Patent Publication Number: US-10321097-B2

Title: Perimeter monitoring device for work vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 13/818,353, filed Feb. 22, 2013, the entire disclosure of which is hereby incorporated herein by reference. Likewise, this application claims priority to Japanese Patent Application No. 2011-127476 filed on Jun. 7, 2011, and Japanese Patent Application No. 2011-129461 filed on Jun. 9, 2011, the disclosures of which are hereby incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a perimeter monitoring device, and particularly to a perimeter monitoring device configured to monitor the surrounding of a work vehicle and display a monitored result on a display device. 
     BACKGROUND ART 
     Dump trucks, used for carrying crushed stones in mines or etc., have been provided as supersized work vehicles. Compared to vehicles such as normal trucks, dump trucks of this type have a remarkably large vehicle width and a large longitudinal length. Therefore, it is difficult for operators thereof to grasp surrounding statuses by means of side mirrors and etc. In view of this, as described in U.S. Patent Application Publication No. US 2009/0259400 A1, for instance, a perimeter monitoring system is provided for efficiently checking the surrounding status of a vehicle. 
     The system disclosed in the above mentioned publication includes an obstacle detection system, an operator interface, an interface module and a controller connected to these components. When an obstacle is detected, the system is configured to display such dangerous obstacle on a display device and issue a warning to an operator. 
     On the other hand, Japan Laid-open Patent Application Publication No. JP-A-2010-198519 describes a device configured to present a risk of an obstacle within an active positional range of a working unit. It is herein detected whether or not an obstacle exists in the surrounding of a working machine. When an obstacle exists, a positional relation between a working machine main body and the obstacle is computed, and further, a risk level of making contact with the obstacle is computed based on an operational action of the working machine. Then, alert contents are configured to be outputted to a monitor and an audio output device in accordance with the risk. 
     SUMMARY 
     In the system of U.S. Patent Application Publication No. US 2009/0259400 A1, respective areas, corresponding to a plurality of blind spots in the surrounding of the vehicle, are configured to be schematically displayed with icons on a single monitor, and further, an image of a single camera capturing an obstacle is configured to be displayed on the monitor. Further, a blind area (icon), including the position that the obstacle exists, is configured to be flashed (see FIG. 7 and its related description in the specification of U.S. Patent Application Publication No. US 2009/0259400 A1). 
     However, the number of obstacles in the surrounding of a vehicle is not limited to one, and a plurality of obstacles may exist in some cases. U.S. Patent Application Publication No. US 2009/0259400 A1 does not mention a processing for such case at all. 
     On the other hand, when a plurality of obstacles exist, the device of Japan Laid-open Patent Application Publication No. JP-A-2010-198519 is configured to display all the obstacles on a monitor (see FIG. 9 and its related description in the specification of Japan Laid-open Patent Application Publication No. JP-A-2010-198519). As described in this publication, an operational range covers all the directions in the case of a working machine such as a hydraulic excavator. Therefore, it is required to display all of the plural obstacles on a monitor. 
     However, in applying a structure as described in Japan Laid-open Patent Application Publication No. JP-A-2010-198519 to a work vehicle such as a dump truck, even an object, which is not required to be recognized as an obstacle, is supposed to be displayed on a monitor and this makes it difficult to grasp an obstacle that should be normally most watched out. 
     It is an object of the present invention to enable an operator to easily grasp an object that should be most watched out when a plurality of obstacles are detected in the surrounding of a work vehicle. 
     A perimeter monitoring device for a work vehicle according to a first aspect is a device configured to monitor a surrounding of the work vehicle and display a monitored result on a display device, and includes a plurality of cameras, a bird&#39;s-eye image display unit, a plurality of obstacle detecting sensors, a camera image specifying unit and a camera image displaying unit. The plural cameras are attached to the work vehicle and obtain a plurality of sets of image data regarding the surrounding of the work vehicle. The bird&#39;s-eye image display unit displays a bird&#39;s-eye image of the surrounding of the work vehicle on the display device based on the sets of image data obtained by the plural cameras. The plural obstacle detecting sensors are attached to the work vehicle and respectively detect obstacles in the surrounding of the work vehicle. The camera image specifying unit is configured to specify a single or plurality of camera images in which a single or plurality of obstacles are shot when the single or plurality of obstacles are detected by the obstacle detecting sensors. The camera image displaying unit is configured to display a relevant camera image, ranked in a high priority ranking based on a priority order set in accordance with travelling states, in alignment with the bird&#39;s-eye image on the display device when a plurality of camera images are specified by the camera image specifying unit. 
     In the present device, the sets of camera image data regarding the surrounding of the vehicle are obtained by the plural cameras. Further, the bird&#39;s-eye image of the surrounding of the vehicle is created based on the plural sets of camera image data, and is displayed on the display device. On the other hand, when a single or plurality of obstacles are detected by the plural obstacle detecting sensors, a single or plurality of camera images are specified, in which the single or plurality of obstacles are shot. When a plurality of camera images are herein specified, a camera image, ranked in a high priority ranking based on the priority order set by travelling states, is displayed in alignment with the bird&#39;s-eye image on the display device. 
     When a plurality of obstacles are herein detected in the surrounding of the work vehicle, a plurality of cameras are supposed to exist in which the plural obstacles are shot. In such case, a single camera image, ranked in the highest priority ranking based on the priority order preliminarily set in accordance with travelling states, is displayed in alignment with the bird&#39;s-eye image. Therefore, an operator can easily grasp the obstacle that should be watched out most. 
     A perimeter monitoring device for a work vehicle according to a second aspect relates to the first aspect, and wherein the work vehicle includes an operating room disposed in a position displaced either rightwards or leftwards from a vehicle width directional center. Further, the camera image display unit is configured to display a front directional camera image on an opposite side to a side on which the operating room is disposed in alignment with the bird&#39;s-eye image on the display device when no obstacle is detected by the obstacle detecting sensors. 
     Here, especially in some of work vehicles such as supersized dump trucks, the operating room is disposed in a position displaced either rightwards or leftwards from the vehicle center. It is quite difficult for an operator of such work vehicle to grasp the status of the opposite side to the side on which the operating room is disposed. 
     In view of the above, in the perimeter monitoring device of the second aspect, the front directional camera image on the opposite side to the side on which the operating room is disposed is configured to be displayed in alignment with the bird&#39;s-eye image when it is not detected that an obstacle exists. 
     With a camera image, it is herein possible to easily grasp an area, the status of which can be hardly grasped by an operator. 
     A perimeter monitoring device for a work vehicle according to a third aspect relates to the device of the first or second aspect, and wherein the bird&#39;s-eye image display unit is configured to set a plurality of areas corresponding to the plural cameras on a one-to-one basis in the bird&#39;s-eye image and display the respective areas in a sectioned manner with frames. Also, the perimeter monitoring device further includes a highlighted frame displaying unit configured to highlight one or more relevant ones of the frames, respectively sectioning one or more areas in which one or more obstacles are located, when the one or more obstacles are detected by the obstacle detecting sensors. 
     In the present device, the bird&#39;s-eye image has a plurality of areas set correspondingly to the plural cameras, and the respective areas are sectioned by the frames. Further, the frame sectioning the area in which an obstacle is positioned is highlighted. Therefore, the position of the obstacle can be more easily and quickly checked. 
     A perimeter monitoring device for a work vehicle according to a fourth aspect relates to the device of any of the first to third aspects, and further includes a warning unit configured to emit different types of alert sounds in accordance with travelling states and obstacle positions when one or more obstacles are detected by the obstacle detecting sensors. 
     Here, a warning is issued when an obstacle is detected. Therefore, existence of the obstacle can be easily and quickly checked. Further, different types of alert sounds are emitted depending on the travelling states and the obstacle positions. Therefore, an operator can easily grasp the magnitude of risk based on an alert sound, for instance, when different types of alert sounds are set in accordance with the magnitude of risk of the obstacle. 
     In the present invention as described above, when a plurality of obstacles are detected in the surrounding of a work vehicle, it is possible to easily grasp an obstacle that should be most watched out. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an entire structure of a dump truck including a perimeter monitoring device according to an exemplary embodiment of the present invention. 
         FIG. 2  is a block diagram representing a configuration of the perimeter monitoring device included in the dump truck. 
         FIG. 3  is a diagram illustrating attached positions of and shooting ranges of six cameras. 
         FIG. 4  is a diagram illustrating attached positions of and detection ranges of eight radar devices. 
         FIG. 5  is a diagram illustrating a display example of a monitor on which a bird&#39;s-eye image and a camera image are displayed. 
         FIG. 6  is a diagram illustrating areas, sectioned correspondingly to the plural cameras, in the bird&#39;s-eye image. 
         FIG. 7  is a flowchart of a display control processing. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     An exemplary embodiment of the present invention will be hereinafter explained with reference to drawings. It should be noted that in the following explanation, “front”, “rear”, “left” and “right” are terms set based on an operator seated on an operator seat and “a vehicle width direction” is a synonym for “a right-and-left direction”. 
     Overall Structure of Dump Truck 
       FIG. 1  is a perspective view illustrating an entire structure of a dump truck  1  including a perimeter monitoring device according to an exemplary embodiment of the present invention. The dump truck  1  is a self-propelled supersized work vehicle to be used for mining works and etc. 
     The dump truck  1  mainly includes a vehicle body frame  2 , a cab  3  as an operating room, a vessel  4 , two front wheels  5 , two rear wheels  6  and a base  7  for installing thereon a pantograph for power supply. Further, the present dump truck  1  includes a perimeter monitoring device  10  (see  FIG. 2 ) configured to monitor the surrounding of the vehicle and display the result on a monitor. The structure and the action of the perimeter monitoring device  10  will be described below. 
     The vehicle body frame  2  supports components (not illustrated in the figures) such as power mechanisms (a diesel engine, a transmission, etc.) and other auxiliary machineries. Further, the front wheels  5  (only a right front wheel is illustrated in  FIG. 1 ) are supported on the right and left sides of the front part of the vehicle body frame  2 , while the rear wheels  6  (only a right rear wheel is illustrated in  FIG. 1 ) are supported on the right and left sides of the rear part thereof. The vehicle body frame  2  includes a lower deck  2 A on a side closer to the ground, and includes an upper deck  2 B over the lower deck  2 A. For example, movable ladders  2 C are mounted between the lower deck  2 A and the ground, whereas an oblique ladder  2 D is mounted between the lower deck  2 A and the upper deck  2 B. Fence-like banisters are fixed to the right and left portions of the front part, the lateral parts and a portion of the rear part on the upper deck  2 B. 
     The cab  3  is disposed on the upper deck  2 B while being displaced leftwards from the center in the vehicle width direction. An operator seat, a shift lever, a controller for display, a monitor, a handle, an accelerator pedal, a brake pedal and etc. are mounted within the cab  3 . As described below, the controller, the monitor and the shift lever form a part of the perimeter monitoring device  10 . 
     The vessel  4  is a container for loading heavy loads such as crushed stones. The rear-side bottom part of the vessel  4  is pivotally coupled to the rear end portion of the vehicle body frame  2  through pivot pins (not illustrated in the figures) attached to the right and left sides thereof. Accordingly, an actuator such as a hydraulic cylinder (not illustrated in the figure) can cause the vessel  4  to take an uprising posture in which the front part thereof is upwardly pivoted for discharging loads and a loading posture in which the front part thereof is located on the upper part of the cab as illustrated in  FIG. 1 . 
     Structure of Perimeter Monitoring Device  10   
       FIG. 2  is a block diagram representing a configuration of the perimeter monitoring device  10  included in the dump truck  1 . The perimeter monitoring device  10  includes six cameras  11  to  16 , eight radar devices  21  to  28 , a vehicle information detecting part  30 , an obstacle information collecting part  32 , an obstacle processing part  33  and a monitor  34  disposed forwards of the operator seat within the cab  3 . Further, the perimeter monitoring device  10  includes a display controller  40  configured to generate an image to be displayed on the monitor  34  based on sets of camera image data from the cameras  11  to  16  and a set of data from the obstacle processing part  33 . It should be noted that frame memories  11 A to  16 A are respectively mounted between the display controller  40  and the respective cameras  11  to  16  in order to temporarily store camera images. Further, an operator switch  41 , mounted within the cab  3 , is connected to the display controller  40 . The operator switch  41  includes a boot switch, a switch for specifying which one should be displayed on the monitor  34  among a plurality of camera images, and etc. 
     Cameras 
     The six cameras  11  to  16  are attached to the outer periphery of the dump truck  1  for obtaining images of the surrounding of the dump truck  1 .  FIG. 3  is a plan view of the dump truck  1  and illustrates the attached positions and the shooting ranges of the six cameras  11  to  16 . It should be noted that the shooting ranges are represented by converting actual camera shooting ranges into the respective ranges on a bird&#39;s-eye image (see  FIG. 6 ). 
     The first camera  11  is disposed on the upper end portion of the oblique ladder  2 D, and a first shooting range  11 C thereof covers the forward direction of the vehicle. The second camera  12  is disposed on the right end portion of the front-side lateral surface of the upper deck  2 B, and a second shooting range  12 C thereof covers the obliquely right forward direction of the vehicle. The third camera  13  is disposed in a position bilaterally symmetric to the second camera  12 , i.e., on the left end portion of the front-side lateral surface of the upper deck  2 B, and a third shooting range  13 C thereof covers the obliquely left forward direction. The fourth camera  14  is disposed on the front end portion of the right-side lateral surface of the upper deck  2 B, and a fourth shooting range  14 C thereof covers the obliquely right rear direction. The fifth camera  15  is disposed in a position bilaterally symmetric to the fourth camera  14 , i.e., on the front end portion of the left-side lateral surface of the upper deck  2 B, and a fifth shooting range  15 C thereof covers the obliquely left rear direction. The sixth camera  16  is disposed above an axle shaft coupling the two rear wheels  6  while being disposed in the vicinity of the rotational axis of the vessel  4 , and a sixth shooting range  16 C thereof covers the rear direction. 
     According to the aforementioned six cameras  11  to  16 , it is possible to obtain an image of the roughly entire surrounding of the dump truck  1  as illustrated in a center diagram of  FIG. 3 . Each of the six cameras  11  to  16  is configured to transmit a set of data of a camera image, i.e., an image shot by each camera, to the display controller  40  through a corresponding one of the frame memories  11 A to  16 A. 
     Radar Devices 
     The eight radar devices  21  to  28  are attached to the outer periphery of the dump truck  1  and are configured to detect relative positions of obstacles existing in the surrounding of the dump truck  1 .  FIG. 4  is a plan view of the dump truck  1  and illustrates attached positions and detection ranges of the eight radar devices  21  to  28 . It should be noted that the detection ranges are represented by converting actual detection ranges of the radar devices into ranges on the bird&#39;s-eye image (see  FIG. 6 ). 
     The first radar device  21  is disposed on the lower deck  2 A, and a first detection range  21 R thereof covers a range from the front direction to the obliquely left front direction of the vehicle. The second radar device  22  is disposed leftwards of the first radar device  21 , and a second detection range  22 R thereof covers a range from the front direction to the obliquely right front direction of the vehicle. The third radar device  22  is disposed on the front end portion of the right-side lateral surface of the lower deck  2 A, and a third detection range  23 R thereof covers a range from the obliquely right front direction to the right lateral direction. The fourth radar device  24  is disposed laterally to the vehicle while being disposed in the middle of the lower deck  2 A and the upper deck  2 B, and a fourth detection range  24 R thereof covers a range from the right lateral direction to the rear direction. The fifth radar device  25  is disposed in the vicinity of an axle shaft coupling the two rear wheels  6 , and a fifth detection range  25 R thereof covers a range from the obliquely right rear direction to the rear direction. The sixth radar device  26  is disposed rightwards of the fifth radar device  25 , and a sixth detection range  26 R thereof covers a range from the rear direction to the obliquely left rear direction. The seventh radar device  27  is disposed in a position bilaterally symmetric to the fourth radar device  24 , and a seventh detection range  27 R thereof covers a range from the rear direction to the left lateral direction. The eighth radar device  28  is disposed in a position bilaterally symmetric to the third radar device  22 , and an eighth detection range  28 R thereof covers a range from the left lateral direction to the obliquely left front direction. 
     According to the aforementioned eight radar devices  21  to  28 , relative positions of obstacles with respect to the dump truck  1  can be detected over the roughly entire surrounding of the dump truck  1  as illustrated in a center diagram of  FIG. 4 . The eight radar devices  21  to  28  are respectively configured to transmit sets of data regarding detected obstacles to the obstacle processing part  33  through the obstacle information collecting part  32 . 
     Vehicle Information Detecting Part 
     The vehicle information detecting part  30  is configured to detect the operating position of the shift lever disposed in the periphery of the operator seat within the cab  3  and the vehicle speed to be obtained by a vehicle speed sensor. Further, the vehicle information detection part  30  is configured to transmit these sets of data to the display controller  40 . 
     Obstacle Information Collecting Part and Obstacle Processing Part 
     The obstacle information collecting part  32  is configured to collect pieces of information received by the respective radar devices  21  to  28  and output the pieces of information to the obstacle processing part  33 . The obstacle processing part  33  is configured to compare values of preliminarily set parameters and preliminarily set thresholds and output a piece of obstacle information to the display controller  40 . A relative speed between the dump truck  1  and an object, a relative angle therebetween, a relative distance therebetween, and an intensity of a signal from the obstacle (intensity of a radar reflected signal) are set as parameters. The positional coordinate of an object (obstacle) is configured to be computed based on the signal from the object, when the relative speed, the relative angle and the relative distance, amongst these parameters, have values less than or equal to the preliminarily set thresholds while the signal intensity exceeds its threshold. Further, it is determined whether or not the positional coordinate is included in a range that an object is considered as an obstacle and therefore a warning should be issued. When the positional coordinate is included in the warning range, the fact is outputted to the display controller  40  as a piece of obstacle detection information. 
     Display Controller 
     The display controller  40  includes a bird&#39;s-eye image synthesizing part  42 , a camera image switching part  43 , an image display controlling part  44  and a monitor image generating part  45 . 
     The bird&#39;s-eye image synthesizing part  42  is configured to receive a plurality of sets of camera image data from the respective six cameras  11  to  16 . Then, the bird&#39;s-eye image synthesizing part  42  is configured to synthesize the plural sets of camera image data in order to generate a bird&#39;s-eye image  50  regarding the surrounding of the dump truck  1  as illustrated in the left part of  FIG. 5 . Specifically, the bird&#39;s-eye image synthesizing part  42  is configured to execute coordinate conversion with respect to the respective plural sets of camera image data in order to generate a set of bird&#39;s-eye image data representing the bird&#39;s-eye image  50  in which a plurality of camera images are projected onto a predetermined projection plane. In  FIG. 5 , the bird&#39;s-eye image is displayed on the left part of the single monitor  34 , while a single camera image  52  (herein, a front view, i.e., a real-time image being shot by the first camera  11 ) is displayed on the right part thereof. 
     It should be noted that a plurality of areas, corresponding to shooting ranges  11 C to  16 C of the respective cameras  11  to  16 , are sectioned and displayed on the bird&#39;s-eye image  50 .  FIG. 6  illustrates the sectioned areas. In  FIG. 6 , “FRONT”, “RIGHT FRONT”, “LEFT FRONT”, “RIGHT REAR”, “LEFT REAR” and “REAR” respectively correspond to the shooting ranges  11 C to  16 C of the first to sixth cameras  11  to  16 . 
     The camera image switching part  43  is configured to select a camera image to be displayed in alignment with the bird&#39;s-eye image. Specifically, the image display controlling part  44  controls and causes the camera image switching part  43  to select one of the plural camera images based on predetermined options. The processing of selecting one of the plural camera images will be described below. 
     The image display controlling part  44  is configured to receive pieces of information from the operator switch  41  and the vehicle information detecting part  30 , and accordingly, control the camera image switching part  43  and the monitor image generating part  45 . Further, the image display controlling part  44  is configured to receive the piece of information from the obstacle processing part  33 , and accordingly, execute a control of highlighting a frame  54  (see  FIG. 5 ), enclosing an area in which an obstacle exists, on the bird&#39;s-eye image  50 . Frames, enclosing the respective areas, are normally displayed with dashed lines. It should be noted that the respective areas are basically determined based on the shooting ranges  11 C to  16 C of the respective cameras  11  to  16 , but sizes and shapes thereof can be arbitrarily set. 
     The monitor image generating part  45  is configured to generate a monitor image so that a set of bird&#39;s-eye image data obtained by the bird&#39;s-eye image synthesizing part  42  and a single camera image selected by the camera image switching part  43  can be displayed in alignment with each other on the single monitor  34 . The set of monitor image data, generated by the monitor image generating part  45 , is configured to be outputted to the monitor  34 . 
     Image Display Control 
     A control processing of displaying an image on the monitor  34  will be explained with respect to the flowchart of  FIG. 7 . It should be noted that the following sets of data have been preliminarily set and stored in a storage part (not illustrated in the figures) in executing the image display control processing. 
     Preliminarily Set Data 
     Vehicle speed: speed V 1 —a threshold for determining whether or not an image should be displayed on the monitor  34 . 
     Priority order in detecting obstacles: the following Table 1 represents a priority order of cameras to be displayed on the monitor  34  when a plurality of cameras are shooting obstacles. Reference signs enclosed with parentheses “( )” represent the shooting ranges of the respective cameras  11  to  16 . Further, “HIGH”, “MIDDLE” and “LOW” represent types of alert sounds for an operator. “HIGH” is a type of alert sounds for calling the strongest attention of an operator and the sound on/off cycle thereof is set to be the shortest. The sound on/off cycle of “MIDDLE” is set to be longer than that of “HIGH”. The sound on/off cycle of “LOW” is the longest, and the warning level of “LOW” is the weakest. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 FORWARD 
                 REARWARD 
                   
               
               
                   
                 TRAVELLING 
                 TRAVELING 
               
               
                   
                 MODE 
                 MODE 
                 STOP 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 1ST 
                 FRONT 
                 REAR 
                 BACK-AND- 
               
               
                 RANKING 
                 (11C) • HIGH 
                 (16C) • HIGH 
                 FORTH 
               
               
                   
                   
                   
                 DIRECTION • 
               
               
                   
                   
                   
                 MIDDLE 
               
               
                 2ND 
                 RIGHT FRONT 
                 RIGHT REAR 
                 OTHER THAN 
               
               
                 RANKING 
                 (12C) • HIGH 
                 (14C) • HIGH 
                 BACK-AND- 
               
               
                   
                   
                   
                 FORTH 
               
               
                   
                   
                   
                 DIRECTION • 
               
               
                   
                   
                   
                 LOW 
               
               
                 3RD 
                 RIGHT FRONT 
                 LEFT REAR 
               
               
                 RANKING 
                 (13C) • HIGH 
                 (15C) • HIGH 
               
               
                 4TH 
                 RIGHT REAR 
                 RIGHT FRONT 
               
               
                 RANKING 
                 (14C) • MIDDLE 
                 (12C) • MIDDLE 
               
               
                 5TH 
                 LEFT REAR 
                 LEFT FRONT 
               
               
                 RANKING 
                 (15C) • MIDDLE 
                 (13C) • MIDDLE 
               
               
                 6TH 
                 REAR 
                 FRONT 
               
               
                 RANKING 
                 (16C) • MIDDLE 
                 (11C) • MIDDLE 
               
               
                   
               
            
           
         
       
     
     Control Processing 
     First, in Step S 1 , a variety of data are loaded from the outside. Specifically, the following sets of data are loaded.
         Images: sets of camera image data of the respective cameras  11  to  16 .   Vehicle information: the operating position of the shift lever and the vehicle speed.   Operator SW information: a piece of operating information when the operator switch  41  is operated.   Obstacle information: pieces of obstacle information from the respective radar devices  21  to  28 .       

     Next in Step S 2 , it is determined whether or not the vehicle speed exceeds V 1 . The processing proceeds to Step S 3  when the vehicle speed exceeds V 1 . In Step S 3 , the respective images are set not to be displayed on the monitor  34 , while a warning as an alert sound is set not to be issued. The settings are done for making an operator to concentrate in driving while the vehicle is travelling at a vehicle speed greater than a predetermined vehicle speed. Further, with the Step S 3 , no alert sound is emitted while the vehicle is travelling at a vehicle speed greater than the vehicle speed V 1  even if an oncoming vehicle is recognized as an obstacle. 
     The processing proceeds to Steps S 4  to S 6  when the vehicle speed is less than V 1 . In Step S 4 , a bird&#39;s-eye image is generated based on the respective sets of camera image data. Further in Step S 5 , a camera image to be displayed on the monitor  34  is selected. The processing of selecting a camera image is executed based on the processing of Step S 6  and its subsequent Steps. 
     In Step S 6 , it is determined whether or not a specific camera is selected by the operator. The processing proceeds from Step S 6  to Step S 10  when the specific camera is selected by the operator. In Step S 10 , the camera specified by the operator is selected and the processing proceeds to Step S 11 . 
     On the other hand, the processing proceeds from Step S 6  to Step S 20  when no camera is selected by the operator. In Step S 20 , the operating position of the shift lever is determined. When the operating position of the shift lever is “FPRWARD TRAVELLING” and the vehicle speed exceeds “0”, the processing proceeds to Step S 21  and the second camera  12  shooting the right front direction is selected. When the operating position of the shift lever is “REARWARD TRAVELLING”, the processing proceeds to Step S 22  and the sixth camera  16  shooting the rear direction is selected. Except for the aforementioned states, the processing proceeds to Step S 23  and the first camera  11  shooting the front direction is selected. For example, when the operating position of the shift lever is “FORWARD TRAVELLING” but the vehicle speed is “0”, the first camera  11  shooting the front direction is selected. The processing proceeds to Step S 11  after the aforementioned processing. 
     In Step S 11 , it is determined whether or not an obstacle is detected. The processing proceeds from Step S 11  to Step S 5  when no obstacle is detected. 
     The processing proceeds from Step S 11  to Steps S 30  and  31  when an obstacle is detected. In Step S 30 , it is determined whether or not the obstacle is detected by a plurality of radar devices. The processing proceeds from Step S 30  to Step S 32  when the obstacle is detected by a single radar device. In Step S 32 , a camera shooting the obstacle is specified and selected based on the set of positional data of the obstacle detected by the radar device, and the processing proceeds to Step S 5 . 
     On the other hand, the processing proceeds from Step S 30  to Step S 33  when a plurality of obstacles are detected by a plurality of radar devices. In Step S 33 , a single camera ranked highest in the priority order is selected based on Table 1 for determining the priority order, and the processing then proceeds to Step S 5 . 
     Based on the above, in Step S 5 , a camera image to be displayed in alignment with the bird&#39;s-eye image on the monitor  34  is supposed to be selected in accordance with the following procedure. 
     Option 1: when no obstacle is detected and a camera is specified by the operator, the camera image of the camera specified by the operator will be selected. 
     Option 2: when no obstacle is detected and no camera is specified by the operator, the camera image of the camera selected in accordance with the operating position of the shift lever and the vehicle speed will be selected. 
     Option 3: when a single obstacle is detected, the camera image of the camera shooting the obstacle will be selected. 
     Option 4: when a plurality of obstacles are detected by the shooting ranges of a plurality of cameras, the camera image of the camera ranked highest in the preliminarily set priority order will be selected. 
     With the aforementioned procedure, a monitor image is generated in Step S 35  based on the bird&#39;s-eye image obtained in Step S 4  and the selected camera&#39;s camera image obtained in Step S 5 . Further in Step S 35 , the frames enclosing the areas in each of which an obstacle exists are highlighted amongst the six sections illustrated in  FIG. 5 . 
     Further in Step S 31 , an alert sound of a predetermined type is set in accordance with the travelling state and the position in which an obstacle is detected. The alert sound type is set based on Table 1. For example, when an obstacle is detected in the front direction during forward travelling, an alert sound type is set that has a short on/off cycle and is harsh to the ear of the operator. Further, a control of emitting an alert sound set in Step S 31  is executed aside from the display control represented in  FIG. 7 . 
     Through the aforementioned control processing, the monitor  34  displays the following content. 
     (1) Display in Accordance with Shift Lever Position (Nonexistence of Obstacle) 
     The bird&#39;s-eye image and the front directional camera image are displayed in alignment with each other on the monitor  34  when the shift lever is operated from “STOP” to “FORWARD TRAVELLING” and the vehicle speed is “0”. 
     When the vehicle starts moving forwards, the camera image on the monitor  34  is switched from the front directional camera image to the right front directional camera image. In other words, the bird&#39;s-eye image and the right front directional camera image are displayed in alignment with each other on the monitor  34 . When the vehicle speed is then further increased and exceeds the vehicle speed V 1 , the bird&#39;s-eye image and a camera image are no longer displayed on the monitor  34 . 
     When the shift lever is operated to “REARWARD TRAVELLING”, the bird&#39;s-eye image and the rear directional camera image are displayed on the monitor  34 . 
     (2) Display in Obstacle Detection 
     When a single obstacle is detected, the bird&#39;s-eye image and the image of the camera shooting the obstacle are displayed in alignment with each other on the monitor  34 . At this time, in the bird&#39;s-eye image, the frame of the area corresponding to the shooting range of the camera shooting the obstacle is highlighted while being displayed with a red bold line and blinked. 
     By contrast, when a plurality of obstacles are detected, the monitor  34  displays the bird&#39;s-eye image on the left part thereof. On the other hand, the monitor  34  displays the image of the one ranked highest in the priority order based on Table 1 amongst cameras respectively shooting the plural obstacles on the right part thereof. At this time, in the bird&#39;s-eye image, the frames of the plural areas corresponding to the shooting ranges of the plural cameras respectively shooting the plural obstacles are highlighted while being displayed with red bold lines and blinked. 
     In the example illustrated in  FIG. 5 , obstacles are detected in three areas “FRONT”, “RIGHT FRONT” and “LEFT FRONT”. These obstacles are respectively shot by the first camera  11  shooting the front direction, the second camera  12  shooting the right front direction and the third camera  13  shooting the left front direction. Therefore, the frames  54  enclosing the three areas are respectively highlighted in the bird&#39;s-eye image  50 . Further, the camera image of “FRONT” ranked highest in the priority order based on Table 1, i.e., the camera image of the first camera  11  is displayed on the right part of the monitor  34 . 
     Further, in the example of  FIG. 5 , an obstacle is detected in the front direction. Therefore, based on Table 1, an alert sound is emitted that has a short on/off cycle and is harsh to the ear of the operator. Accordingly, the operator can find out that an obstacle-related risk is high. 
     (1) When an obstacle is detected in the surrounding of the dump truck  1 , the frame enclosing the area in which the obstacle exists is highlighted in the bird&#39;s-eye image, and the image of the camera shooting the obstacle is displayed in alignment with the bird&#39;s-eye image on the monitor  34 . Therefore, an operator can easily grasp existence of the obstacle. 
     Further, when a plurality of obstacles are detected, the frames of the areas in which the obstacles exist are highlighted in the bird&#39;s-eye image, and the camera image, ranked in the highest priority ranking based on a priority order set in accordance with travelling states, is displayed in alignment with the bird&#39;s-eye image on the display device. Therefore, an operator can easily grasp an obstacle that should be watched out most. 
     (2) When no obstacle is detected, the frontal directional camera image on the opposite side of the cab mounted site is displayed in alignment with the bird&#39;s-eye image on the monitor  34 . Therefore, through the camera image, an operator can easily grasp the area, the status of which can be hardly grasped by the operator. 
     (3) When an obstacle is detected, different types of alert sounds are emitted in accordance with the travelling states and the positions of the obstacle. Therefore, an operator can easily grasp the magnitude of risk by the alert sound. 
     Other Exemplary Embodiments 
     The present invention is not limited to the exemplary embodiment as described above, and a variety of changes or modifications can be made without departing from the scope of the present invention. 
     (A) The priority order of cameras, used when a plurality of obstacles are detected or used in accordance with the travelling state, is not limited to that set in the aforementioned exemplary embodiment. The priority order of cameras may be arbitrarily set in accordance with the arrangement of the cab or etc. 
     (B) In the aforementioned exemplary embodiment, a bird&#39;s-eye image and a camera image are configured to be displayed in alignment with each other on a single monitor. However, two monitors may be disposed in alignment with each other, and a bird&#39;s-eye image and a camera image may be separately displayed on the respective monitors. 
     In the perimeter monitoring device of the illustrated embodiment, when a plurality of obstacles are detected in the surrounding of a work vehicle, it is possible to easily grasp the obstacle that should be watched out most.