Abstract:
A high-speed stacker preferably includes dual stacking arms configured to operate complementary to one another. Most preferably, an electronic control system is provided to enable precise control over the speed and positioning of the stacker arms in both horizontal and vertical orientations. Linear motion devices (such as hydraulic cylinders, screw drive linear actuators, or other devices) can be used to position the arms horizontally and vertically in response to instructions from the electronic control system. In operation, the electronic control system preferably controls the speed and ramping of the stacker arms to repeatedly move courses of material from a feed system to a stacking area at a rapid rate with little maintenance. The high-speed stacker can also be configured to operate fewer than all of the stacker arms to facilitate faster stacking of smaller courses of material.

Description:
This application is a continuation of and claims priority from U.S. patent application Ser. No. 11/340,323 (now U.S. Pat. No. 7,201,554), filed Jan. 25, 2006; U.S. patent application Ser. No. 10/801,524 (now U.S. Pat. No. 6,991,423), filed Mar. 12, 2004; and U.S. Provisional Application Ser. No. 60/520,550, filed Nov. 14, 2003, the contents of each of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to methods and machines for stacking material, and more particularly, to methods and machines for stacking elongated planar members, such as sheets of lumber, plywood, or other material, into packages to facilitate subsequent shipping and/or handling. 
     2. Description of Related Art 
     The lumber industry, in particular, uses stacking machines (or stackers) to collect sheets (or pieces) of lumber, plywood, and other wood products into packages (or bundles) to facilitate bulk handling and shipping. Lumber is generally produced in lengths between 4′ to 28′, with thicknesses ranging from 1″ to 12″, and having widths that range between 2″ to 24″. After production, the lumber is generally gathered into layers (or courses) and then supplied to a stacker where it is formulated into packages that are typically approximately 16 to 30+ layers high and range from about 42″ to 96″ in width. 
     The stacking process requires robust machinery. It is also desirable to have a stacker that is capable of efficiently stacking the lumber at a very high speed. It is further desirable to have stacking machinery that is easy to maintain and that requires very little supervision or manual tuning during the stacking process. The longer the machines are kept up and running between down times and the less manual intervention that is required, the better the process efficiency. Greater efficiency results in increased production and enhanced profitability. 
     The industry is therefore in need of faster and more reliable methods and systems for stacking the materials that are to be bundled together. In particular, in sawmills and planer mills that manufacture lumber and other wood products, the speed of equipment that feeds conventional stackers has been increased, without a corresponding increase in the stacking speed. This results in bottlenecks and inefficiencies at the stackers. 
     Conventional stackers are generally unable to meet the high demands placed on them by current lumber feed systems. Typically, a package of lumber is formed in the stacker using a set of forks (or stacker arms) to raise a course of lumber from stacker chains. The arms are then extended to an area containing the accumulated courses. Once the course of lumber has been set on top of the stack, the stacker arms retreat to pick up the next course. This process is repeated until the desired number of courses have been set and a full package has been created. The package can then be bundled and shipped, or subjected to further processing. 
     U.S. Pat. Nos. 4,290,723 and 5,613,827, and Published U.S. Patent Application No. 20030031550, disclose various machinery and methods for stacking courses of lumber into packages. Unfortunately, none of these, or other known conventional stacker designs, are able to stack lumber at the high stacking rates required to keep up with the increased speed of present feed systems. Conventional systems, for example, are only capable of a maximum of about 15 cycles per minute for a single carriage stacker, and around 24 cycles per minute for a dual carriage stacker—not taking into account down time between loads being stacked and general inefficiencies of the infeed and outfeed systems of the stacker. 
     In addition, conventional stackers have not provided the ability to stack shorter courses of lumber at a faster rate. Typical infeed systems are often able to supply shorter courses of lumber at higher speeds, but, with no way to stack them faster, this higher feed capacity is wasted on conventional stackers. There are generally as many short lengths in a formulated layer of lumber as there would be longer lengths. This means that you have to stack 10′ long, 12″ wide pieces of lumber, produced at 120 pieces per minute, with 4 pieces per layer, at 30 layers per minute (120 pieces/4 pieces per layer) in order to keep up with the infeed system. This is in contrast to 20′ long, 12″ wide pieces that are produced at 60 pieces per minute, which would only need to be stacked at 15 layers per minute (60 pieces/4 pieces per layer). Mills therefore need to be able to stack about 30 courses per minute or more in order to keep up with the infeed of smaller lumber courses, and should also have very few timing and maintenance problems. 
     SUMMARY OF THE INVENTION 
     One objective of the present invention is to provide a stacker that is competitive in cost to build but that operates at higher speeds than conventional single or dual carriage stacker designs. According to one embodiment, the increased speed is preferably due to a simplified, positive actuating mechanism utilizing a higher degree of electronic controls. Simplified mechanical and electrical controls can result in a much faster stacking mechanism with little operation down time, lower maintenance, and less manual supervision and tuning requirements. 
     Dual stacking arms help stack faster, but without proper control, dual stacking arms cannot enable a fast, reliable, and efficient stacker. According to various principles of this invention, electronic control of the velocity and ramping of dual stacking arms is preferably provided to simultaneously and precisely control forward and rearward, as well as vertical, movement of the stacking arms. Precise electronic control enables the stacker to meet high production requirements with little supervision and maintenance. According to a preferred embodiment, a stacker can be made capable of cycling at approximately 35 to 40 layers per minute or more, depending on the size of the material pieces being handled. 
     In some traditional lumber stackers, the courses of lumber are picked up and stacked one course at a time. According to a preferred embodiment of this invention, however, the stacking machine picks up a subsequent course of lumber while the previous course is being set. To accomplish this, a high-speed stacking machine preferably includes two sets of forks (or arms) that operate complementary to one another. This significantly speeds up the rate at which the lumber is able to be stacked. 
     In addition to having two sets of arms working complementary to one another, the design of one preferred embodiment utilizes a rack and pinion system, driven by a linear positioning mechanism (such as a hydraulic cylinder, drive screw linear actuator, or other linear positioning device) to horizontally move and position the stacker arms. A vertical positioning device is also preferably included to raise and lower the stacker arms to the proper vertical position at the proper speed. An electronic control system can be used to precisely monitor and control the speed and ramping of the positioning systems. Precise control of the dual stacker arms, enabled through principles of this invention, allows the stacking machine to create packages of lumber at a faster rate than previous methods and machines with fewer timing and maintenance problems. 
     According to yet another aspect of this invention, in addition to providing the ability to stack lumber of all sizes and mixes at higher speeds, a high-speed stacker is also preferably configured to provide the ability to disengage unnecessary stacking arms. Disengaging extra stacking arms can reduce the mass that must be moved by the positioners, and thereby permit an increase in the speed of the remaining stacking arms. This is particularly advantageous in systems where sawmill and planer mill stacker infeed production equipment are able to produce a higher piece count of shorter courses. A clutching mechanism (or any other mechanical or electrical engagement/disengagement system) could be used to disengage unneeded stacking arms from the stacking process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features, and advantages of the present invention will become more readily apparent from the following detailed description of various preferred embodiments, proceeding with reference to the accompanying drawings, in which: 
         FIG. 1  is a somewhat schematic top plan view of a stacking system having a stacker with plurality of stacker arm pairs configured according to a preferred embodiment of the present invention; 
         FIG. 2  is a somewhat schematic side elevation view of the stacking system of  FIG. 1 ; 
         FIG. 3  is a somewhat schematic perspective view of a pair of stacking arms as used in the stacker shown in  FIG. 1 ; 
         FIGS. 4A-4B  are somewhat schematic top plan and side elevation views of a stacker, illustrating use of a linear positioning device to control horizontal movement of stacker arms according to another aspect of the present invention; 
         FIGS. 5A-5C  are somewhat schematic side elevation views of a stacker, illustrating use of a linear positioning device to control vertical movement of stacker arms according to yet another aspect of the present invention; 
         FIGS. 6A-6H  are somewhat schematic side elevation views illustrating the operation of a stacker configured according to the embodiment shown in  FIG. 1 ; 
         FIG. 7  is schematic block diagram illustrating an electronic control system for controlling a stacking system, according to a still further aspect of the present invention; and 
         FIG. 8  is a schematic top plan view of a stacker illustrating a disengagement mechanism for disengaging one or more of the stacker arms from the system according to a still further aspect of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Various aspects and embodiments of the present invention will now be described in greater detail with reference to the accompany drawings. Beginning with  FIGS. 1-3 , a high-speed stacking system  100  according to a preferred embodiment includes a stacker infeed section  10 , a dual carriage stacker  20 , a package lift  40 , and an electronic control system  50 . 
     The stacker infeed section  10  preferably comprises a material conveyor  12 , an even ender (not shown), and a course make-up section  16 . The even ender aligns ends of the material pieces  60  with one another. The material conveyor  12  carries the material  60  (such as lumber, plywood, or other material) from a material supply to the course make-up section  16  (or pre-staging area). The material  60  can, for example, be formulated into courses  60 A, using the course dividing arms  24  and course stop arms  25 , in the course make-up section  16  of the stacker infeed section  10 . Alternatively, however, the courses  60 A can be formulated in a course accumulation area arranged above stacker chains  26  in the stacker  20 . 
     The dual-carriage stacker  20  preferably includes a structural steel frame  22 , course accumulator stops (or dividing arms)  24 , course stop arms (or front dropping stop)  25 , stacker chains  26 , stacker arms  27 ,  28 , forward and rearward hard-coupled mechanics  30  (e.g., a rack and pinion system), and lift arms  33 ,  34 . A horizontal, linear positioner  36  (see  FIGS. 4A and 4B ) is preferably provided in communication with the stacker arms  27 ,  28  to control their horizontal position. A separate vertical, linear positioner  38  (see  FIGS. 5A-5C ) is preferably provided in communication with the lift arms  33 ,  34  to control vertical movement of the stacker arms  27 ,  28 . Course rake-off stops  39  are also preferably included to scrape the courses  60 A off the arms  27 ,  28  onto the package lift  40 . 
     Referring now to  FIGS. 1 through 4B , the stacker arms  27 ,  28  are preferably arranged in opposing sets configured to operate complementary to each other. In this embodiment, for example, the stacker arms  27 ,  28  can be mounted on a rack and pinion system  30 . The rack and pinion system  30  preferably includes racks  270 ,  280  communicating with one or more pinion gears  32 . More specifically, a first set of stacker arms  27  are preferably arranged on racks  270  communicating with a top portion of the pinion gear  32 . A second set of stacker arms  28  are preferably arranged on racks  280  communicating with a bottom portion of a pinion gear  32 . The pinion gear  32  can include separate gear members  272 ,  282  formed or mounted on a common shaft  275  extending transversely through the stacker  20 . 
     Because of their hard-coupled relationship through the rack and pinion system  30 , movement of a first set of stacker arms  27  preferably creates a complementary movement in an opposite set of stacker arms  28 . The horizontal positioner  36  preferably controls movement of the stacker arms  27 ,  28  and can, for example, be coupled to one of the arms  27  or  28  or directly to the rack and pinion system  30  in the stacker  20 . 
     Accordingly, in this embodiment, by virtue of their mechanical relationship through the rack and pinion system  30 , as one of the arms  27  is driven to its unloading position, the other arm  28  is driven to a loading position. Therefore, only one horizontal positioner  36  is required to control movement of all of the stacker arm pairs  27 ,  28  in the stacker  20 . 
     Of course, numerous other embodiments incorporating the principles of this invention are also possible. In one alternative embodiment, for instance, the forward arms could all be connected to each other, with the rearward arms separately connected to each other without a hard-coupled relationship between the forward and rearward sets of arms. Simultaneous movement of the forward arms could be provided using a first horizontal positioner with simultaneous movement of the rearward arms being provided by a separate, second horizontal positioner. Separate connection between the forward set of arms and the rearward set of arms could, for instance, be provided using separate rack and pinion systems for the forward and rearward sets of arms. Alternatively, all of the arms in a set could be mechanically connected together in another mechanical relationship (such as by a rigid or other mechanical connection). 
     In yet another embodiment, separate horizontal positioners could be used to control movement of each stacker arm pair independently. The relationship between the arms in each pair could be controlled via a separate hard-coupled mechanical relationship, such as a rack and pinion system or other mechanical system. In still another embodiment, each stacker arm could be controlled using its own horizontal positioner. Although significantly more expensive, this approach would offer flexibility in terms of the timing and control of the arms and would provide an electronic disengagement mechanism allowing unneeded stacker arms to be disengaged from the system by simply not operating the positioners for the extra stacker arms. 
     Referring now to  FIGS. 5A-5C , the vertical position of all of the stacker arms  27 ,  28  is preferably controlled using a single vertical positioner  38 . Lift arms  33 ,  34  are preferably arranged in pairs to communicate with a pair of stacker arms  27 ,  28 . In this embodiment, each of the pairs of lift arms  33 ,  34  are preferably mounted on a common shaft  350 . A shaft arm  352  is preferably coupled to the shaft  350  to cause rotational movement of the shaft in response to operation of the vertical positioner  38 . For example, as shown in  FIG. 5A , retraction of the vertical positioner  38  can be configured to raise a stacking arm (not shown) using one of the lift arms  33 .  FIG. 5B  shows that arrangement of the vertical positioner  38  in a middle position allows both arms  27 ,  28  to be lowered, and  FIG. 5C  illustrates the vertical positioner  38  in an extended position, which raises a different stacking arm (not shown) using another one of the lift arms  34 . By arranging all of the lift arms  33 ,  34  for the stacking arm pairs  27 ,  28  on a common shaft  350 , a single vertical positioner  38  can be used to operate all of the lift arms  33 ,  34  in the stacker. 
     Of course, numerous other embodiments are also possible. For instance, all of the lift arms for forward stacker arms could be controlled by one vertical positioner, with the lift arms for rearward stacker arms controlled by a separate vertical positioner. Alternatively, a separate vertical positioner for each stacker arm pair could be used. Another embodiment could use a separate vertical positioner for each lift arm. Other embodiments could use a single lift arm to raise both stacker arms in a stacker arm pair. It should also be noted that the lift arms can be configured in any mechanical relationship with an electrically-controlled positioner that would cause them to raise the stacker arms at the appropriate time in the stacking sequence. Accordingly, the invention is not limited to the specific embodiments disclosed herein. 
     In operation, the vertical and horizontal positioners  38 ,  36  are preferably configured to operate in response to instructions from the electronic control system  50  (see  FIGS. 1 and 7 ). Using electrically-controlled positioners  36 ,  38 , the stacker arms  27 ,  28  are able to be raised and lowered to load and unload courses in a precise sequence, timed with the forward and rearward movement of the arms  27 ,  28 . 
     The vertical and horizontal positioners  36 ,  38  are preferably electrically-controlled hydraulic cylinders, controlled through an electronic controller, such as a Temposonics feedback controller. The hydraulic cylinders also preferably include position monitoring devices that are able to precisely determine and report the position of the hydraulic shafts (or pistons) to the electronic control system  50 . The speed and ramping of the hydraulic cylinders are also preferably controllable through the electronic control system  50 . 
     Of course, other types and/or configurations of positioners are also contemplated within the scope of this invention. For instance, screw drive linear actuators, servo type positioning motor drives, electric motor drives with variable frequency internal or external positioning capability, or other positioning devices could be used in place of the hydraulic cylinders. Combinations of any or all of these types (or other types) of positioners could be used to supply the forward and rearward arm movement as well as the raising and lowering of the arms. 
     Regardless of the type of positioning device used, however, it is desirable to know the state of the positioner and to maintain precise electronic control over both positioning and ramping of the device. The horizontal positioner(s), in particular, are preferably configured to include an electronic positioning strip or other position detection device (arranged within or independently of the positioner) to enable determination of the horizontal positions of the stacker arms. The vertical positioners may also include position detection devices. Precise electronic control over positioning devices helps enable the stacker to operate at high-speeds without tossing material or otherwise jeopardizing the integrity of the stacking process. 
       FIGS. 6A-6H  illustrate the operation of the embodiment of the stacking system  100  shown in  FIG. 1 . It is desirable to have a stacking machine that is able to create packages of lumber or other material at a faster rate than previously available. Typically, a package of lumber is created by using a set of stacker arms to raise a course of lumber from the stacker chains and move it to a stacking area. The arms are extended out to the area containing the accumulated courses. Once the course of lumber has been set, the stacker arms retreat and pick up the next course. This process is repeated until the desired number of courses have been set and a full package has been created. 
     The dual-arm stacker  20  configuration according to the embodiment illustrated in  FIGS. 1-3  contains two sets of forks  27 ,  28  that operate complementary to one another. By picking up the next course of lumber while the previous course is being set, the rate at which the lumber is stacked can be significantly increased. To provide complementary-operating sets of arms, this specific embodiment preferably utilizes a hard-coupled mechanical drive system (such as a rack and pinion system) to arrange the stacker arms  27 ,  28  in their back and forth horizontal positions. To achieve the necessary vertical motion of the stacking arms  27 ,  28 , the system preferably utilizes vertical positioning arms  33 ,  34  (lift arms) that raise and lower the stacking arms  27 ,  28  to the proper vertical position. 
     A method of stacking material according to a preferred embodiment of the present invention will now be described in more detail with reference to  FIGS. 6A-6H . Referring first to  FIG. 6A , as the pieces of material  60  (such as lumber) are transferred down the infeed transfer  10  to the stacker  20 , a set of dividing arms  24  are preferably activated to start the formulation of, and the pre-staging of the courses  60 A of material. When the dividing arms  24  are lowered, the sheets of lumber (or other material)  60  travel to a course dividing section which may be located in a separate pre-staging area or at an infeed area of the stacker chains  26 . Course stop arms  25 , in conjunction with the dividing arms  24 , preferably limit the number of pieces of lumber  60  to the appropriate count for the course  60 A. 
     Referring now to  FIG. 6B , once the course of lumber  60 A is created, a rearward set of stacker arms  27  is raised, thereby picking up the course of lumber  60 A. More particularly, lift arms  33  raise the fork arms  27 , which in turn raise the course of lumber  60 A from the chains  26 . 
     As illustrated in  FIG. 6C , after the course  60 A has been lifted from the chains  26 , the rear forks  27  move forward with the course of lumber  60 A, while the front forks  28  are moved in the opposite direction. The front forks  28  ultimately end up in the position where the rearward forks  27  had been previously (see  FIG. 6D ). This opposite, complementary motion can be created by the relationship of the stacker arms  27 ,  28  to the location of the pinion gear  32 . As the pinion gear  32  rotates clockwise, the stacker arms  27  attached to the top of the pinion gear  32  move forward while the stacker arms  28  attached to the bottom portion of the pinion gear  32  move backwards. The opposite motion is created by rotating the pinion gear counterclockwise. 
     A hard-coupled mechanical drive system is preferably used to provide simultaneous complementary forward and rearward movement of the stacking arms. As discussed above, the mechanical drive system can comprise a gear rack and pinion gear activating mechanism for operating all of the arms simultaneously. The drive system could also comprise, however, dual sets of pinion gears on individual rack assemblies, for driving the forward and rearward arms independently. Other types of mechanical drive systems are also contemplated within the scope of this invention. However, those that provide simultaneous forward and rearward motion through a fixed mechanical relationship that does not change substantially with use or wear of the components are most preferred. 
     Referring now to  FIGS. 6E and 6F , once the course of lumber  60 A has reached the location (e.g., the package lift  40 ) where the lumber package is being built, the lift arms  33  lower, thereby lowering the rear stacker arms  27  and setting the course of lumber  60 A in place on the package accumulation support  42  in the package lift  40 . When the lift arms  33  for the rearward stacker arms  27  are lowered, the lift arms  34  for the front stacker arms  28  are lifted, thereby raising the next course  60 B. 
     As shown in  FIGS. 6G and 6H , the forward stacker arms  28  are then brought forward as the rearward stacker arms  27  are returned to their starting position. As the rearward arms  27  retreat from the package lift  40 , the course  60 A is scraped off the arms  27  by the course rake-off stop  39 . When the forward arms  28  reach the package lift  40 , the forward stacker arms  28  are then lowered using the lift arms  34  to deposit their course  60 B in the package lift  40 . The process is repeated until the desired package size has been achieved. 
     The package lift (or hoist)  40  preferably automatically adjusts the height of a package accumulation support  42  based on the number of layers accumulated thereon. In a preferred embodiment, for example, the package lift  40  can utilize positioners (not shown) to precisely place the accumulation support  42  in a proper receiving position with respect to the stacking arms  27 ,  28 . The accumulation support  42  is thereby preferably configured to index the exact amount to accommodate successive layers. This system can reduce lost time and eliminate the need for other sensing devices that might otherwise be required. 
     Referring to  FIG. 7 , an electronic control system  50  for the high-speed stacker  20  preferably includes a PLC or PC type of positioning and sequencing software  52  loaded into a PLC or PC type of controller hardware  54 , and an operators console  56 . The electronic control system  50  is preferably used to control the precise position and sequencing of the stacker components to permit operation of the stacker at high-speed. 
     Referring now to  FIGS. 1 through 7 , in a preferred embodiment, a controller  54  for a dual-arm, high-speed stacking system  20  provides coordinated motion control of seven axes. These seven axes can include, for instance, a primary stacking arm (or reference) axis (which controls the horizontal positioner  36  to provide forward and backward movement of the stacking arms  27 ,  28 ), a stacking arm lift axis (controlling the vertical positioner  38  to lift and lower the stacking arms  27 ,  28 ), an infeed speed axis (providing analog speed control of the infeed chain  12 ), a course set-up axis (providing coordinated control of timing solenoids for the course dividing arms  24  and stop arms  25 ), a stacker chain speed axis (providing analog control of the speed of the stacker chains  26 ), a sticker or tie strip placement axis (providing control of a sticker placement positional timing solenoid (not shown)); and a hoist axis (controlling positioning of the package lift  40 ). Control of these seven axes preferably provides both velocity and positional coordination of each of the system components with the primary stacking arm axis (or reference axis). 
     As discussed earlier, the location and movement of the stacking arms  27 ,  28  (e.g., the primary axis) are preferably controlled through a horizontal positioner  36  (such as a hydraulic cylinder). The horizontal positioner  36  is, in turn, controlled using a controller  54 A (such as a Temposonics feedback controller) in the electronic control system  50 . The primary axis is therefore used to transport a course of lumber  60 A from the stacker chains  26  to a package lift (or hoist)  40  that is used to accumulate multiple courses of lumber to form a complete package. Linear motion of this axis preferably includes a controlled acceleration/deceleration ramp at each end of the stroke, as well as constant velocity over a central portion of the stroke. From this primary motion, positions of the stacking arms  27 ,  28  can be continuously monitored to provide timing and coordination to the other axes. 
     More particularly, as the stacking arms  27 ,  28  stroke, for instance, the vertical positioner  38  (preferably a second hydraulic cylinder) can be controlled using a separate controller  54 B (e.g., a second Temposonics feedback controller) in the electronic control system  50 . The vertical positioner  38  positions the lift arms  33 ,  34  to provide lift control of the stacking arms  27 ,  28  based upon the position/motion of the arms  27 ,  28 . The amount of lift can also be coordinated to maintain the arms at a fixed position above the package in the hoist  40  and, using the control system  50 , can be adjusted continuously over the stroke of the arms  27 ,  28 . In a retracted position, each stacking arm  27 ,  28  is preferably raised to lift a course  60 A from the stacker chains  26 . At full extension, each of the stacking arms  27 ,  28  is preferably lowered onto the top of the package in the hoist  40 . 
     Concurrent with the cycling motion of the stacking arms  27 ,  28 , lumber  60  is being separated into courses  60 A on the infeed chains  12  and the stacker chains  26 . Timed with the position of the stacking arms  27 ,  28 , a complete course of lumber  60 A is sensed from the course separator arm  24  and the front dropping stop  25 . The stacker arms  27 ,  28  are then raised to separate the lumber to build one course of the package. Both the incoming chains and the stacker chains  26  are preferably controlled using separate Variable Frequency Drives (VFDs)  54 C,  54 E to match their speed to the speed of the stacking arms  27 ,  28 . 
     At the correct position in the stroke of the stacking arms  27 ,  28 , the lumber  60  being held behind the course dividing arms  24  is released by lowering the arms  24 . The lumber  60  travels forward to the course stop arm (front dropping stop)  25  at the end of the stacker chains  26 . Again at the appropriate stroke, the course stop arm  25  is lowered to allow a course  60 A to enter the stacker chains  26 . Once the desired number of pieces have passed the stop arm  25 , the stop arm  25  is raised to accumulate another course. The timing for these actions is therefore preferably based upon the position and speed of the stacking arms  27 ,  28 . Photocell detectors can be used to monitor when a complete course has been formulated behind the front dropping stop  25 . Actuation of the course separator arm  24  and stop  25  can be controlled using a positional timing solenoid controller  54 D. As the stacking arms  27 ,  28  rise to pick up the course  60 A at the end of the stacker chains  26 , the stacker chains  26  may be stopped momentarily to insure proper transfer of the course  60 A onto the stacking arms  27  or  28 . 
     When the stacking arms  27 ,  28  carrying a course of lumber (or other material)  60 A reach their full extension, it may be desirable to initiate placement of a set of stickers or tie strips on the top of, or within, the course for package drying or stability. The number of these stickers or tie strips in the package can be based upon package type and size and can be programmed within the controller. The point of initiation of this cycle can also be controlled relative to the cycle position of the stacking arms  27 ,  28  using the control system  50 . A positional timing solenoid controller  54 F can be used to control sticker placement. 
     After the stacking arms  27 ,  28  retract and the course is on top of the package in the hoist  40 , the hoist  40  is preferably lowered until a photocell detector (not shown) senses the top of the package. In a preferred embodiment, the next course is kept from being lowered until the package clears the photocell detector. A positional timing solenoid controller  54 G can be used to control the stacker hoist  40 . 
     These seven axes controllers  54 A- 54 G can provide an overall control package for monitoring and controlling the operation of a high-speed stacker system. Control and monitoring of the motors, hydraulic pumps, transfer chains, and roll cases helps to provide complete control over the timing and sequencing of the stacking system. Master control over each of the various system component controllers  54 A- 54 G can, for instance, be provided by a Programmable Logic Controller (PLC)  54  combined with a Delta Computer Systems, Inc. RMC100 Series Servo Motion Controller  58  for the Temposonic controllers  54 A.  54 B. A local Touchscreen MMI Terminal  59  can also be provided for monitoring and setup of the system. 
       FIG. 8  illustrates an embodiment of yet another aspect of the invention, in which a disengagement mechanism  80  can be provided to the stacking system  20  to allow disengagement of one or more of the stacker arms from the system when desired, such as when stacking shorter courses. Referring to  FIG. 8 , a high speed stacker  20  can include a plurality of stacker arms  27 ,  28  operated through a horizontal positioner (not shown). One or more of the stacker arms  27 ,  28  can be removed from operation by disengaging them from the remaining stacking arms. In this particular embodiment, this is preferably accomplished by using a clutch  80  to disengage the extra stacker arms  27 A,  28 A and  27 B,  28 B from those that are needed to stack the course. 
     In this embodiment, for example, the clutching mechanism  80  can be arranged to disengage the stacker arms  27 A,  28 A for the fourteen foot and longer board lengths and the stacker arms  27 B,  28 B for the eighteen foot and longer board lengths. By disengaging stacker arms for board lengths greater than fourteen feet, shorter board lengths can be stacked more rapidly. Disengaging unnecessary stacker arms reduces the overall weight of the components required to be moved and therefore reduces the force required to be applied by the controlling cylinder(s) to overcome inertia and move the course. This also reduces wear and tear on the components. 
     Of course, other methods of engagement and disengagement are also within the contemplation of this invention. For example, mechanically mating shafts could be engaged and disengaged manually or using a solenoid or other electric device. Disengagement of extra stacking arms can also be accomplished in various other embodiments through other means. For example, when multiple electrically-controlled positioners are used to control separate stacking arms or stacking arm pairs, the positioners connected to the unneeded stacking arms could be caused to remain idle while only those needed for the stacking operation are used. As can be seen from the foregoing description, any mechanical or electrical method for disengaging extra stacking arms can be used to reduce system loads when stacking shorter materials. This, in turn, can provide increased efficiency and speed with respect to the remaining stacking arms. 
     Having described and illustrated the principles of the invention in a preferred embodiment thereof, it should be apparent that the invention can be modified in arrangement and detail without departing from such principles. Among other things, the terms “forward” and “rearward” have been used throughout the foregoing descriptions for reference purposes only and provide no limitation with respect to the structure of the various embodiments described herein. For example, the stacker arms  27 ,  28  are each capable of being in either a forward or rearward position. We claim all modifications and variations coming within the spirit and scope of the following claims.