Abstract:
An apparatus for determining the stiffness of mail pieces includes a bar or similar deflector for contacting a mail piece conveyed past the bar by a conveyor system, and vibrates when in contact with a passing mail piece, either due to the impact of the mail piece or by the action of a device which causes the deflector to vibrate before the mail piece hits it. A sensor measures a parameter indicative of change in the vibrations of the deflector and generates a signal indicative of the measured parameter. A processor, e.g. programmable controller or circuit, receives the signal from the sensor and determines a stiffness value for the mail piece based on decay in vibrations of the deflector caused by contact with the mail piece.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to mail processing and in particular to a method and apparatus for determining the properties of mail pieces in an automated mail processing environment. 
       BACKGROUND OF THE INVENTION 
       [0002]    Postal services utilize specialized equipment to transport, scan, sort and process tens of millions of envelopes and other flat pieces of mail on a daily basis. These automated systems and machines necessarily utilize specialized equipment designed to transport, scan, process and sort envelopes and other flat pieces of mail. To insure reliable operation of these machines, mail pieces having physical properties incompatible with automated processing equipment must be separated from the mail stream at the earliest opportunity. In particular, mail pieces that are too stiff or rigid to be transported must be diverted from automated processing lines to avoid jamming the machines and/or being destroyed during processing. 
         [0003]    U.S. Pat. No. 7,096,743 issued Aug. 29, 2006 to Vogel, discloses an device for measuring the flexural rigidity in the longitudinal direction of flat items of mail a conveying path. The apparatus includes a sensor for detecting the edges of the items of mail and signaling a control device when a mail item reaches a selected bearing point. A deflection apparatus moves into the conveying path under the control of the control device to bend the mail item. The magnitude of the deflection of the mail item and the magnitude of the deflection force are measured to determine the flexural rigidity of the mail item. 
         [0004]    Reisig, et al., U.S. Pat. No. 6,032,517, issued Mar. 7, 2000, discloses an arrangement for measuring the rigidity of flat items by means of elastic conveying belts that extend through a curved section of a conveying path. A rigidity sensor is provided for measuring the deflection of the conveying belts caused by the item passing through the curved section, and a thickness sensor is used to determine the thickness of the item. An evaluation device is provided for determining the rigidity of an item passing through from the values obtained by the thickness and rigidity sensors. 
         [0005]    Redford U.S. Patent Application 20040245158, Dec. 9, 2004, describes a method and apparatus for stiffness and thickness detection in mail sorting systems by conveying a singulated stream of flat mail pieces one at a time though a test curve upstream from the sharpest curve of the mail processing machine, the test curve including an angled section at which each mail piece tends to bend, determining the thickness of each mail piece, determining the stiffness of each mail piece by measuring deflection of one of the belts of the test curve as an end portion of the mail piece is passing through the angled section, which deflection is in excess of deflection caused by the thickness of the mail piece as it passes between the belts, and diverting a mail piece out of the mail processing machine before it reaches the sharpest curve of the mail processing machine if predetermined stiffness and thickness criteria are exceeded by the thickness and stiffness of the mail piece. The angled section defines an angle less severe than the sharpest curve, whereby a mail piece that would likely jam the mail processing machine at the sharpest curve can pass through the test curve without jamming. 
         [0006]    The foregoing devices require bending the mail pieces by transporting the mail pieces through a curved path. This in turn requires moving parts such as conveyors and rollers as well as space in which to install the devices. Elimination of these parts would simplify and reduce the size or length of the processing line. Ideally, a stiffness detector for use in an high speed automated mail processing line will process the mail without damaging individual mail pieces, have a small footprint, a minimum number of moving parts and will be capable of meeting the throughput requires of the line. 
       SUMMARY OF THE INVENTION 
       [0007]    The invention provides an apparatus for determining the stiffness of mail pieces including a bar or similar deflector for contacting a mail piece conveyed past the bar by a conveyor system, and vibrates when in contact with a passing mail piece, either due to the impact of the mail piece or by the action of a device which causes the deflector to vibrate before the mail piece hits it. A sensor measures a parameter indicative of change in the vibrations of the deflector and generates a signal indicative of the measured parameter. A processor, e.g. programmable controller or circuit, receives the signal from the sensor and determines a stiffness value for the mail piece based on decay in vibrations of the deflector caused by contact with the mail piece. 
         [0008]    In one embodiment, the deflector comprises a beam having a distal end projecting into a path defined by an automated mail processing line such that mail pieces conveyed along the path contact the beam. In another embodiment, the deflector comprises a driven or non-driven roller extending across the mail path. A forcing function device such as an electromagnet, piezotransducer, or striker may be employed to induce vibrations in the deflector. 
         [0009]    The invention further provides a method for removing excessively stiff mail pieces being transported on a conveyor system from a mail processing machine such as a mail sorter. The method includes feeding mail pieces to be processed one at a time into a conveyor of the mail processing machine, conveying the mail pieces one at a time past an apparatus for determining the stiffness of passing mail pieces as described above, determining the stiffness of each mail piece using the stiffness determining apparatus, and diverting out of the mail processing machine mail pieces having excessive stiffness. Where the mail processing machine is a mail sorting machine wherein a sorted mail piece is diverted to one of a plurality of bins based on scanned address information, the diverting step is preferably carried out before a mail piece having excessive stiffness reaches a stacker section of the machine housing the bins. These and other aspects of the invention are discussed further in the detailed description that follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    In the accompanying drawing, wherein like numerals denote like elements: 
           [0011]      FIG. 1  is a schematic side view of an apparatus according to the invention; 
           [0012]      FIGS. 2 to 5  each show a pair of traces of vibration amplitude versus time for a vibrating bell plate contacted by one or more mail pieces; 
           [0013]      FIGS. 6 and 7  are front and rear perspective views of a bell plate used in examples according to the invention; 
           [0014]      FIGS. 8 to 10  each show a pair of traces of vibration amplitude versus time for a vibrating bell plate contacted by an object that is held against the plate; 
           [0015]      FIG. 11  is a schematic end view of a further apparatus according to the invention; 
           [0016]      FIG. 12  is a schematic side view of the roller mechanism of  FIG. 11 ; 
           [0017]      FIG. 13  is a schematic representation of a control circuit for a thickness detector according to the invention; and 
           [0018]      FIG. 14  is a perspective view of a mail processing machine according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The present invention provides a stiffness detection system that uses a deflector such as a rod, plate or bar that is placed in the path of a mail processing machine such as sorter (MLOCR or DBCS machine) or automated facer-canceler system (AFCS). The detection system is placed in an appropriate position along a gap in one side of a pinch belt conveyor system, such as upstream of the first sorting gate as described for the system of Redford et al. U.S. Patent Application 20040245158, Dec. 9, 2004, the contents of which are incorporated by reference herein. Preferably, apparatus  10  is positioned at or near the beginning of the mail processing line. Positioning apparatus  10  at or near the beginning of the line enables detection of mail pieces incompatible with down steam equipment so that such mail pieces may be diverted before jamming any processing equipment. For example, the apparatus may be positioned immediately after the pick off feeder that singulates batches of incoming mail for processing. 
         [0020]    Referring to  FIG. 1 , an apparatus  10  according to the invention for detecting the stiffness of a mail piece  12  includes a deflector such as a bar or beam  14  with a curved or beveled distal end  16  for contacting a mail piece  12 . A proximate end  32  of beam  14  is rigidly secured to a fixture (base or bracket)  33 . Beam  14  is made of a material such as steel capable of vibrating at frequencies and amplitudes measurable using the techniques described hereafter, and the dimensions of beam  14  likewise must permit such vibration. 
         [0021]    For a beam held at one end that is subject to a force applied at its free end, Young&#39;s modulus determines the amount of deflection produced by a given force. The normal force created by bending a mail piece  12  that is passing by the free end of the beam  14  along the conveyor changes the way in which the beam  14  vibrates in a manner that varies according to the stiffness of the mail piece. The waveform for a graph of vibration versus time after contact between the beam and a dampening object is sinusoidal and decays exponentially. 
         [0022]    A mail piece  12  is deflected by a small distance “y” in the Young&#39;s modulus equation. The resulting normal “p” force is that necessary to deflect the mail piece distance y. That force will be small for mail pieces that are not very stiff, larger for mail pieces that are stiff. If that force is then applied to a vibrating body, then the damping that occurs will be proportional to the force that was applied to the vibrating body. As shown in the examples which follow, the dampening effect on the decay waveform is approximately proportional to the stiffness of the dampening object. As such, a decision as to whether an object is excessively stiff could be made based on predetermined threshold levels determined mathematically, empirically with reference to reference curves for mail pieces of known stiffness, or a combination of these approaches. 
         [0023]    Beam  14  in this embodiment is an elongated, generally rectangular steel beam; however, deflectors having other geometries such as a plate or round rod may be employed. While the distal end  16  of beam  14  is preferably rounded to avoid damaging mail pieces contacting the bar, other end geometries may be used. A forcing function device  18  is mounted on contact beam  14  for inducing vibrations in the beam. Forcing function device  18  may be an electromagnet similar to a audio speaker coil or a similar device capable of inducing relatively high frequency vibrations in contact beam  14 . In an alternative embodiment, forcing function device  18  is a striker that impacts beam  14  just prior to, or as mail piece  12  comes in contact with the beam. Known position and velocity sensors may be utilized to determine when a mail piece will contact beam  14  and control the timing of impacts on the beam  14  with the striker. According to a third alternative, no device for inducing vibrations prior to contact with a mail piece is provided, and the impact of the mail piece on the bar is used to create vibrations which are then dampened as the mail piece passes by in accordance with the stiffness of the mail piece. 
         [0024]    A sensor  20  is mounted on beam  14  for measuring the amplitude and frequency of vibrations in the contact beam. As illustrated, forcing function device  18  and sensor  20  are mounted directly on contact beam  14 . However, the forcing function device  18  and sensor  20  could be mounted in close proximity to beam  14  without contacting it as long as the device and sensor perform the functions of inducing and measuring vibrations in contact beam  14 . 
         [0025]    Mail pieces  12  are conveyed along a path  22  between first opposed belt conveyors  24  that are guided over rollers  26  which may be conventionally driven with motors, either directly or with a belt, chain or other drive system. Mail pieces  12  discharged from first opposed belt conveyors  24  are received and conveyed by a second set of opposed belt conveyors  28  that are guided over rollers  30 . Apparatus  10  is positioned adjacent a gap between belt conveyors  24  and  28  such that path  22  intersects the distal end  16  of contact beam  14 . 
         [0026]    The proximate end  32  of contact beam  14  is secured in a frame or bracket such that the leading end of a mail piece  12  traveling along path  22  through the gap between belt conveyors  24  and  28  will contact end  16  of beam  14  while a portion of the mail piece is still pinched between belts  24 . Contact beam  14  extends sufficiently into path  22  to displace mail piece  12  a small distance, for example several millimeters. Forcing function device  18  induces vibrations in the beam  14  that change in response to mail piece  12  contacting beam  14  as the mail piece travels along path  22 . The natural frequency of beam  14  is not critical so long as the frequency is high enough to be distinguishable from environmental vibrations such as vibrations generated by normal machine operation. Preferably the natural frequency is high enough to enable a relatively large number of readings to be taken during the time mail piece  12  is in contact with beam  14  and in pinch between opposed belt conveyors  24 . Natural frequency is important because the frequency of the beam  14  will change as force is applied to it. This is another way to determine the amount of force on the beam. 
         [0027]    Conveyor  28  is at an obtuse angle less than 180 degrees relative to conveyor  26  and path  22  in order to smoothly receive mail piece  12  after it deflects off of end surface  16 . The belts  28  downstream from the vibrating beam or plate  16  are the next step in transporting a letter through the machine to a final sorting pocket. Belts  28  should not touch mail piece  12  while it is being measured, but serve to guide it back into pinch after the measurement. Since the thickness and size of the mail piece  12  and the amount of deflection of the mail piece  12  are unknown, the downstream belts  28  are set at different angles to converge and thereby handle a range of incoming mail pieces and reposition each mail piece back into the transport system. 
         [0028]    As an alternative, device  10  may be positioned in front of a leveler. A leveler does not hold the letter in pinch. Instead, it has a belt below the mail piece that moves at the same speed as the two side belts. At the end of the leveler, the mail piece is guided back into pinch. 
         [0029]    In order to determine a stiffness value for mail piece  12  conveyed along path  22 , forcing function device  18  is activated to induce high frequency vibrations in contact beam  14 . Forcing function device  18  may be operated continuously or activated only when a mail piece is approaching the gap between belt conveyors  24  and  28 . When mail piece  12  contacts beam  14 , the normal force applied to the beam as a result of displacing the mail piece from path  22  will change the vibration of the beam according to the stiffness or flexibility of the mail piece. A stiffer mail piece in contact with beam  14  will dampen vibrations in the beam at a different rate than a more flexible mail piece. 
         [0030]    Sensor  20  senses the change in the magnitude and frequency of vibrations in beam  14  as mail piece  12  contacts the beam and transmits the results to a processor  34  that correlates the results to determine a stiffness value for the mail piece. Processor  34  preferably operates by first deriving a parameter from the measured vibration decay curve, which parameter is indicative of the stiffness of the mail piece. In the illustrated embodiment, a simple form of stiffness index was derived using a circuit that begins counting the number of readings taken between an upper set point and lower set point. The counter stops when the value of the readings below the lower limit. The resulting decay rate, expressed as a hexadecimal number, is recorded as a stiffness index. A greater number of readings indicates slower decay of the curve and thus a more flexible mail piece, whereas a smaller number of readings between the same two set points indicates a stiffer mail piece. This is but one possible means of evaluating relative stiffness of passing objects, and if a processor is available that can perform computations on the curve data, then an actual rate of decay can be derived. 
         [0031]    Using the calculated stiffness index, processor  34  may use either an algorithm or an empirically derived lookup table to determine whether the stiffness level is acceptable or unacceptable. Such a lookup table may be generated by passing sample mail pieces of known stiffness thorough apparatus  10  to determine threshold values and ranges for acceptable and unacceptable levels of stiffness. In order to accurately determine the dampening effect of the mail piece on vibrations in beam  14 , only readings taken while mail piece  12  is in pinch between belts  24  and rollers  26  are used to determine the stiffness value. 
         [0032]    The stiffness value generated by the processor may be used in conjunction with a predetermined maximum threshold to identify mail pieces to be diverted from the mail stream as too stiff to process. Alternatively, as disclosed in U.S. Patent Publication No. 20040245158 cited above, the stiffness value may be used in connection with other characteristics of the mail piece such as thickness and length to determine whether the mail piece is compatible with downstream process equipment. Known sensors such as photocells, laser measuring devices, proximity devices, motion detectors and similar devices may be used to determine the position, height, length and thickness of mail piece  12  as it travels along path  22 . 
         [0033]    To illustrate the dampening effect,  FIG. 2  graphically shows vibrations induced in a stationary bell plate placed in a mail processing path when the plate is impacted with three different letters in succession. Two of the letters were approximately 5 mm thick and filled with card stock of thickness 178, 205 mm respectively while the third was approximately 5 mm thick and filled with 178 mm thick foam. The first impact is the envelope filled with a 178 mm card stock that is not in pinch (held between belts or rollers) when it contacts the plate. The second impact is the letter filled with 305 card stock that is in pinch when it contacts the plate. The third impact is the envelope filled with 178 mm foam that impacts the plate while not in pinch. The top trace in each of the figures is generated with a peak detect circuit to illustrate the decay rate of vibrations caused by the impact of the letters on the bell plate. A distinct peak corresponding to the impact of each letter can be seen, with the thicker stiffer letter (the second) producing the strongest impact. The results illustrate that the method of the invention can be used on a series of mail pieces passing by in succession, even when the gap between mail pieces is not controlled and/or the trailing end is not in pinch. 
         [0034]      FIG. 3-6  show the results of contacting a bell plate with envelopes and striking the plate with a spoon to induce vibrations. The upper trace in each figure represents the rate of decay of the induced vibrations. The stiffness index as measured for the segment between the vertical dotted lines was 1180 hex (4480 decimal).  FIG. 3  shows the rate of decay when the plate is contacted with an empty number 10 envelope. 
         [0035]      FIG. 4  illustrates the rate of decay when the plate is contacted with a number 10 envelope with a piece of card stock in the envelope. The stiffness index was 3872 decimal.  FIG. 5  illustrates the decay rate when the plate is contacted with a number 10 envelope containing two pieces of card stock in the envelope. The stiffness index was 3487 decimal. 
         [0036]      FIGS. 6 and 7  illustrate a bell plate  50  mounted in a base  51 . Bell plate  50  has an attached piezoelectric sensor  52 . An audio speaker  54  (forcing function device) is positioned adjacent to bell plate  50 . A small plastic dowel mounted at the center of the speaker cone touches the bell plate  50 . Speaker  54  is driven at a resonant frequency of 1.4 KHz to induce vibrations in the plate.  FIG. 8  shows the dampening effect as measured with piezoelectric sensor  52  when a thin piece of cardboard is pressed against surface  56  of plate  50 .  FIGS. 9 and 10  illustrate the dampening effect when a thin piece of cardboard and a spoon, respectively, are pressed against the plate. 
         [0037]    Referring to  FIGS. 11 and 12 , in a second embodiment of the invention, an apparatus  40  includes a vibrating, tubular roller  42  having a lengthwise axis  41  in place of contact beam  14 . Roller  42  extends perpendicular to and across path  22  such that a mail piece conveyed along the path contacts the roller. Roller  42  is supported by an external drive bearing  45 , idle bearing  47  and a press bearing  49  disposed above and below path  22 , respectively. Drive bearing  45  may comprise a drive roller powered by an electric motor  48 . Drive bearing  45  is in tangential peripheral contact with roller  42  and is controlled to drive roller  42  to spin at the same rate and in the same direction as the passing mail. Vibrations are induced in roller  42  with a non-contact forcing function device such as an electromagnet  44 . The dampening effect on vibrations induced in roller  42  when contacted with mail piece  12  is measured with a non-contact sensor  46  to determine the stiffness of the mail piece. Roller  42  may be solid or hollow and formed from a suitable material such as steel. Roller  42  is mounted at nodal points to avoid dampening of the oscillations by the mounting device. The nodal position(s) of a rod such as roller  42  are at around 0.22-0.25 of its length. Bearings  45 ,  47  are located at these nodal points (given the vibration frequency, e.g. 2 Khz) along the roller to avoid interference with vibrations induced in the rod for measurement purposes. Alternatively, drive bearing  45  can be omitted and roller  42  mounted to turn freely when contacted by a mail piece traveling along path  22  to reduce frictional forces. 
         [0038]    In another variation, forcing function devices  18 ,  44  are omitted. Mail pieces are conveyed along path  22  with sufficient velocity to generate measurable vibrations in beam  14  or roller  42  when the mail pieces impact the beam or rod. Vibrations induced in beam  14  or roller  42  when impacted by mail piece  12  are measured with contact or non-contact sensors  20 ,  46  to determine the stiffness of the mail piece. The decay rate of the vibrations is correlated to with processor  34  determine the stiffness of the mail piece. 
         [0039]      FIG. 13  is a schematic representation of a control circuit  60  for thickness detectors  10 ,  40 . A processor  62  sends a signal to a numerically controlled oscillator (NCO)  64  that transmits a signal having the desired frequency to an amplifier  66 . Amplifier  66  powers a forcing function actuator  68 , such as an electromagnet, to generate vibrations in a vibrating deflector such as beam  14  or roller  42 . 
         [0040]    Sensor  70  detects the vibrations in the deflector when the deflector is contacted with a mail piece and transmits the corresponding signal to processor  62 . An anti alias filter  72 , amplifier  74 , analog to digital converter  76  and Finite Impulse Response (FIR) filter  78  are used to filter, amplify, convert and condition the signal from sensor  70  for use by processor  62 . The output of processor  62  is a signal corresponding to a stiffness value for the mail piece. The signal may be used to directly control a downstream diverter to divert mail pieces having an unacceptable stiffness value from the main stream and/or sent to a second processor or control computer which utilizes the signal to actuate downstream equipment to divert incompatible mail pieces from the mail stream. 
         [0041]    A stiffness detection system according to the invention can be used in a mail processing machine such as a DBCS machine  80  as shown in  FIG. 14 . Such a machine includes a mail feeder  82  upon which a stack  84  of unsorted mail pieces  86  are loaded for processing. Mail feeder  82  has a jogger□conveyor  88  that advances the stack  84  to a pick off apparatus  90  that feeds a singulated stream of individual mail pieces through a transport section  91  to an automated sorting section  92  which sorts the mail in one or more passes to a plurality of bins  94 . In transport section  91 , each mail piece is scanned for address information. Sorting section  92  is limited in terms of the thickness, stiffness and combined thickness and stiffness of mail pieces that it can process. 
         [0042]    A detector  10  (or  40 ) according to the invention may be incorporated into transport section  91  between pick off  90  and sorting section  92 , so that the singulated stream of mail pieces  86  pass through detector  10  before being conveyed to sorting section  92  for processing. A divert  96  for diverting mail pieces rejected due to excessive thickness is positioned between detector  10  and sorting section  92 . A controller activates divert  96  upon receiving a signal from detector  10  indicating that a mail piece is too stiff. Divert  96  is located at one end of transport section  91  just upstream from an entry end of sorting section  22  and diverts rejected mail pieces by causing them to continue traveling in a straight line and be ejected from one end of sorter  10 , rather than be conveyed around a 90-degree curve as shown for non□diverted mail entering sorting section  92 . This arrangement avoids potential jamming of the reject that might occur if an angled divert were employed. 
         [0043]    While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments will be apparent to persons skilled in the art upon reference to the description. Such variations and additions are specifically contemplated to be with the scope of the invention. It is intended that the appended claims encompass any such modifications or embodiments.