Patent Publication Number: US-2015066199-A1

Title: Robot hand, robot system, and method for depalletizing article

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present disclosure contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2013-182568 filed in the Japan Patent Office on Sep. 3, 2013, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     1. Field of the Invention 
     The embodiments disclosed herein relate to a robot hand, a robot system, and a method for depalletizing an article. 
     2. Description of the Related Art 
     Japanese Unexamined Patent Application Publication No. 2001-317911 discusses an article position recognizing device that detects the position of an article and allows the article to be automatically depalletized. In the article position recognizing device, using contour data of a top article detected by an image processor, a rough position of the top article is determined to recognize the position of the article. 
     SUMMARY 
     According to an aspect of the disclosure, there is provided a robot hand of a robot that handles an article. The robot hand includes a baseplate; a holding member that is disposed at the baseplate and that is configured to hold the article; and a plurality of proximity sensors that are disposed at the baseplate, each of the proximity sensors being configured to detect whether or not the article exists at a side of the holding member. 
     According to another aspect of the disclosure, there is provided a robot system including a robot that handles an article; the robot hand according to the aspect of the robot; and a controller that is configured to control an operation of the robot and an operation of the robot hand. 
     According to still another aspect of the disclosure, there is provided a method for depalletizing an article using a robot including a robot hand that includes a baseplate, a plurality of attraction pads, and a plurality of proximity sensors. The method includes attracting and handling the article using the attraction pad or attraction pads in a predetermined area; recognizing external-form information of the article on the basis of a detection result of the proximity sensor or proximity sensors in an operated state; setting an area of the attraction pad or attraction pads that perform attraction on the basis of the recognized external-form information; and re-attracting and handling the article using the attraction pad or attraction pads in the set area. The plurality of attraction pads are disposed so as to be interspersed in a direction of a surface of the baseplate and are each configured to attract a top surface of the article. The plurality of proximity sensors are disposed at the baseplate, each proximity sensor being configured to detect whether or not the article exists at a side of the holding member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic top view of an exemplary overall structure of a robot system according to an embodiment. 
         FIG. 2  is a schematic side view of the exemplary overall structure of the robot system according to the embodiment. 
         FIGS. 3A ,  3 B, and  3 C are, respectively, a top view, a bottom view, and an end view taken along line IIIC-IIIC of an exemplary structure of a robot hand. 
         FIG. 4  is an explanatory view of a reflective photoelectric sensor. 
         FIG. 5  is an explanatory view of a transmissive photoelectric sensor. 
         FIG. 6  is a block diagram of an exemplary functional structure of a robot controller. 
         FIGS. 7A and 7B  are each an explanatory view of an example of attracting and holding an article when an image recognition error occurs. 
         FIG. 8  is a flowchart of an example of a control procedure performed by the robot controller when an image recognition error occurs. 
     
    
    
     DESCRIPTION OF THE EMBODIMENT 
     An embodiment is hereunder described with reference to the drawings. The terms “front”, “back”, “left”, “right”, “top”, and “bottom” in the description of the specification correspond to directions labeled “front”, “back”, “left”, “right”, “top”, and “bottom” in the drawings. 
     1. Overall Structure of Robot System 
     First, an overall structure of a robot system  1  according to an embodiment is described with reference to  FIGS. 1 and 2 . 
     As shown in  FIGS. 1 and 2 , the robot system  1  according to the embodiment includes a first robot  1 , a second robot  3 , a robot controller  4  (controller), an image processor  5 , and a conveyor  6 . The robot system  1  depalletizes a plurality of articles W that are stacked on a pallet P one at a time from the pallet P. Depending upon the size of the articles W, the articles W may be depalletized two or more articles at a time from the pallet P. 
     The first robot  2  is a robot that handles an article W. The first robot  2  includes a robot hand  10  that is provided with, for example, attraction pads  22  (holding members) as a working tool. The first robot  2  is provided near the pallet P (that is, on the left of the pallet P in the example shown in  FIG. 1 ). The second robot  3  includes a robot hand  20  that is provided with a camera  7  and a laser sensor  8 . The second robot  3  is provided near the pallet P (that is, on the right of the pallet P in the example shown in  FIG. 1 ). 
     The robot controller  4  is formed so as to control the operations of the robots  2  and  3  and the robot hands  10  and  20  of the respective robots  2  and  3 . 
     The first robot  2  and the second robot  3  have basically the same structure except that the structures of the robot hands  10  and  20  differ from each other. Therefore, here, the first robot  2  is described. The main portions of the second robot  3  are given reference numerals and are not described. The first robot  2  corresponds to a robot in the claims, and the robot hand  10  corresponds to a robot hand in the claims. 
     The first robot  2  includes a base  12  that is fixed at a setting location (floor (not shown) in this example) of a working place where a depalletizing operation is performed, a rotary member  13  that is rotatably mounted on a top end portion of the base  12 , and an arm  14  that is mounted on the rotary member  13 . 
     The rotary member  13  is provided on the top end portion of the base  12  so as to be rotatably in a horizontal plane. An actuator Ac1 that rotates and drives the rotary member  13  is placed at or near a connection portion of the rotary member  13  and the base  12 . 
     The arm  14  is formed by connecting a first arm portion  14   a , a second arm portion  14   b , a third arm portion  14   c , a fourth arm portion  14   d , and a fifth arm portion  14   e  in that order from a base end side  13  at the side of the rotary member  13  towards a front end side that is opposite to the base end side  13 . 
     The first arm portion  14   a  is connected to a top end portion of the rotary member  13  so as to be rotatable in a vertical plane. The second arm portion  14   b  is connected to the first arm portion  14   a  so as to be rotatable in a vertical plane. The third arm portion  14   c  is connected to the second arm portion  14   b  so as to be rotatable in a plane that is perpendicular to a direction of extension of the second arm portion  14   b . The fourth arm portion  14   d  is connected to the third arm portion  14   c  so as to be rotatable in a vertical plane. The fifth arm portion  14   e  is connected to the fourth arm portion  14   d  so as to be rotatable in a plane that is perpendicular to a direction of extension of the fourth arm portion  14   d.    
     An actuator Ac2 that rotates and drives the first arm portion  14   a  is provided at or near a connection portion of the first arm portion  14   a  and the rotary member  13 . An actuator Ac3 that rotates and drives the second arm portion  14   b  is provided at or near a connection portion of the second arm portion  14   b  and the first arm portion  14   a . An actuator Ac4 that rotates and drives the third arm portion  14   c  is provided at or near a connection portion of the third arm portion  14   c  and the second arm portion  14   b . An actuator Ac5 that rotates and drives the fourth arm portion  14   d  is provided at or near a connection portion of the fourth arm portion  14   d  and the third arm portion  14   c . An actuator Ac6 that rotates and drives the fifth arm portion  14   e  is provided at or near a connection portion of the fifth arm portion  14   e  and the fourth arm portion  14   d . The structural form of the first robot  2  and the second robot  3  is not limited to this example. The first robot  2  and the second robot  3  may have various structural forms. 
     By controlling the driving of the actuators Ac1 to Ac6 of the first robot  2  and the second robot  3  by the robot controller  4 , the operations of the first robot  2  and the second robot  3  are controlled so that the robot hand  10  of the first robot  2  at an end of the arm  14  and the robot hand  20  of the second robot  3  at an end of the arm  14  reach respective required positions in a predetermined order. 
     The robot hand  20  that is provided with the camera  7  and the laser sensor  8  at the end of the arm  14 , that is, at the end of the fifth arm portion  14   e  is mounted on the second robot  3 . When articles W that are stacked on the pallet P are removed from the pallet P, the second robot  3  is controlled by the robot controller  4  so as to cause the robot hand  20  at the end of the arm  14  to be positioned above the articles W that are stacked on the pallet P. Then, in this state, first, by scanning the top surface of a top article W using the laser sensor  8 , distance information regarding the distance to the top surface of the article W is obtained, to identify the article W whose top surface exists at a highest position. The distance information regarding the distance to the top surface of the identified article W is input to the robot controller  4 . 
     Next, on the basis of the distance information input to the robot controller  4 , the camera  7  of the robot hand  20  performs imaging on the top surface of the identified article W, and generates image information of the top surface of the identified article W. The generated image information is output from the camera  7  and input to the image processor  5 . Then, the image processor  5  performs an image recognition operation on the input image information, to obtain external-form information regarding the external form (dimensions, shape, etc.) of the top surface of the article W. The obtained external-form information regarding the external form of the top surface of the article W is input to the robot controller  4 . 
     When the scanning of the article W on the pallet P by the laser sensor  8  and the imaging operation by the camera  7  are completed, the second robot  3  is controlled by the robot controller  4  so as to cause the robot hand  20  to retreat to a retreating position at a side of the pallet P (that is, a position on the right of the pallet P in the example shown in  FIG. 1 ) from the position above the pallet P. Concurrently with the retreating of the robot hand  20  of the second robot  3 , by control of the robot controller  4  based on, for example, the distance information and the external-form information of the aforementioned article W, the first robot  2  causes the robot hand  10  to move to a location above the pallet P from a retreating position (such as a position above the conveyor  6 ). Then, the first robot  2  causes the robot hand  10  to move downward, the attraction pads  22  to contact the top surface of the identified article W, and the article W to be attracted and held by the attraction pads  22 . Next, the first robot  2  is controlled by the robot controller  4  so as to cause the robot hand  10  to move upward to the location above the conveyor  6  from the location above the pallet P. Then, the first robot  2  causes the robot hand  10  to move downward towards the conveyor  6 , to place the article W held by the attraction pads  22  on a transport surface  6   a  of the conveyor  6 . 
     The conveyor  6  is formed so that, by moving the transport surface  6   a  in the direction of an arrow, the article W placed on the transport surface  6   a  is transported to a take-out position. A photoelectric sensor  28  that detects the position of the bottom surface of the article W that is placed on the conveyor  6  is provided at a location of the conveyor  6  where the article W is placed by the first robot  2 . The photoelectric sensor  28  is a transmissive sensor similarly to a sensor  32  (described below) shown in  FIG. 5 . The photoelectric sensor  28  includes a phototransmitting section  28   a  that is positioned on one side of the conveyor  6  in a width direction thereof and a photoreceiving section  28   b  that is positioned on the other side of the conveyor  6  in the width direction thereof. The phototransmitting section  28   a  and the photoreceiving section  28   b  are disposed so that an optical axis L is positioned above the transport surface  6   a  of the conveyor  6  by a predetermined height H. 
     When the first robot  2  causes the article W held by the attraction pads  22  to move downward towards the conveyor  6 , the photoelectric sensor  28  detects the bottom surface of the article W. Although the robot controller  4  stops the operation of the first robot  2  when the bottom surface of the article W is detected, the robot hand  10  moves downward by a predetermined distance as a result of coasting. A downward-movement distance by which the robot hand  10  moves downward as a result of coasting and the height H are previously set so as to be substantially equal to each other (the height H is slightly larger). Therefore, when the photoelectric sensor  28  has detected the bottom surface of the article W, the robot controller  4  stops the operation of the first robot  2  and, then, causes the attraction pad  22  to stop holding the article W, as a result of which the article W is capable of being smoothly placed on the conveyor  6  regardless of the height of the article W. 
     With the height H being set greater than the downward-movement distance by a predetermined distance D, it is possible for the robot controller  4  to lower the robot hand  10  (article W) by the predetermined distance D after it has stopped the operation of the first robot  2 , and, then, to stop the attraction pads  22  from holding the article W. 
     2. Detailed Structure of Robot Hand 
     A detailed structure of the robot hand  10  of the first robot  2  is described with reference to  FIGS. 3A to 3C .  FIG. 3A  is a top view of the robot hand  10 .  FIG. 3B  is a bottom view of the robot hand  10 .  FIG. 3C  is an end view taken along line IIIC-IIIC of  FIG. 3A . As shown in  FIGS. 3A to 3C , the robot hand  10  of the first robot  2  includes a baseplate  21  that is substantially square-shaped in plan view in this example, the plurality of pads  22  disposed on the baseplate  21 , and a plurality of first to third proximity sensors  24  to  26  disposed on the baseplate  21 . The baseplate  21  has external dimensions (for example, substantially the same external dimensions) corresponding to the dimensions of a largest one of the plurality of articles W that become predeterminate objects to be held. 
     2-1. Structure of Attraction Pads 
     As shown in  FIG. 3B , the plurality of the attraction pads  22  are placed vertically and horizontally so as to be interspersed in a direction of a surface of the baseplate  21 . In this example, with one attraction pad  22  being disposed between inner sides of two outer attraction pads  22 , the attraction pads  22  are disposed inwardly to the center from positions of outer peripheral portions of the baseplate  21  situated along the four sides of the baseplate  21 . The way in which the attraction pads  22  are disposed is not limited to this example. The attraction pads  22  may be variously disposed. As shown in  FIG. 3C , each attraction pad  22  includes a bellows-type attraction section  22   a  disposed below the base plate  21  and a suction tube  22   b  that supports the attraction section  22   a  at the baseplate  21 . A suction tube path extending from a vacuum source (not shown) is connected to the suction tubes  22   b . By sucking inner portions of attraction sections  22   a  via the suction tube path and corresponding suction tubes  22   b , the attraction pads  22  attract the top surface of the article W with which the attraction sections  22   a  contact, and hold the article W. By enabling or disabling suction, the size of an attraction area of the baseplate  21  for attraction by the attraction pads  22  is variously changeable, so that the attraction pads  22  are capable of performing attraction in accordance with the external form of the article W to be held. 
     2-2. Proximity Sensors 
     The plurality of first to third proximity sensors  24  to  26  that are provided at the baseplate  21  are described. 
     The first proximity sensors  24  (first sensors) are interspersed and disposed in the direction of the surface of the baseplate  21 . In the example, the first proximity sensors  24  are provided on the baseplate  21  so as to be positioned between predetermined attraction pads  22  among the attraction pads  22  in the second row, the third row, and the fifth to ninth rows from the top in  FIG. 3A . The arrangement of the first proximity sensors  24  is not limited to this example. The first proximity sensors  24  can be variously arranged. The first proximity sensors  24  are used as load presence sensors that detect the existence of an article W attracted to attraction pads  22 . For example, reflective photoelectric sensors  30  such as that shown in  FIG. 4  are used for the first proximity sensors  24 . Light path holes  24   a  extending vertically through the baseplate  21  are provided at the positions of the baseplate  21  corresponding to the first proximity sensors  24 . 
     As shown in  FIG. 4 , a reflective photoelectric sensor  30  includes a phototransmitting section  30   a  and a photoreceiving section  30   b  disposed on one side of a detection object  31  to be detected. In the photoelectric sensor  30 , the phototransmitting section  30   a  projects a light beam  21 , such as infrared light, onto the detection object  31 . The light beam λ1 is reflected by the detection object  31 , and a reflected light beam λ2 of a smaller quantity is received by the photoreceiving section  30   b . If the quantity of light received by the photoreceiving section  30   b  is greater than or equal to a certain amount, the photoelectric sensor  30  detects that the detection object  31  exists within a certain distance from the photoelectric sensor  30 , and, for example, turns on. Then, when the detection object  31  moves out of a range of the certain distance from the photoelectric sensor  30 , attenuation of the quantity of the reflected light beam λ2 from the detection object  31  is increased, and the quantity of light received by the photoreceiving section  30   b  becomes less than the certain amount, so that the photoelectric sensor  30  detects that the detection object  31  does not exist within the certain distance, and, for example, turns off. 
     Each first proximity sensor  24  is such that the range of the certain distance is set to a range from the position of a lower surface of the baseplate  21  to a position that is below an end of its corresponding attraction pad  22  by a predetermined distance. The first proximity sensors  24  project and receive light via the path holes  24   a , and detect whether or not an article W exists within the range of the certain distance from the lower surface of the baseplate  21 . By scattering and disposing the first proximity sensors  24  having such a structure in the direction of the surface of the baseplate  21 , it is possible to recognize the external-form information (dimensions, shape, etc.) of the article W held by the attraction pads  22 . 
     If an obstacle exists in a path of movement of the robot hand  10  that is moving (downward or horizontally), the first proximity sensors  24  are capable of detecting the obstacle to avoid a collision. 
     Further, when an article W is held by attraction pads  22 , the first proximity sensors  24  at an area corresponding to the external form of the article W are supposed to detect the existence of the article W (that is, are supposed to be turned on). Therefore, if all of the first proximity sensors  24  when the article W is being held detect that the article does not exist (that is, all of the sensors  24  are turned off), it is assumed that the article W has dropped. That is, it is possible to detect that the article W has dropped. 
     In the robot system  1 , since external-form information of an article to be held is obtained as a result of laser scanning and image recognition, the controller is capable of predicting (an area of) the first proximity sensors  24  that detect the existence of an article on the basis of the external-form information. Therefore, when there is a difference between (an area of) the first proximity sensors  24  that have actually detected the existence of an article that is being held and (the area of) the first proximity sensors  24  that are predicted as being sensors that detect the existence of the article that is being held, it is possible to determine that a wrong article other than the specified article is held. That is, it is possible to detect that a wrong article is held. 
     As shown in  FIG. 3A , the second proximity sensors  25  (second sensors) are provided at substantially equal intervals along at least a contour of the baseplate  21 . In this example, the second proximity sensors  25  are disposed at the four corners, at central portions of corresponding outer peripheral portions at the four sides, and at a central portion of the baseplate  21 . The arrangement of the second proximity sensors  25  is not limited to this example. The second proximity sensors  25  can be variously arranged. The second proximity sensors  25  are used as push-in avoiding sensors that do not allow an article W to be pushed in by the attraction pads  24 . For example, transmissive photoelectric sensors  32  such as that shown in  FIG. 5  are used for the second proximity sensors  25 . 
     As shown in  FIG. 5 , a transmissive photoelectric sensor  32  includes a phototransmitting section  32   a  that is disposed on one side of a detection object  31  to be detected and a photoreceiving section  30   b  disposed on the other side of the detection object  31 . In the photoelectric sensor  32 , the phototransmitting section  32   a  projects a light beam λ1 onto the detection object  31 . If the detection object  31  exists in a path of the light beam λ1, a transmitted light beam  23  whose quantity is reduced as a result of transmission of the light beam  21  through the detection object (and interception of the light beam λ1 by the detection object  31 ). If the quantity of light received by the photoreceiving section  32   b  is less than or equal to a certain amount, the photoelectric sensor  32  detects that the detection object  31  exists, and, for example, turns on. Then, when the detection object  31  moves away from the path of the light beam λ1 projected by the detection object  31 , the amount of light received by the photoreceiving section  32   b  becomes greater than or equal to a certain amount, so that the photoelectric sensor  32  detects that the detection object  31  does not exist, and, for example, turns off. 
     As shown in  FIG. 3C , each second proximity sensor  25  includes a phototransmitting section  25   a  and a photoreceiving section  25   b  disposed, respectively, on one side and on the other side in a transverse direction of a rod  22   c  connected to a suction tube  22   b , and is fixed at a predetermined height. Each rod  22   c  is a member that corresponds to the detection object  31  and moves vertically in accordance with a vertical movement of the corresponding attraction pad  22 . When, by this, the attraction pads  22  contact the top surface of an article W, and the baseplate  21  moves downward in such a manner that the lower surface of the baseplate  21  and the top surface of the article W come closer to each other than a predetermined distance, a light path of light that is projected from the phototransmitting sections  25   a  is intercepted by the rods  22   c . As a result, the second proximity sensors  25  detect that the baseplate  21  and the article W are close to each other. 
     Here, in the robot system  1 , distance information regarding the distance to the top surface of a top article W is obtained by laser scanning performed by the laser sensor  8  and the robot hand  10  is moved downward on the basis of the distance information. However, when the distance information is erroneously detected, in particular, when the distance is erroneously detected as being larger than an actual distance, the robot hand  10  (attraction pads  22 ) pushes in the article W, as a result of which the article W may break or may be deformed. 
     Accordingly, by providing the second proximity sensors  25  having the above-described structure at the robot hand  10 , the downward movement of the robot hand  10  is stopped before the baseplate  21  comes close to the article W. This makes it possible to avoid breakage and deformation of the article W caused by the pushing in of the article W by the robot hand  10 . In addition, since the article W is pushed in uniformly over the entire baseplate  21 , the detection can be satisfactorily performed primarily by the sensors disposed at the outer peripheral portions of the baseplate  21 . Therefore, by disposing the second proximity sensors  25  at substantially equal intervals along the contour of the baseplate  21 , it is possible not to allow the article W to be pushed in using the minimum number of sensors required. 
     The third proximity sensors  26  (third sensors) are disposed at outer sides of the contour of the baseplate  21 . In this example, the third proximity sensors  26  are disposed at outer sides of two adjacent sides among the four sides of the baseplate  21 . Rectangular horizontal supporting frames  25  are connected to the outer peripheral portions of the two sides of the baseplate  21 . Two third proximity sensors  26  are disposed at outer portions of each supporting frame  27 . The way in which the third proximity sensors  26  are arranged is not limited to this example. The third proximity sensors  26  are capable of being variously arranged. For example, the third proximity sensors  26  may be disposed at outer sides of the four sides of the baseplate  21 . The third proximity sensors  26  are used for confirming whether or not an article W that is attracted by attraction pads  22  is oversized. Similarly to the first proximity sensors  24 , reflective photoelectric sensors  30  such as that shown in  FIG. 4  are used for the third proximity sensors  26 . Light path holes  26   a  extending through the supporting frames  27  are provided at the positions of the supporting frames  27  corresponding to the third proximity sensors  26 . 
     As described above, the baseplate  21  has external dimensions (for example, substantially the same external dimensions) corresponding to the dimensions of a largest one of the plurality of articles W that become predeterminate objects to be held. By providing the third proximity sensors  26  at the outer sides of the contour of the baseplate  21 , when the third proximity sensors  26  have detected an article W that is being held, it is possible to assume that an article W that is larger than a predetermined maximum size is held. That is, it is possible to detect that the article W is oversized. 
     If an obstacle exists in a path of movement of the robot hand  10  that is moving downward or horizontally, the third proximity sensors  26  are capable of detecting the obstacle before the robot hand  10  collides with the obstacle. Therefore, it is possible to avoid the collision with the obstacle by stopping the movement of the robot hand  10 . 
     3. Functional Structure of Robot Controller 
     As shown in  FIG. 6 , the robot controller  4  includes an external-form recognizing section  34 , an attraction area setting section  35 , and a placement controlling section  36 . On the basis of detection results of the plurality of first proximity sensors  24  when attraction pads  22  hold an article W, the external-form recognizing section  34  recognizes the external-form information (dimensions, shape) of the article W. On the basis of the recognized external-form information, the attraction area setting section  35  sets an area of attraction pads  22  that attract the article W at the baseplate  21 . The placement controlling section  36  performs control so that the lowering of the robot hand  10  is stopped when the photoelectric sensor  28  detects the bottom surface of the article W held by the attraction pads  22 , after which the attraction pads  22  stop holding the article W and the article W is placed on the conveyor  6 . Although not shown in  FIG. 6 , the robot controller  4  has various functions for controlling the operations of the robots  2  and  3  in addition to the above-described operation. 
     4. Attraction and Holding of Article when Image Recognition Error Occurs 
     When attraction pads  22  of the robot hand  10  of the first robot  2  hold an article W on the pallet P, the camera  7 , provided at the robot hand  20  of the second robot  3 , performs imaging on the top surface of the article W on the pallet P. Here, when a plurality of articles W having the same shape, such as cardboard boxes, are disposed side by side without any gaps therebetween, the articles are not capable of being image-recognized as a plurality of articles W, that is, the articles W may be erroneously recognized as a single article. 
     In the embodiment, as described above, when the plurality of articles W is image-recognized as a single article, the dimensions of the erroneously recognized article exceed the dimensions of a largest one of the plurality of articles W that become predeterminate objects to be held, as a result of which an image recognition error occurs. When such an image recognition error occurs, the robot controller  4  operates attraction pads  22  in an area corresponding to an article W having predetermined minimum dimensions among the plurality of attraction pads  22  of the baseplate  21 , so that the operated attraction pads  22  provisionally attract and hold the article W. On the basis of the detection results of the first proximity sensors  24  for this time, external-form information (dimensions, shape) of the article W is recognized. Then, on the basis of the recognized external-form information of the article W, the area of the attraction pads  22  that attract the article W at the baseplate  21  is set, and the article W is re-attracted and held by the attraction pads  22  in the set area. These operations are hereunder described in detail. 
       FIGS. 7A and 7B  are each an explanatory view of an example of attracting and holding an article when the image recognition error occurs. In  FIG. 7A , two articles W1 and W2 that are stacked on the pallet P are arranged side by side without any gap therebetween. In an image recognition operation performed by carrying out imaging using the camera  7 , the articles are erroneously recognized as one article W′, as a result of which the image recognition error occurs. In this case, the robot controller  4  causes the attraction pads  22  that are positioned in a minimum area  38  corresponding to the article W having the predetermined minimum dimensions among the plurality of attraction pads  22  of the baseplate  21  to be operated, so that, with, for example, the minimum area  38  being disposed at a corner of the pallet P (that is, an upper left corner in  FIG. 7A ), the minimum area  38  attracts the top surface of the article W′, and the article W′ is held. Then, when the robot hand  10  is moved upward, only one of the two articles W1 and W2 that is attracted and held by the attraction pads  22 , that is, only the article W1 (left article in  FIG. 7A ) is lifted, whereas the other article W2 (right article in  FIG. 7A ) remains on the pallet P. Therefore, when the plurality of first proximity sensors  24  of the baseplate  21  are used to detect the article, only the first proximity sensors  24  corresponding to the attracted article W1 detect the article and turns on. The other first proximity sensors  24  that are positioned at the outer sides of the first proximity sensors  24  corresponding to the attracted article W1 do not detect the article and turn off. This causes the external-form recognizing section  34  of the robot controller  4  to recognize external-form information (dimensions, shape) of the article W1 on the basis of detection results of the first proximity sensors  24 . 
     Thereafter, as shown in  FIG. 7B , on the basis of the recognized external-form information of the article W1, the attraction area setting section  35  sets an area  39  of the attraction pads  22  that perform attraction at the baseplate  21  as a suitable area that is neither too large or too small with reference to the external dimensions of the article W1. Then, the robot controller  4  re-operates the attraction pads  22  at the set suitable area  39 , so that the attraction pads  22  re-attract and hold the article W. 
     5. Procedure of Control Using Robot Controller 
     An example of a control procedure performed by the robot controller  4  when the above-described image recognition error occurs is shown in  FIG. 8 . The robot controller  4  causes the laser sensor  8  to measure the distance to the top surface of a top article W on the pallet P, and the camera  7  to perform imaging on the top article W identified by measuring the distance. If the above-described image recognition error occurs, the steps of this flowchart are started. 
     First, in Step S 10 , the robot controller  4  outputs a control signal to the first robot  2 , and controls a position based on, for example, distance information and external-form information of the article W, to move the robot hand  10  of the first robot  2  to a position above the pallet P. Then, the robot controller  4  lowers the robot hand  10 , causes the attraction pads  22  in an area of the baseplate  21  corresponding to an article having predetermined minimum dimensions to operate, causes the operated attraction pads  22  to provisionally attract and hold the identified article W, and causes the operated attraction pads  22  to, for example, lift the article W. 
     Then, in Step S 20 , the robot controller  4  obtains detection results of the plurality of first proximity sensors  24  at the baseplate  21  while the attraction pads  22  hold the article W. 
     Next, in Step S 30 , the external-form recognizing section  34  of the robot controller  4  recognizes the external-form information (dimensions, shape) of the article W on the basis of the detection results of the first proximity sensors  24 . As described above, only the first proximity sensors  24  corresponding to the held article W among the plurality of first proximity sensors  24  are turned on, whereas the other first proximity sensors  24  that are positioned at the outer sides of the held article W1 are turn off. Therefore, the external-form information (dimensions, shape) of the article W is recognized. After recognizing the external-form information, the robot controller  4  lowers the robot hand  10 , stops the attraction pads  22  from holding the article W, and causes the article W to be placed on the pallet P. 
     In Step S 40 , on the basis of the recognized external-form information of the article W, the attraction area setting section  35  of the robot controller  4  sets an area (attraction area) of the attraction pads  22  that attract the article W at the baseplate  21 . After setting the attraction area, the article W may be placed on the pallet P. 
     Thereafter, in Step S 50 , the robot controller  4  re-operates the attraction pads  22  at the set attraction area and causes the attraction pads  22  to re-attract and hold the article W for handling the article W. This makes it possible for the attraction pads  22  to stably hold the article W and to move towards the conveyor  6 . When the Step S 50  ends, this flow ends. 
     6. Advantages of Embodiment 
     As described above, the first robot  2  according to the embodiment depalletizes a plurality of articles W that are stacked on the pallet P one at a time. Here, by scanning the top surface of a top article W on the pallet P using the laser sensor  8  of the robot hand  20  of the second robot  3 , distance information regarding the distance to the top surface of the top article W on the pallet P is obtained, to identify the article W whose top surface exists at a highest position. Then, the camera  7  of the robot hand  20  performs imaging on the top surface of the identified article W, and the image processor  5  performs an image recognition operation, so that external-form information of the top surface is obtained. On the basis of, for example, the distance information and the external-form information, the first robot  2  causes the robot hand  10  to move and hold the article W. 
     Here, when a plurality of articles W having the same shape, such as cardboard boxes, are disposed side by side on the pallet P without any gaps therebetween, the articles W are not capable of being image-recognized as a plurality of articles W, that is, the articles W may be recognized as a single article. When such an erroneous recognition occurs, for example, the operation of the robot is stopped due to an error or dropping of the article. This may cause a depalletizing operation to be stopped. 
     In the embodiment, a plurality of proximity sensors (first proximity sensors  24  to third proximity sensors  26 ) are arranged on the baseplate  21  of the robot hand  10 . Therefore, when, as described above, an image recognition operation is not capable of being performed, for the time being, the attraction pads  22  hold, lift, and handle an article W, and external-form information (dimensions, shape) of the article W is capable of being recognized on the basis of the detection results of the plurality of proximity sensors in an operated state. As a result, on the basis of the recognized external-form information of the article W, it is possible to set a suitable holding mode of the attraction pads  22 , and to re-hold and handle the article W in the set holding mode. In this way, even if the article W to be depalletized is erroneously recognized, it is possible to continue the depalletizing operation without, for example, the operation of the robot being stopped due to an error or dropping of the article W. 
     In the embodiment, in particular, the plurality of first proximity sensors  24  arranged so as to be interspersed in the direction of the surface of the baseplate  21  are included among the plurality of proximity sensors. Using on/off information of the first proximity sensors arranged so as to be interspersed in the direction of the surface of the baseplate  21 , it is possible to clarify the external-form information (dimensions, shape) of the handled article W and to increase recognition precision. 
     As described above, when an obstacle exists in a path of movement of the robot hand  10  that is moving (downward or horizontally), the first proximity sensors  24  are capable of detecting the obstacle to avoid a collision. Further, it is possible to detect that an article W has dropped and that a wrong article is held. 
     In the embodiment, in particular, the second proximity sensors  25  arranged at substantially equal intervals along the contour of the baseplate  21  are included among the plurality of proximity sensors. As described above, this makes it possible to avoid breakage and deformation of the article W occurring when it is pushed in by the robot hand  10 . 
     In the embodiment, in particular, the third proximity sensors  26  arranged at an outer side of the contour of the baseplate  21  are included among the plurality of proximity sensors. As described above, this makes it possible to detect that an article W is oversized. If an obstacle exists in a path of movement of the robot hand  10  that is moving (downward or horizontally), the third proximity sensors  26  are capable of detecting the obstacle to avoid a collision. 
     In the embodiment, in particular, a plurality of attraction pads  22 , serving as holding members, arranged so as to be interspersed in the direction of the surface of the baseplate  21  and formed so as to attract the top surface of an article W are provided. This makes it possible to select where appropriate the attraction pads  22  that attract the article W, serving as a holding object, in accordance with the external form of the article W. Therefore, it is possible to hold articles having various sizes and shapes. In addition, since it is possible to change an attraction position at the baseplate  21  in accordance with where the article W, serving as a holding object, is placed within the pallet P, it is possible to increase depalletizing efficiency. 
     In the embodiment, in particular, the robot system  1  includes a conveyor  6  that transports an article W placed on the conveyor  6  by the first robot  2 , and a photoelectric sensor  28  that is disposed above the transport surface  6   a  of the conveyor  6  where an article W is placed and that includes a phototransmitting section  28   a  and a photoreceiving section  28   b . The phototransmitting section  28   a  is positioned on one side of the conveyor  6  in the width direction thereof, and the photoreceiving section  28   b  is positioned on the other side of the conveyor  6  in the width direction thereof. 
     By this, the placement controlling section  36  of the robot controller  4  performs control so that the lowering of the robot hand  10  is stopped when the photoelectric sensor  28  detects the bottom surface of an article W held by the attraction pads  22 , after which the attraction pads  22  stop holding the article W to place the article W on the conveyor  6 . As a result, the article W is capable of being smoothly placed on the conveyor  6  regardless of the height of the article W. Therefore, it becomes unnecessary to provide devices, such as a camera and a sensor, for detecting the height of the article W. This simplifies the structure of the robot system  1 . 
     7. Modification 
     The disclosure is not limited to the above-described disclosed embodiment. Various modifications are possible without departing from the gist and technical ideas of the disclosure. 
     In the above-described embodiment, in addition to the first robot  2 , the second robot  3  is provided for mounting the camera  7  and the laser sensor  8  on the second robot  3 . However, for example, by mounting the camera  7  and the laser sensor  8  on the first robot  2 , only one first robot  2  may be provided, that is, the second robot  3  does not have to be provided. 
     Although the first proximity sensors  24  to the third proximity sensors  26  are photoelectric sensors, they may be, for example, capacitive sensors or ultrasonic sensors. 
     In addition to what are already described above, it is possible to combine techniques according to the embodiment, etc. where appropriate. 
     Although not exemplified one by one, the embodiment, etc. can be variously modified without departing from the gist thereof. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.