JP4390758B2 - Work take-out device - Google Patents

Description

  The present invention relates to a workpiece take-out device that takes out a workpiece using a hand attached to a robot, and more particularly to a workpiece take-out device provided with a gripping means that can be used to take out a workpiece in a container.

  In recent years, the number of cases in which peripheral devices such as workpiece supply / positioning devices are simplified by making intelligent robots that perform work on workpieces has increased. For example, the work of taking out workpieces stacked in a container or on a pallet is one of the typical applications where a robot with a hand attached to the tip of an arm can be considered. Therefore, it is necessary to detect the position / posture of each workpiece by a visual sensor, determine the work position / posture of the robot based on the result, and execute the workpiece by hand.

  When workpieces stacked in bulk are stored in a container, it is required to prevent the robot arm and hand from interfering with the container when removing the workpiece. Restrictions arise in the work. In other words, workpieces that can be taken out without worrying about interference with the container are limited to those that are sufficiently separated from the inner wall of the container and satisfy the limiting conditions such as not tilting toward the inner wall of the container. It was not easy to take out.

  As a proposal regarding measures for avoiding such interference, there is, for example, the following Patent Document 1 (interference avoidance device). In Patent Document 1, in order to avoid interference between the container, the robot arm, and the hand, an interference region is set in advance, and the robot arm posture and wrist rotation posture are set so that the robot does not enter the interference region. A technique for actively changing is shown. However, in such a conventional technique, although interference between the robot arm and the hand and the container can be avoided, when the work to be taken out is located near the inner surface of the container or when it is inclined toward the inner wall side of the container, Or, when other workpieces are piled up high around the workpiece to be taken out, none of the workpieces can be taken out.

JP 2002-331480 A

  Accordingly, an object of the present invention is to provide a workpiece take-out apparatus that can greatly relieve the conditions of a work that can be taken out, particularly when taking out a work in a container.

The present invention is applicable to "a workpiece take-out apparatus including a robot whose robot arm tip is rotatable about a rotation axis and a hand attached to the robot arm tip and having a gripping means for gripping a workpiece". is intended to, in the apparatus, the base of "the gripping means extends into a rod along the central axis of the central portion have penetration force extending away from the robot arm tip gripping of said gripping means The center is offset from the central axis, and the angle formed by the grip direction in which the grip means is facing and the axial direction of the rotary shaft is configured to be a predetermined angle other than 0 degrees. The above-mentioned problem is solved by imposing.

  As a result, it is possible to grip a workpiece close to the container inner wall without bringing the center of the base of the hand close to the container inner wall. Without this offset, it is necessary to match the center axis of the work with the center axis of the base of the hand, and the condition that the distance between the center of the work and the inner wall of the container exceeds the interference radius of the hand must be satisfied. For example, the workpiece cannot be gripped without interference between the hand and the inner wall of the container, and according to the present invention, such a situation can be avoided.

Here, the direction of the central axis can be made variable by pivotally supporting the base of the gripping means. Further , a visual sensor may be further provided so that the overlapping state of the workpieces is detected by the visual sensor, and the gripping position of the workpiece may be changed based on the result. The use of the visual sensor is particularly effective when a part of the work to be taken out overlaps the surrounding work. That is, by identifying a grippable part using a visual sensor and gripping the part, it is possible to grip a workpiece partially overlapping with the surrounding workpiece without interfering with the surrounding workpiece.

  When a visual sensor is provided and imaging is performed using the imaging means, the imaging means of the visual sensor is attached via a slider mechanism that moves in a direction away from and approaching the tip of the robot arm. The risk of interference can be reduced by sometimes moving the robot arm away from the tip of the robot arm and moving it in the direction approaching the tip of the robot arm when gripping the workpiece.

  That is, when the surrounding workpiece is stacked higher than the workpiece to be taken out, it is necessary to approach the workpiece in order to accurately pick up the workpiece to be picked up by the visual sensor. Although there is an increased risk of interference between the workpiece and the hand, the imaging means of the visual sensor is attached via the slider mechanism as described above, and at the time of imaging, it is moved away from the tip of the robot arm to prevent interference during imaging. it can. Further, by retracting in the reverse direction during gripping, it is possible to avoid the imaging means from interfering with gripping.

As described above, according to the workpiece take-out device according to the present invention, the workpiece close to the inner wall of the container can be gripped. In addition, the workpiece can be taken out while preventing interference between the hand or the visual sensor and other workpieces or containers. Therefore, the efficiency of the taking-out operation is improved and it is economical. Further, the problem that the work that can be taken out due to interference between the hand or the visual sensor and the container or other work can be solved, so that the hand, the visual sensor, the work or the like can be prevented from being damaged.
Furthermore, there is a possibility that the position / posture of the image pickup unit may be shifted during interference. In this case, the work must be interrupted and recalibration must be performed, but such a situation can be avoided.

  DESCRIPTION OF EMBODIMENTS Embodiments relating to a workpiece picking apparatus according to the present invention will be described below in detail with reference to the drawings. First, FIG. 1 shows an overall configuration of a workpiece picking apparatus according to an embodiment. Reference numeral 1 denotes, for example, a vertical articulated robot (hereinafter simply referred to as “robot”), which is connected to the robot control device 2 by a cable 6, and its operation is controlled by the robot control device 2. A hand 3 and a three-dimensional visual sensor (sensor head including an imaging unit; hereinafter, represented by an imaging unit) 4 are attached to the arm tip of the robot 1. The hand 3 has a gripping mechanism (work gripping means) to be described later, and the control is performed by the robot control device 2. Note that a signal and electric power for controlling the hand 3 are supplied by a cable 8 connected to the robot control device 2.

  As the three-dimensional visual sensor, for example, a well-known sensor that combines a projector that projects so-called pattern light such as slit light or spot light and a video camera (imaging means) that detects reflected light can be used. A CCD video camera can also be used to obtain a two-dimensional image in normal shooting (no light projection).

  The imaging means (sensor head) 4 of the three-dimensional visual sensor is connected to a control processing device 5 for the visual sensor by a cable 9. The control processing device 5 controls a sensing operation (light projection, imaging, etc.) by the three-dimensional visual sensor and processes a light detection signal (video video signal) obtained by sensing (including normal imaging), which will be described later. In an aspect, the apparatus includes hardware and software for sending required information to the robot controller 2 via the LAN network 7, and a personal computer, for example, can be used.

  In this example, the workpieces taken out by the hand 3 of the robot 1 are a large number of workpieces 13 accommodated in a stacked state in a basket-like container 11 placed near the robot 1. For example, a container having a rectangular opening defined by the inner wall 12 is used as the container 11, but the shape is not particularly limited.

  Next, the structure and function of the hand 3 will be described with reference to FIG. As shown in FIGS. 2A and 2B, the hand 3 is attached to the mount 41 of the robot arm tip 10 using a connecting member 31. In the connecting member 31, for example, an expansion / contraction mechanism including an expansion / contraction means 32 driven by a pneumatic cylinder or the like is disposed in parallel, and a gripping means (hand main body portion) 35 that performs an operation of gripping a workpiece is connected to the connection member 31. In addition, it is pivotally supported by rotation support portions (pivotal axes) 33 and 34 provided near the tip of the expansion / contraction means 32.

  Examples of the gripping means 35 include a chuck type that grips an object by opening and closing a plurality of claws, a type that uses a suction pad that utilizes vacuum pressure, and a type that attracts an object by magnetic force using an electromagnetic magnet. Yes, which one to use is selected by design according to conditions such as the shape, material, and weight of the workpiece to be grasped. Here, the thing using the opening-and-closing members 35a and 35b is used as an example. Regardless of which is used, a command for gripping (for example, a closing command / opening command, a suction command / suction stop command, etc.), power feeding, and the like are sent from the robot controller 2.

  Now, such a gripping means 35 can rotate about the rotation support portion 33 with the connecting member 31 and change its posture by causing the expansion / contraction means 32 to perform an expansion / contraction operation. A plurality of expansion / contraction positions of the expansion / contraction means 32 are set in advance, and can be switched by a command from the robot controller 2. These expansion / contraction positions include the expansion / contraction positions that cause the gripping means 35 to take the first posture shown in FIG. 2A and the second posture shown in FIG. 2B.

  Here, the first posture is configured such that the angle formed by the gripping direction A of the gripping means 35 and the axial direction B of the rotation axis of the robot arm tip 10 is a predetermined angle other than 0 degrees. Hereinafter, the angle formed between the directions AB is referred to as “grip angle” for convenience. The grip angle corresponding to the first posture (a) is referred to as “first grip angle”. Similarly, the grip angle corresponding to the second posture (b) is referred to as a “second grip angle”.

Typically, the first grip angle is greater than 0 degrees (for example, 10 degrees or more) and 45 degrees or less. Such setting has a meaning that it is provided when the work to be taken out is inclined toward the inner wall 12 side of the container 11. This will be described with reference to FIG.
As shown in FIG. 3, when the work 13a to be taken out is inclined toward the inner wall 12 side of the container 11, the work can be avoided while avoiding interference by setting the first gripping angle to be substantially greater than 0 degrees. 13a can be grasped and taken out. On the other hand, if the first gripping angle is 0 degree, the normal direction set up in front of the workpiece 13a and the axis direction of the rotation axis of the robot arm tip coincide with each other. If the inner wall 12 or the like of the container 11 is present in the vicinity, there is a high possibility that a part of the hand 3 or the robot arm interferes with the container 11. However, if the inclination of the workpiece 13a is small, interference hardly occurs even if the first gripping angle is 0 degree.

  Next, the second posture shown in FIG. 2B is such that the gripping angle (the angle formed by the gripping direction of the gripping means 35 and the axial direction of the rotation axis of the robot arm tip 10) is approximately 90 degrees. Is set. By setting the second posture in this way, the gripping means 35 of the hand 3 approaches the workpiece from a direction not occupied by other workpieces in the vicinity space of the workpiece to be taken out, The workpiece can be gripped while avoiding the interference.

Further, when imaging is performed in order to know the position and orientation of the workpiece with the visual sensor, it is necessary to bring the imaging means 4 of the visual sensor close to the workpiece, but by switching the gripping means 35 to the second orientation, Interference with the group can be avoided. Furthermore, as shown in FIG. 2A, the grip center C of the grip means 35 of the hand 3 is configured to be offset from the center axis D of the base portion of the grip means 35 . Thus, when gripping, it is not necessary to match the center axis of the workpiece with the center axis of the base of the gripping means 35 , so even when the work is close to the inner wall of the container, the center of the gripping means base is offset to the inside of the container. Can take the posture. This situation is shown in FIG. In FIG. 4, reference numeral 13 b is a work to be taken out by the gripping means 35 and is in a position close to the inner wall 12 of the container.

  Now, in the hand 3 having the structure shown in FIG. 3, the imaging means (which may include a projector) 4 is fixedly attached to the mount 41 of the robot arm tip 10. That is, the positional relationship between the gripping means 35 and the imaging means (which may include a projector) 4 is invariable except for the posture change of the gripping means 35. For this reason, the hand 3 automatically approaches the work when the image pickup means 4 is brought close to the work for image pickup, and the image pickup means 4 automatically approaches the work when the gripping means 35 is held close to the work. In general, there is no need to approach the gripping means at the time of imaging, and there is no need to approach the imaging means at the time of gripping. In either case, this only increases the risk of interference with surrounding workpieces.

  Therefore, in this embodiment, in addition to the above-described hand structure, it is also proposed to provide a slide mechanism that enables the imaging unit to move between the hand tip side and the base side. FIG. 5 shows an example of a hand structure having such a slide mechanism. In the hand structure shown in the figure, the imaging means 4 of the visual sensor is not fixedly attached to the mount 41 of the robot arm tip 10 but is imaged parallel to the axial direction of the rotation axis of the robot arm tip 10. It is attached via a slider mechanism 42 for moving the means 4.

  Reference numeral 43 denotes a carrier of the image pickup means 4, and the image pickup means 4 moves when the carrier 43 moves on the slider mechanism 42. Although not shown, the slider mechanism 42 is connected to the robot control device 2, and the position (slide position) of the carrier 43 to the imaging means 4 is controlled by a command from the robot control device 2. .

  As a result, when it is necessary to approach the target workpiece for imaging, the approach of the hand 3 can be minimized by sliding the imaging means 4 to the position closest to the target workpiece. Thereby, when approaching for imaging, the possibility that the hand 3 may cause interference with surrounding workpieces or the like can be reduced. Similarly, when it is necessary to approach the target workpiece for gripping, the approach of the imaging unit 4 can be minimized by retracting the imaging unit 4 to a position farthest from the target workpiece. Thereby, even when approaching for gripping, it is possible to reduce the possibility that the imaging unit 4 causes interference with surrounding workpieces.

  As shown in FIG. 6, this interference avoidance method is particularly effective when the workpiece 13c to be imaged or grasped is at a position deeper (sunk) than the surrounding workpiece group. In FIG. 6, the position of the imaging means 4 indicated by a solid line indicates an example of “sliding position when imaging the workpiece 13 c”, and the position of the imaging means 4 indicated by a broken line is “when gripping the workpiece 13 c”. An example of “slide position” is shown.

  Using the workpiece take-out device having the above configuration and functions, an operation of taking out the workpieces 13 in the container 11 one by one is executed. The outline of the procedure until the operation of actually grasping the workpiece 13 using the grasping means 35 of the hand 3 is as follows, for example.

  (1) Move the robot 1 to the first imaging position. Here, the first imaging position is a robot position suitable for accommodating the distribution area of the work 13 (the inner area of the inner wall 12 of the container 11) with some margin using the imaging means (video camera) 4 of the visual sensor. It is assumed that the robot controller 2 is taught in advance.

  (2) At the first imaging position, an image containing the distribution area of the work 13 (the inner area of the inner wall 12 of the container 11) is acquired using the imaging means 4.

  (3) Attempt to detect individual workpieces 13 using the control processing device 5. Various methods for detecting individual workpieces are known, but here, a two-dimensional image of a workpiece is taught to the control processing device 5 in advance, and one or more workpieces 13 are found by a matching method to perform three-dimensional measurement. It can be detected by performing.

  (4) “Work to be taken out this time” is selected from the detected works according to an appropriate rule. There are various methods of selection, and which one is used is selected by design. For example, there is a rule for preferentially selecting a work existing at the highest position. The detected height of each workpiece can be detected, for example, by sequentially performing slit light projection on each workpiece and performing three-dimensional measurement. There is also a rule that gives priority to a workpiece close to the center of the container 11.

  (5) A second imaging position for obtaining detailed information on the work to be taken out this time and the surrounding situation is determined. This second imaging position is based on, for example, the two-dimensional position information (obtained by imaging at the first imaging position) of the work to be taken out this time (the three-dimensional position) previously taught to the robot controller 2. It may be corrected (the height is as taught). Alternatively, the second imaging position may be determined using the three-dimensional position data of the workpiece to be extracted this time among the three-dimensional measurement data obtained by projecting slit light onto each workpiece in (4) above. .

  (6) Interference between the workpiece group and the hand 3 is avoided by causing the gripping means 35 to take the second posture and folding it compactly. Note that the second orientation may also be adopted for the imaging in (3) above.

  (7) When the hand shown in FIG. 5 is used, the imaging means 4 is slid to the direction away from the tip of the robot arm, that is, the position closest to the workpiece. In this case, in the above (5), the second imaging position is determined in consideration of this slide position. Further, when the hand shown in FIGS. 2A and 2B is used, the step (7) is naturally omitted.

  (8) The robot 1 is moved to the second imaging position. Here, as described above, even if the work to be taken out sinks more than the surrounding work, the direction shown in FIG. 5 is used to separate the imaging means 4 from the tip of the robot arm, that is, the most work side. If it is slid to the position, the possibility of interference can be greatly reduced.

  (9) The control processing device 5 is used to acquire a two-dimensional image of the work to be taken out and the vicinity thereof. In addition, three-dimensional measurement of the workpiece and the vicinity thereof is executed by slit light projection by a projector.

  (10) The result is analyzed in the control processing device 5, the work to be taken out and the situation in the vicinity of the work are judged, and the gripping mode matching the situation is determined. This situation determination is performed in the form of determining which of the cases classified to cover the assumed situation. FIG. 7 exemplifies the main points (flow chart) of these cases and the determination process, and the gripping style selected according to the determination result. Here, as an example of the workpiece shape, a circular ring shape (for example, a tire for a vehicle) is assumed.

  First, it is determined whether or not a work to be taken out is standing (step S1). In order to make a determination of “Yes” / “No” in step S1, the imaging means 4 images a workpiece from a plurality of directions in advance, and prepares a teaching image model in the control processing device 5 based on this. Keep it. Then, the work image acquired at the second imaging position is compared with the teaching image models (a plurality), the teaching image model having the highest matching degree is selected, and it is determined whether the work is standing or standing. .

  If the determination in step S1 is “No”, it is determined whether or not the workpiece to be taken out overlaps with another workpiece (step S2). In the case of a circular ring-shaped workpiece, for example, this determination may be “Yes” if the elliptical arc contour of the workpiece to be taken out is cut by another elliptical arc contour, and “No” otherwise. .

  If the determination in step S2 is “No”, it is determined that the workpiece to be taken out is in a state of sleeping without overlapping with other workpieces, as indicated by reference numeral 21. Here, “middle grip” is selected as the grip mode matching this situation (step S3). “Intermediate grip” is a gripping mode in which the workpiece 21 is gripped from the inside of the workpiece 21 by opening the opening and closing members 35 a and 35 b of the gripping means 35. That is, in this “intermediate grip” mode, the side surface of the inner ring of the work 21 is used as a contact surface, and the gripping means 35 is brought close to the top of the work 21 and then the open / close members 35a and 35b are opened to grip the work.

  If the determination in step S <b> 2 is “Yes”, the workpiece to be taken out overlaps with another workpiece 23, as indicated by reference numeral 22, and is partially covered with the other workpiece 23. judge. Here, “external grip” is selected as the gripping mode that matches this situation (step S4). Here, the “external grip” is a mode in which the open / close members 35a and 35b (see FIG. 2A) are closed to grip the arc-shaped portion on the side not covered with the other work 23 from both sides. .

  That is, when there is an overlap with another work 23, the side surfaces of the inner ring and the outer ring of the work 22 are used as contact surfaces, the gripping means 35 is brought closer to the top of the work, the open / close members 35a and 35b are closed, and the work 22 is gripped. To do.

  On the other hand, if the determination in step S1 is “Yes”, it is determined whether there is an empty area on both sides of the workpiece to be taken out (step S5). In the case of a circular ring-shaped workpiece, this determination is made, for example, on both sides of the workpiece 24 to be picked up by detecting the outlines of thin strip portions (other standing workpieces) at the same height as the workpiece to be picked up. Otherwise, “vacant on both sides” (work 24), “one side free” (work 25) if other standing work 26 is detected on one side, other standing work 27, 29 on both sides. May be detected as “no space” (work 28).

The gripping modes selected corresponding to these situations are “upper grip” (step S6), “lateral grip” (step S7), and “ungripable” (step S8) in this order.
“Upper grip” (step S6) and “lateral grip” (step S7) belong to the category of so-called “vertical grip”. In “upper gripping”, the workpiece is gripped by closing both the open / close members 35a and 35b after the gripping means 35 is brought close to the top of the workpiece 24 using both surfaces in the thickness direction of the workpiece as contact surfaces.

  In “lateral grip” when there is a space only on one side of the work 25, the side surfaces of the inner ring and the outer ring of the work 25 are used as contact surfaces, and the gripping means 35 is brought closer to the work 25 from the direction where there is an empty space on the side of the work 25 After that, the work 25 is gripped by closing the opening and closing members 35a and 35b.

If it is determined that the gripping is impossible (step S8), in order to change the work to be taken out this time, a work that is determined to be the second best in the application rule in the above (4) is determined, and the processes in the above (5) repeat. Thereafter, the same process is repeated until one of steps S3, S4, S6, and S7 is reached. If all the workpieces detected in (3) above reach step S8, an alarm is output to stop the system. However, such a situation should rarely occur.
Even if a workpiece that cannot be gripped once, as described above, if another workpiece is taken out first, there is generally a possibility that a space will be generated on one side of the workpiece that could not be gripped. It is considered extremely high.

  (11) The posture of the gripping means 35 is selected according to the determined gripping mode. When approaching the workpiece from above, the gripping means 35 is set to the first posture (case other than side gripping). When approaching the workpiece from the side, the gripping means 35 is set to the second posture (side gripping case).

  (12) When the hand shown in FIG. 5 is used, the imaging means 4 is slid (retracted) to the direction approaching the tip of the robot arm, that is, the position farthest from the workpiece. When the hand shown in FIGS. 2A and 2B is used, the step of (12) is naturally omitted.

  (13) A work position (robot position) for carrying out the gripping work is determined based on the position / posture of the work to be taken out and the determined gripping mode. In addition, the approach point (one or more) before that is determined as necessary.

(14) The robot 1 is moved to a predetermined work position, and gripping is executed in the above-described manner according to the determined gripping mode. When the approach point is determined, the robot 1 is moved to the work position via the approach point, and gripping is executed. The gripped work is carried to a designated place, and the work is released by releasing the restraint by the gripping means 35.
(15) Return to (1) above, and repeat the process cycle until no workpiece is detected in (3).

  As mentioned above, although typical embodiment was described, this invention is not limited to the said embodiment. For example, the imaging means of the visual sensor does not necessarily have to be attached to the robot arm, and the imaging means may be fixedly provided above the container. Further, the present invention can be applied to a case using a container having a low wall surface such as a tray or a pallet as a container, or a case in which workpieces are stacked on a mounting plate, a table, a floor surface or the like without a wall surface. Needless to say.

  The expansion / contraction means for changing the posture of the gripping means of the hand may be an expansion / contraction mechanism using an electric motor as a drive source, in addition to the pneumatic cylinder described in the embodiment. Furthermore, instead of the expansion / contraction mechanism, in order to change the posture of the gripping means, a mechanism that directly rotates the gripping means by providing a rotation mechanism using an electric motor as a drive source at the end of the connecting member may be used.

It is the figure which showed the whole structure of the workpiece picking apparatus which concerns on embodiment of this invention. 2A and 2B are diagrams illustrating a schematic structure of a hand employed in an embodiment of the present invention, in which FIG. 1A shows a state in which the gripping mechanism of the hand takes a first posture, and FIG. 2B shows a second posture in the gripping mechanism. This represents the state where It is a figure which shows the holding | grip attitude | position of a hand when a workpiece | work inclines to the container inner wall side. It is a figure which shows the holding | grip attitude | position of a hand when a workpiece | work is adjoining to the container inner wall. It is the figure which showed the outline of the hand which enabled the imaging means of the visual sensor to move. FIG. 6 is a diagram for explaining a technique for avoiding interference with surrounding workpieces when imaging / gripping a workpiece located at a position lower than the surrounding workpieces using the hand shown in FIG. 5. In an embodiment, it is a figure which illustrated correspondence with a flow chart explaining a procedure which chooses a grasping method based on a result of image pick-up and three-dimensional visual measurement, and a situation on the work side.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Robot 2 Robot control apparatus 3 Hand 4 3D visual sensor (imaging means)
5 Personal computer (control processing device for visual sensor)
6 Cable (between robot controller and robot)
7 LAN network 8 Cable (between robot controller and hand)
9 Cable (between camera and personal computer)
DESCRIPTION OF SYMBOLS 10 Robot arm front-end | tip 11 Container 12 Inner wall 13, 13a, 13b, 13c, 21-29 Workpiece | work 31 Connection member 32 Expansion / contraction means 33, 34 Rotation support part (pivot)
35 gripping means 41 mount A gripping direction of gripping means B axis direction of rotation axis of robot arm tip C gripping center of gripping means D center axis of base of gripping means

Claims (4)

In a workpiece unloading apparatus comprising: a robot whose tip of a robot arm can rotate around a rotation axis; and a hand attached to the tip of the robot arm and having a gripping means for gripping a workpiece.
The base of the gripping means extends into a rod along the central axis of the central portion have penetration force extending away from the robot arm tip,
The gripping center of the gripping means is offset from the central axis,
A workpiece picking device configured such that an angle formed by a gripping direction to which the gripping unit is directed and an axial direction of the rotation shaft is a predetermined angle other than 0 degrees.   2. The workpiece picking apparatus according to claim 1, wherein the base of the gripping means is pivotally supported so that the direction of the central axis is variable.   The workpiece according to claim 1, further comprising a visual sensor, wherein the visual sensor detects an overlapping state of the workpieces, and changes a gripping position of the workpiece based on the result. Take-out device.   A visual sensor is further provided, and the imaging means of the visual sensor is attached via a slider mechanism that moves in a direction away from and toward the robot arm tip, and in a direction away from the robot arm tip during imaging. The workpiece picking apparatus according to claim 1 or 2, wherein the workpiece picking device moves and moves in a direction approaching the tip of the robot arm when gripping the workpiece.

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