Abstract:
A sheet transport system ( 20 ) comprising at least one sheet transporter ( 30,36 ) that receives and hands off a sheet being transported by the system so as to transport the sheet from a first position to a second position, wherein the at least one transporter ( 30,36 ) comprises at least one orifice ( 44 ) through which air is aspirated to create a vacuum that grips a sheet when it is received by the transporter ( 30,36 ); a vacuum system coupled to the at least one orifice ( 44 ) controllable to aspirate air through the at least one orifice ( 44 ) to grip the sheet; at least one vacuum sensor ( 60 ) that generates signals responsive to magnitude of vacuum of the at least one orifice ( 44 ); and a controller ( 49 ) that receives the signals generated by the at least one vacuum sensor ( 60 ) and provides a signal indicative of a location of the sheet in the transport system ( 20 ) from the signals.

Description:
RELATED APPLICATIONS 
   The present application is a U.S. national application of PCT/IL00/00231, filed 18 Apr. 2000. 

   FIELD OF THE INVENTION 
   The invention relates to sheet transport systems and in particular to monitoring the position of a sheet in a sheet transport system and determining the location of a sheet that jams in the system. 
   BACKGROUND OF TE INVENTION 
   A printing machine generally comprises a sheet transport system that receives sheets from a sheet feeder, moves the sheets through various printing stations in the printer and after the sheets are printed transports the sheets to an output tray. Sensors that “watch” for the passage of a sheet as the sheet transport system moves the sheet through the printer monitor the position of the sheet. Usually the sensors are optical sensors or contact sensors that sense a leading and/or trailing edge of the sheet as the edge passes through their respective fields of view or contract positions. The sensors do not provide continuous real time information as to where the sheet is at all times as it passes through the sheet transport system. As a result, if the sheet jams it is often difficult to locate the position at which it jammed and a position of a cause of the jam. In addition, optical and contact sensors are readily dirtied by dust, such as paper dust, from sheets transported by a transport system and have to be cleaned regularly. 
   U.S. Pat. No. 4,369,964 describes a sheet feed apparatus that senses if a sheet fed to an impression roller of a printer seats properly on the roller. The roller is formed with two longitudinal slots that communicate with a source of vacuum that aspirates through the slots. When a sheet to be printed is transferred to the roller, the sheet covers the slots and vacuum generated by the vacuum source at the slots as a result of aspiration of the vacuum source secures the sheet in position on the roller surface. If the sheet doesn&#39;t seat properly on the roller surface the sheet doesn&#39;t completely cover both slots. As a result, the vacuum developed by the vacuum pump at the slots drops below or doesn&#39;t attain full magnitude. A pressure sensor senses that the vacuum is below what it should be and generates an alum indicating a malfunction. 
   The system comprises the impression roller and a set of vacuum suckers that deliver sheets to the roller. The system does not indicate a jam or malfunction of the sucker “delivery system”. The system does not determine if a sheet is improperly seated on the roller as a result of the sheet jamming at a position of the suckers or the suckers improperly holding the sheet and/or transferring the sheet to the roller improperly. 
   SUMMARY OF THE INVENTION 
   An aspect of some embodiments of the present invention relates to providing a sheet transport system comprising a sheet position monitoring system that senses the position of a sheet passing through the system at all times during which the sheet is being transported by the transport system. 
   An aspect of some embodiments of the present invention relates to providing a sheet transport system that senses when a sheet jams or is improperly transported by the transport system and determines where the jam or faulty transport occurs. 
   An aspect of some embodiments of the present invention relates to providing a sheet transport system that provides a real time display of location of a sheet being transported by the system as the sheet moves through the system. 
   A sheet transport system in accordance with an exemplary embodiment of the present invention comprises a plurality of sheet transporters that transport sheets from a first position to a second position. The transporters receive and hand off one to the other sheets being transported from the first position to the second position. Bach transporter is coupled to a source of vacuum controllable to aspirate air through at least one orifice formed in a structure of the transporter. The transporter grips and holds a sheet when the sheet covers the at least one orifice and the vacuum source is controlled to aspirate through the orifice so as to create a vacuum at the orifice. 
   In some embodiments of the invention, when a first transporter hands off a sheet it is holding to a second transporter, the sheet is positioned so that it covers the at least one orifice of the second transporter. Vacuum holding the sheet to the at least one orifice of the first transporter is deceased so that the first transporter releases the sheet. Vacuum at the at least one orifice of the second transporter is increased so that the second transporter grips the sheet and removes it from the first transporter. 
   The magnitude of the vacuum near to or at the at least one orifice of each transporter is monitored by a suitable sensor, hereinafter referred to as a “vacuum sensor”. The vacuum sensor may be a sensor such as a pressure sensor that measures vacuum directly or a sensor, such as a flow meter or other sensor that indicates presence of a vacuum, that provides measurements from which the vacuum can be inferred. 
   In some embodiments of the invention, at any one time during the passage of a sheet through the sheet transport system, which of the transporters in the system is holding the sheet can be determined from vacuum sensed by the vacuum sensors. When a transporter is holding the sheet and the sheet is properly seated on the transporter, its at least one orifice is covered by the sheet and a maximum vacuum suitable for holding the sheet is generated by the vacuum source at the orifice. This maximum vacuum is hereinafter referred to as a “gripping vacuum”. On the other hand if a transporter is not holding the sheet, vacuum at the transporter&#39;s at least one orifice is substantially equal to zero. (The vacuum system is usually controlled not to aspirate air through the at least one orifice of a transporter not intended to hold the sheet and vacuum at the at least one orifice is of course substantially equal to zero. It should be noted however that even if air is aspirated through the transporters at least one orifice, vacuum at the orifice will still be substantially equal to zero or very low because the orifice is not covered.) The position of the sheet can be determined from the position and orientation of the transporter holding the sheet. In some embodiments of the present invention positions of transporters in the sheet transport system are determined using methods and devices known in the art, such a suitable encoder. 
   In addition, vacuum readings may, for some embodiments of the invention, indicate if the sheet being held by a transporter is properly seated on the transporter&#39;s at least one orifice. If the sheet does not cover all of the at least one orifice properly, as might happen for example if the sheet jams in the transport system or falls off the transporter, vacuum at the at least one orifice is reduced below the gripping vacuum. The occurrence of the jam is indicated by a low, aberrant vacuum reading by a vacuum sensor associated with the transporter, which is, or should, be holding the sheet. The location of the jam is indicated by which pressure sensor is sensing the aberrant vacuum and the position of the transporter holding the jammed sheet. 
   In some embodiments of the present invention, the sheet transport system comprises a video monitor and vacuum readings by the vacuum sensors are used to provide a real time display of progress of the sheet on the monitor as the sheet moves through the transport system. If the transport system malfunctions and the sheet is improperly transported, a location in the transport system at which the sheet is improperly transported may be indicated on the video monitor. 
   Some sheet transport systems, in accordance with an embodiment of the present invention, may comprise only a single transporter for moving a sheet from a first position to a second position. The transporter comprises at least one orifice formed in a structure of the transporter coupled to a source of vacuum for holding a sheet that the transporter transports. At least one vacuum sensor senses vacuum at the at least one orifice to monitor transport of sheets by the transporter. 
   In some embodiments of the present invention, sheet transporters are rotating sheet transporters. Each transporter comprises a rotatable shaft and in some embodiments at least one array of suction cups, i.e. “orifices”, for gripping a sheet. The suction cups are mounted to the shaft and communicate with a source of vacuum. Sheet transport systems for printers comprising rotating sheet transporters having suction cups for holding a sheet are described in PCT Applications PCT/IL98/00553, PCT/IL99/00600 and PCT/IL00/00081 which are incorporated herein by reference. 
   There is therefore provided in accordance with an embodiment of the present invention, a sheet transport system comprising: at least one sheet transporter that receives and hands off a sheet being transported by the system so as to transport the sheet from a first position to a second position, wherein the at least one transporter comprises at least one orifice through which air is aspirated to create a vacuum that grips a sheet when it is received by the transporter, a vacuum system coupled to the at least one orifice controllable to aspirate air through the at least one orifice to grip the sheet; at least one vacuum sensor that generates signals responsive to magnitude of vacuum at the at least one orifice; and a controller that receives the signals generated by the at least one vacuum sensor and provides a signal indicative of a location of the sheet in the transport system from the signals. In some embodiments the vacuum system is controllable to release air to the at least one orifice to release the sheet. 
   Additionally or alternatively, the controller uses signals from the at least one vacuum sensor to determine if a sheet being transported by a transporter of the at least one transporter is being transported properly. 
   In some embodiments of the present invention, the sheet transport system includes a video monitor on which the controller displays the location of sheets being transported by the transport system. If a malfunction occurs in the transport system and at a particular location in the transport system a sheet is improperly transported, the controller may indicate the particular location on the video monitor. 
   In some embodiments of the present invention, the transport system comprises a transporter position monitor that continuously monitors position of a transporter of the at least one transporter and generates signals responsive thereto. The controller may use signals from the transporter position monitor to determine a location of a sheet being transported by the sheet transport system. 
   In some embodiments of the present invention the controller determines that a transporter of the at least one transporter is gripping a sheet being transported by the transport system and that the sheet is gripped properly if signals from a vacuum sensor of the at least one vacuum sensor indicate that vacuum sensed by the sensor is greater than a first pre-determined level of vacuum. 
   In some embodiments of the present invention the controller determines that a transporter of the at least one transporter is not gripping a sheet being transported by the transport system if signals from the at least one vacuum sensor indicate that vacuum sensed by the at least one vacuum sensor is less than a second pre-determined level of vacuum. 
   In some embodiments of the present invention the controller determines that a transporter of the at least one transporter is gripping a sheet being transported by the transport system, but that the sheet is gripped improperly, if signals from a vacuum sensor of the at least one vacuum sensor indicate that the vacuum sensed by the sensor is between the first and second vacuum levels. 
   In some embodiments of the present invention the at least one transporter comprises a plurality of sheet transporters. In some embodiments of the present invention, sheet transporters of the plurality of sheet transporters seriatim receive and hand off a sheet being transported by the system so as to transport the sheet from the first position to the second position. 
   In some embodiments of the present invention, a transporter of the at least one transporter is a rotary transporter, which rotates about an axis to receive and hand off a sheet being transported by the transport system. 
   In some embodiments of the present invention, an orifice of the at least one orifice is a suction cup. 
   In some embodiments of the present invention, the vacuum sensor is a pressure sensor. 
   In some embodiments of the present invention, the vacuum sensor is a flow meter. 
   In some embodiments of the present invention, the sheet transport system is a transport system that transports sheets printed in a printing system comprising at least tone printing station and wherein the first and second positions are positions of the sheets in the printing system. At least one of the first and second positions may be a position in a printing station of the at least one printing station. The printing station may comprise an impression roller. In some embodiments of the present invention, at the position in the printing station, a sheet is removed from the impression roller. In some embodiments of the present invention, at the position in the printing station a sheet is mounted on the impression roller. 
   There is further provided in accordance with an embodiment of the present invention, a sheet transport system comprising: at least one sheet transporter that receives and hands off a sheet being transported by the system so as to transport the sheet from a first position to a second position; a plurality of sensors that generates signals responsive to a position of a sheet as it is transported by the transport system; a display screen; and a controller that receives the signals generated by the sensors and uses the signals to provide a real time visual display on the screen of a sheet as it moves through the transport system. In some embodiments of the present invention, the visual display shows the sheet in continuous motion moving through the transport system. 

   
     BRIEF DESCRIPTION OF FIGURES 
     Non-limiting embodiments of the present invention are described below with reference to figures attached hereto. In the figures, identical structures, elements or parts that appear in more than one figure are generally labeled with the same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below. 
       FIG. 1  schematically shows a sheet transport system comprising rotary transporters and a sheet position monitoring system for sporting sheets in a tandem printer, in accordance with an embodiment of the present invention; 
       FIG. 2  schematically shows a perspective view of a rotary transporter comprised in the transport system shown in  FIG. 1 ; 
       FIG. 3  schematically shows transport of a sheet by transporters of the transport system shown in FIG.  1  and vacuum readings from pressure sensors of the transporters that monitor the transport, in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
     FIG. 1  schematically shows a side view of an example of a sheet transport system  20 , in accordance with an embodiment of the present invention, for transporting sheets from a first impression roller  22  of a tandem printer to a second impression roller  24  of the tandem printer. 
   Transport system  20  may comprise four rotary transporters  30 ,  32 ,  34 ,  36  that remove a sheet from impression roller  22  after a first side of the sheet is printed and seriatim hand the sheet off one to the other to transfer the sheet to impression roller  24  where a second side of the sheet is printed. In some embodiments a transporter  38  removes the sheet from impression roller  24  after the second side is printed and transports the sheet to an output tray or another impression roller (not shown). 
   By way of example, rotary transporters  30 ,  34 ,  36  and  38  are similar in construction and when transport system  20  is transporting a sheet from impression roller  22  to impression roller  24  each rotates with a substantially constant rotation. In the embodiment shown, transporter  32 , which moves a sheet from transporter  30  to transporter  34  is a perfector transporter. Perfector transporter  32  operates to turn a sheet over as it transfers the sheet from transporter  30  to transporter  34  and during transfer of the sheet, perfector transporter  32  changes direction rotation from counterclockwise to clockwise. Direction of rotation of each transporter  30 - 38  is indicated by curved arrows inside the transporter. Features and mode of operation of sheet transport systems similar to sheet transport system  20  are described in PCT applications PCT/IL98/00553, PCT/IL99/00600 and PCT/IL00/00081 referenced above. Whereas the embodiment of transport system  20  in  FIG. 1  uses a perfector to reverse the sheet, other means known in the art for inverting a sheet where such reversal is desired, may be used in place of the perfector system. 
   Each transporter  3038  comprises a shaft  40 , about which it rotates, and by way of example at least one linear array  42  of suction cups  44  for gripping a sheet being transported by the transporter. In some embodiments transporters  30 - 38  comprise two or more support rims  46  on which a sheet gripped by the transporter lies. Suction cups  44  in a suction cup array  42  of a transporter  30 - 38  may be mounted on a manifold  48  mounted to the transporter&#39;s support rims  46 . To avoid clutter only some elements common to all transporters  30 - 38  are labeled with their identifying numeral. In the side view of transport system  20  shown in  FIG. 1  only one suction cup  44  of a linear suction cup array  42  and only one support rim  46  of a transporter  30 - 38  is shown  FIG. 2  shows features of the embodiment of transporter  30  in perspective view. 
   In an embodiment of the present invention manifold  48  of each suction cup array  42  comprised in a transporter  30 - 38  is connected via a pressure hose  50  to a different “vacuum” channel (not shown) in the shaft  40  of the transporter. The vacuum channels are coupled to an appropriate vacuum system (not shown) using devices known in the art such as rotary joints or in some embodiments, a vacuum distributor of a type described in PCT Application PCT/IL00/00081. In  FIGS. 1 and 2  vacuum channels are schematically shown coupled to a vacuum system via vacuum distributors  52 .  FIG. 2  shows transporter  30  with a vacuum distributor  52  and a pressure hose  50  for each of two vacuum channels (not shown) in its shaft  40  that couples the vacuum channel and thereby one of manifolds  48  to the vacuum system. A controller  49  ( FIG. 1 ) controls the vacuum system to aspirate air through or release air to suction cups  44  of each suction cup array  42  so that the suction cups respectively grip and release a sheet being transported by transport system  20  at appropriate times. 
   In some embodiments, for each suction cup array  42 , a vacuum sensor  60  measures (directly or indirectly) vacuum at suction cups  44  of the array. Vacuum sensor  60  may be coupled to the vacuum distributor  52  that communicates vacuum to the array. Vacuum sensors  60  can be any suitable sensors useable for determining level of vacuum at suction cups  44 . Vacuum sensors  60  can, for example, be pressure sensors that measure vacuum directly or flow meters that provide measurements of air flow through suction cups  44 , from which presence of vacuum can be determined. 
   Each vacuum sensor  60  generates signals responsive to vacuum it senses and transmits the signals to controller  49 . When a suction cup array  42  is gripping a sheet, pressure at suction cups  44  of the array is at a minimum and signals from vacuum sensor  60  monitoring vacuum at the suction cup array indicate a maximum gripping vacuum at the suction cups. When a suction cup array  42  is not gripping a sheet, pressure at the suction cup array is high and may be substantially equal to atmospheric pressure and signals from its vacuum sensor  60  indicate this situation. If a sheet being held by a suction cup array  42  is not properly seated on the array&#39;s suction cups, for example as a result of the sheet jamming, vacuum sensor  60  of the suction cup array will indicate an aberrant vacuum intermediate zero vacuum and the gripping vacuum. 
   In some embodiments, controller  49  uses vacuum readings from vacuum sensors  60  to continuously monitor the location and orientation of a sheet being transported by sheet transport system  20  as the sheet progresses through the system. Vacuum readings from vacuum sensors  60  indicate which suction cup array, and therefore which transporter  3038 , is holding the sheet at a given time. Angular position of the sheet is determined from the rotational orientation of the transporter holding the sheet. In some embodiments, rotational orientation of each transporter  3038  at any given time may be known from a structure of a transmission system used to rotate the transporters and a suitable encoder coupled to the transmission system, collectively referred to as a transporter position monitor. The encoder may be coupled to an appropriate moving structure of the transmission system or to a transporter, such as for example a shaft  40  of one of the transporters, using methods and devices known in the art. An aberrant vacuum reading by a vacuum sensor  60  indicates a transport system malfunction and/or that the sheet is not properly seated on a suction cup array. Location of the malfunction and/or jam may be determined from a position of the suction cup array  42  whose vacuum sensor  60  indicates the aberrant vacuum. 
   In some embodiments of the present invention, transport system  20  comprises a video monitor  120 . Controller  49  may use signals from vacuum sensors  60  and from the system encoder to display in real time on video monitor  120  positions of sheets being transported by transport system  20 . In some embodiments, controller  49  controls video monitor  120  to show an image  122  of transport system  20  and displays positions of sheets being transported by transport system  20  by displaying images of the sheets on the transport system image  122 . In some embodiments, a number or other designation identifies each sheet transported by transport system  20  and the same number identifies an image of the sheet displayed on transport system image  122 . Video display  120  in  FIG. 1  shows three sheets  124 ,  126  and  128  being transported by transport system  20 . Transport system identification numbers, “1001”, “1002” and “1003” for sheets  124 ,  126  and  128  are shown in a rectangular fame  130  at the bottom of video monitor  120 . Bach of the transport system identification numbers is located in frame  130  under the image of its corresponding sheet. In some embodiments of the present invention controller  49  displays, or can be programmed to display, on monitor  120 , vacuum readings from suction cup arrays  42 , as a function of time. 
     FIG. 3  schematically shows transport of a sheet  70  by transporters  34  and  36  of the embodiment of transport system  20  shown in  FIG. 1  to impression roller  24  and vacuum readings from vacuum sensors of the transporters that monitor the transport. Suction cup array  42  of transporter  34  ( FIG. 1 ) that is involved in the transport of sheet  70  is labeled  42 A in FIG.  3  and its suction cups are labeled  44 A. Vacuum sensor  60  that monitors vacuum at suction cups  44 A is labeled  60 A. Suction cup array  42  of transporter  34 , which is involved in the transport of sheet  70 , its suction cups  44  and its corresponding vacuum sensor  60  are similarly labeled  42 B,  44 B and  60 B respectively. Features of transport system  20  not germane to the discussion of the hand off of sheet  70  are not shown in FIG.  3 . 
   Insets  72 ,  74  and  76  in  FIG. 3  show position of sheet  70  and rotational orientations of transporters  34  and  36  at times t 0 , t 1  and t 2  respectively. In inset  72 , at time to, sheet  70  is being held by transporter  34  which is rotating sheet  70  towards a handoff position  80  between transporter  34  and transporter  36 . When sheet  70  reaches handoff position  80 , transporter  34  hands off the sheet to transporter  36 . In inset  74  at time t 1  sheet  70  has reached handoff position  80  and transporter  34  is handing off sheet  70  to transporter  36 . In inset  76 , at time t 2  after transporter  36  has received sheet  70  from transporter  34 , transporter  36  has rotated sheet  70  to a handoff position  82  between transporter  36  and impression roller  24  and is handing off sheet  70  to impression roller  24 . 
   Vacuum readings from vacuum sensors  60 A and  60 B for a faultless transport of sheet  70  to impression roller  24  are shown as a function of time by solid line curves  90  and  92  respectively on time lines  94  and  96  of a graph  98 . Between times t 0  and t 1  suction cup array  42 A is gripping sheet  70 . Sheet  70  is seated properly on suction cups  44 A and vacuum sensor  60 A indicates that vacuum at suction cup array  42 A is at a maximum gripping vacuum “V G ”. Shortly before time t 1 , the vacuum system is controlled to aspirate air through suction cups  44 B of suction cup array  42 B and vacuum as indicated by curve  92  at suction cups  44 B begins to increase rapidly. At time t 1 , when sheet  70  is at handoff position  80 , the vacuum system is controlled to rapidly reduce vacuum at suction cups  44 A to zero and transporter  34  releases sheet  70 . At handoff position  80  suction cups  44 A are opposite suction cups  44 B and sheet  70  covers suction cups  44 B as well as well as covering suction cups  44 A. Vacuum readings from vacuum sensor  60 B shown by solid curve  92  show vacuum at suction cups  44 B rising rapidly to the gripping vacuum V G , indicting that sheet  70  is properly seated on suction cups  44 B. As transporters  34  and  36  rotate away from handoff position  80  transporter  36  removes sheet  70  from transporter  34 . Vacuum at suction cups  44 B remains stable at substantially V G  until time t 2  at which time transporter  36  has rotated sheet  70  to handoff position  82  and the vacuum system is controlled to rapidly reduce vacuum at suction cups  44 B to zero. Transporter  36  releases sheet  70  and impression roller  24  grips the sheet, by way of example with conventional sheet grippers (not shown) and removes sheet  70  from transporter  36 . 
   Dashed curves  100  and  102  indicate vacuum readings from vacuum sensors  60 A and  60 B when a transport system malfunction causes a faulty handoff of sheet  70  from transporter  34  to transporter  36 . 
   The malfunction is assumed to be, by way of example, a timing error in transport system  20  that causes a delay in reduction of vacuum at suction cups  44 A. The reduction, which should occur at time t 1 , is delayed and occurs slightly after time t 1  as indicated by curve  100 . (Height of curve  100  is shown slightly lower than that of curve  90  for clarity.) Therefore, sheet  70  is not released on time by transporter  34  and both suction cup arrays  42 A and  42 B grip sheet  70  as transporters  34  and  36  rotate the suction cup arrays away from handoff position  82 . As a result, when vacuum at suction cups  44 A finally drops to zero, sheet  70  does not seat properly on suction cups  44 B and after the handoff of sheet  70  to transporter  36  there is air leakage through some of suction cups  44 B. Vacuum at suction cups  44 B therefore does not increase to the gripping vacuum V G . Vacuum readings from vacuum sensor  60 B shown by dashed curve  102  are low and indicate the faulty handoff. 
   From graph  98  it is seen that vacuum readings from vacuum sensors  60 A and  60 B indicate at any given time during transport of sheet  70  from transporter  34  to impression roller  24  on which transporter  34  or  36  sheet  70  is located. In addition, the vacuum readings indicate if the transport of sheet  70  is performed properly. In a case where there is a malfunction in the transport of sheet  70  the pressure readings indicate the malfunction and are used, in accordance with an embodiment of the present invention, to analyze the malfunction and determine its cause. For example, in the faulty transport of sheet  70  described above vacuum readings from vacuum sensor  60 B indicate that the cause of the faulty handoff between transporters  34  and  36  is the delayed reduction in vacuum at suction cup array  42 A. 
   Whereas transporters  30 ,  34 ,  36  and  38  ( FIG. 1 ) are shown, by way of example, gripping sheets that they transport using a single suction cup array, some transporters in accordance with an embodiment of the present invention, grip a sheet using more than one suction cup array. For such a transporter, information as to whether or not a sheet being transported by the transporter is gripped properly may be determined from vacuum readings from the more than one suction cup array gripping the sheet. For example in sheet transport system  20  perfector transporter  32  simultaneously grips a sheet that it transports and turns over with both its suction cup arrays  42 . A first suction cup array  42  holds the sheet along a leading edge of the sheet and a second suction cup array  42  holds the sheet along a trailing edge of the sheet. Proper transport of the sheet by perfector transporter  32  from transporter  30  to transporter  34  depends upon proper synchronization of vacuum at the suction cup arrays so that the leading and trailing edges of the sheet are gripped and released at appropriate times. Vacuum readings from the suction cup array  42  gripping the leading edge of the sheet and vacuum readings from the suction cup array  42  griping the trailing edge of the sheet are used to monitor proper functioning of perfector transporter  32 . 
   It should be noted that whereas a sheet position monitoring system is shown for sheet transport system  20 , sheet position monitoring systems, in accordance with an embodiment of the present invention, are useable in sheet transport systems having configurations different from that of sheet transport system  20 . For example, a sheet transport monitoring system similar to that shown for sheet transport system  20  can be used, in accordance with an embodiment of the present invention, with a “re-feed sheet transport system”. A re-feed sheet transport system is described in PCT Applications PCT/IL98/00553 and PCT/IL00/00081 referenced above and shown in FIGS. 4-5F in the latter application. 
   Furthermore, whereas sheet transporters  3038  are shown as comprising “rim mounted” suction cup arrays for gripping sheets that they transport, other types of configurations for transporters, in accordance with embodiments of the present invention are possible and can be advantageous. For example, a rotary sheet transporter, in accordance with an embodiment of the present invention, can comprise a circularly cylindrical surface with at least one slot or circular shaped orifice therein, through which air is aspirated to grip a sheet. Or a sheet transporter can shuttle back and forth with a cyclic linear motion to transport a sheet from a first position to a second position. Such a “shuttle” transporter might comprise a planar surface with at least one orifice therein for griping a sheet. Other configurations for sheet transporters, in accordance with an embodiment of the present invention, will occur to persons of the art. 
   In the description and claims of the present application, each of the verbs, “comprise” “include” and “have”, and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of members, components, elements or parts of the subject or subjects of the verb. 
   The present invention has been described using detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the present invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the present invention that are described and embodiments of the present invention comprising different combinations of features noted in the described embodiments will occur to persons of the art. The scope of the invention is limited only by the following claims.