Patent Publication Number: US-2022226878-A1

Title: Robot gripper, industrial robot, handling system and method for removing plate-shaped workpieces from a stack

Description:
TECHNICAL FIELD 
     The invention relates to a robot gripper configured to be connected to an industrial robot. Moreover, the invention relates to an industrial robot with this gripper, and to a handling system comprising this industrial robot. In addition, the invention relates to a method for removing plate-shaped workpieces, which preferably consist at least in sections of wood, wood materials, plastic, aluminum or the like, from a stack. 
     PRIOR ART 
     In the furniture and components industry, an increase in demand for individualized products is currently apparent. For example, end consumers are increasingly buying furniture items that are customized to the dimensions of a specific room, or that feature a particular color and/or material combination. Despite a high degree of individualization and the associated small batch size, such products can be manufactured on an industrial scale. 
     In the case of a piece of furniture, for example, it is conceivable to first prepare all the required plates and then stack them on a storage device. Since a piece of furniture usually consists of a large number of various plates, the prepared plates that are stacked on the storage device mostly form an uneven stack. An uneven stack, also referred to as chaotic stack, has individual layers, the composition and/or arrangement of which differs. As shown in  FIG. 1 , a chaotic stack  210  on a storage device  200  may comprise workpieces  220   b  lying in an edge section of the chaotic stack  210 . Moreover, a chaotic stack  210  may comprise workpieces  220   a  lying in a central section of the chaotic stack  210 . In addition, it is clear that the workpieces  220  of the chaotic stack  210  may have different dimensions. 
     In order to remove parts from a chaotic stack in an automated manner (i.e. to separate plates), industrial robots, e.g. gantry robots or six-axis robots, with suction grippers can be used. A suction gripper is configured to pick up and hold objects by negative pressure. It has been shown, however, that individual, in particular individual superimposed plates, are difficult to separate due to adhesion effects or other surface adhesion effects. Specifically, when lifting one plate, there is the risk of also lifting the plate underneath. This effect is intensified if the workpieces to be sucked have a certain porosity, i.e. pass on the negative pressure of the gripper at least in part to plates underneath. 
     In order to nevertheless be able to remove an individual plate from a chaotic stack, it has proven useful to push the plate to be removed off the plate underneath using a suction gripper. The plate to be removed is usually displaced parallel to the top surface of the plate underneath. However, with this method, the plates lying in an edge section of the chaotic stack must first be removed. Only after these outer plates have been removed is there sufficient space to also remove the inner plates by displacement. Therefore, a removal sequence is specified which does not necessarily correspond to the optimal sequence. 
     DESCRIPTION OF THE INVENTION 
     The object of the invention is to provide a simple and efficient possibility for removing plate-shaped workpieces from a stack. 
     According to the invention, this object is solved by a robot gripper according to claim  1 , an industrial robot according to claim  14 , a method according to claim  15 , and a handling system according to claim  21 . Preferred embodiments are specified in the subclaims. 
     A robot gripper according to the invention comprises a receiving area configured to be connected to an industrial robot. Moreover, the robot gripper comprises at least one suction unit configured to be connected to a vacuum source. The suction unit comprises a suction surface on which a preferably plate-shaped workpiece, preferably consisting at least in sections of wood, wood materials, plastic, aluminum or the like, can be held by negative pressure. In addition, the robot gripper comprises a substantially straight pressing edge in an edge section. A robot gripper according to the invention moreover comprises an actuator that is configured to tilt the suction unit about a first tilt axis relative to the receiving area. 
     In the present context, an industrial robot is a device that is suitable for picking up a workpiece, changing its position and laying down the workpiece. An industrial robot may be, for example, a six-axis robot, a linear robot, a gantry robot, a robot with parallel kinematics, or a comparable device. 
     A suction unit can be configured, for example, as a suction box or as a plurality of suction cups. For example, a suction box can be configured substantially cuboid in shape. A suction surface can e.g. be a surface of a suction box which has openings. However, a suction surface can also be a virtual surface that is defined by a plurality of suction cups. 
     An actuator can be, for example, a linear motor, a spindle drive, a hydraulically or pneumatically operable cylinder, or a magnetically operable actuator; 
     A robot gripper according to the invention advantageously makes it possible to apply a method in which even centrally arranged plates of a chaotic stack can be removed from the chaotic stack without first having to remove plates arranged in edge sections of the chaotic stack. For this purpose, the gripper according to the invention can first be moved by an industrial robot into the vicinity of a workpiece to be removed which lies on a chaotic stack. The workpiece to be removed can be a plate-shaped workpiece, for example, which lies in the uppermost stack layer of a chaotic stack in a central section of the chaotic stack. The suction unit can then be tilted such that the suction surface assumes an angle relative to the workpiece to be removed, e.g. an angle of between 1° and 2° inclusive. Furthermore, the robot gripper can be moved such that the pressing edge of the robot gripper lies substantially on an edge of the workpiece to be removed. The suction unit and the workpiece to be removed may enclose a substantially wedge-shaped gap in this state. If a negative pressure is applied to the suction unit in this state, the workpiece lying under the robot gripper can be pulled to the suction surface of the suction unit without another workpiece lying underneath adhering to the pulled workpiece. During pulling, the workpiece substantially performs a tilting movement towards the suction surface. The pressing edge of the robot gripper can be regarded as the tilt axis with respect to the tilting movement of the workpiece. 
     A robot gripper according to the invention may comprise a plurality of suction units, e.g. two suction units. A first suction unit can be longer than a second suction unit with respect to a first direction. The suction surface of the first suction unit and the suction surface of the second suction unit are preferably in the same plane. The first and second suction units are preferably arranged side by side with respect to the first direction. 
     If a robot gripper comprises first and second suction units, as described above, the plurality of suction units can define a suction unit group. A robot gripper according to the invention may comprise a plurality, but preferably two, of these suction unit groups. Preferably, the suction surfaces of the suction units of the plurality of suction unit groups are in the same plane. Preferably, the suction unit groups are arranged side by side with respect to a second direction that is different from the first direction. The second direction can be perpendicular to the first direction. 
     If a robot gripper comprises a plurality of suction units, the robot gripper is preferably configured such that individual suction units or each individual suction unit can be selectively supplied with negative pressure. 
     A plurality of suction units can be associated with increased flexibility of the robot gripper. In particular if the suction units can be selectively supplied with negative pressure, such a robot gripper is suitable for removing plates of different dimensions. For instance, only one of the suction units of the robot gripper, namely a suction unit with a small suction surface, can be supplied with negative pressure in order to remove a small workpiece from a stack. However, another suction unit of the robot gripper, namely a suction unit with a larger suction surface, can also be supplied with negative pressure in order to remove a larger workpiece from a stack. In addition, a plurality of suction units of a robot gripper can also be simultaneously supplied with negative pressure in order to remove a very large workpiece from a stack. 
     A robot gripper according to the invention can be configured to tilt the suction unit relative to the receiving area by an angle of between 0.1° and 35° inclusive. Preferably, a robot gripper according to the invention is configured to tilt the suction unit relative to the receiving area by an angle of between 0.5° and 10° inclusive. Particularly preferably, a robot gripper according to the invention is configured to tilt the suction unit relative to the receiving area by an angle of between 1° and 2° inclusive. 
     It has been shown that the above-mentioned intervals are particularly suitable for picking up a plate-shaped workpiece in a process-safe manner and at the same time preventing that a workpiece located underneath adheres to the workpiece to be removed. 
     With a robot gripper according to the invention, the pressing edge may be an edge extending substantially parallel to the first tilt axis. For instance, the pressing edge may be an edge that is formed between the suction surface and a lateral surface of the suction unit and extends substantially parallel to the first tilt axis. Preferably, the pressing edge is provided with a radius smaller than or equal to 10 mm, and particularly preferably with a radius smaller than or equal to 1 mm. For example, the radius is in a range of between 0.2 mm and 1 mm inclusive. 
     If a pressing edge is provided with a radius, this can have an advantageous effect on possible interactions with the workpiece to be removed. Specifically, a radius is conducive to the damage-free removal of the workpiece. 
     However, the pressing edge may also be an element that is not directly connected to the suction surface of a suction unit. For example, a pressing edge may be an element that is attached to the receiving area of the robot gripper or to another structural element thereof. Moreover, a robot gripper may have a plurality of pressing edges. 
     Furthermore, a robot gripper according to the invention may comprise an identification system configured to read data from an identification means of a workpiece. An identification means may be a barcode, a QR code, an RFID tag, or a comparable identification means, for example. In this way, it can be made possible for a production control system, for example an IVIES system, to plan upcoming production steps and/or to monitor the production process. 
     A robot gripper according to the invention may comprise a recognition system configured to recognize the size and/or position of workpieces and/or workpiece edges. For example, the robot gripper may be configured to recognize the size and/or position of workpieces and/or workpiece edges by means of an optical image recognition system. Alternatively or additionally, the robot gripper may be configured to determine the size and/or position of workpieces and/or workpiece edges by means of the identification means and/or by means of data provided by a central device. 
     Using such a recognition system, the position of workpiece edges can be detected with a relatively high degree of accuracy, allowing the pressing edge of the robot gripper to be placed relatively accurately on a workpiece edge. A relatively accurate placing of the pressing edge of the robot gripper on a workpiece edge is in turn conducive to process-safe removal of the workpiece to be removed from the stack. 
     A robot gripper according to the invention may be configured to recognize whether the pressing edge is in contact with an object. Preferably, the robot gripper is also configured to obtain information on the pressing force resulting from the contact. Thus, firstly, a certain pressure can be created on the workpiece edge, which is necessary for removing the workpiece. Secondly, the workpiece can thus be protected from damage, e.g. due to an increased pressing force. 
     With a robot gripper according to the invention, the actuator may be pretensioned to a first zero position with a spring element, and the actuator may be configured to perform a discrete stroke opposite to the pretension when pressurized. In this way, a simple structure and a simple control of the robot gripper can be made possible. Specifically, with this robot gripper, a second pressurization is not necessary to bring the actuator from a stroke state into its zero position. Rather, it is sufficient to remove the pressurization with which the actuator performs a stroke opposite to the pretension. The spring element mentioned above may be, for example, a torsion spring, a coil spring, a bending spring or a gas pressure spring. 
     The robot gripper described above can comprise at least two serially connected actuators, wherein each of the at least two serially connected actuators is configured to perform a discrete stroke opposite to the pretension when pressurized, and wherein the robot gripper is configured such that each of the at least two serially connected actuators can be selectively pressurized. In this manner, a cost-effective robot gripper can be provided, in which the suction surface of the suction unit may take different tilt angles. The robot gripper can thus be used flexibly for different workpiece types and workpiece sizes. 
     Furthermore, one of the aforementioned robot grippers can comprise at least two actuators arranged in parallel, wherein each of these at least two actuators is configured to perform a discrete stroke opposite to the pretension when pressurized, and wherein the robot gripper is configured such that each of the at least two actuators connected in parallel can be selectively pressurized. In this manner, it can be made possible to tilt the suction surface of the suction unit in a first direction and/or in a second direction. The first direction and the second direction may be opposite directions. Consequently, the flexibility of the robot gripper can be further increased. 
     Furthermore, a robot gripper according to the invention can be configured such that the at least one suction unit can be tilted about a second axis, wherein the suction unit is pretensioned to a second zero position with respect to the second axis, the second axis being different from the first axis. Preferably, the second axis is perpendicular to the first axis. Moreover, the second axis is preferably parallel to the first direction. With such a robot gripper, it can be made possible to remove a workpiece from a stack, which lies on the stack at an angle with respect to the horizontal plane. The horizontal plane is a plane with respect to which the direction of gravity is perpendicular. Moreover, such a robot gripper can be used to remove workpieces from a stack which do not have a constant thickness. 
     An industrial robot according to the invention is configured as a linear robot with at least two, but preferably three, purely translatory degrees of freedom and with no more than two rotatory degrees of freedom. The industrial robot according to the invention is provided with one of the robot grippers described above. 
     An advantage of the invention lies in the interaction of the industrial robot according to the invention with the robot gripper. Since the actuator for tilting the suction surface is integrated in the robot gripper, a cost-effective linear robot can be used instead of a cost-intensive six-axis robot. 
     A method according to the invention for removing plate-shaped workpieces, which preferably consist at least in sections of wood, wood materials, plastic, aluminum or the like, from a stack comprises the following steps:
         moving a previously described robot gripper into the vicinity of a workpiece to be removed, wherein the workpiece to be removed preferably lies between other workpieces;   tilting the suction unit such that the suction surface assumes an angle of between 0.1° and 35° inclusive, preferably between 0.5° and 10° inclusive, and particularly preferably between 1° and 2° inclusive, relative to the workpiece to be removed;   moving the robot gripper such that the pressing edge of the robot gripper lies substantially on an edge of the workpiece to be removed;   applying negative pressure to the suction unit such that the workpiece to be removed is pulled to the suction surface of the suction unit;   lifting the robot gripper together with the workpiece to be removed held on the suction surface.       

     A method according to the invention can moreover comprise the step of tilting back the suction unit, together with the workpiece to be removed. 
     Preferably, the method according to the invention is carried out with the industrial robot according to the invention. 
     The method according to the invention can be assigned the same and/or comparable advantages as the previously described robot gripper or industrial robot. 
     Preferably, the method step of applying negative pressure to the suction unit is carried out after the step of tilting the suction unit. In this way, the risk of also sucking further workpieces that are arranged under the workpiece to be removed is minimized. 
     A method according to the invention can further comprise the following step: detecting the size of the workpiece using a recognition system configured to detect the position of the edges of the workpiece, and/or using an identification system configured to read the size of the workpiece from an identification means assigned to the workpiece, wherein the identification means is preferably a barcode, a QR code, or a comparable code. 
     Using such a recognition system, the position of workpiece edges can be detected with a relatively high degree of accuracy, allowing the pressing edge of the robot gripper to be placed relatively accurately on a workpiece edge. A relatively accurate placing of the pressing edge of the robot gripper on a workpiece edge is in turn conducive to process-safe removal of the workpiece to be removed from the stack. 
     In addition, a method according to the invention can comprise the following step: determining the position of the edge of a workpiece on the basis of the position of an identification means assigned to the workpiece, wherein the identification means is preferably a barcode, a QR code, or a comparable code. 
     The method described above may have cost advantages over other, comparable methods. For example, configurations are conceivable in which a system for determining the position of an identification means is more cost-effective than a direct system for determining the position of an edge of a workpiece. Moreover, an aforementioned method can be more accurate than a direct system for determining the position of an edge of a workpiece. The reason is that an identification means may be provided with reproducible landmarks, whereas edges of a workpiece may have a different appearance depending on the nature of the material. 
     Furthermore, a method according to the invention can comprise the following step:
         selecting a suction unit suitable for the size of the workpiece to be removed; or selecting a combination of a plurality of suction units suitable for the size of the workpiece to be removed;   selectively supplying the selected suction unit or the selected suction units with negative pressure, wherein the non-selected suction units are not supplied with negative pressure.       

     The flexibility of the method according to the invention can be promoted by these additional method steps. Specifically, this also allows plates of different dimensions to be efficiently removed. For instance, only one of the suction units of the robot gripper, namely a suction unit having a small suction surface, can be supplied with negative pressure in order to remove a small workpiece from a stack. However, another suction unit of the robot gripper, namely a suction unit with a larger suction surface, can also be supplied with negative pressure in order to remove a larger workpiece from a stack. In addition, a plurality of suction units of a robot gripper can also be simultaneously supplied with negative pressure in order to remove a very large workpiece from a stack. 
     A handling system according to the invention comprises an industrial robot according to the invention and a measurement means. The handling system is configured to recognize whether a plurality of workpieces are held on a suction unit of the robot gripper. The measurement means is preferably an area scanner. The plurality of workpieces can be, for example, two plate-shaped workpieces that adhere to one another in the thickness direction of the workpieces due to adhesion effects and/or due to the suction effect of the robot gripper. 
     If another workpiece adheres to the underside of a workpiece to be removed, there is the risk that this workpiece falls down in an uncontrolled manner during handling. This can disrupt the further removal routine, for example. Furthermore, there is also the risk that workpieces falling down may injure persons or damage plant components. If, however, it is recognized whether a plurality of workpieces are held on a suction unit, the specific removal process can be stopped and/or reversed, for example. Alternatively or additionally, a machine operator can be called to rectify the fault. Therefore, the handling system according to the invention can be associated with the effect of improving the efficiency of the removal process, reducing the risk of injury to persons and/or protecting plant components from damage. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a perspective view of a storage device with a chaotic stack; 
         FIG. 2  is a schematic illustration of a first embodiment of a robot gripper according to the invention; 
         FIG. 3 a    is a schematic illustration of a second embodiment of a robot gripper according to the invention in a first state; 
         FIG. 4 a    is a schematic illustration of a third embodiment of a robot gripper according to the invention; 
         FIG. 4 b    is a schematic sectional view of the third embodiment of a robot gripper according to the invention along line A-A in  FIG. 4   a;    
         FIG. 5 a    schematically shows a step of an embodiment of a method according to the invention for removing plate-shaped workpieces from a stack; 
         FIG. 5 b    schematically shows a further step of an embodiment of a method according to the invention for removing plate-shaped workpieces from a stack; 
         FIG. 5 c    schematically shows a further step of an embodiment of a method according to the invention for removing plate-shaped workpieces from a stack; 
         FIG. 5 d    schematically shows a further step of an embodiment of a method according to the invention for removing plate-shaped workpieces from a stack. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention described below are merely examples, and should not be seen as limiting. Identical reference numbers specified in different figures designate identical, corresponding, or functionally similar elements. 
       FIG. 1  shows a chaotic stack  210  lying on a storage device  200 . The chaotic stack  210  comprises individual layers with workpieces  220 , the individual layers differing in composition and/or arrangement. In the illustrated case, the chaotic stack  210  consists exclusively of plate-shaped workpieces  220 . However, it is also conceivable that the chaotic stack alternatively or additionally comprises deviating workpiece types, for example bar-shaped workpieces. The chaotic stack  210  may also comprise workpieces of non-constant thickness, for example wedge-shaped workpieces and/or workpieces of any shape. The storage device  200  may be, for example, a pallet, or a parts carrier for an automated guided vehicle (AGV). The chaotic stack  210  shown in  FIG. 1  comprises workpieces  220   b  lying in an edge section of the chaotic stack  210 . In addition, the chaotic stack  210  shown in  FIG. 1  comprises workpieces  220   a  lying in a central section of the chaotic stack  210 . The workpieces  220  of the chaotic stack  210  shown in  FIG. 1  have different dimensions. 
       FIG. 2  schematically shows a first embodiment of a robot gripper according to the invention. The robot gripper  100  comprises a receiving area  10  configured to be connected to an industrial robot  300 . In addition, the robot gripper  100  comprises at least one suction unit  20  configured to be connected to a vacuum source. The suction unit  20  comprises a suction surface  21  on which a preferably plate-shaped workpiece, which preferably consists at least in sections of wood, wood materials, plastic, aluminum or the like, can be held by negative pressure. In addition, the robot gripper  100  comprises a substantially straight pressing edge  140  in an edge section. The robot gripper  100  further comprises an actuator  30  configured to tilt the suction unit  20  about a first tilt axis relative to the receiving area  10 . 
     In the first embodiment shown, the tilt axis is the axis of a joint  90 . The tilt axis is parallel to the X direction. The actuator  30  is a cylinder, for example a hydraulically or pneumatically operable cylinder. The actuator  30  is connected to a first support element  40  via a joint  110 , and via a further joint  110  to a second support element  41 . The first support element  40  is connected to the receiving area  10  without a degree of freedom. The second support element  41  is connected to the receiving area  10  via a joint  90 . The suction unit  20  is attached to the second support element  41 . In the present case, the suction unit  20  is configured as a suction box  20  that has suction openings  50  on its underside. The suction openings  50  are in fluidic connection with a space inside the suction box  20 . The space inside the suction box in turn can be fluidically connected to a vacuum source (not shown). 
       FIGS. 3 a  and 3 b    show a schematic illustration of a second embodiment of a robot gripper  100  according to the invention. The robot gripper  100  of the second embodiment comprises a receiving area  10  configured to be connected to an industrial robot  300 . Furthermore, the robot gripper  100  comprises at least two suction units  20   a ,  20   b , each configured to be connected to a vacuum source. The suction units  20   a ,  20   b  each comprise a suction surface  21  on which a preferably plate-shaped workpiece  220 , which preferably consists at least in sections of wood, wood materials, plastic, aluminum or the like, can be held by negative pressure. The robot gripper  100  of the second embodiment comprises a substantially straight pressing edge  140   a  in a first edge section, and a substantially straight pressing edge  140   b  in a second edge section. The robot gripper  100  of the second embodiment further comprises at least four actuators  30   a ,  30   b ,  30   c ,  30   d  configured to tilt the suction units  20   a ,  20   b  about a first tilt axis relative to the receiving area  10 . 
     In the case shown, the first tilt axis is an axis that is parallel to the X direction. The first tilt axis may be, for example, an axis of the joint  90 , an axis of a joint  110  of an actuator  30   a ,  30   b ,  30   c ,  30   d , or an axis that is substantially coincident with one of the pressing edges  140   a ,  140   b . In the robot gripper  100  of the second embodiment, the actuators are pretensioned to a first zero position with spring elements  60 . The robot gripper  100  of the second embodiment further comprises at least four actuators  30   a ,  30   b ,  30   c ,  30   d , with two of these actuators  30   a ,  30   b  and  30   c ,  30   d  each being serially connected. Each of the serially connected actuators  30   a ,  30   b ,  30   c ,  30   d  is configured to perform a discrete stroke opposite to the pretension when pressurized. The robot gripper  100  of the second embodiment is configured such that each of the actuators  30   a ,  30   b ,  30   c ,  30   d  can be selectively pressurized. The serially arranged actuators  30   a ,  30   b  are arranged parallel to the serially arranged actuators  30   c ,  30   d . In this way, the robot gripper  100  can be tilted in two opposite directions. Due to the serial arrangement of the actuators  30   a ,  30   b  and  30   c ,  30   d , respectively, it is possible for the suction units  20   a ,  20   b  or the suction surface  21  of the robot gripper  100  to assume at least four different tilt angles relative to the receiving area  10 . The robot gripper of the second embodiment may comprise spring elements  60  that are configured, for example, as gas pressure springs, torsion springs, bending springs, or the like. The variant of the second embodiment shown in  FIGS. 3 a  and 3 b    comprises gas pressure springs  60 . 
     As shown in  FIGS. 3 a  and 3 b   , a suction unit  20   a  of the robot gripper  100  may be longer in a first direction, in the present case the Y direction, than another suction unit  20   b  of the robot gripper  100 . The suction units  20   a ,  20   b  of the robot gripper have a common, flat suction surface  21 . In addition, the suction units are arranged side by side with respect to the first direction, in the present case with respect to the Y direction. In the present case, the suction units  20   a ,  20   b  are configured as suction boxes  20   a ,  20   b  which have suction openings  50  on their underside. The suction openings  50  are each in fluidic connection with spaces inside the suction boxes  20   a ,  20   b . The spaces inside the suction boxes  20   a ,  20   b  in turn can be fluidically connected to a vacuum source (not shown). Each space inside a suction box  20   a ,  20   b  can be selectively fluidically connected to a vacuum source (not shown). 
     The actuators  30   a ,  30   c  are each attached to a first support element  40  by a joint  110 . The actuators  30   b ,  30   d  are each attached to a second support element  41  by a joint  110 . The suction units  20   a ,  20   b  are attached to the second support element  41 . The first support element  40  is connected to the receiving area  10  without a degree of freedom. The second support element  41  is connected to a spline shaft hub  80  via the joint  90 . The spline shaft hub  80  engages a spline shaft  70  which in turn is connected to the receiving area  10  without a degree of freedom. The receiving area  10  has a receiving surface  11 . 
     The embodiment shown with a spline shaft  70  and a spline shaft hub  80  is advantageous in that suction units  20   a ,  20   b  can be set to a plurality of different tilt angles with few actuators  30   a ,  30   b ,  30   c ,  30   d . In addition, the system, consisting of spline shaft  70  and spline shaft hub  80 , secures the suction units  20   a ,  20   b  against transverse forces (in the case shown: forces in the X direction and in the Y direction) and against torsion with respect to the spline shaft axis (in the case shown: torsion with respect to the Z axis). 
     The states of the robot gripper  100  of the second embodiment shown in  FIGS. 3 a  and 3 b    differ solely by the tilt angle α, by which the suction units  20   a ,  20   b , or their suction surface  21 , are tilted relative to the receiving area  10  that has a receiving surface  11 . Specifically,  FIG. 3 a    shows a state in which the suction surface  21  of the robot gripper  100  is oriented substantially parallel to the receiving surface  11  of the receiving area  10 .  FIG. 3 b   , on the other hand, shows a state in which the suction surface  21  of the robot gripper  100  assumes an angle α relative to the receiving surface  11  of the receiving area  10 . For example, the angle α can assume a value of between 0.1° and 35° inclusive. Preferably, the angle α assumes a value of between 0.5° and 10° inclusive, and particularly preferably between 1° and 2° inclusive. 
       FIG. 4 a    shows a schematic top view of a third embodiment of a robot gripper  100  according to the invention.  FIG. 4 b    shows a schematic sectional view of the third embodiment along line A-A in  FIG. 4 a   . The third embodiment of the robot gripper  100  substantially corresponds to the second embodiment described above. However, the third embodiment of the robot gripper  100  comprises a plurality, but preferably two, groups of suction units. In the case shown, the suction units  20   a ,  20   b  form a first suction unit group. The suction units  20   c ,  20   d  form a second suction unit group (cf.  FIG. 4 a   ). The suction surfaces  21  of the suction units of the suction unit groups lie in the same plane. The suction unit groups are arranged side by side with respect to a second direction that is different from the first direction. In the case shown, the second direction is the X direction. 
     The robot gripper  100  of the second embodiment is configured such that individual suction units  20   a ,  20   b ,  20   c ,  20   d  can be selectively supplied with negative pressure. The robot gripper  100  of the third embodiment is also configured such that individual combinations of suction units, for example the suction units  20   b  and  20   c , can be supplied with negative pressure. 
     The robot gripper  100  of the third embodiment is suitable for removing plates  220  of different dimensions from a chaotic stack  210 . For instance, only one of the suction units  20   a ,  20   b ,  20   c ,  20   d  of the robot gripper  100 , e.g. suction unit  20   b , can be supplied with negative pressure in order to remove a small workpiece from a stack. However, another one of the suction units  20   a ,  20   b ,  20   c ,  20   d  of the robot gripper  100 , e.g. the suction unit  20   a , can also be supplied with negative pressure in order to remove a larger workpiece  220  from a chaotic stack  210 . In addition, a plurality or all of the suction units  20   a ,  20   b ,  20   c ,  20   d  of a robot gripper  100  can also be simultaneously supplied with negative pressure in order to remove a very large workpiece  220  from a chaotic stack  210 . 
     The robot gripper  100  of the third embodiment is configured such that the suction units  20   a ,  20   b ,  20   c ,  20   d  can be tilted about a second axis. In the case shown, the second axis is the Y axis. The suction units  20   a ,  20   b ,  20   c ,  20   d  are pretensioned to a second zero position with respect to the second axis. Pretensioning is performed with spring elements  60 ′ (cf.  FIG. 4 a   ). Both the tilting about the first axis and the tilting about the second axis can be realized withe the joint  90 . The joint  90  may be configured as a universal joint for this purpose. 
     Thus, a robot gripper  100  of the third embodiment can be used to remove workpieces  220  from a chaotic stack  210  which lie on the chaotic stack  210  at an angle with respect to the horizontal plane. The horizontal plane is a plane with respect to which the direction of gravity is perpendicular. In the case shown, the horizontal plane is the XY plane. In addition, a robot gripper  100  of the third embodiment can be used to remove workpieces  220  from a chaotic stack  210  which do not have a constant thickness. 
     The spring elements  60  of the robot gripper  100  of the third embodiment are schematically shown as coil springs in  FIG. 4 b   . However, a robot gripper  100  of the third embodiment may alternatively or additionally comprise any other types of springs, for example, torsion springs, bending springs, or gas pressure springs. The same applies to the spring elements  60 ′. 
       FIGS. 5 a  to 5 d    schematically show steps of an embodiment of a method according to the invention for removing plate-shaped workpieces  220  from a chaotic stack  210 .  FIG. 5 a    schematically shows the step of moving a robot gripper  100 , which is attached to an industrial robot  300 , into the vicinity of a workpiece  220   a  to be removed. The workpiece  220   a  to be removed is substantially centered on the uppermost layer of a chaotic stack  210  between workpieces  220   b  lying in edge sections of the uppermost layer of the chaotic stack  210 . The robot gripper  100  shown comprises at least one suction unit  20   a . For example, the robot gripper  100  shown may be a robot gripper  100  of the first, second, or third embodiments. The workpieces of the chaotic stack  210  lie on a storage device  200 . 
       FIG. 5 b    shows a state of the robot gripper  100  attached to the industrial robot  300 , of the storage device  200  and of the chaotic stack  210  after the following method steps have been carried out: tilting the suction unit  20   a  such that the suction surface  21  assumes an angle of between 0.1° and 35° inclusive, preferably between 0.5° and 10° inclusive, and particularly preferably between 1° and 2° inclusive, relative to the workpiece  220   a  to be removed; moving the robot gripper  100  such that the pressing edge  140  of the robot gripper lies substantially on an edge of the workpiece  220   a  to be removed. 
       FIG. 5 c    shows a state of the robot gripper  100  attached to the industrial robot  300 , of the storage device  200  and of the chaotic stack  210  after the following method step has been carried out: applying negative pressure to the suction unit  20   a  such that the workpiece  220   a  to be removed is pulled to the suction surface  21  of the suction unit  20   a.    
       FIG. 5 d    shows a state of the robot gripper  100  attached to the industrial robot  300 , of the storage device  200  and of the chaotic stack  210  after the following method step has been carried out: lifting the robot gripper together with the workpiece to be removed held on the suction surface. 
     REFERENCE NUMBERS 
     
         
           10  Receiving area 
           11  Receiving surface 
           20  Suction unit 
           21  Suction surface 
           30  Actuator 
           40  First support element 
           41  Second support element 
           50  Suction opening 
           60  Spring element 
           70  Spline shaft 
           80  Spline shaft hub 
           90  Joint 
           100  Robot gripper 
           110  Joint 
           140  Pressing edge 
           200  Storage device 
           210  Chaotic stack 
           220  Workpiece 
           220   a  Central workpiece 
           220   b  Workpiece in edge section 
           300  Industrial robot