Abstract:
A method and apparatus for controlling gaps between articles traveling on a conveyor system includes providing a plurality of tandem conveying surfaces defining a series of sequentially arranged conveying surface interfaces. At least one of the conveying surfaces associated with each of said conveying surface interfaces is an adjustable-speed conveying surface. At least one article sensor is provided adjacent at least one of the conveying surface interfaces. The article sensor provides information on positions of the articles. The speeds of the tandem conveying surfaces are controlled in a manner that establishes controlled gaps between articles, by identifying a controllable gap that can be adjusted between articles at a particular conveying surface interface and adjusting the controllable gap at the particular conveying surface interface. The control adjusts the controllable gap by adjusting the speed of the adjustable speed conveying surface associated with the particular conveying surface interface using a feedback controller. Gaps between articles are sequentially adjusted at the series of sequentially arranged conveying surface interfaces.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 09/851,021, filed May 8, 2001, and entitled Conveyor Induct System, now U.S. Pat. No. 6,629,593, which claims priority to commonly assigned, U.S. provisional application Ser. No. 60/203,301, filed May 11, 2000, and entitled Induct for Next Generation Sorter. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to conveyor systems, and in particular to the induction section of a conveyor sortation system. The induction section of a conveyor system generally refers to the portion of the conveying system in which articles are inducted, or initiated, into the conveying sortation system. The induction section typically performs the function of providing the proper gaps between packages, or other articles traveling on the conveying system, so that the packages can be sorted for proper distribution. The proper gapping between articles on the conveying system is important for a variety of reasons. First, the gapping of the articles affects the throughput of the conveying system, which is often a factor of high commercial significance. By reducing the gaps between articles on the conveying system, the number of articles that can be placed on the conveying system at a given time is increased. By being able to place more articles on the conveying system at a given time, more articles are moved over a given time period, thus increasing the throughput of the conveying system. 
     Second, the gapping of the articles is highly important because the gapping affects the sortation functions of the conveying system. Where the conveying system functions to sort the articles being conveyed, prior art conveying systems have often used pusher shoes or other diverting means to push the articles off of a main conveyor onto one or more branch conveyors. In order for these pusher shoes to sort the packages correctly, it is important that sufficient space be provided between articles so that the pushers do not inadvertently push against another article while they are in the process of diverting a second article. Further, pusher shoes on some sorters tend to rotate the article, at least partially, when they are in the process of diverting the article onto a branch conveyor. This rotation ensures that the articles are properly aligned with the branch conveyor when they are diverted. In order for the article to be rotated, however, it is necessary for there to be a space behind the article in order to provide room for the rotation of the article. If insufficient space is allotted, the article may contact another article while being rotated, thus causing it to enter the branch conveyor misaligned, or to not be diverted at all. 
     The sortation functions of the conveying system therefore weigh in favor of providing a certain amount of gapping between articles while the throughput considerations weigh in favor of reducing this gapping to as small a space as possible. In order to best balance these competing interests, the induct portion of the conveying system would ideally be able to consistently and accurately produce gaps that were just large enough to accommodate the gapping requirements of the sortation section of the conveying system while rarely, if ever, exceeding these minimum gapping requirements. Prior art induct systems, however, have left room for improvement as to the consistency, accuracy, and speed at which at which gapping of the articles can be performed. Particularly as increases in throughput have been based on increasing conveyor speed while reducing gapping, the necessity for precise gapping in conveyor systems has increased. 
     One reason for the less than desirable performance of prior induct systems has been the inaccuracies that result in controlling an article as it transitions from one conveyor to another. Because gapping between articles can only be changed by changing the speed of one article with respect to another article, the creation and control of gaps tends to occur by utilizing at least two conveyors. For example, if an upstream conveyor is traveling at a first speed and a downstream conveyor is traveling at a different speed than the first conveyor, an article moving from the upstream conveyor to the downstream conveyor will increase the gap between it and any trailing package behind it as it passes onto the downstream conveyor. This increase in gap is due to the acceleration of the article as it initially arrives on the second conveyor. For a certain amount of time, the leading article is traveling at a higher speed (the speed of the downstream conveyor) than the trailing article, which is traveling at the speed of the upstream conveyor. This difference in speeds enlarges the gap between the two articles. 
     In the past, the controlling of gaps by using multiple conveyors traveling at different speeds has resulted in inaccuracies of the created gap partially because the point at which an article switches from having its speed controlled by the upstream conveyor to the downstream conveyor has not been able to be accurately determined. While this point is often assumed to correspond to the moment when the center of gravity of the article reaches the midpoint between the upstream and the downstream conveyors, this assumption often proves incorrect, thus leading to inaccuracies in the created gaps. 
     The creation of gaps in prior art conveying systems has also been inaccurate because these prior art conveying systems are not able to accurately determine the position of articles on the conveyors. For example, in some prior art conveying systems, the position of the article was determined by sensing the passing of the article by a single photo-detector positioned alongside the conveyor. As the article moved past the single photo-detector, its position was computed by computing how far the conveyor belt had moved since the article had been detected. Determining how far the conveyor belt had moved often was carried out by way of an encoder that measured the amount of rotations of the motor that powered the conveyor belt. Due to measurement inaccuracies, slippage, and other factors, this calculation of the article&#39;s position on the conveyor has a significant uncertainty. This uncertainty of the article&#39;s position on the conveyor makes controlling the created gaps in prior art induct systems difficult. 
     In light of the foregoing disadvantages of the prior art, the need for an induct system that improves the accuracy and consistency of the created gaps between articles can therefore be seen. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides an improved method and apparatus for inducting articles into a conveying system. The method and apparatus allow the position of articles traveling on individual conveyors to be more accurately determined, thus aiding the accuracy and precision of the gapping process. The method and apparatus of the present invention also allow an accurate determination, rather than an assumption, to be made as to the point at which an article moving from one conveyor to another changes from having its speed controlled by the upstream conveyor to that of the downstream conveyor. 
     According to one aspect of the present invention, a method is provided for determining the control point of an article traveling on a conveyor system. The method includes providing a first and a second conveyor. The first conveyor includes a downstream end that is aligned with an upstream end of the second conveyor. The method further includes detecting when a change occurs in the speed of the article due to the article having reached the second conveyor. According to another aspect of the present invention, a method for controlling a gap between a first and a second article on a conveyor is provided. The method includes providing a first and a second conveyor which are aligned with each other. The speed of the second conveyor is set to be different than the speed of the first conveyor. The control point of the first article is measured as the first article passes from the first conveyor to the second conveyor. Any gap between the first article and the second article is measured while the first article is traveling at least partially on an adjustable speed conveyor. The measured gap is compared to a desired gap, and the speed of the adjustable speed conveyor is adjusted in order to reduce any difference between said measured gap and said desired gap. 
     According to still another aspect of the present invention, a method is provided for creating a gap between a first article and a second article. The method includes providing a first and a second conveyor which are aligned with each other. The first conveyor is an adjustable speed conveyor. Any gap between the first and the second article is repetitively measured while the first article is at least partially positioned on the first conveyor. Any difference between the measured gap and a desired gap is calculated. A feedback controller is provided which takes the difference between the measured gap and the desired gap and outputs a command for adjusting the speed of the adjustable speed conveyor. The outputted command is based upon the difference between the measured gap and the desired gap. 
     According to still further aspects of the present invention, apparatuses for carrying out the above-described methods are provided. The measuring of the gap between articles may be accomplished by way of a horizontal array of photo-detectors. The determination of the control point of an article may also be carried out by way of a horizontal array of photo-detectors. The determination of the size of a gap, as well as the issuance of a speed command based upon the size of that gap, may also be repeated multiple times for an individual gap. In this manner, direct feedback is provided to the speed controller for the adjustable speed conveyor as the gap is either maintained or changed. The feedback controller may be a proportional-integral-derivative type controller. In still further aspects of the present invention, the methods and apparatuses for determining the control point of an article can be combined with, or utilized separately from, the methods and apparatuses for creating a gap. 
     The methods and apparatuses of the present invention provide an improved induction system that allows gaps to be created between packages that are more accurate and consistent, as well as allowing the induction system to operate at higher speeds while maintaining this improved accuracy and consistency. These and other advantages of the present invention will be apparent to one skilled in the art in light of the following specification when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a combination plan view and block diagram of a conveyor system and controller according to one aspect of the present invention; 
         FIG. 2  is a plan view of a conveyor system according to another aspect of the present invention illustrated at a first moment in time; and 
         FIG. 3  is a plan view of the conveyor system of  FIG. 2  illustrated at a later moment in time. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described with reference to the accompanying drawings wherein like reference numerals correspond to like elements in the several drawings. A conveyor induct system  20  is generally depicted in plan and block diagram form in  FIG. 1 . Conveyor induct system  20  comprises two subsystems which can be utilized independently from each other or in combination with each other. One such subsystem is a control point determination module  38  which serves to accurately determine the location of the control points on individual articles, as will be explained in more detail below. The other subsystem of conveyor induct system  20  is a gap control system  40  which controls the speed of one or more of the conveyors in order to create a desired gap between articles as they exit onto a take-away conveyor  32 . By way of background, such controlling of gaps is also described in commonly-owned U.S. Pat. Nos. 5,267,638 and 5,038,911, the disclosures of which are hereby incorporated herein by reference. The disclosure of commonly-assigned U.S. provisional application Ser. No. 60/203,301, filed May 11, 2000, to which this application claims priority, is also hereby incorporated herein by reference. 
     Conveyor induct system  20  includes a first conveyor  22  and a second conveyor  24 , both of which carry packages or other articles on their conveying surfaces  26  in the direction indicated by arrow  28 . The particular type of conveying surface  26  is not limited within the scope of the present invention, but can include belts, rollers, slats, and any other type of conveying structure capable of transporting goods. A feed conveyor, which may include an accumulation conveyor  30 , feeds packages to induct system  20 . A take-away conveyor  32  carries packages that have been properly gapped to a sortation system (not shown) or other downstream junction. Conveyors  30  and  32  are only partially illustrated in  FIG. 1 . Take away conveyor  32  is located downstream of, and aligned with, second conveyor  24  and transports articles that are transferred onto it from second conveyor  24 . It will be understood that the dimensions of conveyors  22 ,  24 ,  30 , and  32  are not necessarily drawn according to scale, and that the lengths and widths of these conveyors can vary as would be understood by one skilled in the art. A series of articles  34   a - h  ride on the conveying surfaces  26  of the various conveyors and move in the direction of arrow  28 . Articles  34 , which are not part of the invention, are depicted in various sizes and are again not necessarily drawn to scale but are depicted for purposes of description herein. 
     As can be seen from articles  34   a, b , and  c , the articles  34  that enter into conveyor induct system  20  from accumulation conveyor  30  may be generally closely packed together such that no gaps exist between these articles. While it is not necessary within the scope of the invention that such lack of gaps exists, it is desirable that excessive gaps be removed or minimized prior to the articles reaching first conveyor  22 . Because articles are often manually loaded onto the conveyor system by workers unloading trucks, or other vehicles, there is often a significant amount of spacing that initially exists between articles as they are first unloaded. This spacing may be on the order of several feet and is desirably eliminated or reduced before the articles reach first conveyor  22 . To that end, it may be desirable to merge several incoming conveyors into a single conveyor line so that gaps can be reduced or eliminated. While such merging of conveyors may cause a reduction in the average gap between packages, there still may exist individual gaps of varying size. An accumulation conveyor helps remove these gaps, provided articles are being fed into the conveyor system at a fast enough rate. It will be understood, however, that an accumulation conveyor is not a necessary component of the present invention. If an accumulation conveyor is used, it can be any of a variety of types of conventional accumulation conveyors, or other types of accumulation conveyors, as would be known by one skilled in the art. 
     First conveyor  22  preferably, although not necessarily, operates at a speed that is no greater than the highest speed of the immediately upstream conveyor, which, as illustrated in  FIG. 1 , is accumulation conveyor  30 . By limiting the speed of first conveyor  22  to that of the maximum speed of accumulation conveyor  30 , articles that travel from accumulation conveyor  30  onto first conveyor  22  will not have either their leading or trailing gaps increased. The gapping between articles changes when the articles transition from first conveyor  22  to second conveyor  24 . First conveyor  22  has a speed which is generally less than that of second conveyor  24  so that any gaps between articles will be enlarged as they pass from first conveyor  22  to second conveyor  24 . While not required, first conveyor  22  may have its speed set at a fixed ratio to that of second conveyor  24  so that changes in the speed of second conveyor  24  will cause proportionate changes in the speed of first conveyor  22 . Alternatively, it may be desirable to control the speeds of first and second conveyors  22  and  24  so that at certain times they operate at the same speed, while at certain other times they operate at different speeds. 
     An example of a gap created by the passage of articles from first conveyor  22  to second conveyor  24  can be seen between articles  34   e  and  34   d . This gap will continue to increase until article  34   d  has passed sufficiently onto second conveyor  24  such that article  34   d  is carried at the same speed as second conveyor  24 . After article  34   d  has transitioned onto second conveyor  24 , the created gap between article  34   d  and  e  may or may not be equal to the desired gap. The creation of the desired gap is accomplished during the transition of articles from second conveyor  24  to take-away conveyor  34 . An example of a desired gap is illustrated in  FIG. 1  between the trailing edge of article  34   h  and the leading edge of article  34   g.    
     In order to more accurately control the gap between articles as they travel between two conveyors of different speeds, it is helpful to know precisely when the article will shift from moving at the speed of the first conveyor to moving at the speed of the second conveyor. This information can be gained by using control point determination module  38  in order to find the location of the control point of each article. The control point of an article is the point on the article which, when it is aligned with a specified reference point on the conveyor, will designate the moment in time that the article&#39;s speed switches from being controlled by one belt to another belt. It is important to know the control point of an article during the gapping phase of the articles because adjustments in the speed of a conveyor during gapping will be ineffective in changing the speed of the article unless the article is still being controlled by the particular conveyor whose speed is being adjusted. 
     Control point determination module  38  includes a horizontal array of photo-detectors  42  which comprise a first array of photo-emitters  44  and a second set of photo-receptors  46 . The emitters  44  are positioned on an opposite side of first conveyor  22  across from, and aligned with, receptors  46 . As would be understood by one skilled in the art, the particular side on which emitters  44  and receptors  46  is immaterial so long as the emitters are opposite the receptors. Photo-detector array  42  emits electromagnetic signals, such as light, infrared, or other signals, from emitters  44  across first conveyor  22  to receptors  46 , which detect the emitted signal provided there is no article on first conveyor  22  that obstructs the line of sight between an emitter  44  and a receptor  46 . Photo-detector array  42  is thus able to determine where gaps between articles are located by determining which particular photo-detectors are not obstructed at a given moment. Horizontal array of photo-detectors  42 , which is often referred to as a light curtain, can be any type of conventional array of photo-detectors, such as those that are commercially available from Kore Computing of Comstock Park, Mich., or SICK, Inc. of Bloomington, Minn. Other types of photo-detectors can also be used. Photo-detector array  42  should include photo-detectors that are spaced relatively closely together so that accurate information can be gathered about the location of gaps and the position of articles traveling on first conveyor  22 . While other spacings are within the scope of the invention, a photo-detector positioned every five millimeters along the length of horizontal array  42  is contemplated. More compactly spaced photo-detectors would, of course, give more precise information about the location of articles and gaps, if all other factors remain equal. 
     Photo-detector array  42  should give updated information about the entire array of photo-detectors multiple times a second, such as once every two milliseconds, although this update rate can vary widely within the scope of the present invention, particularly with respect to faster update rates. The height at which photo-detector array  42  is positioned above conveying surface  26  should be very slight so that photo-detector array  42  does not overlook articles traveling on the conveyor that are of very low height. The precise height of array  42  can therefore be varied depending upon the types of articles being sorted. In general, a height above conveying surface  26  of a few millimeters should be appropriate for most situations. It may also be desirable to include multiple arrays of photo-detectors  42  which are vertically stacked upon each other to give height information about articles, as well as information about article shape. The length of photo-detector array  42  is preferably, although not necessarily, at least half as long as the greatest expected article length. Where first conveyor  22  is expected to be used in an application that transports articles of considerable length, the length of array  42  should therefore be correspondingly increased. Photo-detector array lengths that are less than half of the greatest expected article length still fall within the scope of the invention, but do increase the probability of not being able to accurately determine an article&#39;s control point, as the following discussion will illustrate. 
     Control point determination module  38  determines the location of control points on individual articles with respect to a particular reference point. In  FIG. 2 , a one dimensional frame of reference  48  is illustrated and oriented parallel to first and second conveyors  22  and  24 . A reference point  50  is designated on frame of reference  48 . Reference point  50  is located halfway between the downstream end of first conveyor  22  and the upstream end of second conveyor  24 . A control point  52  is depicted on article  34   d . When control point  52  reaches reference point  50 , i.e. it becomes aligned with reference point  50 , the control of article  34   d  will switch from first conveyor  22  to second conveyor  24 . Thus article  34   d , in the position illustrated in  FIG. 2 , is being controlled by first conveyor  22  despite the fact that a portion of article  34   d  is located on second conveyor  24 . The precise moment when control switches from first conveyor  22  to second conveyor  24  is largely dependent upon the frictional characteristics of the article and the conveyor, as well as the center of gravity of the particular article. While in the past the control point was often assumed to be the center of the article, control point determination module  38  actually measures the location of the control point. 
     Control point determination module  38  measures the location of an article&#39;s control point by monitoring the gap, if there is one, between the trailing edge of article  34   d  and the leading edge of article  34   c . As long as article  34   d  is still being controlled by first conveyor  22 , this gap will not change. However, as soon as article  34   d  transitions to being controlled by second conveyor  24 , which is traveling at a higher speed than first conveyor  22 , article  34   d  will begin to advance forwardly from article  34   c , thus either creating a gap between articles  34   c  and  d , or enlarging whatever gap may have already pre-existed.  FIG. 3  depicts the moment when the control point  52  of article  34   d  has reached reference point  50 . At this moment, article  34   d  begins to separate from article  34   c , thus creating a gap between articles  34   c  and  d . This gap is illustrated in  FIG. 3  and detected by an unobstructed beam of electromagnetic energy  54  from photo-detector array  42 . By monitoring which particular photo-detector in array  42  detects this newly created or expanded gap, the control point of article  34   d  can be determined by a calculator  56 . Because the distance that each individual photo-detector is positioned away from reference point  50  is known, calculator  56  can simply calculate the control point  52  as being this same distance. This calculation of the location of control point  52 , however, is based upon the longitudinal distance of control point  52  from the trailing edge of article  34   d . Typically, it will be desirable to know the location of control point  52  with respect to a different landmark such as, for example, the leading edge of article  34   d . Calculator  56  can easily compute this location of control point  52  by subtracting the longitudinal distance of control point  52  to the trailing edge of article  34   d  from the overall length of article  34   d . Alternative calculation methods are, of course, also possible. 
     The length of article  34   d  can be determined in a variety of different manners. One way is to have photo-detector array  42  monitor the leading edge of article  34   d  while it is within the detection zone of array  42 . Once out of this detection zone, the speed of first conveyor  22  is recorded until the change in the gap between articles  34   d  and  34   c  occurs. At the moment at which this change in gap occurs, the recorded speed of conveyor  22  can be used to determine the distance article  34   d  has traveled between the time its leading edge left the photo-detector detection zone and the speed change occurred. This distance will be equal to the distance of control point  52  from the leading edge of article  34   d.    
     The determination of the location of control point  52  is dependent upon what reference point is used to define control point  52 . As discussed above, reference point  50  was used to define the location of control point  52 . Other reference points could be used. For example, reference point  58  could alternatively be used to define control point  52 . Reference point  58  is located at the very edge of the downstream end of first conveyor  22 . If reference point  58  is used as the reference point, the control point  52  will be the location on article  34  that is aligned with reference point  58  at the moment the control of article  34  transitions from that of first conveyor  22  to that of second conveyor  24 . This control point will correspond to a different physical location on article  34  due to the different locations of reference points  50  and  58 . Reference point  60 , which is located at the very edge of the upstream end of conveyor  24 , could also be used as a reference point. And, as noted, any other reference point can be used in defining the control point so long as the particular reference point is known and used consistently throughout the rest of the conveyor system. 
     The determination of control point  52  as described herein involves, in one embodiment, the creation or enlargement of gaps between articles as they transition from conveyor  22  to conveyor  24 , and the creation or enlargement of these gaps may be desirably controlled. These gaps are created or enlarged as a result of the higher speed of second conveyor  24  with respect to first conveyor  22 . If conveyor  22  is set to run at a fixed fraction of the speed of conveyor  24 , then the gaps that are created between articles transitioning from conveyor  22  to conveyor  24  will vary as the length of the articles vary. For example, if only long articles are moved over conveyor  22 , they will tend to have relatively large gaps. If only small articles are moved, they will tend to have relatively smaller gaps. If both small and large articles are conveyed by conveyors  22  and  24 , they will tend to have gaps of varying length. The varying lengths are due to the different amounts of time long and short articles spend moving at the speed of first conveyor  22  while the immediately downstream article is moving at the higher speed of second conveyor  24 . This disparity in gap lengths is often desirably reduced or eliminated. 
     The reduction in the disparity of gap lengths between articles exiting control point determination module  38  may be desirable for several reasons. First, any downstream gap controlling system, such as gap control system  40 , may not operate as effectively when articles are being input into the system with widely varying gaps. This may be become more of a problem as conveyor speeds increase. Second, if the gaps created in control point determination module  38  were larger than the desired gaps, they would have to be eliminated downstream by the gap control system. Having to eliminate gaps created by control point determination module  38 , of course, makes the gap control system have to work harder, and therefore makes the system less efficient. 
     In order to carry out this reduction in gap variation, a controller could be added that controls the speed of first conveyor  22  such that, as soon as control point  52  is measured, the speed of first conveyor  22  is immediately brought up to that of second conveyor  24 . This will substantially prevent the gaps from getting any larger after the control point is determined, and will tend to reduce the disparity in gaps that would otherwise result for large and small articles if the conveyor speeds remained at a fixed ratio. After the speed of conveyor  22  has been brought up to that of conveyor  24 , the existing gaps will not change. The controller would maintain conveyor  22  at the same speed as conveyor  24  up until the moment the control point reached the reference point. At that moment, the controller would decrease the speed of conveyor  22  with respect to conveyor  24  down to an acceptably lower speed such that the control point of the next article could be measured. After this was measured, the speed of conveyor  22  would then be increased to that of conveyor  24  again, and so on. Alternatively, it would be possible to implement this controller by having it make adjustments to the speed of second conveyor  24 , rather than first conveyor  22 . 
     After calculator  56  has determined the control point of an article, this information can be fed to whatever other systems, modules, or structures that can make use of this information. In the embodiment depicted in  FIG. 1 , this information is fed to gap control system  40  where it is used to more accurately control the gapping of articles exiting on take-away conveyor. Alternatively, this information could be used in an induct process such as that described in commonly assigned, U.S. patent application Ser. No. 09/669,170, filed Sep. 25, 2000, and entitled High Rate Induction System, the disclosure of which is hereby incorporated by reference. 
     Gap control system  40  includes a horizontal array of photo-detectors  62  which may be of the same type of photo-detectors as array  42 . Array  62  is located alongside a portion of conveyor  24 , a portion of conveyor  32 , and the gap between conveyor  24  and conveyor  32 . Array  62  preferably, although not necessarily, extends for a distance equal to the maximum expected length of articles that will be conveyed. A gap detector  64  receives the output from array  62  and uses it to determine the gap or gaps between articles that are traveling within the zone of detection of array  62 . As illustrated in  FIG. 1 , gap detector  64  would detect both the gap between articles  34   e  and  f  and the gap between articles  34   f  and  g . Gap detector  64  detects these gaps by determining which individual photo-detectors are obstructed by articles, and which are not. Based either upon the number of photo-detectors that are not obstructed between articles, or the distance between the most widely separated, non-obstructed set of continuous photo-detectors, the length of a gap can be determined. Also, by determining which individual photo-detectors are not obstructed, the location of the gap with respect to conveyor  24  can be determined based upon the known position of each of the photo-detectors. 
     After gap detector  64  detects the one or more gaps which are within the detection zone of array  62 , it passes this information to a gap selector  66 . Gap selector  66  chooses which of the gaps detected by detector  64 , if there are more than one, to use in the feedback control loop that controls the speed of conveyor  24 . The decision of which gap to use is based upon the location of the control point  52 . In the position illustrated in  FIG. 1 , the control point  52  of article  34   f  has not yet reached the center of the space between conveyors  24  and  32 . The speed of article  34   f  is therefore still being controlled by conveyor  24 . By adjusting the speed of conveyor  24 , it is therefore still possible to adjust the gap between article  34   f  and article  34   g . Gap selector  66  would therefore choose this gap (between articles  34   f  and  g ) as the gap to be used within the feedback control loop. The dimensions of this gap would then be fed into a comparator which also receives an input from a desired gap  70 . The desired gap  70  can be chosen according to the needs of the rest of the conveying system, or can be chosen in any other manner. As illustrated in  FIG. 1 , the gap between articles  34   g  and  34   h  should correspond to the desired gap. Comparator  68  compares the measured gap from gap selector  66  to the desired gap  70  and outputs the difference between these gaps to a feedback controller  72 . Feedback controller  72  outputs a command to a variable speed motor controller  74  that causes controller  74  to change the speed of conveyor  24  as commanded. This command is generated in response to the difference between the measured gap and the desired gap  70 . Variable speed motor controller  74  outputs a signal to a motor  76  that powers conveyor  24 . The particular type of motor  76  is not limited by the present invention and the location of motor  76  with respect to conveyor  24  can vary from that depicted in  FIG. 1 . 
     Feedback controller  72  may be any type of feedback controller, such as a proportional-integral-derivative (PID) controller, or any other type of feedback controller. The particular form of the feedback equation or equations used by feedback controller  72 , along with the particular constants or gains used in the equation(s), will vary depending upon the type of controller used, the responsiveness of the controlled motor or motors, the feedback rate, and other factors, as would be understood by one skilled in the art. While other command rates can be used, having feedback controller  72  update and issue commands approximately twenty times per second should generally be sufficient. The commands issued from feedback controller  72  may be either digital or analog. If digital, the operating range of the motor is preferably digitized such that it can be commanded to operate at over one hundred different speeds. Less digitization, however, can also be used within the scope of the invention. The repetitive measuring of the article gaps by array  62  may take place at the same rate as the commands are issued from controller  72 , or a different rate. 
     As article  34   f  moves along conveyor  24 , feedback controller  72  will repeatedly issue commands to motor  76  to adjust its speed in order to create the desired gap between articles  34   f  and  34   g . Feedback controller  72  will continue to output commands to adjust the gap between article  34   f  and article  34   g  until control point  52  reaches the midpoint between conveyors  24  and  32  (assuming the control point to be based on a reference point midway between conveyors). When control point  52  has reached this midpoint, article  34   f  is no longer being controlled by conveyor  24 , despite the portion of article  34   f  that is still physically located on conveyor  24 . Adjustments to motor  76  in order to change the gap between articles  34   f  and  34   g  are therefore ineffective. Thus, when control points  52  of article  34   f  has reached the midpoint between conveyors  24  and  32 , gap selector  66  chooses the gap between articles  34   e  and  f  as the gap whose dimensions are being input into comparator  68 . Thereafter, any adjustments in the speed of motor  76  will affect the gap between articles  34   e  and  f , rather than articles  34   f  and  g . Any necessary adjustments to the gap between articles  34   e  and  f  will continue to be implemented by gap control system  40  until the control point of article  34   e  reaches the midpoint between conveyors  24  and  32 . At that point, the gapping control will switch to the next upstream gap, and so on. 
     By being able to accurately determine the control point of each article, such as by using control point determination module  38 , the control of the gapping performed by gap control system  40  is enhanced. Such control point information allows the gap control system  40  to continue to make any necessary changes in the gap of interest right up until such control is no longer possible. Such control point information also helps ensure that the changes being made to the speed of conveyor  76  will affect only the intended gap and not others. As stated previously, the invention contemplates that such control point information may come from control point determination module  38 , or it may come from other sources as well. For example, it would be possible to utilize gap control system  40  wherein the control point information was merely an assumption as to the location of the control point, such as the center point of the article. While such a system would likely be more inaccurate than a system in which the control point was actually measured, gap control system  40  finds equal applicability to such a system. 
     It will be understood by one skilled in the art that the particular conveyor arrangement indicated in  FIG. 1  could vary significantly from that depicted. For example, it would alternatively be possible to implement gap control system  40  in a manner in which the conveyor whose speed was modified to control the gaps was the downstream conveyor, rather than the upstream conveyor. It would also be possible to implement multiple gap control systems  40  sequentially. Such a sequential array of gap control systems might further increase the accuracy of the final gap, particularly where the desired gap differs substantially from the gaps that pre-exist between the articles being fed into gap control system  40 . In still another alternative, gap control system  40  could be modified to include one or more additional conveyors upstream of conveyor  24  that used an open loop type of control in order to make refinements to the article gaps such that gapping control system  40  only had to make minor adjustments, if any, to create the desired gaps. In still another alternative embodiment, the horizontal photo-detector arrays could be replaced by other sensors that provided repeated updates about the status of gaps and the position of articles. One such sensor could be a camera or cameras appropriately mounted to visually record the movement of articles. Such video images could be processed by the appropriate software in order to measure the gaps and article lengths in order to provide input into the feedback loop. An acoustic sensor could also be used as an alternative to the photo-detector array, as well as still further types of sensors. 
     As still another variation, it would possible to adapt feedback controller  72  to limit the acceleration that motor  76  is commanded to undertake. Such limits on acceleration would primarily be designed to avoid tipping any of the articles traveling on conveyor  22 . Such limits could be modified based on the determination of control point  52 . The more forwardly control point  52  is located on the article to be accelerated, the more acceleration the article can generally withstand without tipping, when all other factors remain equal. If provisions are made to determine the height of the article, this information can also be used to determine the appropriate acceleration that an article can undergo without tipping. The length of the article could also be factored into the determination of the acceptable acceleration for an article, as would be understood by one skilled in the art. 
     While the present invention has been described in terms of the preferred embodiments depicted in the drawings and discussed in the above specification, along with several alternative embodiments, it will be understood by one skilled in the art that the present invention is not limited to these particular embodiments, but includes any and all such modifications that are within the spirit and the scope of the present invention as defined in the appended claims.