Abstract:
A system for automatically adjusting a fastener screw of a pivot joint between first and second parts of a pair of scissors, each part having a blade and a respective handle. A part clamp engages and holds the handle of the first part. A torque arm engages and moves the handle of the second part relative to the first part about the pivot joint. A driver adjusts the fastener screw to provide a desired resistance to relative movement between the first and second parts. A position encoder is connected to the part clamp and torque arm to generate a position signal indicative of the position of the second part relative to the first part. A torque transducer is connected to the parts clamp and torque arm to generate a torque signal indicative of the resistance to relative movement between the first and second parts. A controller has inputs operatively connected to the position encoder and to the torque transducer and an output operatively connected to the driver. The controller is responsive to the position signal and to the torque signal and controls the driver.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Patent Application No. 60/426,920, filed Nov. 15, 2002, which application is hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to automatic assembly systems and more particularly to systems for automatically fastening mating parts of an assembly having a dynamic joint. 
   Various attempts are shown in the prior art to deal with the problem of assembling a pair of mating parts having a pivot joint, such as a pair of scissors, with the desired tightness of the pivot joint. 
   U.S. Pat. No. 6,594,879 to Wheeler et al. is directed to setting the spacing between two or more moving, e.g., reciprocating, cutting elements of a power cutting tool. Wheeler teaches controlling the “clearance” between elements as they are fastened together. 
   U.S. Pat. No. 6,161,273 to Rivera et al. is directed to a method and apparatus for forming rivet joints that allow pivotal motion of the parts that are interconnected by such joints with a desired amount of clearance. Parts to be riveted together are aligned with each other and held in place on a parts support anvil, and a rivet is placed into aligned holes. A rivet support anvil is positioned against the head of the rivet to establish an initial condition. The rivet support is adjusted a required amount with respect to the parts support anvil prior to formation of the second head on the opposite end of the rivet. The rivet is allowed to move a controlled amount prior to formation of the second head, to provide the desired amount of clearance. The required amount of adjustment is determined empirically and is used thereafter in riveting a particular type of assembly, using fairly uniform parts and rivets of known composition. Once the correct amount of adjustment has been determined, the same adjustment relative to the initial condition will result in the desired clearance in each similar joint made thereafter. 
   U.S. Pat. No. 5,694,694 to Roskam is directed toward a pair of scissors having a screw holding the legs of the scissors together at the pivot joint to permit readjustment of the action by the user. A second screw provides for adjustment of the inclination of one blade relative to the other to adjust the tension and friction along the cutting edges. 
   U.S. Pat. No. 5,461,765 to Linden et al. is directed toward a method of manufacturing a pair of scissors wherein at least part of the pivot joint is molded of plastic material integrally with the plastic handle while the metal blades are held together. As a last step, the clearance at the joint is adjusted by applying a force sufficient to longitudinally displace the pivot joint to establish the desired amount of clearance, or by appropriately loosening the fastener prior to removing the assembly from the mold. 
   SUMMARY OF THE INVENTION 
   The present invention overcomes significant disadvantages of the prior art by providing, as one aspect thereof, a system that can adjust the pivot joint of an assembly dynamically, i.e., as the joint is “exercised,” to provide that the desired resistance to movement is obtained at a predetermined point or points in the range of motion, or throughout the range of motion, as desired. As a result, the performance of the pivot is not highly dependent upon the manufacturing tolerances of the parts that comprise the assembly. 
   According to one aspect of the invention, a system is provided to automatically fasten mating parts of an assembly, measure the torque or tension between the mating parts as the parts are moved in relation to each other, and set the torque or tension by tightening or loosening the fastener to achieve the proper fit and function of a dynamic joint. Preferably, a predetermined torque or tension between the mating parts is set in at least one predetermined relative position, or configuration, of the mating parts. 
   According to another aspect of the invention, a system for automatically adjusting a fastener of a pivot joint between first and second parts of an assembly includes a first fixture configured to engage the first part and a second fixture configured to engage and move the second part relative to the first part about the pivot joint. A driver is configured to adjust the fastener to provide a desired resistance to relative movement between the first and second parts. A position encoder is connected to the first and second fixtures to generate a position signal indicative of the position of the second part relative to the first part. A torque transducer is connected to the first and second fixtures to generate a torque signal indicative of the resistance to relative movement between the first and second parts. A controller has inputs operatively connected to the position encoder and to the torque transducer and an output operatively connected to the driver. The controller is responsive to the position signal and to the torque signal, and the controller controls the driver. 
   According to yet another aspect of the present invention, a method for automatically adjusting a fastener of a pivot joint between first and second parts of an assembly is provided. The method includes the steps of moving the second part relative to the first part about the pivot joint while monitoring the relative angular displacement of the parts, driving the fastener while monitoring the torque required to move the second part relative to the first part, and adjusting the tightness of the fastener to achieve a monitored torque that is within predetermined limits. 
   Objects and advantages of the present invention will be more apparent upon reading the following detailed description in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of one embodiment of an automatic dynamic joint tensioning system according to the present invention. 
       FIG. 2  is a front view of the system of  FIG. 1 . 
       FIG. 3  is a top view of a portion of the system taken along line A—A of  FIG. 1 , and further showing an assembly mounted thereon in a first dynamic configuration. 
       FIG. 4  is a top view of a portion of the system taken along line A—A of  FIG. 1 , and further showing the assembly mounted thereon in a second dynamic configuration. 
       FIG. 5  is an example of a torque curve as used by an automatic dynamic joint tensioning system according to the present invention. 
       FIGS. 6A and 6B  together comprise a flow chart illustrating the operational steps of the system of  FIG. 1 . The bottom of  FIG. 6A  is continued at the top of  FIG. 6B . 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. 
   One embodiment of a system according to the present invention is designed to fasten together the cutting blades of a pair of scissors and automatically tighten the fastener, e.g., a screw, to the proper level for desired scissors action, i.e., tight enough to begin cutting at the proper point but not so tight as to inhibit normal intended use. 
   Referring to  FIGS. 1–4 , there is illustrated a system  10  including a frame  12  supporting a table  14  and a fastening device  16  disposed above table  14 . Fastening device  16  can be a model no. SA-36 screwdriver from Weber Screwdriving Systems, Mount Kisco, N.Y. Table  14  supports a vertical shaft  18  that is coaxial with a drive axis B. Attached to shaft  18 , above table  14 , is a torque transducer  20  and a torque arm  22  having fixture pins  24  proximate the free end thereof. Torque transducer  20  can be a model no. RTS-100 from Transducer Techniques, Temecula, Calif. Attached to shaft  18 , below table  14 , is a servo motor  26  and a position encoder  28 . Servo motor  26  can be a model no. SGMAH-04AAF41 servo motor from Yaskawa Electric Corporation, Waukegan, Ill. A home switch  30  is connected to table  14  and shaft  18 . Also supported by table  14  in cantilever fashion is a joint support  32  disposed above shaft  18 , but not connected thereto, in alignment with drive axis B. A part clamp  34  is also supported by table  14 . Part clamp  34  and fixture pins  24  are offset from drive axis B. Part clamp  34  can be a model no. RP-10 parallel gripper from Robohand, Inc., Monroe, Conn. Fastening device  16  includes a driver  40  mounted for rotation about drive axis B and driven by a drive servo motor  42  connected to a drive shaft (not shown). Drive servo motor  42  can be a model no. SGMPH-04AAE410 servo motor from Yaskawa Electric Corporation, Waukegan, Ill. Also connected to the drive shaft, and hence to driver  40 , is a drive encoder  44 . 
   A controller  50  has input signal line  52  connected to the output of torque transducer  20 , input signal line  54  connected to the output of home switch  30 , input signal line  56  connected to the output of position encoder  28 , and input signal line  58  connected to the output of drive encoder  44 . Controller  50  has output signal line  60  connected to the input of servo motor  26 , output signal line  62  connected to the input of part clamp  34 , and output control line  64  connected to the input of drive servo motor  42 . Controller  50  may be a programmable logic controller (PLC), e.g., a PLC in the DL205 series from Automation Direct, Cumming, Ga. The PLC may be used with an Automation Direct EZTouch 8″ STN color panel operator interface. 
   Referring in particular to  FIGS. 3 and 4 , an assembly  70 , such as a pair of scissors, having a fastener  72  that functions as a pivot dynamic joint, is shown in place on system  10 . Preferably, the assembly  70 , including fastener  72 , would have been retrieved from another location, such as a carrying pallet on a conveyor, and placed on system  10  by a pick-and-place mechanism, not shown. In  FIG. 3 , assembly  70  is shown in a first configuration with the blades  74  and  76  closed and the respective handles  78  and  80  in a close relationship. In  FIG. 4 , assembly  70  is shown in a second configuration with the blades  74  and  76  separated and the respective handles  78  and  80  in a separated relationship. Blade  74  and handle  78  comprise a first part of assembly  70 , and blade  76  and handle  80  comprise a second part of assembly  70 . Assembly  70  is placed with pivot fastener  72 , which can be a threaded screw, located on joint support  32  and aligned coaxially with drive axis B. Handle  78  is engaged by part clamp  34  and handle  80  is engaged by fixture pins  24  of torque arm  22 . As torque arm  22  rotates about drive axis B, handle  80  and blade  76  are moved from the first configuration of  FIG. 3  to the second configuration of  FIG. 4 , and vice versa. 
   Prior to system  10  automatically adjusting the fastener  72  to achieve the desired resistance to movement of the second part of assembly  70  relative to the first part of assembly  70 , the operator has the capability of inputting via a touchscreen associated with controller  50  certain parameters such as driver backlash, offset angle, test angle, desired torque, torque tolerance, and number of attempts or tries. The input for driver backlash compensates for the backlash or “play” in the fastener driver, which allows for a more accurate result. The offset angle automatically positions the torque arm to accept a particular product configuration, and allows the system to adjust a range of product styles. The test angle, desired torque, and torque tolerance are variables which allow the system to qualify the torque at a specific point, and to accept the product only if the torque is within a specific range. The number of attempts is adjustable to allow for a predetermined number of tries to adjust the tension. If the proper tension is not met after a set number of tries, it is assumed that there is an inherent problem with the product that does not allow it to be tensioned properly. For example, there could be insufficient or excessive blade camber. 
   In operation, mating first and second parts of assembly  70  are retrieved from a pallet on a conveyor by a pick-and-place mechanism and placed into the system  10  with the joint fastener  72  positioned directly over the joint support  32 . The home or starting position of the system is established with a home switch  30 . The part clamp  34  rigidly holds the first part, or stationary member, of the assembly  70  in position. The second part, or dynamic member, of the assembly  70  is placed between the fixture pins  24  on the torque arm  22 . Driver  40  of fastening device  16 , which is powered by drive servo motor  42  and monitored by drive encoder  44  feeds and drives fastener  72  into the joint of the mating parts. The second dynamic member of the assembly  70  and the torque arm  22  are rotated or moved relative to the first stationary member of the assembly  70  by a servo motor  26 . The relative position of the second dynamic member and the torque arm are monitored by an encoder  26  and stored in a data table associated with controller  50 . The torque or tension between the mating parts is monitored by torque transducer  20  and stored in the data table. 
   Based upon feedback from the torque transducer  20 , the fastening device  16  will automatically adjust the fastener via a PID loop to achieve the proper fit and function of the dynamic joint. A predetermined torque or tension of the joint will be set at a predetermined position of the mating parts in relation to each other. More specifically, the screw is tightened until the torque required to move the unrestrained blade relative to the restrained blade, as measured by torque transducer  20 , is within predetermined limits, such as those shown in  FIG. 5 , at a predetermined angle, e.g., the angle which corresponds to the point during closing of the scissors where the ground edges of the two blades should meet to begin cutting. With the screw tightness set at that level, the tension between the blades is measured by torque transducer  20  through the range of operating angles of the scissors and any out-of-range measurement through the range of motion is indicated to a machine operator by means of an alarm, indicator light or the like so that appropriate action can be taken. As an example, the camber or other characteristic of one blade or the other may be out of tolerance such that the scissors tension, while correct at the point of initial contact between the cutting edges of the blades during closure thereof, is higher or lower than desired at some later point of scissors operation. 
   The torque and relative position of the joint are recorded and a torque curve is plotted to show the relationship between the torque and the relative position. The torque curve is plotted in real time to allow the operator to witness the resulting torque, not only at the critical position, but over the full stroke of the product. In this case, the desired torque is set at a predetermined point. However, it is desirable to witness the resulting torque over the full range of motion. For example, if there is a burr or “snag” in the cutting edge, it will cause a spike in the torque. By establishing a high and low torque tolerance, any product having such a spike can be distinguished as defective. 
   The scissors blades may be delivered to joint support  32  in a closed state, as shown in  FIG. 3 , by a pick-and-place mechanism, for example, from a moving conveyor line on which prior blade processing and pre-assembly steps have performed. The tension may be measured and compared to the full range of the above-referenced limits during an initial opening operation after initial insertion and tightening of a screw. Preferably, however, as a preliminary step, one blade is restrained by the grippers of part clamp  34 , and the other blade is moved to a fully opened position in one continuous motion and then moved back to the closed position in order to properly defoil or debur the cutting edges of the blades. The scissors are then opened and closed again and, during such exercising operation, the tension is measured and compared at predetermined points to the above-referenced limits. It should be noted in this regard that  FIG. 5  depicts torque with respect to relative displacement, i.e., angular displacement relative to an open state. Very little force is required to begin closing the scissors, but the force rapidly increases when the cutting edges of the blades come into contact with each other (as indicated at approximately 35° C. of relative displacement in  FIG. 5 . 
   The operational steps of system  10  are summarized and illustrated in the flow chart of  FIGS. 6A and 6B . Control of these steps is preferably implemented in ladder logic in controller  50 . With reference to the flow chart of  FIG. 6A , to which the parenthetical reference numerals that follow correspond, the operational steps of the system are as follows. The home routines are initiated ( 100 ) wherein the offset angle, desired torque, torque tolerance, and number of tries are inputted ( 102 ) and the data is stored in a table ( 104 ). A torque graph, maximum torque, minimum torque, test angle and measured torque are displayed ( 106 ) to complete the home routines ( 108 ). The controller sends the variables to drive servo  42  and torque arm servo  26  ( 110 ) such that the torque arm servo is moved to the offset angle ( 112 ) which may be zero, e.g., in cases where only one model of scissors is assembled and the home position is set accordingly, or may be some nonzero angle such as a desired offset angle for a particular model of scissors selected from a menu of models which the system is programmed to assemble. The assembly or scissors  70  is loaded onto joint support  32  ( 114 ), e.g., with a pick-and-place mechanism, and the first part or stationary member  78  of the assembly  70  is held in position by part clamp  34  ( 116 ). The fastening device  16  is extended and the screw fastener  72  is driven into the pivot joint of the assembly ( 118 ). The screw may be driven in until the driver stalls in response to the screw bottoming out in the joint, and then backed off a desired amount dependent upon the blade length and shape, e.g., blade camber. 
   Turning now to  FIG. 6B , the controller retrieves a predetermined screw back-off value ( 120 ) and backs off the screw fastener ( 122 ). The assembly, or scissors,  70  is opened and closed completely to de-foil the blades ( 124 ). The controller retrieves the test angle, desired torque and torque tolerance ( 126 ) and exercises the assembly, or scissors,  70  by opening and closing the scissors ( 128 ). The torque curve, measured torque and torque angle are displayed for viewing by the operator ( 130 ). The torque is evaluated at the test angle ( 132 ). If the torque is too low ( 134 ), the screw fastener is tightened ( 136 ) and the steps ( 128 ,  130  and  132 ) are repeated. If the torque is too high ( 138 ), the screw fastener is loosened ( 140 ) and the steps ( 128 ,  130  and  132 ) are repeated. After the second evaluation of the torque at the test angle ( 132 ), if the torque is within the torque tolerance, the scissors assembly is noted as “good” and unloaded ( 142 ) by the pick-and-place mechanism to an unload conveyor. If the torque is not within the torque tolerance, the scissors assembly is noted as “bad” and is unloaded ( 144 ) by the pick-and-place mechanism to the unload conveyor. A diverter on the unload conveyor diverts the scissors assemblies that have been noted as “bad” to an alternate path to be scrapped or reprocessed. Alternatively, the pick-and-place mechanism could place “bad” assemblies directly in a scrap bin. 
   It will be appreciated from the foregoing description of the principles of the invention that the system may be suitable in a number of applications utilizing joined parts, e.g., slides, guides, bearings, and swivels for use in such products as automotive parts, electrical/electronic parts, appliances, medical devices, and consumer products. 
   While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only preferred embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.