Curl detection through pneumatic acquisition sensing

Abstract: A machine includes a scheme for detecting the level of curl in sheets in a stack (7) by measuring the time it takes to acquire a sheet with a bottom corrugation vacuum feeder (BVCF)(9). A vacuum switch (80) changes state at a preset vacuum level thereby detecting any significant rise in vacuum which occurs when a sheet has been acquired by the BCVF (9). A digital control circuit (86) senses the change of state which takes place in the vacuum switch (80) and feeds a signal to the machine's microprocessor (88) which in turn signals an air knife (12) in the BVCF to increase or decrease air pressure toward the sheet stack (7) to compensate for the stressed state of the sheets.

Description: The present invention relates to sheet feeding and more particularly to a vacuum corrugation feeder with the capability of detecting the degree of curl in sheets. With the advent of high speed xerographic copy reproduction machines wherein copies can be produced at a rate in excess of three thousand copies per hour, there is a need for a document and sheet feeder to, for example, feed documents to the platen of a copier in rapid succession in a reliable and dependable manner in order to utilize the full capabilities of the copier. A number of document handlers are currently available to fill that need. These document handlers must operate flawlessly to virtually eliminate the risk of damaging the original document and generate minimum machine shutdowns due to misfeeds or document multifeeds. It is in the initial separation of the individual documents from the document stack where the greatest number of problems occur which, in some cases, can be due to upcurl and downcurl in documents. Various approaches have been highly successful in answering the above problems, for example US-A-4,305,576 discloses a typical vacuum separating and feeding system wherein a plurality of friction belts is arranged to run over a vacuum plenum placed at the bottom of a sheet supply tray which has a"U" shaped pocket formed in it. The pocket serves to provide space for the bottom sheet to be captured by the vacuum feed belt assembly, to provide an air seal between the bottom sheet and the edges of the pocket and to provide a high pressure seal between the bottom sheet and the remainder of the stack. This high pressure seal is achieved by supporting a major portion of the stack weight on the edge regions of the pocket. However, this "U" shaped configuration was found to not permit deformation of the sheet in a geometrically developable shape which results in a reduction in the degree of levitation of the sheet stack. The bottom sheet vacuum corrugation feeder in US-A-4,411,417 answered this problem by including a differently designed stack support tray that has a planar base portion defining a base plane, the front of the base portion having a opening within which the bottom sheet separator is positioned. The tray also includes two sloping planar side wings, one at each side of the opening in the base portion. The sloping planar side wings are angled upward from the base plane and are angled outward from front to rear of the tray and intersect the base plane such that the intersection at the rear of the tray is in the approximate location of the rear corners of a rectangle the size of a sheet to be fed and the intersection of the planar wings and the base plane at the front of the tray is approximately midway between the front corners of a sheet to be fed and the center line of a sheet to be fed. Even though the above two document handlers in US-A-4,305,576 and 4,411,417 have been highly successful, an improvement of the document feeder in US-A-4,411,417 is still needed because of the presence of unpredictable curl in sheets which has been known to cause problems throughout the xerographic process. In present day intelligent paper handling devices, it is useful to know as much as possible about the qualities of the paper which is passing through a machine. Various parameters such as timing, airflow, normal force and velocity can be adjusted to compensate for the presence of detectable levels of input stress, such as, sheet curl to ensure that the paper will arrive safely and in time to meet the interface requirements of the various subsystems of the machine. Copy quality parameters, such as, transfer or detack current, as well as, fuser and decurler settings can also benefit from this information. Accordingly, in an aspect of this invention, a means of detecting the level of curl is disclosed by means of measuring the time it takes to acquire the sheet with a vacuum feeder. The acquisition time for capture of a sheet by the vacuum feeder is measured by use of a low cost pressure switch that changes state depending upon the degree of vacuum behind feeder belt holes and a timer. The timer value at the point that the pressure switch changes state and basis weight of the sheet are compared to a lookup table to yield the curl level of the sheet. Various aspects and embodiments of the present invention are defined in the appended claims. The present invention will be described further, by way of examples, with reference to the accompanying drawings, in which: FIG. 1 is a flow chart showing sheet curl determination in accordance with embodiments of the present invention; FIG. 2 is a cross sectional side view of an exemplary sheet separator-feeder employing the present invention; FIG. 3 is a plan view of the sheet separator-feeder showing the sheet stacking and holed belts surrounding a vacuum plenum; and FIG. 4 is a chart showing sheet acquisition sensing times for different curl levels in accordance with embodiments of the present invention. The invention will now be described by reference to a preferred embodiment of the bottom vacuum corrugation feeder apparatus for a copier/printer in FIG. 2. However, it should be understood that the curl detection method and apparatus of the present invention could be used with a top vacuum corrugation feeder or vacuum feeders in general. In general, the amount of curl in sheets can be determined in accordance with the present invention as shown in the flow diagram of FIG. 1 which illustrates the sequence of operation of a vacuum feeder in determining the curl level in sheets fed from the vacuum feeder. The vacuum feeder includes a vacuum plenum onto which sheets are drawn and forwarded out of the feeder for further processing. The feeder is started in block 100 and simultaneously a timer 105 is started in block 105. Once a sheet is drawn against the vacuum plenum in the the vacuum feeder, a pressure switch is actuated in block 110. Actuation of the pressure switch stops the timer in block 120 and the time it took for the sheet to be drawn to the vacuum plenum is read in block 130. This timer value is shown in block 140 and is sent along with the basis weight in block 145 of the sheet to lookup table 150 where a comparison of the input data is made with curl amounts that are contained in the lookup table to yield a curl level in block 160. This curl level has many uses, for example, with the curl level detection scheme of the present invention being used in a duplex tray, the signal from block 160 which indicates the amount of curl in the sheet is sent to microprocessor 88 of FIG. 2 which in turn signals the fuser of the machine to decrease the heat used in fixing images to the sheet since the heat from the fuser could have contributed to the level of curl in the sheet when the sheet passed through the fuser for the fusing of the first side image. Referring now particularly to FIG. 2, there is illustrated an exemplary automatic sheet separator-feeder for installation over the exposure platen 3 of a conventional xerographic reproduction machine, however, the principle of this invention and document handler 1 could also be used as a copy sheet feeding apparatus or duplex tray feeder with obvious modifications. This is merely one example of a document handler with which the exemplary sheet separator-feeder improvements of the present invention may be combined. The document handler 1 is provided with a document tray 5 which will be described in greater detail later, adapted for supporting a stacked set of documents 7. A vacuum belt corrugating feeding mechanism 9 is located below the front or forward area of the document tray for acquiring and corrugating the bottom document sheet in the stack and for feeding out that document sheet to take-away roll pair 11 through document guides 13 to a feed-roll pair 15 and under platen roll 17 onto the platen of the copy machine for reproduction. A retractable registration edge 18 is provided here to register the lead edge of the document fed onto the platen. Following exposure of the document, the edge 18 is retracted by suitable means such as solenoid and that document is fed off the platen by roll 17 onto guide 19 and feed-roll pair 21 and returned back to the top of the document stack through a feed-roll pair 23. Gross restacking lateral realignment is provided by a edge guide (not shown) resettable to a standard sheet site distance from an opposing fixed edge guide. In the event it is desired to present the opposite side of a document for exposure, the document is fed from the stack 7 through guides 13 until the trail edge passes document diverter 24. Document diverter 24 is then rotated counterclockwise, i.e., into the document sheet path. The document direction is reversed and the document is diverted by diverter 24 through guides 26 and feed-roll pair 25 onto the platen 3. The document handler 1 is also provided with a sheet separator finger 35 as is well known in the art, to sense and indicate the documents to be fed versus those documents returned to the document handler, i.e., to count each set circulated. Upon removal (feed out) of the last document from beneath sheet separator finger 35, the finger drops through a slot provided in the tray 5 to actuate a suitable sensor indicating that the last document in the set has been removed from the tray. The finger 35 is then automatically rotated in a clockwise direction or otherwise lifted to again come to rest on top of all the documents in the stack 7, for the start of the next circulation of document set 7. Referring more particularly to FIGS. 2 and 3, and the document sheet separator-feeder 9, there is disclosed a plurality of feed belts 37 supported for movement on feed belt rolls, 39 and 40. Spaced within the run of the belts 37 there is provided a vacuum plenum 41 having a support plate and openings therein adapted for cooperation with perforations 45 of about 3 mm in the belts 37 to provide a vacuum for pulling the bottom document in the document stack onto the belts 37. The plenum 41 is bi-level sloped and provided with three raised members 60-64 on a portion of the bi-leveled portion of the support plate that are below the belts 37 so that upon capture of the bottom document in the stack against the belts a corrugation will be developed in the sheet thereby enhancing its separation from the rest of the stack. This increased separation is due to the corrugation gaps placed in the sheet that reduce the vacuum pressure levels between the sheets due to porosity in the first (bottom) sheet and provide for entry of the separating air flow from the air knife 12. One of the raised members 61 is positioned in the center of the support plate. The air knife 12 is comprised of a pressurized air plenum 50 having a plurality of separated air orifices 51 to inject air between the bottommost document pulled down against the feed belts and the documents in the stack thereabove to provide a air cushion or bearing between the stack and the bottom document to minimize the force needed for removing the bottom document from the stack. By suitable valving and controls, it is also desirable to provide a delay between the time the vacuum is applied to pull the document onto the feed belts and the start up of the feed belts, to assure that the bottom document is captured on the belts before belt movement commences and to allow time for the air knife to separate the bottom sheet from any sheets that were pulled down with it. Turning now to the present invention more particularly, present vacuum corrugation feeders sometimes have difficulties feeding stressed or curled sheets with the consequence that sheets do not reach particular subsystems within required time spans. An answer to this problem is shown in FIG. 1 and includes a conventional low cost vacuum switch 80 placed within vacuum transport plenum 41 that enhances performance of the feeder through closed loop air system control as described herein below. Switch 80 is a commercially available pressure switch which is diaphragm actuated and can be set to open or close an electrical control circuit at a given pressure (or vacuum) level. It can be installed either directly in the vacuum plenum as shown or in the duct which connects the plenum to the blower, as desired. The switch will detect any significant rise in vacuum and changes state at a preset vacuum level when the bottommost non-stressed sheet in the document stack 7 is pulled onto the perforated belts 37 that surround the vacuum plenum 41, thus sealing the air system and creating the characteristic closed port pressure. The degree of curl whether upcurl or downcurl is determined by measuring the time it takes to acquire a sheet to belts 37. A conventional digital control circuit 86 senses the change of state which takes place in the vacuum switch and feeds a signal to the machine's microprocessor 88. The microprocessor in turn sends a signal to air knife 12 which reacts to adjust the air pressure of the air knife to increase the flow of air through nozzles 51 sufficient to compensate for the particular stress level of the sheets. Studies have shown a distinct correlation between the curl level present in a sheet and the time required to acquire the sheet. For example, in FIG. 4, the chart shows a preferential sensitivity to upcurl, part of which is inherent due to interactions with the air knife. The characteristic response exhibits sufficient sensitivity suitable for microprocessor control. The apparent overlap which can be seen, for example, in the acquisition time for flat 14.5 kg (32 pound) paper and a 9.07 kg (20 pound) paper with a curl radius of 0.84 m (33 inches (1000 ÷ 30)) requires that the basis weight be known independently. This is enabled through combining the basis weight input either from a basis weight sensing scheme as shown in US-A-5,138,178 or through direct input by the operator, with the acquisition time input that the curl level may be determined with absolute confidence. Although lightweight sheets show relatively little change in acquisition time as a function of curl, this is not viewed as problem, since the curl level of lightweight sheets is generally seen to be of little consequence in setting machine parameters. In conclusion, a curl detection system has been disclosed that uses the time needed to acquire a curled sheet by a bottom vacuum corrugation feeder to detect the degree of sheet curl for actions to be taken for reliable sheet handling. The acquisition time by the vacuum feeder is measured by a low cost pressure switch, in conjunction with a digital control circuit, determines the degree of vacuum behind perforations in feed belts that surround a vacuum plenum and signals a microprocessor to adjust various parameters, such as, timing, airflow, and velocity to compensate for the detected levels of curl. It is, therefore, evident that there has been provided in accordance with the present invention a nip sheet sensing scheme has been disclosed which fully satisfies the aims and advantages hereinbefore set forth.

Claims: A system for determining the amount of upcurl or downcurl in sheets fed from a stack, including: a vacuum feeder (9) having a vacuum plenum (41) for drawing a sheet from the stack to said vacuum plenum (41) for feeding along a predetermined path; characterised in that the system further includes : a vacuum switch (80) positioned within said vacuum plenum (41) and adapted to change state at a preset vacuum level due to a sheet being pulled against said vacuum plenum (41); digital control circuitry (86) for sensing the change of state of said vacuum switch (80); and for determining a time needed to acquire a curled sheet by the vacuum feeder (9); and a microprocessor (88) adapted to receive a signal from said control circuitry (86) once said vacuum switch (80) changes state and determine the amount of curl in the sheet. A system as claimed in claim 1, including a duplex tray and a fuser and wherein said microprocessor (88) is adapted to signal said fuser to decrease the heat level of said fuser based on the amount of curl in the sheet fed from said duplex tray. A system as claimed in claim 1 or claim 2, including an air knife (12) for separating individual sheets from the stack and wherein said microprocessor (88) is adapted to receive a signal from said control circuitry (86) once said vacuum switch (80) changes state and signal said air knife (12) to increase or decrease air pressure toward the stack to compensate for the particular curl in the sheet. A method of determining the curl level in sheets fed from a vacuum feeder (9) that attracts the sheets individually to a vacuum plenum (41), including: actuating the vacuum feeder (9) to feed sheets individually from the stack; characterised in that the method further includes: simultaneously starting a timer (105) with the actuation of the vacuum feeder (9); actuating (110) a vacuum pressure switch (80) upon once a sheet is captured by the vacuum plenum (41) ; determining the time lapse (140) between actuating the vacuum feeder (9) and actuation of the vacuum pressure switch (80); determining the basis weight of the sheets in the stack (145); providing a lookup table of curl levels of sheets (150); and determining the curl level in the sheet from the lookup table (160) based on the basis weight of the sheet and the time lapse between actuating the vacuum feeder and actuation of the vacuum pressure switch. A sheet separator-feeder (9) for separating and forwarding sheets seriatim from a stack of sheets to be fed, said sheet separator-feeder (9) including a stacking tray (5) having a surface for supporting a stack of sheets to be fed an air knife (12) positioned opposite the sheet stack and adapted to separate a sheet in the stack from the remainder of the stack, a plurality of apertured endless vacuum feed belts (37) extending through at least one end of said sheet stacking tray (5) for acquiring and advancing the sheet of the stack, said plurality of apertured endless vacuum feed belts (37) extending across a vacuum chamber (41) that support said belts (37), said vacuum chamber (41) having vacuum ports (45) therein for applying a negative pressure through said belts (37), characterised by an arrangement for detecting the level of curl in the sheets and adjusting the air pressure of said air knife (12) against the stack based on the degree of curl detected, said arrangement including a vacuum switch (80) positioned within said vacuum chamber (41) and adapted to change state at a preset vacuum level due to a sheet being pulled against said vacuum chamber (41), digital control circuitry (86) for sensing the change of state of said vacuum switch (80); and for determining a time needed to acquire a curled sheet by the sheet separator-feeder (9); and a microprocessor (88) adapted to receive a from said control circuitry (86) once said vacuum switch (80) changes state and signal said air knife (12) to increase air pressure toward the stack. A sheet separator-feeder (9) as claimed in claim 5, characterised in that the separator-feeder (9) is a bottom sheet separator-feeder for separating and forwarding sheets seriatim from the bottom of the stack of sheets to be fed. A sheet separator-feeder (9) as claimed in claim 5 or claim 6, characterised in that the vacuum chamber (41) includes a bi-level support plate for supporting said belts (37); said support plate having a plurality of corrugation means for corrugating the sheet in the stack extending along a sloped bi-level portion of said support plate. A sheet separator-feeder (9) as claimed in claim 7, characterised in that said corrugation means on said support plate of said vacuum chamber (41) includes three raised members (60-64) on a portion of said bi-level portion of said support plate with one of said raised members (61) being positioned in the center of said support plate.

Inventors: SIEGEL ROBERT P, SIEGEL, ROBERT P.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1995
Main classification: G03G15
IPC codes: B65H3, G03G15, B65H7