Singulation device for mail
CROSS-REFERENCE TO RELATED APPLICATION 
     In copending patent application Ser. No. 864,258 for &#34;Mail Singulation and Culling System&#34; by James R. Hunter et al., there is described a mail processing system in which the singulator of the present invention finds particular utility. Reference should also be made to U.S. Pat. No. 3,927,876 entitled &#34;Device for Singulating Documents&#34; by Edward A. Wojtowicz in which there is described and claimed a device for feeding documents one at a time from a stack thereof. Both the aforementioned application and patent are assigned to the same assignee as the present application. 
     BACKGROUND OF THE INVENTION 
     In mail processing, it is usually necessary at some point to separate or singulate the documents one from the other. Most present day devices are designed to singulate mail pieces which fall within a given size range, such as letter mail. As a result, these devices are not capable of handling mixed mail comprising, for example, (in addition to letters), flats, magazines, newspapers, etc. 
     The singulation device of the reference patent separates documents, including flat mail pieces, arranged and presented to it in stack form, by using multiple stages. The latter includes a drop ledge, a friction roller, and a vacuum roller. The stacks of mail are previously oriented and faced so that an operator can view them after singulation. The singulation stages are situated in a confined area and are necessarily limited in number. 
     It will be appreciated that the singulator of the reference patent may be used successfully in many applications. However, a need existed for a singulator capable of processing same time, it must be highly reliable with a negligible doubles rate and must be easy to maintain. 
     The singulator of the present invention described hereinafter fills these requirements. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, the singulator incoporates a slide having a compound slope which enhances the gravitational forces acting upon the documents. The slide incorporates a single registration wall and the documents are guided down the incline in an orderly and uniform manner with one common side edge registration. Situated in spaced-apart relation along the slide are singulating stations, each including a friction roller and document sensing means on both sides thereof. Documents travelling down the slide are also leading edge registered as they impact and momentarily park at each singulating roller in preparation for further processing in equipment located beyond the singulator. The number of singulation stations is a function of the desired singulation reliability and is limited only by physical space constraints. 
     In a typical singulation operation, the rotation of a first friction roller associated with a first singulation station is controlled by a pair of photocell sensor assemblies and clock timing means. The first sensor assembly is located downstream adjacent the first roller and the second sensor assembly, upstream adjacent a second roller associated with the second singulation station. 
     Initially, it is assumed that one or more documents are in the first station and that both rollers are stationary. The sensing of the absence of a document at the second station by the second sensor assembly initiates the rotation of the first roller. This rotation is limited to a predetermined angle, for example 90 degrees, unless halted in response to the first sensor assembly&#39;s detection of a document moving toward the second station. If rotation has progressed through the predetermined angle and has stopped, a dwell period is initiated, during which detection of a document by the first assembly will temporarily halt the resumption of rotation of the first roller. On the other hand, if no document is detected, the second station remains unoccupied, and following the dwell period, rotation of the first roller will recommence until halted by the second sensor assembly&#39;s detection of a document at the second station. 
     It should be noted that should more than one document arrive at a singulation station, succeeding stations operated in the manner described hereinbefore will separate them and produce the desired singulation output. The number of stations employed are limited only by space constraints. 
     These and other features of the invention will become more fully apparent in the detailed description of the singulator and its mode of operation, which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a pictorial view of a singulator slide having three singulation stations. 
     FIGS. 2A, 2B, and 2C indicate in schematic form the sequence of events involved in the separation of the mail pieces by the friction roller in one of the stations shown in FIG. 1. 
     FIG. 3 is a block diagram indicating the electronic subassemblies and mail flow associated with complete singulation. 
     FIG. 4 is a flow diagram indicating the control sequence associated with one of the singulation stations and representative of all of the stations along the slide. 
     FIG. 5 is a logic circuit diagram applicable to each of the singulation stages for implementing the flow diagram of FIG. 4. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A singulator 10 having three singulation stations 10a, 10b and 10c positioned along a slide 12 is illustrated in FIG. 1. In an actual embodiment, the singulation stations include respective rollers 14, 16 and 18, each covered with a high coefficient of friction compound. Each roller is mounted on sheet metal guides. The back 12a and edge 12b guides of the slide 12 form a 90-degree &#34;V&#34; trough which is sloped downwards at approximately 45 degrees from the horizontal. Singulation photocell assemblies 14a, 14b; 16a, 16b; 18a, 18b are associated with the respective rollers 14, 16 and 18. The photocell assemblies are illuminated by a fluorescent lamp assembly 30. The rollers 14, 16 and 18 are independently driven by respective controller stepping motors 24, 26 and 28 in order that they can be rapidly started and stopped. 
     Mail flow through the singulation stations beings with the advancement of mail to the upper singulation station 10a. As seen in FIG. 1, the mail carried on edge by a conveyor 20 may be caused to curve around the transition between the conveyor and singulator and to slide down to the first roller 14. The sequence of singulation by a roller is illustrated in FIGS. 2A, 2B and 2C. In FIG. 2A two pieces of mail are shown resting on the roller 14. A signal calling for mail derived from photocells 14b upstream of roller 16 (FIG. 1) causes the roller 14 to rotate approximately 90 degrees and then stop. The two mail pieces advance to the position illustrated in FIG. 2B. During the roller dwell period the top mail piece continues to slide while the bottom piece is restrained by the roller 14. As seen in FIG. 2C, the top mail piece continues to slide down to the next singulation stage 10b, the leading edge of the document interrupts the photocell assembly 14a which generates a signal to prevent the roller 14 from restarting its rotation. The bottom mail piece remains in this position until the next feed signal from photocells 14b is received. If the photocells 14a are interrupted before the roller has moved 90 degrees, the rotation of roller 14 will be stopped at that time. 
     If there had been only a single mail piece on the roller in the previous example, then at the end of the roller dwell period the photocells 14a would not have been interrupted and the roller 14 would have been restarted and rotated until the single mail piece had been advanced to roller 16 of the next singulation station 10b. 
     If there had been three mail pieces on the roller, the top two mail pieces might have advanced to the second singulation station 10b and would be singulated there. The third singulation station 10c is an additional stage to reduce to a minimum the number of double documents which might enter the succeeding processing equipment 22. 
     FIG. 3 is a block diagram indicating the electronic subassemblies and mail-flow associated with complete singulation. Three singulation stages, or stations 10a, 10b and 10c, are indicated. The singulation performed in each station is under the control of a pair of photocell assemblies, for example, photocell assemblies 16a and 16b provide input signals to the control logic circuit of stage 10b. The logic on the other hand applies a signal to the roller stepper motor (26 in this case) to control its operation. The output mail stream from the third and last singulation stage is delivered to the succeeding processing equipment 22. 
     FIG. 4 is a flow diagram illustrating the control sequences associated with the second singulation stage 10b which is typical of all three stages. FIG. 5 is a logic circuit diagram applicable to each of the singulation stages, representing the &#34;control logic&#34; of FIG. 3 and implementing the flow diagram of FIG. 4. 
     In the description which follows, reference may be made to both FIGS. 4 and 5, the former for general operation, the latter for specific details. 
     Considering the operation of the second singulation stage 10b, it has been observed hereinbefore that photocell assembly 16b is used to detect the need for mail from roller 16. Photocell assembly 16a, on the other hand, is used in discriminating between single and multiple pieces of mail above roller 16. 
     In order to initiate a feed cycle from roller 16, it is assumed that photocell 16b which was formerly dark, that is, covered by a document parked at roller 18 of the third singulation stage 10c, has gone light as the last mentioned document is removed by roller 18. A signal corresponding to the dark-to-light transition of photocell assembly 16b is applied to and triggers single-shot 32. Another single-shot 34 is unaffected since it triggers only on signals derived from light-to-dark transitions in photocell assembly 16b. The output pulse of single-shot 32 is applied via line 36 to OR gate 38, the output of which appears on the SET terminal of motor control flip-flop 40. This action energizes solid-state relay 42 to apply AC line voltage to roller motor 26, thereby turning it on. Concurrently, the output pulse from single-shot 32 is applied via line 44 to the START terminal of clock counter 46. As will be noted hereinafter, the clock counter had been previously RESET by the action of photocell assembly 16b in the last singulation cycle. Thus, the clock counter 46 starts to run when photocell assembly 16b indicates that the trailing edge of a mail piece leaving station 10b has been detected. The clock counter provides distinct output pulses on lines 48 and 50 indicative of two time periods, &#34;T&#34; and &#34;T plus delta t&#34;. The first of these represents the time elapsed for the stepper motor 26 to turn roller 16 through 90 degrees. The other output pulse occurs at the end of &#34;T&#34; plus a dwell period, &#34;delta t&#34;. 
     It will first be assumed that while motor 26 is turning through 90 degrees, a document moves off roller 16 and momentarily causes photocell assembly 16a to go from light to dark, as it falls down the slide toward roller 18. The light-to-dark transition signal from photocell assembly 16a triggers single-shot 52 which in turn is applied on line 54 and via OR gate 56 to the RESET terminal of flip-flop 40. This action de-energizes relay 42, stopping motor 26. The output pulse from single-shot 52 also sets via line 58 a second flip-flop 60, which had been placed in the RESET state during the last singulation cycle. The operation of flip-flop 60 will become apparent hereinafter. At this time, the detection of a document by photocell assembly 16a has resulted in a complete cycle, since it is assumed that the document has been delivered to roller 18. 
     If during the 90 degree rotation of roller 16, photocell assembly 16a had not detected a document, the operation would have been as follows. 
     At the end of time &#34;T&#34; (90 degrees rotation), a pulse from clock counter 46 on line 48 is applied via OR gate 56 to the RESET terminal of flip-flop 40, de-energizing relay 42 and stopping motor 26. The cycle of operation is now within the &#34;delta t&#34; period. This period allows for the time it takes the top piece of mail, that is, a double on roller 16 to slip by the bottom piece which remains in contact with the roller, and fall the distance from the roller to photocell assembly 16a. If it is assumed that a double is present on roller 16, the detection of a document by photocell assembly 16a  during the &#34;delta t&#34; period, generates a light-to-dark transition signal which triggers single-shot 52. The output of single-shot 52 is applied to OR gate 56 which tends to RESET flip-flop 40 and stop motor 26, had these conditions not already occurred at the end of the &#34;T&#34; time period as discussed hereinbefore. Flip-flop 60 is placed in the SET state. The singulation cycle for stage 10b is complete. 
     On the other hand, it will now be assumed that during the &#34;delta t&#34; period, no document was detected by photocell assembly 16a. It is therefore desirable to restart motor 26. This is accomplished by the pulse on the &#34;T plus delta t&#34; output line 50 of the clock counter 46. This pulse is applied to one input of AND gate 62. As mentioned previously, flip-flop 60 is normally in a RESET state and its output on line 64 is enabling for AND gate 62. The output of this last gate is applied on line 66 via OR gate 38 to set flip-flop 40 and to start motor 26. 
     It will be recalled that if during &#34;delta t&#34;, photocell assembly 16a detects a document, flip-flop 60 is placed in the SET state. Under this condition, AND gate 62 is disabled and will remain so at the end of the &#34;delta t&#34; period, precluding the restarting of motor 26. 
     Continuing from the restarting of motor 26 after the elapse of time &#34;T plus delta t&#34;, roller 16 will continue to turn until photocell assembly 16b detects the arrival of a document at roller 18. When this occurs, the signal from photocell assembly 16b during the light-to-dark transition triggers single-shot 34. The latter applies a pulse to three areas: to a first, on line 68 via OR gate 56 to RESET flip-flop 40 and stop motor 26; to a second, on line 70 to reset flip-flop 60; and to a third, on line 72 to reset clock counter 46. The present singulation cycle is complete. Thereafter, the trailing edge of a document passing photocell assembly 16b (mail leaving roller 18) will again initiate logical activity for a new cycle of operation. 
     In conclusion, a singulator has been described for separating into single items a variety of mail pieces including letters, flats, magazines and newspapers for further processing. It should be understood that changes and modifications of the device may be needed to suit particular requirements. Such changes and modifications insofar as they are not departures from the true scope of the invention, are intended to be covered by the following claims.