Patent Publication Number: US-6655677-B2

Title: Active gap controlled feeder

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to document feeders, and, more specifically, to document feeders in commercial high speed document processing equipment. 
     Document feeders are common in various types of equipment in which a stack of documents or sheets of paper are to be processed one-by-one in sequence. Common sheet feeders are found in copying machines and fax machines which typically operate at relatively low sheet feeding speeds. 
     As the feeding speed increases, the document feeders configured therefor typically increase in complexity and cost in view of the increased difficulty associated therewith. For example, banking institutions or other processing centers use sophisticated check processing machines for reading, imaging, encoding, and sorting commercial checks at considerable speed. Four hundred checks or documents per minute (400 dpm) carried through the processing machine is a relatively high rate which is typically maintained continuous for one or more business operating shifts per day. 
     The checks are loaded in stacks into a hopper portion of the machine which individually transports the checks in turn through the remainder of the machine along a check feedpath defined by driven rollers and belts and associated guiding elements. A controller in the form of a digitally programmable computer controls the entire operation of the machine including the speed of the various motors driving the various transport rollers and belts for maximizing check processing throughput, without unacceptable jamming of the checks or other malfunctions in any one of the modules of the machine. 
     Optical sensors are disposed along the entire check feedpath through the machine for detecting the presence of the checks and the motion thereof for use in controlling operation and detecting any jamming which may occur. Malfunction or jamming must be instantaneously detected by the sensors for correspondingly interrupting operation of the driving motors to limit the adverse affects thereof. 
     Any interruption in normal operation of the check processing machine correspondingly reduces its effective check processing throughput, and is commercially undesirable as a loss of valuable processing time with a corresponding increase in associated costs. 
     Maximum throughput of the series of checks transported through the machine is limited in practice by a minimum desirable spacing or gap between the traveling checks. For example, the encoding module of the machine must have sufficient time to print along the bottom edge of the check a suitable code indicative of the check processing or clearing operation. 
     Correspondingly, the desired processing throughput rate controls the maximum gap between traveling checks since the checks may be more widely spaced apart at slower transport speeds if desired. 
     Nevertheless, it is common practice to use the optical sensors in the machine for determining intercheck gaps during operation and adjust those gaps as desired. Since relatively large gaps provide sufficient time for processing each check in each of the various modules of the machine, the control thereof does not require either high precision or fast response. For example, one check processing machine enjoying years of successful commercial use in this country is designed for a throughput rate of about 400 checks per minute for continuous operation of the machine without malfunction or jamming due to excessively small intercheck gaps. 
     Commercial checks vary in length from about 4.8-9 inches (12.2-22.9 cm) in length and about 2.75-5 inches (7-12.7 cm) in width or vertical height. With a nominal check length of about 6 inches (15.2 cm) a nominal intercheck gap of about 9 inches (22.9 cm) will occur at the 400 dpm throughput rate. This relatively large gap may be effectively measured by optical sensors placed several checks downstream in the feedpath from the inlet hopper tray. Measurement of the size of the downstream gaps may be used to adjust gap size upstream therefrom, typically by intermittently halting operation of the advance drive motor for correspondingly increasing intercheck gaps as desired. 
     However, in a higher speed check processing machine being developed, the nominal intercheck gap is closer to the minimum permitted value for the machine which can therefore randomly result in occasional intercheck gaps less than the permitted minimum. When sub-minimum gaps occur, downstream monitoring sensors can lead to operational warnings or automatic interruption in the check processing machine requiring remedial correction. And, sub-minimum gaps may also cause undesirable jamming of the checks which also requires remedial operator intervention which substantially reduces the overall throughput rate of the machine during use. 
     Accordingly, it is desired to provide an improved document feeder and method of operation for actively controlling interdocument gaps. 
     BRIEF SUMMARY OF THE INVENTION 
     A document feeder includes advance, separation, and transport rollers for driving documents in turn along a feedpath from an input tray containing a stack of the documents. A sensor is disposed between the separation and transport rollers for detecting presence of the documents therebetween. A controller is joined to the rollers for controlling speed thereof, and is operatively joined to the sensor for measuring interdocument gaps between the separation and transport rollers. The controller actively adjusts the interdocument gaps by temporarily changing speed of the separation roller at the beginning of the feedpath near the input tray. The interdocument gaps are measured and corrected between the separation and transport rollers for precise control thereof while maximizing throughput. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, in accordance with preferred and exemplary embodiments, together with further objects and advantages thereof, is more particularly described in the following detailed description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is an isometric view of an exemplary check processing machine having a document feeder in accordance with an exemplary embodiment of the present invention. 
     FIG. 2 is an enlarged schematic view of respective portions of adjoining separation and transport rollers found within the dashed circle labeled  2  in FIG. 1, between which intercheck gaps are measured and corrected in accordance with an exemplary embodiment of the present invention. 
     FIG. 3 is a schematic top view of a portion of the document feeder illustrated in FIG. 1 including an exemplary feedpath therethrough. 
     FIG. 4 is a flowchart representation of an exemplary method of operating the document feeder illustrated in FIG. 3 in accordance with one embodiment of the present invention. 
     FIG. 5 is a flowchart representation of an exemplary method of operating the document feeder of FIG. 3 in accordance with a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Illustrated in FIG. 1 is an apparatus or processor  10  for processing at high speed a stack of documents  12  in the exemplary form of conventional commercial checks being processed or cleared by a typical banking institution or processing center. The check processor includes various modules suitably joined together in a row, including at one end a document feeder  14  followed in turn by modules for reading, imaging, encoding, endorsing, verifying, and sorting the checks in a conventional manner. 
     The individual checks  12  are transported through the various modules at a relatively high rate of speed for maximizing the throughput processing rate thereof for substantially continuous operation in working shifts. Maximum throughput is typically limited by the minimum allowable interdocument or intercheck gaps between the checks as they are transported through the machine. 
     For example, the processor includes a check encoder  16  as one of the intermediate modules which is conventionally configured for printing a processing code along the bottom of each check being processed. The encoder is typically the one module which sets the minimum intercheck gap acceptable for continuous operation of the machine without premature malfunction warnings or undesirable check jamming. Accordingly, the speed and throughput rate of the individual checks may only be maximized provided that the intercheck gap does not decrease below the specified minimum. 
     A portion of the document feeder is illustrated schematically in FIG. 2 in which a first one of the documents or checks designated  12   a  is being transported in sequence ahead of a following second document or check designated  12   b , with succeeding checks being transported in sequence one-by-one. In this way, any two checks being transported in turn through the processor include a leading check  12   a  followed directly behind by a trailing check  12   b.    
     And, each check includes a leading edge  18  traveling first through the processor followed in turn by a longitudinally opposite trailing edge  20 . An interdocument or intercheck gap G is thusly defined by the spacing between the trailing edge  20  of the first check  12   a  and the leading edge  18  of the following second check  12   b.    
     FIG. 3 illustrates schematically an exemplary embodiment of the major components of the transport mechanism for the document feeder  14  illustrated in FIG. 1, which corresponds in part with the portion thereof illustrated in FIG.  2 . The document feeder  14  is also referred to as a hopper since it includes an input hopper or tray  22  in which a stack of the checks  12  is placed for processing in a vertically standing position as shown in top view in FIG.  3 . The checks may have any conventional configuration, and are typically thin sheets of paper which vary in length L and width or height as indicated above. 
     The feeder includes a picker  24  in the exemplary form of a driven urethane belt for frictionally engaging each of the checks in turn for driving them through a feedpath commencing at the input tray  22  and terminating at the opposite end of the machine illustrated in FIG. 1 at any one of several output trays used for sorting the checks. The feedpath is defined by cooperating transport rollers or belts and guide rails specifically configured for the precision transport of the individual checks through the processor. 
     In the preferred embodiment illustrated in FIG. 3, an advance roller  26  adjoins the input tray  22  for first driving the checks sequentially in turn along the feedpath as they are delivered thereto by the picker  24 . The advance roller is preferably in the form of a silicone wheel surrounding a central hub which is driven by a timing belt from a corresponding first drive motor  28 . The drive motor is preferably a stepper motor which precisely drives both the advance roller  26  and the picker belt  24  through the common timing belt. 
     A separation roller  30  adjoins the advance roller downstream therefrom for further driving the checks in turn along the common feedpath from the advance roller. The separation roller  30  is in the exemplary form of a conventional aluminum drum having a textured drive surface directly driven by a second drive motor  32  suitably joined thereto. The second drive motor is preferably also a stepper motor for precisely controlling movement of the separation roller  30 . 
     A transport roller  34  is spaced downstream from the separation roller  30  along the common feedpath for further driving the checks downstream therefrom. The transport roller is in the preferred form of a neoprene belt suitably supported on guide wheels and suitably driven by a third stepper drive motor  36  operatively joined thereto by another timing belt. 
     Cooperating with the advance roller  26  is a retard roller  38  in the exemplary form of a urethane foam belt suitably mounted on supporting wheels in a conventional manner for adjusting the tension thereof. The retard roller  38  is driven by a fourth drive motor  40  with a cooperating reduction gearbox for driving the retard roller at a substantially lower speed than that of the advance roller  26  in a conventional manner for retarding feeding of more than one check at a time from the input tray  22 . The advance roller  26  and retard roller  38  cooperate with each other on opposite sides of the feedpath for initially receiving a single check at a time into the feedpath. 
     The separation roller  30  cooperates with an idler roller  42  in the exemplary form of a urethane belt suitably mounted on guide wheels on opposite sides of the feedpath. The idler belt  42  presses and holds checks against the driven separation roller  30  during operation. And, the transport roller  34  cooperates with corresponding spring loaded idler rollers  44  disposed on opposite sides of the feedpath corresponding with the guide wheels supporting the belt roller  34 . 
     An electrical controller  46  in the exemplary form of a digitally programmable computer is operatively joined to the advance, separation, and transport rollers through their corresponding stepper motors  28 , 32 , and  36  which control their rotary operation. In this way, the speed of the individual stepper motors may be accurately controlled by the controller for in turn controlling transport speed of the individual checks along the feedpath. 
     In basic operation, the corresponding stepper motors for the advance, separation, and transport rollers  26 , 30 , 34  are driven to obtain the desired throughput rate for transporting the individual checks in sequence through the check processing machine. The separation and transport rollers  30 , 34  preferably both run at the same transport speed which is maximized to maximize the throughput rate. The advance roller  26  is run at a suitable fraction of the speed of the separation roller  30  to generate separation or the interdocument gaps between successive checks. The first stepper motor  28  preferably has variable speed control effected by the controller to adjust the intercheck gap at its creation. 
     The controller  46  is thusly configured to operate the separation roller  30  at a speed suitably greater than that of the advance roller  26  to effect a natural intercheck gap between successive checks carried from the advance roller to the separation roller, and in turn downstream to the transport roller. 
     The retard roller  38  is operated slower than the advance roller  26  to ensure feeding of one check at a time from the input tray into the feedpath. In this way, the natural intercheck gap is created as each check leaves the slower moving advance roller  26  and is driven downstream by the faster moving separation roller. 
     As shown in FIG. 2, the separation roller creates the intercheck gaps which vary in size according to the length of the individual checks. Since longer checks remain longer under the control of the slower advance roller, the gap behind the leading check under control of the faster moving separation roller  30  increases. Correspondingly, shorter checks remain under control of the slower advance roller for a shorter time leading to a smaller gap with the downstream leading check being driven by the higher speed of the separation roller. 
     The document feeder as described above is conventional in configuration and operation and works effectively without unacceptably small intercheck gaps up to a limited high throughput rate of about 400 dpm as indicated above. For a nominal check length of about 6 inches (15.2 cm) the nominal intercheck gap G is about 9 inches (22.9 cm) which is more than adequate for proper operation of all the modules of the check processor. 
     Any variation in that nominal gap due to variations in check length or to variations in the friction driving from the various rollers in the feedpath do not significantly change the large intercheck gap. Intercheck control of such a large gap may therefore be effected in any conventional manner including control of the relative speeds of the advance and separation rollers by gap detection several checks downstream in the feedpath past the transport roller. 
     However, and as indicated above, further increases in document throughput rate correspondingly result in a reduction of the nominal intercheck gap which may then become closer to the permissible minimum gap for proper operation without malfunction or jamming, and the intercheck gap control becomes more critical. 
     For example, by increasing the throughput rate to 600 dpm a nominal intercheck gap of about 4 inches (10.2 cm) for the same nominal check length results which is relatively close to the minimum permitted intercheck gap. Variation in check length in addition to variation in the friction driving capability of the driven rollers randomly affects the actual intercheck gap which may occasionally fall below the permitted minimum. 
     In such an event, the sub-minimum gaps will be detected by optical sensors in the downstream modules of the check processor leading to an operator warning or alert, and possibly shutdown of the check processor for corrective action. The sub-minimum intercheck gap may also cause undesirable jamming of checks during the high speed transport thereof also requiring operator intervention and correction. 
     In accordance with the present invention as initially illustrated in FIG. 2, a proximity sensor  48  in the preferred form of an optical sensor is fixedly mounted in the check feedpath between the separation and transport rollers  30 , 34  for detecting presence, or lack thereof, of each of the checks as they are transported therebetween. The optical sensor  48  may be identical to those optical sensors conventionally located along the entire feedpath of the check processor, and includes a phototransistor on one side of the feedpath cooperating with a light emitter  48   b  in the exemplary form of a light emitting diode on the opposite side of the feedpath. 
     As the checks are transported past the optical sensor  48 , the light beam is periodically interrupted by the passing checks indicating the presence thereof. The intercheck gaps do not interrupt the light beam and are thusly correspondingly detected. 
     The single optical sensor  48  placed between the separation and transport rollers provides a manner for directly measuring the intercheck gap at the beginning of the high speed transport feedpath to control the size thereof and prevent sub-minimum gaps from traveling downstream through the processor modules, including the encoder module  16  with its corresponding minimum intercheck gap requirement. 
     As illustrated in FIG. 3, the optical sensor  48  is operatively joined to the controller  46  which is configured in accordance with the present invention for measuring the intercheck gaps G on the fly between the separation and transport rollers. The controller  46  is also configured for correspondingly actively adjusting the intercheck gaps by temporarily changing speed of the separation roller  30  as required based on the measured value of the specific intercheck gap. In this way, active gap correction is immediately effected at the beginning of the transport portion of the feedpath for precisely controlling intercheck gaps downstream therefrom irrespective of variation in check length or frictional characteristics of the advance and separation rollers  26 , 30  initially forming the intercheck gaps. 
     Since the controller  46  is preferably in the form of a digitally programmable computer, it may be conventionally programmed in suitable software for measuring the magnitude of the intercheck gaps using the optical sensor  48 , and then calculating the necessary correction in the measured gap which is immediately implemented by temporarily adjusting the speed of the separation roller  30  while the corresponding trailing check  12   b  is within the control thereof. Simultaneously, the speed of the advance roller  26  is also temporarily adjusted to maintain constant the speed fraction with the separation roller  30 . 
     For example, if the gap between the leading check  12   a  and the trailing check  12   b  is too small, the trailing check  12   b  may be temporarily retarded for increasing the gap at its leading edge  18  to meet the desired gap requirement. If the gap is initially too large, the trailing check  12   b  may be temporarily accelerated in speed to reduce the gap at its leading edge  18  to the desired gap requirement. After each gap correction, the speed of the separation roller  30  is returned immediately to its nominal transport speed for maximizing the throughput rate of the checks in the preferred embodiment. In this way, active gap control occurs check-by-check at the commencement of the transport feedpath. 
     Accordingly, by the simple introduction of the optical sensor  48  and associated changes in the controller  46 , active gap correction may be effected in a relatively simple manner for accommodating the different intercheck gaps created by variation in check length and variation in roller frictional materials. The optical sensor  48  is used for measuring the actual intercheck gaps between the separation and transport rollers  30 , 34 , and then the controller  46  is used for temporarily changing speed of the separation roller  30  by precisely controlling its driving stepper motor  32  for actively adjusting the size of the actual gap measured by the sensor. 
     FIG. 4 illustrates in flowchart form an exemplary method of operating the document feeder illustrated schematically in FIG.  3 . Cooperating with the separation roller  30  is a first rotary encoder  50  for measuring rotary speed thereof, as well as position in a preferred embodiment. The rotary encoder may have any conventional configuration and may be suitably joined to the separation roller  30  in various manners. For example, the encoder may be attached to the drive shaft of the stepper motor  32  which in turn drives the separation roller  30 . 
     The rotary encoder typically includes a disk having a multitude of slits around the perimeter thereof which cooperate with a conventional optical sensor in the form of a light emitter and detector bridging the disk whose light beam is interrupted by the rotating encoder disk. In this way, the circumferential position of the disk and the drive shaft may be readily determined along with the rotary speed thereof in a conventional manner. Since the stepper motor  32  drives the separation roller  30 , the rotary encoder  50  may also be used for determining the rotary position of the roller itself and, in particular, the longitudinal position of the checks being transported thereby. 
     Correspondingly, a second rotary encoder  52  cooperates with the transport roller  34  in a manner substantially identical with that of the first encoder and the separation roller for measuring rotary position of the transport roller  34  by rotary position of its drive motor  36 , for in turn measuring the longitudinal position of the checks being driven by the transport roller. 
     The two encoders  50 , 52  are preferably identical in configuration and encoding accuracy and are both operatively joined to the controller  46  illustrated in FIG. 3 for use in precisely controlling the transport speeds of both separation and transport rollers  30 , 34  during operation. The controller  46  is further configured to measure or calculate the intercheck gap G between the trailing edge  20  of the first or leading check  12   a  passing the sensor  48 , and the leading edge  18  of the second or trailing check  12   b  which passes the sensor  48  immediately behind the leading check. 
     One manner of measuring the intercheck gap G using the single sensor  48  is by also measuring corresponding difference in position of the trailing edge of the first check as indicated by the second encoder  52 . 
     Preferably the first and second encoders  50 , 52  are high resolution encoders effective for accurately measuring both rotational position and rotary speed. Each encoder may include, for example, 1,000 slits around the disk circumference cooperating with a pair of optical sensors in quadrature for providing corresponding 4,000 count resolution for each revolution. In this way, the rotary position of the second encoder  52  may be used for determining the precise position of the first check  12   a  being carried by the transport roller  34  with sufficient precision for the desired 600 dpm rate of travel of the checks along the feedpath. 
     The preferred method of operation of the check feeder is to initially operate the various rollers including the transport roller  34  at the desired transport speed preferably corresponding with the maximum feedrate of about 600 dpm for example. In this way, the nominal intercheck gap will be about 4 inches (10.2 cm) for the nominal 6 inch (15.2 cm) check length. 
     As the first check  12   a  is being transferred from the separation roller  30  to the transport roller  34 , the light beam to the optical sensor  48  is interrupted until the trailing edge  20  travels downstream past the sensor. In this way, the trailing edge of the first document may be readily detected by the sensor  48 , at which time the controller is used for position stamping the second encoder  52  for indicating the corresponding rotary position of the encoder disk thereof corresponding with detection of the trailing edge of the first document. This position stamp location is suitably stored in memory associated with the controller. 
     As the first check continues its downstream travel, the next following second check  12   b  is carried by the separation roller  30  toward the sensor  48 . The sensor may then detect the leading edge  18  of the second check  12   b  as the light beam to the sensor is interrupted, and the second encoder  52  is again position stamped for determining the corresponding position of the trailing edge of the first check being driven by the transport roller  34  which corresponds with detection of the leading edge of the second check. 
     By calculating in the controller the positional difference between the position stamped trailing and leading edges of the successive checks  12   a,b  the corresponding gap G may be determined. The intercheck gap G is simply the difference in longitudinal position of the trailing edge  20  of the first check  12   a  as it is driven by the transport roller  34  during the time interval between detection of its trailing edge  20  and detection of the following leading edge  18  of the second check  12   b.    
     Once the size of the intercheck gap G is determined, any required correction thereof may be effected. For example, if the gap G is too small, the separation roller  30  may be retarded for increasing the gap as the first check continues to move downstream. If the measured gap G is too large, the separation roller  30  may be accelerated to decrease the gap as the second check is driven closer to the preceding first check. 
     Accordingly, by temporarily changing speed of the separation roller  30  in response to the measured gap, the intercheck gap may be actively adjusted as both the first and second checks  12   a,b  are independently driven by their respective transport and separation rollers  34 , 30 . 
     In the preferred embodiment it is desired to operate the document feeder at maximum throughput rate near the minimum desired intercheck gap for the checks of maximum length. Shorter checks will then effect a correspondingly smaller intercheck gap which may be actively increased check-by-check as required by temporarily reducing the speed of the separation roller  30  to correspondingly increase the intercheck gap as the second check  12   b  is driven by the separation roller. 
     The desired amount of intercheck gap change is simply the difference between the measured gap and the desired gap which is preferably slightly greater than the minimum gap design requirement. And, the active gap correction may be effected by increasing speed of the separation roller for decreasing gaps, or decreasing speed of the separation roller for increasing gaps relative to the nominal transport speed of the transport roller  34 . 
     In order to precisely adjust the intercheck gap, the leading edge  18  of the second document  12   b  is detected by the sensor  48  upon commencement of interruption of the light beam thereto, at which time the first encoder  50  corresponding with the rotary position of the separation roller  30  is suitably position stamped by the controller. As shown in FIG. 4, position stamping of the leading edge  18  of the second check  12   b  corresponds with a constant advance length A between the optical sensor  48  and the last pinch point at the end of the driving control of the separation roller  30  for the second check prior to transferring transport control to the downstream transport roller  34 . The minimum check length B is another constant value, and the difference C between the minimum check length and the advance length A is the minimum remaining length of the second check subject to control of the separation roller  30  prior to being discharged downstream therefrom. 
     Accordingly, active correction of the intercheck gap G may occur in some or all of the remaining length C of the second check under control of the separation roller  30 . The speed of the separation roller  30  may be changed in a myriad of combinations within the available remaining check length C for actively adjusting the intercheck gap G from its initial measured value. 
     For example, the separation roller  30  may be temporarily halted to increase the intercheck gap as desired. The duration of the roller halt is simply the time required to increase the intercheck gap the desired amount based on the transport speed of the downstream first check  12   a . The halt time is simply the desired gap increase divided by the linear speed of the downstream check  12   a , with any suitable adjustment for electrical and mechanical response delays. 
     However, in the preferred embodiment illustrated in FIG. 4, the intercheck gap spacing between the two checks  12   a,b  is increased by simply reducing the speed of the separation roller  30  to a speed below the nominal transport speed thereof, but greater than zero. Since the remaining length C for active gap correction is relatively long, the desired gap correction may be effected over the entire length C which typically requires only slight reduction in the speed of the separation roller  30 . 
     After the speed of the separation roller  30  is temporarily changed for effecting the desired active correction of the intercheck gap G, the speed change of the separation roller is terminated to return the separation roller to its nominal transport speed, which is preferably equal to the transport speed of the transport roller  34 , prior to actively adjusting the intercheck gap of the next check being carried by the separation roller. In this way, the gap between each two successive checks being carried between the separation and transport rollers is initially measured, compared with the desired intercheck gap, and adjusted either up or down as required by correspondingly changing the speed of the separation roller  30  on the fly. 
     The active gap correction implemented in the embodiment illustrated in FIG. 4 simply introduces the single optical sensor  48  between the separation and transport rollers, the high precision rotary position encoders  50 , 52 , and a suitably high speed controller  46  for effecting the corrections in the milliseconds required as each check is transported at the relatively high rate of 600 dpm in the preferred embodiment. Although rotary encoders and controllers are conventional, the high precision rotary position encoders  50 , 52  and cooperating controller  46  used in the FIG. 4 embodiment have increased costs associated therewith. Cost is a significant competitive factor in the production of commercial high speed check processing machines, and therefore it is desired to effect active gap correction at reduced costs. 
     Accordingly, FIG. 5 illustrates an alternate embodiment of the present invention corresponding with the hardware illustrated in FIG. 3, but with significant savings in the cost thereof. In this embodiment, instead of using the high precision rotary position encoders  50 , 52  illustrated in the FIG. 4 embodiment, less costly first and second rotary speed encoders  50 B, 52 B are used in conjunction with the corresponding stepper drive motors  32 , 36 . Instead of having the high 4,000 count capability of the rotary position encoders  50 , 52  illustrated in FIG. 4, the rotary speed encoders  50 B, 52 B illustrated in FIG. 5 may have a substantially lower count capability, 200 counts for 50 slits per revolution in quadrature, which is sufficient for accurately measuring speed of the corresponding drive motors  32 , 36 . 
     The rotary speed encoders are similarly operatively joined to the controller  46  illustrated in FIG. 3 for providing feedback thereto of the measured speeds of the separation and transport rollers. As shown schematically in FIG. 5, the controller includes a cooperating precision clock  54  and is operatively joined to the optical sensor  48 . 
     With these elements, the controller is preferably configured for measuring the intercheck gap G between the trailing edge  20  of the first check  12   a  that passes the optical sensor  48  and the leading edge  18  of the second check  12   a  passing the sensor in turn, by the product of the corresponding difference in time or elapsed time between detection of the trailing and leading edges and the speed of the first check  12   a  driven by the transport roller. Since the transport roller  34  preferably operates at a constant rotary speed, the corresponding linear transport speed of the checks being driven thereby is also constant. By measuring the time difference between the successive trailing and leading edges of the checks, the corresponding intercheck gaps therebetween is simply the product of that time difference and the linear speed of the downstream first check  12   a.    
     As shown schematically in FIG. 5, the optical sensor  48  is used for detecting the trailing edge  20  of the first check  12   a  passing the sensor under control of the transport roller  34 , and the controller is used for time stamping the clock corresponding with the location of the trailing edge. 
     As the checks continue to be transported along the feedpath, the same optical sensor  48  is again used for detecting the leading edge  18  of the following second check  12   b  as it reaches the sensor, and the controller again time stamps the clock for determining the corresponding time of leading edge detection. 
     The length of the intercheck gap G may then be readily calculated by the product of the differential or elapsed time between detection of the time stamped trailing and leading edges and the speed of the first check  12   a  driven by the constant speed transport roller  34 . 
     Since an accurate measurement of the intercheck gap G has now been determined, that gap is compared in the controller to the desired gap stored in memory and adjusted either up or down as required. As indicated above, the speed of the separation roller  30  may again be temporarily changed after time stamping of the leading edge of the second check  12   b  to actively adjust the intercheck gap as the second check  12   b  is driven by the separation roller for the remaining portion of the length thereof. 
     In the preferred embodiment illustrated in FIG. 5, the separation roller  30  is temporarily halted to a complete stop of zero speed for a suitable portion of the remaining length of the second check  12   b  being driven thereby to correspondingly increase the intercheck gap to a value greater than or equal to the desired minimum gap. The temporary halting of the separation roller  30  occurs for a time duration equal to the required increase in intercheck gap divided by the linear transport speed of the downstream first check  12   a . That gap correction can be effected within a portion of the remaining length of the upstream second check  12   b  within the control of the separation roller  30 , with suitable adjustments as desired for the electrical and mechanical time lags associated with temporarily halting the separation roller  30  and re-accelerating the separation roller back to its nominal transport speed. 
     This active gap control process is readily effected with less costly speed encoders  50 B, 52 B, and with a correspondingly less expensive controller  46  which requires less computational capability and response time for implementing the time-based active gap correction. 
     A particular advantage of the active gap correction apparatus disclosed above is its ability to be operated for further maximizing the throughput rate of the document feeder while maintaining relatively small intercheck gaps substantially equal to or only slightly greater than the desired minimum intercheck gap. As indicated above, the encoder  16  illustrated in FIG. 1 typically sets the minimum acceptable intercheck gap for the entire check processing machine. That minimum gap may be about 3 inches (7.6 cm) for example while the check processor is operated at the 600 dpm high throughput rate. 
     Accordingly, the separation and transport rollers  30 , 34  illustrated in FIG. 3 may be initially operated to drive the checks  12  between the feeder and the encoder at the maximum desired throughput rate, with an initial intercheck gap initially selected to be less than the minimum acceptable gap for continuous operation of the encoder  16  without malfunction or failure thereof in accordance with its design specifications. The advance roller  26  is correspondingly operated at a slower rotational speed than the separation roller  30  to effect the less than minimum intercheck gap. 
     The optical sensor  48  and the rotary encoders may then be used as described above for measuring the actual intercheck gap between successive checks as they are transferred between the separation and transport rollers. 
     The controller  46  uses the feedback from the optical sensor and rotary encoders for temporarily reducing the speed of the separation roller  30  as described in the various embodiments disclosed above for each of the checks in sequence as they pass the optical sensor to suitably increase the intercheck gap to a value either equal to or slightly greater than the desired minimum gap for maintaining the maximum throughput rate. 
     In this way, the check processing machine may be configured for maximizing throughput while minimizing the intercheck gap according to that minimum gap required by any one of the modules thereof. The checks are initially transported with sub-minimum intercheck gaps and actively corrected on the fly in the feeder module to precisely achieve the desired minimum intercheck gap for effective operation of the various modules downstream therefrom. 
     Irrespective of the variation in lengths of the checks being processed, and irrespective of the frictional characteristics of the advance and separation rollers  26 , 30  initially effecting the intercheck gaps, the active gap control is effected at the beginning of the transport portion of the feedpath for ensuring maximum throughput operation of the entire check processing machine for maintaining the continuous operation thereof without premature malfunction or jamming due to sub-minimum intercheck gaps. 
     Although various embodiments of the document feeder have been described above for maximizing the throughput rate while minimizing the intercheck gaps, the apparatus may be otherwise operated to advantage. For example, excessively large intercheck gaps may be reduced by temporarily accelerating the separation roller to a higher speed. Acceleration correction of gap length will require a corresponding separation drive motor with sufficient torque and time response, typically at increased cost. However, decreased equipment cost is associated with reducing or temporarily halting operation of the separation roller  30  as required between successive checks for increasing the too small intercheck gaps therebetween. 
     While there have been described herein what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein, and it is, therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention.