Multiple Sheet Detection System

Abstract

A multiple-sheet-detecting system for use in a sheet feed path to provide signals to a machine control to indicate when superposed sheets are fed past a detection station. The detection system has a light source and photosensor located on opposite sides of the sheet path such that the illumination reaching the photosensor is interrupted by the presence of a sheet or sheets in the sheet path. An electronic circuit is coupled to the photosensor which experiences a change in resistance depending upon the transmittance of a sheet or sheets in the sheet path and provides an output signal when there is a multiple sheet condition. The electronic circuit includes a motor-driven potentiometer which is utilized as a memory for a previously fed sheet in a feedback circuit for comparison with a signal representative of the next sheet. The circuit may be calibrated for varying circuit parameters to accommodate different sheet weights and types.

Description

This invention relates to a multiple sheet detection system to detect the presence of superposed sheets advanced along a sheet path and to indicate to a machine control that a multiple sheet feed condition has occurred.

In feeding copy sheet material along a path, as for example, through a copy processor, it is essential to advance one sheet at a time and to prevent the forwarding of multiple or superposed sheets. When multiple sheets are forwarded accidentally, jams occur at the processing stations which result in a machine malfunction or possibly a jam at the heat fusing station. With the advent of high-speed reproduction machines incorporating complex sheet feed systems feeding different sheet sizes, weights, etc., problems associated with multiple sheets have become increasingly apparent.

Usually, the detection of multiple or superposed sheets is accomplished by mechanical sensors as described, for example, in U.S. Pat. No. 3,396,965. While these devices are suitable in some cases, they are not entirely satisfactory in many present-day reproduction systems.

It is therefore the principal object of this invention to improve multiple sheet detection systems.

It is another object of the present invention to enable detection of superposed sheets fed in a sheet path and to indicate this condition to a machine control irrespective of the weight or thickness of the sheet stock being fed.

It is another object of the invention to enable detection of multiple sheets being fed past a detection station at very high rates.

It is another object of the present invention to detect the feeding of multiple sheets by measuring the light transmission of sheet material and sensing differences in the transmission of sheets being fed.

It is another object of the present invention to utilize electronic circuitry having feedback and memory for the detection of multiple sheets being fed along a sheet path.

It is another object of the present invention to facilitate the detection of superposed sheets being fed along a sheet path in a manner more simple and more rapid than that used heretofore.

The detection system of the invention, generally speaking, is accomplished by comparing the light transmittance of each fed sheet with that of a previously fed sheet and utilizing feedback and memory in an electronic circuit for comparing differences of the transmissions and providing appropriate signals to machine control to indicate the presence of superposed sheets.

For a better understanding of the invention as well as other objects and features thereof, reference is had to the following detailed description of the invention which is to be read in connection with the accompanying drawings wherein:

FIG. 1 illustrates schematically a xerographic recording apparatus incorporating a multiple sheet detection system constructed in accordance with the present invention;

FIG. 2 is an enlarged view of a sheet feed section of the apparatus illustrating lamp and photosensor assemblies of the present invention and their relationship to the sheet feed path;

FIG. 3 is a circuit diagram of the multiple sheet detection system; and

FIG. 4 is a diagram illustrating signal voltage variations indicative of single and multiple sheet feed conditions.

Referring now to FIG. 1, there are shown various components of a xerographic recording machine which produces enlarged copies of microfilm aperture cards on cut sheets of different sizes and weights at high rates and which utilizes a multiple sheet detection system according to the present invention. As in all xerographic recording systems, a light image of an original to be reproduced is projected onto the sensitized surface of a xerographic plate to form an electrostatic latent image thereon. Thereafter, the latent image is developed with an oppositely charged developing material to form a powder image corresponding to the latent image on the plate surface. The powder image is then electrostatically transferred to a support surface such as paper to which it is permanently affixed.

In the recording machine of FIG. 1, aperture cards are placed in a card feed section 16 from which each card is fed in seriatim in a path transverse to the optical path to scan a film frame on the card in timed relation to the movement of a xerographic plate which is in the form of a drum 20. Xerographic drum 20 comprises a layer of photoconductive material on a conductive backing and is mounted in suitable bearings in a machine frame and driven by a suitable drive. The drum passes first a charging station A at which a uniform electrostatic charge is deposited by a corona-generating device 21. Next at an exposure station B, the drum is exposed to a light image to discharge the photoconductive layer in the areas struck by light whereby there remains on the drum at latent electrostatic image corresponding to the film frame on the aperture card being scanned. As the drum continues its movement, the electrostatic latent image passes through a development station C at which a developing material including toner particles having electrostatic charge opposite to that of the electrostatic latent image are cascaded across the drum surface whereby the toner particles adhere to the latent image to form a powdered image. As toner powder images are formed, additional toner particles are supplied to the developing material in proportion to the amount of toner deposited on the drum. Positioned adjacent to the development stating C is an image transfer station D at which the powder images are electrostatically transferred from the drum surface to sheet material supplied from a sheet feed station past a sheet detection station in a manner as will be explained more fully hereinafter.

A sheet transport apparatus, generally designated 25, serves to transport the sheet material received from a sheet feed section 27 toward a transfer station D where a corona transfer device 29 similar to the corona-charging device 21 is located to effect the electrostatic transfer of the developed image onto the sheet material. After transfer, the powder image is permanently affixed to the sheet material by a heat fuser apparatus 31 and the sheet material conveyed onto a vertical transport 33 for delivery to a copy tray 35. A drum-cleaning station E comprises a corona discharge device 37 and brush 39 to remove residual toner particles remaining on the drum surface after image transfer, and a light source 40 to complete discharge of any residual electrostatic charge remaining on the surface whereby the drum may be ready for another cycle to pass through the processing stations as just described.

It is believed that the foregoing description of the xerographic recording apparatus is sufficient for purposes of showing a general operation of a typical recording apparatus using a multiple sheet detection system constructed in accordance with the present invention. For further details concerning the specific construction of the xerographic machine components previously described, reference is made to copending applications Ser. No. 824,541 filed on May 14, 1969, entitled Multiple Output Electrostatic Recording System; and Ser. No. 824,542 filed on May 14, 1969, entitled Copier Machine Feeding Multiple Size Copy Sheets. It should be understood, however, that the detection system of the present invention is not limited to use in copier machines and may find use in any suitable environment.

In FIG. 2, there is shown a sheet detection station located at the exit side of sheet feed section 27 which feeds sheets of different sizes and/or types as selected from a conveniently located control panel. Sheets are fed desirably one at a time toward guide members 45 which converge and terminate at the exit of the sheet feed sections where the multiple sheet detection station is positioned.

It will be appreciated that the sheet feed section is modular and is of the type which can be easily pulled away from the rest of the xerographic recording machine as described more fully in the previously mentioned copending application if desired.

At the multiple sheet detection station, there is positioned a lamp assembly 51 situated below the sheet feed path and a photosensor assembly 53 located just above the sheet path which assemblies are mounted on the frame of the sheet feed section. Lamp assembly 51 includes a lamp DS1 or any suitable source of illumination in a housing 57 formed with an aperture 59 such that illumination from the lamp is directed across the sheet path to be sensed on a photosensitive element V1 through the aperture 63 located in a housing 65. Photosensitive element V1 may be any suitable light-sensitive device. Typically, the photosensitive element may comprise a photocell, photovoltaic cell, photoconductor, phototransistor, photodiode, light activated SCR, etc. Guide members 45 are notched or opened to provide an uninterrupted flow of illumination in a direction across the sheet path. Lamp DS1 is adjustable relative to the lamp housing as by screws 67 received in the housing against the action of springs 68. It will be appreciated that the overall configuration of the lamp and its aperture and the photosensitive element, and its aperture is such that any variation in sensitivity due to differences in the sheet path is minimized.

In accordance with the invention as a sheet advances past the lamp and photosensor assemblies, illumination from the lamp DS1 is interrupted which causes an increase in the resistance of the photosensitive element which is coupled to an electronic circuit as will be described. The photosensitive element resistance level normally generated by advancing single sheets is such as to be repeatable if the light transmission of the sheet is the same or nearly the same. However, when superposed or multiple sheets are fed past the lamp and photosensor assemblies, the drop in light transmission causes the resistance of the photosensitive element to increase significantly above the resistance generated by a single sheet. When this occurs, the electronic circuit coupled to the photosensitive element utilizes this resistance change to provide an output signal to a machine control to indicate the presence of multiple sheets.

A better understanding of the invention may be had in connection with the circuit illustrated in FIG. 3.

The circuit has a regulated well-filtered power supply which is made up of transformer T1, bridge rectifier CR1, resistors R1, R3 and R4, capacitors C1 and C2 and zener diodes CR2 and CR3. It has been found that output voltages from such a power supply of about plus and minus 12 volts performs well for purposes of the present invention. Transformer T1 also provides a supply voltage for lamp DS1. A resistor R20 is used to provide an unfiltered power source for relays which are energized in a manner and a purpose to be described hereinafter. Resistors R2 and R6 and a trim pot R5 serve to make up a reference voltage divider. In this manner, voltage values are selected to place a voltage at point C with respect to point B of 1/2 the normal supply voltage which in this case is 6 volts. The trim pot R5 enables an offset voltage Y to be selected which can accommodate different sheet weights and types in a unitary system. Offset voltage Y should not be too low so that a dense single sheet will actuate the detection system to indicate a double sheet condition or so high that very light or high-transmittance superposed sheets being fed past the detecting station do not indicate a single sheet condition. For a supply of 12 volts, it has been found that an offset voltage Y is desirable which ranges from about 0.5 volts to about 2 volts and preferably about 1.2 volts (FIG. 4).

In order to provide a useful voltage variation for the sensing of multiple sheets, the photosensitive element V1, which in this case is a photocell, a resistor R19 and a multiturn pot R37 serve as the active transmittance-sensitive input to the circuit. When no sheet is present in the feed path at the detection station, the resistance of the photocell is very low or in the order of about 1 K ohms. For this condition, the voltage at point E will be approximately 11 volts. However, when a single sheet is present at the detection station, the photocell resistance will be greatly increased and the voltage at point E decreases to about 6 volts. It will be appreciated that the exact voltage level will depend upon such things as the transmittance of each particular sheet of material and that this will vary with the density of any given material, as by about one-half to about 1 volt.

When two sheets are fed past the detection station, the voltage at point E drops to approximately 4 volts due to a much greater increase in the photocell resistance.

It will be noted that point E is connected to the negative input of a differential amplifier Q3 and also to the positive input of a differential amplifier Q7. It will be further noted that point C is connected to a positive input of a differential amplifier Q3 and to the negative input of a differential amplifier Q7. Now it will be appreciated that the differential amplifiers Q3 and Q7, together, serve to make up a null detector. Thus, when point E of the circuit is positive with respect to point C, Q7 will be turned on and Q3 will be turned off. Likewise, when point C of the circuit is positive with respect to point E, Q3 will be turned on and Q7 will be turned off.

Furthermore, when the voltage between point C and point E is about 0 volt, a null condition exists and both Q3 and Q7 are turned off.

It will be appreciated that relays K2 and K3 are driven by differential amplifiers Q2 and Q7 and driver transistors Q6 and Q8, respectively. Relays K2 and K3, in turn, operate bidirectional motor B20 which drives multiturn potentiometer R37.

In accordance with the invention, null detector Q3 and Q7 and bidirectional motor B20 and potentiometer R37 serve as a feedback circuit which is responsive to the light transmission of each sheet being fed past the detection station. Thus, in order to achieve a null condition between point E and point C when there is a single sheet at the detection station, the potentiometer R37 will be driven by bidirectional motor B20 in a direction toward the null.

When a null condition is achieved between point C and point E, energization of the bidirectional motor B20 ceases and the potentiometer R37 comes to a stop. It has been found that where R37 has a 100 K-ohm total resistance, that a potentiometer drive speed of 10 r.p.m. performs well.

Desirably, bidirectional motor B20 is energized when one or more sheets are present at the detection station. To accomplish this, a differential amplifier Q2 serves as a switch which turns on transistor Q5 only when there is sheet material at the detection station. Resistors R7 and R21 make up a voltage divider which sets the voltage at point M to about 8 volts. Point M is connected to the positive input of Q2 and point E is connected to the negative input. Thus, Q2 is turned on only when point M is positive with respect to point E of the circuit and can occur only when there is sheet material at the detection station. It will be noted that Q5 provides a path for transistors Q6 and Q8 and relays K1 and K2. Thus, Q5 which is driven by Q2 must be on before K1 or K2 can be actuated. By this arrangement, bidirectional motor B20 can drive potentiometer R37 only when sheet material is present at the detection station.

In the invention when a multiple sheet condition occurs point E is driven negative with respect to point D of the circuit. When this happens, differential amplifier Q1 is turned on. The negative input of Q1 is connected to point E. Due to the feedback loop mentioned above, the voltage at point E is approximately equal to the voltage at point C. It will be noted that the voltage at point D which is connected to the positive input of Q1 is about 1.2 volts negative with respect to point C. By this arrangement, the voltage at point D is about 1.2 volts negative with respect to point E. It will now be appreciated that the voltage at point E must be driven negative with respect to point D in order for transistor Q1 to be turned on. Furthermore, point E is driven negative with respect to point D only when a multiple sheet condition exists.

Thus, relay K1 which is driven by Q9 provides an output to machine control only when Q1 is turned on or when multiple sheets are fed past the detection station. When this occurs, a visual indication of the multiple sheets is provided by a lamp DS2 in the circuit and conveniently located on the machine.

It is desirable that motor B20 be prevented from driving potentiometer R37 when a multiple sheet condition occurs. To accomplish this, Q4 will be turned on by Q1, thereby preventing Q5 from turning on, thereby precluding motor B20 from driving potentiometer R37. By this arrangement, the motor B20 cannot drive when there either is no sheet material at the detection station or when multiple sheets are detected.

In order to accommodate great differences in weights and types of sheet material in the detection system of the invention, the system is adapted for calibration to meet such varying conditions. To accomplish this, a calibration switch S38 is provided to be manually actuated during the time that a single sheet of the material of the type to be fed is introduced at the detection station. Switch S38 is held actuated until such time as the motor B20 turns off which is indicated when lamp DS2 is deenergized. Wen this occurs, calibration is complete since the feedback circuit previously described is adapted to store the transmittance information derived during calibration of a single sheet. By this arrangement, it is possible to use different types and weights of sheet material in a multiple sheet detection system without an unduly involved calibration procedure.

The invention described above enables multiple sheets to be detected in a rapid and simple manner with a high reliability. Moreover, a great latitude of sheet weights and types can be accommodated. It will be appreciated that the sheet detection system of the invention is highly desirable for present-day copier/duplicator machines.

Claims

What is claimed is:

1. A multiple sheet detection system comprising photosensitive means positioned on one side of a sheet path at a sheet detection station,

illumination means located on the opposite side of the sheet path for directing illumination towards said photosensitive means,

circuit means coupled to said photosensitive means operative to determine the presence of multiple sheets including

first circuit means for generating discrete electrical signals representative of the transmittance of sheet material advanced along the sheet path, second circuit means for storing a previous signal supplied from said first circuit means, and third circuit means for comparing the stored signal with a succeeding signal generated from said first circuit means to determine the presence of multiple sheets and generating an output signal when a multiple sheet condition occurs.

2. A system according to claim 1 including means to prevent said second circuit means from storing signals when sheet material is not present at the sheet detection station.

3. A system according to claim 2 wherein said second circuit means comprises a potentiometer operatively connected to bidirectional motor.

4. A system according to claim 3 including switching means coupled to said second circuit means to enable the setting of said potentiometer to be varied to establish a reference level for different sheet weights and types.

5. In a recording machine in which copy sheets are advanced through copy-processing stations along a predetermined path, an improved detection apparatus for detecting multiple sheets at a sheet detection station comprising

means for feeding sheet material along a sheet path past copy-processing stations,

photosensitive means positioned on one side of the sheet path at the sheet detection station,

illumination means located on the opposite side of the sheet path for directing illumination towards said photosensitive means,

circuit means coupled to said photosensitive means to determine the presence of multiple sheets at said detection station and indicate such condition to machine control,

said circuit means including first circuit means for generating discrete electrical signals representative of the transmittance of sheet material advanced along the sheet path, second circuit means for storing signals supplied from said first circuit means, and third circuit means for comparing each stored signal with a succeeding signal generated from said first circuit means to determine the presence of multiple sheets and generating an output signal to machine control when a multiple sheet condition occurs.