Patent Publication Number: US-11642800-B2

Title: Apparatus and method for use with robot

Description:
FIELD 
     Embodiments of present disclosure generally relate to the field of industrial robots, and more particularly, to an apparatus and a method for use with a robot. 
     BACKGROUND 
     Industrial robots are widely used in various manufacturing applications, such as assembling objects such as household appliance components, automobile parts, or the like. A robot for assembling objects is able to repetitively move objects, gripped by a gripper connected to a robot arm of the robot, from a position to another position. In order to carry out such movements precisely, the robot must be calibrated in advance. In other words, the robot must first be taught about the above positions. 
     Traditionally the robot teaching is done manually by a human operator. The operator manually controls the robot arm to position the gripper of the robot or an object gripped by the gripper in desired positions. Once so positioned, the positions of the gripper are recorded so that the robot can return the gripper to these positions in later process of assembling objects. 
     However, the robot teaching often requires a big engineering effort in robot applications. The accuracy of the positioning is subject to the skill and visual acuity of the operator. Especially for accurate assembly, precise positioning of the gripper is hardly achieved without a skillful and patient engineer. Accordingly, there is a need for a new solution for use with a robot that will allow the calibrating or teaching of the robot to be performed quickly and accurately. 
     SUMMARY 
     In a first aspect of the present disclosure, an apparatus for use with a robot is provided. The apparatus comprises a reflective photoelectric sensor arranged on a gripper of the robot; and a controller configured to: cause the reflective photoelectric sensor to scan over a target object; monitor changes in an output signal from the reflective photoelectric sensor; for each detected change exceeding a threshold, determine a coordinate of a gripping component on the gripper in a robot coordinate system, to obtain a set of coordinates; determine a position of the target object in the robot coordinate system based on the set of coordinates and a predefined offset value between the reflective photoelectric sensor and the gripping component; and store the position of the target object for future use in assembling objects. 
     In some embodiments, the position of the target object is stored as a gripping position of the gripping component. 
     In some embodiments, the position of the target object is stored as a dropping position of the gripping component. 
     In some embodiments, the gripping component includes a clamping jaw, a vacuum chuck, or an electromagnet. 
     In some embodiments, the apparatus further comprises a camera, and the controller is further configured to: cause the gripping component to grip the target object; cause the camera to capture an image containing the reflective photoelectric sensor and the target object gripped by the gripping component; and determine, from the image, an actual offset value between the reflective photoelectric sensor and the target object gripped by the gripping component, as the predefined offset value. 
     In some embodiments, the reflective photoelectric sensor is a first reflective photoelectric sensor, the apparatus further comprises a second reflective photoelectric sensor arranged on the gripper, and the controller is further configured to: align the gripping component with the target object in orientation based on output signals from the first and second reflective photoelectric sensors. 
     In some embodiments, the controller is configured to align the gripping component with the target object in orientation by: causing the first and second reflective photoelectric sensors to move towards a side of the target object; determining a first time point when a change in the output signal from the first reflective photoelectric sensor exceeds the threshold; determining a second time point when a change in the output signal from the second reflective photoelectric sensor exceeds the threshold; and if the first time point is different from the second time point, causing the gripper to rotate to align the gripping component with the target object in orientation. 
     In some embodiments, the reflective photoelectric sensor is a first reflective photoelectric sensor, the apparatus further comprises a third and a fourth reflective photoelectric sensors arranged on the gripper, and the controller is further configured to: cause a lower surface of the gripping component to be parallel to an upper surface of the target object based on output signals from the first, third and fourth reflective photoelectric sensors. 
     In some embodiments, the controller is configured to cause the lower surface of the gripping component to be parallel to the upper surface of the target object by: causing the first, third and fourth reflective photoelectric sensors to locate above the target object; determining respective distances between the upper surface of the target object and the first, third and fourth reflective photoelectric sensors based on the output signals from the first, third and fourth reflective photoelectric sensors; and if at least one of the distances is different from the others, causing the gripper to rotate such that the lower surface of the gripping component is parallel to the upper surface of the target object. 
     In some embodiments, the reflective photoelectric sensor is a reflective optical fiber sensor or a laser displacement sensor. 
     In a second aspect of the present disclosure, a method for use with a robot is provided. The method comprises causing a reflective photoelectric sensor arranged on a gripper of the robot to scan over a target object; monitoring changes in an output signal from the reflective photoelectric sensor; for each detected change exceeding a threshold, determining a coordinate of a gripping component on the gripper in a robot coordinate system, to obtain a set of coordinates; determining a position of the target object in the robot coordinate system based on the set of coordinates and a predefined offset value between the reflective photoelectric sensor and the gripping component; and storing the position of the target object for future use in assembling objects. 
     In some embodiments, the position of the target object is stored as a gripping position of the gripping component. 
     In some embodiments, the position of the target object is stored as a dropping position of the gripping component. 
     In some embodiments, the gripping component includes a clamping jaw, a vacuum chuck, or an electromagnet. 
     In some embodiments, the method further comprises: causing the gripping component to grip the target object; causing a camera to capture an image containing the reflective photoelectric sensor and the target object gripped by the gripping component; and determining, from the image, an actual offset value between the reflective photoelectric sensor and the target object gripped by the gripping component, as the predefined offset value. 
     In some embodiments, the reflective photoelectric sensor is a first reflective photoelectric sensor, and the method further comprises: aligning the gripping component with the target object in orientation based on output signals from the first reflective photoelectric sensor and a second reflective photoelectric sensor arranged on the gripper. 
     In some embodiments, aligning the gripping component with the target object in orientation comprises: causing the first and second reflective photoelectric sensors to move towards a side of the target object; determining a first time point when a change in the output signal from the first reflective photoelectric sensor exceeds the threshold; determining a second time point when a change in the output signal from the second reflective photoelectric sensor exceeds the threshold; and if the first time point is different from the second time point, causing the gripper to rotate to align the gripping component with the target object in orientation. 
     In some embodiments, the reflective photoelectric sensor is a first reflective photoelectric sensor, and the method further comprises: causing a lower surface of the gripping component to be parallel to an upper surface of the target object based on output signals from the first reflective photoelectric sensor, and a third and a fourth reflective photoelectric sensors arranged on the gripper. 
     In some embodiments, causing the lower surface of the gripping component to be parallel to the upper surface of the target object comprises: causing the first, third and fourth reflective photoelectric sensors to locate above the target object; determining respective distances between the upper surface of the target object and the first, third and fourth reflective photoelectric sensors based on the output signals from the first, third and fourth reflective photoelectric sensors; and if at least one of the distances is different from the others, causing the gripper to rotate such that the lower surface of the gripping component is parallel to the upper surface of the target object. 
     In some embodiments, the reflective photoelectric sensor is a reflective optical fiber sensor or a laser displacement sensor. 
     In a third aspect of the present disclosure, a robot comprising the apparatus according to the first aspect of the present disclosure is provided. 
     In a fourth aspect of the present disclosure, a device is provided. The device comprises: a processing unit; and a memory coupled to the processing unit and storing instructions thereon, the instructions, when executed by the processing unit, causing the device to perform the method according to the second aspect of the present disclosure. 
     According to various embodiments of the present disclosure, the apparatus and method for use with the robot provide a new solution for calibrating or teaching the robot. Such sensor-based robot calibrating or teaching scheme can detect a plurality of edge points on the target object automatically and hence determine the position of the target object precisely for use in assembling objects. In this way, the calibrating or teaching of the robot can be performed quickly and accurately. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       Through the following detailed descriptions with reference to the accompanying drawings, the above and other objectives, features and advantages of the example embodiments disclosed herein will become more comprehensible. In the drawings, several example embodiments disclosed herein will be illustrated in an example and in a non-limiting manner, wherein: 
         FIG.  1    schematically illustrates a scanning motion of an apparatus for use with a robot according to some example embodiments over a target object; 
         FIG.  2    schematically illustrates an example process of scanning the target object using the reflective photoelectric sensor; 
         FIG.  3    schematically illustrates an example of obtaining the predefined offset value between the reflective photoelectric sensor and the target object gripped by a gripping component; 
         FIG.  4    schematically illustrates a scanning motion of the apparatus as shown in  FIG.  1    over a target object; 
         FIG.  5    schematically illustrates an example process of gripping the target object by the gripping component; 
         FIG.  6    schematically illustrates an example process of dropping the target object gripped by the gripping component onto the target object; 
         FIG.  7    schematically illustrates an example process of aligning a gripping component of the gripper with the target object in orientation using an apparatus for use with a robot according to another example embodiment; 
         FIG.  8    schematically illustrates an example process of causing a lower surface of the gripping component to be parallel to an upper surface of the target object using an apparatus for use with a robot according to a further example embodiment; and 
         FIG.  9    is a flow chart of a method for use with a robot according to embodiments of the present disclosure. 
     
    
    
     Throughout the drawings, the same or similar reference symbols are used to indicate the same or similar elements. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Principles of the present disclosure will now be described with reference to several example embodiments shown in the drawings. Though example embodiments of the present disclosure are illustrated in the drawings, it is to be understood that the embodiments are described only to facilitate those skilled in the art in better understanding and thereby achieving the present disclosure, rather than to limit the scope of the disclosure in any manner. 
       FIG.  1    schematically illustrates a scanning motion of an apparatus  200  for use with a robot  100  according to some example embodiments over a target object  210 . As shown, the target object  210  is supplied by a feeder  310  onto a work table  410  and supported by the work table  410 . The robot  100  includes a robot arm  110  and a gripper  120  connected to the robot arm  110 . The gripper  120  is driven by the robot arm  110  so as to move between different positions above the work table  410 . The gripper  120  includes a gripping component  140  operable to grip and drop an object, such as the target object  210 . 
     In an embodiment, the gripping component  140  may be a clamping jaw having two or more fingers for grasping the target object  210 . Alternatively, in another embodiment, the gripping component  140  may be an adhesive component, such as a vacuum chuck or an electromagnet. It is to be understood that the gripping component  140  can be of suitable types other than the examples as described above. The present disclosure does not intend to limit the type of the gripping component  140 . 
     In order to grip the target object  210  for assembling objects, a position of the target object  210  needs to be determined in advance and recorded as a gripping position of the gripping component  140 . Hereafter, the apparatus  200  for use with the robot  100  to determine the position of the target object  210  will be described in detail with reference to  FIG.  1   . 
     In general, the apparatus  200  includes a reflective photoelectric sensor  130  and a controller  300 . It is to be understood that the components in the drawings are not drawn in scale. Rather, it is just for illustration. 
     The reflective photoelectric sensor  130  is arranged on the gripper  120 . In operation, the reflective photoelectric sensor  130  is configured to emit a light beam L towards the target object  210  and the work table  410 , and to receive a reflected light beam. In an embodiment, the reflective photoelectric sensor  130  may be a reflective optical fiber sensor. In response, the reflective optical fiber sensor generates an output signal representative of light intensity of the reflected light beam. Changes in the output signal from the reflective optical fiber sensor can represent at least one of the following: color change of the objects, distance change between the reflective optical fiber sensor and the objects, texture change of the objects, and angle change of the objects. 
     In another embodiment, the reflective photoelectric sensor  130  may be implemented by a laser displacement sensor. The laser displacement sensor generates an output signal representative of respective distances between the laser displacement sensor and the objects. Changes in the output signal from the laser displacement sensor can represent distance change between the laser displacement sensor and the objects. 
     It is to be understood that the reflective photoelectric sensor  130  can be of suitable types other than the examples as described above. The present disclosure does not intend to limit the type of the reflective photoelectric sensor  130 . 
     The controller  300  of the apparatus  200  may be implemented by any dedicated or general-purpose processor, controller, circuitry, or the like. In some embodiments, the controller  300  may be the controller for the robot  100  as well. 
     In order to determine the position of the target object  210 , the controller  300  is configured to cause the reflective photoelectric sensor  130  to scan over the target object  210 . In an example, under the control of the controller  300 , the reflective photoelectric sensor  130  may scan over the target object  210  along a predefined direction S, as shown in  FIG.  1   . In another example, the reflective photoelectric sensor  130  may scan over the target object  210  along the predefined direction S first and then along other directions different from the predefined direction S. 
     It is to be understood that the reflective photoelectric sensor  130  may scan over the target object  210  along any direction other than the examples as described above. The present disclosure does not intend to limit the scanning direction of the reflective photoelectric sensor  130 . Hereinafter, example scanning directions of the reflective photoelectric sensor  130  will be described in detail with reference to  FIG.  2   . 
     During the scanning, the controller  300  monitors changes in the output signal from the reflective photoelectric sensor  130 . As an example, in those embodiments where the reflective photoelectric sensor  130  is implemented by a reflective optical fiber sensor, the changes in the output signal from the reflective optical fiber sensor can represent at least one of the following: color change of the objects, distance change between the reflective optical fiber sensor and the objects, texture change of the objects, and angle change of the objects. When the reflective photoelectric sensor  130  is implemented by a laser displacement sensor, the changes in the output signal from the laser displacement sensor can represent distance change between the laser displacement sensor and the objects. 
     If a change is detected to exceed a threshold, then it can be considered that an edge point of target object  210  is found. In response, the controller  300  may determine a coordinate of the gripping component  140  at that moment in a robot coordinate system. By determining and recording the coordinate of the gripping component  140  each time when the change of the output signal of the reflective photoelectric sensor  130  exceeds the threshold, a set of coordinates are obtained. These coordinates represent the positions of the gripping component  140  when edge points of the target object  210  are found by the reflective photoelectric sensor  130 . 
     The threshold may be pre-stored in any suitable storage or memory accessible to the controller  300 . During the scanning, a change in the output signal from the reflective photoelectric sensor  130  may be tiny. Such a tiny change would not imply that an edge point of the target object  210  is found. Only when the detected change exceeds the threshold, it is determined that the edge point of target object  210  is detected. In this way, the edge points of the target object  210  may be precisely detected. 
     Then, based on the set of coordinates and a predefined offset value PO between the reflective photoelectric sensor  130  and the gripping component  140 , the controller  300  determines the position of the target object  210  in the robot coordinate system. The predefined offset value “PO” represents the distance and orientation between the reflective photoelectric sensor  130  and the gripping component  140 . Accordingly, based on the predefined offset value PO, the controller  300  may convert the obtained set of coordinates of the gripping component  140  into the position of the target object  210  in the robot coordinate system. 
     The predefined offset value PO may be determined or measured and stored in advance. In an example, the predefined offset value PO may be preset by an operator in view of the distance and orientation between the reflective photoelectric sensor  130  and the gripping component  140 . 
     In another example, the apparatus  200  may further include a camera  400  for determining the predefined offset value PO. An example process for determining the predefined offset value PO using the camera  400  will be described in detail hereinafter with reference to  FIG.  3   . It is to be understood that the predefined offset value PO can be measured or set in any other suitable manners. The present disclosure does not intend to limit the manners of obtaining the predefined offset value PO. 
     Upon determination of the position of the target object  210 , the controller  300  stores the position of the target object  210  into any accessible storage or memory, for future use in assembling objects, which will be discussed later. 
     In the embodiment as depicted in  FIG.  1   , the position of the target object  210  in the robot coordinate system may represent the gripping position of the gripping component  140 . In future assembling process, based on the stored positions, the gripping component  140  may return to the gripping position repetitively and grip the target object  210  at the gripping position. 
     With the apparatus  200  as described above, the position of the target object  210  can be well-taught and then determined precisely for future use in assembling objects. In this way, the calibration or teaching of the robot  100  can be performed quickly and accurately without the need of human intervention. 
     In the following, an example process of scanning the target object  210  using the reflective photoelectric sensor  130  will be described in detail with reference to  FIG.  2   . As shown, the target object  210  may be of a circular shape. The reflective photoelectric sensor  130  starts the scanning at point sp 0  and moves along a predefined direction S. As such, a pair of edge points sp 1  and sp 2  of the target object  210  can be found. The reflective photoelectric sensor  130  then moves to a middle point cp 1  between the edge points sp 1  and sp 2  and scans along a direction S′ orthogonal to the predefined direction S. As such, a further pair of edge points sp 3  and sp 4  of the target object  210  can be found. The middle point cp 2  between the edge points sp 3  and sp 4  is the center of the target object  210 . In some embodiments, the position of the target object  210  may be represented by the coordinate of the middle point cp 2  in the robot coordinate system. 
     It is to be understood that the embodiment as shown in  FIG.  2    is just for illustration, without suggesting any limitations as to the scope of the present disclosure. In other embodiments, the target object  210  may be of any other shapes, such as triangle, square, hexagon or irregular shape. The present disclosure does not intend to limit the shape of the target object  210 . 
       FIG.  3    schematically illustrates an example of obtaining the predefined offset value “PO” between the reflective photoelectric sensor  130  and the target object  210  gripped by the gripping component  140 . For ease of discussion, the gripping component  140  is described as a clamping jaw in this embodiment. 
     In general, in order to determine the PO, the controller  300  may cause the gripping component  140  to grip the target object  210  and then reposition the target object  210  by the fingers of the clamping jaw. In this event, an actual offset value “AO” between the reflective photoelectric sensor  130  and the target object  210  can be determined and used as the predefined offset value PO. 
     Specifically, as described above, the apparatus  200  may further include a camera  400 . After the target object  210  is gripped by the gripping component  140 , the controller  300  may cause the camera  400  to capture an image containing the reflective photoelectric sensor  130  and the target object  210  gripped by the gripping component  140 . In an example, the camera  400  may be disposed on the work table  410 . In another example, the camera  400  may be disposed at other positions. Then, the controller  300  may determine, from the image, the actual offset value AO and store the actual offset value AO in the memory as the predefined offset value PO. 
     It is to be understood that the value of PO can be determined by any suitable means other than the one as described with reference to  FIG.  3   . For example, it is possible to manually measure the value and then input to the apparatus  200 . 
       FIG.  4    schematically illustrates a scanning motion of the apparatus  200  as shown in  FIG.  1    over a target object  210 ′. In this example, the target object  210 ′ is held by a fixture  320  on a work table  420 . The target object  210 ′ may be used to receive another object (e.g., the target object  210  shown in  FIG.  1   ) for assembling. In an embodiment, the position of the target object  210 ′ may be determined by the controller  300  in the similar manner as determining the target object  210  and stored as a dropping position of the gripper  120 . In assembling objects, the gripping component  140  may move to the dropping position and drop the target object  210  onto the target object  210 ′ at the dropping position. 
     In some embodiments, with the stored gripping position and dropping position of the gripping component  140 , the robot  100  may assemble objects using these positions. In this regard,  FIG.  5    schematically illustrates an example process of gripping the target object  210  by the gripping component  140 , and  FIG.  6    schematically illustrates an example process of dropping the target object  210  gripped by the gripping component  140  onto the target object  210 ′. 
     The gripping component  140  as shown in  FIGS.  5  and  6    may be an adhesive component, such as a vacuum chuck or an electromagnet, for example. As shown in  FIG.  5   , the gripping component  140  grips the target object  210  at the gripping position determined based on the predefined offset PO. If the predefined offset PO is not accurate, an aligning error ERR may exist between the gripping component  140  and the gripped target object  210 . As shown in  FIG.  6   , since the dropping position of the gripping component  140  is determined based on the same predefined offset PO, the aligning error ERR in gripping the target object  210  can be compensated when dropping the target object  210  onto the target object  210 ′. Thus, the target object  210  and the target object  210 ′ can be properly aligned with each other. 
     In some cases, the orientation of the gripping component  140  may be not aligned with the target object  210  in orientation. This would result in that the gripping component  140  may be unable to grip the target  210  precisely.  FIG.  7    schematically illustrates an example process of aligning the gripping component  140  with the target object  210  in orientation using an apparatus  200  for use with the robot  100  according to another example embodiment. 
     The apparatus  200  as shown in  FIG.  7    differs from the apparatus  200  as shown in  FIG.  1    in that in addition to the reflective photoelectric sensor  130 , there is another reflective photoelectric sensor  150  arranged on the gripper  120 . For ease of discussion, the reflective photoelectric sensor  130  may be referred to as a first reflective photoelectric sensor, and the reflective photoelectric sensor  150  may be referred to as a second reflective photoelectric sensor. The first and second reflective photoelectric sensor  130 ,  150  may be of the same or different types. With the first and second reflective photoelectric sensor  130 ,  150 , the controller  300  may align the gripping component  140  with the target object  210  in orientation based on output signals from the first and second reflective photoelectric sensors  130 ,  150 . 
     Specifically, in an embodiment, the controller  300  may cause the first and second reflective photoelectric sensors  130 ,  150  to move towards a side  230  of the target object  210 . During the moving, the controller  300  may determine a first time point when a change in the output signal from the first reflective photoelectric sensor  130  exceeds the threshold. Likewise, the controller  300  may determine a second time point when a change in the output signal from the second reflective photoelectric sensor  150  exceeds the threshold. 
     If the first time point is the same as the second time point, then it can be determined that the gripping component  140  is already aligned with the target object  210  in orientation. Otherwise, if the first time point is different from the second time point, the controller  300  may cause the gripper  120  to rotate to align the gripping component  140  with the target object  210  in orientation. In this way, the gripping component  140  can be aligned with the target object  210  in orientation properly. 
     In some cases, a lower surface  180  of the gripping component  140  may be not parallel to an upper surface  240  of the target object  210 . This would result in that the gripping component  140  may be unable to grip the target  210  precisely.  FIG.  8    schematically illustrates an example process of causing a lower surface  180  of the gripping component  140  to be parallel to an upper surface  240  of the target object  210  using an apparatus  200  for use with the robot  100  according to a further example embodiment. 
     The apparatus  200  as shown in  FIG.  8    differs from the apparatus  200  as shown in  FIG.  1    in that a third and a fourth reflective photoelectric sensors  160 ,  170  are arranged on the gripper  120 . In this embodiment, the reflective photoelectric sensor  130  may be still referred to as the first reflective photoelectric sensor. The first, third and fourth reflective photoelectric sensors  130 ,  160 ,  170  may be of the same or different types. In an embodiment, the first, third and fourth reflective photoelectric sensors  130 ,  160 ,  170  may be arranged to be non-collinear. With the first, third and fourth reflective photoelectric sensors  130 ,  160 ,  170 , the controller  300  may cause the lower surface  180  of the gripping component  140  to be parallel to the upper surface  240  of the target object  210  based on output signals from the first, third and fourth reflective photoelectric sensors  130 ,  160 ,  170 . 
     In an embodiment, the controller  300  may cause the first, third and fourth reflective photoelectric sensors  130 ,  160 ,  170  to locate above the target object  210 . Then, the controller  300  may determine respective distances between the upper surface  240  of the target object  210  and the first, third and fourth reflective photoelectric sensors  130 ,  160 ,  170  based on the output signals from the first, third and fourth reflective photoelectric sensors  130 ,  160 ,  170 . If the distances are the same as each other, then it can be determined that the lower surface  180  of the gripping component  140  is parallel to the upper surface  240  of the target object  210 . Otherwise, if at least one of the distances is different from the others, the controller may cause the gripper  120  to rotate such that the lower surface  180  of the gripping component  140  is parallel to the upper surface  240  of the target object  210 . In this way, the gripping component  140  can be better aligned with the target object  210 . 
     In other embodiments, one or more additional reflective photoelectric sensors may be arranged on the gripper  120  so as to calculate the robot  100  with more degrees of freedom. 
       FIG.  9    is a flow chart of a method for use with a robot according to embodiments of the present disclosure. The method  900  can be carried out by, for example the apparatus  200  for use with the robot  100  as illustrated in  FIGS.  1  and  3 - 8   . 
     At block  910 , a reflective photoelectric sensor  130  arranged on a gripper  120  of the robot  100  is caused to scan over a target object  210 ,  210 ′. For example, in some embodiments, the reflective photoelectric sensor  130  is a reflective optical fiber sensor or a laser displacement sensor. 
     At block  920 , changes in an output signal from the reflective photoelectric sensor  130  are monitored. As described above, significant changes of the output signals may represent the detection of edges of the target object  210 ,  201 ′. 
     At block  930 , for each detected change exceeding a threshold, a coordinate of a gripping component  140  on the gripper  120  in a robot coordinate system is determined, to obtain a set of coordinates. In some embodiments, the gripping component  140  includes a clamping jaw, a vacuum chuck, or an electromagnet. 
     At block  940 , a position of the target object  210 ,  210 ′ in the robot coordinate system is determined based on the set of coordinates and a predefined offset value PO between the reflective photoelectric sensor  130  and the gripping component  140 . 
     In some embodiments, the method  900  further comprises: causing the gripping component  140  to grip the target object  210 ; causing a camera to capture an image containing the reflective photoelectric sensor  130  and the target object  210  gripped by the gripping component  140 ; and determining, from the image, an actual offset value AO between the reflective photoelectric sensor  130  and the target object  210  gripped by the gripping component  140 , as the predefined offset value PO. 
     At block  950 , the position of the target object  210 ,  210 ′ is stored for future use in assembling objects. 
     In some embodiments, the position of the target object  210  is stored as a gripping position of the gripping component  140 . In some embodiments, the position of the target object  210 ′ is stored as a dropping position of the gripping component  140 . 
     In some embodiments, the reflective photoelectric sensor  130  is a first reflective photoelectric sensor, and the method further comprises: aligning the gripping component  140  with the target object  210  in orientation based on output signals from the first reflective photoelectric sensor and a second reflective photoelectric sensor  150  arranged on the gripper  120 . 
     In some embodiments, aligning the gripping component  140  with the target object  210  in orientation comprises: causing the first and second reflective photoelectric sensors  130 ,  150  to move towards a side  230  of the target object  210 ; determining a first time point when a change in the output signal from the first reflective photoelectric sensor  130  exceeds the threshold; determining a second time point when a change in the output signal from the second reflective photoelectric sensor  150  exceeds the threshold; and if the first time point is different from the second time point, causing the gripper  120  to rotate to align the gripping component  140  with the target object  210  in orientation. 
     In some embodiments, the reflective photoelectric sensor  130  is a first reflective photoelectric sensor, and the method further comprises: causing a lower surface  180  of the gripping component  140  to be parallel to an upper surface  240  of the target object  210  based on output signals from the first reflective photoelectric sensor, and a third and a fourth reflective photoelectric sensors  160 ,  170  arranged on the gripper  120 . 
     In some embodiments, causing the lower surface  180  of the gripping component  140  to be parallel to the upper surface  240  of the target object  210  comprises: causing the first, third and fourth reflective photoelectric sensors  130 ,  160 ,  170  to locate above the target object  210 ; determining respective distances between the upper surface  240  of the target object  210  and the first, third and fourth reflective photoelectric sensors  130 ,  160 ,  170  based on the output signals from the first, third and fourth reflective photoelectric sensors  130 ,  160 ,  170 ; and if at least one of the distances is different from the others, causing the gripper  120  to rotate such that the lower surface  180  of the gripping component  140  is parallel to the upper surface  240  of the target object  210 . The subject matter described herein may be embodied as a device comprising a processing unit and a memory. The memory is coupled to the processing unit and stores instructions for execution by the processing unit. The instructions, when executed by the processing unit, cause the device to perform the method as described above. 
     In the context of the subject matter described herein, a memory may be any tangible medium that can contain, or store a program for use by or in connection with an instruction execution system, apparatus, or device. The memory may be a machine readable signal medium or a machine readable storage medium. A memory may include but not limited to an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples of the memory would include an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. 
     It should be appreciated that the above detailed embodiments of the present disclosure are only to exemplify or explain principles of the present disclosure and not to limit the present disclosure. Therefore, any modifications, equivalent alternatives and improvement, etc. without departing from the spirit and scope of the present disclosure shall be included in the scope of protection of the present disclosure. Meanwhile, appended claims of the present disclosure aim to cover all the variations and modifications falling under the scope and boundary of the claims or equivalents of the scope and boundary.