Abstract:
A seed delivery apparatus and methods in which a seed conveyor delivers seed from a metering device to a furrow in a controlled manner to maintain seed placement accuracy within the furrow.

Description:
BACKGROUND 
       [0001]    In recent years, the agricultural industry has recognized the need to perform planting operations more quickly due to the limited time during which such planting operations are agronomically preferable or (in some growing seasons) even possible due to inclement weather. However, drawing a planting implement through the field at faster speeds increases the speed of deposited seeds relative to the ground, causing seeds to roll and bounce upon landing in the trench and resulting in inconsistent plant spacing. The adverse agronomic effects of poor seed placement and inconsistent plant spacing are well known in the art. 
         [0002]    As such, there is a need for apparatus, systems and methods of effectively delivering seed to the trench while maintaining seed placement accuracy at both low and high implement speeds. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]      FIG. 1  is a side elevation view of a prior art row unit of an agricultural row crop planter. 
           [0004]      FIG. 2A  is a side elevation view of an embodiment of a seed conveyor in cooperation with a seed disc. 
           [0005]      FIG. 2B  is a partial side elevation view of an embodiment of a seed conveyor in cooperation with a seed disc. 
           [0006]      FIG. 2C  is a partial side elevation view of an embodiment of a seed conveyor depositing seeds in a seed trench. 
           [0007]      FIG. 2D  is a side elevation view of an embodiment of a seed conveyor in cooperation with a seed disc. 
           [0008]      FIG. 2E  is a side elevation view of an embodiment of a seed conveyor in cooperation with a seed disc. 
           [0009]      FIG. 3  is a partial side elevation view of an embodiment of a seed conveyor in cooperation with a seed disc. 
           [0010]      FIG. 4A  is a side elevation view of an embodiment of a seed conveyor in cooperation with a seed disc. 
           [0011]      FIG. 4B  is a partial side elevation view of an embodiment of a seed conveyor in cooperation with a seed disc. 
           [0012]      FIG. 4C  is a partial side elevation view of an embodiment of a seed conveyor in cooperation with a seed disc. 
           [0013]      FIG. 5A  is a partial side elevation view of an embodiment of a seed conveyor in cooperation with an embodiment of a seed sensor. 
           [0014]      FIG. 5B  is a partial front elevation view of an embodiment of a seed conveyor in cooperation with an embodiment of a seed sensor. 
           [0015]      FIG. 5C  is a side elevation view of an embodiment of a seed conveyor. 
           [0016]      FIG. 5D  is a partial side elevation view of an embodiment of a seed conveyor in cooperation with an embodiment of a seed sensor. 
           [0017]      FIG. 5E  is a view of an embodiment of a seed sensor in cooperation with an embodiment of a seed conveyor along section  5 E- 5 E of  FIG. 5D . 
           [0018]      FIG. 5F  is a partial side elevation view of an embodiment of a seed conveyor in cooperation with an embodiment of a seed sensor and a seed disc. 
           [0019]      FIG. 6A  is a partial side elevation view of a seed disc in cooperation with an embodiment of a seed sensor in cooperation with an embodiment of a seed disc and an embodiment of a seed conveyor. 
           [0020]      FIG. 6B  is a partial front elevation view of an embodiment of a seed disc in cooperation with an embodiment of a seed sensor. 
           [0021]      FIG. 6C  is a partial front elevation view of an embodiment of a seed disc in cooperation with an embodiment of a seed sensor. 
           [0022]      FIG. 7A  is a partial side elevation view of an embodiment of a seed conveyor in cooperation with an embodiment of a seed sensor. 
           [0023]      FIG. 7B  is a partial front elevation view of an embodiment of a seed sensor in cooperation with an embodiment of a seed conveyor. 
           [0024]      FIG. 8A  is a schematic illustration of an embodiment of a seed conveyor control system. 
           [0025]      FIG. 8B  illustrates an embodiment of a seed conveyor control system. 
           [0026]      FIG. 9A  illustrates an embodiment of a process for controlling a seed conveyor. 
           [0027]      FIG. 9B  is a top view of a tractor in cooperation with an embodiment of a planter. 
           [0028]      FIG. 9C  is a top view of a tractor in cooperation with an embodiment of a planter. 
           [0029]      FIG. 9D  illustrates an embodiment of a process for determining a local speed along a toolbar. 
           [0030]      FIG. 9E  illustrates a calibration curve for controlling a seed conveyor. 
           [0031]      FIG. 10A  illustrates an embodiment of a process for controlling a seed conveyor. 
           [0032]      FIG. 10B  is a side elevation view of an embodiment of a seed conveyor traversing a field. 
           [0033]      FIG. 10C  illustrates an embodiment of a process for controlling a seed conveyor. 
           [0034]      FIG. 10D  is a side elevation view of an embodiment of a seed conveyor traversing a field. 
           [0035]      FIG. 11A  is a side elevation view of an embodiment of a planter row unit in cooperation with an embodiment of a seed conveyor. 
           [0036]      FIG. 11B  is a perspective view of a seed conveyor in cooperation with an embodiment of a seed meter. 
           [0037]      FIG. 11C  is a perspective view of a seed conveyor in cooperation with an embodiment of a seed meter. 
           [0038]      FIG. 11D  is a front elevation view of an embodiment of a seed conveyor in cooperation with an embodiment of a seed disc. 
           [0039]      FIG. 11E  is a side elevation view of an embodiment of a seed conveyor in cooperation with an embodiment of a seed disc. 
           [0040]      FIG. 12A  is a side elevation view of another embodiment of a seed conveyor with certain components removed for clarity. 
           [0041]      FIG. 12B  is a side perspective view of the seed conveyor of  FIG. 12A  with certain components removed for clarity. 
           [0042]      FIG. 12C  is a cross-sectional view of the seed conveyor of  FIG. 12A  in communication with an embodiment of a seed disc. 
           [0043]      FIG. 12D  is a cross-sectional view of the seed conveyor of  FIG. 12A  in communication with another embodiment of a seed disc. 
           [0044]      FIG. 12E  is a perspective cross-sectional view of the seed conveyor of  FIG. 12A  in communication with the seed disc of  FIG. 12C . 
           [0045]      FIG. 12F  is a perspective view of the seed conveyor of  FIG. 12A  with certain components removed for clarity. 
           [0046]      FIG. 12G  is a left side elevation view of the seed conveyor of  FIG. 12A  with certain components removed for clarity. 
           [0047]      FIG. 12H  is a right side elevation view of the seed conveyor of  FIG. 12A  with certain components removed for clarity. 
           [0048]      FIG. 12I  is a perspective view of the gearbox of the seed conveyor of  FIG. 12A . 
           [0049]      FIG. 12J  is a partial right elevation view of the seed conveyor of  FIG. 12A  in communication with an embodiment of a seed meter. 
           [0050]      FIG. 12K  is a partial right perspective view of the seed conveyor of  FIG. 12A  in communication with the seed meter of  FIG. 12J . 
           [0051]      FIG. 12L  is a partial left elevation view of the seed conveyor of  FIG. 12A  in communication with the seed meter of  FIG. 12J , with certain components removed for clarity. 
           [0052]      FIG. 12M  is a cross-sectional view of the seed conveyor of  FIG. 12A  in communication with the seed disc of  FIG. 12C . 
           [0053]      FIG. 13  is partial side elevation view of a row unit shank supporting the seed conveyor of  FIG. 12A . 
           [0054]      FIG. 14  is a partial side elevation view still another embodiment of a seed conveyor including a loading wheel. 
           [0055]      FIG. 15  illustrates a process for operating a seed conveyor having loading wheels. 
       
    
    
     DESCRIPTION 
       [0056]    Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views,  FIG. 1  illustrates a side elevation view of a single row unit  10  of a conventional row crop planter such as the type disclosed in U.S. Pat. No. 7,438,006, the disclosure of which is hereby incorporated herein in its entirety by reference. As is well known in the art, the row units  10  are mounted in spaced relation along the length of a transverse toolbar  12  by a parallel linkage  14 , comprised of upper and lower parallel arms  16 ,  18  pivotally mounted at their forward ends to the transverse toolbar  12  and at their rearward end to the row unit frame  20 . The parallel linkage  14  permits each row unit  10  to move vertically independently of the toolbar  12  and the other spaced row units in order to accommodate changes in terrain or rocks or other obstructions encountered by the row unit as the planter is drawn through the field. 
         [0057]    The row unit frame  20  operably supports a seed hopper  23  which may be adapted to receive seed from a bulk hopper (not shown), a seed meter  26  and a seed tube  28  as well as a furrow opener assembly  30  and furrow closing assembly  40 . The furrow opening assembly  30  comprises a pair of furrow opener discs  32  and a pair of gauge wheels  34 . The gauge wheels  34  are pivotally secured to the row unit frame  20  by gauge wheel arms  36 . A coil spring  50  is disposed between the parallel arms  16 ,  18  to provide supplemental downforce to ensure that the furrow opener discs  32  fully penetrate the soil to the desired depth as set by a depth adjusting member (not shown) and to provide soil compaction for proper furrow formation. Rather than a coil spring, supplemental downforce may be provided by actuators or other suitable means such as disclosed in U.S. Pat. No. 6,389,999 to Duello, the entire disclosure of which is hereby incorporated herein by reference. 
         [0058]    In operation, as the row unit  10  is lowered to the planting position, the opener discs  32  penetrate into the soil. At the same time, the soil forces the gauge wheels  34  to pivot upwardly until the gauge wheel arms  36  abut or come into contact with the stop position previously set with the furrow depth adjusting member (not shown) or until a static load balance is achieved between the vertical load of the row unit and the reaction of the soil. As the planter is drawn forwardly in the direction indicated by arrow  39 , the furrow opener discs cut a V-shaped furrow  60  into the soil while the gauge wheels  34  compact the soil to aid in formation of the V-shaped furrow. Individual seeds  62  from the seed hopper  23  are dispensed by the seed meter  26  into an upper opening in the seed tube  28  in uniformly spaced increments. As seeds  62  fall through the seed tube  28 , the seeds move downwardly and rearwardly between the furrow opener discs  32  and into the bottom of the V-shaped furrow  60 . The furrow  60  is then covered with soil and lightly compacted by the furrow closing assembly  40 . 
         [0059]    It should be appreciated that because seeds  62  fall freely through the seed tube  28  in the row unit  10  described above, the path of travel of the seeds and the velocity of the seeds at the exit of the seed tube are relatively unconstrained. It would be preferable to constrain the path of travel of seeds  62  in order to reduce errors in spacing between seeds; i.e., placing seeds in the field at non-uniform spacing. Additionally, it would be preferable to control the velocity of seeds  62  such that the seeds have a decreased horizontal velocity relative to the ground upon landing in the furrow  60 . 
         [0060]    A seed conveyor  100  is illustrated in  FIG. 2A . The seed conveyor  100  includes a belt  140  stretched around upper and lower pulleys  152 , 154  and preferably driven by the upper pulley  152 ; in other embodiments the seed conveyor may be driven by the lower pulley  154 . The belt  140  includes flights  142 . The seed conveyor  100  additionally includes a guide surface  110  disposed adjacent to the flights  142  on one side of the seed conveyor. The seed conveyor  100  preferably includes a backing plate  130  disposed to maintain the position of belt  140 . 
         [0061]    In operation, the seed conveyor  100  receives seeds  62  from a seed disc  50  and conveys them to an exit  164 . The seed disc  50  is preferably housed in a seed meter  26  similar to that illustrated in  FIG. 1  and rotates in a direction indicated by arrow  56  about a shaft  54  rotatably mounted in the seed meter. Turning to  FIG. 2B , the seed meter  26  is preferably of the vacuum type as is known in the art, such that a vacuum source (not shown) creates a vacuum behind the seed disc  50  (on the perspective of  FIG. 2B ), thus creating a pressure differential across apertures  52  in the disc. As the apertures  52  rotate past a pool of seeds in the location generally indicated by reference numeral  58 , the pressure differential causes individual seeds  62  to become entrained on each aperture  52  such that the seeds are carried by the disc as illustrated. As the apertures cross a boundary such as axis  196 , preferably at approximately the 3 o&#39;clock position of the seed disc  50 , the vacuum source is substantially cut off (e.g., by termination of a vacuum seal as is known in the art) such that the seeds  62  are released from the disc as they cross axis  196 . Seeds  62  preferably fall from the disc in a substantially vertical fashion along an axis  192 . Guide surface  110  includes an angled portion  112 , along which each seed  62  slides downward and rearward before passing between two flights  142  at a seed inlet generally indicated by reference numeral  162 . Each seed  62  is then conveyed downward by seed conveyor  100 . 
         [0062]    The belt  142  is preferably driven at a speed proportional to the groundspeed St ( FIG. 2C ) of the row unit  10 . For example, in some embodiments the seed conveyor  100  is driven such that the linear speed of belt  142  at the bottom of the lower pulley  154  is approximately equal to the groundspeed St. 
         [0063]    As illustrated in  FIG. 2B , each seed  62  is initially accelerated downward by the flight  142  above the seed. Turning to  FIG. 2C , as each seed  62  moves downward along the seed conveyor  100 , it may fall away from the flight  142  above it. However, as each seed  62  nears the bottom of the seed conveyor, the flights  142  accelerate in order to travel around lower pulley  154  such that the flights  142  contact the seed and impart a rearward horizontal velocity to the seed. Additionally, an angled portion  114  of the guide surface  110  guides the seed rearward, imparting a rearward horizontal velocity to the seed. Thus, as the seed  62  exits the seed conveyor at a seed exit generally indicated by reference numeral  164 , the seed has a downward vertical velocity component Vy and a horizontal velocity component Vx, the magnitude of which is less than the speed of travel St of the row unit  10 . It should be appreciated that a smaller horizontal velocity component Vx is preferable because the seed  62  will experience less fore-aft roll as it lands in the furrow  60 , leading to more uniform seed placement. The angled portion  114  preferably is disposed 20 degrees below horizontal. 
         [0064]    Returning to  FIG. 2B , it should be appreciated that flights  142  travel faster as they travel around the upper end of upper pulley  152 , e.g., above an axis  194 . Additionally, the flights  142  have a substantial horizontal velocity component above axis  194 . As a result, attempting to introduce seeds  62  between the flights above axis  194  may result in seeds being knocked away from the belt  140 . Thus, the seed inlet  162  at which seeds  62  pass between flights  142  is preferably below the axis  194 . This result is preferably accomplished by positioning of the axis  196  at which seeds are released from the disc  50  below the axis  194  and/or by configuring angled portion  112  of guide surface such that seeds  62  slide below axis  194  before entering the inlet  162 . 
         [0065]    Turning to the embodiment of  FIGS. 11A-11E , a seed conveyor  100  is illustrated in cooperation with a row unit  10 . The row unit  10  includes a shank portion  35 . Referring to  FIG. 11A , the seed conveyor  100  is mounted to the shank portion  35  by attachment ears  106 , 108 . Turning to  FIG. 11B , the seed conveyor  100  includes sidewalls  82 ,  84 . A conveyor motor assembly  1022  is mounted to the sidewall  82 . The conveyor motor assembly includes a conveyor motor  1020 . The conveyor motor drives an output shaft  1026 . The output shaft  1026  preferably drives the input shaft  1024 ; in some embodiments the output shaft is coupled to an input shaft by a drive belt (not shown), while in other embodiments the output shaft and input shaft may be operably coupled by one or more gears. The input shaft  1024  is operably coupled to the upper pulley  152  of the seed conveyor  100 . Turning to  FIG. 11C , the seed conveyor is shown with guide surface  110  removed for clarity, revealing the flights  142 . Turning to  FIG. 11D , the seed conveyor  100  is preferably disposed transversely adjacent the seed disc  50 . Turning to  FIG. 11E , the seed conveyor  100  is disposed to receive seeds  62  released from the seed disc  50  onto the angled portion  112  of the seed guide  110  ( FIG. 11B ). In operation, seeds  62  are released from the surface of the seed disc  50  at approximately the three o&#39;clock position. Seeds  62  slide along the angled portion  112  of the seed guide  110  between the flights  142  of the belt  140 . 
         [0066]    As illustrated  FIG. 2D , the orientation of the seed conveyor  100  with respect to the seed meter  50  may be varied. In the embodiment of  FIG. 2D , the orientation of the seed conveyor  100  has been reversed from that illustrated in  FIG. 2A , reducing the space claim of the combination. In such alternative embodiments, seeds are preferably discharged from the seed conveyor  100  in a direction opposite to the direction of travel  39 . Additionally, the seed conveyor  100  is preferably positioned to receive seeds from the seed meter  50 . 
         [0067]    In the embodiment of  FIG. 2E , the seed conveyor includes an unconstrained belt region  147 . The unconstrained belt region  147  is preferably located adjacent the seed guide  110 . The unconstrained belt region  147  is preferably located between the seed inlet  162  and the seed exit  164 . As the belt  140  travels through the unconstrained belt region  147 , the belt is free to undergo small fore-aft deflections (to the right and left on the perspective of  FIG. 2E ). It should be appreciated that in the embodiment of  FIG. 2E , the backing plate is preferably omitted or located at a predetermined aft (rightward on the perspective of  FIG. 2E ) distance from the seed guide  110  to allow the belt  140  to undergo fore-aft deflections. 
         [0068]    In an alternative embodiment illustrated in  FIG. 3 , a modified seed conveyor  200  includes a belt  240  having modified flights  242  having bevels  244 . As the belt  240  moves past a seed inlet  262 , seeds  62  are more easily introduced between the flights  242  because a larger vertical gap exists between flights at the seed inlet due to the bevels  244 . Similar to the embodiment of  FIGS. 2A-2C , a gap  118  between the guide surface and the belt is preferably of a predetermined size large enough to allow consistent clearance between the guide surface and the belt, but small enough to prevent seeds  62  from escaping from between flights. 
         [0069]    In an alternative embodiment illustrated in  FIG. 4A-4B , a modified seed conveyor  300  includes a modified belt  340  without flights. Referring to  FIG. 4A , the belt  340  is disposed adjacent a modified guide surface  310 . Backing plates  330 , 332  preferably retain the desired position of the belt  340 . Turning to  FIG. 4B , the belt  340  preferably includes roughness elements  344  such that the outer surface of the belt has a relatively high effective coefficient of friction. Guide surface  310  includes an inner face  314  which is smooth (i.e., has a relatively low coefficient of friction) and is preferably substantially free from burrs, warping, and other surface imperfections. Thus, as seeds  62  are released from the seed disc  50  and into a modified seed inlet generally indicated by reference numeral  362 , the seeds are drawn between the belt  340  and the guide surface and held static with respect to the belt while sliding downward along the guide surface  314 . 
         [0070]    In some embodiments, the seed conveyor  300  of  FIGS. 4A-4B  is modified as illustrated in  FIG. 4C . The modified seed conveyor  300 ′ includes a modified guide surface  310 ′ having an angled portion  312 ′. In a preferred embodiment, the conveyor  300 ′ is disposed with respect to the seed disc  50  such that angled portion  312 ′ is adjacent to the axis  196  at which seeds  62  are released from the disc  50  (by vacuum cut-off as discussed elsewhere herein). Thus as each seed  62  is released from the disc  50 , the seed is pulled between the angled portion  312 ′ and the belt  340 . The belt  340  then continues to draw the seed  62  downward against a smooth interior face of the guide surface  310 ′ and discharged as in the embodiment of  FIGS. 4A-4B . Thus the guide surface  310 ′ cooperates with the belt  340  to pull seeds  62  from the disc  50  at approximately the same time that each seed is released from the disc. In alternative embodiments, the angled surface  312 ′ is disposed just above the axis  196  such that the guide surface and belt begin to pull each seed from the disc just before the seed is released from the disc. In other embodiments, the angled surface  312 ′ may be disposed just below the axis  196  such that the guide surface and belt catch each seed just after the seed is released from the disc. In still other embodiments, the seed conveyor  300 ′ may be located farther frontward or rearward (to the right or left as viewed in  FIG. 4C ) such that seeds  62  are pushed from the apertures  52  by contact with either the belt  340  or with the angled surface  312 ′. 
       Seed Sensing 
       [0071]    As described further herein, the seed conveyor embodiments described above are preferably provided with seed sensors for detecting the time at which each seed  62  passes known locations. 
         [0072]    Turning to  FIG. 5A , a bottom portion of a seed conveyor  400  similar to the seed conveyor  100  of  FIG. 2  is illustrated. The seed conveyor  400  includes a guide surface  130  having an opening  490 . A seed sensor  500  is mounted to guide surface  130 . The seed sensor  500  may include an optical sensor  510  disposed to detect light passing through a sensing region  495  between the flights. It should be appreciated that the height of measuring region  495  is less than or equal to the height of opening  490 . The height of measuring region  495  is preferably greater than the height of the flights and less than the gap between the flights. The optical sensor  510  may additionally include a light source such as an LED for providing light waves to be reflected by the belt for detection by the sensor. Alternatively, a separate light source (not shown) may be disposed behind the belt (to the right in the perspective of  FIG. 5A ) so as to transmit light through apertures (not shown) in the belt toward sensor  510 . In any case, the sensor  500  generates a signal which changes due to the presence of a seed  62  in measuring region  495 . 
         [0073]    Turning to  FIG. 5B , a central portion of a seed conveyor  450  similar to the seed conveyor  400  is viewed from the front (from the left in the perspective of  FIG. 5A ), with the guide surface not shown for clarity. The seed conveyor  450  includes sidewalls  482 , 484  that cooperate with the guide surface to enclose the belt and flights  142 . Sidewalls  482 , 484  include openings  452 , 454  respectively, which openings are preferably aligned along a horizontal axis. A seed sensor  550  includes a transmitter  520  mounted to sidewall  484  and a receiver  515  mounted to sidewall  482 . In some embodiments, the seed sensor  550  is an optical sensor. The transmitter  520  is disposed to transmit light through aperture  454 , through a sensing region  497 , and through aperture  452 . The receiver  515  is disposed to detect light transmitted through the sensing region  497  and aperture  452 . The height of sensing region  497  is preferably equal to the height of apertures  452 , 454 . The height of sensing region  497  is preferably greater than the height of flights  142  and less than the vertical spacing between the flights. The depth (on the perspective of  FIG. 5B ) of sensing region  497  is preferably the same as the depth of flights  142 . The sensor  550  generates a signal which changes due to the presence of a seed  62  in measuring region  497 . 
         [0074]    Turning to  FIG. 5C , it should be appreciated in light of this disclosure that in either of the seed conveyor embodiments  400 , 450 , the vertical location of the seed sensors  500 , 550  may be selected in order to select the location of each seed  62  relative to the flights  142  at the point where the seed is detected. 
         [0075]    In order to detect seeds while the seeds are positively constrained against a flight  142 , the seed sensor are preferably placed along an upper portion of the belt in a zone A ( FIG. 5C ). In zone A, each seed  62  is in contact with the flight above the seed until the seed is accelerated by gravity to a speed in excess of the belt speed. To achieve a similar result, in other embodiments, the seed sensor is placed in a zone C, in which the flights have accelerated and again push the seeds along the seed path. 
         [0076]    Alternatively, in order to detect the seed when it is separated from the flights  142 , the sensor is preferably located in a zone B. In zone B, the seed has been accelerated by gravity to a speed faster than the belt speed and separated from the flight above it, but has not yet contacted the flight below. 
         [0077]    In other embodiments, the seed conveyor may incorporate an electromagnetic seed sensor. In one such embodiment, referring to  FIG. 5D , a seed conveyor  150  includes a seed guide  187  incorporating an electromagnetic seed sensor  800 . In such embodiments, seeds  62  slide along an inner face  164  of the seed guide  187 , passing through a sensor arc  810  before exiting the seed conveyor  150 . Turning to  FIG. 5E , which illustrates the electromagnetic seed sensor  800  along the section  5 E- 5 E of  FIG. 5D , the sensor arc  810  houses an electromagnetic energy transmitter  822  and a receiver  824 . A circuit board  830  and associated circuitry is housed in the seed guide  187 . The circuit board  830  is in electrical communication with the transmitter and receiver  822 , 824 . The transmitter  822  generates electromagnetic energy which crosses a sensing region  850  within the sensor arc  810 . The detector  824  generates a signal related to a characteristic of the electromagnetic energy received from the transmitter  822 . As each seed  62  passes through the sensing region  850 , a characteristic of the electromagnetic energy transmitted to the detector  824  is modified such that the signal generated by the detector is likewise modified. The seed sensor  800  may be substantially similar to any of the electromagnetic seed sensors disclosed in Applicant&#39;s U.S. patent application Ser. No. 12/984,263, the disclosure of which is hereby incorporated herein in its entirety by reference. 
         [0078]    In other embodiments, turning to  FIG. 5F , a similar electromagnetic seed sensor  800  is mounted to the angled portion  112  of the seed conveyor  100 . In such embodiments, seeds  62  pass through the sensor arc  810  after being released from the seed meter  50  and before entering between flights  142  of the seed conveyor. It should be appreciated that in various embodiments, the sensor arc  810  may be positioned such that seeds  62  pass through the sensor arc either before or after contacting the angled portion  112 . In other embodiments, an optical sensor may be disposed to sense the passage of seeds in the same location as the sensor arc  810  of  FIG. 5F . 
         [0079]    Turning to  FIG. 6A , an additional seed sensor  600  may be used to detect the presence of seeds  62  on the disk  50 . The seed sensor  600  is preferably disposed to detect passing seeds  62  on the surface of the disc. The seed sensor  600  may comprise an optical transmitter  610  configured to emit light to an optical receiver  620 , which is preferably configured to produce a signal related to the amount of light received from transmitter  610 . The transmitter and receiver  610 , 620  are preferably mounted to a seed meter housing  20  of the seed meter  26  enclosing the seed disc  50 . As illustrated in  FIG. 6A , the transmitter and receiver  610 , 620  are preferably disposed below and above the seed path, respectively, such that passing seeds cause a light interruption and modify the signal produced by the receiver  620 . Thus when a seed is not present on an aperture  52  (e.g., aperture  52   a ), the receiver  620  produces a modified signal. It should be appreciated in light of this disclosure that where a seed stripper or singulator  22  is incorporated in the seed meter  26  in order to remove excess seeds from apertures  52 , such devices may occasionally “strip” an aperture such that no seed is carried to the seed conveyor  100 . Thus the seed sensor  600  is preferably disposed downstream along the seed path with respect to the singulator  22 . 
         [0080]    In other embodiments, as illustrated in  FIGS. 6B and 6C , a transverse seed sensor  700  preferably comprises a transmitter  710  and receiver  720  disposed to transmit and receive light across the apertures  52  in a transverse direction, such that light from transmitter  710  is transmitted to the receiver  720  if no seed is present on the aperture (e.g., aperture  52   a ). In such an embodiment, the receiver  720  receives light and emits a modified signal when a “skip” (i.e., a failure to load or retain at least one seed on the disk) occurs. 
         [0081]    A transverse seed sensor may also be incorporated in the seed conveyor  300  of  FIGS. 4A-4B . Referring to  FIG. 7A , a seed sensor  900  is incorporated into a modified seed conveyor  350 . The seed sensor  900  is transversely disposed to detect the passage of seeds through a sensing region  997  between the belt  340  and an inner face  354  of the seed conveyor  350 . Turning to  FIG. 7B , the seed conveyor  350  includes spaced-apart transverse sidewalls  382 , 384 . The sidewalls  382 , 384  include apertures  352 , 354 , respectively. A transmitter  910  is mounted to sidewall  382 . Transmitter  910  is configured to transmit light (or other electromagnetic energy) through the aperture  352 , through the sensing region  997 , and through the aperture  354 . A receiver  920  is mounted to sidewall  384 . Receiver  920  is configured to generate a signal which changes due to the presence of a seed in measuring region  997 . 
       Loading Wheel Seed Conveyor Embodiments 
       [0082]    Turning to  FIGS. 12A-13 , a seed conveyor  1200  including loading wheels is illustrated. Referring to  FIGS. 12A and 12B , the seed conveyor  1200  includes a housing  1210  in which a first loading wheel  1202  and a second loading wheel  1204  are rotatably supported by the meter housing  1210 , preferably above the apex of the belt  140 . The loading wheels are preferably driven to rotate as described later herein; on the view of  FIG. 12A , loading wheel  1202  preferably rotates in the clockwise direction and loading wheel  1204  preferably rotates in the counter-clockwise direction. The loading wheels  1202 ,  1204  are preferably spaced to leave a gap  1201  between the loading wheels, preferably above the apex of the belt  140 . The gap  1201  is preferably sized to permit seeds to pass through with a small amount of compression of each loading wheel, such that a seed placed in the gap is positively constrained by the loading wheels  1202 ,  1204 . The gap is preferably 0.01 inches wide for seed conveyors used to plant corn and soybeans. The loading wheel  1202  preferably includes vanes  1207  and the loading wheel  1204  preferably includes vanes  1209 . The loading wheels  1202 ,  1204  are preferably made of a material having relatively low compressibility. In some embodiments, the loading wheels  1202 ,  1204  are made of polyurethane. It should be appreciated that the vanes in each loading wheel make the loading wheel more compressible than a solid piece of relatively incompressible material such that the loading wheels may be compressed to receive seeds in the gap  1201 . In other embodiments each loading wheel is comprised of a solid annular or cylindrical piece of a more compressible material; such embodiments are not preferred because more compressible materials tend to wear more quickly from repeated engagement of seeds. As illustrated, the loading wheels  1202 ,  1204  preferably include roughness elements (e.g., ribbing) disposed substantially around the perimeters of the loading wheels. 
         [0083]    Referring to  FIG. 12C , the seed conveyor  1200  is illustrated in communication with a seed disc  50  having a single radial array of seed apertures  52 . The seed conveyor  1200  is preferably disposed adjacent the seed disc  50 . In operation, as described elsewhere herein, the seed apertures pick up seeds  62  from a seed pool  58  located at approximately the six o&#39;clock position on the view of  FIG. 12C  and are carried in a clockwise seed path. As the seeds  62  approach the housing  1210 , they preferably pass through a notch in a brush  1230  disposed to contact and clean the seed disc and then enter the housing  1210 . 
         [0084]    Referring to  FIGS. 12C and 12E , seeds  62  preferably enter the housing  1210  through a throat  1215  defined by a lower surface  1206  and an upper surface  1211 . The upper surface  1211  preferably comprises a lower surface of an insert  1208  removably attached (e.g., by screws as illustrated herein) to the housing  1210 . It should be appreciated that the upper surface  1211  is preferably part of a removable insert because frequent repeated contact with seeds  62  may cause appreciable wear depending on the material used to form the upper surface  1211 . The upper surface  1211  is preferably normal to the surface of the seed disc  50 . The upper surface  1211  preferably includes a curvilinear portion  1281  concentric with the seed apertures  52  and a subsequent curvilinear portion  1283  along which the upper surface  1211  curves continuously from concentricity with the seed apertures  52  to become approximately tangential with the outer perimeter of the loading wheel  1202 . The surface  1211  preferably terminates adjacent to the gap  1201 . Turning to  FIG. 12M , the seed apertures  52  define an outer radius Ro, a median radius Rm and an inner radius Ri from the center of the seed disc  50 . The curvilinear portions  1281  and  1283  preferably have radii between Ro and Rm. The curvilinear portion  1283  preferably has a radius approaching Rm toward the terminal end of the upper surface  1211 . The lower surface  1206  preferably has a radius less than Ri. In operation, each seed  62  is preferably dislodged inwardly from the seed aperture  52  by contact with the curvilinear portion  1281  but preferably remains entrained on the seed aperture while in contact with the curvilinear portion  1281 . The seed  62  is further dislodged inwardly from the seed aperture  52  by contact with the curvilinear portion  1283 . 
         [0085]    Turning to  FIG. 12D , the seed conveyor  1200  is illustrated in communication with a seed disc  51  having an array of inner seed apertures  52   i  arranged concentrically with an array of outer seed apertures  52   o . Those skilled in the art will recognize that such discs are conventionally used to plant soybeans and other crops. The seed conveyor  1200  is preferably configured to partially dislodge seeds from both aperture arrays and subsequently constrain or “pinch” them between the loading wheels. For example, the loading wheel  1204  is disposed to intersect the path of the array of inner seed apertures  52   i  such that the loading wheel  1204  urges seeds from the inner seed apertures toward the gap  1201 . As illustrated, the upper surface  1211  is preferably disposed similarly with respect to the outer seed apertures  52   o  as described herein with respect to the apertures  52  in  FIG. 12M . 
         [0086]    Returning to  FIG. 12C , after the seeds  62  pass the curvilinear portion  1283 , they enter the gap  1201  between the loading wheels  1202 ,  1204 . The loading wheels  1202 ,  1204  are slightly compressed by the introduction of each seed into the gap  1201  such that the wheels positively constrain the seed in the gap. The vacuum seal imposing a vacuum on the apertures  52  preferably terminates adjacent to the gap  1201  at an axis  196 ′ such that seeds  62  are released from the disc  50  just before entering the gap. Due to the rotation of the loading wheels, the seed  62  is then ejected downward toward the belt. 
         [0087]    Returning to  FIG. 12A , seeds  62  ejected by the loading wheels  1202 ,  1204  travel along a nominal seed path Ps which is tangential to both of the loading wheels. Seeds ejected by the loading wheels  1202 ,  1204  preferably freefall along the seed path Ps under the influence of gravity and the velocity imparted on the seeds by ejection from the loading wheels  1202 ,  1204 . Seed traveling along seed path Ps preferably enters between flights of the belt  240  forward (to the left on the view of  FIG. 12A ) of a plane Ad dividing the ascending and descending portions of the belt. Thus the seed path Ps intersects a descending portion of the belt  240 . 
         [0088]    Returning to  FIG. 12C , seeds  62  enter the belt  52  between flights  242  and pass by a surface  1225 , which preferably comprises a surface of an insert removably attachable (e.g., by screws as illustrated) to the housing. The surface  1225  preferably includes agitation elements (e.g., ribbing) sized to agitate seeds  62  which may occasionally be accidentally trapped between the flight  242  and the inner wall of the housing  1210  instead of being introduced between flights as desired; upon agitation against the surface  1225 , the seeds are released from being trapped between the flight  242  and the inner wall of the housing  1210  and pass in between adjacent flights. It should be appreciated that allowing a seed  62  to remain trapped between the flight  242  and the inner wall of the housing  1210  causes unnecessary wear on the housing  1210 , damages the seed, damages the belt  240 , and causes seed spacing errors due to reflexive action of the flight upon release of seed from the conveyor  1200 . 
         [0089]    Turning to  FIGS. 12I, 12J, 12K, and 12L , the seed conveyor  1200  preferably includes a seed conveyor motor  1020 . The seed conveyor motor  1020  is preferably housed within a motor housing  1212  of the housing  1210 . The motor  1020  preferably drives the seed conveyor via a gearbox  1250 . The motor  1020  preferably also drives the loading wheels  1202 ,  1204  via the gearbox  1250 . 
         [0090]    Referring to  FIG. 12J , the motor  1020  drives an output gear  1258 . The output gear preferably drives an idler gear  1257 . The idler gear  1257  preferably drives an idler gear  1253 . The idler gear  1253  preferably drives a conveyor input gear  1256 . Thus the output gear  1258  indirectly drives the conveyor drive gear  1256 . 
         [0091]    The conveyor input gear  1256  preferably drives an idler gear  1255 . The idler gear  1255  preferably drives a loading wheel drive gear  1254 . Thus the output gear  1258  indirectly drives the loading wheel drive gear  1254 . 
         [0092]    The idler gear  1257  preferably drives a loading wheel drive gear  1252 . Thus the output gear  1258  indirectly drives the loading wheel drive gear  1252 . 
         [0093]    Turning to  FIG. 12I , the loading wheel drive gear  1252  preferably drives the loading wheel  1202  via a shaft  1251 - 2 . The loading wheel drive gear  1254  preferably drives the loading wheel  1204  via a shaft  1251 - 4 . The conveyor drive gear  1256  preferably drives the upper pulley  152  via a shaft  1251 - 6 . 
         [0094]    The gears constituting the gearbox  1250  are preferably relatively sized as illustrated in  FIG. 12J . The gears constituting the gearbox  1250  are preferably relatively sized such that the angular speeds of the perimeters of the loading wheels  1202 ,  1204  are substantially equal. The gears constituting the gearbox  1250  are preferably relatively sized such that a ratio between the linear speed of the perimeter of the loading wheel  1204  and the linear speed of the outer perimeter of flights  242  on the descending portion of the belt  240  is approximately 0.73. In other embodiments, the gears constituting the gearbox  1250  are relatively sized such that a ratio between the linear speed of the perimeter of the loading wheel  1204  and the linear speed of the outer perimeter of flights  242  rounding the top belt  240  is approximately 0.73. 
         [0095]    Referring to  FIGS. 12I and 12K , the gearbox  1250  is preferably enclosed by a cover  1249  securing a seal  1259  against the meter  26 . 
         [0096]    In other embodiments, the seed disc  50  is also indirectly driven by the motor  1020 , e.g., by a drive belt connecting a gear driven by output gear  1258  to a shaft on which the seed disc is mounted for rotation. In still other embodiments, the loading wheels  1202 ,  1204  are driven by a separate motor from the motor  1020 . As illustrated, the seed disc  50  is preferably driven by a separate meter drive motor  27  which preferably comprises an electric motor disposed to drive gear teeth provided on the perimeter of the seed disc  50  as disclosed in Applicant&#39;s co-pending U.S. application Ser. No. 61/675,714, the disclosure of which is hereby incorporated herein in its entirety by reference. 
         [0097]    Turning to  FIGS. 12F, 12G, and 12H , the seed conveyor  1200  is illustrated from top to bottom. As with the other seed conveyor embodiments described elsewhere herein, the belt  240  conveys seeds  62  downwardly toward a seed exit  164  at which an angled portion  114  imports a rearward horizontal velocity to the seeds as the seeds are released sequentially into the trench. 
         [0098]    Turning to  FIGS. 12G, 12H, and 12K , the seed conveyor  1200  preferably includes a housing portion  1232  and a housing portion  1234  which cooperate to enclose the belt  240  during operation. The housing portions  1232 ,  1234  preferably comprise three walls each. Referring to  FIG. 12K , the housing portion  1232  preferably engages the housing  1234  such that two fore-aft walls of the housing portion  1232  are received within two fore-aft walls of the housing portion  1234 . 
         [0099]    To assemble the seed conveyor  1200 , the user first attaches the housing portion  1232  to the housing  1210  using attachment ears  1233 . Referring to  FIG. 12K , the user then slides the housing portion  1234  over the housing portion  1232  in a transverse direction and then slides the housing portion  1234  downwardly such that attachment ears  1235  in the housing portion  1234  engage protrusions  29  in the housing  1210 . When the housing portions  1232 ,  1234  are relatively positioned such that the attachment ears  1235  engage protrusions  29 , a spring  1236  mounted to the housing portion  1234  is allowed to relax such that a portion of the spring extends through openings in the housing portions  1232 ,  1234 , thus retaining the relative vertical position of the housing portions  1232 ,  1234 . To disassemble the seed conveyor  1200 , the user first pulls back the spring  1236  to allow the housing portions  1232 ,  1234  to slide vertically relative to one another, then slides the housing portion  1234  upwards and then away from the housing portion  1232 . 
         [0100]    Turning to  FIG. 15 , process  1500  for planting seeds using the seed conveyor  1200  is illustrated. At step  1505 , the seed disc  50  is preferably rotated through the seed pool and a seed is preferably captured by the seed meter. In the implementation of process  1500  using a vacuum-type seed meter or positive air seed meter, the step of capturing seeds is accomplished by entraining seeds onto the seed apertures  52  of a seed disc  50 . In the implementation of process  1500  using a finger pickup-style meters such as those disclosed in U.S. Pat. No. 6,273,010, the entire disclosure of which is hereby incorporated herein by reference, the step of capturing a seed is accomplished by capturing each seed with a spring-loaded mechanical finger. At step  1510 , the loading wheels  1202 ,  1204  are preferably driven to rotate in opposite directions. At step  1515 , the seed conveyor  1200  is driven such that flights  142  circulate around the belt  240 . At step  1520 , a seed is released (e.g., from an aperture  52  of the seed disc  50 ), preferably adjacent to the loading wheels  1202 ,  1204  and preferably above the loading wheels  1202 ,  1204 . At step  1525 , the seed is preferably captured between the loading wheels  1202 ,  1204 . At step  1525 , one of the loading wheels is preferably deformed to receive the seed in the gap  1201 . At step  1530 , the seed is preferably ejected from between the loading wheels  1202 ,  1204 . At step  1530 , one of the loading wheels preferably returns to a relaxed state. At step  1530 , the seed is preferably ejected downward into the belt  240 , i.e., between flights  142 . At step  1535 , the seed is conveyed to a lower end of the belt  240  between flights  142 . At step  1540 , the seed is released from the belt with a rearward horizontal velocity, e.g., by releasing the seed along surface  114 . 
         [0101]    Turning to  FIG. 13 , the seed conveyor  1200  is illustrated mounted to a row unit  1300 . The row unit  1300  preferably includes a closing wheel attachment portion  1302  for pivotally mounting a closing wheel assembly (not shown) to the row unit and parallel arm attachment apertures  1320  for pivotally mounting a parallel arm arrangement (not shown) to the row unit. The parallel arm arrangement is pivotally mounted to a toolbar (not shown) such that the row unit  1300  is allowed to translate vertically with respect to the toolbar as the row unit traverses a field. The row unit  1300  preferably includes two transversely spaced sidewalls  1304 , preferably located below the mounting location of the meter  26 . The row unit  1300  preferably includes a downwardly extending shank  1306  having a pair of opener disc axles  1310  for pivotally mounting a pair of opener discs to either side of the shank  1306 . A bracket  1340  is preferably mounted to a lower portion of the shank  1306 . The bracket  1340  preferably includes two transversely spaced sidewalls  1342  extending rearwardly and joined at a rearward end of the bracket  1340 . A seed firmer  1307  is preferably mounted to the rearward end of the bracket  1340 . The seed firmer  1307  is preferably disposed to resiliently contact the bottom of the trench (not shown) opened by the. The seed firmer  1307  is preferably made of a resilient material. In some embodiments, the seed firmer  1307  comprises seed firmers such as those described in U.S. Pat. No. 5,425,318, the disclosure of which is hereby incorporated in its entirety herein by reference. 
         [0102]    The user preferably mounts the seed conveyor  1200  to the row unit  1300  by extending the seed conveyor between the sidewalls  1304  of the row unit and the sidewalls  1342  of the bracket  1340 . The seed conveyor  1200  is preferably mounted to the row unit  1300  via structure (not shown) adjacent the sidewalls  1304 . Referring to  FIGS. 12F, 12G and 12H , the seed conveyor  1200  preferably includes two transversely extending spacers  1248  which contact interior surfaces of the sidewalls  1342  of the bracket  1340 , maintaining a lower end of the seed conveyor in substantial alignment with the trench opened by the opening discs and in substantial alignment with the seed firmer  1307 . 
         [0103]    The seed conveyor  1200  preferably includes a seed sensor  550  comprised of a transmitter  520  mounted to the housing portion  1232  and a receiver  515  mounted to the housing portion  1234 . The housing portions  1232 ,  1234  preferably include openings (not shown) aligned along a transversely extending axis such that light (or other signals) transmitted by the transmitter  520  pass through the openings and between flights of the belt  240  to the receiver  515 . 
         [0104]    Turning to  FIG. 14 , a seed conveyor  1400  having a single loading wheel  1420  is illustrated. The seed conveyor  1400  is preferably disposed such that the path of seed apertures  52  intersects the descending portion of the belt  140 . The vacuum imposed on the seed apertures  52  is preferably substantially cut off (e.g., by the terminal end of a vacuum seal) adjacent to a plane Pv intersecting the location at which seeds enter the belt  140 . Thus seeds are released from the disc just prior to entering the belt (i.e., passing between flights  142  of flight). The loading wheel  1420  is preferably located adjacent to the location at which seed enter the belt  140 . The loading wheel  1420  is preferably driven for rotation about a central axis in the direction indicated by the arrow in  FIG. 14 . The surface of the loading wheel thus urges the seeds into the belt and prevents seeds from being stuck between the tips of flights  142  and a wall  1430  adjacent to the belt  142 . The surface of the loading wheel  1420  preferably includes roughness elements as illustrated in  FIG. 14  such that the loading wheel exerts greater frictional forces on the passing seeds. A guide  1410  preferably guides seeds into contact with the loading wheel  1420 . 
       Conveyor Control Systems and Methods 
       [0105]    A control system  1000  for controlling and monitoring the seed conveyor  100  as well as any other seed conveyor embodiment disclosed herein is illustrated schematically in  FIG. 8A . The control system  1000  includes a planter monitor  1005 . The planter monitor  1005  preferably includes a CPU and user interface, and may comprise a monitor such as that disclosed in Applicant&#39;s co-pending U.S. patent application Ser. No. 12/522,252. The planter monitor  1005  is preferably in electrical communication with a seed conveyor motor  1020 . The seed conveyor motor  1020  is operably coupled to the seed conveyor  100  to drive the seed conveyor. For example, in some embodiments the seed conveyor motor  1020  includes a driven output shaft mechanically coupled to a central shaft of the upper pulley  154  or the lower pulley  152 . The seed conveyor  1020  preferably includes an encoder (e.g., a hall-effect sensor) for sensing the rotational speed of the conveyor  100 . The planter monitor  1005  is preferably in electrical communication with a meter drive motor  27 . The meter drive motor  27  may comprise any apparatus known in the art for driving seed meters at a desired speed such as a hydraulic drive or electric drive. As an example, the meter drive motor  27  may comprise an electric motor mounted on or near the seed meter  50 , the electric motor having an output shaft operably coupled to the shaft  54  of the seed meter; in such an embodiment, the meter drive motor  27  preferably includes an encoder (e.g., a hall-effect sensor) for sensing the rotational speed of meter  50 . In other embodiments, the meter drive motor  27  may comprise a ground drive driven by the rotation of planter wheels  8  ( FIG. 9B ). The planter monitor  1005  is also preferably in electrical communication with a speed source  1010 . The speed source may comprise a GPS system, a radar speed sensor, or a wheel speed sensor. The planter monitor may choose between multiple speed sources by predicting reliability as disclosed in Applicant&#39;s co-pending PCT Patent Application No. PCT/US2011/045587, incorporated herein in its entirety by reference. 
         [0106]    Continuing to refer to  FIG. 8A , the planter monitor is preferably in electrical communication with one or more seed sensors adapted for mounting to the seed conveyor  100 . The seed sensors may comprise one or more of the seed sensors  500 ,  550 ,  700 ,  800 ,  900  described herein. The seed sensors may also be in electrical communication with the meter drive motor  27  and the seed conveyor motor  1020 . 
         [0107]    Turning to  FIG. 8B , one embodiment of a planter monitor control system  1000  is illustrated. The planter monitor control system  1000  of  FIG. 8B  includes a seed sensor  550  mounted to the sidewalls of the seed conveyor  100 . The meter drive motor  27  in the planter monitor control system  1000  of  FIG. 8B  comprises an electric drive. The speed St of seed conveyor  100  is generally to the left along the perspective of  FIG. 8B  and has a magnitude which varies with the speed and direction of the planting implement. 
         [0108]    A process  1100  for controlling the rotational speed of the seed conveyor  100  is illustrated in  FIG. 9A . At block  1102  the planter monitor  1005  obtains a speed of the planting implement from the speed source  1010 . At block  1103 , the planter monitor  1005  preferably obtains the current commanded planting population (i.e., the number of desired seeds planted per acre) from a memory contained within the planter monitor  1005 . At block  1105 , the planter monitor  1005  preferably commands a rotational speed of meter  50  based on the desired population and the current implement speed. 
         [0109]    Continuing to refer to  FIG. 9A , at block  1110 , the planter monitor  1005  preferably determines an operating speed of the seed conveyor  100 . This step may be accomplished using a Hall-effect or other sensor adapted to measure the driving speed of the electric motor or the rotational speed of the driven shaft of the seed conveyor  100 . This step may also be accomplished by measuring the time between flights  142  passing the seed sensor  550 . It should be appreciated in light of the instant disclosure that step of block  1110  does not require measuring an actual operational speed but may comprise measuring a criterion related to the operational speed. 
         [0110]    Continuing to refer to  FIG. 9A , at block  1500  the planter monitor  1005  preferably determines the ground speed St of the seed conveyor  100 . In some embodiments, this step may be accomplished by assuming that the tractor or implement speed reported by the speed source  1010  is equal to the ground speed St of the seed conveyor  100 . Such a method is accurate when the tractor and toolbar  12  are not turning, but becomes inaccurate when the tractor and toolbar  12  are turning. In other embodiments the step of block  1500  may be performed more accurately by determining the local ground speed St of each conveyor  100  along the toolbar  12 . Such embodiments are described herein in the section entitled “Conveyor Ground Speed Determination.” 
         [0111]    Returning to  FIG. 9A  and process  1100 , at block  1117  the planter monitor  1005  preferably determines a conveyor motor speed command using a calibration curve. A preferred calibration curve  990  is illustrated in  FIG. 9E . The calibration curve  1200  relates the ground speed St to a desired operational speed So. It should be appreciated in light of the instant disclosure that the calibration curve  990  could also relate a criterion related to ground speed (such as a measured voltage or commanded voltage) to a criterion related to a desired conveyor speed (such as a measured voltage or commanded voltage). The calibration curve  990  preferably includes a sloped portion  992  (e.g., having a slope approximately equal to 1) in which operational speed is directly related to ground speed. The calibration curve  990  preferably includes a zero-slope portion  991  in which operational speed does not decrease as the ground speed decreases. The constant portion  991  is preferably below a minimum ground speed St- 1  (e.g., 1 mile per hour). A slope of the calibration curve  990  preferably changes below the minimum ground speed St- 1 . The calibration curve  990  preferably has a non-zero minimum operational speed So- 1  (e.g., 100 rpm at the upper pulley  152 ). It should be appreciated in light of the instant disclosure that a zero-slope portion is not required to ensure a non-zero minimum operational speed. It should also be appreciated in light of the instant disclosure that a non-zero minimum operational speed is preferable in order to simplify control of the seed conveyor when stopping and starting the planting implement. The minimum operational speed So- 1  is preferably small enough that seeds  62  exiting the seed conveyor  100  do not have sufficient rearward horizontal velocity Vx ( FIG. 2C ) to cause substantial seed bounce or roll at low ground speeds (e.g., less than 1 mile per hour). 
         [0112]    Returning to  FIG. 9A  and the process  1100 , at block  1120  the planter monitor  1005  preferably commands the new desired conveyor speed. It should be appreciated in light of the instant disclosure that the change in conveyor speed command may be deferred until the actual conveyor speed is outside of a preferred range, e.g. 5%, with respect to the desired conveyor speed. 
         [0113]    Turning to  FIG. 10A , a process  1600  is illustrated for shutting off and turning on the seed conveyor  100  at planting boundaries. Turning to  FIG. 10B , the seed conveyor is illustrated at three locations indicated by  100 ,  100 ′, and  100 ″ along direction of travel  39 . As illustrated, the meter  50  has introduced several seeds  62  into the seed conveyor  100 ; the earliest seed introduced into the seed conveyor  1200  is identified as seed  62 - 1 . The seed conveyor  100  first crosses a first planting boundary  1710 , thus entering into a no-planting region  1715  (e.g., a waterway), and then crosses a second planting boundary  1720 , thus exiting the no-planting region  1715 . In overview, the process  1600  shuts off the seed conveyor  100  at the first planting boundary  1710 , advances the earliest seed  62 - 1  a distance De to the exit while the conveyor is in the no-planting region, and starts the seed conveyor at the second planting boundary  1720 . 
         [0114]    Returning to  FIG. 10A  to describe the process  1600  in detail, at block  1610  the planter monitor  1005  preferably determines whether the seed conveyor is within a predetermined distance or time from crossing a planting boundary. The current distance to a planting boundary is preferably estimated by comparing the position reported by a GPS receiver  5  ( FIG. 9B ) to the position at which a planting boundary intersects a line along the direction of travel. The time to a planting boundary is preferably estimated by dividing the distance to a planting boundary by the speed currently reported by the speed source  1010 . Once the seed conveyor  100  is within a predetermined time or distance of a planting boundary, at block  1615  the planter monitor  1005  preferably begins to record the distance De between the earliest seed  62 - 1  in the seed conveyor and the seed exit  164 . The distance De is preferably recorded by recording the time of each seed pulse from the seed sensor  550  ( FIG. 8B ) and then estimating the position of the seed by integrating the speed of the conveyor motor  1020 . When De equals zero, it is assumed that the earliest seed  62 - 1  in the conveyor has exited the conveyor and the planter monitor  1005  preferably identifies the next earliest seed as the earliest seed  62 - 1 . At block  1620 , the planter monitor  1005  determines whether the seed conveyor  100  has crossed a planting boundary (e.g., planting boundary  1710  in  FIG. 10B ). Once the conveyor has crossed a planting boundary into a no-planting region (e.g., no-planting region  1715  in  FIG. 10B ), at block  1625  the planter monitor  1005  commands the meter drive motor  27  ( FIG. 8B ) to shut off or alternatively commands a clutch associated with the seed meter  50  to disengage. At block  1628 , the planter monitor  1005  preferably allows a predetermined delay to pass before commanding the conveyor motor  1020  to stop at block  1630 . The predetermined delay may vary with ground speed and planting population and may be based on empirically determined delays between meter stop commands and the last seed deposited by the meter  50  into the seed conveyor  100 . 
         [0115]    Continuing to refer to  FIG. 10A , at block  1635  the planter monitor  1635  preferably advances the seed conveyor  100  such that the belt  140  travels through a distance De, thus moving the last seed  62 - 1  adjacent to the seed exit  164 . At block  1640 , the planter monitor  1005  preferably determines whether the seed conveyor  100  has crossed a planting boundary (e.g., planting boundary  1720  in  FIG. 10B ). Once a planting boundary has been crossed, the planter monitor  1005  preferably starts the conveyor motor  1020  at block  1645  and preferably subsequently starts the meter drive motor  27  (or alternatively commands a clutch associated with the meter  50  to engage) at block  1650 . 
         [0116]    Turning to  FIG. 10C , another process  1600 ′ is illustrated for shutting off and turning on the seed conveyor  100  at planting boundaries. Turning to  FIG. 10D , the seed conveyor is illustrated at three locations indicated by  100 ,  100 ′, and  100 ″ along direction of travel  39 . As with process  1600 , process  1600 ′ shuts off the seed conveyor  100  at the first planting boundary  1710 , advances the earliest seed  62 - 1  to the seed exit while the conveyor is in the no-planting boundary, and starts the seed conveyor at the second planting boundary  1720 . However, rather than calculating and storing the distance De as in process  1600 , process  1600 ′ uses a seed sensor  1800  to determine the location of the earliest seed  62 - 1 . The seed sensor  1800  is preferably be an optical seed sensor mounted to the seed conveyor  100  in a fashion similar to the seed sensor  550  described herein. The seed sensor  1800  is preferably disposed to sense seeds  62  adjacent the seed exit. The seed sensor  1800  is preferably disposed to sense seeds  62  prior to release; i.e., before the flight  142  below the seed is sufficiently separated from seed guide  110  to allow the seed to exit the seed conveyor  100 . 
         [0117]    Returning to  FIG. 10C  to describe the process  1600 ′ in detail, at block  1620  the planter monitor  1005  preferably determines whether the seed conveyor  100  is at a planting boundary (e.g., first planting boundary  1710  in  FIG. 10D ). Once the conveyor has crossed a planting boundary into a no-plant region (e.g., no-plant region  1715  in  FIG. 10D ), at block  1625  the planter monitor  1005  commands the meter drive motor  27  ( FIG. 8B ) to shut off or alternatively commands a clutch associated with the seed meter  50  to disengage. At block  1632 , the planter monitor  1005  preferably commands the conveyor motor  1020  to advance. Once a seed pulse has been received from seed sensor  1800 , the planter monitor  1005  preferably commands the conveyor motor  1020  to stop at block  1637 . At block  1640 , the planter monitor  1005  preferably determines whether the seed conveyor  100  has crossed a planting boundary (e.g., planting boundary  1720  in  FIG. 10B ). Once a planting boundary has been crossed, the planter monitor  1005  preferably starts the conveyor motor  1020  at block  1645  and preferably subsequently starts the meter drive motor  27  (or alternatively commands a clutch associated with the meter  50  to engage) at block  1650 . 
       Conveyor Ground Speed Determination 
       [0118]    As noted elsewhere herein, in order to match the operating speed of the seed conveyor  100  to the ground speed St of the conveyor, it is desirable to determine the ground speed of each seed conveyor at each row unit  10 . This determination becomes more complex when the implement is turning, because the speed of each seed conveyor  100  varies according to its distance from the center of the turn. Thus several alternative systems and methods of determining individual conveyor ground speed St are disclosed herein. 
       Conveyor Ground Speed Determination—Systems 
       [0119]    Turning to  FIG. 9B , the toolbar  12  is drawn through the field by a tractor  2 . The toolbar  12  is preferably mounted to the tractor  2  by a hitch  13  near the transverse center of the toolbar. Toolbar  12  is supported by wheels  8 , which are mounted in transversely spaced relation along the toolbar. A right wheel  8 - 1  is mounted at a transverse distance Dw- 1  from the center of the toolbar  12 . A left wheel  8 - 2  is mounted at a transverse distance Dw- 2  from the center of the toolbar  12 . Wheels  8  may be mounted to the toolbar  12  in a fashion similar to the wheel and tire assemblies disclosed in U.S. patent application Ser. No. 12/270,317 (Pub. No. US 2010/0116974). Row units  10 , each preferably including a seed conveyor  100 , are mounted in transversely spaced relation along the toolbar  12 . A right row unit  10 - 1  is located at a transverse distance D 1  from the center of toolbar  12 . A left row unit  10 - 2  is located at a transverse distance D 2  from the center of toolbar  12 . 
         [0120]    Continuing to refer to  FIG. 9B , several data-gathering devices are preferably mounted to the tractor  2  and the toolbar  12 . A gyroscope  6  is preferably mounted to the toolbar  12 . The gyroscope  6  is preferably in electrical communication with the planter monitor  1005 . A three-axis accelerometer  7  is preferably mounted to the toolbar  12 . The accelerometer  7  is preferably mounted to the toolbar  12 . The gyroscope and accelerometer  6 ,  7  are mounted to the toolbar at a transverse distance Da from the center of the toolbar  12 . A GPS receiver  5  is preferably mounted to the tractor  2 . The GPS receiver  5  is preferably in electrical communication with the planter monitor  1005 . A radar speed sensor  11  is preferably mounted to the underside of the tractor  2 . The radar speed sensor  11  is preferably in electrical communication with the planter monitor  1005 . Wheel speed sensors  9  are preferably mounted to wheels  8  and configured to measure the rotational speed of wheels  8 . Wheel speed sensors  9  are preferably in electrical communication with the planter monitor  1005 . Wheel speed sensors  9  may be similar to the rotation sensors described in U.S. patent application Ser. No. 12/270,317 (Pub. No. US 2010/0116974). In other embodiments, a GPS receiver and radar speed sensor are mounted to the toolbar  12 . 
         [0121]    Continuing to refer to  FIG. 9B , while traveling through the field, the tractor  2  has a velocity Vt, while the right and left row units  10 - 1 ,  10 - 2  have velocities V 1 , V 2  respectively. It should be appreciated that the ground speed St of each seed conveyor  100  is equal to the speed component of the associated row unit velocity; e.g., the magnitude of V 1  is equal to the ground speed St of the seed conveyor associated with row unit  10 - 1 . Additionally, wheels  8 - 1 ,  8 - 2  travel at longitudinal speeds Sw 1 , Sw 2 . As illustrated in  FIG. 9B , when the tractor  2  is traveling in a consistent direction (i.e., not turning), velocities Vt, V 1  and V 2  are equal. As illustrated in  FIG. 9C , as the tractor  2  turns, the direction of velocity Vt changes and the velocities V 1  and V 2 . The toolbar  12  has an angular velocity w about a center of rotation C. The center of rotation C is a distance Rc from the center of the toolbar  12 . It should be appreciated that the longitudinal speed of each point along the toolbar  12  increases with the distance of each point from the center of the toolbar. 
       Conveyor Ground Speed Determination—Methods 
       [0122]    Turning to  FIG. 9D , process  1500  includes multiple methods of determining conveyor ground speed St. It should be appreciated that process  1500  of  FIG. 9D  is a detailed illustration of the block  1500  of  FIG. 9A . 
         [0123]    At block  1506 , the planter monitor  1005  preferably obtains the geometry relevant to the available groundspeed determination method, e.g., distances D 1 , D 2 , Da, Dw 1 , Dw 2 , the transverse and longitudinal offsets between the GPS receiver  5  and the hitch  13 , and the longitudinal offset between the hitch  13  and the center of the toolbar  12 . To accomplish this step, the planter monitor  1005  preferably prompts the user to enter the relevant distances and offsets via a series of graphical user interface screens similar to those disclosed in Applicant&#39;s co-pending PCT Patent Application No. PCT/US2011/045587, previously incorporated herein by reference. 
         [0124]    At block  1508 , the planter monitor  1005  preferably selects the desired method of ground speed St. In some embodiments step may be accomplished by simply choosing the only available method. In other embodiments, the method may be selected based on the stability of the signals used in certain methods (e.g., a method other than GPS may be selected during periods of GPS signal instability). 
         [0125]    Turning first to the wheel speed method, at block  1520  the planter monitor  1005  preferably determines the longitudinal speeds Sw 1 , Sw 2  of wheels  8 - 1 ,  8 - 2  from the signals generated by wheel speed sensors  9 - 1 ,  9 - 2 , respectively. At block  1522 , the planter monitor  1005  preferably determines the angular speed w of the toolbar  12  by a relation such as: 
         [0000]    
       
         
           
             w 
             = 
             
               
                 
                   S 
                   
                     w 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 - 
                 
                   S 
                   
                     w 
                      
                     
                         
                     
                      
                     2 
                   
                 
               
               
                 
                   D 
                   
                     w 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 + 
                 
                   D 
                   
                     w 
                      
                     
                         
                     
                      
                     2 
                   
                 
               
             
           
         
       
     
         [0126]    At block  1524 , the planter monitor  1004  preferably determines the longitudinal speed at a row unit, e.g., row unit  10 - 1 , using a relation such as: 
         [0000]        V   1   =S   w1   +w ( D   1   −D   w1 ) 
         [0127]    In this and each of the following methods described herein, the planter monitor  1005  preferably stores the speed Vn of each row unit  10 - n  as the groundspeed St of the seed conveyor  100  associated with the row unit  10 - n.    
         [0128]    Turning next to the gyroscope method, at block  1530  the planter monitor  1005  preferably determines the angular speed w of the toolbar  12  from the signal generated by the gyroscope  6 . At block  1532 , the planter monitor  1005  preferably determines the longitudinal speed of one location along toolbar  12 . In some embodiments, the longitudinal speed of the center of the toolbar  12  may be determined from the signal generated by the radar speed sensor  11 . In other embodiments, the longitudinal speed of the accelerometer  7  may be determined by integrating the signal from the accelerometer. At block  1534 , the planter monitor  1005  preferably calculates the velocity of, e.g., the row unit  10 - 1  based on the angular speed w and the known longitudinal speed of a location on the toolbar. Assuming the accelerometer-integrated speed (Sa) is used, the planter monitor  1005  preferably uses a relation such as: 
         [0000]        V   1   =S   a   w ( D   1   −D   a ) 
         [0129]    Turning next to the GPS method, at block  1510  the planter monitor  1005  preferably records the GPS position over a period of time. At block  1514 , the planter monitor  1005  preferably determines the distance Rc from the center of the toolbar  12  to the center of rotation of the toolbar. At block  1516 , the planter monitor  1005  preferably determines the longitudinal speed of the center of the toolbar (Vc) from the tractor speed Vt reported by the radar speed sensor  11 . At block  1518 , the planter monitor  1005  preferably determines the velocity of a row unit  10 - 1  using a relation such as: 
         [0000]    
       
      
       V 
       1 
       =V 
       c 
       +wD 
       1  
      
     
         [0130]    It should be appreciated that the methods disclosed herein for determining a ground speed St of each seed conveyor effectively determine a row-unit-specific speed. Thus the row-unit-specific speed could also be used to implement a desired application rate in implements having sectional or row-by-row application rate control. For example, in some embodiments the meter drive motor  27  is driven at a rate based upon the row-unit-specific speed determined by one or more of the methods described herein with respect to  FIG. 9D , rather than based upon the tractor speed reported by GPS or radar as is conventional. It should be appreciated that the increase in application rate accuracy resulting from the use of a row-unit-specific speed is most significant when the implement is executing a turn or otherwise traveling in a curvilinear path. It should also be appreciated that such use of a row-unit-specific speed to control application rate could be implemented in row units without a seed conveyor (e.g., using a conventional seed tube or depositing seeds directly from the metering device into the seed trench). 
         [0131]    The foregoing description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment of the apparatus, and the general principles and features of the system and methods described herein will be readily apparent to those of skill in the art. Thus, the present invention is not to be limited to the embodiments of the apparatus, system and methods described above and illustrated in the drawing figures, but is to be accorded the widest scope consistent with the spirit and scope of the appended claims.