Abstract:
The present invention relates to a gripper system which can be secured to a hand of a robot and has at least two mutually independent gripper elements; each gripper element is provided with at least one force sensor for ascertaining a force in a predetermined direction. With this gripper system, it is possible to grasp components more reliably.

Description:
FIELD OF THE INVENTION 
     The invention relates to a gripper system having at least two mutually independent gripper elements, and to a vacuum gripper system having at least one independent gripper element. 
     BACKGROUND OF THE INVENTION 
     In handling workpieces, it is currently unavoidably necessary for the workpiece to have a defined outset position or a defined position in space, so that the workpiece can be grasped exactly with a gripper system that is secured to a robot hand. The permissible positional deviations are as a rule in the range of ±0.2 mm. Such exactly positioned workpieces can be grasped blind by a robot and can be handled and joined together blind. However, it has been found that large workpieces in particular, such as molded sheet-metal parts, while exactly positioned still cannot be grasped without error, for instance because they are warped or deformed. There is also the danger that because of its warping, a workpiece once grasped cannot be joined or can be joined only defectively. Such a handling system therefore requires permanent monitoring. 
     It also requires relatively expensive workpiece receptacles that assure that the workpieces will have a defined position of repose. Another problem is considered to be that if a gripper crashes, the gripper may become deformed. The gripper then moves onward, thus deformed, which can lead either to an inaccurate reception of the workpiece and/or an imprecise or defective joning operation. 
     SUMMARY OF THE INVENTION 
     It is therefore the object of the present invention to furnish a gripper system which requires less monitoring and with which even workpieces that are positioned outside the tolerance range can be grasped and nevertheless joined exactly. 
     This object is attained according to the patent invention with a gripper system which has at least two mutually independent gripper elements, and each gripper element is provided with at least one force sensor for ascertaining a force in a predetermined direction. This object is also attained according to the patent invention with a vacuum gripper system which has at least one independent gripper element that is provided with at least one force sensor for ascertaining a force in a predetermined direction. 
     In the first of these embodiments, the two mutually independent gripper elements enable secure grasping of the workpiece, and the quality of the gripping operation is ascertained via the force sensors. For instance, if a gripper element is located correctly on the workpiece or if this gripper element engages the workpiece in accordance with the specifications, then the value ascertained by the force sensor is also within the predetermined tolerance range. If the workpiece is incorrectly positioned, however, then the gripper element engages the workpiece out of line. As the workpiece is being lifted, the force sensor ascertains a value that is outside the tolerance range. The possibility now exists of putting down the workpiece and grasping it again, with the gripper system now shifted slightly, and in particular as a function of the force values ascertained by the force sensors. This can be done with recourse to the data stored in a data memory. From the forces ascertained, however, the positioning of the workpiece can also be calculated. 
     The gripper system can advantageously be secured to a robot hand. Another option is for the workpiece, despite being grasped wrong, to be transported onward, with the robot hand triggered via the values of the force sensors in such a way that the gripping error is compensated for, so that the workpiece is nevertheless joined exactly. 
     The gripper system according to the present invention has the substantial advantage that even workpieces that are outside the specified tolerance of ±0.2 mm can be grasped. In other words, this means that with the gripper system of the present invention a substantially greater positional tolerance for the workpieces can be specified; it can be in the range of ±2 cm, or in other words can be higher by a factor of 100. 
     Another advantage is considered to be that in handling the grasped workpiece, that is, in swiveling the workpiece that is grasped, the forces acting on the gripper elements, the force of gravity, and the dynamic mass or centrifugal forces in all the degrees of freedom, can be ascertained, and as a result the position of the workpiece at the gripper elements or in the gripper system can be determined. The robot hand or arm can also be braked, if the forces acting on the workpiece exceed limit values, and hence the workpiece will not come loose from the gripper elements. Because of the possibility of ascertaining dynamic mass forces or centrifugal forces, the possibility also exists of grasping a workpiece that has been positioned rotationally incorrectly, detecting the misrotated position, and correcting it. Both position error detection and rotary position detection are possible. The rotary position error can also be ascertained, in one embodiment, directly from the measured forces. 
     In order to grasp a beam, for instance, two gripper elements can be used, with which the force and the X position of the beam are ascertained. If three gripper elements are used, then both the introduced force at right angles to the X-Y direction and the X-Y position of the workpiece can be ascertained. If four gripper elements are used, the force of gravity, the X-Y position, and the position angle or in other words the rotary position of the workpiece can be ascertained. 
     The gripper system of the present invention furthermore has the substantial advantage that with it, different workpieces can be grasped, and that the gripper system can tell which workpieces have been grasped. If a gripper element is grasping empty air, for instance, or if its overall force deviates from the norm, then for instance it can be assigned to a particular component or workpiece. 
     In a further refinement, it is provided that the force sensor is disposed on the gripper element in such a way that with it, the force component acting in the receiving direction of the gripper element can be ascertained. As a result, in a simple way, the optimal contact of the gripper system with the workpiece to be grasped is ascertained. Via the ascertained force component in the receiving direction, the contact force of the gripper system on the workpiece can also be detected, so that the workpiece on being grasped is not damaged even if the workpiece is not in its intended position. It is furthermore possible in joining to monitor the requisite joining forces, that is, a controlled pressing of a front panel onto a passenger car body, or to monitor a force path, for instance in snap-in operations, and as a result integrated monitoring of successful joining is achieved. 
     An especially advantageous embodiment provides that the gripper elements are embodied as modules, and in particular as plug-in modules. Plug-in modules can be put together relatively simply and in a way secure against mistakes, so that the gripper system can be constructed quickly and without problems by putting the individual components together. Furthermore, modules have the particular advantage that because of their identical design they can be manufactured relatively inexpensively, and virtually any desired configuration can be created with them. 
     To make it possible to grasp large components or workpieces, the gripper elements are connected to one another via connecting components. The connecting components are either embodied rigidly, for instance as straight or curved connecting pieces, corner pieces or shunts. Furthermore, the components may have their own code for their shape and size. These connecting components make it possible to construct different structures, so that the gripper system can be optimally adapted to the shape of the workpiece. All the gripper elements, however, have identical connection hookups. 
     However, the connecting components can also be variable in their three-dimensional structure and can for instance be embodied as a two- or three-dimensional joint or as a telescoping piece, and in particular they are motor-driven. This has the substantial advantage that the gripper system is not rigid but instead is flexible, so that workpieces can be grasped not merely on a side but also all the way around; this may be advantageous for the sake of better handling, better joining, or the detection of the workpiece in the gripping operation. For instance, the gripper system can be moved to approach the workpiece until the first gripper element contacts the workpiece with the predetermined force. Then, by adjusting the flexible connecting components, the other gripper elements are made to approach the workpiece until they, too, contact the workpiece with the predetermined force. The workpiece can accordingly be grasped and held all the way around, in the same way as if it were grasped by a human hand. Furthermore, because of the flexibility of the gripper system, different workpieces can be grasped using a single structure, because the system can automatically adapt to the workpiece geometry, within a limited context. Preferably, the gripper system can be reproduced on an external screen, so that the instantaneous position or motion sequences can be monitored. Measuring the angle allows adaptive or controlled gripping. Adjusting the angle can be done manually or by motor and can be terminated for instance by a predetermined force, a predetermined negative pressure, a predetermined adjustment time, a predetermined angle, or by other sensors. The angle adjustment can also be done by electrochemical actuators or by rheological fluids. 
     To further increase the gripping capacity of the gripper system and to make the system more flexible, the gripper elements can be connected indirectly or directly to end plates or adapter plates. Other components, function elements, or tools, such as cameras, welding devices, and the like can also be secured to these plates. These additional components or tools can, however, also be secured to the connecting elements, for instance in clamping rails provided on the connecting elements. 
     To enable triggering the gripper elements and/or the active connecting components individually, these elements and components are connected to a bus system and have their own address. In this way, it is possible over a relatively low number of data lines, such as a CAN bus, to trigger the individual gripper elements or components. The only additional requirement is a power supply for the electronics and the mechanical drives of the various elements. 
     Advantageously, each gripper element is equipped with a negative pressure generator, such as an ejector. This has the substantial advantage that a central negative pressure system is not needed; instead, recourse can be had to the compressed-air system, which as a rule is present anyway. By measuring the negative pressure, particularly during gripping, it can be ascertained whether the gripper element has grasped the workpiece, and how well the workpiece has been grasped. It is also possible to regulate the vacuum. 
     Advantageously, each gripper element is connected to a central arithmetic and data storage unit, with which the gripper element communicates. This arithmetic and data storage unit triggers each gripper element and brings about the sequence of motion of the gripper system. 
     Preferably, each gripper element has its own microcontroller for computation tasks and a data memory. In this way, each gripper element can perform computation tasks, such as whether the particular gripper element is correctly contacting the workpiece (measurements of the negative pressure in the case of a suction cup), independently of the first numerical arithmetic unit. Depending on this, other suction elements or connecting components can for instance be triggered. The gripper elements can exchange data both with the central arithmetic unit and with one another and can make decisions. 
     In a preferred embodiment, the force sensor is a strain gauge. Via this force sensor, forces can be measured relatively simply and exactly. A piezoelectric element as the force sensor is also conceivable. Other sensors can operate on an inductive, pneumatic, magnetostrictive, or optical basis. 
     A simple design of the gripper system is attained in that all the elements and components can be plugged in, in particular via a plug and socket system. In this way, the individually needed gripper elements can be connected to the required connecting components by inserting them into one another in a way that precludes mistakes. Not only the supply lines, such as electric lines and compressed-air lines, but also the data lines and in particular the CAN bus (or some other bus) are extended all the way through each component. Termination of the individual lines is made possible via end plates, for instance. 
     Advantageously, the gripper elements have display elements, for instance for displaying the status of various functions and for error display. For instance, defective gripper elements or other information can also be displayed visually in the gripper system. 
     Advantageously, the gripper elements are embodied as suction cups, magnets, mechanical grippers such as fingers or the like, or suction cushions, or form- and/or force-locking grippers. An adhesive or electrostatic connection is also conceivable. Suction cups, in particular, have the substantial advantage that not only two-dimensionally but also three-dimensionally curved, rigid or flexible bodies can be grasped equally well. Furthermore, gas-tight and porous bodies can be grasped. In addition, the suction cups can be optimally adapted in their shape to the workpiece or workpieces to be grasped. 
     A training mode is possible, in which the system can learn the position detection, part detection, and/or force detection, for instance by a linear ratchet in a snap-in operation, a collision, or adhesive bonding operations. The apparatus of the present invention, particularly because of its modular design, can be constructed quickly and reconfigured quickly and has high availability. The system is redundant with regard to the vacuum, the electrical system, and the pneumatic system. 
     A safety shutoff of the system is also possible, for instance if a predetermined force, such as a joining force, is exceeded, if a crash occurs, or if the wrong course of the force is ascertained. The safety shutoff can also be done, however, if the negative pressure drops, if some malfunction in the (CAN) bus occurs, or if there is a power failure. 
     Further advantages, characteristics and details of the present invention will become apparent from the dependent claims and the ensuing description, in which with reference to the drawing, particularly preferred exemplary embodiments of the invention are described in detail. The characteristics shown in the drawing and mentioned in the drawing and mentioned in the specification and recited in the claims can each be essential to the present invention individually or in arbitrary combination. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1, is a plan view of a first embodiment of the gripper system of the present invention; 
     FIG. 2, is a side view of the gripper system of FIG. 1; 
     FIG. 3, is a plan view of a further embodiment of the gripper system of the present invention; 
     FIG. 4, is a plan view on a further variant of the gripper system of the present invention; 
     FIG. 5, is a side view of the gripper system of FIG. 4; 
     FIG. 6, is a further embodiment of the gripper system of the present invention; 
     FIG. 7, is a side view of a gripper element; 
     FIG. 8, is partly in section, the support of a suction cup of a gripper element; 
     FIG. 9, is a plan view of a connecting component embodied as a joint; 
     FIG. 10, is a side view of the joint of FIG. 9; 
     FIG. 11, is a plan view of a second embodiment of a connecting component embodied as a joint; and 
     FIG. 12, is a side view of the joint of FIG.  11 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, a first exemplary embodiment of a gripper system, identified overall by reference numeral  1 , is shown. This exemplary embodiment of a gripper system  1  is embodied as a frame gripper  2 . This frame gripper  2  has a robot flange  3 , to which a robot arm can be secured. This robot flange is supported by two struts  4 ; the struts  4  are screwed to the frame, designated overall by reference numeral  5 . This frame  5  comprises a plurality of components, and in the exemplary embodiment shown it has four gripper elements  6 , four first connecting components  8  embodied as connecting pieces  7 , and four second connecting components  8  embodied as corner pieces  9 . There is also one electronics hookup  10  and one compressed-air hookup  11  on one corner piece  9 . The gripper elements  6  and the connecting components  8  together form a rigid frame, which can be moved in space via the robot arm attached to the robot flange  3 . The frame  5  can for instance be lowered onto generally flat workpieces  37  (FIG. 5) and placed on them so that via the suction cups  12 , which are shown in more detail in FIG. 2, the workpiece  37  can be picked up by suction. 
     It can also be seen from FIG. 2 that the connecting components  8  laterally have a securing groove  13 , with which other tools, such as cameras, welding devices, and so forth can be secured. FIG. 3 shows a further embodiment of the gripper system of the present invention, which again has a frame-like structure. The robot flange  3  is connected to a connecting component  8  embodied as a joint  14 . This joint  14  has two articulated arms  15  and  16 , which are pivotable in the X-Y plane, the plane of the drawing, about two pivot pins  17 . Connected to the two articulated arms  15  and  16  are connecting pieces  18 , which in turn are connected to gripper elements  6 . The gripper element  6  associated with the articulated arm  15  is coupled to an articulated arm  19  of a further joint  14 , whose second articulated arm  20  is connected via a further connecting piece  18  to a gripper element  6 . This further gripper element  6  is connected in turn, via an articulated arm  21 , to a third joint  14 , whose second articulated arm  22  is connected, via a further connecting piece  18 , to a fourth gripper element  6 . This fourth gripper element  6 , finally, is connected via a connecting piece  18  to an articulated arm  23  of a fourth joint  14 , whose second articulated arm  24  is connected, via a connecting piece  18 , to the gripper element  6  that is associated with the articulated arm  16  via a further connecting element  18 . 
     This frame  5  shown in FIG. 3, in contrast to the exemplary embodiment of FIG. 1, is not rigid, since its configuration is adjustable by way of the location of the articulated arms  15 ,  16 ,  20 - 24  of the joints  14 . The joints  14  can be adjusted manually or by motor. This gripper system  1  can be adapted to special shapes or configurations of workpieces  37 . With this gripper system  1  and motor- adjustable components, it is accordingly possible for different workpieces, previously detected and recognized by means of a camera, for instance, to be grasped. 
     This gripper system  1  can also be expanded without problems; this is indicated particularly at the joints  14  by dot-dashed lines  25 , where for example for the joint  14  that has the articulated arms  23  and  24 , one further gripper element  6  is added. This makes this gripper system  1  variable in the X-Y plane. 
     FIGS. 4 and 5 show a further exemplary embodiment of a gripper system  1 , in which once again a joint  14 , which has articulated arms  26  and  27  that are pivotable in the X-Y plane of FIG. 4, is secured to the robot flange  3 . Two connecting pieces  18  and  28  are connected to the articulated arms  26  and  27 ; the connecting piece  18  is connected to a gripper element  6 , and the connecting piece  28  is connected to a swivel joint  29 . Further gripper elements are connected both to this swivel joint  29  and to the robot flange  3 . The free ends of the gripper elements  6  are terminated via end pieces  30  or adapter plates  31 . It can be seen clearly from FIG. 5 that the swivel joint  29  has two articulated arms  32  and  33 , which are pivotable in the direction of the arrow  34 , or in other words in the direction of the longitudinal axis of the connecting piece  28  and the Z plane. In this way, the arm  35  of the gripper system  1  can be pivoted in such a way that the gripper element  6  connected to the articulated arm  33  can be placed against a beveled portion  36  of an otherwise flat workpiece  37 . This gripper system is variable in space. 
     In addition to the gripper system  1  shown in FIGS. 1 and 5 and embodied as a three-finger gripper, this system can also be designed as a four-finger gripper, which is shown in FIG.  6 . Two joints  14  are connected to the robot flange  3 , and one connecting piece  18 , one gripper element  6  and one end piece  30  are secured to each of their articulated arms  15  and  16 . The joints  14  have supply lines  38  for compressed air and lines  39  for transmitting both data and electrical energy to the gripper arms  15 ,  16 ,  19 - 24 ,  26  and  27  (FIG.  3 ). 
     FIG. 7 shows a gripper element in side view. This gripper element  6  has a housing  40 , which in particular has a transparent housing cover  41  fitted over it. Through the housing cover  41 , electronic components  42 , such as LEDs for monitoring the function and status, can be seen. These electronic components  42  form, among other elements, a microcontroller for computation tasks and a data memory. 
     Located on one side face  43  of the gripper element  6  is a plug connector  44  and an opening  45  into which a sealing ring can be inserted and which serves to deliver compressed air. On the side of the gripper element  6  opposite the side face  43 , there is a socket  61  (FIG. 9) complimentary to the plug connector  44 , as well as an opening that fits the opening  45 , so that by simply inserting them into one another the individual components can be connected to one another. The plug connector  44  provides electrical connection of all the electrical or electronic components; by way of it, data can be exchanged and the components can be supplied with energy. The suction cup  12  is located on the underside of the gripper element  6 . 
     The suction cup  12 , as shown in FIG. 8, has a threaded connector  46 , which is secured to an elastic suction bell and into which a threaded disk  47  is screwed. A shaft  48 , which is guided without play in a ball bushing  49 , is glued into this threaded disk  47 . Slide bearings are also conceivable instead of the ball bushing  49 . A V-ring  50  is seated on the shaft  48 , and the shaft  48  is supported on the end face of the ball bush  49  via this ring. Two arms  51  and  52  are also provided, which are stationary on the housing  40  and extend in the direction of the threaded disk  47  and extend over it slightly at the edge. The arm  51  contacts the underside of the threaded disk  47 , and the arm  52  contacts its top. Strain gauges  53 , which detect any deformation of the arms  51  and  52 , are disposed on both sides of each of the arms  51  and  52 . Thus if the suction cup  52  is moved in the direction of the double arrow  54 , then the arms  51  and  52  are deformed by the threaded disk  47 , which is recorded via the strain gauges  53 . In this way, forces in the receiving direction can be detected. Forces in other directions are supported by the ball bush  49 . 
     FIG. 9 shows the swivel joint  29 , which has already been mentioned in conjunction with FIGS. 4 and 5. This swivel joint  29  has two articulated arms  32  and  33 , which are each provided with a respective flange plate  55 . The plug connector  44  can be seen on one flange plate. The articulated arm  32  is provided with a gear wheel segment  56 , which meshes with a pinion  57  of a drive motor  58  (stepping motor). This drive motor  58  is secured to a motor flange  59 , and the motor flange  59  is connected to the articulated arm  33 . By actuation of the drive motor  58 , the position of the articulated arms  32  and  33  relative to one another can be varied; that is, the two articulated arms  32  and  33  can be pivoted about the pivot pin  60 . In FIG. 9, the socket  61  for receiving a plug connector of an adjacent component can also be seen; this plug connector has the same construction as the plug connector  44 . Threaded swivel connections  62  and a compressed-air line  63  can also be seen; the threaded swivel connections  62  open out into the openings  45 . 
     FIGS. 11 and 12 show the joint  14  with its articulated arms  15  and  16  both extended straight out and in a position pivoted by an angle α. The two articulated arms are pivotable about pivot pins  64  and  65 . The face end  66  shown in FIG. 12 has the connection socket  61  for the electrical connection with a plug connector  44  and also has the opening  45  for the compressed-air hookup. A compressed-air hookup  11  for attaching a compressed-air supply hose is also provided on the top side of the joint  14 . However, the joint  14  may also be provided with a flange, which has a side face  43  and thus has the connections  44  and  45 . The joint  14  and the swivel joint  29  are self-locking and can also have an incremental encoder or rotary transducer, for instance, for measuring angles. As the joints, purely manually adjustable units are also conceivable. 
     The drawing shows merely exemplary embodiments of the gripper system  1  of the present invention; the present invention is not intended to be limited to the combinations shown in the drawing, and on the contrary, all combinations of these components are intended to be encompassed by the present invention. By permanently monitoring the force of gravity, the manipulation force and the joining force, errors and tendencies to error can be detected immediately. It is also possible to keep uninterrupted records and to monitor the manual manipulation. A further advantage is that the forces can be monitored not only when the workpiece is being grasped but also when it is being joined, which particularly when components are adhesively bonded is important in terms of clamping forces and clamping times.