) in  FIG. 3  by inclining a mounting angle of the motor  13   a  by 16°. As a result, a size of the movable mirror  12   b  required for securing the same imaging range can be reduced. 
     Since the moment of inertia of the movable mirror  12   b  can be reduced by miniaturizing the movable mirror  12   b , the power consumption required for driving the movable mirror  12   b  can be reduced, and further, the movable mirror  12   b  can be driven at a higher speed. 
     Further, in the moving object imaging device  1  according to the embodiment, the captured image  107  obtained at the mounting position of the camera  11  is inclined by a mounting angle of the rotary shaft of the movable mirror  12   a . Therefore, by inclining the camera with respect to the optical axis and mounting the camera, the horizontal and vertical directions of the acquired captured image  107  and the scanning direction coincide with each other, and the operation of the present device can be intuitively performed. Further, even though the camera  11  is horizontally mounted, what is described just above can be realized by adding numerical calculation processing such as coordinate conversion to the acquired captured image  107 , however, since the computation processing is required, an update period of image information to be sent to the display device deteriorates. 
     The present invention is not limited to the embodiments described above, and includes various modifications. For example, the above-mentioned embodiments are described in detail so as to describe the present invention in an easy-to-understand manner, and are not necessarily limited to those including all of the configurations described herein. 
     REFERENCE SIGNS LIST 
       1 : moving object imaging device 
       2   a : flying object 
       2   b : traveling object 
       3 : optical axis 
       11 : camera 
       12   a ,  12   b : movable mirror 
       121   a ,  121   b : reflection mirror part 
       122   a ,  122   b : mounting part 
       13   a ,  13   b : motor 
       14 : controller 
       20 : command input device 
       21   a ,  21   b : switch 
       22   a ,  22   b : storage part 
       23   a ,  23   b ,  24   a ,  24   b : adder 
       25   a ,  25   b : compensator 
       26   a ,  26   b : amplifier 
       27 : image processing part 
       101   a ,  101   b : driving current 
       102   a ,  102   b : detection angle 
       103 : imaging trigger signal 
       104 : imaging end signal 
       105   a ,  105   b : target angle command 
       106   a ,  106   b : operation amount 
       107 : captured image 
       108   a ,  108   b : deviation amount