EP0403523B1 - Electrophotographic printing device with regulated electrophotographic process - Google Patents

Abstract

An electrophotographic printing device has a closed-cycle regulator for acquiring and regulating the essential operational parameters of the electrophotographic process. It includes a first regulating step for stabilising the electrophotographic process on the photoconductor (12) by regulating the charging potential (18) and the discharge exposure (17), and by acquiring and monitoring the residual potential (SL). It further includes a second regulating step for ensuring and optimising the development of the charge image by regulating the toner supply to the development region (14) and the inking of the charge image; and a third regulating step for ensuring and optimising duplication by acquiring the specific values of the recording media and regulating the corona device (UK).

Description

The invention relates to an electrophotographic printing device according to the features of claim 1.

There is a significant difference between the acceptance of the copying result of electrophotographic copying machines and the printing result of printing devices working according to the principle of electrophotography by the operators: While in copying machines the copying result is measured on the master copy and the operator generally also accepts poorer copies, this is the case with electrophotographic printing devices are not the case.

Electrophotographic printing devices are generally used in connection with EDP systems and the possibility of influencing the print quality is low or the operator expects the printer to deliver an optimal printing result under all conditions. This results in different demands on the quality of the electrophotographic process between printers and copiers.

In order to achieve this high requirement with regard to print quality for printers, it is necessary to minimize the tolerances in the electrophotographic process.

The quality of the consumables, such as toner and developer, and the manufacturing quality of the photoconductor, respectively, also have a significant influence on the print quality. The printer manufacturer has less influence on the quality of these materials when operating the printing device.

In the case of copying machines, it is known to regulate the individual units involved in the electrophotographic process to predetermined standard values via control devices.

It is known from Patent Abstracts of Japan, Volume 10, No. 288 (P-502) (2344), Sept. 30, 1986 and JP-A-61 105 578, the charging device for a photoconductor drum for a certain period of time during and after to control the switch-on phase so that the fluctuations in the surface potential generated are compensated for by the switch-on process.

Furthermore, it is known from Patent Abstracts of Japan, Volume 7, No. 101 (P-194) (1246) April 28, 1983 JP-A-58 25 677 to determine the resistance value of the paper web with the aid of a multi-stage comparison device in front of the transfer printing station and to control the corona discharge of the transfer corona in the transfer printing station in stages.

A control method for an electrophotographic copier is described in Patent Abstracts of Japan, Volume 7, No. 184 (P-216) (1329) August 13, 1983 and JP-A-58 86 562. The toner density and the residual charge on the surface of a photoconductor are scanned using a toner image generated using a standard image. The values thus detected and calculated are compared with predetermined standard values and, depending on this, a developer circuit, an exposure circuit, a toner supply circuit and a developer process are controlled via a microcomputer circuit. Among other things, a reflection density measuring device and a surface charge sensor are used as sensors.

With the known arrangement, a standard template is imaged on the photoconductor and the developer station is regulated depending on the values of the standard template. This means that the norm template ensures average norm values of the electrophotographic process and, based on these norm values, different templates are copied in relation to these norm values.

This has the disadvantage that an adaptation to different templates is not possible. Bad templates are considered bad originals formed, regulation of the standard values even depending on the copying result is not provided.

From US-A-3 788 739 an electrophotographic device is also known, in which a section with maximum exposure intensity is exposed on a photoconductor drum in the writing area and then scanned with the aid of a charge detector. The measured potential is then compared with a predetermined value. The potential values during charging, exposure and the transfer pressure corona are adjusted via a control device depending on the measured potential.

In the case of electrophotographic printing and copying devices such as are described, for example, in JP-A-58115453 and JP-A-58221858, the operating parameters which are essential for image generation are determined, but each operating parameter is then compared with a predetermined fixed manipulated variable and from it determines the manipulated variable for the image generation process.

The aim of the invention is to provide an electrophotographic printing device which, regardless of quality fluctuations in the consumables and independent of changing operating conditions, delivers optimum print quality.

Another object of the invention is to design an electrophotographic printing device in such a way that the tolerances in the electrophotographic process can be significantly reduced in order to achieve maximum print quality. The entire process should run automatically if possible.

This object is achieved in an electrophotographic printing device according to the features of patent claim 1.

Advantageous embodiments of the invention are characterized in the dependent claims.

The process-controlled, multi-stage control arrangement for optimizing the electrophotographic process depending on the process results and the process flow of the individual process steps results in a guaranteed constant print quality even when the process itself changes. Closed internal control loops first stabilize the electrophotographic process itself and then the operating parameters the printing device, including the process parameters, is controlled for optimum print quality.

Changes in operating conditions and fluctuations in the consumables cannot have any effect. This increases the print quality and the entire printing device becomes operationally reliable.

Embodiments of the invention are shown in the drawings and are described in more detail below, for example.

• FIG 1 eine schematische Schnittdarstellung einer elektrofotografischen Druckeinrichtung für Einzelblätter mit Duplex- und Simplexdruck
• FIG 2 ein schematisches Blockschaltbild einer Ansteueranordnung für die Druckeinrichtung
• FIG 3 ein schematisches Blockschaltbild des in der Ansteueranordnung der FIG 2 verwendeten Hauptprozessors
• FIG 4 eine Prinzipdarstellung des Regelkreises zur Regelung des Aufladepotentials
• FIG 5 eine schematische Darstellung der Struktur der Regelanordnung zur programmgeführten Elektrofotografie
• FIG 6 eine schematische Darstellung einer Gesamtübersicht des Regelkonzeptes
• FIG 7 ein schematisches Blockschaltbild der Regelanordnung zur programmgeführten Elektrofotografie und
• FIG 8 eine schematische Darstellung der auf dem Fotoleiter erzeugten Testmarken und Testmuster.

Show it

• 1 shows a schematic sectional view of an electrophotographic printing device for single sheets with duplex and simplex printing
• 2 shows a schematic block diagram of a control arrangement for the printing device
• 3 shows a schematic block diagram of the main processor used in the control arrangement of FIG
• 4 shows a schematic diagram of the control circuit for regulating the charging potential
• 5 shows a schematic representation of the structure of the control arrangement for program-guided electrophotography
• 6 shows a schematic representation of an overall overview of the control concept
• 7 shows a schematic block diagram of the control arrangement for program-guided electrophotography and
• 8 shows a schematic representation of the test marks and test patterns generated on the photoconductor.

A single sheet page printer shown schematically in FIG. 1 and working according to the principle of electrophotography contains three paper storage containers V1, V2 and V3 with different capacities for holding single sheets. The paper storage containers V1, V2 and V3 are constructed in a conventional manner and are connected to a pressure channel DK via the paper feed channels 11 Printing device in connection. The pressure channel DK contains the actual printing station DS with a motor-driven photoconductor drum 12 around which the individual units of the electrophotographic printing station are arranged. An aggregate is a character generator 13 with a character-dependent controllable LED comb with individual controllable lighting elements, not shown here, which can be constructed, for example, in accordance with US Pat. No. 4,780,731 and whose light intensity can be regulated by varying the control voltage or the control current. A charging sensor SL is connected to the exposure station 13, which measures the surface potential on the photoconductor drum and emits a signal as a function thereof. The charge image generated on the photoconductor with the character generator 13 depending on the character is colored using a developer station 14. The developer station 14 contains a toner reservoir TV for receiving toner and a metering device D in the form of a metering roller. Depending on the toner consumption, the metering roller D supplies toner to the actual developer station. The toner is mixed by means of two mixing screws MS and the developer mixture of ferromagnetic carrier particles and toner particles is then fed to a developer roller E. The developer roller E acts as a so-called magnetic brush roller and consists of a hollow roller with magnetic strips arranged therein. The developer roller transports the developer mixture of ferromagnetic carrier particles and toner particles to the development gap ES between the photoconductor drum 12 and developer roller E. Excess developer is transported back to the developer station 14 via the developer roller E.

Immediately downstream of the developer station 14 is a toner mark scanner TA in the form of a reflection scanner. This scanning device TA is described later and is used when scanning a test routine or to scan test marks generated and colored automatically and regularly on the photoconductor and to evaluate these test patterns, for example with regard to color density and color saturation.

The inked charge image is then transferred to a recording medium, in this case to single sheets, in a transfer printing station 15. For this purpose, the transfer printing station 15 has a transfer printing corona device UK. The transfer printing corona device UK loosens the colored charge image on the photoconductor drum 12 so that it can be transferred to the recording medium (single sheet).

The single sheet is then transported via a suction table S to a fixing station with electrically heated fixing rollers FX, which are driven by an electric motor, and the toner image on the recording medium is thermally fixed.

A cleaning station 16 follows in the direction of rotation of the photoconductor drum 12. The cleaning device 16 is constructed in a conventional manner and contains e.g. a stripping element RE, which removes the excess toner or the carrier particles from the photoconductor drum 12. This cleaning process is supported by a corona device KR.

The surface of the photoconductor drum 12 is then discharged using an exposure device 17. This exposure device contains a light source which is homogeneous over its entire spatial length and whose intensity can be controlled in a targeted manner.

Thereafter, the surface of the photoconductor drum discharged by the discharge exposure is uniformly recharged in a charging device 18 with a charging corotron arranged therein.

To transport the single sheets through the print channel, the pressure channel DK contains paper transport elements in the form of a belt-shaped suction table S and paper transport rollers P.

Coupled to the pressure channel DK on the input and output sides is a paper transport element P in the form of a return channel RF containing motor-driven roller pairs. The return channel RF has a turning device W1, in which in the so-called duplex mode the front and back of the single sheets are described, the single sheets are turned before being fed back to the pressure channel DK.

Controlled by a paper switch, a paper transport channel system PK is connected to the pressure channel DK and feeds the single sheets, not shown here, printed in the simplex or duplex process.

To determine the position of the continuous single sheets and to control the paper transport elements P, all paper channels have paper scanning sensors LS (shown as black triangles), which consist of light barriers. For reasons of clarity, only a few light barriers are shown here.

The page printer shown schematically in FIG. 1 is controlled with the aid of a control arrangement as shown in FIGS. 2 and 3.

control

The controller for the page printer is basically divided into a controller part C and the actual device controller G. The controller C is principally constructed in accordance with US Pat. No. 4,593,407. It has the task of accepting the print data coming in from a computer H, preparing them page by page and controlling the character generator 13 of the printing station as a function of the characters to be displayed. The device control G in turn serves for the coordinated execution of all printer functions. It has a modular structure and consists of a main processor HP and various submodules SUB1 to SUB5, which ensure independent monitoring of the assigned printer units. Communication between the individual control parts takes place via a hardware / software interface that is uniform for all parts (network-like coupling, serial bus). Each submodule SUB1 to SUB5 has its own Processor equipped and can operate the associated unit of the printing device independently and is itself testable. This self-test capability means that independent test routines are carried out both when the device is switched on and when requested by the main processor HP. All control boards of the printer in the device control are registered with regard to their status in a non-volatile memory. The controller can access these values. In addition, the contents of the non-volatile memory can be printed out if necessary. There are also interfaces for additional devices.

2 and 3 show the basic structure of the device control in the form of a block diagram. 3 shows a block diagram of the structure of the main processor HP.

All submodules SUB1 to SUB5 and the main processor HP are connected to one another with a serial interface INT1, which is controlled via line drivers. The serial interface INT1 is controlled under the control of the main processor HP via a BIT bus. The interface protocol corresponds to the usual HDLC / SDLC description (fast data transfer). To relieve the load on the interface and to simplify the cable routing to the individual units, the units are controlled by the associated submodules SUB1 to SUB5 directly via power amplifiers (not shown here). The main processor HP periodically checks the function of the individual submodules SUB1 to SUB5. A monitoring circuit (hardware / watchdog) checks the process in the main processor. The sequence control is synchronized with the peripheral speed of the photoconductor drum 12 via the output signals of a rotary pulse generator DI. The output of this rotary pulse generator DI (FIG. 1) is connected to all submodules SUB1 to SUB5 and supplies a synchronization signal F at cyclical intervals.

3, the main processor has the following structure:
A central processing unit CPU is connected to three memories SP1 to SP3 and an input / output unit EA. The memory SP1 is a read-write memory, the memory SP2 is an electrically programmable read-only memory and the memory SP3 is a non-volatile data memory. The input / output unit EA detects, among other things, the synchronization pulse F.

In the non-volatile memory SP3, consumption metabolism, printed / fixed page, maintenance intervals, error statistics and deviations from guide values etc. entered by the operator are stored. The connection to controller C is established via a common interface INT2.

The main processor HP has the task of coordinating all messages, commands and measurement data from the outdoor stations SUB1 to SUB4, checking for plausibility and forwarding them. It also establishes the connection to controller C via the interface INT2 and the system bus BUS2. In doing so, bidirectional commands and messages are transferred. The correct program sequence in the device control is continuously monitored via the monitoring circuit U (watchdog noise).

As already explained, five submodules SUB1 to SUB5 independently monitor and control the aggregates assigned to them. Communication between the individual modules SUB1 to SUB5 and the main processor HP takes place via a hardware / software interface INT1 which is uniform for all parts. Each submodule has its own processor with an input buffer that transmits the data supplied via input I to the processor and power levels that drive the associated units via output O. The submodules can be tested themselves, ie test routines are carried out independently when the device is switched on or when requested by the main processor HP.

The submodule SUB1 monitors all sensors LS of the storage containers V1 to V3, the feed channels 11 and the pressure channel DK and in particular the pressure start signal of the sensor LS SYN. The submodule SUB1 controls all units in this area. It detects and reports paper run errors.

The submodule SUB2 detects all sensors LS in the paper output area, i.e. in the area of the dispensing container and in the dispensing channel AK. Paper run errors are detected and reported to the main processor HP.

The submodule SUB3 monitors the sensors LS in the paper channel system and in the feedback channel RF. It controls the paper flow in these channels and detects paper flow errors.

The SUB4 submodule controls a control panel AZ on the printer. The control panel AZ contains a keyboard and a display device, the display showing the paper flow in the printer or, in the event of a paper transport fault, the fault location.

The sub module SUB4 in connection with the control panels AZ represents the interface between the operator or maintenance technician and the printing device. All operator inputs and all information from the device are made via the control panel. This essentially consists of a display for displaying the information and a keyboard for entering various commands and parameters. In addition, it has some special controls and indicators.

The submodule SUB5 detects the sensors of the printing station DS and the fixing station FX. These sensors are, for example, the charge sensor SL for detecting the surface potential of the photoconductor 12, transport monitoring sensors in the developer station 14, temperature sensors and microswitches in the fixing station FX, the toner mark sensor TA between the developer station 14 and transfer printing station UK. The submodule SUB5 controls the units, the fixation lamps, Motors, fans, charging corotrons etc. The errors that occur are reported to the main processor HP.

The submodule SUB5 in connection with the main processor HP also contains the process-controlled control arrangement according to the invention for detecting and regulating the essential operating parameters of the electrophotographic process.

This control arrangement is a process-controlled control arrangement which is constructed in several stages and in principle consists of three blocks (control stages) CC1, CC2, CC3. In accordance with the control strategy on which the control is based, the entire electrophotographic process is first subdivided into a sequence of process steps which take place or mesh with one another, namely the photoconductor process, the development process and the transfer printing process. Now an attempt is made to regulate the individual process steps independently using individual control blocks, based on the result of the individual process step and the course of the process in the process step. The aim is to stabilize the individual process steps with regard to their operating parameters, in order to build up the next process step on the continuous stabilized process step.

When optimizing the entire electrophotographic process, the results of the individual steps are used as the starting point. However, this can only serve as the basis for a first approximation optimization, because the three control blocks CC1, CC2, CC3 in turn form their own control system, e.g. a change in the light intensity of the character generator 13 has a direct influence on the residual potential of the surface charge of the photoconductor 12, in turn leads to a change in contrast during the coloring in the developer station 14. If a change to be corrected is found in the process step "development", it may be necessary to regulate parameters whose changes have an effect on the process step "photoconductor".

In the first control stage CC1, the electrophotographic parameters are stabilized as a prerequisite for optimizing the development process. The electrophotographic parameters are understood in particular to be the parameters influencing the charge balance on the photoconductor. In order to be able to safely regulate this charge balance in the photoconductor, the first regulating stage contains a regulating circuit shown in FIG. 4 for regulating the charging potential on the photoconductor.

Test runs and experience in operation have shown that the tolerances of the charging of the photoconductor drum, in particular, can greatly reduce quality and give rise to faults. Influencing factors are, in particular, drum specimen scatter, temperature and air humidity, fatigue of the photoconductor, aging of the toner, influence of the cleaning station, device adjustment and corotron condition in the charging station 18. In order to become independent of these influencing variables, it is necessary to regulate the charging potential of the photoconductor. For this purpose, a charge sensor SL is located directly in front of the developer station, for example in the form of an electro-volt meter, with which the charging potential of the photoconductor drum can be continuously detected. The output signal of this measuring probe is queried at defined intervals using a conventional interrogation arrangement AF. The interrogation arrangement AF compares the measured values obtained with stored reference measured values and corrects the charging current at the charging corotron 18. After a time delay of approx. This cyclical detection enables an almost instantaneous correction of the charging current of the charging corotron 18. The regulation of the charging potential is very important for the print quality. Fluctuations in the charging potential have a direct impact on the print quality. The constant automatic detection and correction of the charging potential enables safe operation within the permissible bandwidth. With the invention Control arrangement, it is possible to reduce the tolerance of the charging potential that occurs by a factor of 5, for example from absolutely 400 V to approximately 80 V. The remaining 80 V potential tolerances are mainly due to the non-adjustable charge fluctuations on the circumference of the photoconductor drum. An achievable reduction in tolerance from 400 V to 80 V already leads to considerable quality stabilization and assurance. For example, it is possible to increase the pretension at the developer station for better coloring of large areas and at the same time to ensure sufficient security against background coloring.

In a further control circuit assigned to the first control stage, the light output of the discharge lamps 17 is controlled in the exposure station. The light output of the discharge lamps strongly depends on the lamp aging, the number of specimens and the temperature. In order to become independent of these tolerances, the light output is e.g. detected by a photo sensor PS arranged in the light channel of the discharge lamp 17 and corrected by raising or lowering the lamp current. In order to be able to better regulate the light output, a light source that is homogeneous over its entire length and whose intensity can be specifically controlled is used.

The contrast or residual potential of the photoconductor drum 12 has a further significant influence on the print quality when it is discharged from, for example, a regulated charging potential with a defined exposure. Despite the regulated charging potential, there are very clear deviations in the residual potential and the discharge capacity from the photoconductor spectra. Some of these tolerances correspond to deviations that can arise with uncontrolled charging. In addition to the scatter of specimens of the photoconductor drum, the overall tolerances of the residual or contrast potential also depend on fluctuations in the performance of the writing light and, under certain circumstances, also on the influences of the toner (developer mixture). This ensures a constant quality of the print result, especially of full surfaces or not always guaranteed when printing bar codes.

Too high a residual potential leads to insufficient large-scale coloring.

However, it is difficult to regulate the residual potential. In addition, regulation is not without risk for e.g. printing quality possible. The residual potential can, however, be detected with the aid of a monitoring device.

This monitoring device uses two sensors, namely the charging sensor SL, which is also used to measure the charging potential, and the toner mark sensor TA.

The charging sensor SL and the toner mark sensor TA are located in the area of the photoconductor 12 on a single movement track. In this way, a test mark that is preferably generated outside the actual writing area on the photoconductor first reaches the area of the charge sensor SL and then the area of the toner mark sensor TA.

The charging sensor SL has several functions:
It is initially used in the manner described to measure the charging potential, wherein it detects the unexposed areas after charging.

It also serves to measure the residual charge of the residual charge potential. This is done in that, as shown in FIG. 8, an elongated full-area mark 31 is produced outside the writing area 29 by exposure at the edge of the photoconductor drum. All the LEDs of the character generator required to generate the full-area mark are activated with a predetermined light output, this light output being dependent on the type and temperature of the photoconductor. If the full-area mark 31 is produced by exposure, but has not yet been colored, the charging sensor SL measures in the area of the full area the residual potential. The elongated full-area mark is necessary, among other things, because the charge sensor SL has a certain inherent inertia and, because of the rotational speed of the photoconductor drum, a reliable measurement is only possible after a certain time and thus after a certain pass through the full-area mark.

The optical scanner TA in the form of a reflection light barrier is located in the same movement track of the photoconductor 12, downstream of the developer station. The reflection light barrier is constructed in the usual way and consists of a light source and a photo transistor as a receiver. The output signal of the phototransistor depends on the degree of reflection of the toner mark applied to the photoconductor and now colored by the developer station and thus on the color saturation, i.e. the optical density of the mark applied and colored by the developer station. The wavelength of the reflection light barrier is chosen so that the scanning light has no influence on the function of the photoconductor drum. This is necessary because the light barrier is constantly activated and therefore also scans areas that have not been exposed.

To detect the residual potential, test routines for generating the described full-area marks are called up from time to time via test programs stored in the control arrangement. Then the residual potential is determined in the exposed and non-inked full-area mark via the charge sensor SL and this signal is compared with a limit value stored in the storage device and, depending on this comparison process, a warning signal is then triggered on the display device AZ when the residual potential is exceeded. The maintenance personnel can now stabilize the residual potential, for example, by changing the pretension at the developer station (BIAS voltage) or by other measures. However, this regulation can also be carried out automatically by the control arrangement.

However, it is also possible to influence the residual potential by changing the light intensity of the character generator 13 and thus to regulate the residual potential. For this purpose, the intensity of the writing light of the character generator 13 is changed depending on the comparison process. This is done by changing the control current or the control voltage of the LED.

If a character generator with a laser beam is used instead of a character generator with activatable single points (LED comb), it is necessary to change the intensity of the laser beam, this can e.g. also take place via filters or other measures.

The development device for securing and optimizing the development of the charge pattern is regulated with a second control stage CC2.

In order to regulate the conveyance of toner from the storage container TV via the metering device D to the developer station 14, a toner mark 30 is constantly generated on the photoconductor 12 outside the actual writing area via the character generator 13 in short time intervals, specifically with a defined exposure intensity and this toner mark 30 is colored via the developer station . The colored toner mark 30 is then scanned on the photoconductor 12 with the aid of the optical scanning device TA and, depending on the degree of inking of this brand, the regulation of the conveyance of the toner from the storage container TV via the metering device D to the developer station 14 takes place. Depletion of the developer supply in the developer station 14 is directly reflected in the color density of the toner marking. If the developer supply in the developer station is used up, the color density of the toner marking is changed significantly, this cannot be compensated for by additional funding. This state of consumption is recognized by the control arrangement and a warning signal is activated on the display device AZ.

In other larger time intervals by calling a test routine "large area coloring" e.g. a test pattern can be generated via the control panel, e.g. can consist of a bar extending over the entire width of the record carrier. This test pattern can also be scanned on the photoconductor using the optical scanning device TA. also several scanners can be arranged side by side. However, this can also be done using a single scanner, e.g. an elongated bar corresponding to the full-area mark 31 is used as the test pattern, which is arranged outside the actual writing zone, a continuous scanning being carried out as the test mark passes through. However, this scanning can also take place in sections at short intervals. A value for the large area coloring can be derived from this. If the degree of coloring of the test pattern is too low, the coloring of the background areas on the photoconductor drum and / or on the paper must first be checked. If this is too high, this indicates a device malfunction or a very aged developer mix. Appropriate activities to compensate for this can then be undertaken.

In the case of a correct degree of coloring of the background area, an improvement in the large-area coloring can be achieved again by correcting the developer roller bias or the operating point of the toner delivery control.

The background area of printed images can also be monitored via the scanning device TA. This background monitoring can be done continuously. If the background coloring exceeds a permissible level, the degree of coloring of the large area is first checked again. If this is within the permissible limits, it can be corrected as described for the measurement of the large area coloring.

Another way to check the print quality is to capture raster reproduction.

Because of the different discharge characteristics of the photosensitive recording material in the fine range, a defined raster reproduction can be impaired. For example, a very easily unloadable photoconductor layer changes a raster to higher or darker values, while a somewhat poorly unloadable photoconductor layer hinders raster printing. Since the human eye is very sensitive on this point and therefore high demands have to be made in this regard, it is necessary to correct this tolerance.

The image-based representation with electrophotographic printers takes place in the dot pattern in different gray values, the gray value representation being carried out by corresponding configuration of the individual dots having the same size.

In order to be able to check this gray value display, it is possible to generate a raster mark at certain time intervals by calling a test routine via the control arrangement. As shown in FIG. 8, the raster mark consists of a raster area which has a 50% optical density (black area), i.e. 50% black, 50% white. However, this can vary in a range from 25 to 75% area coverage. The raster mark is generated via the character generator 13 and colored via the developer station 14. It is then scanned in the manner described using the optical scanner TA.

The sampled value is compared with a stored target value and the light intensity of the character generator 13 is changed in accordance with the deviation, for example by increasing or decreasing the LED voltage. The stored setpoint can, however, also be changed as a function of various machine parameters, in order to achieve an adjustment, for example, depending on the recording medium material used, the photoconductor drum used or the type of recording medium itself. For this purpose, the corresponding correction values or characteristic data can be entered via the display device AZ, or corresponding sensors independently detect these values.

The transfer station is controlled in principle with a third control stage CC3 for securing and optimizing the transfer printing.

It has been found that the setting of an optimal transfer corotron current in the corona device UK of the transfer station 15 is strongly dependent on the paper weight class used and on the paper width, and also on the corotron contamination itself. In order to be able to set the transfer corona device optimally, the control panel AZ is used to Entered the paper width and the paper thickness into his keyboard-like input device and set the assigned optimal transfer corotron current determined from empirical values via the device software. This can also be done automatically with a detection device, not shown here, which e.g. when the individual sheets leave the feed channels 11, the thickness and size of the paper are detected by an optoelectronic scanning device.

The three control levels capture and stabilize all parameters that are important for print quality. This makes it possible to place the working points of the various parameters in optimal areas without considering the worst case conditions and thus to ensure the maximum achievable quality at all times.

Furthermore, the data recorded and determined in the course of the control processes can be used for testing and service purposes.

The structure of this control process, referred to as program-guided electrophotography, is listed in FIG. 5. An overall overview of the control concept can be seen in FIG. 6. The control loops shown in FIG. 6 are largely self-contained in order to rule out a clear and undefined control behavior. The individual control loops are influenced depending on the results of the individual process steps, for example changing a parameter.

In summary, essential functions of the microprocessor-controlled control arrangement are the following:

Regulation of the charging potential of the photoconductor drum

In addition to a clear reduction in tolerance, the information about whether the conditions in the electrophotographic printing process are still regular is available via the setting value of the charging corotron current determined in the microprocessor for diagnostic purposes.

A strong reduction or increase in the charging capacity of the photoconductor drum, caused by external influences such as temperature, toner, etc., can be recognized, evaluated and corrected.

Furthermore, various test programs can run routinely or on command for diagnostic and remote diagnosis purposes, gray fog test, background test.

Detection of the residual potential (discharge potential) or regulation of the residual potential e.g. about the light output of the character generator.

The information about the residual potential of the photoconductor drum provides valuable information about the current state of the electrophotographic printing unit. The residual potential can be regulated within limits via the light output of the character generator.

For example, the value of the residual potential can provide information as to whether the printing of sophisticated programs (barcode) or raster printing with high quality is possible. The light output of the character generator can also be regulated by scanning the raster marks. Is e.g. the grid mark is too dark, the light output is reduced and the mark becomes brighter.

Furthermore, deterioration of the discharge capacity caused by toner can be recognized and monitored.

Regulation of the coloring ability

In view of the relatively large fluctuations in the coloring of large areas, the information about the degree of coloring can be used to different parameters such as adjust the bias of the developer station within certain limits.

Claims (8)

1. Electrophotographic printer in which during an electrophotographic process charge images are generated on a photoconductor (12) by means of a character generator (13) in a sequence of process steps which occur successively or overlap one another, developed in a developer station (14) and transferred onto a recording medium in a print transfer station (15), having

   - a process-controlled control arrangement (SUB5) for optimising the various operating parameters of the electrophotographic process by stabilising the individual process steps with respect to their operating parameters, the next process step building on a completed stabilised process step;

   - control stages (CC1, CC2, CC3) which are assigned to the individual process steps and arranged one behind the other for independent control of the individual process steps based on the operating parameters of the individual process step and the preceding process steps;

   - sensors (SL, TA, PS) for detecting the operating parameters of the individual process steps and input means (AZ) for specific characteristic variables of the electrophotographic process; and having

   - means for generating test marks and/or test patterns of process-relevant structures on the photoconductor (12) outside the actual printing region by means of the character generator (13) as a function of the operating state of the printer, the charge state of said structures being detected after exposure and their inking density being detected after development on the photoconductor (12) by means of sensors (SL, TA).
2. Electrophotographic printer according to Claim 1, having a first control stage (CC1) for stabilising the electrophotographic process on the photoconductor (12) by controlling and/or monitoring the operating parameters of the photoconductor (12) such as charging potential (18), discharge exposure (17) and residual potential (SL), having a second control stage (CC2) for safeguarding and optimising the development of the charge image by controlling and/or monitoring the operating parameters of the developer station (14) such as toner supply to the development region (ES), inking of the charge image, cleaning of the photoconductor (12) and light intensity of the character generator (13) and having a third control stage (CC3) for safeguarding and optimising the print transfer by controlling and/or monitoring the operating parameters of the print transfer station (15) by detecting the specific recording medium variables and adaptation of the corona device (UK).
3. Electrophotographic printer according to Claim 1, having a charge sensor (SL), arranged between character generator (13) and developer station (14), and having an optical sensor (TA) arranged downstream of the developer station (14) in the direction of movement of the photoconductor (12), charge sensor (SL) and optical sensors (TA) being arranged one behind the other in a motion track of the photoconductor (12).
4. Electrophotographic printer according to Claim 3, the optical sensor (TA) being constructed as a reflection light barrier whose scan light has such a wavelength that the scan light does not photoelectrically influence the photoconductor (12).
5. Electrophotographic printer according to Claim 1, a toner test mark (30) being generated at regular time intervals, the inking density of which mark being sensed by an optical sensor (TA) and being communicated to the control arrangement which controls the supply of toner to the developing region (ES) as a function of the inking density and/or actuates a warning device (AZ).
6. Electrophotographic printer according to Claim 1, a solid-area test mark (31) being first generated by exposure, after call-up of a test routine via the control arrangement, with an exposure intensity that, on the one hand, permits the residual charge potential to be identified by means of a charge sensor (SL) and, on the other hand, then permits the inking density to be sensed by means of an optical sensor (TA) after inking of the solid-area mark (31) as required.
7. Electrophotographic printer according to Claim 1, screen marks (32) with a defined optical density being generated after call-up of a test routine by means of the control arrangement and being sensed by an optical sensor (TA), and the control arrangement preferably setting, in addition to other control parameters, the light power of the character generator as a function of the output signal of the optical sensor (TA).
8. Electrophotographic printer according to one of Claims 1 to 7, the character generator (13) being constructed as a character generator (13) whose light intensity is controllable.

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