Method and system for quickly and efficiently transferring a workpiece from a first station to a second station
TECHNICAL FIELD 
     This invention relates to methods and systems for quickly and efficiently transferring a workpiece from a first station to a second station and, in particular, to methods and systems for quickly and efficiently transferring a workpiece from a first work station to a second work station utilizing a pendulum-type robot. 
     BACKGROUND ART 
     Dual-axis load/unload robots, such as illustrated in FIGS. 1A and 1B, are the traditional and most widely used automation for press to press transfer. This type of automation has a minimum of two axis of motion and a third linear slide used to move the entire unit out from between the presses for die changeover or maintenance. A first axis gives the arm its vertical motion to lift or lower the part. A second axis gives the arm its horizontal motion in and out of the press. The axis motion can be programmable, but in most cases the axes are non-programmable. 
     The non-programmable models use hard stops for the different positions and the stops must be repositioned for each different part set up. A robot mounting frame 11 attaches to a linear slide which mounts onto the front or back of each press. This type of robot must have a part transfer station including a part transfer unit having a shuttle or a conveyor between the presses to transfer the part from the unload robot to the load robot. 
     With such dual-axis load/unload robots, a part/blank P is presented to a load robot 10 by a destacker unit (not shown), as illustrated in FIGS. 1A and 1B. After a press, generally indicated at 12, completes its cycle (with its dies 14 in an up position), an unload robot 16 moves to a position 18 and picks up part P. At the same time, the robot 10 has picked up another part/blank at the destacker and has moved to a position 20. As the robot 16 retracts out of the press 12 to a position 24, the robot 10 extends in and releases the part/blank at the position 18. The robot 10 retracts clear and the press 12 cycles. The robot 16 drops the part P onto a nest 26 of a part transfer unit, generally indicated at 28, at the position 24 and the part P is then shuttled to the next press, generally indicated at 30, at a position 32. A robot 34 picks up the part P at the position 32 and is ready to load the press 30. A robot 36 picks up the part P in the press 30 at a position 38 and retracts. Then, the part P is loaded into the press 30 in the same manner as described with respect to the press 12. 
     When a different part is to be run on the press line all the dies such as dies 14 and 22 of the press 12 and 40 and 42 of the press 30 must be removed and different dies set in the presses 12 and 30. This process must be performed as quickly as possible, so that the press line can be put back into production. To accommodate this process, the robots 10, 16, 34 and 36 and the part transfer unit 28 are typically mounted to slide units or tracks such as slide units 44 and 46 and track 48. This allows for quicker removal and replacement of the equipment from a die change area 33. Once the new dies are set, the robot positions, tooling, and part transfer nest 26 must be manually adjusted before production can begin. 
     Such systems have the following features: 
     Each robot has minimum two axis of motion; 
     Robots mount to the front or back of the press; 
     Three mechanisms are required to transfer parts between two presses: loader robot, unloader robot and part transfer station; 
     • Parts are transferred between the presses by a shuttle or conveyor; 
     • Non-programmable or programmable axis motion available; and 
     • Slides or tracks are used to move (3) mechanisms out of the die change area. 
     Such systems having the following shortcomings: 
     • Relatively expensive; 
     • Multiple units required between each press; 
     • Mechanisms must be moved out from between the presses for die changeover; 
     • Removal, replacement, and calibrating these mechanisms after a change significantly increases the part/die changeover time; therefore, the press line productivity is reduced; 
     • Limited flexibility to adapt to changes in the part and die shapes or locations; 
     • Any changes in part orientation is done on the part transfer unit; and 
     • Part quality is reduced because of the number of times the part is handled. 
     Swing arm robots, such as illustrated in FIGS. 2A and 2B are also used in press-to-press applications that require more flexibility and where only one mechanism between the presses is desired. This type of automation consists of a standard six axis robot 50 mounted to an auxiliary swing arm 52. The addition of the swing arm 52 allows the standard robot 50 to reach the large center-to-center distances of most press lines. The robot 50 is typically a standard six axis articulated arm robot. The robot 50 mounts to the swing arm 52 which transports it back and forth between two presses, generally indicated at 54 and 56. The robot 50 is programmable and has a high degree of flexibility to accommodate the variation in parts or die positions. The swing arm 52 is typically mounted to a slide or track 58 which allows the robot 50 and the swing arm 52 to be manually moved out from between the presses 54 and 56 during die changeover. 
     With such a swing arm robot 50, a part/blank P&#39; is presented to a robot 60 by a destacker unit (not shown) as illustrated in FIGS. 2A and 2B. After the press 54 completes its cycle (with its die 62 in an up position), the robot 50 moves to a position 64 and picks up the part P&#39;. At the same time, the robot 60 has picked up a part/blank at the destacker unit and has moved to a position 66. As the robot 50 retracts out of the press 54 to a position 68, the robot 60 extends in and releases the part/blank P&#39; at the position 64. The robot 60 retracts clear and the press 54 cycles. The swing arm 52 rotates clockwise as the base of the robot 50 rotates counterclockwise (as shown in FIG. 2A) transporting the robot 50 and the part P&#39; to a position 70. A robot 72 moves into the press 56 (at a position 74), picks up another part and retracts out of the press 56. The robot 50 proceeds to load the part P&#39; into the press 56, in the same manner as described with respect to press 54. When the robot 50 finishes loading the press 56, the robot 50 moves back to the press 54 and repeats the sequence. 
     When a different part is to be run on the press line all the dies, such as dies 62 and 63 of the press 54 and the dies 76 and 78 of the press 56 must be removed and different dies set in the presses 54 and 56. This process must be performed as quickly as possible, so the press line can be put back into production. To accommodate this process, the swing arm 52 is typically mounted on the slide or track 58. This allows for quicker removal and replacement of the robot 50 from a die change area 71. Once the new dies are set, the robot 50 is repositioned between the presses 54 and 56 and their pickup and dropoff positions are reprogrammed before production can begin. 
     Such systems have the following features:: 
     • Seven axis of motion; 
     • Standard six axis robot mounted to a swing arm axis; 
     • One robot required to transfer part between two presses; 
     • Robot mounts to the floor between two presses; 
     • Programmable axis motion with high flexibility; and 
     • Slides or tracks are used to move the robot out of the die change area. 
     Such systems have the following shortcomings: 
     • Relatively expensive; 
     • The swing arm axis motor or motors require high torque to enable it to move the heavy robot at high speed, press to press; 
     • Swing arm robots must be moved out from between the presses for die changeover; 
     • Removing and replacing these robots significantly increases the part/die changeover time; therefore, the press line productivity is reduced; 
     • The swing arm axis is redundant and requires special controls to accommodate; 
     • A minimum of six axes are required to transport the part, press to press; 
     • Seven axes are required for full flexibility; and 
     • Reliability is reduced due to the effects of high inertia on the standard robot&#39;s wrist axis. 
     The U.S. Pat. No. 4,695,027 to Lindholm et al. discloses a robot installation for use in a production line including at least one pendulum-type robot, including a primary arm and a secondary arm. A counterweight is displaceably arranged on the primary arm so that a center of gravity of the arm system lies on or is immediately proximate to the pivot axis independently of the position of the secondary arm on the primary arm. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a method and system for quickly and efficiently transferring a workpiece from a first station to a second station utilizing a pendulum-type robot located above and between the first and second stations. 
     Another object of the present invention is to provide a method and system for quickly and efficiently transferring a workpiece, such as a sheet metal part, from a first work station, such as a press, to a second work station, such as another press utilizing a pendulum-type robot located above and between the first and second work stations. 
     Yet still another object of the present invention is to provide a method and system for quickly and efficiently transferring a workpiece, such as a sheet metal part, from a first work station, such as a press, to a second work station, such as a second press, utilizing a pendulum-type robot in which swinging motion from the first work station to the second work station and rotational movement of the robot about second and third axes of the robot cause a center of gravity of the workpiece to travel in a substantially straight line path between the first and second work stations. 
     In carrying out the above objects and other objects of the present invention, a system is provided for quickly and efficiently transferring a workpiece from a first work station to a second work station. The system includes a support structure and a robot supported on the support structure. The robot includes an arm assembly pivotally supported proximal a first end of the arm assembly to swing about a first axis located above and between the first and second work stations. The system also includes an end effector supported on a second end of the arm assembly distal the first axis to pick up the workpiece at the first work station and place the workpiece at the second work station. The arm assembly swings from the first work station to the second work station about the first axis, at least partially under the action of gravity. 
     Still further in carrying out the above objects and other objects of the present invention, a method is provided for quickly and efficiently transfer a workpiece from a first work station to a second work station. The method includes the steps of providing a support structure and providing a robot including an arm assembly rotatable about the first, second and third axes. The second axis is oblique to both the first and third axes. A robot is supported on the support structure so that the arm assembly is located at a position above and between the first and second work stations to swing from the first work station to the second work station about the first axis at least partially under the action of gravity. The method also includes the steps of providing an end effector on the arm assembly, swinging the arm assembly about the first axis to the first work station and utilizing the end effector to pick up the workpiece at the first work station. Then the method includes the steps of swinging the arm assembly from the first work station to the second work station while rotating the arm assembly about the second and third axes during the step of swinging the arm assembly from the first work station to the second work station to allow a center of gravity of the workpiece to follow a substantially straight line path between the first and second work stations. Finally, the method includes the step of utilizing the end effector to place the workpiece at the second work station. 
     Preferably, the system includes a counterbalance mechanism attached to the arm assembly so that the amount of torque required to swing the arm assembly at each of the first and second work stations is substantially reduced. Also, preferably, the counterbalance mechanism is a cable balancing device, including a tension cable for supporting the arm assembly. 
     Further, preferably, at least one of the work stations includes a press having a pair of opposing dies and wherein the workpiece is a sheet metal part. 
     The advantages of the method and system of the present invention are numerous. For example, parts can be transferred between the work stations quickly and efficiently by utilizing small torque motors for the robot, which is consequently lighter to allow faster cycle times and, hence, higher productivity. Also, die changeover is a relatively simple matter requiring relatively little time, hence also increasing productivity of the presses. 
    
    
     The above objects, features and advantages are readily apparent from the following detailed description of the present invention when taken in connection with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     FIG. 1A is a top plan schematic view, illustrating a prior art, dual axis load/unload robot system; part flow and a die change area between two presses are indicated; 
     FIG. 1B is a side elevation view of the system of FIG. 1A; 
     FIG. 2A is a second prior art top plan schematic view illustrating a swing arm robot system and also illustrating part flow and a die change area; 
     FIG. 2B is a side elevational view of the system of FIG. 2A; 
     FIG. 3A is top plan view illustrating a method and system of the present invention utilizing a pendulum-type robot; 
     FIG. 3B is a side elevational view of the system of FIG. 3A; 
     FIG. 4 is a side elevational view of the pendulum-type robot mounted on a support structure illustrating various axes of rotation and end effector, such as a gripper mounted on an arm assembly of the robot; 
     FIG. 5 is a top plan view of the pendulum-type robot and support structure of FIG. 4; 
     FIG. 6 is a side elevational view, partially broken away, of the arm assembly of FIG. 4 but modified to include a wrist mechanism having a pivotal axis of rotation; and 
     FIG. 7 is a side elevational view, partially broken away, of the arm assembly of FIG. 4, but modified to include a wrist mechanism having fifth and sixth axes of rotation to provide pitch and roll motions. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     System Description 
     Referring to FIGS. 4 and 5, in combination with FIGS. 3A and 3B, a support frame, generally indicated at 100, is mounted to a floor outside of die change area 102 by brackets 104. Alternatively, the frame 100 can be mounted to the side frames of presses, generally indicated at 106 and 108, respectively. 
     As best shown in FIGS. 4 and 5, the support frame 100 carries a bracket 110 to which a robot pendulum arm 112 is pivoted at one end and rotates by means of a motor 114 about an axis, as indicated by arrows 116. The other end of the pendulum arm 112 is pivoted at a platform 118 which is kept in a fixed orientation, preferably horizontal, by means of an arm 120. The parts 110, 112, 118 and 120 form a four-bar linkage. 
     A motor 122 drives a link 124 in a pivoting rotation relative to the platform 118 about a second axis, as indicated by arrows 126. The link 124 is also coupled to a part 128 in a four-bar linkage arrangement by means of coupling links 130 and 132. The second axis 126 is generally oblique to the first axis 116 though, preferably, the second axis 126 is orthogonal to the first axis 116 and, also preferably, vertical. 
     A motor 134 rotates the link 130 in a vertical plane about another substantially horizontal axis, as indicated by arrows 136, to raise or lower the part 128. 
     A motor 138 rotates a part 140 relative to the part 128 about another substantially vertical axis, as indicated by arrows 142 to move an end effector or gripper 144 in a generally horizontal plane. Accordingly, a robot, generally indicated at 146, having four degrees of freedom is described. The motions of the robot 146 are controlled by means of a conventional programmable robotics controller (not shown). 
     This four-axis arrangement is sufficient to move the end effector or gripper 144 and an attached object or part P&#34; within the work envelope of the robot 146 while maintaining parallelism between the object and a fixed plane, preferably horizontal. This is usually sufficient for a majority of the press-to-press, or press-to-conveyor, transfer operations, especially those addressed by the prior art of FIGS. 1A and 1B. 
     For the flexibility addressed by the prior art of FIGS. 2A and 2B, which may require rotation of the transferred object between two press locations, the robot 146 can be fitted at the part 140 with any type of robotic wrist mechanism having one or more degrees of freedom as shown in FIGS. 6 and 7 to obtain robots 146&#39; and 146&#34;, respectively. Generally, a two-axis wrist 148 having two orthogonal axes with its first axis being orthogonal to a vertical axis is sufficient to allow the robot 146&#34; full flexibility for positioning and orienting an object in a three-dimensional space with six degrees of freedom, as shown in FIG. 7. A one-axis wrist 150 with its axis orthogonal to the vertical axis provides five degrees of freedom, as shown in FIG. 6. 
     In the other embodiment of FIG. 3B, the robot 146 is supported by a cable balancing device, generally indicated at 152. This cable balancing device 152 is used to balance gravity by means of a tension cable 154 that supports the robot arm assembly. The cable 154 is put under continuous tension by a cylinder 156 that compensates for the variation in cable length during pendulum motion. The cable 154 runs between pulleys 158. The two pulleys 158 are mounted to the support frame 100 or can be mounted to the presses 106 and 108. One of the pulleys 158 is mounted to a tension compensating cylinder, such as a cylinder 156, substantially close to the center of gravity of the arm assembly of the robot. By balancing gravity, motor torque required to drive the pendulum arm of the robot 146 is reduced. It is also possible to replace the lower pulley 158 by attaching the ends of the cable 154 to the platform 118. 
     Process and Method Description 
     Referring to FIGS. 3A and 3B, a part/blank P&#34; is presented to a first one of the robots 146 in the illustrated system by a destacker unit (not shown). After the press 106 completes its cycle (its die 160 in an up position), a second one of the robots 146 moves to a position 162 and picks up the part using its second horizontal axis, as indicated by arrows 136. 
     At the same time, the robot 146 has picked up a part/blank at the destacker and has moved to a position 164. As the second robot 146 retracts out of the press 106 to a position 166, the first robot 146 extends in and releases the part/blank at the position 162. The first robot 146 then retracts clear and the press 106 cycles. 
     The second robot 146 pivots about its first horizontal or pendulum axis, as indicted by arrows 116, to move the robot arm through a position 168, while rotation occurs clockwise about the first vertical axis, as indicated by arrows 126 and counterclockwise about the second vertical axis, as indicated by arrows 142, thereby transporting the part P&#34; to a position 170. This combination of axes motion allows the center of gravity of the part P&#34; to follow a path that approximates a straight line between the two presses 106 and 108. Any height adjustment from one press to the other is compensated by rotation about the second horizontal axis, as indicated by arrows 136. 
     A third pendulum-type robot (not shown) moves into the press 108 at position 172, picks up the part, and retracts to a position 174. Another part is then loaded into the press 108 by the second robot 146, in the same manner as described for the press 106. When the second robot 146 finishes loading the press 108, the second robot 146 moves back to the press 106 and repeats the sequence. 
     When a different part is to be run on the press line, all the dies, such as dies 160, 161 of the press 106 and the dies 171 and 173 of the press 108 must be removed and different dies set in the presses 106 and 108. This process must be performed as quickly as possible, so the press line can be put back into production. To accommodate this process, the second pendulum-type robot 146 moves to a preprogrammed rest position that is outside of the die change area 102. The second robot 146 and the support structure 100 do not have to be physically moved out from between the presses 106 and 108 because the vertically extending supports of the support structure 100 are located off to the side of the presses 106 and 108. This allows for the quickest die changeover because removal and replacement of the second robot 146 is not required. Once the new dies are set, the robots&#39; pickup and dropoff positions are calibrated by reteaching a reference frame located on each die. This frame consists of a maximum of three points which can be located anywhere on the die. The method shortens the calibration and part setup time for the new part. 
     The method and system of the present invention have the following features: 
     • A minimum number of axes are used to transfer a part press-to-press; 
     • The basic unit consists of four axes of motion, with up to six axes available; 
     • Two additional wrist motions are available to add flexibility; 
     • One robot is required to transfer a part between two presses; 
     • The robot does not have to be removed from between the presses during die changeover; 
     • The robot mounts to an overhead support structure located off to the left or right side of the press line; 
     • The support structure can attach to the floor, ceiling, presses, or existing structural members; 
     • Programmable axis motion with high flexibility; 
     • The design focus is on moving the part, not a large robot, this allows smaller torque motors to be used; 
     • Electronic interface-to-press sensors, resolver or encoder mounted to the press ram to interlock programs, initiative cycle, and coordinate motion between the robots and the presses is possible; and 
     • These interfaces allow faster unload/load time in the press, hence the part throughput can be increased. 
     The method and system of the present invention solve the shortcomings of the system of FIGS. 1A and 1B by: 
     • Lower cost by using one robot, instead of three mechanisms; 
     • The unique combination of axes allows the robot to be mounted off to the side of the press line; 
     • The robot does not have to be physically removed from between the presses for die changeover; 
     • The time for die changeover is shortened, hence productivity is increased; 
     • Calibration of the robot program to new dies after changeover is quicker with the use of reference frames; 
     • No tracks or slides required between presses; 
     • The basic four axes of motion allows the end effector/gripper to be oriented horizontally in X, Y and Z directions; 
     • Higher quality parts are produced, because they are handled fewer times; 
     • No modification is required to the presses for mounting the units; and 
     • Higher system uptime because there are fewer mechanisms. 
     The method and system of the present invention solves shortcomings of the system of FIGS. 2A and 2B by: 
     • Lower cost because fewer number of axes are used to transfer part; 
     • The unique combination of axes allows the robot to be mounted off to the side of the press line; 
     • The robot does not have to be physically removed from between the presses for die changeover; 
     • No tracks or slides required between presses; 
     • The time for die changeover is shortened, hence productivity is increased; 
     • Calibration of the robot program to new dies after changeover is quicker because the robot location is not disturbed; 
     • One less axis is required to achieve the same flexibility; 
     • The design allows for small torque motors; 
     • Lighter mechanism allows faster cycle time, hence high productivity; and 
     • Application specific custom designed wrists can be mounted to accommodate the die loading requirements of large sheet metal parts at a minimum of cost. 
     The invention has been described in an illustrative manner and, it is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.