Electrophotographic copier including copy sheet detach detector

Abstract

 A detach detector for detecting whether or not a copy sheet has been stripped from a photoconductor (19) after transfer comprises a discharge lamp (40) and an electrostatic probe (41). In operation, the lamp discharges the photoconductor during correct operations of the machine to provide a low voltage level at the probe. If, however, a copy sheet remains attached to the photoconductor, the rear side remains at a high voltage level which is detected by the probe.

Description

[0001] The present invention relates to electrophotographic copiers, and in particular to such copiers employing copy sheet detach detection means.

[0002] In electrophotographic machines, copies of documents or other subjects are produced by creating an image of the subject on a photoreceptive surface, developing the image, and then fusing the developed image to copy receiving material. In machines utilizing plain bond copy paper, or other ordinary image-receiving material not specially coated, the electrophotographic process is of the transfer type where a photoreceptive material is placed around a rotating drum or arranged as a belt to be driven by a system of rollers. In the typical transfer process, photoreceptive material is passed under a stationary charge generating station to place a relatively uniform electrostatic charge, usually several hundred volts, across the entirety of the photoreceptive surface. Next, the photoreceptor is moved to an exposure station where it receives light rays reflected from the document. Since white areas of the original document reflect large amounts of light, the photoreceptive material is discharged in white areas to relatively low levels, while the dark areas continue to contain high voltage levels, even after exposure. In that manner, the photoreceptive material is caused to bear a charge pattern which corresponds to the printing, shading, etc. present on the original document.

[0003] After receiving the image, the photoreceptor is moved to a developing station where a developing material called toner is placed on the image. This material may be in the form of a black powder which carries an electric charge opposite in polarity to the charge pattern on the photoreceptor. Because of the attraction of the oppositely charged toner, it adheres to the surface of the Photoreceptor in proportions related to the shading of the original. Thus, black character printing should receive heavy toner deposits, white background areas should receive none, and gray or otherwise shaded half-tone character portions of the original should receive intermediate amounts.

[0004] The developed image is moved from the developing station to a transfer station, where a copy receiving material, usually paper, is juxtaposed to the developed image on the photoreceptor. A charge is placed on the back side of the copy paper so that when the paper is stripped from the photoreceptor, the toner material is held on the paper and removed from the photoreceptor.

[0005] The remaining process steps call for permanently bonding the transferred toner material to the copy paper and cleaning the residual toner left on the photoreceptor so that it can be reused for subsequent copy production.

[0006] As noted above, the sheet of copy-receiving material must be stripped away from the photoreceptor subsequent to passing through the transfer station where the charge is received on the back side of the copy paper. Should a sheet of copy-receiving material be unsuccessfully detached from the photoreceptive surface, the moving photoreceptor will carry the paper into operating stations such as the charge corona station, the exposure station, the developer station, and so on. At any of these points, the paper may crumple and jam into one of these apparatus. When that occurs, a serious jam condition is caused, necessitating the use of maintenance personnel to clear the jam.

[0007] To prevent paper from moving into these operating stations, detectors have been placed next to the stripping station so that an unsuccessfully detached sheet of copy-receiving material can be detected and the machine can be immediately shut down. In that manner, it is hoped to enable the machine key operator to clear the jam condition and avoid the necessity of calling maintenance personnel.

[0008] The typical document detection equipment in use at the current time is an optical detector in which light rays are reflected from the photoconductive surface to a photodetecting cell. In that manner, the cell is arranged to normally view the photoconductive surface and receive a certain level of reflected light from that surface. When an unsuccessfully detached sheet of copy-receiving material is present, the detected light is usually much greater since the sheets are normally white. As a result, the increased detected illumination results in a higher detected voltage, which enables the detecting circuitry to immediately shut down the copier machine. U.S. Patent Specification Nos. 3,521,060 and 3,791,729 describe such detectors.

[0009] While the current technique is satisfactory for most purposes, it is obvious that it will not work when transparencies are being produced and an unsuccessfully detached transparency moves into the detecting station. In this situation, the light rays from the light source will pass through the transparency and reflect off of the photoconductive surface into the photodetecting cell. Consequently, the reflected light level sensed by the photodetecting cell will not be materially changed, even though an undetached sheet of copy-receiving material is present on the surface of the photoconductor. For that reason, it has been customary to provide transparencies with a white strip along the leading edge thereof. In that manner, the photodetecting cell will sense the white leading edge and shut down the machine before the transparency can move into the operating stations; this solution to the problem is subject to failure where the operator of the machine loads the transparencies into the paper drawer in a manner such that the white edge is not the leading edge; or where the transparency contains no white edge. It is also a relatively expensive solution, since transparencies with white edges must be produced.

[0010] Another arrangement is shown in U.S. Patent Specification No. 3,650,616 relates to locating a corona generator next to a photoconductive surface in order to detect an undetached copy paper. Instead of measuring an interruption in current as in the previous patent, an unbalance in corona voltage is sensed when paper is present on the surface thereby creating a situation which detects the presence of the paper.

[0011] U.S. Patent Specification No. 3,852,668 discloses a particular design for an electrostatic probe which is used in detecting the charge level on the surface of the photoconductor. This specification is representative of many which show the utilisation of electrostatic probes to detect charge level in order to alter the developer bias voltage, the illumination voltage level, or the charge corona voltage level, to adjust the charge level of the photoconductor.

[0012] The present invention employs a similar probe to detect the unwanted presence of a copy sheet on the photoconductive surface after image transfer.

[0013] According to the invention, there is provided an electrophotographic copier including a movable photoconductive imaging element and, positioned in order adjacent the path of movement of the element, a charging station, an imaging station, a development station, a transfer station for electrostatically transferring developed images from the imaging element to a copy sheet brought into contact therewith, and detach failure detection means to detect failure of copy sheet stripping from the imaging element after image transfer, characterised in that said detection means comprises a light source positioned adjacent the transfer station to discharge the imaging element as it exits from the transfer station and an electrostatic probe positioned to sense the charge on the imaging element, or a copy sheet still thereon, subsequent to its passage past the light source.

[0014] The invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is an overall view of a typical electrophotographic copier machine;

FIG. 2 illustrates the paper path of the machine shown in FIG. 1; and

FIG. 3 shows a copy sheet detach detection arrangement.

[0015] FIG. 1 shows a schematic diagram of a typical electrophotographic copier machine of the transfer type. An original document is placed on a document glass 10 and imaged through optics module 11 onto a rotating photoconductor at exposure station 12. The image of the document is transferred to copy-receiving material at transfer station 15 under the influence of transfer corona 16. The copy-receiving material, usually paper, is forwarded to the transfer station 15 from one or the other of paper bins 13 and 14. The image is fused to the copy paper by rolls 17 and 18 and is then deposited into one of the bins of collator 18.

[0016] The photoreceptive surface 19 is rotated in direction "A" past a charge corona 20 and a backcharge corona 21 in order to provide a smooth electrostatic charge on the surface of photoreceptor 19 prior to its entry into the exposure station 12. After exposure, the latent image is developed by developer 22 prior to reaching the transfer station 15. Subsequent to the transfer station, the photoreceptor moves under a preclean corona 23, prior to beginning a subsequent copy cycle.

[0017] An electrostatic probe 24 is shown in FIG. 1 is situated adjacent the transfer station in order to detect an undetached sheet of copy-receiving material.

[0018] FIG. 2 shows the paper path of the machine of FIG. 1 in more detail. Copy-receiving material is placed in bins 13 and 14, and is fed one sheet at a time by sheet feeding means 30 or 31 along the paper path through rollers 32 to a gate mechanism 33. This includes a finger 38 which intercepts the copy-receiving material when positioned as shown by the phantom lines in FIG. 2, and releases the copy-receiving material at the correct point in the machine cycle so that the leading edge of the copy sheet mates with the leading edge of the image on the surface of photoreceptive material 19. When released by gate 33, the copy receiving material progresses to the transfer station where corona 16 generates a charge which is placed on the back side of the copy-receiving material. In a typical machine, the developed image may carry positive toner particles, and in such a case, a negative charge is placed by corona 16 on the back side of the copy-receiving material in order to attract the positive toner from the surface of the photoreceptor 19 to the sheet of copy-receiving material. In that manner, the developed image is transferred from the photoconductor 19 to the copy-receiving material. Subsequent to the transfer operation, a detach finger 34 is positioned in close juxtaposition to the photoreceptive material 19 in order to intercept the leading edge of the copy-receiving material. Upon interception, finger 34 is moved in an arc around pivot point 35 by the moving copy-receiving material until the finger drops under the transport path to a position shown in phantom in FIG. 2. Meanwhile, the copy sheet is brought under the influence of star wheel 36, which causes the copy sheet to continue its movement toward the nip of fuser rollers 17 and 18. Detector 24, which is designed to detect unsuccessfully detached copy sheets, is located near the photoreceptive material 19 just subsequent to the detach finger 34.

[0019] FIG. 3 shows the detector 24 in detail. A light source 40 is positioned just prior to an electrostatic probe 41 in order to discharge the photoreceptive surface to a value which may, for example, be approximately -50 volts. The probe 41 acts as one plate of a capacitor Cl with the photoreceptive surface 19 acting as the other plate of that capacitor. The effect of this capacitor is to sense the voltage on the photoreceptor. A second capacitor C2 is provided such that the voltage sensed by capacitor Cl is divided between the two capacitors so that the voltage across capacitor C2 is applied at input terminal 43 of operational amplifier (op amp) 44 with a reference voltage of similar value applied to the second input terminal 45. The output of op amp 44 is applied to a peak detecting network comprising diode 46, resistor 47, and capacitor 48. The peak detecting network senses the change which may occur at capacitor Cl and holds that effect, even though the charge at capacitor Cl may be extinguished.

[0020] The light source 40 may comprise a light emitting diode (LED) or a group of LED's, and the value of capacitance Cl may be in the order of 20-30 pico farads. The resistor 51 should be large, approximately one megohm.

[0021] FET 49 is provided to initialize the capacitive voltage divider prior to a photoreceptor voltage sensing test so that any stray voltages which may have found their way into the capacitive voltage divider, may be extinguished prior to the test.

[0022] In operation, lamp 40 discharges the photoreceptive surface 19 such that probe 41 detects a repeatable voltage level of, for example, -50 volts. In a typical embodiment, the capacitor voltage dividing network may be in the order of 100:1, thus providing a voltage of -.5 volts on capacitor C2. That voltage is applied at terminal 43, and the reference voltage at 45 is designed to be of similar value so that the resulting output of op amp 44 is low. To ensure low output from the op amp, the reference voltage should be high enough to equal the expected worst case conditions to be encountered during the machine life.

[0023] Should a copy-receiving material be unsuccessfully detached by detach finger 34, the copy-receiving material would proceed on the surface 19 under the voltage sensing probe 41. Since the back side of copy-receiving material has passed under the transfer corona 16, the probe 41 will now sense the voltage present on the copy receiver. In this case, the lamp 40 would have no effect in reducing the voltage to -50 volts, since the back side of copy-receiving material is not a photoreceptive medium. As a consequence, the probe 41 will sense a relatively high voltage which may be, for example, -450 volts for a sheet of copy paper ranging upwardly to perhaps -2,000 volts for a highly insulative copy-receiving medium such as transparency material. In any event, a relatively large voltage is sensed by the probe 41 as compared to the -50 volts on the discharged photoreceptive material. This large change is reflected into the input 43 of op amp 44, causing the output of op amp 44 to go high. In that manner, an unsuccessfully detached sheet of copy-receiving material is sensed as an output voltage at terminal 50. This voltage may then be fed into logic circuitry which shuts down the operation of the electrophotographic copier machine.

[0024] It should be noted that the present arrangement is insensitive to the relative opaqueness of the copy-receiving material since the light from source 40 has no effect on discharging a transparency. For that reason, the white or otherwise opaque edge on transparencies, which is required for optical detection, may be dispensed with.

Claims

1. An electrophotographic copier including a movable photoconductive imaging·element (19) and, positioned in order adjacent the path of movement of the element, a charging station (21), an imaging station (11), a development station (22), a transfer station (16) for electrostatically transferring developed images from the imaging element to a copy sheet brought into contact therewith, and detach failure detection means (24) to detect failure of copy sheet stripping from the imaging element after image transfer, characterised in that said detection means comprises a light source (40) positioned adjacent the transfer station to discharge the imaging element as it exits from the transfer station and an electrostatic probe (41) positioned to sense the charge on the imaging element, or a copy sheet still thereon, subsequent to its passage past the light source.

2. A copier as claimed in claim 1, further characterised in that said detection means includes an operational amplifier (44) coupled to the probe and a reference voltage to compare the probe voltage with the reference voltage.

3. A copier as claimed in claim 1 or claim 2 further characterised in that the probe comprises a conducting plate positioned adjacent the surface of the imaging element to act as a capacitor.

4. A copier as claimed in claim 3, further characterised in that said probe is connected to a capacitor (C2) to form a capacitve voltage divider network to provide an output smaller than, but proportional to, the voltage sensed at the probe.

5. A copier as claimed in claim 4, further characterised by initialisation means (49) operable to set the network output to a predetermined voltage level immediately prior to a detach sensing operation.

Drawing