Abstract:
A control system and control method for use in a machine for wrapping an article in plastic film are disclosed, the machine including a sealing head and a corresponding anvil movable by a pneumatic cylinder from an initial separated position to a contact position. The cylinder has an air exhaust outlet for exhausting air from the cylinder during operation of the cylinder from its initial state to an actuated state. A selectively actuatable valve disposed in the outlet selectively directs exhausting air through a first path of relatively low airflow resistance and a second path of relatively high airflow resistance. The valve directs exhaust air flow from the first path into the second path during operation of the cylinder and just prior to the cylinder being placed in the actuated state to cushion contact between the head and anvil.

Description:
This application claims the benefit of U.S. Provisional Application Ser. No. 60/048,302, filed Jun. 2, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to a packaging machine, and more particularly to a packaging machine in which articles to be packaged are wrapped in heat-shrinkable plastic film. More specifically, the present invention relates to a system and a method for controlling the film cutting and sealing operation of such a machine. 
     Packaging machines which wrap articles in heat-shrinkable plastic film are well-known. One example of a machine of this general type can be seen in U.S. Pat. No. 4,658,569. A web of film is provided in which the film is folded to form a two-ply web having a fold connecting the layers along one edge of the web. The web is drawn from the roll, and turned through 90° to form an open cavity within the web into which the article to be packaged is introduced. The film and article are together advanced beyond a sealing station. At the sealing station, a cooperating cutting/sealing head and anvil are positioned for vertical movement to bring the sealing/cutting head into contact with the anvil and to subsequently separate them. The head is heated, and the contact is made with the web located between the head and anvil. This causes the web to be both severed and sealed at the sealing station. Vertical movement of the sealing apparatus usually encompasses movement of both the sealing/cutting head and the anvil toward and away from each other, the movement typically being caused by one or more pneumatic cylinders. 
     Other machine operations seal the film along the unfolded side edge of the web, producing an article which is encased in the film. The sealed package is then moved into a heated shrink tunnel which causes the film to shrink so as to closely conform to the packaged article. 
     With regard to the lateral seal formed at the sealing station, it has generally been found that the seal should be positioned at the mid-point of the article height so that the upper and lower film layers are fitted around the article in a generally symmetrical manner. In one known machine of this type, this can be accomplished by moving the entire sealing head/anvil/pneumatic cylinder assembly up or down to properly position the meeting point between the sealing/cutting head and anvil which continue to be moved the full stroke of the cylinders. 
     It is desirable to minimize the time required for the sealing operation in order to enhance the overall speed of the process, especially when long continuous runs are made. One way to achieve this is to reduce the time required for the sealing head and anvil to complete their downward/upward travel for each cycle. This can be done, for packages which are of less than maximum acceptable height, by mechanically holding the anvil at its position below the plane of the conveyor system, while lowering the cutting/sealing head. This reduces the up/down stroke distance for each cycle, and also satisfies the desire to position the resulting seal along the mid-point of the package height. 
     However, this in turn creates a problem because the seal head speed and cushioning cannot be controlled by conventional methods. In conventional machines in which the seal head executes its full stroke for all package heights, the speed of the seal head is slowed (cushioned) just prior to contact with the anvil to provide smooth yet firm sealing pressure. This is accomplished through the conventional construction of the pneumatic cylinder itself, which pneumatically slows cylinder piston movement near the full limit of either direction of stroke. However, this method is not usable for less than the full stroke of the cylinder. 
     Slowing of the cutting/sealing head and anvil just prior to meeting is desirable. Otherwise, the full-speed “crashing” of the cutting/sealing head into the anvil can cause premature wearing of the cutting knife, as well as poor sealing quality due to “bounce” of the sealing mechanisms. Since many articles to be packaged on a machine can be expected to be less than the maximum article size accepted by the machine, a need exists for a control system for the cutting and sealing operation that permits high speed operation of the sealing station even when the articles to be packaged are of less than full height. 
     SUMMARY OF THE INVENTION 
     The present invention provides a control system for use in a machine for wrapping an article in plastic film, including a sealing station having a sealing head and a corresponding anvil, and means for moving the head and anvil from an initial separated position to a contact position and back to the initial position. The moving means includes a pneumatic cylinder having an initial state in which the head and anvil are in the initial position and an actuated state in which the head and anvil are in the contact position. The cylinder has an air exhaust outlet line for exhausting air from the cylinder during operation of the cylinder from the initial state to the actuated state. 
     An adjustment means is provided for adjusting the initial position to move the head and anvil closer together while in the initial position. A selectively actuatable valve disposed in the outlet line selectively directs exhausting air through a first path including relatively low airflow resistance and a second path including relatively high airflow resistance. Means is provided for controlling the valve to direct exhaust air flow from the first path into the second path during operation of the cylinder and just prior to the cylinder being placed in the actuated state to cushion contact between the head and anvil. 
     The means for controlling the valve may include a microprocessor programmed to control the valve to direct exhaust air flow from the first path into the second path just prior to the mutual contact between the head and the anvil. The microprocessor controls the valve to direct exhaust air flow from the first path into the second path after a predetermined time interval during operation of the cylinder. 
     A memory may be included for storing the predetermined time interval, and wherein the microprocessor reads the predetermined time interval from the memory. The memory may store a plurality of the predetermined time intervals, in which case the microprocessor reads an appropriate one of the predetermined time intervals as a function of distance between the head and the anvil when the cylinder is in the initial state. 
     The present invention also includes a method for controlling a machine for wrapping an article in plastic film. The machine includes a sealing station having a sealing head and a corresponding anvil, and means for moving the head and anvil from an initial separated position to a contact position and back to the initial position, the moving means including a pneumatic cylinder having an initial state in which the head and anvil are in the initial position and an actuated state in which the head and anvil are in the contact position, the cylinder having an air exhaust outlet line for exhausting air from the cylinder during operation of the cylinder from the initial state to the actuated state, and a selectively actuatable valve disposed in the outlet line for selectively directing exhausting air through a first path including relatively low airflow resistance and a second path including relatively high airflow resistance. 
     The control method comprises operating the cylinder to move from the initial state into the actuated state, and controlling the valve to direct exhaust air flow from the first path into the second path during operation of the cylinder just prior to the cylinder being placed in the actuated state to cushion contact between the head and the anvil. 
     The method may further comprise the step of adjusting the initial position to move the head and the anvil to change the distance therebetween while the cylinder is in the initial state. The valve may be controlled to direct exhaust air flow from the first path into the second path after a predetermined time interval during operation of the cylinder. The machine may include a memory for storing the predetermined time interval, in which case the predetermined time interval is read from the memory. The memory may also store a plurality of the predetermined time intervals, wherein an appropriate one of the predetermined time intervals is read as a function of distance between the head and the anvil when the cylinder is in the initial state. 
     In the method, following adjusting of the initial position, the distance between the head and the anvil may be determined by controlling the valve to direct exhaust air flow from the first path into the second path, thereafter operating the cylinder from the initial state to the actuated state to cause contact between the head and the anvil, and measuring the time required for the operation of the cylinder. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a film wrapping machine with which the present invention may be used. 
     FIG. 2 is a schematic view of the sealing station of the machine of FIG.  1 . 
     FIG. 3 is a cut-away, schematic view of a conventional pneumatic cylinder and pneumatic control system for use with a film wrapping machine of the type shown in FIG.  1 . 
     FIG. 4 is a view similar to FIG. 3 showing the control system of the present invention. 
     FIG. 5 is a schematic view of a portion of the electronic control system for use with the present invention. 
     FIG. 6 is a flowchart diagram illustrating the stroke measurement portion of the control system of the present invention. 
     FIG. 7 is a flowchart diagram illustrating the speed calibration portion of the control system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a film-wrapping machine of the type with which the present invention may be used is shown. An article feeding conveyor  1  advances a series of articles  2  to be packaged onto transfer conveyor  3 . Conveyor  3  in turn advances the packages  2  to sealing station  5 . 
     Supported over transfer conveyor  3  is a film roll F. The plastic film carried on roll F is a heat-sealable, heat-shrinkable plastic film commercially available from a number of suppliers. It is provided on roll F as a two-ply film, which is formed by folding the film to create a folded side edge for the web. Film is drawn from the roll F to provide film web f, with the folded edge e away from the direction of movement of the articles  2 . A conventional turning/folding guide assembly  4  turns the film through 90° to direct it in the same direction of movement as the transfer conveyor  3 . At the same time, guide assembly  4  inverts the web layers so that the inner surfaces of the web f as it is drawn from roll F become the outer surfaces, and fold edge e is disposed on the far side of the machine as viewed in FIG.  1 . 
     Guide assembly  4  further spreads apart the layers of the film web f so that transfer conveyor  3  and the articles carried thereon can be directed between the layers of web f. 
     Web f is advanced by the combination of feed rollers  6 ,  7 ,  8  and  9 , and an article  2  is advanced by transfer conveyor  3 , in a coordinated manner until the web f and the encased article are advanced past the sealing station  5 . Movement of the web and article are halted, and a heated cutting/sealing head  10  and a cooperating anvil  11  are moved toward each other until head  10  and anvil  11  contact each other with the film web f disposed therebetween. The head  10  severs the film surrounding the article from the remainder of the incoming web f and seals the transverse cut edge. Additional sealing apparatus (not shown) seals the lateral edge of the web at the near side as viewed in FIG. 1, producing a sealed, loosely-fit film wrapped package  12 . 
     After sealing of the transverse edge is completed, head  10  and anvil  11  are moved apart. The package  12  is advanced by conveyor  14  to carry the package  12  into a heat-shrink tunnel  15  to cause the sealed film to shrink into close-fitting conformance with the article. The completed package  16  is conveyed from tunnel  15 . 
     Sealing station  5  may be seen schematically in greater detail by reference to FIG.  2 . Anvil  11  is mounted on a mounting block  20 . Block  20  is connected to the rod  22  of a pneumatic cylinder  24 , which is in turn fixed to the machine frame  26 . Cylinder rod  22  is shown in its fully retracted position, the result of which is that anvil  11  is shown in its lowermost position. This places the working surface of anvil  11  slightly below the level of the travel path of transfer conveyor  3 , so that articles to be packaged may pass freely over anvil  11  when in its lowered position. 
     The anvil mounting block  20  is also fixedly connected to chain  28 , which passes around upper sprocket  30  and lower sprocket  32  to form a vertically-oriented travel path for chain  28 . Connected to the opposite side of the travel path for chain  28  is a head mounting block  31 , to which cutting/sealing head  10  is mounted. Sprockets  30  and  32  are connected to a sprocket supporting structure  34 . Structure  34  is in turn supported by the machine frame through appropriate connections (not shown), but is mounted for vertical movement. As shown schematically in FIG. 2, the means for vertical movement of structure  34  may preferably include a hand crank wheel  36  attached to a screw shaft  38 , which is in turn engaged by a threaded connection with the machine frame  40 . Actuation of crank wheel  36  causes shaft  38  to rotate within the threaded connection with frame  40 , thereby raising or lowering structure  34  as may be desired. 
     As depicted in FIG. 2, the sealing station is shown in an open position, as is the case when an article and surrounding film is being moved past the sealing station for subsequent severing and sealing. Further, the anvil  11  and cutting/sealing head  10  are shown separated by their maximum separation distance D, used when the article to be packaged is of the maximum height accommodated on the wrapping machine. 
     After an article is moved beyond the sealing station (to the left as shown in FIG.  2 ), the web of film (not shown) extends through the sealing station. To sever and seal the web, pneumatic cylinder  24  is actuated, thereby extending cylinder rod  22  to raise anvil  11 . Because mounting block  20  is connected to chain  28 , the movement of rod  22  also moves chain  28  in a counter-clockwise direction about its path, thereby moving mounting block  31  and lowering head  10 . It will be seen that after movement of anvil  11  (and as a result, head  10 ) through a distance of D/2, head  10  and anvil  11  will meet, severing and sealing the film web. The cylinder  24  is then actuated to withdraw rod  22 , returning head  10  and anvil  11  to their initial positions as shown in FIG.  2 . 
     When designing and constructing the wrapping machine, pneumatic cylinder  24  and its rod  22  are selected so that the full stroke of the cylinder is the distance D/2. Thus, in the case of an article of maximum height for the machine (i.e., a height of slightly less than D), the head and anvil meet at the end of the full extension of rod  22 . 
     In general, it is desirable to position the seal between the film layers at approximately the midpoint of the height of the article to be packages for maximum package quality. To adjust the sealing station for articles of less than maximum height, hand crank  36  is used. Upon rotation of hand crank  36 , screw shaft  38  operating within the threaded connection with frame  40  moves structure  34  in a downward direction. Because mounting block  20  is fixed through cylinder  24  to frame  26 , the movement of structure  34  through an adjustment distance a does not produce any change in vertical position of anvil  11 . However, chain  28  will be moved around its travel path by a distance equal to the distance a, so that head  10  advances downwardly by a total distance 2a. Head  10  and anvil  11  are now separated by a distance D−2a, and will meet after actuation of cylinder  24  and extension of rod  22  through a distance of D/2−a, which is less than the full stroke of the cylinder. 
     An appropriate scale (not shown) is provided in conjunction with hand crank  36 , so that an operator of the wrapping machine may adjust the separation distance between head  10  and anvil  11  as appropriate for the article to be packaged. 
     Before describing the control system for controlling the actuation of cylinder  24 , reference is made to FIG. 3, in which a cut-away view of a conventional pneumatic cylinder  50  is shown. A piston  52  is connected to rod  54 , which passes through cylinder end wall  56 . An air line  58  is provided for allowing pressurized air through check valve  60  into the cylinder to extend rod  54 , and to allow air to exhaust through flow regulator valve  62  as air is directed into the opposite end of the cylinder to retract rod  54  during the opposite piston stroke. A similar air line  64 , including inlet check valve  66  and an adjustable outlet flow regulator valve  68  are provided at the opposite end of cylinder  50  on the opposite side of piston  52 . 
     A cavity  70  is formed into end wall  56 , and a projection  72  is formed on piston  52  which fits in airtight relationship into cavity  70 . An auxiliary (cushion) air exhaust  74  connects the cavity  70  to the outside, with an adjustable flow resistance regulator valve  76  disposed within the auxiliary (cushion) exhaust outlet. Valve  76  is set to provide a greater flow resistance than valve  68 . 
     As piston  52  approaches the upper end of its stroke, projection  72  seals cavity  70 . The trapped air within cavity  70  must exhaust through exhaust valve  76 . The greater resistance offered by this valve slows the movement of piston  52 , thereby cushioning it as it completes its stroke. 
     A similar cushion is provided at the lower end of cylinder  50 . 
     A cylinder constructed as shown by cylinder  50  in FIG. 3 is used as cylinder  24  in the wrapping machine shown in FIGS. 1 and 2. When the sealing station is set for its maximum height, piston rod  22  will be fully extended when head  10  and anvil  11  meet. Thus, the cushion which is built into the cylinder will cause rod  22  to be slowed in its movement at the end of its stroke. This will in turn slow the movement of head  10  and anvil  11 , and will produce a cushioned meeting between the two elements as the cutting and sealing operations are performed on the film web. 
     Movement of the cylinder rod is also cushioned at the end of its downward stroke to return the sealing station machine to its ready position. 
     If the sealing station is adjusted for an article of less than maximum height, head  10  and anvil  11  are brought closer together and will meet during cutting and sealing before piston rod  22  has travelled its full stroke. Thus, the cushioning mechanism built into cylinder  24  cannot be used to cushion or slow the movement of the head and anvil just prior to their meeting. 
     Accordingly, cylinder  24  is provided with the pneumatic control system shown schematically in FIG.  4 . Parts which correspond to those shown with cylinder  50  in FIG. 3 are identified with identical reference numerals. However, upper air line  64  as shown in FIG. 3 is replaced for cylinder  24  with air line  80 , which branches into air lines  82  and  84 . Disposed within air line  82  is a compressed air inlet line including check valve  86 , and an air exhaust line including flow regulation valve  88 . This branch  82  of air line  80  functions in the usual manner similar to air line  64  of FIG.  3 . 
     Located at the point at which air line  80  branches into lines  82  and  84  is an electronically controlled switching valve  90 . The valve  90  is used to switch the air exhaust from the upper portion of the cylinder  24  from branch path  82  to branch path  84 . Branch path  84  is provided with a flow regulator valve  92  which is adjusted to provide a relatively high flow resistance value similar to that provided by valve  76  disposed in the cushion exhaust  74 . 
     At the beginning of a closing stroke for the sealing station (during which piston direction in FIG. 4 is upward), valve  90  is positioned to exhaust through path  82  so that the piston (and thus sealing head  10  and anvil  11 ) move at normal speed. However, just prior to sealing contact between the head  10  and anvil  11 , valve  90  is actuated to direct exhaust through the branch cushion path  84 . The increased resistance to the air exhaust caused by resistance valve  92  slows the piston and sealing head to an appropriate speed for contact. 
     As has been explained herein, the point at which contact occurs between head  10  and anvil  11  depends upon the initial separation of the head and anvil, which can be adjusted depending upon package size. Thus, valve actuation timing for valve  90  must also be variable to cause the cushioning action to occur just prior to contact between the head and anvil. Therefore, the control system for the sealing station must be capable of recognizing a change of the initial seal head height, as well as provide a means to “measure” such height. 
     Commercially available sealing machines with which the present invention is intended to be used operate under the control of a microprocessor. The microprocessor is used to control the valve  90  as well. Accordingly, as shown in FIG. 5, microprocessor  100  is provided, communicating with a memory  102 . A variety of inputs and outputs (not shown) are provided so that microprocessor  100  can control the general functioning of the wrapping machine in a known manner. 
     Valve  90  is operatively connected to microprocessor  100  to be controlled thereby. Additionally, a motion sensor  104  is connected to microprocessor  100  and mounted adjacent crank wheel  36  (see FIG.  2 ). Sensor  104  detects any adjusting movement made by wheel  36 , although it does not measure the amount of motion but only that motion has occurred. (Alternatively, a sensor could be provided to detect movement of support structure  34 .) A further sensor  106  is connected to microprocessor  100  and provided to detect motion of cutting/sealing head  10  and/or anvil  11 . Sensor  106  can thus detect that contact between head and anvil have occurred by determining that movement of either or both head and anvil has stopped. 
     The measurement operation of the present invention can be seen by reference to the flowchart shown in FIG.  6 . In block  120 , microprocessor  100  monitors sensor  104  to determine whether any adjusting movement of the crank wheel has occurred. If this is detected, the valve  90  is actuated to direct all exhaust air flow through path  84  and cushion valve  92 , shown at block  122 . A measurement operation is also carried out upon machine start-up, block  124 , and valve  90  is actuated as indicated in block  122 . 
     The cylinder  24  is next actuated to carry out a closing operation for the sealing station, block  126 . Because all air flow is through cushion valve  92 , the movement of the head and anvil are at the relatively slow “cushion” speed. The end of movement is detected by sensor  106 , and the time required to move the head and anvil from their starting positions to a closed position is measured, block  128 . This time value is directly related to the distance travelled. 
     The measured time value is applied against a look-up table stored in memory  102 , block  130 , to determine an appropriate timing value for actuation of valve  120  during subsequent cycles. This value is used, block  132 , to set the actuation time for valve  90  during subsequent, normal-operation cycles. 
     In the preferred embodiment, the look-up table is created empirically. The sealing head/anvil spacing is adjusted to ¼″ increments. For each increment, an appropriate actuation time is determined by trial-and-error and entered into the table. 
     During the subsequent machine cycles, cylinder  24  is first moved at the higher speed resulting from exhaust through path  82 . At the appropriate time (as determined by the actuation time from the look-up table), valve  90  is actuated, thereby slowing the sealing head by directing exhaust through the valve  92  located in branch  84 . 
     A standardized look-up table is used for all machines of the same model. Therefore, the main seal head speed and cushion speed must be standardized. For this reason, a calibration procedure is incorporated into the system, shown in flowchart form in FIG.  7 . 
     The system is manually placed in calibration mode and the cutting/sealing head and anvil are set to a known separation. The operator initiates a calibration cycle, block  134 , and valve  90  is actuated to direct all exhaust during the closing stroke through branch  84  and valve  92 , block  136 . Cylinder  24  is actuated to close the head and anvil, block  138 . The time required to close head  10  and anvil  11  (which corresponds to movement speed) is measured, block  140 , and at block  142  compared against a standard time which has been predetermined for the particular model machine. The result of this comparison is provided on a display panel (not shown), block  144 , for the information of the operator. 
     The operator then manually adjusts valve  92  to increase or decrease the resistance to air flow through branch  84 , thereby increasing or decreasing the movement speed of the piston  52  within cylinder  24 . The calibration cycle is repeated and the result is again displayed. The operator continues through successive iterations until the difference between actual stroke time and standard stroke time is rendered either zero or acceptably small. 
     A similar calibration may be performed for the high-speed, normal piston movement by using the full stroke of piston  52  so that the mechanical cushion built into the cylinder may be used. Valve  90  is left deactivated in such a case, and movement time is compared against a standard. Valve  88  may be adjusted following each calibration cycle until the difference between actual stroke time and standard stroke time becomes either zero or acceptably small.