Patent Publication Number: US-2023140321-A1

Title: Distance measurement system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Japanese Patent Application No. 2021-178101 filed on Oct. 29, 2021. The entire disclosure of Japanese Patent Application No. 2021-178101 is hereby incorporated herein by reference. 
     BACKGROUND 
     Technical Field 
     The present invention relates to a distance measurement system including a distance measurement device that measures the distance to an object according to the phase difference between an emitted light wave emitted toward the object and a received light wave. 
     Description of the Related Art 
     In recent years, a TOF (time-of-flight) sensor, which measures the distance to a measurement object by receiving the reflection of light emitted from an LED (light emitting diode) toward the measurement object, has been used as a light source, for example. For example, in order to correct deviation in the emission direction of a laser beam emitted by an object detection device, Patent Literature 1 discloses an object detection device, comprising: an emission means for emitting a beam; a reception means for receiving a reflected beam obtained when the beam emitted by the emission means hits an object and is reflected; a determination means for determining whether or not the object that reflected the reflected beam received by the reception means is a road surface; a measurement means for measuring the distance to a reflection position on the road surface on the basis of the reflected beam received by the receiving means; a calculation means for calculating the inclination angle of the road surface on the basis of the distance to the reflection position on the road surface measured by the measurement means; and a control means for controlling the emission angle of a beam on the basis of the inclination angle of the road surface calculated by the calculation means. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP-A 2006-276023 
     SUMMARY 
     Problem to be Solved by the Invention 
     However, the above-mentioned conventional object detection device has the following problem. 
     The object detection device disclosed in the above publication is used as a laser radar installed in an automobile, and calculates the inclination angle of the road surface on the basis of the distance to the reflection position of the reflected beam, which is reflected when the beam emitted from the emission means hits the road surface, and adjusts the beam emission angle. 
     However, with this configuration, although the emission direction of the laser can be adjusted according to the deviation in the optical axis direction of the laser that has been distorted due to collision or the like, it is impossible to ascertain the attachment orientation of the laser radar. 
     Consequently, in order to ascertain the attachment orientation of the laser radar, a provided separately orientation sensing device such as an inclination sensor or a level ends up being necessary. 
     It is an object of the present invention to provide a distance measurement system capable of sensing the attachment orientation of a distance measurement device attached to a main body unit, without using a orientation sensing device such as an inclination sensor or a level. 
     Means for Solving Problem 
     The distance measurement system according to the first invention is a distance measurement device that measures the distance to an object according to the phase difference between emitted light waves emitted toward the object and the received light waves, the distance measurement system comprising a main body unit, a distance measurement device, and an attachment orientation sensing unit. The distance measurement device has an emission unit that emits the light toward a specific reference surface, a detection unit that detects the light emitted from the emission unit, a distance information acquisition unit that acquires distance information about the distance to a reference point on the reference surface according to the phase difference between the received light waves and the emitted light waves detected by the detection unit, and an angle information acquisition unit that acquires angle information about the angle to the reference point. This distance measurement device is mounted to the main body unit. The attachment orientation sensing unit senses the attachment orientation of the distance measurement device with respect to the reference surface on the basis of the distance information and the angle information acquired by the distance information acquisition unit and the angle information acquisition unit. 
     Here, for example, in order to sense the attachment orientation of the distance measurement device mounted on a specific device such as a conveyance device, the distance information and the angle information measured by the distance measurement device are used to sense the attachment orientation of the distance measurement device with respect to the reference surface. 
     Here, the distance measurement device is, for example, a TOF (time-of-flight) sensor, LiDAR (light detection and ranging), SC (structural camera), or the like that can acquire information about the distance to a reference point on a reference surface, and makes use of a sensor that obtains angle information. 
     Also, the “attachment orientation” of the distance measurement device means, for example, the inclination angle of the distance measurement device with respect to the reference surface, the distance from the reference surface, the rotation angle with respect to the reference surface, and so forth. 
     The “reference surface is,” for example, the floor surface on which a specific device is installed, or a flat surface such as a wall disposed in the vertical direction, and the “reference point” on the reference surface means, for example, a specific point on a floor surface or a wall surface. 
     The light emitted from the emission unit includes, for example, light in the broad sense (ultraviolet light, visible light, infrared light) and the like. 
     The distance information acquisition unit may be configured to detect the reflection of light and calculate distance information, or may be configured to acquire distance information from a distance sensor or the like provided as an external device, for example. 
     The attachment orientation sensing unit may be provided, for example, inside the distance measurement device, or may be provided separately from the distance measurement device. 
     Also, the specific thing to which the distance measurement device is mounted may be, for example, a vehicle such as a conveyance device or a passenger vehicle, or may be an indoor wall surface, a ceiling surface, an outdoor support, or the like. 
     Consequently, the attachment orientation of the distance measurement device with respect to a floor surface or other such reference surface can be sensed by using the results (distance information and angle information) measured or acquired by the distance measurement device. 
     As a result, the attachment orientation of a distance measurement device attached to any of various devices can be sensed without the use of an orientation sensing device such as an inclination sensor or a level. 
     The distance measurement system according to the second invention is the distance measurement system according to the first invention, wherein the main body unit has a wheel that can travel on the reference surface, a drive unit that rotationally drives the wheel, and a drive control unit that controls the drive unit. Consequently, in a system configuration in which a distance measurement device is mounted to a conveyance device that can travel on rotating wheels over a reference surface, the attachment orientation of the distance measurement device can be sensed by using the distance information and angle information measured or acquired by the distance measurement device itself. 
     The distance measurement system according to the third invention is the distance measurement system according to the second invention, wherein the drive control unit controls the drive unit so as to rotationally drive the wheel, and moves to a specific return position. 
     Consequently, in a system configuration in which a distance measurement device is mounted to a conveyance device capable of traveling on rotating wheels over a reference surface, when the user finishes using the system, for example, the device can be controlled to automatically return to a specific return position and go into standby mode. 
     The distance measurement system according to the fourth invention is the distance measurement system according to the third invention, further comprising a charging station that is provided at the return position and to which a part of the main body unit is connected. 
     Consequently, in a system configuration in which a distance measurement device is mounted to a conveyance device capable of traveling on rotating wheels over a reference surface, the device can be controlled so as to return to the charging station provided at a specific return position and go into standby mode. 
     The distance measurement system according to the fifth invention is the distance measurement system according to the fourth invention, wherein the main body unit has a secondary battery that supplies electric power to the drive unit. The charging station has a charging device that charges the secondary battery. 
     Consequently, in a system configuration in which a distance measurement device is mounted to a conveyance device capable of traveling on rotating wheels over a reference surface, after use, for example, the secondary battery provided to the main body unit can be connected to the charging station and charged. Consequently, the secondary battery is always kept in a charged state, so electric power can be stably supplied to the drive unit that drives the wheels. 
     The distance measurement system according to the sixth invention is the distance measurement system according to any of the third to fifth inventions, wherein the distance information acquisition unit acquires distance information with respect to a reference point acquired at the return position. 
     Consequently, the attachment orientation can be sensed more stably and accurately by acquiring the distance information used for sensing the attachment orientation of the distance measurement device at a specific position (return position). 
     The distance measurement system according to the seventh invention is the distance measurement system according to the sixth invention, wherein the attachment orientation sensing unit senses the attachment orientation by using the distance information and the angle information with respect to the reference surface acquired at the return position. 
     Consequently, the attachment orientation can be sensed more stably and accurately by sensing the attachment orientation of the distance measurement device at a specific position (return position). 
     The distance measurement system according to the eighth invention is the distance measurement system according to any of the first to seventh inventions, wherein the attachment orientation sensing unit senses at least one of the inclination angle of the distance measurement device with respect to the reference surface, the distance from the reference surface, and the rotation angle with respect to the reference surface, as the attachment orientation. 
     Consequently, at least one of the inclination angle, the distance, and the rotation angle with respect to the reference surface of the distance measurement device can be sensed as the attachment orientation. 
     The distance measurement system according to the ninth invention is the distance measurement system according to any of the first to eighth inventions, wherein the attachment orientation sensing unit senses the attachment orientation by using the angle information and the distance information to two reference points on the reference surface. 
     Consequently, for example, the attachment orientation of the above-mentioned distance measurement device can be sensed by using information about the distance and the angle with respect to two reference points on the reference surface, such as a floor surface. The distance measurement system according to the tenth invention is the distance measurement system according to any of the first to ninth inventions, wherein the distance measurement device further includes a distance image generation unit that generates a distance image including the reference surface on the basis of the acquisition results of the distance information acquisition unit and the angle information acquisition unit. The system further comprises a distance image acquisition unit that acquires the distance image from the distance image generation unit. 
     Consequently, distance information and angle information are given to each pixel included in the acquired distance image, which means that the attachment orientation of the distance measurement device can be sensed by using specific pixels as reference points. 
     The distance measurement system according to the eleventh invention is the distance measurement system according to the tenth invention, wherein the attachment orientation sensing unit senses the attachment orientation of the distance measurement device by using a first distance to a first reference point on the reference surface at a first pixel included in the distance image acquired by the distance image acquisition unit, and a first angle with respect to the reference surface, as well as a second distance to a second reference point on the reference surface at a second pixel that is different from the first pixel, and a second angle with respect to the reference surface. 
     Consequently, the attachment orientation of the distance measurement device can be sensed by using the first distance to the first reference point and the first angle with respect to the reference surface, which the first pixel included in the distance image has as information, and the second distance to the second reference point and the second angle with respect to the reference surface, which the second pixel included in the distance image has as information. 
     The distance measurement system according to the twelfth invention is the distance measurement system according to the tenth or eleventh invention, wherein the attachment orientation sensing unit senses rotation with respect to the reference surface as the attachment orientation of the distance measurement device by using a first angle with respect to the emission axis of the light emitted from the emission unit at the first pixel included in the distance image acquired by the distance image acquisition unit, and a second angle with respect to the emission axis of the light emitted from the emission unit at a second pixel that is different from the first pixel. 
     Consequently, the attachment orientation of the distance measurement device (whether or not there is rotation with respect to the reference surface) can be sensed by using the first angle with respect to the emission axis of the light emitted from the emission unit at a first pixel included in the distance image, and the second angle with respect to the emission axis at another, second pixel. 
     The distance measurement system according to the thirteenth invention is the distance measurement system according to any of the tenth to the twelfth inventions, wherein the attachment orientation sensing unit senses the rotation of the attachment orientation of the distance measurement device on the basis of whether or not the positions of pixels at the same distance to the reference surface in the distance image acquired by the distance image acquisition unit are moving from a specific reference position. 
     Consequently, rotation of the attachment orientation of the distance measurement device can be sensed according to whether or not there is movement of the positions of pixels at the same distance to the reference surface in the distance image acquired by the distance image acquisition unit. 
     The distance measurement system according to the fourteenth invention is the distance measurement system according to any of the tenth to thirteenth inventions, wherein the attachment orientation sensing unit senses the rotation angle of the attachment orientation of the distance measurement device on the basis of how many degrees the positions of pixels at the same distance to the reference surface in the distance image acquired by the distance image acquisition unit have rotated from a specific reference position. 
     Consequently, the rotation angle of the positions of pixels at the same distance to the reference surface in the distance image acquired by the distance image acquisition unit can be sensed as the rotation angle of the attachment orientation of the distance measurement device. 
     The distance measurement system according to the fifteenth invention is the distance measurement system according to any of the first to the fourteenth inventions, further comprising a correction possibility determination unit that determines whether or not to correct the measurement result in the distance measurement device on the basis of the sensing result in the attachment orientation sensing unit. 
     Consequently, it can be determined whether or not to correct the distance information measured by the distance measurement device according to whether or not the attachment orientation (attachment angle, rotation angle, etc.) of the distance measurement device is within a specific permissible range. 
     This means that, for example, in a situation where the distance measurement device is inclined so much that the distance cannot be corrected, some other measure, such as notifying the user, can be taken without performing distance correction. 
     The distance measurement system according to the sixteenth invention is the distance measurement system according to any of the first to fifteenth inventions, further comprising a memory unit for storing information related to the attachment orientation of the distance measurement device sensed by the attachment orientation sensing unit. 
     Consequently, information related to the attachment orientation of the distance measurement device, such as the attachment angle and the rotation angle, is stored, which means that information related to the attachment orientation can be used to correct the distance information measured by the distance measurement device. 
     The distance measurement system according to the seventeenth invention is the distance measurement system according to any of the first to the sixteenth inventions, wherein the reference surface is a floor surface. 
     Consequently, the attachment orientation of the above-mentioned distance measurement device can be sensed by setting the floor surface as the reference surface, and a reference point on the floor surface. 
     The distance measurement system according to the eighteenth invention is the distance measurement system according to any of the first to the seventeenth inventions, wherein the distance measurement device is any one of a TOF (time-of-flight) sensor, a LiDAR (light detection and ranging), or an SC (structural camera). Consequently, the attachment orientation can be sensed by using the distance information and angle information measured by various distance measurement devices, such as a TOF sensor, LiDAR, or SC. 
     Effects 
     With the distance measurement system according to the present invention, the attachment orientation of a distance measurement device attached to the main body unit can be sensed without using a orientation sensing device such as an inclination sensor or a level. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an oblique view of the configuration of a conveyance system in which a TOF sensor equipped with the attachment orientation sensing device according to an embodiment of the present invention is installed on a conveyance device; 
         FIG.  2 A  is a conceptual diagram showing the configuration of a conveyance system in a state in which the conveyance device of  FIG.  1    has been put in a dock, and  FIG.  2 B  is a top view of  FIG.  2 A ; 
         FIG.  3    is a conceptual diagram showing the polar coordinates, Cartesian coordinates, and an orthogonal coordinate system parallel to the floor surface of the TOF sensor mounted on the conveyance device in  FIG.  2 A ;  FIG.  4    is a control block diagram of a TOF sensor or the like included in the conveyance system of  FIG.  1   ; 
         FIG.  5    is a diagram illustrating the principle that the TOF sensor in  FIG.  1    calculates the distance to an object by TOF method; 
         FIG.  6    is a control block diagram showing the configuration of the attachment orientation sensing device included in the TOF sensor in  FIG.  4   ; 
         FIG.  7    is a diagram illustrating the principle of sensing the attachment angle and the attachment height of the TOF sensor in the attachment orientation sensing device in  FIG.  6   ; 
         FIG.  8    is a diagram illustrating the principle of sensing the rotation angle of the TOF sensor in the attachment orientation sensing device in  FIG.  6   ; 
         FIG.  9    is a diagram illustrating the principle of sensing the rotation angle of the TOF sensor in the attachment orientation sensing device in  FIG.  6   ; 
         FIGS.  10 A and  10 B  are diagrams illustrating the principle of sensing the rotation angle of the TOF sensor in the attachment orientation sensing device in  FIG.  6   ; 
         FIGS.  11 A and  11 B  are diagrams illustrating the principle of sensing the attachment angle and the attachment height when the TOF sensor is rotated in the attachment orientation sensing device in  FIG.  6   ; 
         FIG.  12    is a flowchart of the flow of processing to sense the attachment angle and the attachment height of the TOF sensor in the attachment orientation sensing device in  FIG.  6   ; 
         FIG.  13    is a flowchart of the flow of processing to sense the rotation angle of the TOF sensor in the attachment orientation sensing device in  FIG.  6   ; 
         FIG.  14    is a flowchart of the flow of processing performed when the conveyance device in  FIG.  1    returns to the dock; 
         FIG.  15    is a diagram illustrating a state which the TOF sensor including the attachment orientation sensing device according to another embodiment of the present invention is attached to the wall of a room as a monitoring device; 
         FIG.  16    is a control block diagram showing the configuration of a conveyance system including the attachment orientation sensing device according to yet another embodiment of the present invention; and 
         FIG.  17    is a control block diagram showing the configuration of a conveyance system including the attachment orientation sensing device according to yet another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT 
     A conveyance system (distance measurement system)  50  comprising a conveyance device (specific object)  30  in which is installed a TOF sensor (distance measurement device)  20  including an attachment orientation sensing device  10  according to an embodiment of the present invention will now be described through reference to  FIGS.  1  to  14   . 
     (1) Conveyance System  50  The conveyance system (distance measurement system)  50  is a system that controls such that the conveyance device  30  shown in  FIG.  1    automatically carries out a desired conveyance operation, and comprises the conveyance device  30 , the TOF sensor (distance measurement device)  20 , the attachment orientation sensing device  10  provided in the TOF sensor  20 , and a dock (return position, charging station)  40  (see  FIG.  2 A , etc.). 
     In the conveyance system  50 , the conveyance device  30  automatically travels in the travel direction while obstacles and the like are recognized by the TOF sensor  20 , and carries out a specific conveyance operation. Once the conveyance work is finished, or when the remaining charge of the conveyance device  30  is low, for example, as shown in  FIGS.  2 A and  2 B , the conveyance device  30  is controlled so as to return to the dock  40  installed at a specific standby position (return position). 
     The attachment orientation sensing device  10  is provided in the interior of the TOF sensor  20 , and senses the attachment orientation of the TOF sensor  20  with respect to the floor surface FL by using angle information and distance information with respect to reference points P 1  and P 2  (see  FIG.  7   , etc.) on a floor surface FL detected by the TOF sensor  20 . 
     The detailed configuration of the attachment orientation sensing device  10  will be described in detail below. 
     As shown in  FIG.  1   , etc., the TOF sensor  20  is attached to the upper surface of the main body unit  31  of the conveyance device  30 , and senses information about the distance to obstacles in the travel direction of the conveyance device  30 , cargo to be conveyed, and so forth. 
     The detailed configuration of the TOF sensor  20  will be described in detail below. 
     The conveyance device (specific thing)  30  is an example of a specific thing to which the TOF sensor  20  is attached, and is, for example, an AGV (automatic guided vehicle), AMR (autonomous mobile robot), or other such automated conveyance machine that is controlled by a specific travel program. The conveyance device  30  carries out unmanned or manned conveyance work in a factory or a warehouse, for example. 
     As shown in  FIGS.  1 ,  4   , etc., the conveyance device (specific thing)  30  comprises a main body unit  31 , a drive unit  32 , wheels  32   a , forks  33 , a drive control unit  34 , a charging terminal  35 , and a secondary battery  36 . 
     The main body unit  31  is a substantially cylindrical housing, for example, and the TOF sensor  20  is attached to the upper surface thereof. Also, a plurality of the wheels  32   a  are provided, which are rotatably attached to the lower part of the main body unit  31  and allow the conveyance device  30  to move in the desired direction. 
     The drive unit  32  is an electric motor, for example, and the conveyance device  30  is made to travel in the desired direction by rotationally driving at least one of the wheels  32   a  attached to the lower part of the main body unit  31 . 
     In this embodiment, three wheels  32   a  are provided to the lower part of the main body unit  31 , and at least one of these is rotationally driven by the drive unit  32 . Also, at least one of the wheels  32   a  is provided as a steerable wheel that determines the travel direction of the conveyance device  30 . 
     The forks  33  are provided at the front of the main body unit  31 , and a load is placed on these forks during conveyance work. The forks are controlled for up and down, tilt angle, and so forth by a conveyance control unit (not shown) provided to the conveyance device  30 . 
     The drive control unit  34  controls the rotation speed and the rotation direction of the drive unit  32  that rotationally drives the plurality of wheels  32   a . This allows the conveyance device  30  to move in the desired direction at the desired speed to carry out the conveyance job. 
     As shown in  FIG.  1   , the charging terminal  35  is provided on the back side (opposite side from the forks  33 ) of the main body unit  31 . As shown in  FIGS.  2 A and  2 B , when the conveyance device  30  is connected to the dock  40 , the charging terminal  35  is connected to a connection portion  41  on the dock  40  side, and power is supplied from a charger (charging device)  42  to the conveyance device  30 . 
     As shown in  FIG.  1   , the secondary battery  36  is provided in the interior of the main body unit  31  of the conveyance device  30 . When the conveyance device  30  is connected to the dock  40 , the secondary battery  36  is repeatedly charged by the electric power supplied from the dock  40  side via the charging terminal  35 . The secondary battery  36  then supplies this stored electric power to the drive unit  32 . 
     As shown in  FIGS.  2 A and  2 B , the dock  40  is installed at a specific standby position (return position) to which the conveyance device  30  returns upon finishing a conveyance job. The conveyance device  30  is connected to the dock  40  at the standby position, and the installed secondary battery  36  is charged. 
     Also, as shown in  FIGS.  2 A and  2 B , a mark M made on the floor surface FL is disposed in front of the conveyance device  30  connected to the dock  40 . 
     The mark M has a line segment L 2  that is substantially parallel to the front surface of the conveyance device  30  provided with the forks  33 . The line segment L 2  is disposed substantially perpendicular to a straight line connecting the dock  40  and the conveyance device  30  connected to the dock  40 . 
     Consequently, the attachment orientation sensing device  10  can sense the attachment orientation of the TOF sensor  20  mounted on the conveyance device  30  by referring to the line segment L 2  of the mark M. 
     In this embodiment, an example is given in which the sensing of the attachment orientation of the TOF sensor  20 , the determination of whether or not correction is possible, the processing to correct the measured distance information, and so forth are carried out in a state in which the conveyance device  30  is connected to the dock  40 , but processing such as the sensing of the attachment orientation of the TOF sensor  20  may instead be performed in a state in which the TOF sensor  20  is not connected to the dock  40 . 
     (2) TOF Sensor  20   
     As shown in  FIG.  3   , the TOF sensor (distance measurement device)  20  is attached to the upper surface of the main body unit  31  of the conveyance device  30  so as to face downward from the horizontal plane. The TOF sensor  20  uses a preset angle table and a measured distance value to perform first coordinate transformation to transform a polar coordinate system into a rectangular coordinate system (the TOF optical axis coordinate system (X T , Y T , Z T ) indicated by the solid lines in  FIG.  3   ). Also, the TOF sensor  20  uses the attachment angle and attachment height obtained by the sensing processing described below to perform second coordinate transformation in which the TOF optical axis coordinate system (X T , Y T , Z T ) is transformed into a rectangular coordinate system that is parallel to the floor surface FL (the three axes (X TH , Y TH , Z TH ) indicated by the one-dot chain lines in  FIG.  3   ). 
     Furthermore, the TOF sensor  20  uses the rotation angle of the TOF sensor  20  obtained by the rotation angle sensing processing (discussed below) to perform third coordinate transformation in which the rectangular coordinate system (X TH , Y TH , Z TH ) parallel to the floor surface FL is matched to the rectangular coordinate system (X A , Y A , Z A ) of the conveyance device  30  to which the  20  is attached. 
     After the third coordinate transformation is performed, the conveyance device  30  (TOF sensor  20 ) is disposed such that the Z A  axis of the rectangular coordinate system is perpendicular to the above-mentioned line segment L 2  of the mark M (see  FIGS.  2 A and  2 B ). 
     The “rotation angle” of the TOF sensor  20  is an angle indicating the positional deviation in the rotation direction around the emission axis of the light emitted from an emission unit  21 . 
     As shown in  FIG.  4   , the TOF sensor  20  comprises the emission unit  21 , a light receiving lens  22 , an imaging element  23 , a control unit  24 , a memory unit  25 , and the attachment orientation sensing device  10 . The emission unit  21  has an LED, for example, and irradiates an object such as a load or the floor surface FL with light L 1  of the desired wavelength. The emission unit  21  is provided with a projection lens (not shown) that guides the light L 1  emitted from the LED toward the object. 
     The light receiving lens  22  is provided to receive the light emitted from the emission unit  21  toward the object and reflected by the object, and guide this reflected light to the imaging element  23 . 
     The imaging element  23  has a plurality of pixels, receives at each of the plurality of pixels the reflected light received by the light receiving lens  22 , and transmits a photoelectrically converted electrical signal to the control unit  24 . Also, the electrical signal corresponding to the received amount of reflected light sensed by the imaging element  23  is used by the control unit  24  to calculate distance information. The control unit  24  reads various control programs stored in the memory unit  25  and controls the emission unit  21  that irradiates the object with light. Also, the control unit  24  adjusts the exposure time of the imaging element  23  for sensing the amount of light emitted from the emission unit  21  and the amount of reflection of the light emitted from the emission unit  21 , according to the distance to the object, for example. More specifically, the control unit  24  adjusts the exposure time to be shorter when the distance to the object is short, and adjusts the exposure time to be longer when the distance to the object is long. 
     As shown in  FIG.  4   , the control unit  24  has a distance information calculation unit  24   a , an angle information acquisition unit  24   b , a distance image generation unit  24   c , and a distance correction processing unit  24   d.    
     The distance information calculation unit  24   a  calculates information about the distance to the object for each pixel, on the basis of the electrical signal corresponding to each pixel received from the imaging element  23 . 
     Here, the calculation of information about the distance to the object by the TOF sensor  20  in this embodiment will now be described with reference to  FIG.  5   . 
     Specifically, in this embodiment, so-called TOF (time-of-flight) method is used by the distance information calculation unit  24   a  to calculate the distance to the object on the basis of the phase difference D (see  FIG.  4   ) between the emitted light wave with a specific AM-modulated frequency, such as a sine wave or a square wave, emitted from the emission unit  21  and the light wave received by the imaging element  23 . 
     Here, the phase difference  0  is represented by the following relational expression (1). 
       Φ=atan(y/x)   (1)
 
     (where x=a 2 −a 0 , y=a 3 −a 1 , and a 0  to a 3  are amplitudes at points where the received light wave was sampled four times at 90-degree intervals) 
     The transformation formula from the phase difference D to the distance D is shown by the following relational formula (2). 
         D =( c /(2 ×f LED))×(Φ/2π)+DOFFSET   (2)
 
     (where c is the speed of light (≈3×108 m/s), fLED is the modulation frequency of the LED emitted light wave, and DOFFSET is the distance offset) 
     Consequently, the distance information calculation unit  24   a  can easily calculate the distance to the object by receiving the reflected light of the light emitted from the emission unit  21  and comparing the phase difference thereof, and using the speed of light c. 
     The angle information acquisition unit  24   b  acquires the angle (angle information) with respect to the emission axis of the light emitted from the emission unit  21  for each of the pixels constituting the imaging element  23  of the TOF sensor  20 . The angle information acquisition unit  24   b  can also acquire angle information for each pixel stored in the memory unit  25  as a table in advance from the memory unit  25 , for example. 
     The distance image generation unit  24   c  uses the distance information and the angle information calculated and acquired by the distance information calculation unit  24   a  and the angle information acquisition unit  24   b , respectively, to generate a distance image in which the distance information and the angle information have been assigned to each pixel. 
     The distance correction processing unit  24   d  performs correction processing as necessary, on the basis of the attachment orientation (attachment angle, rotation angle, etc.) of the TOF sensor  20  sensed by the attachment orientation sensing device  10  (discussed below), for the distance information calculated by the distance information calculation unit  24   a.    
     The memory unit  25  stores, for example, various programs for controlling the operation of the TOF sensor  20 , and also stores the distance information calculated by the distance information calculation unit  24   a , angle information corresponding to each pixel stored in advance as a table, the distance image generated by the distance image generation unit  24   c , the distance information corrected by the distance correction processing unit  24   d , and so forth. 
     (3) Attachment Orientation Sensing Device  10   
     As shown in  FIG.  4   , the attachment orientation sensing device  10  according to this embodiment is provided inside the TOF sensor  20 , and uses the angle information and the information about the distance to the reference points P 1  and P 2  on the floor surface FL sensed by the TOF sensor  20  to sense the attachment orientation of the TOF sensor  20  itself. 
     As shown in  FIG.  6   , the attachment orientation sensing device  10  comprises a distance information acquisition unit  11 , an angle information acquisition unit  12 , a distance image acquisition unit  13 , an attachment orientation sensing unit  14 , a correction possibility determination unit  15 , and a memory unit  16 , and a notification unit  17 . The distance information acquisition unit  11  acquires from the control unit  24  the information about the distance to the object calculated by the distance information calculation unit  24   a.    
     The angle information acquisition unit  12  acquires from the control unit  24  the information about the angle to the object acquired by the angle information acquisition unit  24   b.    
     The distance image acquisition unit  13  acquires from the control unit  24  the distance image generated by the distance image generation unit  24   c.    
     The attachment orientation sensing unit  14  uses the angle information and the information about the distance to the floor surface FL measured by the TOF sensor  20  to sense the attachment orientation of the TOF sensor  20  with respect to the floor surface FL. 
     More specifically, as shown in  FIG.  6   , the attachment orientation sensing unit  14  has an attachment angle sensing unit  14   a , an attachment height sensing unit  14   b , and a rotation sensing unit  14   c.    
     The attachment angle sensing unit  14   a  senses information related to the attachment angle of the TOF sensor  20  with respect to the floor surface FL as information related to the attachment orientation. More specifically, the attachment angle sensing unit  14   a  senses the attachment angle θa of the TOF sensor  20  attached to the conveyance device  30  with respect to the floor surface FL, by using angle information θ 1  and θ 2  corresponding to each of the pixels of the imaging element  23  and the measurement results up to the two reference points P 1  and P 2  (distance information d 1  and d 2 ). 
     The attachment height sensing unit  14   b  senses information related to the attachment height of the TOF sensor  20  from the floor surface FL. More specifically, the attachment height sensing unit  14   b  senses the attachment height da of the TOF sensor  20  attached to the conveyance device  30  with respect to the floor surface FL by using the angle information θ 1  and θ 2  corresponding to each of the pixels of the imaging element  23  and the measurement results up to the two reference points P 1  and P 2  (distance information d 1 , d 2 ). Here, the sensed attachment orientation (attachment angle θa, attachment height da) is calculated by using the results (d 1 , D 2 , θ 1 , θ 2 ) of measuring the distance to any two reference points P 1  and P 2  on the floor surface FL, as shown in  FIG.  7   . 
     That is, if we let: 
     da: the attachment height of the TOF sensor from the floor surface FL (where da is a vertical line at 90° to the floor surface FL), 
     θa: the angle between the floor surface FL and the optical axis of the TOF sensor  20 , 
     θ 1 : the angle of the first pixel of the TOF sensor  20  with respect to the center of the TOF (sensor specifications), 
     d 1 : the distance (measured value) from the first pixel of the TOF sensor  20  to the reference point P 1  on the floor surface FL, 
     θ 2 : the angle of the second pixel of the TOF sensor  20  with respect to the TOF center (sensor specifications), 
     d 2 : the distance from the second pixel of the TOF sensor  20  to the reference point P 2  on the floor surface FL (measured value), 
     then the following relational expressions are valid. 
       cos (θa)=da/d
 
       cos(θ a−θ 1)= da/d 1
 
       cos (θ a−θ 2)= da/d 2
 
     Consequently, the attachment height da is expressed by the following two equations, using the attachment angle θa, the angle information (θ 1 , θ 2 ), and the information about the distance (d 1 , d 2 ) to the reference points P 1  and P 2 . 
         da=d 1cos (θ a−θ 1)   (1)
 
         da=d 2cos (θ a−θ 2)   (2)
 
     Here, since θ 1  and θ 2  are known values determined by the sensor specifications, and dl and d 2  are values obtained by measurement, the attachment height da and the attachment angle θa can be calculated from the equations (1) and (2). 
     The rotation sensing unit  14   c  senses information related to the rotation angle around the optical axis of the TOF sensor  20 . More specifically, as shown in  FIG.  8   , in the rotation sensing unit  14   c , all of the pixels lying on a circle C centered on the center pixel P 0  of the frame of the distance image generated by the distance image generation unit  24   c  of the TOF sensor  20  attached to the conveyance device  30  should have the same angle of view (such as θ 1 ). Consequently, as shown in  FIG.  9   , the rotation sensing unit  14   c  calculates the rotation angle θb along with detecting whether or not there is rotation of the TOF sensor  20 , according to whether or not the position of the pixels on the circle C centered on the image center of the frame image are moving. 
     That is, as shown in  FIG.  10 A , with the rotation angle θb sensed by the rotation sensing unit  14   c , when there is no rotation of the TOF sensor  20 , the sensed distance to the pixels P 3  and P 4  intersecting the horizontal line passing through the center pixel P 0  (x 0 , yO) is the same. On the other hand, when there is rotation of the TOF sensor  20 , as shown in  FIG.  10 B , the pixels at the same sensed distance to the pixels P 3  and P 4  move by the amount of the rotation angle θb. 
     Consequently, the rotation angle θb of the TOF sensor  20  can be found from whether there is a change in the positions of the pixels P 3  and P 4  at the same distance, and the rotation angle thereof. 
     Regarding the attachment angle θa and the attachment height da when the TOF sensor  20  is rotating, as shown in  FIGS.  11 A and  11 B , the pixels of θ 1  and θ 2  can be found in the same way by letting d 1  and d 2  be the distances of the pixels at the intersections with the vertical line passing through the center of the diameter line a connecting the above-mentioned same distances and the same angle circle. 
     The correction possibility determination unit  15  determines whether to correct the measurement result (distance information) in the distance information calculation unit  24   a  of the control unit  24  on the basis of information about the installation angle and the rotation angle sensed by the attachment angle sensing unit  14   a  and the rotation sensing unit  14   c  of the attachment orientation sensing device  10 . 
     Here, a case in which correction is not possible is, for example, a case in which the attachment orientation of the TOF sensor  20  has been greatly distorted as a result of the conveyance device  30  colliding with an unexpected obstacle or the like while traveling. 
     Then, whether or not correction is possible is determined according to whether or not the attachment angle and the rotation angle sensed by the attachment angle sensing unit  14   a  and the rotation sensing unit  14   c  of the attachment orientation sensing device  10  are within the preset correctable reference range. 
     Consequently, when the sensing result in the attachment orientation sensing device  10  indicates a large amount of distortion of the attachment orientation of the TOF sensor  20 , measures can be taken such as sending a notice prompting the user to adjust the attachment orientation of the TOF sensor  20 , without correcting the distance value, which is the measurement result. 
     The memory unit  16  stores information about the attachment orientation (attachment angle, rotation angle, etc.) of the TOF sensor  20  sensed by the attachment orientation sensing unit  14 . 
     Consequently, the TOF sensor  20  can use the information related to the attachment orientation of the TOF sensor  20  stored in the memory unit  16  to correct the measurement result (distance information). 
     For example, if the correction possibility determination unit  15  has determined that the distance information cannot be corrected, it is highly probable that the attachment orientation of the TOF sensor  20  will be extremely distorted, etc., so the notification unit  17  notifies the user to adjust the attachment orientation of the TOF sensor  20 . 
     The method for sensing the attachment orientation of the TOF sensor  20  in this embodiment will now be described through reference to the flowchart shown in  FIG.  12   . 
     Here, a step of sensing the attachment angle θa and the attachment height da as the attachment orientation of the TOF sensor  20  will be described. 
     First, as shown in  FIG.  12   , in step S 11  it is determined whether or not the center pixel P 0  of the TOF sensor  20  is within the floor surface FL. Here, if the center pixel P 0  is within the floor surface FL, the processing proceeds to step S 13 , and if the center pixel P 0  is outside of the floor surface FL, the processing proceeds to step  512   a.    
     Regarding the determination in step S 11 , it is not essential that the determination be made on the basis of the center pixel, and some pixel other than the center pixel may be used, but in this embodiment the center pixel is used in order to simplify the description. 
     Here, in step  512   a , since it was determined in step S 11  that the center pixel P 0  was outside of the floor surface FL, the notification unit  17  notifies the user that information related to the attachment orientation of the TOF sensor  20  cannot be sensed. 
     Next, in step S 13 , since it was determined in step S 11  that the center pixel P 0  was within the floor surface FL, light is emitted from the emission unit  21  and the reflected light is received by the imaging element  23 , and the measured value (distance information) of the center pixel P 0  of the TOF sensor  20  is set as d. Next, in step S 14 , an arbitrary pixel P 1  having the same x coordinate as the center pixel P 0  is selected. Here, P 1  is within the floor surface FL, the angle formed by the center pixel P 0  and an arbitrary pixel P 1  is  01 , and the measured value (distance) of the arbitrary pixel P 1  is d 1  (distance and angle information acquisition step). 
     Next, in step S 15 , an arbitrary pixel P 2  having the same x coordinate as the center pixel P 0  is selected. Here, the arbitrary pixel P 2  is within the floor surface FL, the angle formed by the center pixel P 0  and the arbitrary pixel P 2  is  02 , and the measured value (distance) of the arbitrary pixel P 2  is d 2 . 
     Next, in step S 16 , as described above, the attachment angle θa and the attachment height da of the TOF sensor  20  are calculated from the following equations (1) and (2) (attachment orientation sensing step). 
         da=d 1cos (θ a−θ 1)   (1)
 
         da=d 2cos (θ a−θ 2)   (2)
 
     Next, in step S 17 , it is determined whether or not the attachment angle θa and the attachment height da of the TOF sensor  20  are within the reference range. 
     The reference range may be set as desired, according to the user&#39;s preference and to the type, shape, performance, and so forth of the TOF sensor  20 . 
     Here, in step S 12   b , since it was determined in step S 17  that the attachment angle θa and the attachment height da are outside of the reference range, the notification unit  17  notifies the user that the measurement result measured by the TOF sensor  20  cannot be corrected. 
     Next, in step S 18 , since it was determined in step S 17  that the attachment angle θa and the attachment height da are within the reference range, the attachment angle θa and the attachment height da are stored in the memory unit  16 . 
     Next, in step S 19 , the measurement result of the TOF sensor  20  is corrected on the basis of the values of the attachment angle θa and the attachment height da, and the processing is ended. 
     After step S 19 , coordinate transformation may be performed at the time of distance measurement with the TOF sensor  20  using the values for the attachment angle θa and the attachment height da. Alternatively, the user may adjust the attachment orientation of the TOF sensor  20  by referring to the values of the attachment angle θa and the attachment height da. 
     Next, the step of sensing the rotation angle θb as the attachment orientation of the TOF sensor  20  will be described with reference to  FIG.  13   . 
     First, as shown in  FIG.  13   , in step S 21 , it is determined whether or not the center pixel P 0  of the TOF sensor  20  is within the floor surface FL. Here, if the center pixel P 0  is within the floor surface FL, the processing proceeds to step S 23 , but if the center pixel P 0  is outside of the floor surface FL, the processing proceeds to step S 22   a.    
     Here, in step S 22   a , since it was determined in step S 21  that the center pixel P 0  is outside of the floor surface FL, the notification unit  17  notifies the user that information related to the attachment orientation of the TOF sensor  20  cannot be sensed. 
     Next, in step S 23 , since it was determined in step S 21  that the center pixel P 0  is within the floor surface FL, a circle C centered on the center pixel P 0  of the TOF sensor  20  is defined as the floor surface FL. 
     Next, in step S 24 , the distance values of the pixels lying on the circumference of the circle C are read (distance information acquisition step). 
     Next, in step S 25 , of the distance values obtained in step S 24 , the pixels P 3  and P 4  at the same distance are used. 
     Next, in step S 26 , it is determined whether or not the pixel P 3 , the center pixel P 0 , and the pixel P 4  are aligned on the same Y coordinate. Here, if the pixel P 3 , the center pixel P 0 , and the pixel P 4  are not aligned on the same Y coordinate, the processing proceeds to step S 28 , but if they are aligned, the processing proceeds to step S 27 . 
     Next, in step S 27 , since it was determined in step S 26  that the pixel P 3 , the center pixel P 0 , and the pixel P 4  are aligned on the same Y coordinate, the rotation of the TOF sensor  20  is judged to be zero degrees (there is no deviation in the attachment orientation in the rotation direction), and the processing is ended. At this point, the user may be notified via the notification unit  17  that there is no need for correction due to the rotation of the TOF sensor  20 . 
     Next, in step S 28 , the coordinates of the center pixel P 0  are set to (x 0 , y 0 ), and the angle formed by the straight line of Y=y 0  and the line connecting the pixels P 3 , P 0 , and P 4  is defined as the rotation angle θb in the optical axis direction (attachment orientation sensing step). 
     Next, in step S 29 , it is determined whether or not the rotation angle θb is within the reference angle range, and if it is within the reference angle range, the processing proceeds to step S 30 , but if it is outside the reference angle range, the processing proceeds to S 22   b.    
     Here, in step S 22   b , since it was determined in step S 29  that the rotation angle θb is outside of the reference angle range, the notification unit  17  notifies the user that the measurement result of the TOF sensor  20  cannot be corrected. 
     Next, in step S 30 , since it was determined in step S 29  that the rotation angle θb is within the reference angle range, the rotation angle θb is stored in the memory unit  16 . 
     Next, in step S 31 , the result (distance value) measured by the TOF sensor  20  is corrected on the basis of the value of the rotation angle θb, and the processing is ended. After step S 31 , coordinate transformation may be performed at the time of distance measurement with the TOF sensor  20  by using the rotation angle θb. Alternatively, the user may adjust the rotation angle of the TOF sensor  20  by referring to the value of the rotation angle θb. 
     Attachment Orientation Sensing Method upon Return to Dock 
     In the method for sensing the attachment orientation of the TOF sensor  20  in this embodiment, the processing performed when the conveyance system  50  returns to the dock  40  will now be described through reference to the flowchart shown in  FIG.  14   . 
     Here, the step of adjusting the attachment orientation using the attachment angle θa, the attachment height da, and the rotation angle θb sensed in a state in which the conveyance device  30  to which the TOF sensor  20  is attached has finished a specific job and returned to the dock  40  will be described. 
     First, as shown in  FIG.  14   , in step S 41 , it is determined whether or not the conveyance device  30  is recognized as being connected to the dock  40 . Here, if it is recognized that the conveyance device  30  is connected to the dock  40 , the processing proceeds to step S 43 , and if this is not recognized, the processing proceeds to step S 42 . 
     Here, in step S 42 , since it was determined in step S 41  that the conveyance device  30  is not recognized as being connected to the dock  40 , steps S 41  and S 42  are repeated until the conveyance device  30  is connected to the dock  40 . 
     Next, in step S 43 , since it was determined in step S 41  that the conveyance device  30  is connected to the dock  40 , the initial setting of the exposure time Inti of the imaging element  23  of the TOF sensor  20  is performed. 
     Next, in step S 44 , it is determined whether or not the mark M of the chart can be identified by the TOF sensor  20 . Here, if the mark M can be identified, the processing proceeds to step S 46 , and if the mark M cannot be identified, the processing proceeds to step S 45 . 
     Next, in step S 45 , since it was determined in step S 44  that the mark M of the chart cannot be identified by the TOF sensor  20 , the exposure time Inti of the imaging element  23  of the TOF sensor  20  is adjusted. This adjustment processing for the exposure time Inti is repeated until the mark M on the chart is recognized. Next, in step S 46 , since it was determined in step S 44  that the mark M of the chart can be identified by the TOF sensor  20 , the TOF sensor  20  captures an image of the floor surface FL along with the mark M made substantially parallel to the front surface of the conveyance device  30 . 
     At this point, since the TOF sensor  20  is aligned so that the front surface of the conveyance device  30  is substantially parallel to the line segment L 2  of the mark M, the attachment orientation can be sensed more accurately by measuring the distance to the two reference points P 1  and P 2  on the floor surface FL in this state. 
     Next, in step S 47 , the two reference points P 1  and P 2  are set on the imaged floor surface FL, and the attachment angle θa and the attachment height da of the TOF sensor  20  are calculated from the above equations (1) and (2) (distance information acquisition step, angle information acquisition step, and attachment orientation sensing step). Next, in step S 48 , it is determined whether or not the attachment angle θa of the TOF sensor  20  calculated in step S 47  is within the reference range. Here, if it is determined that the attachment angle θa is within the reference range, the processing proceeds to step S 50 , but if it is determined that the attachment angle θa is outside of the reference range, the processing proceeds to step S 49 . 
     The reference range may be set as desired, according to the user&#39;s preference and the type, shape, performance, and so forth of the TOF sensor  20 . 
     Next, in step S 49 , since it was determined in step S 48  that the attachment angle θa is outside of the reference range, the notification unit  17  notifies the user that the measurement result measured by the TOF sensor  20  cannot be corrected using the attachment angle θa. 
     Next, in step S 50 , since it was determined in step S 48  that the attachment angle θa is within the reference range, the above-mentioned rotation sensing unit  14   c  performs calculation processing for the rotation angle θb (attachment orientation sensing step). 
     Next, in step S 51 , it is determined whether or not the rotation angle θb is within the correctable reference angle range. If it is within the reference angle range, the processing proceeds to step S 53 , but if it is outside of the reference angle range, the processing proceeds to S 52 . 
     Here, in step S 52 , since it was determined in step S 51  that the rotation angle θb is outside of the reference angle range, the notification unit  17  notifies the user that the measurement result of the TOF sensor  20  cannot be corrected using the rotation angle θb. 
     Next, in step S 53 , since it was determined in step S 51  that the rotation angle θb is within the reference angle range, the optical axis coordinate system of the TOF sensor  20  is transformed into a rectangular coordinate system parallel to the floor surface FL. 
     More specifically, a transformation coefficient for transforming from the TOF optical axis coordinate system (X T , Y T , Z T ) indicated by the solid lines in  FIG.  3    into the three axes (X TH , Y TH , Z TH ) indicated by the one-dot chain lines in  FIG.  3    is found and is stored in the memory unit  16 . 
     Next, in step S 54 , the rectangular coordinate system parallel to the floor surface FL transformed in step S 53  is transformed into the rectangular coordinate system of the conveyance device  30 . 
     More specifically, the transformation coefficient for transforming the three axes (X TH , Y TH , Z TH ) indicated by the one-dot chain lines in  FIG.  3    into the rectangular coordinate system (X A , Y A , Z A ) of the conveyance device  30  is found and is stored in the memory unit  16 . 
     Next, in step S 55 , it is determined whether or not the difference as compared to the previous transformation coefficient is at or above a specific threshold value. Here, if the difference from the previous ratio of the transformation coefficient is at or above a specific threshold value, the processing proceeds to step S 56 , but if it is below the threshold value, it is determined that re-adjustment is unnecessary and the process is ended. 
     Next, in step S 56 , since it was determined in step S 55  that the difference from the previous ratio of the transformation coefficient is at or above a specific threshold value, the notification unit  17  notifies the user that the attachment orientation of the TOF sensor  20  has significantly deviated from that at the time of the previous adjustment. Next, in step S 57 , since it was learned that the attachment orientation of the TOF sensor  20  has significantly deviated from that at the time of the previous adjustment, the attachment angle θa, the attachment height da, and the rotation angle θb of the TOF sensor  20  attached to the conveyance device  30  are adjusted. 
     Main Features 
     The conveyance system  50  of this embodiment comprises the main body unit  31 , the TOF sensor  20 , and the attachment orientation sensing device  10 . The TOF sensor  20  has the emission unit  21  that irradiates the floor surface FL with light, the imaging element  23  that detects the light emitted from the emission unit  21 , the distance information acquisition unit  11  that acquires information about the distance to the reference points P 1  and P 2  on the floor surface FL according to the phase difference between the received light wave and the emitted light wave detected by the imaging element  23 , and the angle information acquisition unit  12  that acquires information about the angle to the reference points P 1  and P 2 . The TOF sensor  20  is mounted to the main body unit  31 . The attachment orientation sensing unit  14  senses the attachment orientation of the TOF sensor  20  with respect to the floor surface FL on the basis of the distance information and the angle information acquired by the distance information acquisition unit  11  and the angle information acquisition unit  12 . 
     Consequently, the attachment orientation of the TOF sensor  20  with respect to a reference surface such as the floor surface FL can be automatically sensed by using the results (distance information and angle information) measured by the TOF sensor  20 . 
     Therefore, the attachment orientation of the TOF sensor  20  attached to any of various devices can be sensed without performing measurement with an orientation sensing device such as an inclination sensor or a level, and the measurement result of the TOF sensor  20  can be corrected as needed, according to the deviation in the attachment orientation. Other Embodiments 
     An embodiment of the present invention was described above, but the present invention is not limited to or by the above embodiment, and various modifications are possible without departing from the gist of the invention. 
     (A) 
     In the above embodiment, an example was given in which the TOF sensor  20  (distance measurement device) was attached to the conveyance device  30 , but the present invention is not limited to this. 
     For example, the configuration may be such that the distance measurement device  120  (attachment orientation sensing device  110 ) is provided inside a surveillance camera or the like, or to a monitoring device mounted on a wall surface in a room, as shown in  FIG.  15   , instead of the conveyance device. 
     In this case, the attachment orientation of a monitoring device can be automatically sensed by disposing the device facing toward the camera optical axis AX with respect to the floor surface, so that the floor surface will serve as the reference surface. 
     Also, the attachment orientation sensing device of the present invention may be attached to another device, such as an automobile, a motorcycle, an electric bicycle, or another such vehicle. 
     (B) 
     In the above embodiment, an example was given in which the attachment orientation sensing device  10  was provided inside the TOF sensor  20 , but the present invention is not limited to this. 
     For example, the attachment orientation sensing device  10  may be configured to be provided outside the TOF sensor  20 , as shown in  FIG.  16   . 
     Alternatively, as shown in  FIG.  17   , the attachment orientation sensing device  10  may be configured to be provided inside the conveyance device  30  to which a distance measurement device such as a TOF sensor is attached. 
     (C) 
     In the above embodiment, an example was given in which the attachment angle, the attachment height, and the rotation angle with respect to the floor surface FL were sensed as the attachment orientation of the TOF sensor  20 , but the present invention is not limited to this. 
     For example, some other attachment orientation, such as twisting, may be sensed instead of the above-mentioned attachment angle. 
     (D) 
     In the above embodiment, an example was given in which the attachment angle, the attachment height, etc., of the TOF sensor  20  were sensed by using information about the distance from the TOF sensor  20  to two points on the floor surface FL, but the present invention is not limited to this. 
     For example, the attachment angle, the attachment height, etc., may be sensed by using the distance to three or more points on a reference surface such as a floor surface. 
     (E) 
     In the above embodiment, an example was given in which the floor surface FL was used as the reference surface for automatically sensing the attachment orientation of the TOF sensor  20 , but the present invention is not limited to this. 
     For example, a wall surface, a ceiling surface, or other such surface may be used instead of a floor surface as the reference surface. 
     (F) 
     In the above embodiment, an example was given in which the attachment orientation of the TOF sensor  20  was sensed by using the position where the dock  40  is installed as a specific return position, but the present invention is not limited to this. 
     For example, if the floor surface or other such reference surface does not have any inclination, etc., there will be no need to sense the attachment orientation at a specific position, and the attachment orientation may be sensed at the desired position and timing. 
     (G) 
     In the above embodiment, an example was given in which the TOF sensor  20  was used as a distance measurement device, but the present invention is not limited to this. 
     For example, instead of a TOF sensor, some other distance measurement device that can acquire information about the distance to a reference point, and that has information about the angle to the reference point, such as LiDAR (light detection and ranging) or an SC (structural camera), may be used. 
     INDUSTRIAL APPLICABILITY 
     The distance measurement system of the present invention has the effect of allowing the attachment orientation of the distance measurement device mounted on the main body unit to be sensed without having to use a device for orientation sensing, such as an inclination sensor or a level, and therefore can be widely applied to various systems that include a distance measurement device. 
     REFERENCE SIGNS LIST 
     
         
           10  attachment orientation sensing device 
           11  distance information acquisition unit 
           12  angle information acquisition unit 
           13  distance image acquisition unit 
           14  attachment orientation sensing unit 
           14   a  attachment angle sensing unit 
           14   b  attachment height sensing unit 
           14   c  rotation sensing unit 
           15  correction availability determination unit 
           16  memory unit 
           17  notification unit 
           20  TOF sensor (distance measurement device) 
           21  emission unit 
           22  light receiving lens 
           23  imaging element (sensing unit) 
           24  control unit 
           24   a  distance information calculation unit 
           24   b  angle information acquisition unit 
           24   c  distance image generation unit 
           24   d  distance correction processing unit 
           25  memory unit 
           30  conveyance device (specific thing) 
           31  main body unit 
           32  drive unit 
           32   a  wheel 
           33  forks 
           34  drive control unit 
           35  charging terminal 
           36  secondary battery 
           40  dock (return position, charging station) 
           41  connection portion 
           42  power supply unit (charging device) 
           50  conveyance system (distance measurement system) 
           110  attachment orientation sensing device 
           120  TOF sensor (distance measurement device) 
         AX optical axis 
         C circle 
         d, d 1 , d 2  distance 
         da height (distance) 
         FL floor surface (reference surface) 
         L 1  light 
         L 2  line segment 
         P 0  image center (pixel) 
         P 1 , P 2  reference point 
         P 3 , P 4  pixel 
         S 1  object 
         θ 1 , θ 2  angle information 
         θan attachment angle 
         θb rotation angle