Abstract:
Veneer sheet lay-up apparatus with rearward and forward tablets, about which rearward and forward conveyors are respectively entrained. A controller longitudinally reciprocates the tablets toward and away from one another, between open and closed configurations. A veneer sheet delivered to the rearward tablet is forwardly conveyed to the forward tablet, which forwardly conveys the sheet&#39;s forward portion. The controller detects the sheet&#39;s arrival at the desired lay-up position and thereupon simultaneously rearwardly retracts the rearward tablet and forwardly advances the forward tablet from beneath the sheet, creating a lay-up space into which the veneer sheet is gravity-displaced.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/350,075 filed 23 Jan. 2002. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates to apparatus and methods for manipulation of veneer sheets. Particular embodiments of the invention have application to the manufacture of plywood and laminated veneer lumber (LVL).  
         BACKGROUND  
         [0003]    Veneer sheets are used to form plywood and LVL.  
           [0004]    Producing plywood or LVL from individual veneer sheets typically involves layering a plurality of glue-covered veneer sheets and then processing the sheets using a combination of pressure and heat to set the glue and fuse the veneer layers together.  
           [0005]    The process of layering veneer sheets is known as “lay-up” or “laying up” and requires that individual veneer sheets be aligned precisely with one another. Improperly aligned veneer layers may result in excess waste, reduced yield and inferior quality products. Manual lay-up techniques involve placing glue covered veneer sheets against at least two solid fences in order to align the sheets. Manual veneer lay-up is labor intensive, dependent on worker performance and may lead to worker injury, because of the repetitive motion involved.  
           [0006]    Because of inconsistencies between individual veneer sheets, the lay-up process is difficult to automate. Even if individual veneer sheets are cut to approximately the same size, there may be variations in their size, which can adversely affect their lay-up. For example, typically sized veneer sheets used in the production of four foot by eight foot plywood sheets may vary between 48 to 52 inches wide, 96 to 102 inches long and {fraction (1/16)} to ⅙ of an inch thick. These size discrepancies may be due to variations in the shrinkage rate of wood from different sections of the tree stem that occur during processing of individual veneer sheets. In addition to size variation, each veneer sheet may vary in surface quality, waviness, location and size of defects (i.e. knot holes and splits).  
           [0007]    This invention assists in automating the laying up of veneer sheets during the production of multi-layer wood products, such as plywood and LVL, notwithstanding the aforementioned veneer sheet size deformations. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0008]    In drawings, which illustrate non-limiting embodiments of the invention:  
         [0009]    [0009]FIG. 1 is a top plan schematic illustration of a veneer lay-up apparatus incorporating a dual tablet lay-up device in accordance with the invention;  
         [0010]    [0010]FIG. 2 is a top plan view of the FIG. 1 dual tablet lay-up device in its open configuration;  
         [0011]    [0011]FIG. 3 is a partially sectioned side elevation view of the apparatus depicted in FIG. 2;  
         [0012]    [0012]FIG. 4 is partially sectioned end view of a portion of the FIG. 2 device depicting the rearward tablet translation mechanism;  
         [0013]    [0013]FIG. 5 is a top plan view of the FIG. 1 dual tablet lay-up device in its closed configuration;  
         [0014]    [0014]FIG. 6 is a partially sectioned side elevation view of the apparatus depicted in FIG. 5;  
         [0015]    [0015]FIGS. 7A and 7B are partial side elevation views of the rearward conveyor belt mechanism of the FIG. 2 device shown respectively in open and closed configurations;  
         [0016]    [0016]FIGS. 8A and 8B are respectively top plan and front elevation views of the rearward tablet portion of the FIG. 2 device;  
         [0017]    [0017]FIG. 9 is a partially sectioned end view of a portion of the FIG. 2 device depicting the forward tablet translation mechanism;  
         [0018]    [0018]FIGS. 10A and 10B are partial side elevation views of the forward conveyor belt mechanism of the FIG. 2 device shown respectively in open and closed configurations;  
         [0019]    [0019]FIGS. 11A and 11B are respectively top plan and rear elevation views of the forward tablet portion of the FIG. 2 device;  
         [0020]    [0020]FIG. 12 is a schematic isometric illustration of a lay-up carriage; and,  
         [0021]    [0021]FIG. 13 is a schematic illustration of a control system for controllably operating the FIG. 2 device. 
     
    
     DESCRIPTION  
       [0022]    Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practised without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense.  
         [0023]    [0023]FIG. 1 schematically depicts veneer lay-up apparatus  10  incorporating input conveyor  20 , dual tablet lay-up device  100  and lay-up carriage  500 .  
         [0024]    Throughout this description, the terms “forward”, “forwardly” and “forwardmost” correspond to the normal direction of movement of input conveyor  20  as indicated by arrow  28 A. The terms “rear”, “rearward”, “rearwardly” and “rearwardmost” correspond to the direction indicated by arrow  28 B. The terms “longitudinal” and “longitudinally” correspond to either forward direction  28 A or rearward direction  28 B. The terms “transverse” and “transversely” correspond to the direction substantially perpendicular to the longitudinal direction (i.e. either of the directions indicated by double-headed arrow  26 ).  
         [0025]    Lay-up apparatus  10  lays up veneer sheets  22 . The laid up sheets can then be used to form plywood or LVL. Veneer sheets  22  are transported along input conveyor  20  until they reach dual tablet lay-up device  100 , which includes a rearward tablet assembly  102  and a forward tablet assembly  202 . Depending on whether plywood or LVL is being produced, lay-up carriage  500  may remain stationary, such that successive veneer sheets  22  are laid on top of one another; or, lay-up carriage  500  may be indexed transversely between successive veneer sheets  22  to produce a billet of sheets in which each sheet is offset by a predefined distance relative to the immediately adjacent sheet(s).  
         [0026]    Veneer sheets  22  are substantially rectangular in shape and may have various industry standard dimensions. The lumber grain of veneer sheets  22  may be oriented longitudinally or transversely without affecting the operation of lay-up apparatus  10 . With minor modifications which will be readily apparent to those skilled in the art, the invention may be applied to veneer sheets of any size and is thus independent of the size of veneer sheets  22 . The drawings and description of the invention depict certain orientations of veneer sheets  22 . For example, FIG. 1 depicts veneer sheets  22  with their longer sides disposed in the transverse direction. Notwithstanding the drawings and accompanying description, the invention is independent of the orientation of veneer sheets  22 .  
         [0027]    One of the transverse edges of each veneer sheet  22  will be referred to as transverse edge  22 A; the forwardmost edge of each veneer sheet  22  will be referred to as forward edge  22 B; and, the rearwardmost edge of each veneer sheet  22  will be referred to as rearward edge  22 C.  
         [0028]    Input conveyor  20  transports veneer sheets  22  forwardly along conveyor belts  24  toward dual tablet lay-up device  100 . If veneer sheets  22  have been spaced apart from one another or transverse edges  22 A have been pre-aligned relative to device  100  (as is preferred), then input conveyor  20  may incorporate a mechanism or technique to maintain this spacing and/or alignment. One well known technique used to maintain such spacing and/or alignment of veneer sheets  22  involves the application of suction force to the undersides of veneer sheets  22 , such that veneer sheets  22  do not move relative to conveyor belts  24 .  
         [0029]    When veneer sheets  22  reach the forwardmost end of input conveyor  20 , they are transferred from the forwardmost ends of conveyor belts  24  to the rearwardmost ends of conveyor belts  112  on dual tablet lay-up device  100 .  
         [0030]    Dual tablet lay-up device  100  incorporates a pair of substantially parallel longitudinally extending rearward frame members  106 ,  108  which support rearward tablet assembly  102  and a pair of substantially parallel longitudinally extending forward frame members  206 ,  208  which support forward tablet assembly  202 . The forward ends of rearward frame members  106 ,  108  abut directly against the rearward ends of forward frame members  206 ,  208 . As explained in more detail below, rearward and forward tablet assemblies  102 ,  202  mutually reciprocate with one another such that dual tablet lay-up device  100  is positionable in an open configuration and a closed configuration. Dual tablet lay-up device  100  is depicted in its open configuration in FIG. 2 and in its closed position in FIG. 5.  
         [0031]    Rearward tablet assembly  102  is depicted in greater detail in FIGS.  2  to  8 . Rearward tablet assembly  102  includes: a flat, moveable, rearward tablet  110  which extends between and is longitudinally reciprocable relative to frame members  106 ,  108 ; and, a plurality of parallel endless conveyor belts  112 . Preferably, at least some of conveyor belts  112  are apertured as indicated at  116 ; the remainder are non-apertured belts as indicated at  114  (see FIGS. 2, 5). In the illustrated embodiment, apertured conveyor belts  116  extend forwardly beyond the forwardmost extent of non-apertured conveyor belts  114 , allowing rearward conveyor belts  112  to interleave with forward conveyor belts  212  when dual tablet lay-up device  100  is in its closed configuration (see FIG. 5).  
         [0032]    Frame member  106  is a C-shaped channel member having a horizontal flange  107  which defines a lower recess  106 A and an upper recess  106 B in frame member  106 , as depicted in FIG. 4.  
         [0033]    Referring to FIGS. 3 and 4, rearward tablet assembly  102  includes a rearward tablet translation mechanism  132 , which facilitates longitudinal reciprocation of rearward tablet  110  with respect to frame member  106 . Rearward tablet translation mechanism  132  includes a drive motor  134  having a sprocket  136  on a rotational shaft thereof. Belt  138 , which is preferably a gear belt, is entrained over sprocket  136  and exterior sprocket  140 , such that the rotation of drive motor  134  (and sprocket  136 ) causes a corresponding rotation of exterior sprocket  140 . Exterior sprocket  140  is coupled by a shaft (not shown) to interior sprocket  142 , such that rotation of exterior sprocket  140  causes a corresponding rotation of interior sprocket  142 , which is rotatably mounted to a rearward end of frame member  106  inside lower recess  106 A.  
         [0034]    A second sprocket  144  is also rotatably mounted in lower recess  106 A of frame member  106 . Second sprocket  144  is forwardly spaced apart from sprocket  142 . In the illustrated embodiment, sprocket  144  is an idler sprocket. Belt  146 , which is preferably a gear belt, is entrained over sprockets  142  and  144 , such that belt  146  travels within lower recess  106 A of frame member  106 . Rearward tablet  110  is rigidly attached to an upper segment of belt  146  via horizontally extending clamp  148  and vertically extending bracket  150 .  
         [0035]    Wheel  152  is rotatably mounted to the rearward end of rearward tablet  110  via axle bolt  153  which extends through an upper portion of vertically extending bracket  150 , such that wheel  152  is located in upper recess  106 B of frame member  106 . Inside upper recess  106 B, wheel  152  rolls along track  154 , which is fixed to the upper surface of flange  107 . A second wheel  156  is rotatably mounted to a forward end of rearward tablet  110  through a similar vertically extending bracket (not shown), such that wheel  156  rolls along track  154  in upper recess  106 B of frame member  106 .  
         [0036]    Rearward tablet translation mechanism  132  also includes similar components (not shown) on opposing frame member  108  (see FIG. 2) and/or the opposing side of rearward tablet  110 . Such components include: an interior sprocket rotatably mounted inside a lower recess at a rearward end of frame member  108 ; a second idler sprocket rotatably mounted inside the lower recess of frame member  108  at a position forwardly spaced apart from the interior sprocket; a gear belt entrained over the interior and idler sprockets which travels in the lower recess of frame member  108 ; a clamp which rigidly attaches rearward tablet  110  to the belt; and, a pair of wheels rotatably mounted at spaced apart locations to the forward and rearward ends of rearward tablet  110 , which roll along a track provided in an upper recess of frame member  108 . A drive shaft (not shown) extends transversely between exterior sprocket  140  on frame member  106 , through interior sprocket  142  and to the corresponding interior sprocket rotatably mounted on frame member  108 . When drive motor  134  causes interior sprocket  142  to rotate, the drive shaft rotates, thereby causing a corresponding rotation of the interior sprocket on frame member  108 .  
         [0037]    The operation of rearward tablet translation mechanism  132  is explained with reference to the components depicted in FIGS. 2, 3 and  4 , it being understood that the opposing components (i.e. those mounted to frame member  108  and/or on the transversely opposing side of rearward tablet  110 ) operate in a similar manner to facilitate the reciprocation of rearward tablet  110 . Drive motor  134  causes longitudinal movement of rearward tablet  110  relative to frame member  106 . More particularly, drive motor  134  drivingly rotates sprockets  136 ,  140  and  142  as previously explained. As interior sprocket  142  rotates, it drives belt  146  over sprockets  142 ,  144 , such that the upper segment of belt  146  moves in the longitudinal direction. Since rearward tablet  110  is fastened to the upper segment of belt  146  by clamp  148 , movement of belt  146  causes corresponding movement of rearward tablet  110  in the longitudinal direction relative to frame member  106 . Such movement is facilitated by wheels  152  and  156  which roll along track  154 . Drive motor  134  and the corresponding movement of rearward tablet  110  relative to frame member  106  are controlled by suitable control hardware and software, as explained below.  
         [0038]    [0038]FIGS. 2 and 3 depict dual tablet lay-up device  100  in its open configuration, with rearward tablet  110  positioned at (or near) the rearwardmost extent of its longitudinal travel. The closed configuration of dual tablet lay-up device  100  is depicted in FIGS. 5 and 6. In the closed configuration, rearward tablet  110  is positioned at (or near) the forwardmost extent of its longitudinal travel. As can be seen by comparing FIGS. 2 and 5, the forward ends of conveyor belts  112  extend forwardly when device  100  is in the closed configuration; and, retract rearwardly when device  100  is in the open configuration, thereby providing a continuous surface for transport of veneer sheets  22 .  
         [0039]    Rearward conveyor belt mechanism  160  facilitates the movement of conveyor belts  112 . As depicted in FIGS. 7A and 7B, rearward conveyor belt mechanism  160  allows conveyor belts  112  to extend and retract to accommodate longitudinal movement of rearward tablet  110 . FIG. 7A shows the disposition of rearward conveyor belt mechanism  160  when rearward tablet  110  is at or near its rearwardmost position (i.e. the open configuration) and FIG. 7B shows the disposition of rearward conveyor belt mechanism  160  when rearward tablet  110  is at or near its forwardmost position (i.e. the closed configuration). Some elements of rearward tablet assembly  102  are not shown in FIGS. 7A and 7B in order to avoid obscuring detail of rearward conveyor belt mechanism  160 .  
         [0040]    Conveyor belts  112  are entrained over forward pulleys  118  and rearward pulleys  120 . Rearward pulleys  120  are fixed at spaced apart intervals on rearward shaft  122 , which is rotatably mounted between the rearward ends of frame members  106 ,  108 . Forward pulleys  118  are rotatably mounted on pulley supports  124 , which are fixed to the forward end of rearward tablet  110  at spaced apart positions, such that each forward pulley  118  is longitudinally aligned with a corresponding one of rearward pulleys  120 . Drive motor  126  (see FIG. 2) rotates sprocket  128  via belt  130 , which is preferably a gear belt, to drive rearward shaft  122 . Shaft  122  in turn rotates rearward pulleys  120  and causes corresponding longitudinal movement of conveyor belts  112 . Drive motor  126  is controlled by suitable control hardware and software, as explained below.  
         [0041]    Shaft  122  and rearward pulleys  120  are constrained to rotational movement and do not reciprocate relative to frame members  106 ,  108 . However, since forward pulleys  118  are attached to rearward tablet  110 , they reciprocate longitudinally with rearward tablet  110  as shown in FIGS. 7A and 7B. Conveyor belts  112  are also entrained over reciprocable rearward pulleys  162 ,  164  and fixed idler pulley  166 . Reciprocable rearward pulleys  162 ,  164  are rotationally coupled to the rearward end of rearward tablet  110  by pulley supports  170  at positions longitudinally aligned with corresponding ones of rearward pulleys  120  and forward pulleys  118 . Because they are attached to rearward tablet  110 , reciprocable rearward pulleys  162 ,  164  reciprocate longitudinally with rearward tablet  110 . Idler pulleys  166 , however, are rotationally mounted to a transverse cross brace (not shown), which extends between frame members  106 ,  108 , at positions longitudinally aligned with corresponding ones of rearward pulleys  120  and forward pulleys  118 . Idler pulleys  166  rotate, but do not reciprocate with rearward tablet  110 .  
         [0042]    In the open configuration of FIG. 7A, tablet  110  is at (or near) the rearwardmost extent of its travel. In this open configuration, pulleys  162 ,  164 ,  118  are in their rearward positions, such that the top segments of conveyor belts  112  extend a relatively short distance in the longitudinal direction. In contrast, in the closed configuration of FIG. 7B, rearward tablet  110  is at (or near) the forwardmost extent of its travel. In this closed configuration, pulleys  162 ,  164 ,  118  are in their forward positions, with pulleys  162 ,  164  positioned just rearward of idlers  166  and with forward pulleys  118  extended to engage forward tablet assembly  202  (see FIG. 5). In the closed configuration of FIG. 7B, the top segments of conveyor belts  112  extend a relatively long distance in the longitudinal direction, as is revealed by comparing FIGS. 7A and 7B.  
         [0043]    Rearward tablet assembly  102  uses suction to maintain alignment of veneer sheets  22  as they are transported by conveyor belts  112 . This suction is explained with reference to FIG. 8 which shows rearward tablet  110  without conveyor belts  112  to more clearly depict certain components of the suction pressure system. FIG. 3 shows that rearward tablet assembly  102  includes a vacuum source  182  and a vacuum conduit  184 . Vacuum conduit  184  extends from vacuum source  182  through a Y-junction (not shown), from which it diverges to form a pair of vacuum conduits (not shown) which extend longitudinally along each transverse side of rearward tablet  110 . Each longitudinally extending portion of vacuum conduit  184  extends into and is slidably coupled to a corresponding one of vacuum pipes  176 ,  178  which extend along opposite transverse sides of rearward tablet  110 , such that horizontally extending vacuum pipes  176 ,  178  are in communication with vacuum source  182 . As rearward tablet  110  reciprocates, each of vacuum pipes  176 ,  178  move slidably over the longitudinally extending portions of vacuum conduit  184 .  
         [0044]    A plurality of apertures  180  are provided in the upper surface of rearward tablet  110 . Vacuum pressure is applied by vacuum source  182  through vacuum conduit  184  and vacuum pipes  176 ,  178  to apertures  180 . As shown in FIGS. 2 and 5, at least some of conveyor belts  112  have apertures  186 . When veneer sheets  22  are transferred to conveyor belts  112 , suction pressure is applied through apertures  180 ,  186  to the undersides of veneer sheets  22 . Additionally or alternatively, suction pressure may be applied to the undersides of veneer sheets  22  through apertures  180  in rearward tablet  110  and through the gaps between conveyor belts  112 .  
         [0045]    Referring to FIGS. 2, 3,  5 ,  6 ,  9  and  10 , dual tablet lay-up device  100  also has a forward tablet assembly  202 , which includes: a flat, moveable, forward tablet  210  that extends between and is longitudinally reciprocable relative to forward frame members  206 ,  208 ; and a plurality of parallel endless conveyor belts  212 . Some of conveyor belts  212  are apertured as indicated at  216 ; the remainder are non-apertured belts as indicated at  214 . In the illustrated embodiment, apertured conveyor belts  216  extend rearwardly beyond the rearwardmost extent of non-apertured conveyor belts  214 , allowing forward conveyor belts  212  to interleave with rearward conveyor belts  112  when dual tablet lay-up device  100  is in its closed configuration (see FIG. 5). A transversely oriented fence  204  is mounted between frame members  206  and  208  above forward tablet assembly  202 .  
         [0046]    Referring to FIGS. 3 and 9, forward tablet assembly  202  includes a forward tablet translation mechanism  232 , which facilitates the longitudinal reciprocation of forward tablet  210  with respect to frame member  206 . Forward tablet translation mechanism  232  includes a drive motor  234  having a sprocket  236  on a rotational shaft thereof. Belt  238 , which is preferably a gear belt, is entrained over sprocket  236  and exterior sprocket  240 , such that the rotation of drive motor  234  (and sprocket  236 ) causes a corresponding rotation of exterior sprocket  240 . Exterior sprocket  240  is coupled by a shaft (not shown) to interior sprocket  242 , such that rotation of exterior sprocket  240  causes a corresponding rotation of interior sprocket  242 , which is rotatably mounted to a forward end of frame member  206  inside upper recess  206 B.  
         [0047]    A second sprocket  244  is also rotatably mounted in upper recess  206 B of frame member  206 . Second sprocket  244  is rearwardly spaced apart from sprocket  242 . In the illustrated embodiment, sprocket  244  is an idler sprocket. Belt  246 , which is preferably a gear belt, is entrained over sprockets  242  and  244 , such that belt  246  travels within upper recess  206 B of frame member  206 . Forward tablet  210  is rigidly attached to a lower segment of belt  246  via horizontally extending clamp  248  and vertically extending bracket  250 . Vertically extending bracket  250  may also extend longitudinally to position forward tablet  210  in a desirable longitudinal location relative to belt  246 .  
         [0048]    Wheel  252  is rotatably mounted to the rearward end of forward tablet  210  via axle bolt  253  which extends through a lower portion of vertically extending bracket  250 , such that wheel  252  is located in lower recess  206 A of frame member  206 . Inside lower recess  206 A, wheel  252  rolls along track  254 , which is fixed to the upper surface of a bottom flange  105  of frame member  206 . A second wheel  256  is rotatably mounted to a forward end of forward tablet  210  through a similar vertically extending bracket (not shown), such that wheel  256  rolls along track  254  in lower recess  206 A of frame member  206 .  
         [0049]    Forward tablet translation mechanism  232  also includes similar components (not shown) on opposing frame member  208  (see FIG. 2) and/or the opposing side of forward tablet  210 . Such components include: an interior sprocket rotatably mounted inside an upper recess at a forward end of frame member  208 ; a second idler sprocket rotatably mounted inside the upper recess of frame member  208  at a position rearwardly spaced apart from the interior sprocket; a gear belt entrained over the interior and idler sprockets which travels in the upper recess of frame member  208 ; a clamp which rigidly attaches forward tablet  210  to a lower segment of the belt; and a pair of wheels rotatably mounted at spaced apart locations to the forward and rearward ends of forward tablet  210 , which roll along a track provided in an lower recess of frame member  208 . A drive shaft  203  (see FIG. 2) extends transversely between exterior sprocket  240  on frame member  206 , through interior sprocket  242  and to the corresponding interior sprocket rotatably mounted on frame member  208 . When drive motor  234  causes interior sprocket  242  to rotate, drive shaft  203  rotates, thereby causing corresponding rotation of the interior sprocket on frame member  208 .  
         [0050]    The operation of forward tablet translation mechanism  232  is explained with reference to the components depicted in FIGS. 2, 3 and  9 , it being understood that the opposing components (i.e. those mounted to frame member  208  and/or on the transversely opposing side of forward tablet  210 ) operate in a similar manner to facilitate reciprocation of forward tablet  210 . Drive motor  234  causes longitudinal movement of forward tablet  210  relative to frame member  206 . More particularly, drive motor  234  drivingly rotates sprockets  236 ,  240  and  242  and shaft  203  as previously explained. As interior sprocket  242  rotates, it drives belt  246  over sprockets  242 ,  244 , such that the lower segment of belt  246  moves in the longitudinal direction. Since forward tablet  210  is fastened to the lower segment of belt  246  by clamp  248 , movement of belt  246  also moves forward tablet  210  in the longitudinal direction relative to frame member  206 . Such movement is facilitated by wheels  252  and  256  which roll along track  254 . Drive motor  234  and the corresponding movement of forward tablet  210  relative to frame member  206  are controlled by suitable control hardware and software, as explained below.  
         [0051]    [0051]FIGS. 2 and 3 depict dual tablet lay-up device in its open configuration, with forward tablet  210  positioned at (or near) the forwardmost extent of its longitudinal travel. The closed configuration of dual tablet lay-up device  100  is depicted in FIGS. 5 and 6. In the closed configuration, forward tablet  210  is positioned at (or near) the rearwardmost extent of its longitudinal travel. As can be seen by comparing FIGS. 2 and 5, both forward tablet  210  and conveyor belts  212  translate rearward when device  100  is in the closed configuration and translate forward when device  100  is in the open configuration.  
         [0052]    Forward conveyor belt mechanism  260  facilitates the movement of forward conveyor belts  212 . As depicted in FIGS. 10A and 10B, forward conveyor belt mechanism  260  allows conveyor belts  212  to reciprocate longitudinally with forward tablet  210 . FIG. 10A shows the disposition of forward conveyor belt mechanism  260  when forward tablet  210  is at or near its forwardmost position (i.e. the open configuration) and FIG. 10B shows the disposition of forward conveyor belt mechanism  260  when forward tablet  210  is at or near its rearwardmost position (i.e. the closed configuration). Some elements of forward tablet assembly  202  are not shown in FIGS. 10A and 10B in order to avoid obscuring detail of forward conveyor belt mechanism  260 .  
         [0053]    Conveyor belts  212  are entrained over forward pulleys  218 , rearward pulleys  224  and drive pulleys  220  and are entrained under forward idler pulleys  258  and rearward idler pulleys  209 .  
         [0054]    Rearward pulleys  224  are rotatably mounted to pulley supports  264  which are fixed to the rearward end of forward tablet  210  at transversely spaced apart positions. Forward pulleys  218  are rotatably mounted to pulley supports  262  which are fixed to the forward end of forward tablet  210  at spaced apart locations, such that each forward pulley  218  is longitudinally aligned with a corresponding one of rearward pulleys  224 . Conveyor belts  212  are also entrained over drive pulleys  220 . Drive pulleys  220  are fixed at spaced apart locations on drive shaft  222 , which extends transversely and is rotatably mounted between frame members  206 ,  208 . The position of each drive pulley  220  is longitudinally aligned with a corresponding forward pulley  218  and a corresponding rearward pulley  224 .  
         [0055]    Conveyor belts  212  are also entrained under forward idler pulleys  258  and rearward idler pulleys  209 . Forward idler pulleys  258  are rotatably mounted to pulley supports  259  which are fixed, at transversely spaced apart locations, to transverse cross-member  257  (see FIGS. 2 and 5). Rearward idler pulleys  209  are rotatably mounted to drive shaft  203  of forward tablet translation mechanism  232  at transversely spaced apart locations (see FIG. 2). Rearward idler pulleys  209  rotate independently of the rotation of shaft  203 . Each forward idler pulley  258  and each rearward idler pulley  209  are located in longitudinal alignment with corresponding ones of drive pulleys  220 , forward pulleys  218  and rearward pulleys  224 .  
         [0056]    Conveyor belts  212  are driven by drive motor  226  (see FIGS. 2 and 5), which rotates sprocket  228  through belt  230  to drive shaft  222 . Belt  230  is preferably a gear belt. Drive shaft  222  in turn rotates drive pulleys  220 , which cause corresponding longitudinal movement of conveyor belts  212 . Drive motor  226  is controlled by suitable control hardware and software, as explained below.  
         [0057]    Shaft  222  and drive pulleys  220  are constrained to rotational movement because they are mounted to frame members  206  and  208 . Rearward and forward idler pulleys  209 ,  258  are also constrained to rotational movements because they are respectively mounted to transverse shaft  203  and transverse cross-member  257 . Consequently, shaft  222 , drive pulleys  220 , rearward idler pulleys  209  and forward idler pulleys  258  do not reciprocate relative to frame members  206 ,  208 . However, since forward pulleys  218  and rearward pulleys  224  are attached to forward tablet  210 , they reciprocate longitudinally with forward tablet  210  as shown in FIGS. 10A and 10B.  
         [0058]    In the open configuration of FIG. 10A, forward tablet  210  is at (or near) the forwardmost extent of its travel. In this open configuration, forward pulleys  218  are positioned near the forward end of dual tablet lay-up device  100  and rearward pulleys  224  are positioned just rearward of rearward idlers  209 . In contrast, in the closed configuration of FIG. 10B, forward tablet  210  is at (or near) the rearwardmost extent of its travel. In the closed configuration, forward pulleys  218  are positioned just forward of forward idlers  258  and rearward pulleys  224  are extended rearward to engage rearward tablet assembly  102  (see FIG. 5).  
         [0059]    Forward tablet assembly  202  uses suction to maintain alignment of veneer sheets  22  when they move over top of conveyor belts  212 . This suction mechanism is explained with reference to FIG. 11, which shows forward tablet  210  without conveyor belts  212  to more clearly depict certain components of the forward tablet suction pressure system. FIG. 3 shows that forward tablet assembly  202  includes a vacuum source  282  and a vacuum conduit  284 . Vacuum conduit  284  extends from vacuum source  282  through a Y-junction (not shown), from which it diverges (see FIG. 2) to form a pair of vacuum conduits which extend longitudinally along each transverse side of forward tablet  210 . Each longitudinally extending portion of vacuum conduit  284  extends into and is slidably coupled to a corresponding one of vacuum pipes  276  and  278  which extend along opposite transverse sides of forward tablet  210 , such that vacuum pipes  276  and  278  are in communication with vacuum source  282 . As forward tablet  210  reciprocates, each of vacuum pipes  276 ,  278  move slidably over the longitudinally extending portions of vacuum conduit  284 .  
         [0060]    A plurality of apertures  280  are provided in the upper surface of forward tablet  210 . Vacuum pressure is applied by vacuum source  282  through vacuum conduit  284  and vacuum pipes  276 ,  278  to apertures  280 . As shown in FIGS. 2 and 5, at least some of conveyor belts  212  have apertures  286 . When veneer sheets  22  move over top of conveyor belts  212 , suction pressure is applied through apertures  280 ,  286  to the undersides of veneer sheets  22 . Additionally or alternatively, suction pressure may be applied to the undersides of veneer sheets  22  through apertures  280  in forward tablet  210  and through the gaps between conveyor belts  212 .  
         [0061]    [0061]FIG. 13 schematically depicts a control system  300  used for controlling the operation of dual tablet lay-up device  100 . Control system  300  includes a controller  302 , which may be, for example, a programmable computer, an embedded processor or the like. Controller  302  may include more than one data processor. Controller  302  may also include memory (not shown) which stores program information and the like. In a preferred embodiment (not shown), controller  302  includes an embedded processor and a programmable logic circuit (PLC).  
         [0062]    As shown schematically in FIG. 13, controller  302  is connected to control the movement of: (i) rearward tablet  110 ; (ii) rearward conveyor belt  112 ; (iii) forward tablet  210 ; and, (iv) forward conveyor belt  212 .  
         [0063]    Controller  302  is independently connected to each of a plurality of variable speed drive controllers  304 ,  306 ,  308 ,  310 . Each drive controller  304 ,  306 ,  308 ,  310  is associated with a corresponding one of drive motors  126 ,  134 ,  226 ,  234 . Controller  302  controllably transmits: (i) rearward conveyor belt drive signal  312  to variable speed drive controller  304 ; (ii) rearward tablet drive signal  314  to variable speed drive controller  306 ; (iii) forward conveyor belt drive signal  316  to variable speed drive controller  308 ; and, (iv) forward tablet drive signal  318  to variable speed drive controller  310 . Drive signals  312 ,  314 ,  316 ,  318  are preferably analog control signals which are amplified by variable speed drive controllers  304 ,  306 ,  308 ,  310  to actuate their associated drive motors  126 ,  134 ,  226 ,  234 . Alternatively, power amplifiers can be used in the place of variable speed drive controllers  304 ,  306 ,  308 ,  310 . As another alternative, drive signals  312 ,  314 ,  316 ,  318  can be digital signals, with drive controllers  304 ,  306 ,  308 ,  310  configured to convert the drive signals into corresponding analog signals to actuate their associated drive motors  126 ,  134 ,  226 ,  234 .  
         [0064]    Controller  302  is electronically coupled to optical sensor  328  and to inductive proximity detectors  332 ,  334 ,  336 ,  338 ,  340 ,  342 ,  344 ,  346 . Optical sensor  328  detects the forward edge  22 B of a veneer sheet  22  (not shown in FIG. 13) when the sheet is transferred from input conveyor  20  to dual tablet lay-up device  100  (see FIG. 1). As explained below, controller  302  uses inductive proximity detectors  332 ,  334 ,  336 ,  338 ,  340 ,  342 ,  344 ,  346  to limit and control the longitudinal motion of rearward tablet  110  and forward tablet  210 .  
         [0065]    Reflector  330  is positioned below rearward tablet  110  in alignment with optical sensor  328 . Optical sensor  328  is positioned above rearward tablet  110  and emits a downwardly oriented optical beam (not shown), which is normally reflected by reflector  330 . Optical detector  328  detects the reflected beam. When a veneer sheet  22  travelling forwardly on conveyor belts  112  interrupts the beam, optical sensor  328  produces a veneer sheet position signal  329  which is electronically transmitted to controller  302 .  
         [0066]    Inductive proximity detectors  332 ,  334 ,  336 ,  338 ,  340 ,  342 ,  344 ,  346  are positioned at longitudinally spaced apart locations. Preferably, detectors  332 ,  334 ,  336 ,  338 ,  340 ,  342 ,  344 ,  346  are mounted on either of frame members  106 ,  108 . Each proximity detector  332 ,  334 ,  336 ,  338 ,  340 ,  342 ,  344 ,  346  establishes a localized electromagnetic field using an oscillator and a coil (not shown). When a metal object (such as rearward tablet  110  or forward tablet  210 ) enters a region proximate to one of detectors  332 ,  334 ,  336 ,  338 ,  340 ,  342 ,  344 ,  346 , the detector senses a change in its corresponding electromagnetic field and outputs a corresponding signal  333 ,  335 ,  337 ,  339 ,  341 ,  343 ,  345 ,  347 .  
         [0067]    Four proximity detectors  332 ,  334 ,  336 ,  338  are associated with rearward tablet translation mechanism  132  and assist controller  302  to control the motion of rearward tablet  110 . A rearward pair of proximity detectors  332 ,  334  is positioned proximate to the rearward translational limit of rearward tablet  110 . A forward pair of proximity sensors  336 ,  338  is positioned proximate to the forward translational limit of rearward tablet  110 .  
         [0068]    Of the rearward pair of proximity detectors associated with rearward tablet translation mechanism  132 , detector  332  is located more forwardly than detector  334 . When detector  332  detects the presence of rearward tablet  110 , it transmits a rearward tablet rearward limit signal  333  to controller  302  indicating that rearward tablet  110  is at or near its rearward translational limit. If rearward tablet  110  is travelling rearwardly past detector  332 , then rearward tablet rearward limit signal  333  indicates that rearward tablet  110  should be decelerated to a stop. Rearwardmost detector  334  is spaced apart rearwardly from detector  332 . When rearwardmost detector  334  detects rearward tablet  110 , it transmits an emergency stop signal  335  to controller  302  indicating that rearward tablet  110  has travelled too far in the rearward direction. Controller  302  receives emergency stop signal  335  and immediately shuts off rearward tablet drive signal  314 . Alternatively, emergency stop signal  335  may trigger a switch which directly shuts off rearward tablet drive signal  314  without communicating with controller  302 .  
         [0069]    Of the forward pair of proximity detectors associated with rearward tablet translation mechanism  132 , detector  336  is located more rearwardly than detector  338 . When detector  336  detects the presence of rearward tablet  110 , it transmits a rearward tablet forward limit signal  337  to controller  302  indicating that rearward tablet  110  is at or near its forward translational limit. If rearward tablet  110  is travelling forwardly past detector  336 , then rearward tablet forward limit signal  337  indicates that rearward tablet  110  should be decelerated to a stop. Forwardmost detector  338  is spaced apart forwardly from detector  336 . When forwardmost detector  338  detects rearward tablet  110 , it transmits an emergency stop signal  339  to controller  302  indicating that rearward tablet  110  has travelled too far in the forward direction. Controller  302  receives emergency stop signal  339  and immediately shuts off rearward tablet drive signal  314 . Alternatively, emergency. stop signal  339  may trigger a switch which directly shuts off rearward tablet drive signal  314  without communicating with controller  302 .  
         [0070]    Four proximity detectors  340 ,  342 ,  344 ,  346  are associated with forward tablet translation mechanism  232  and assist controller  302  to control the motion of forward tablet  210 . A rearward pair of proximity detectors  340 ,  342  is positioned proximate to the rearward translational limit of forward tablet  210 . A forward pair of proximity detectors  344 ,  346  is positioned proximate to the forward translational limit of forward tablet  210 . When activated by the presence of forward tablet  210 , the rearward pair of proximity detectors  340 ,  342  respectively transmit signals  341 ,  343  to controller  302 . Forward tablet rearward limit signal  341  indicates that forward tablet  210  is at or near its rearward translational limit and signal  343  is an emergency stop signal, triggered when forward tablet  210  has travelled too far in the rearward direction. Similarly, when activated by the presence of forward tablet  210 , the forward pair of proximity detectors  344 ,  346  respectively transmit signals  345 ,  347  to controller  302 . Forward tablet forward limit signal  345  indicates that forward tablet  210  is at or near its forward travel limit and signal  347  is an emergency stop signal, triggered when forward tablet  210  has travelled too far in the forward direction.  
         [0071]    Control system  300  also includes encoders  320 ,  322 . Encoder  320  measures the shaft angle of rearward conveyor belt drive motor  126  and electronically transmits a “rearward conveyor movement” signal  324  to controller  302 . Rearward conveyor movement signal  324  is representative of the position and speed of rearward conveyor belts  112 . In a similar manner, encoder  322  measures the shaft angle of forward conveyor belt drive motor  226  and electronically transmits a “forward conveyor movement” signal  326  to controller  302 . Forward conveyor movement signal  326  is representative of the position and speed of forward conveyor belts  212 .  
         [0072]    The operation and control of dual tablet lay-up device  100  is now described with reference to FIGS. 1, 2,  3 ,  5 ,  6  and  13 . Input conveyor  20 , rearward tablet assembly  102  and forward tablet assembly  202  are aligned to position the top segments of conveyor belts  24 ,  112  and  212  at substantially the same height. A veneer sheet  22 , which has been pre-aligned on its transverse edge  22 A, travels forwardly on input conveyor  20  toward the forwardmost ends of conveyor belts  24 . Conveyor belts  24  run at a known speed. The speed of conveyor belts  24  is preferably controlled by a controller (not shown) or human operator. Alternatively, conveyor belts  24  may be provided with a constant drive signal so that they run at a preset speed.  
         [0073]    Prior to veneer sheet  22  reaching the forwardmost ends of conveyor belts  24 , controller  302  moves dual tablet lay-up device  100  into its open configuration (see FIGS. 2 and 3) by rearwardly retracting rearward tablet  110  and forwardly advancing forward tablet  210 .  
         [0074]    To move dual tablet lay-up device  100  into its open configuration, controller  302  outputs rearward tablet drive signal  314  to variable speed drive controller  306 , which drives rearward tablet drive motor  134 , causing the rearward retraction of rearward tablet translation mechanism  132  and rearward tablet  110 . As rearward tablet  110  nears its rearward translational limit, proximity detector  332  detects the presence of rearward tablet  110  and transmits rearward tablet rearward limit signal  333  to controller  302 . In response to the receipt of signal  333 , controller  302  controllably reduces the amplitude of rearward tablet drive signal  314 , causing the rearward retraction of rearward tablet  110  to decelerate to a stop.  
         [0075]    Rearward tablet drive signal  314  is provided as a constant amplitude drive signal during the rearward retraction of rearward tablet  110 , until controller  302  receives rearward tablet rearward limit signal  333  from proximity detector  332 . When controller  302  receives signal  333 , controller  302  controllably reduces the amplitude of rearward tablet drive signal  314 , so that rearward tablet  110  comes to a stop in a position just forward of emergency stop proximity detector  334 . This longitudinal position is the open configuration position for rearward tablet  110 . If, for some unforeseen reason, rearward tablet  110  travels too far in the rearward direction, then proximity detector  334  outputs emergency stop signal  335 , which cuts off rearward tablet drive signal  314 , immediately stopping any further rearward retraction of rearward tablet  110 .  
         [0076]    At substantially the same time as the rearward retraction of rearward tablet  110 , controller  302  outputs forward tablet drive signal  318  to variable speed drive controller  310 , which drives forward tablet drive motor  234 , causing the forward advance of forward tablet translation mechanism  232  and forward tablet  210 . As forward tablet  210  nears its forward translational limit, proximity detector  344  detects the presence of forward tablet  210  and transmits forward tablet forward limit signal  345  to controller  302 . In response to the receipt of signal  345 , controller  302  controllably reduces the amplitude of forward tablet drive signal  318 , causing the forward advance of forward tablet  210  to decelerate to a stop.  
         [0077]    Forward tablet drive signal  318  is provided as a constant amplitude drive signal during the forward advance of forward tablet  210 , until controller  302  receives forward tablet forward limit signal  345  from proximity detector  344 . When controller  302  receives signal  345 , controller  302  controllably reduces the amplitude of forward tablet drive signal  318 , so that forward tablet  210  comes to a stop in a position just rearward of emergency stop proximity detector  346 . This longitudinal position is the open configuration position for forward tablet  210 . If, for some unforeseen reason, forward tablet  210  travels too far in the forward direction, then proximity detector  346  outputs emergency stop signal  347 , which cuts off forward tablet drive signal  318 , immediately stopping any further forward advance of forward tablet  210 .  
         [0078]    Prior to veneer sheet  22  reaching the forwardmost ends of conveyor belts  24 , controller  302  also controllably adjusts the speed of rearward conveyor belt drive motor  126 , such that rearward conveyor belts  112  are brought to substantially the same speed as conveyor belts  24 . More specifically, controller  302  monitors rearward conveyor movement signal  324  (produced by encoder  320 ) and uses rearward conveyor movement signal  324  to controllably generate rearward conveyor belt drive signal  312 . Controller  302  outputs rearward conveyor belt drive signal  312  to variable speed drive controller  304 , which controllably drives rearward conveyor belt drive motor  126  until the speed of conveyor belts  112  reaches substantially the same speed as conveyor belts  24 . Alternatively, controller  302  may output a predetermined rearward conveyor belt drive signal  312 , such that the speed of conveyor belts  112  approaches the speed of conveyor belts  24  in an “open loop” manner (i.e. without incorporating feedback information from rearward conveyor movement signal  324 ).  
         [0079]    Controller  302  controllably adjusts the speed of forward conveyor belt drive motor  226 , such that the speed of forward conveyor belts  212  continually tracks the speed of rearward conveyor belts  112 . More specifically, controller  302  monitors rearward conveyor movement signal  324  (produced by encoder  320 ) and forward conveyor movement signal  326  (produced by encoder  322 ) and uses these encoder signals  324 ,  326  to controllably generate forward conveyor belt drive signal  316 . Controller  302  outputs forward conveyor belt drive signal  316  to variable speed drive controller  308 , which controllably drives forward conveyor belt drive motor  226  in such a manner that the speed of forward conveyor belts  212  tracks the speed of rearward conveyor belts  112 .  
         [0080]    The speed of forward conveyor belts  212  need not track the speed of rearward conveyor belts  112  at all times. For example, it is only necessary that forward conveyor belts  212  track the speed of rearward conveyor belts  112  when a forward portion of a veneer sheet  22  is located on forward conveyor belts  212  and a rearward portion of the veneer sheet  22  is located on rearward conveyor belts  112 . The speeds of rearward and forward conveyor belts  112 ,  212  can alternatively be independently controlled, such that both rearward and forward conveyor belts  112 ,  212  independently track desired target speeds.  
         [0081]    Dual tablet lay-up device  100  is in its open configuration when veneer sheet  22  reaches the forwardmost ends of conveyor belts  24 . As a veneer sheet  22  is conveyed forwardly past the forwardmost ends of conveyor belts  24 , it passes onto conveyor belts  112  and is thereby transferred from input conveyor  20  to rearward tablet assembly  102 . As discussed above, the speeds of conveyor belts  112 ,  24  are approximately the same during the transfer to avoid bending or stretching veneer sheet  22 . Alignment of the sheet&#39;s transverse edge  22 A is maintained during the transfer by suction applied through apertures  180 ,  186  as previously explained.  
         [0082]    When the forward edge  22 B of sheet  22  intersects the notional plane formed by optical detector  328  and reflector  330 , optical detector  328  transmits veneer sheet position signal  329  to controller  302 , which indicates that a veneer sheet  22  has been transferred onto rearward conveyor belts  112 . In response to veneer sheet position signal  329 , controller  302  causes veneer sheet  22  to be conveyed forwardly on dual tablet lay-up device  100  until sheet  22  reaches its desired lay-up position.  
         [0083]    Upon receipt of veneer sheet position signal  329 , controller  302  causes dual tablet lay-up device  100  to advance to is closed configuration (see FIGS. 5 and 6) by forwardly translating rearward tablet  110  and rearwardly translating forward tablet  210 .  
         [0084]    To forwardly translate rearward tablet  110  into its closed configuration position, controller  302  outputs rearward tablet drive signal  314  to variable speed drive controller  306 , which drives rearward tablet drive motor  134 , causing the forward translation of rearward tablet translation mechanism  132  and rearward tablet  110 . As rearward tablet  110  nears its forward translational limit, proximity detector  336  detects the presence of rearward tablet  110  and transmits rearward tablet forward limit signal  337  to controller  302 . In response to the receipt of signal  337 , controller  302  controllably reduces the amplitude of rearward tablet drive signal  314 , causing the forward translation of rearward tablet  110  to decelerate to a stop.  
         [0085]    Rearward tablet drive signal  314  is provided as a constant amplitude drive signal during the forward translation of rearward tablet  110 , until controller  302  receives rearward tablet forward limit signal  337  from proximity detector  336 . When controller  302  receives signal  337 , controller  302  controllably reduces the amplitude of rearward tablet drive signal  314 , so that rearward tablet  110  comes to a stop in a closed configuration position just rearward of emergency stop proximity detector  338 . This longitudinal position is the closed configuration position for rearward tablet  110 . If, for some unforeseen reason, rearward tablet  110  travels too far in the forward direction, then proximity detector  338  outputs emergency stop signal  339 , which cuts off rearward tablet drive signal  314 , immediately stopping any further forward translation of rearward tablet  110 .  
         [0086]    At substantially the same time as the forward translation of rearward tablet  110 , controller  302  causes the rearward translation of forward tablet  210  into its closed configuration position. Controller  302  outputs forward tablet drive signal  318  to variable speed drive controller  310 , which drives forward tablet drive motor  234 , causing the rearward translation of forward tablet translation mechanism  232  and forward tablet  210 . As forward tablet  210  nears its rearward translational limit, proximity detector  340  detects the presence of forward tablet  210  and transmits forward tablet rearward limit signal  341  to controller  302 . In response to the receipt of signal  341 , controller  302  controllably reduces the amplitude of forward tablet drive signal  318 , causing the rearward translation of forward tablet  210  to decelerate to a stop.  
         [0087]    Forward tablet drive signal  318  is provided as a constant amplitude drive signal during the rearward translation of forward tablet  210 , until controller  302  receives forward tablet rearward limit signal  341  from proximity detector  340 . When controller  302  receives signal  341 , controller  302  controllably reduces the amplitude of forward tablet drive signal  318 , so that forward tablet  210  comes to a stop in a closed configuration position just forward of emergency stop proximity detector  342 . This position is the closed configuration position for forward tablet  210 . If, for some unforeseen reason, forward tablet  210  travels too far in the rearward direction, then proximity detector  342  outputs emergency stop signal  343 , which cuts off forward tablet drive signal  318 , immediately stopping any further rearward translation of forward tablet  210 .  
         [0088]    After veneer sheet  22  is transferred to rearward conveyor belts  112  and controller  302  receives veneer sheet position signal  329 , controller  302  continues to controllably actuate rearward and forward conveyor belt drive motors  126 ,  226  and rearward and forward conveyor belt mechanisms  160 ,  260  to control the motion of rearward and forward conveyor belts  112 ,  212 . The controlled motion of rearward and forward conveyor belts  112 ,  212  permits controlled forward conveyance of veneer sheet  22  to its desired lay-up position which is just rearward of fence  204  (see FIGS. 2 and 5).  
         [0089]    Controller  302  is pre-programmed with the desired longitudinal lay-up position for veneer sheet  22 . This desired lay-up position is preferably expressed as the number of encoder counts (i.e. of encoder  320 ) required to move a veneer sheet  22  between the longitudinal position of optical sensor  328  and the desired lay-up position. When controller  302  receives veneer sheet position signal  329 , controller  302  monitors rearward conveyor movement signal  324  and uses rearward conveyor movement signal  324  along with the desired lay-up position to controllably generate rearward conveyor belt drive signal  312 . Controller  302  outputs rearward conveyor belt drive signal  312  to variable speed actuator  304 , which drives rearward conveyor belt drive motor  126  to controllably convey veneer sheet  22  in a forward direction until veneer sheet  22  reaches the desired lay-up position.  
         [0090]    Preferably, the control algorithms used by controller  302  are configured such that, after the receipt of veneer sheet position signal  329 , dual tablet lay-up device  100  moves between its open configuration and its closed configuration relatively quickly in comparison to the movement of veneer sheet  22  along rearward conveyor belts  112 . In this manner, dual tablet lay-up device  100  is in its closed configuration before veneer sheet  22  reaches the forwardmost ends of rearward conveyor belts  112 .  
         [0091]    When dual tablet lay-up device  100  is in its closed configuration, the forwardmost ends of conveyor belts  112  are interleaved with the rearwardmost ends of conveyor belts  212  (see FIG. 5). As the forward edge  22 B of veneer sheet  22  is conveyed forwardly towards and past the forwardmost ends of conveyor belts  112 , a forward portion of veneer sheet  22  passes onto conveyor belts  212 . As discussed above, controller  302  preferably actuates forward conveyor belt drive motor  226  and forward conveyor belt assembly  260 , such that forward conveyor belts  212  continually track the speed of rearward conveyor belts  112 . Because the speed of conveyor belts  212  tracks the speed of conveyor belts  112 , veneer sheet  22  is not stretched or bent as its forward portion is transferred to conveyor belts  212 . The alignment of veneer sheets  22  is maintained during the passage of the forward portion of veneer sheet  22  onto conveyor belts  212  by the combined suction force applied through apertures  180 ,  186 ,  280 ,  286  as previously explained.  
         [0092]    As veneer sheet  22  approaches its desired lay-up position, controller  302  causes rearward and forward conveyor belts  112 ,  212  to controllably decelerate until veneer sheet  22  comes to rest in its desired lay-up position.  
         [0093]    Once veneer sheet  22  has reached its desired lay-up position, controller  302  actuates rearward and forward tablet drive motors  134 ,  234  and rearward and forward tablet translation mechanisms  132 ,  232  to controllably move dual tablet lay-up device  100  into its open configuration again. As described above, controller  302  moves dual tablet lay-up device  100  to its open configuration by rearwardly retracting rearward tablet  110  and forwardly advancing forward tablet  210 . As forward tablet  210  advances forwardly from under forward edge  22 B of veneer sheet  22  and rearward tablet  110  retracts rearwardly from under the rearward edge  22 C of veneer sheet  22 , veneer sheet  22  drops onto lay-up carriage  500  (see FIG. 1). During the initial movement of dual tablet lay-up device  100  towards its open configuration (i.e. while veneer sheet  22  remains atop of conveyor belts  112 ,  212 ), conveyor belts  112 ,  212  remain motionless. Because conveyor belts  112 ,  212  are motionless and because suction force is continually applied through apertures  180 ,  186 ,  280 ,  286 , veneer sheet  22  does not move transversely or longitudinally during the movement of dual tablet lay-up device  100  towards it open configuration. Consequently, veneer sheet  22  drops onto lay-up carriage  500  precisely in the desired lay-up position.  
         [0094]    The process of transferring veneer sheets  22  from input conveyor  20  to rearward tablet assembly  102  and controlling the reciprocating movement of the components of rearward tablet assembly  102  and forward tablet assembly  202  to drop veneer sheet  22  onto lay-up carriage  500  is repeated for a plurality of veneer sheets  22  in order to lay-up a multi-ply lumber product having the desired number of plies.  
         [0095]    A particular embodiment of lay-up carriage  500  is depicted in FIG. 12. Lay-up carriage  500  is generally located underneath dual tablet lay-up device  100  (see FIG. 1). Lay-up carriage  500  includes a pair of substantially parallel transversely extending base members  502 ,  504 , which support a mobile frame assembly  506  for transverse movement relative to dual tablet lay-up device  100  (see FIG. 1). Mobile frame assembly  506  supports a plurality of vertically moveable tines  508 , which extend generally forwardly from mobile fame assembly  506  at transversely spaced apart locations. The upper surfaces of tines  508  are preferably substantially horizontally oriented.  
         [0096]    Lay-up carriage  500  includes a drive motor  510 , control hardware and software (not shown) and suitable mechanism(s) (not shown) for controllably moving mobile frame assembly  506  in a transverse direction relative to dual tablet may up device  100 . Lay-up carriage  500  also includes a hydraulic cylinder  512 , control hardware and software (not shown), and suitable mechanism(s) (not shown) for controllably moving tines  508  is a vertical direction relative to dual tablet lay-up device  100 .  
         [0097]    Lay-up carriage  500  cooperates with dual tablet lay-up device to lay-up veneer sheets  22 . Input conveyor  20  feeds individual veneer sheets  22  onto dual tablet lay-up device  100  and dual tablet lay-up device  100  lays up the successive individual sheets  22  onto lay-up carriage  500  as described above. The operation of lay-up carriage  500  depends on the type of multi-ply lumber product being produced.  
         [0098]    In the case of plywood, it is desirable to lay-up multiple veneer sheets  22  directly on top of one another. During the lay-up of a plywood product, therefore, mobile frame assembly  506  remains at a single transverse location as successive veneer sheets  22  drop from dual tablet lay-up device  100  and are layed up onto tines  508  (or onto preceding veneer sheets  22 ). As each successive veneer sheet  22  is layed up, hydraulic cylinder  512  is controllably actuated to incrementally lower tines  508 . Alternatively, hydraulic cylinder  512  may be controllably actuated to incrementally lower tines  508  on an intermittent basis. In this manner, lay-up carriage  500  cooperates with dual tablet lay-up device  100  to maintain the precise alignment of each successive veneer sheet  22 . When the desired number of plies (i.e. sheets  22 ) has been layed up onto tines  508 , hydraulic cylinder  512  and drive motor  510  are respectively actuated to move the stack of veneer sheets (not shown) downwardly away from, and transversely out from underneath, dual tablet lay-up device  100 . Spaced apart tine members  508  allow the layed up stacks of veneer to be easily transported off of lay-up carriage  500  for further processing by forklift or other means.  
         [0099]    In the case of LVL, it is desirable to lay-up successive veneer sheets  22  that are transversely offset by a predetermined amount with respect to one another. Such inter-ply displacement may be used to form butt joints between two or more pieces of LVL. To fabricate LVL, drive motor  510  is controllably actuated between each successive veneer sheet  22 , such that mobile frame assembly  506  (and hence tines  508 ) move incrementally in a transverse direction with respect to dual tablet lay-up device  100 . Hydraulic cylinder  512  is also controllably actuated to incrementally lower tines  508  between each successive veneer sheet  22 . Alternatively, hydraulic cylinder  512  may be controllably actuated to incrementally lower tines  508  on an intermittent basis. In this manner, lay-up carriage  500  cooperates with dual tablet lay-up device  100  to align the transverse edge  22 A of each successive veneer sheet  22  in a position which is transversely offset from that of the immediately preceding veneer sheet  22 . When the desired number of plies (i.e. sheets  22 ) has been layed up onto tines  508 , hydraulic cylinder  512  and drive motor  510  are respectively actuated to move the stack of veneer sheets (not shown) downwardly away from, and transversely out from underneath, dual tablet lay-up device  100 , such that the stack of sheets may be removed from lay-up carriage  500  by forklift or other means.  
         [0100]    As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example:  
         [0101]    When dual tablet lay-up device  100  is in its closed configuration (FIG. 5), rearward conveyor belts  112  interleave with forward conveyor belts  212 . Such an interleaved configuration is not a necessary aspect of the invention. Each rearward conveyor belt  112  may extend equidistantly in the forward direction and each forward conveyor belt  212  may extend equidistantly in the rearward direction.  
         [0102]    Rearward frame member  106  and forward frame member  206  may be integral with one another. Similarly, rearward frame member  108  and forward frame member  208  may be integral with one another.  
         [0103]    The actuation mechanisms of forward conveyor belt mechanism  260 , forward translation mechanism  232 , rearward conveyor belt mechanism  160  and rearward translation mechanism  132  are depicted as pulleys and belts. Other mechanisms, such as gears and/or chains may be used. In addition, any type of controllable actuators may be used for these mechanisms, including without limitation: electric motors, combustion engines, hydraulic motors, hydraulic cylinders and the like.  
         [0104]    Optical sensor  328  may generally be any type of edge-detect sensor capable detecting a forward edge  22 B of veneer sheets  22  at precise locations. For example, acoustic sensors, imaging sensors, mechanical edge detectors and/or capacitive edge detectors may be used in place of optical sensor  328 .  
         [0105]    Inductive proximity detectors  332 ,  334 ,  336 ,  338 ,  340 ,  342 ,  344 ,  346  may generally be implemented using any type of proximity or distance detectors. For example, acoustic, infra red or imaging sensors may be used in place of inductive proximity detectors  332 ,  334 ,  336 ,  338 ,  340 ,  342 ,  344 ,  346 .  
         [0106]    Other types of sensors could replace proximity detectors  332 ,  334 ,  336 ,  338 ,  340 ,  342 ,  344 ,  346  to provide the feedback required to control the motion of rearward and forward tablet mechanisms  132 ,  232 . For example, encoders coupled to the shafts of motors  134 ,  234  could also be used as feedback devices to control the motion of rearward and forward tablet mechanisms  132 ,  232 .  
         [0107]    Controller  302  may use any of a wide variety of well known control algorithms to effect the speed and/or position control of conveyor belt drive motors  126 ,  226 , conveyor belt assemblies  160 ,  260 , rearward and forward tablet drive motors  134 ,  234  and rearward and forward tablet assemblies  132 ,  232 .  
         [0108]    Input conveyor  20  and lay-up carriage  500  are not essential to the invention. In general, dual tablet lay-up device  100  may receive veneer sheets  22  from any source and lay-up veneer sheets  22  onto any surface. Preferably, the surface onto which veneer sheets  22  are layed up is substantially horizontal.  
         [0109]    When laying up veneer sheets for the production of plywood, lay-up carriage  500  may be implemented as a stationary hoist. Such a hoist is not required to move transversely. Preferably, however, the hoist may be incrementally lowered between the lay-up of successive veneer sheets  22 . Alternatively, the hoist may be intermittently lowered.  
         [0110]    To fabricate LVL according to the embodiment of FIG. 12, lay-up carriage  500  is described as moving transversely by a predetermined amount between successive veneer sheets  22 . Lay-up carriage  500  may alternatively be constructed to move longitudinally by a predetermined amount between successive veneer sheets  22 , so as to create LVL wherein successive plies are longitudinally offset from one another.  
         [0111]    Vertical movement of fork assembly  534  is not required. Tines  538  may be positioned at a suitable height (relative to dual tablet lay-up device  100 ) that a sufficient number of veneer sheets  22  can be layed up onto one another without interfering with the operation of dual tablet lay-up device  100 .  
         [0112]    Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.