Abstract:
Apparatus and methods for applying flexible straps around objects include a feed and tension unit a feed drive wheel and a feed pinch wheel, a primary tension drive wheel and a primary tension pinch wheel, and a secondary tension drive wheel and a secondary tension pinch wheel, wherein at least one of the pinch wheels is controllably biased against the respective drive wheel by a solenoid that is controlled in two stages: a first stage that provides a full feed or tensioning force and a second stage that provides a reduced feed or tensioning force by altering the pulse width modulation of the solenoid. In another embodiment, the three sets of wheels of the feed and tension unit are configured to provide a simplified “V-shaped” strap path that reduces bending of the strap, thereby reducing friction and consequent feeding difficulties. In another embodiment, the feed and tension unit includes inner and outer guides that form a strap channel through the feed and tension unit to provide easy access to the strap path for clearing the strap path in the event of a jam. In another embodiment, a track assembly includes a plurality of sections providing modularity of construction. Each section includes a backplate attached to at least one support member, and a slotted cover pivotably attached to the at least one support member proximate the backplate and moveable between an open position spaced apart from the backplate and a closed position proximate the backplate. In another embodiment, a cutting assembly for severing strap material includes a press platen and a cutter having a first cutting blade along a first edge thereof and a second cutting blade along a second edge thereof, the cutter being removably and variably engaged to the press platen.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to apparatus and methods for applying flexible straps around bundles of objects.  
         BACKGROUND OF THE INVENTION  
         [0002]    Many high-speed, automatic strapping machines have been developed, such as those disclosed in U.S. Pat. Nos. 3,735,555; 3,884,139; 4,120,239; 4,312,266; 4,196,663; 4,201,127; 3,447,448; 4,387,631; 4,473,005; 4,724,659, 5,379,576, 5,414,980, 5,613,432, and 5,809,873. As disclosed by the devices in these patents, a conveyor belt typically conveys a bundle at high speed to a strapping station where straps are automatically applied before the conveyor belt moves the strapped bundle away from the device.  
           [0003]    Typical strapping machines employ an initial or primary tensioning apparatus that provides an initial tensioning of the strap about the bundle. A secondary tensioning apparatus thereafter provides increased or enhanced tension of the strap. A sealing head then seals the strap, typically through the use of a heated knife mechanism, to complete the bundling operation.  
           [0004]    [0004]FIG. 1 is a strapping machine  100  in accordance with the prior art, as shown and described in U.S. Pat. No. 5,414,980, issued to Shibazaki et al. The strapping machine  100  includes the following major components, all mounted to a housing or frame  110 : a strap dispenser  112 , an accumulator  114 , a feed and tension unit  116 , a track  118 , a sealing head  122 , and a control system  124 . In addition, some devices also have a secondary tension unit  120  (not shown), such as the type disclosed in U.S. Pat. No. 3,552,305 issued to Domey et al. The basic operation of the machine involves a feeding cycle and a strapping cycle. In the feeding cycle, strap is pulled from a strap coil mounted on the dispenser  112  by a feed and tension motor and is fed through the accumulator  114 , the feed and tension unit  116 , the sealing head  122 , and the track  118 . After the strap has been fed around the track  1   18  and back into the sealing head  122 , the strapping cycle begins.  
           [0005]    During the strapping cycle, the strapping machine performs several functions. First, the sealing head  122  of the strapping machine grips the free end of the strap, holding it securely. Next, in a primary tensioning sequence, a track guide mechanically opens and the strap is pulled from the track  118  as the strap is drawn around the bundle by a feed and tension motor.  
           [0006]    As the primary tensioning sequence is completed, additional strap tension may be applied by the secondary tension unit  120 . As this secondary tensioning process is completed, the sealing head  122  grips the supply side of the strap. The overlapping strap sections are then heated by a heater blade, pressed together by a press platen, and severed from the supply by a strap cutter  140 .  
           [0007]    Following the sealing process, the strap path through the sealing head  122  is once again aligned and the feeding sequence can begin. The sealing head  122  continues to rotate allowing the seal to cool while the feeding sequence continues. At the end of the strapping cycle, the sealed strap is released and the strapping machine  100  is ready to repeat the feeding cycle.  
           [0008]    Although desirable results are achievable using the prior art strapping machines  100 , some operational drawbacks exist. For example, the prior art feed and tension unit  116  typically includes a complicated series of strap guides. The strap must be fed through the strap guides, undergoing several bends and turns between the dispenser  112  and the sealing head  122 . Existing strapping machines typically turn the strap through a total of 360 degrees or more before reaching the track. The bends and turns in the strap path may induce kinks in the strap that may subsequently lead to feeding difficulties. If the strap becomes jammed in the feed and tension unit  116 , the process of clearing the strap path from the complicated series of strap guides may be time-consuming and may require machine downtime.  
           [0009]    Another disadvantage of the prior art strapping machines is that the drive assemblies of the sealing head  122  and the feed and tension unit  120  are typically complicated designs featuring a one or more gear boxes. Often these gear boxes are complicated and must transfer the drive forces through a 90 degree angle. Generally, the cost of fabricating the drive assembly increases with the design complexity, adding to the ultimate cost of the strapping machine.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention improves upon prior strapping devices, and provides additional benefits, such as by providing variability in the apparatus that can be easily altered to fit various production and package requirements and by employing a control system that monitors operating signals and transmits control signals accordingly.  
           [0011]    A feed and tension unit under one aspect of the invention includes three sets of wheels: (1) a feeding set including a feed drive roller and a feed pinch roller, (2) a primary tensioning set including a primary tension drive roller and a primary tension pinch roller, and (3) a secondary tensioning set including a secondary tension drive roller and a secondary tension pinch roller, and wherein at least one of the feed pinch roller, the primary tension pinch roller, or the secondary tension pinch roller is coupled to a solenoid that controllably biases the pinch roller against the respective drive roller based on a pinch signal supplied to the solenoid, the pinch signal having a first pulse width modulated stage that provides a full pinch force and a second pulse width modulated stage that provides a reduced pinch force.  
           [0012]    During a primary tensioning operation, a control system monitors position signals from a feed pinch roller position sensor and terminates primary tensioning when a slippage condition is determined. The control system then initiates a secondary tensioning operation. The secondary tensioning operation lasts for a predetermined amount of time, then the control system initiates a joining operation that secures the strap around the bundle.  
           [0013]    In another aspect of the invention, the three sets of wheels or rollers of the feed and tension unit are configured to provide a simplified strap path that reduces bending of the strap, thereby reducing friction and consequent feeding difficulties. Alternately, the drive wheels of the feed and tension unit may be positioned on the side of the strap opposite from the bundle to reduce adverse effects of debris from the bundle. In another aspect, the feed and tension unit includes inner and outer guides that form a strap channel through the feed and tension unit. The inner and outer guides are configured to provide easy access to the strap path for clearing the strap path in the event of a jam.  
           [0014]    In a further aspect of the invention, a strap material accumulating compartment includes a first sidewall having a plurality of mounting posts projecting therefrom, each mounting post having a plurality of mounting holes disposed therethrough, a second sidewall having a plurality of mounting apertures alignable with and slideably engageable with the mounting posts, and a plurality of pin holders positioned proximate the mounting apertures, and a plurality of mounting pins removably and adjustably engageable with the mounting holes and the pin holders. The first and second sidewalls approximately form a chamber therebetween wherein the strap may accumulate. The width of the chamber may be adjusted easily and quickly to accommodate varying widths of strap by removal of the retaining pins, repositioning the second sidewall at the desired location, and replacement of the retaining pins within the desired holes.  
           [0015]    In yet another aspect of the invention, the track assembly includes a plurality of sections providing modularity of construction. Each section includes a backplate attached to at least one support member, and a slotted cover pivotably attached to the at least one support member proximate the backplate and moveable between an open position spaced apart from the backplate and a closed position proximate the backplate, and a biasing member engaged with the slotted cover that exerts a biasing force on the slotted cover to urge the slotted cover toward the closed position. The biasing force is small enough that a tensioning force in the strap material may overcome the biasing force and thereby actuate the slotted cover toward the open position to allow the strap material to escape from the guide passage during a tension cycle. During a feed cycle, the strap material exerts a closing force on an outer surface of the slotted cover, urging the slotted cover into the closed position. In another aspect, the slotted covers are pivotably mounted on guide pins that are approximately parallel to the path of the strap material within the guide passage.  
           [0016]    In another aspect, a cutting assembly for severing strap material includes a press platen and a cutter having a first cutting blade along a first edge thereof and a second cutting blade along a second edge thereof, the cutter being removably and variably engaged to the press platen such that at least one of the first or second cutting blades is engageable with the strap material. In another aspect, at least one of the first and second edges is slanted at a slant angle with respect to an adjacent edge of the cutter.  
           [0017]    These and other benefits of the present invention will become apparent to those skilled in the art based on the following detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a front elevational view and partial fragmentary view of a strapping machine under the prior art.  
         [0019]    [0019]FIG. 2 is an isometric view of a strapping machine in accordance with an embodiment of the invention.  
         [0020]    [0020]FIG. 3 is an isometric view of a sealing head in accordance with an embodiment of the invention.  
         [0021]    [0021]FIG. 4 is a top elevational view of the sealing head of FIG. 3.  
         [0022]    [0022]FIG. 5 is a back elevational view of the sealing head of FIG. 3.  
         [0023]    [0023]FIG. 6 is an isometric view of a press platen and a cutter of the sealing head of FIG. 3.  
         [0024]    [0024]FIG. 7 is an isometric view of a main drive assembly in accordance with an embodiment of the invention.  
         [0025]    [0025]FIG. 8 is a top elevational view of the main drive assembly of FIG. 7.  
         [0026]    [0026]FIG. 9 is a side elevational view of the main drive the assembly of FIG.  
         [0027]    [0027]FIG. 10 is a first isometric view of a feed and tension unit in accordance with an embodiment of the invention.  
         [0028]    [0028]FIG. 11 is a second isometric view of the feed and tension unit of FIG.  
         [0029]    [0029]FIG. 12 is a partial front elevational view of a strap path of the feed and tension unit of FIG. 10.  
         [0030]    [0030]FIG. 13 is a partial isometric view of a primary pinch wheel and a proximity switch of the feed and tension unit of FIG. 10.  
         [0031]    [0031]FIG. 14 is an exploded isometric view of an accumulator in accordance with an embodiment of the invention.  
         [0032]    [0032]FIG. 15 is a front elevational view of the accumulator of FIG. 14.  
         [0033]    [0033]FIG. 16 is a top elevational view of the accumulator of FIG. 14.  
         [0034]    [0034]FIG. 17 is an isometric view of a dispenser in accordance with an embodiment of the invention.  
         [0035]    [0035]FIG. 18 is a top elevational view of the dispenser of FIG. 17.  
         [0036]    [0036]FIG. 19 is an isometric view of a track in accordance with an embodiment of the invention.  
         [0037]    [0037]FIG. 20 is a partial sectional view of a straight section of the track of FIG. 19 taken along line  20 - 20 .  
         [0038]    [0038]FIG. 21 is an isometric view of a corner section of the track of FIG. 19.  
         [0039]    [0039]FIG. 22 is an exploded isometric view of the press platen and cutter of FIG. 6.  
         [0040]    [0040]FIG. 23 is an enlarged partially-exploded isometric view of a pair of inner and outer strap guides of the feed and tension unit of FIG. 10.  
         [0041]    [0041]FIG. 23A is a cross-sectional view of the inner and outer guides of FIG. 23 to illustrate the guide slot created by the inner and outer guides.  
         [0042]    [0042]FIG. 24 is a cross-sectional view of the accumulator of FIG. 15 taken along line  24 - 24 .  
         [0043]    [0043]FIG. 25 is a partially exploded isometric view of a straight section of the track of FIG. 19.  
         [0044]    In the drawings, identical reference numbers identify identical or substantially similar elements or steps. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0045]    The present disclosure is directed toward apparatus and methods for strapping bundles of objects. Specific details of certain embodiments of the invention are set forth in the following description, and in FIGS.  2 - 25 , to provide a thorough understanding of such embodiments. A person of ordinary skill in the art, however, will understand that the present invention may have additional embodiments, and that the invention may be practiced without several of the details described in the following description.  
         [0046]    [0046]FIG. 2 is an isometric view of a strapping machine  200  in accordance with an embodiment of the invention. The strapping machine  200  includes seven major subassemblies: a frame  210 , a control system  220 , a dispenser  250 , an accumulator  300 , a feed and tension unit  350 , a sealing head  400 , a drive assembly  500 , and a track  450 . The subassemblies are of modular construction, which allows them to be used in multiple frame configurations.  
         [0047]    Throughout the following discussion and in the accompanying figures, the strap material is shown and referred to as a particular type of material, namely, a flat, two-sided, tape-shaped strip of material. This practice is adopted herein solely for the purpose of simplifying the description of the inventive methods and apparatus. It should be understood, however, that several of the methods and apparatus disclosed herein may be equally applicable to various types of strap material, and not just to the flat, two-sided, tape-shaped material shown in the figures. Thus, as used herein, the terms “strap” and “strap material” should be understood to include all types of materials used to bundle objects.  
         [0048]    The overall operation of the strapping machine  200  will first be described with reference to various figures, and thereafter, the individual components will be described in detail. In brief, the operation of the strapping machine  200  involves paying off strap  202  from a strap coil  204  located on the dispenser  250  (FIGS.  17 - 18 ), and feeding a free end  206  of the strap  202  through the accumulator  300  (FIGS.  14 - 16 ), the feed and tension unit  350  (FIGS.  10 - 13 ), the sealing head  400  (FIGS.  3 - 5 ), and around the track  450  (FIGS.  19 - 20 ). After the strap  202  is fed around the track  450 , the free end  206  is fed back into the sealing head  400 . At this point the strap  202  is in position to start a strapping cycle.  
         [0049]    Upon the start of the strapping cycle, several sealing head cams  402  in the sealing head  400  (FIGS.  3 - 5 ) begin to rotate, forcing a left-hand gripper  404  to pinch the free end  206  of the strap  202  against an anvil  406 . After gripping the strap  202  in the sealing head  400 , the feed and tension unit  350  (FIGS.  10 - 13 ) retracts the strap  202  from the track  450 . As the strap  202  is pulled from the track  450 , the strap  202  is tensioned around a bundle of objects (not shown) located in a strapping station  208  (FIG. 2) by a feed and tension motor  361  (FIG. 10). As the strap  202  becomes tight around the bundle, a primary tension pinch wheel  352  (FIG. 10) stops rotating. A proximity sensor  354  (FIG. 11) detects the lack of rotation of the primary tension pinch wheel  352  (FIG. 12) and starts a secondary tension process.  
         [0050]    Preferably, the cams  402  operate as cycloidal cams allowing the sealing head  400  to operate smoothly at increased speeds and the cam follower pressure angles are minimized to extend cam life. As used herein, the term cycloidal cam means a cam with cycloidal displacement generated by taking a sinusoidal acceleration function that has a magnitude of zero at its beginning and end, and integrating the function to obtain the velocity and displacement of the follower.  
         [0051]    Secondary tension is applied until a drive wheel clutch  356  (FIGS.  7 - 8 ) slips, at a predetermined set-point, and the sealing head  400  rotates far enough to grip the strap  202  with a right-hand gripper  408 . After the strap  202  is gripped by the right-hand gripper  408 , the tension on the free end  206  of the strap  202  is released and the strap  202  around the bundle is cut free from the coil  204  by a cutter  414  (FIGS. 3 and 6). The two overlapping ends of the strap  202  are then heated by inserting a heater blade  410  (FIG. 3) between them and lightly pressing the straps against the blade  410  with a press platen  412  (FIG. 3). The press platen  412  then lowers slightly and the heater blade  410  is removed from between the strap ends. Next, the press platen  412  presses both ends against the anvil  406  (FIG. 3) for bonding and cooling. As the sealing head cams  402  continue to rotate, the press platen  412  lowers slightly allowing the anvil  406  to open and release the sealed strap. After the strap is released, the anvil  406  is closed and the strapping cycle is completed by feeding strap  202  through the sealing head  400 , around the track  450 , back into the sealing head  400  and finally actuating a feed stop switch  416  (FIG. 3).  
         [0052]    Two modes of operation are available: manual and automatic. The manual mode applies single or multiple straps while an operator actuates a switch. The automatic mode applies a single strap or multiple straps when a switch is actuated by a moving bundle. The automatic mode is used in conveyor lines and in conjunction with other automated machinery.  
         [0053]    As shown in FIG. 2, the frame  210  consists of a main support  212 , adjustable legs  214 , and cover plates  216 . The frame  210  provides structural support for all of the other sub-assemblies of the strapping machine  200 . In this view, the strap  202  is fed about the track  450  in a strap-feed direction  209  that is generally counter-clockwise.  
         [0054]    The strapping machine  200  is controlled by a control system  220  that may include a programmable logic controller  222  (FIG. 3) that operates in conjunction with various input and output devices and controls the major subassemblies of the strapping machine  200 . Input devices may include, for example, momentary and maintained push buttons, selector switches, toggle switches, limit switches and inductive proximity sensors. Output devices may include, for example, solid state and general purpose relays, solenoids, and indicator lights. Input devices are scanned by the controller  222 , and their on/off states are updated in a controller program  224 . The controller  222  executes the controller program  224  and updates the status of the output devices accordingly. Other control functions of the controller  222  are described below in further detail.  
         [0055]    In one embodiment, the programmable controller  222  and its associated input and output devices may be powered using a 24 VDC power supply. The controller  222 , power supply, relays, and fuses may be contained within a control panel (not shown). The momentary and maintained push buttons, selector switches, and toggle switches may be located on a control pendant or a control panel cover. The limit switches, inductive proximity sensors, and solenoids are typically located within the strapping machine  200  at their point of use. At least one indicator light may be mounted on the top of the track  450  and may light steadily to indicate an out-of-strap condition, and may flash to indicate a strap misfeed condition.  
         [0056]    One commercially-available programmable controller  222  suitable for use with the strapping machine  200  is the T100MD1616+ PLC manufactured by Triangle Research International Pte Ltd in Singapore. This device includes sixteen NPN-type digital outputs, four of which are NPN Darlington Power Transistor types and twelve of which are N-channel power MOSFET types. Two of the outputs are capable of generating a Pulse Width Modulated (PWM) signal with a frequency and duty cycle determined in the programming software. Also included are four input channels of 10-bit analog-to-digital converters. Two of the input channels are buffered by operational amplifiers with a ×5 gain accepting analog signals of 0-1 full scale. The remaining two channels are unbuffered and accept 0-5 V full scale analog signals. The unit includes a stable 5 V (+/−1% accuracy) regulated DC power supply to be used as a voltage reference for the analog inputs. A single channel 8-bit digital-to-analog output utilizing a 0-20 mA current loop signal, also resides on the PLC.  
         [0057]    The T100MD1616+ PLC has communication ports, including an RS232C port for program uploads, downloads and monitoring, a two-wire RS485 network port, a 14-pin LCD display port for possible future use as a diagnostic display driver, and a port for expansion. The PLC itself is controlled by a custom CPU that has both EEPROM and RAM memory backup. The controller program  224  used to program the controller  222  may, for example, include Trilogi programming software available from Triangle Research International Pte Ltd, and may include both ladder logic and Tbasic type code (described more fully at www.tri.com.sg/index.htm).  
         [0058]    [0058]FIG. 3 is an isometric view of the sealing head  400  of the strapping machine  200  of FIG. 2. FIGS. 4 and 5 are top elevational and back elevational views, respectively, of the sealing head  400  of FIG. 3. FIG. 6 is an isometric view of the press platen  412  and the cutter  414  of the sealing head  400  of FIG. 3. The sealing head  400  comprises a motor-driven main shaft  418  and a series of cams  402  which perform gripping, sealing and cutting functions. These cams  402  drive three sliding members  422  and three rotating arms  424  (FIG. 5). One slide member  422  is coupled to the right-hand gripper  408 , another slide member  422  is coupled to the left-hand gripper  404 , and the third slide member  422  is coupled to the press platen  412 . The sliding members  422  perform the gripping, sealing and cutting functions, while the pivoting arms  424  move an inner slide  420 , the anvil  406 , and the heater blade  410  into and out of a strap path as required during a strapping cycle.  
         [0059]    [0059]FIG. 22 is an exploded isometric view of the press platen  412  and cutter  414  of FIG. 6. As shown in this view, the press platen  412  includes a pair of mounting nubs  411 , and the cutter  414  includes mounting recesses  413 . A spring  415  is disposed between the cutter  414  and the press platen  412 , one end of the spring  415  being partially disposed within a seating hole  417  disposed in the press platen  412 . The cutter  414  has cutting edges  419  at both ends, allowing the cutter  414  to be reversibly positioned on the press platen  412  for added operational life. In the embodiment shown in FIG. 22, the cutting edges  419  are slanted at an angle α. Although a wide variety of cutting edge angles α may be used, a cutting edge angle in the range of approximately 9 degrees or less is preferred.  
         [0060]    During assembly, the spring  415  is compressed between the cutter  414  and the press platen  412  until the two mounting recesses  413  slideably engage two of the mounting nubs  411 . One may note that the cutter  414  has a pair of mounting recesses  413  situated near each end of the cutter  414  which allows the cutter  414  to be reversibly mounted onto the press platen  412 . The cutter  414  and the press platen  412  are then positioned securely between the left and right-hand grippers  404 ,  408  with the pressure from these parts maintaining the compression of the spring  415 . The cutter  414  and press platen  412  are then engaged with the third slide member  422 . This arrangement provides the necessary scissors action to sever the strap  202 .  
         [0061]    An advantage of the cutter  414  and press platen  412  assembly shown in FIGS. 6 and 22 is that the cutter  414  is removably and replaceably mounted to the press platen  412  by slideably engaging onto the press platen  412 . This allows the cutter  414  to be more easily removed for replacement or maintenance than in the prior art devices. The reversibility of the cutter  414  also essentially doubles the useful life of the component.  
         [0062]    [0062]FIG. 7 is an isometric view of a main drive assembly  500  in accordance with an embodiment of the invention. FIGS. 8 and 9 are top and side elevational views, respectively, of the main drive assembly  500  of FIG. 7. The main drive assembly  500  includes a main drive motor  502  that drives a sealing head drive belt  508  and a drive wheel belt  510 . The sealing head drive belt  508  and the drive wheel belt  510  are preferably “toothed” belts. The sealing head drive belt  508  is directly coupled to a spring clutch  504 . The drive wheel belt  510  is turned approximately 90 degrees on a pair of drive pulleys  512  and is coupled to the drive wheel clutch  356 . As shown in FIG. 7, the main drive motor  502 , the spring clutch  504 , and the drive wheel clutch  356  are operatively coupled to the controller  222 , such as, for example, by electrically conductive leads  223 .  
         [0063]    One advantage of the main drive assembly  500  is that the drive wheel clutch  356  is driven by the drive wheel belt  510 , which is turned at an approximately 90 degree angle on the drive pulleys  512 . This arrangement, commonly referred to as a “mule drive,” eliminates a 90-degree gearbox commonly found in drive systems of prior art strapping machines. Thus, the complexity and costs of fabrication of the main drive assembly  500  are reduced, and reliability and maintainability is improved.  
         [0064]    In the embodiments shown in the accompanying figures, the spring clutch  504  is a wrap spring clutch and the drive wheel clutch  356  is an electromagnetic clutch. Alternately, other spring clutch  504  and drive wheel clutch  356  embodiments may be used. The spring clutch  504  stops the sealing head cams  402  at the proper degree of rotation during each stage of the cycle and stops the cams  402  in their home position at the end of each cycle. As stated above, the drive wheel clutch  356  slips at a torque that is determined by the voltage supplied to a coil located within the electromagnetic drive wheel clutch  356 . The slip in the drive wheel clutch  356  determines the amount of secondary tension that is applied to the strap  202 .  
         [0065]    The main drive motor  502  drives the sealing head  400  by means of the sealing head drive belt  508  and the spring clutch  504  (FIGS. 7 and 8) which is mounted over an end of the sealing head main shaft  418  (FIG. 3). Rotation of the main shaft  418  causes the keyed cams  402  (FIGS. 3 and 5) to rotate and perform the necessary gripping, sealing and cutting functions. During a first period of rotation, the main shaft  418  rotates to the first of three stops on the spring clutch  504 , causing a cutter-gripper assembly  426  to grip the strap  202  and the inner slide  420  to move out of the strap path. The main drive motor  502  then tensions the strap about the bundle, as will be described more fully below. When the strap tensioning is complete, the controller  222  pulses the spring clutch  504  allowing the cams  402  to rotate in a second period of rotation.  
         [0066]    During the second period of rotation the right-hand gripper  404  grips the tensioned strap just ahead of the feed stop switch  416  and the tension in the strap is then released. After the tension is released, the platen  412  and the cutter  414  (FIGS. 6 and 22) rise to cut the strap  202  and press the strap against the heater blade  410 . The cams  402  continue to rotate through a dwell section as the strap  202  melts on the heater blade  410 . After a predetermined time for melting has passed, the press platen  412  and the cutter  414  retract slightly allowing the heater blade  410  to retract.  
         [0067]    After the heater blade  410  retracts, the press platen  412  rises again to press the two melted ends of the strap  202  together for cooling and sealing. The sealing head main shaft  418  continues to rotate during a third period of rotation until a clutch trigger  428  disengages the spring clutch  504 . The sealing head  400  maintains this position for a predetermined time until the controller  222  again energizes a spring clutch solenoid  506  (not shown) located within the spring clutch  504 . The continued rotation of the cams  402  releases the press platen  412  and drops the left and right-hand grippers  404 ,  408  to their home positions. One of the cams  402  then pivots the anvil  406  out of the strap line past a pair of strippers  430 . As the anvil  406  pivots, the strippers  430  push the strap off of the anvil  406 . After the strap  202  is out of the sealing head  400 , the anvil  406  closes, and the cams  402  reach their home positions. At the home position the spring clutch  504  reaches the third and final stop as the feed stop switch  416  (FIG. 3) signals the controller  222  to begin another feed sequence.  
         [0068]    [0068]FIG. 10 is a first isometric view of the feed and tension unit  350  in accordance with an embodiment of the invention. FIGS. 11 and 12 are a second isometric view and a partial front elevational view, respectively, of the feed and tension unit  350  of FIG. 10. As best seen in FIG. 12, there are three sets of wheels in the feed and tension unit  350 : (1) a primary tensioning set including a primary tension drive wheel  360  and a primary tension pinch wheel  352 , (2) a secondary tensioning set including a secondary tension drive wheel  362  and a secondary tension pinch wheel  364 , and (3) a feeding set including a feed drive wheel  366  and a feed pinch wheel  368 .  
         [0069]    The feed and tension unit  350  pinches the strap  202  between each of the three sets of drive wheels and pinch wheels. The feed, primary tension, and secondary tension pinch wheels  366 ,  360 ,  362  are engaged against the strap  202  by a feed pinch solenoid  370   a , a primary tension pinch solenoid  370   b , and a secondary tension pinch solenoid  370   c , respectively. The drive wheel clutch  356  is powered by a drive wheel belt  510  from the main drive motor  502 . The primary tension and feed drive wheels  360 ,  366  are powered by a secondary drive belt  372  mounted on a feed and tension motor  361 . The secondary tension drive wheel  362  is powered by the drive wheel clutch  356  that is driven by the drive wheel belt  510  from the main drive motor  502 . As shown in FIGS. 10 and 11, the feed and tension motor  361 , and the solenoids  370   a ,  370   b ,  370   c  are operatively coupled to the controller  222  by conductive leads  223 .  
         [0070]    Unlike prior art strapping machines which feed the strap around several bends in the feed and tension unit prior to reaching the track, the strapping machine  200  features a simplified strap path (FIG. 12) allowing the strap to be fed in a straighter path than previously achievable. The path begins at the supply dispenser  250  that is located on the opposite side of the strapping machine from the feed and tension unit. This position further enables the strap to travel in a less tortuous path. As shown in FIG. 12, the drive wheels  360 ,  366 , and  362  are positioned in an approximately triangular orientation, with the strap  202  traversing an approximately “V-shaped” strap path having an included angle of in the range of approximately 20 degrees to approximately 40 degrees. Less bending of the strap reduces friction throughout the system, increasing the reliability of strap feeding. Less bending also reduces the tendency of the strap to permanently deform and cause feeding difficulties. Thus, the feed and tension unit  350  of the present invention advantageously reduces or eliminates kinks in the strap which lead to feeding difficulties. While the strapping machines of the prior art typically turned the strap through a total of 360 degrees or more prior to reaching the track, the feed and tension unit  350  greatly reduces the amount of turning of the strap. For example, in the embodiment shown in the accompanying figures, the strap is turned through between approximately 180 and approximately 220 degrees as the strap is initially fed from the dispenser  250  across the strapping machine to the sealing head  400 .  
         [0071]    As the strap  202  passes through each set of pinch wheels, a plurality of inner guides  374  and a plurality of outer guides  376  keep the strap  202  in line with the sealing head  400 . FIG. 23 is an enlarged partially-exploded isometric view of a pair of inner and outer strap guides  374 ,  376  of the feed and tension unit  350  of FIG. 10. As best viewed in FIG. 23, each “L-shaped” inner guide  374  has a roughly L-shaped cross-section and is coupled to a matching “L-shaped” outer guide  376  to form a strap channel  380  through which the strap  202  passes. FIG. 23A is a cross-sectional view of the inner guide  374  and outer guide  376  and illustrates the guide chamber formed by the inner and outer guides to guide the strap material  202 .  
         [0072]    The inner and outer guides  374 ,  376  are secured in position on a plurality of guide pins  378  which project from a back plate  382  (FIG. 10) of the feed and tension unit  350  by a plurality of retaining knobs  379 , although a variety of other securing devices may be used. In FIG. 10, one of the outer guides  376  is removed from the strap path adjacent to the primary tension pinch and drive wheels  352 ,  360  to provide a view of one of the “L-shaped” inner guides  374 .  
         [0073]    During a feeding sequence, the strap  202  is pinched between the feed drive and pinch wheels  366 ,  368 . In one embodiment, a feed force applied by the feed drive and pinch wheels  366 ,  368  is regulated by a pulse width modulated solenoid  370   a  in two stages: a first stage that provides a full feed force and a second stage that provides a reduced feed force by altering the pulse width modulation of the feed pinch solenoid  370   a . Because the pinch force exerted by a solenoid  370   a  on the strap  202  varies with supplied voltage, supplying a pulse width modulated voltage signal to the solenoid  370   a  provides the ability to vary the force exerted by the solenoid  370   a.  As the force exerted by the solenoid  370   a  is decreased, the strap  202  is permitted to slip on the feed drive wheel  366  more easily with a decreased amount of feed drive force. Commercially-available solenoids suitable for this purpose include those solenoids available from Ledex® Actuation Products of Vandalia, Ohio.  
         [0074]    It should be noted that the frequency of the pulses which are fed to the solenoid affects the operation and performance of the solenoid. Generally, as the frequency of the pulses is increased, the adjustability of the pinch force exerted by the solenoid is improved. For example, using the above-referenced solenoids available from Ledex® Actuation Products, a pulse frequency of 8000 Hz has been successfully used.  
         [0075]    The feed drive and pinch wheels  366 ,  368  feed the strap through the sealing head  400 , around the track  450 , and back into the sealing head  400 . When the free end  206  of the strap  202  reaches the sealing head  400 , the arrival of the free end  206  is detected by feed stop switch  416 , which transmits a feed stop signal to the controller  222 . The controller  222  then sends a feed pinch signal to the feed pinch wheel  368  to disengage the feed pinch wheel  368  from the strap  202 , and the feeding sequence is complete.  
         [0076]    During a primary tensioning sequence, the strap  202  is pinched between the primary tension drive wheel  360  and the primary tension pinch wheel  352 . In a first primary tension stage, the primary tension solenoid  370   b  engages the primary tension pinch wheel  352  against the primary tension drive wheel  360  with full pinch force to ensure that the primary tensioning solenoid engages and the strap  202  is pulled free of the track  450 . The pinch force is then reduced during a second primary tension stage by altering the pulse width modulation of the primary tension solenoid  370   b.  As the strap  202  is pulled tightly around the bundle during the primary tensioning sequence, the primary tension pinch wheel  352  stops rotating due to the slippage of the strap on the primary tensioning drive wheel  360 .  
         [0077]    Using pulse width modulation to control the pinch forces exerted by the solenoids  370   a ,  370   b  during feeding and primary tensioning of the strap advantageously allows the operator a larger range of adjustment than is possible with a mechanical, single force adjustment system of the prior art. The two-stage force operation provides improved controllability of the strap  202  movement, including allowing the strap  202  to be quickly accelerated and to be easily stopped as required by the operator.  
         [0078]    [0078]FIG. 13 is an isometric view of the primary tension pinch wheel  352  and the proximity sensor  354  of the feed and tension unit  350  of FIG. 10. The proximity sensor  354  is operatively coupled to the controller  222 . The proximity sensor  354  monitors the primary tension pinch wheel  352  during primary tensioning, such as by monitoring the passing of notches in the wheel  352 , to detect the stall of the primary tension pinch wheel  352 . The proximity sensor  354  transmits signals to the controller  222 . As the signals from the proximity sensor  354  indicate that the primary tension pinch wheel  352  is not turning due to the slippage of the strap  202  on the primary tension drive wheel  360 , the controller  222  starts a secondary tensioning sequence.  
         [0079]    The secondary tensioning sequence begins by pinching the strap between the secondary tension pinch wheel  364  and the secondary tension drive wheel  362 . Then, the secondary tension drive wheel  362  is driven by the drive wheel clutch  356  until the drive wheel clutch  356  starts to slip. After the strap  202  is tensioned to the point that the drive wheel clutch  356  slips, the controller  222  permits a predetermined amount of time to pass to allow the strap to be cut and sealed as described above. The feeding sequence may then be repeated.  
         [0080]    An advantage of the strapping machine  200  is that the pinch wheels  352 ,  364 ,  368  are actuated by the solenoids  370   a ,  370   b ,  370   c.  Using a two-stage pulse width modulated (PWM signal, the solenoids are adjustably controllable by the user during strapping machine  200  operation. During the first stage, the solenoid is given a PWM signal at a constant duty cycle. For the second stage, the solenoid is controlled using a PWM signal with a duty cycle that is user-adjustable via, for example, a potentiometer. Since the average voltage seen by the solenoid is determined by the duty cycle, varying the duty cycle will vary the amount of force the solenoid pulls. Thus, the pinch wheels  352 ,  364 ,  368  may be adjustably controlled during operation of the strapping machine  200 , eliminating the labor-intensive process of mechanical re-adjustment of the pinch wheels  352 ,  364 ,  368  and the associated downtime of the strapping machine.  
         [0081]    [0081]FIG. 14 is an exploded isometric view of an accumulator  300  in accordance with an embodiment of the invention. FIGS. 15 and 16 are front and top elevational views, respectively, of the accumulator  300  of FIG. 14. FIG. 24 is a cross-sectional view of the accumulator  300  of FIG. 15 taken along line  24 - 24 . The accumulator  300  includes a first and second sidewalls  302 ,  304  that substantially enclose a chamber  306  that stores strap for rapid feeding, as well as for temporarily storing of the strap  202  that is drawn back in the tensioning process. The second sidewall  304  is incrementally adjustable by placing retaining pins  308  in a series of holes  310  located in shafts  312  that protrude from the first sidewall  302  to accommodate different sizes of strap  202 . Pin holders  309  are attached to the second sidewall  304  which engage the retaining pins  308  and fix the position of the second sidewall  304  on the shafts  312 .  
         [0082]    The chamber  306  is substantially enclosed by the first sidewall  302  and the adjustable second sidewall  304 . A pair of end walls  320  extend vertically between the first and second sidewalls  302 ,  304 . A top wall  322  extends horizontally along between the first and second sidewalls  302 ,  304 , the top wall  322  having the top entrance  316  where strap  202  is fed into and pulled out of the accumulator unit  300 . An “L” shaped wand  324  extends between the first and second sidewalls  302 ,  304  along the bottom of the chamber  306 . The wand  324  is pivotally attached to the first sidewall  302 .  
         [0083]    In operation, an accumulator motor  330  (FIG. 14) drives an accumulator drive wheel  332  to feed the strap  202  between the accumulator drive wheel  332  and an accumulator pinch wheel  334 . An accumulator feed switch  336  (FIG. 14) is positioned proximate the accumulator drive and pinch wheels  332 ,  334  to detect the presence of the strap  202  and to transmit a control signal to the accumulator motor  330 . As the chamber  306  fills with strap  202 , the wand  324  is pushed downwardly by the weight of the strap  202 , pivoting the wand  324  into contact with an indicator switch  326  (FIG. 15). The indicator switch  326  then transmits a signal to the controller  222  to shut off the accumulator motor  330 , as described more fully below.  
         [0084]    Alternately, during an automatic feeding mode, a strap diverter  314  covers a top entrance  316  of the chamber  306 . When strap  202  is fed into the strapping machine  200  by the accumulator motor  330 , a diverter solenoid  318  (FIG. 14) pulls the strap diverter  314  over the top entrance  316  of the chamber  306 , diverting the strap  202  directly into the feed and tension unit  350  and around the track  450 .  
         [0085]    As best seen in FIG. 24, the accumulator  300  advantageously allows the width w of the chamber  306  and the top entrance  316  to be adjusted easily and quickly to accommodate varying widths of strap  202 . Unlike prior art apparatus that have accumulator sidewalls that are solidly affixed to form a single chamber size, the accumulator  300  of the present invention includes shafts  312  having a plurality of holes  310  placed at increments to match various commonly used strap sizes. Thus, the position of the second sidewall  304  with respect to the first sidewall  302  may be quickly and easily varied by removal of the retaining pins  308 , repositioning the second sidewall  304  at the desired location, and replacement of the retaining pins  308  within the desired holes  310 . The pin holders  309  then engage against the retaining pins  308  and fix the position of the second sidewall  304  on the shafts  312 . This mounting configuration allows the adjustment of the accumulator without having any additional parts, such as spacers between the first and second sidewalls  302 ,  304 .  
         [0086]    [0086]FIG. 17 is an isometric view of a dispenser  250  in accordance with an embodiment of the invention. FIG. 18 is a top elevational view of the dispenser  250  of FIG. 17. The dispenser  250  includes a mounting shaft  252  extending outwardly from the frame  210  between an inner hub  254  and an outer hub  256 . A spring brake  258  is operatively coupled to the mounting shaft  252  and to the frame  210 . When actuated, the brake  258  allows the rotation of the mounting shaft  252 . A mandrel  260  is rotatably mounted on the mounting shaft  252  and supports the inner hub  254  and the outer hub  256 . Strap  202  is routed from the strap coil  204  around a first pulley  262  and a second pulley  264  and over a strap exhaust switch  266 .  
         [0087]    As strap  202  is required in the accumulator  300 , the accumulator motor  330  is energized and the dispenser brake  258  released, allowing the strap coil  204  to spin freely and strap  202  to feed into the chamber  306 . In this embodiment, the brake  258  releases the strap coil  204  to spin only when power is supplied to the brake  258 . When the strap coil  204  is depleted, the strap exhaust switch  266  is no longer actuated which stops the strapping machine  200  until the strap coil  204  is replenished. A braking circuit is used to prevent the accumulator motor  330  from drawing the free end  206  of the strap into the accumulator  300 . The remaining loose tail of strap can then be pulled out of the accumulator  300  before a new strap coil is installed. The empty strap coil  204  is replaced by removing an outer hub securing nut  268  and the outer hub  256 , and then removing the strap coil core (not shown) from the mandrel  260 . Next, a fresh strap coil  204  is placed on the mandrel  260  with the strap  202  wound in a clockwise direction. Finally, the outer hub  256  and the outer hub securing nut  268  are replaced and the nut tightened securely.  
         [0088]    To begin feeding the strap  202 , the free end  206  is removed from the strap coil  204 , threaded around the first pulley  262 , through the strap exhaust switch  266 , around the second pulley  264  and between the accumulator drive wheel  332  and the accumulator pinch wheel  334 . As the strap  202  is placed between the accumulator wheels  332 ,  334 , the accumulator feed switch  336  is actuated causing the accumulator feed solenoid to actuate, thus feeding the strap over the accumulator and into the track.  
         [0089]    When enough force is applied to the wand  324  by the weight of the strap  202  accumulating in the chamber  306 , the wand  324  moves downwardly to actuate the indicator switch  326 , indicating that the accumulator unit  300  is full. In response to this signal, the controller  222  de-energizes the accumulator motor  328  and the dispenser brake  330  to halt the accumulator filling sequence. A time delay occurs between when the dispenser brake  330  is de-energized and when the accumulator motor  328  is de-energized to take up any slack in the strap coil  204 .  
         [0090]    [0090]FIG. 19 is an isometric view of a track  450  in accordance with an embodiment of the invention. FIG. 20 is a partial sectional view of a straight section  452  of the track  450  of FIG. 19 taken along line  20 - 20 . FIG. 21 is an isometric view of a corner section  454  of the track  450  of FIG. 19. FIG. 25 is a partially exploded isometric view of a straight section  452  of the track  450  of FIG. 19. During feeding, after the strap  202  exits from the sealing head  400 , it is pushed completely around the track  450  and then back into the sealing head  400 . The track  450  directs the strap  202  around the strapping station  208 .  
         [0091]    The track  450  includes a plurality of straight sections  452  and a plurality of corner sections  454 . As shown in FIGS. 19 and 20, each straight section  454  includes a guide support  455  at each end of the straight section  454 . A straight slotted cover  456  and a straight backplate  457  are coupled to the straight supports  455  to form a portion of a guide passage  462  that retains the strap  202  during feeding. Each straight slotted cover  456  includes a straight inner surface  472  on the inner circumference of the guide passage  462 , and a straight outer surface  474  on the outer circumference of the guide passage  462 .  
         [0092]    As best seen in FIGS. 20 and 21, the straight supports  455  and the corner supports  454  are keyed to fit on a raised “T” section  459  of an outer arch  458 . The outer arch  458  forms a frame for the other components of the track  450 . As the strap  200  is tensioned around the bundle, the straight and corner slotted covers  456 ,  463  open, allowing the strap  202  to pull clear of the guide passage  462 . FIG. 20 illustrates the open position of the slotted cover  456  in phantom to assist in a more complete understanding of the invention. As the strap  202  clears the guide passage  462 , each of the straight and corner slotted covers  456 ,  463  is closed by the springs  461  and becomes ready for the strap  202  to be fed again. The V-shape of the guide passage  462  in the corner section  454  helps assure that the strap removal begins in the corner sections  454  rather than in the straight sections  452  of the track  450 . When the strap  202  (see FIG. 20) is removed from the track  450 , the V-shape of the guide passage  462  in the corner section  454  causes the track cover  463  to begin opening in the corner section  454 . As the strap  202  begins to separate from the track  450  in the corner sections  454 , the V-shaped guide passage  462  imparts a slight twist to the strap to start opening the straight slotted  456  (see FIG. 20) in the straight sections  452  of the track  450 .  
         [0093]    As shown in FIG. 21, each corner section  454  includes a corner slotted cover  463  and a corner backplate  465  coupled to a plurality of guide supports  455 . The corner slotted cover  463  and corner backplate  465  form a portion of the guide passage  462  therebetween. Each corner slotted cover  453  includes a corner inner surface  476  on the inner circumference of the guide passage  462 , and a corner outer surface  478  on the outer circumference of the guide passage  462 . In this embodiment, the corner slotted cover  463  and the corner backplate  465  are coupled to the guide supports  455  using a four-bar linkage assembly  469  that permits the corner slotted cover  463  to pivotably open to release the strap  202  from the guide passage  462 . Although alternate embodiments for pivotably mounting the corner slotted covers  463  may be conceived, in the embodiment shown in FIG. 21, the inner bars  468  (one shown) of the four-bar linkage assembly  469  have an enlarged opening  470  to permit the corner slotted cover  463  to pivotably open about an axis of rotation that is oriented approximately 45 degrees from the horizontal.  
         [0094]    As best shown in FIG. 25, the straight slotted cover  456  and the straight backplate  457  are spring-loaded by a plurality of springs  461 . The straight slotted covers  456  and the straight backplates  457  are hingeably engaged on pivot pins  467  that are approximately parallel to the path of the strap  202  in the guide passage  462 . The pivot pins  467  are inserted through corresponding apertures  467   a  and  467   b  in the straight slotted cover  456  and straight backplate  457 , respectively, and rotate about an axis defined by the longitudinal axis of the pivot pins  467 . The pivot pins  467  are retained in position by snap-on retainers or any other convenient retainer element.  
         [0095]    The springs  461  are inserted through a corresponding aperture  461  a in the straight backplate  457  and are coupled to the straight slotted cover  456  by a spring retaining pin  466 . In an exemplary embodiment, the spring retaining pins  466  are identical to the pivot pins  467  and are retained within corresponding apertures  466   a  in the straight slotted cover  456  by the snap-on retainers. The springs  461  are thus coupled on a proximal end to the straight slotted cover  456  by the spring retaining pins  466  and are retained within the aperture  461   a  by an enlarged distal end, sometimes referred to as a circle cotter. This arrangement allows the straight slotted cover  456  to pivot open and release the strap  200  (see FIG. 20) and automatically close due to the spring force exerted on the straight slotted cover by the springs  461 . Although various sizes of straight slotted covers  456  may be employed, in the embodiment shown in FIGS. 20 and 25, the guide passage  462  is sized to receive strap sizes varying from approximately 5 mm to approximately 15 mm.  
         [0096]    One advantage of the track  450  of the present invention is the modular construction of the straight and corner sections  452 ,  454  which allows the track  450  to be incrementally extended in length and height. Because the straight and corner sections  452 ,  454  are keyed to fit a raised section  459  of the outer arch  458 , these components form an easily assembled slide-together arch system, enabling the size of the track  450  to be easily modified for various combinations of length and height. Thus, the size of the strapping station  208  may be quickly and efficiently modified for a variety of bundle sizes.  
         [0097]    Another advantage of the track  450  is that by pivoting the straight slotted covers  456  parallel to the strap path, and by pivoting the corner slotted covers  463  on the four-bar linkage assemblies  469 , each individual straight and corner section  452 ,  454  may open using only the forces exerted by the strap  202  as it is tightened during tensioning. During the tension cycle, the strap  202  is drawn against the straight inner surfaces  472  and the corner inner surfaces  476 , forcing the straight slotted covers  456  and corner slotted covers  463  to pivotably open in the manner described above. Thus, the track  450  does not require complex hydraulic or pneumatic actuation systems to open the track to release the strap during tensioning. This reduces costs and simplifies maintenance of the track and strapping machine.  
         [0098]    A further advantage of the track  450  is that, in the embodiment shown in FIGS. 19 through 22, the forces exerted by the strap on the straight slotted covers  456  and corner slotted covers  463  during the feed cycle assist in keeping the track closed during feeding. During the feed cycle, the strap  202  pushes outwardly on the straight outer surfaces  474  and the corner outer surfaces  478  to create a moment (i.e., a force vector) that forces the straight slotted covers  456  and the corner slotted covers  463  toward the closed position. This aspect of the invention reduces misfeeds of the strap, and eliminates the need for complex hydraulic or pneumatic actuation systems to close the track and keep it closed during the feed cycle.  
         [0099]    The detailed descriptions of the above embodiments are not exhaustive descriptions of all embodiments contemplated by the inventors to be within the scope of the invention. Indeed, persons skilled in the art will recognize that certain elements of the above-described embodiments may variously be combined or eliminated to create further embodiments, and such further embodiments fall within the scope and teachings of the invention. It will also be apparent to those of ordinary skill in the art that the above-described embodiments may be combined in whole or in part with prior art methods to create additional embodiments within the scope and teachings of the invention.  
         [0100]    Thus, although specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. The teachings provided herein of the invention can be applied to other methods and apparatus for strapping bundles of objects, and not just to the methods and apparatus for strapping bundles of objects described above and shown in the figures. In general, in the following claims, the terms used should not be construed to limit the invention to the specific embodiments disclosed in the specification. Accordingly, the invention is not limited by the foregoing disclosure, but instead its scope is to be determined by the following claims.