GB1559021A - Copier - Google Patents

Description

(54) COPIER (71) We, XEROX CORPORATION, of Xerox Square, Rochester, New York, United States of America, a corporation organized under the laws of the state of New York, United States of America, do herebv declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to line-by-line scanning system for copying machines.

The formation and development of images on the surface of photoconductor material by electrostatic means is well known. The basic xerographic process as taught by C. F. Carlson in US Patent No.

2,297,691 involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic charge pattern image by depositing on the image a finely divided marking material referred to in the art as 'toner'; The toner will normally be attracted to those areas of the layer which retains a charge thereby forming a toner image corresponding to the electrostatic charge pattern. The powder image may then be transferred to a support surface such as paper and permanently affixed to the support by any suitable means such as heat fixing or solvent fixing.

Alternatively, the powder image may be fixed to the photoconductive layer if elimination of the power transfer step is desired. In addition instead of forming a charge pattern by uniformly charging a photoconductor followed by image-wise light exposure, a charge pattern may be formed by directly charging the layer in image configuration. Other methods are known for applying marking particles to the imaging surface. Included within this group are the 'cascade' development technique disclosed by E N Wise in US Patent 2,618,552; the powder cloud development technique disclosed by C. F. Carlson in US Patent 2,221,776; and the magnetic brush process disclosed, for example, in US Patent 2,874.063.

A liquid technique for developing electrostatic charge patterns is the liquid development process disclosed by R. W.

Gundlach in US Patent 3,084,043. In this method, a charge pattern is developed or made visible by presenting to the imaging surface a liquid developer on the surface of a developer dispensing member having a plurality of raised portions defining a substantially regular patterned surface and a plurality of portions depressed below the raised portions. The depressed portions contain a liquid developer which is maintained out of contact with the electrostatic imaging surface. When the raised areas of the developer application are brought into contact with the imaging surface bearing a charge pattern, the developer creeps up the sides of raised portions in contact only with the charged area of the imaging surface, and is deposited thereon.

In a compact electrostatographic copying device employing the development techniques by R. W. Gundlach in US Patent 3,084,043 or in US Patent 2,811,465 by H. G.

Crieg the imaging surface and the liquid developer application are desirably small diameter cylinders or the like, to facilitate the cooperative movement of the surface in contact during development in a confined space. Such moving contact between the imaging surface and the applicator resulting in the transfer of liquid developer from the applicator to the photoreceptor occurs at development speeds ranging generally from about 2 to about 80 inches per second.

It can be readily appreciated that the quality of the print is, in the large part, dependent on the exposure of the charged xerographic plate to the radiation image.

The largest single factor effecting exposure latitude, i.e., range of illumination intensity, is the efficiency of the developer system. In other words if the developer system is highly sensitive so as to develop background or image portions as "grey" areas when in reality these are white, then illumination control must be commensurately sensitive to provide the proper exposure of the charged xerographic surface. With modern day improvement to xerographic developer systems, the desirability of maintaining proper illumination becomes increasingly apparent.

A uniformly high level of illumination as required for exacting exposure is complicated by many factors. For example, variation in lamp output due to lamp aging or deterioration is sufficient to cause development of white areas thereby detracting from overall quality of the print.

It has been determined for example, that deterioration of some lamps is dependent on properties of their phosphor coating. The deterioration characteristics of aperture lamps having the same type of phosphor coating do not differ significantly. The deterioration of certain types of such lamps can be as much as 40% after approximately 1000 hours of use. Such a large change in illumination level cannot be tolerated in modern copying systems. Thus, such lamps are generally replaced after a time period much earlier than the 1000 hours deterioration period mentioned hereinabove. Some prior art proposals for compensating for variation in lamp output utilize photosensitive devices, such as photocells, which measure lamp output and adjust various machine parameters to compensate for the variation in lamp output.

According to the invention there is provided a line-by-line scanning system for a document to be copied comprising a stationary platen, document illumination means supported on a scan carriage, optical projection means for projecting light from a an illuminated document and on to a moving photosensitive surface through an exposure slit, and grey-shutter slidable mounted on the scan carriage arranged so as to be movable into the line of sight of the photosensitive surface at the exposure slit during scanning to enable development of a calibrating grey image.

'Ihe line-by-line scanning system may include buffer stops mounted adjacent each end of the platen and arranged to impact the grey-shutter and move the grey-shutter into and out of the line of sight of the photosensitive surface respectively.

The line-by-line scanning system may include a light intensity detector mounted to detect the intensity of light reflected from the illuminated document, said grey-shutter including a cut-out portion to allow a light to be reflected from the document to be projected on to the detector whenever the grey-shutter is in the line of sight of the photosensitive surface.

Embodiments of the invention will now be described by way of example with refernece to the accompanying drawings in which: Fig. 1 shows schematically the layout of an electrostatographic copier; Figs. 2 and 3 show the layout of an optical scanning system of the copier; Fig. 4 shows diagrammatically three positions of the optical scanning system together with a grey shutter; Fig. 5 shows a development characteristic of the copier; and Fig. 6 shows a circuit of an automatic developer bias and developer current control circuit.

Referring now to Fig. 1, there is shown a photoreceptor which in operation is charged by a corotron 3 and then exposed to a light image at the exposure station 5 in order to form a charge pattern. Both charging and exposing are disclosed by C. F.

Carlson in US Patent 2,297,691. The electrostatic image thus formed is then made visible at a developing station where liquid developer is applied to the photoconductive surface. At the developing station an applicator 7 having a pattern of recesses on its surface is pressed against the photoreceptor 1. The image, now visible on the surface of the photoreceptor is then transferred to a paper sheet 10 at a transfer station 9 by pressing the sheet 10 into contact with the image on the photoreceptor 1 so that the image is transferred to the paper sheet 10 to form the final copy. Any developer material remaining on the surface of the photoreceptor is removed by a cleaning blade 14.

Developer material cleaned from the surface of the photoreceptor 1 is collected in a tray 15.

A copying machine such as that described in British Patent specification 995413 uses an optical system comprising two fixed mirrors with a lens between the mirrors, the stationary original being illuminated by lamps on a movable lamp carriage, and light from the original being screened from the projection system except for a small slit between the lamps. British Patent specification 1,122,622 described a document copying machine in which the whole of the stationary original is illuminated during exposure, and scanning is achieved by oscillating one of the mirrors of the projection system about an axis in its plane. It has been proposed, for example in the United States patent specification 3,642,366 to have a more compact image projection system in which two mirrors are moved in different directions at speeds relating to the speed of movement of the photosensitive surface.

In copiers having optical systems of the kind already mentioned and generally any copiers relying on a photosensitive response, it may be desirable to adjust the illumination of the document to be copied to maintain as far as possible a constant irradiance at the image plane, that is at the photosensitive surface of photoreceptor.

This constant irradiance is desirably achieved for various original document background reflectivities and as far as practical in some cases despite aging or other forms of deterioration of the optical system and deterioration of lamps.

In many copiers, copies provided depend for their definition on the difference of light intensity between light and dark part of the original document to be copied so that adjustment of the illumination of the document may not be so critical. However, in a copier as described in UK Patent 880,597, the definition of the copies made depends in effect on the actual value of the illumination received, rather than a differential value, so that ensuring near constant irradiance received at the photoreceptor surface for differing types of original, that is, for each individual onginal, becomes even more important.

Adjustment of the optical system parameters, such as lamp illumination output and aperture changes, provide compensation primarilY to meet deterioration in copies due to changes in the optical system in the copying machine. In practice other parameters of the machine can change, particularly in the development system of the copying machine due to changes in ambient conditions for example, causing deterioration in copy quality. It will be noted that other parameters of the development system and other systems may be individually monitored and adjusted to maintain copy quality.

In practice, especially in copying machines in which development is dependent on actual values, rather than differential values of illumination received by the photoreceptor, copy quality is particularly dependent on correct calibration of the copying machine. It is an object of the present invention to provide an automatic calibration system for a copying machine.

The apparatus of Fig. 1 depicts a typical apparatus and typical configurations of imaging surface and applicator. It is to be understood that other configurations are possible, for example, one surface could be a flat plate while the other surface is arcuate at the point of contact between the surfaces.

In another example, one of the surfaces could be a belt.

The development of the image on the surface of the photoreceptor 1 is dependent on the relative potential of the applicator.

roller 7 and the photoreceptor 1. As such, this is controlIed by electrically biassing the applicator 7 with respect to the photoreceptor 1 to prevent spurious development of non-imaged areas. General characteristics of the biassing potential plotted against transfer of toner of the developer material is known for various types of copying machines and a flow of current from applicator to photoreceptor 1 with transfer of toner is generally indicative of the amount of toner being transferred at any time.

Referring to Figs. 2 and 3, a platen 110 is provided to support a document 111. A scanning mirror system includes two movable mirrors 112 and 113 shown in their extreme left and right positions in full and dotted outline respectively. The mirror 113 is arranged to move at half the speed of the mirror 112 during scanning or imaging to maintain the optical distance constant between the document 111 and a lens 114.

A tubular lamp 115 extending across the platen 10 parallel to the mirror 112 moves with the mirror 112. The lamp 115 is provided as illumination means to illuminate the document 111 through the the platen I10 during scanning.

An optical path extending from the platen 110 to the lens 114 continues beyond the lens to be reflected in sequence by mirrors 116 and 117 towards the photoreceptor 1.

An optical slit 119, better seen in Fig. 3, is provided in a cover plate 120. The slit is used to restrict the image field and thus preserve image quality. A light intensity detector 121, see Fig. 3, is mounted on the plate 120 adjacent the slit 119. A platent cover 122 is laid over the document 111.

In general operation, the document 111 is scanned by the sweep of the mirrors 12 and 13 from left to right forming a latent image of the document on the photoreceptor 1 which rotates in synchronism with the movement of the mirrors 112 and 113. The intensity of illumination incident on the document in the present example is determined by the magnitude of the electrical supply to the drive circuit of the lamp 115. To provide good copies of originals of widely differing reflectance properties, we alter the illumination of the originals according to their reflectivity. In the embodiment, this is achieved during a rescan period (ie, a calibration period) of optical scanning system by controlling the supply to the lamp 115 in dependence upon the maximum intensity of light received at the detector 121 during that period. For a further explanation of one such automatic exposure system see co-pending U.K. application 18010/74 (Serial No. 1467985).

In the described embodiment the optical calibration period consists of the rescan period enabling calibration of the system according to the reflectivity of the document to be copied. Optical calibration can however take place during a non-imaging initial forward scan of the document followed by an imaging forward scan. It is more convenient, although not essential, to achieve calibration during a. pre- scan or rescan period as in the embodiment described.

In the described embodiment, the detector 121 is positioned adjacent to the photoreceptor 118 and where the image is in focus. This' is the preferred position so that variations or deteriorations of the components of the optical system will be taken into account by the operation of the detector 121. The detector 121 could be placed in some other part of or adjacent to the optical path provided means are provided to focus an image at that point. An auxiliary lens or mirror may be provided for example. Also, more than one detector could be provided across the width of the slit.

In Fig. 4, a grey shutter 150 is slidably mounted on a scan carriage 151 which carriers the mirror 112 and lamp 115.

During rescan the shutter 150 is positioned so that illumination from the lamp 115 is reflected from the shutter 150 along the optical path to the photoreceptor 1 to enable the formation of a grey image. However, the shutter is provided with a cut-out portion (not shown) to allow light reflected from the document to be reflected directly, along the optical path, onto the detector 121.

At the end of rescan, the shutter 150 impacts a buffer 152 which moves the shutter to the right with respect to the carriage 151. Dunng scanning the shutter 151 is positioned so that the shutter is no longer in line of sight of the photoreceptor.

At the end of scan, the shutter 150 impacts a buffer 153 which moves the shutter to the left in readiness for a next rescan.

In one arrangement, the copier is arranged to come to rest between copy runs parts way through a rescan. In arrangements proposed earlier the detector 121 relied on the actual value of illumination received. In this one arrangement the optical calibration can be modified to use the grey image of the shutter 121 before the start of each copying cycle as a reference level. This enables automatically that measurements of light intensity at detector 121 are compared to a grey image level. Such an arrangement means that automatic exposure system or optical calibration can maintain its effective overall calibration even if changes due to ambient or operating conditions vary component performance because calibration is in accordance with a grey level persisting at the time of measurement and not as compared with an actual value.

In Fig. 5, the developer system characterised is illustrated. The current increases with the density of grey being developed. This is due, at least in part, to the increased transfer of toner taking place from the applicator 7 to the photoreceptor 1. According to particular conditions, a developer current of predetermined magnitude can be assessed for developing, say, a mid-grey density image. In this embodiment as explained earlier, the photoreceptor 1 is exposed through a midgrey shutter during rescan and the apparatus adjusted, as described below, until the desired developer current flows.

It will be appreciated that direct correspondence of a mid-grey image to a mid-grey developer current is usual.

However, in this embodiment, the lamp may be deliberately increased above normal intensity during rescan for calibration of exposure control system (see for example UK Specification No. 1529220) so that a shutter which is somewhat denser than mid-grey is required for a mid-grey image at normal illuminator levels. In any event, the so-called desired developer current referred to herein, means the current required to ensure developer system calibration to mid-grey, say, incorporating as applicable adjustments in the grey shutter for deliberate lamp output changes.

Referring to Fig. 6, the applicator 7 (Fig.

1) is connected through a resistor 200 to a first input of a comparator 201 and to a resistor 202 connected to a busbar 203.

The other input of the comparator 201 is connected to a variable tapping on a potentiometer 204 connected between the busbar 203 and a supply line 205. The supply line is connected through a diode 206, resistor 207 and a supply switch 208 to a 405 volt supply.

A low voltage line 209 is connected to the busbar 203 through parallel circuits comprising an n-p-n transistor 210 in series with a zener diode 211, and in series a zener diode 212, an n-p-n transistor 213 and a p-n-p transistor 214. The base of the transistor 214 is connected to the output of the comparator 201. The bases of transistors 210 and 212 have a common connection through a resistor 215 to the line 209 and through a diode 216 in series with a resistor 217 to a point between the resistor 207 and the diode 206. A zener diode 218 is connected between the line 205 and the busbar 203. A capacitor 219 is connected between the lines 205 and 209. A diode 220 is connected between the first input of the comparator 201 and the collector of the transistor 210 which collector is also connected to the busbar 203 through a resistor 221. The zener diodes 212 and -218 are each rated at 5.6 volts.

In operation, the switch 208 is initially closed so that the capacitor 219 is fully charged. The output of the comparator 201 holds the transistor 214 ON. Current flowing through the zener diode 212 and the transistor 213 ensures that the transistor 210 is ON. The currents flowing through the transistor 210 and 214 discharge the capacitor 219 and the voltage of the busbar 203 falls so reducing the bias on the ap licator roll 7.

Developer current begins to flow through the resistor 200 and developes a voltage across the resistor 202. The desired developer current for mid-grey is known for the development system and corresponds to a voltage difference set on the potentiometer 204. The capacitor continues to discharge and the developer current continues to increase until the input voltages at the comparator 201 are equal, which means the desired developer current magnitude has been reached.

The output of the comparator then swings positive turning OFF the transistor 214 so that the transistor 210 rapidly turn OFF.

Thereafter, the bushbar 203 remains at the desired applicator bias voltage. In practice, the bushbar voltage will rise by 5.6 volts but' this is a fixed variation and in any event small compared to the bias voltage, which is usually around 350 volts in this embodiment.

Thus, the circuit described automatically calibrates the development system by adjusting the applicator bias voltage until a desired developer current flows to develop a mid-grey image. The development on the machine of each copy immediately following is carried out while maintaining the adjusted applicator bias voltage at the adjusted value.

The circuit described includes protection against the applicator shorting to ground (resistor 200, diode 220) and means for turning on transistors 210 and 214 before the start of the capacitor discharge (diode 206, resistor 217 and diode 216). Furthermore, the emitter base junctions of transistor 210 and 212 vary together with temperature such that current through zener diode 211 and 218 is constant.

The arrangement ensures that however badly, within reason, some of the copier machine systems are out of tolerance, ability to copy is within the capability of the developer system so that generally acceptable copies are always produced.

In the example described, the copier is calibrated during rescan and is ready for the first or next, as the case may be, copy to be made. It will be noted that by ensuring proper development of copies is taking place, which operation is the last in the copying cycle (apart from transfer), the copier calibration which is possible is the most effective in practice. Fine tuning of charging and/or exposure may not result in good copies if the developing is not within tolerance. Further, we believe that determination of variations in say charging and their correction prove particularly difficult. By concentrating on the developer ment system We believe we have provided an effective and practical solution allowing some latitude in other systems to take place without individual correction.

Further, the concentration on one system or parameter of the several in the machine simplifies the control of the overall system and, as has been explained, proves effective in maintaining copy quality within calibration or at least acceptable limits.

Any current flow between the developer housing (not shown) and the main frame of the copier (not shown) can adversely effect the performance of the developer system. In the circuit described a guard-ring (not shown) surrounding the applicator housing (not shown) can be connected by a high resistance resistor (shown dotted) to the busbar 203. This removes or at least substantially reduces any leakage current between the applicator 7 and the main frame or drive shafts of the copier which could affect the performance of the described circuit.

WHAT WE CLAIM IS: 1. A line-by-line scanning system for a document to be copied comprising a stationary platen, document illumination means supported on a scan carriage, optical projection means for projecting light from an illuminated document on to a moving photosensitive surface through an exposure slit, and a grey-shutter slidable mounted on the scan carriage arranged so as to be movable into the line of sight of the photosensitive surface at the exposure slit during scanning to enable development of a calibrating grey image.

2. A line-by-line scanning system according to claim 1 including buffer stops mounted adjacent each end of the platen and arranged to impact the grey-shutter and move the grey-shutter into and out of the line of sight of the photosensitive surface respectively.

3. A line-by-line scanning system according to claim 1 or 2, including a light intensity detector mounted to detect the intensity of light reflected from the illuminated document, in which said grey-shutter has a cut-out portion to allow

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   201 and the collector of the transistor 210 which collector is also connected to the busbar 203 through a resistor 221. The zener diodes 212 and -218 are each rated at 5.6 volts.

   In operation, the switch 208 is initially closed so that the capacitor 219 is fully charged. The output of the comparator 201 holds the transistor 214 ON. Current flowing through the zener diode 212 and the transistor 213 ensures that the transistor 210 is ON. The currents flowing through the transistor 210 and 214 discharge the capacitor 219 and the voltage of the busbar 203 falls so reducing the bias on the ap licator roll 7.

   Developer current begins to flow through the resistor 200 and developes a voltage across the resistor 202. The desired developer current for mid-grey is known for the development system and corresponds to a voltage difference set on the potentiometer 204. The capacitor continues to discharge and the developer current continues to increase until the input voltages at the comparator 201 are equal, which means the desired developer current magnitude has been reached.

   The output of the comparator then swings positive turning OFF the transistor 214 so that the transistor 210 rapidly turn OFF.

   Thereafter, the bushbar 203 remains at the desired applicator bias voltage. In practice, the bushbar voltage will rise by 5.6 volts but' this is a fixed variation and in any event small compared to the bias voltage, which is usually around 350 volts in this embodiment.

   Thus, the circuit described automatically calibrates the development system by adjusting the applicator bias voltage until a desired developer current flows to develop a mid-grey image. The development on the machine of each copy immediately following is carried out while maintaining the adjusted applicator bias voltage at the adjusted value.

   The circuit described includes protection against the applicator shorting to ground (resistor 200, diode 220) and means for turning on transistors 210 and 214 before the start of the capacitor discharge (diode 206, resistor 217 and diode 216). Furthermore, the emitter base junctions of transistor 210 and 212 vary together with temperature such that current through zener diode 211 and 218 is constant.

   The arrangement ensures that however badly, within reason, some of the copier machine systems are out of tolerance, ability to copy is within the capability of the developer system so that generally acceptable copies are always produced.

   In the example described, the copier is calibrated during rescan and is ready for the first or next, as the case may be, copy to be made. It will be noted that by ensuring proper development of copies is taking place, which operation is the last in the copying cycle (apart from transfer), the copier calibration which is possible is the most effective in practice. Fine tuning of charging and/or exposure may not result in good copies if the developing is not within tolerance. Further, we believe that determination of variations in say charging and their correction prove particularly difficult. By concentrating on the developer ment system We believe we have provided an effective and practical solution allowing some latitude in other systems to take place without individual correction.

   Further, the concentration on one system or parameter of the several in the machine simplifies the control of the overall system and, as has been explained, proves effective in maintaining copy quality within calibration or at least acceptable limits.

   Any current flow between the developer housing (not shown) and the main frame of the copier (not shown) can adversely effect the performance of the developer system. In the circuit described a guard-ring (not shown) surrounding the applicator housing (not shown) can be connected by a high resistance resistor (shown dotted) to the busbar 203. This removes or at least substantially reduces any leakage current between the applicator 7 and the main frame or drive shafts of the copier which could affect the performance of the described circuit.

   WHAT WE CLAIM IS: 1. A line-by-line scanning system for a document to be copied comprising a stationary platen, document illumination means supported on a scan carriage, optical projection means for projecting light from an illuminated document on to a moving photosensitive surface through an exposure slit, and a grey-shutter slidable mounted on the scan carriage arranged so as to be movable into the line of sight of the photosensitive surface at the exposure slit during scanning to enable development of a calibrating grey image.
2. 2. A line-by-line scanning system according to claim 1 including buffer stops mounted adjacent each end of the platen and arranged to impact the grey-shutter and move the grey-shutter into and out of the line of sight of the photosensitive surface respectively.
3. 3. A line-by-line scanning system according to claim 1 or 2, including a light intensity detector mounted to detect the intensity of light reflected from the illuminated document, in which said grey-shutter has a cut-out portion to allow

   light reflected from the document to be projected on to the detector whenever the grey-shutter is in the line of sight of the photosensitive surface.
4. 4. A line-by-line scanning system substantially as herein described with reference to Figs. 2 to 4 of the accompanying drawings.