EP0406236B1 - Device for monitoring the transport of record carriers in sheet form in an electrographic printer - Google Patents

Abstract

The transport of record carriers in sheet form can be monitored simply, reliably and accurately, in particular in the region of the transfer station of an electrographic printer, thereby ensuring transfer of the toner image to the record carrier in sheet form with faithful reproduction of the format. The reloading and transfer process can be temporally synchronized with the output signal (VER) of an optical sensor (7) for detecting the front edge of a record carrier (2), (a) from the transfer station (6). Side delivery of a record carrier is detected by an optical measuring device (9 to 13) which moves outward in steps from a zero position transversally to the transport path (1) and detects the side edge of the record carrier. The number of steps in an index of the side delivery of the record carrier, the magnitude of which is used to adjust the toner image horizontally at the beginning of a line at a given distance from the side edge. The monitoring device is particularly applicable for single-sheet operation in electrographic printers.

Description

The invention relates to a device for monitoring the position of sheet-shaped recording media with respect to a transport path in an electrophotographic printer according to the preamble of the main claim. Such a device is known from Patent Abstracts of Japan, Vol. 6, No. 88 (P-118) [966] dated May 26, 1982.

It is generally known for printing devices that the feeding in particular also of sheet-like recording media to a printing station is of essential importance for the appearance of the finished printed product. In the case of an endless recording medium which is pulled off a roll, it is only necessary to ensure that the recording medium runs through the printing device with a voltage which does not exceed a critical limit. If this is achieved, straight running and also the position of the side edge of the record carrier with respect to a normal position can be set up. For this purpose, lateral guides are often provided on the transport path, so that the horizontal position of the recording medium can be adjusted with respect to the printing station.

This problem is even more critical in the case of sheet-like recording media which are moved individually along the transport path in as close succession as possible. The sheet-shaped recording media are moved forward by transport rollers which may act on the recording medium unevenly or with a force component displaced by a certain angle from the transport direction. The result is that the record carrier is transported along the transport path with a certain skew. It is therefore unavoidable, especially in front of the printing station, to provide devices that support the sheet-like Align the record carrier as precisely as possible parallel to the transport path in a standard position. Nevertheless, with reasonable design effort, this is only possible within certain tolerances.

In conventional electrophotographic printers, this has hitherto been accepted, even if attempts are made to fit the toner image to be printed onto the sheet-like recording medium as precisely as possible into the format of the recording medium. In particular, a certain tolerance range for the lateral storage of the sheet-shaped recording medium from the desired standard position parallel to the direction of transport is considered inevitable. In modern electrophotographic printers, however, a high quality of the printed image is increasingly being sought and also achieved. That is why more and more fields of application are opened up for electrophotographic printers, in which format compliance, ie a precisely defined vertical distance from the front edge or a horizontal distance from the side edge of the recording medium is required for the printed image. This applies in particular to so-called form printing, for example.

With regard to this problem, a recording device with a device for monitoring the position of sheet-like recording media is now in detail from the document Document Abstracts of Japan, Volume 6, No. 88 (P-118) (966) from May 26, 1982, cited at the beginning known with respect to a transport path, an essential part of this monitoring device is an optical sensor for detecting the front edge of a supplied recording medium. This sensor is probably designed in such a way that it can also determine a certain lateral placement of the conveyed recording medium from a reference position. Delay times can thus be derived from the information content of the output signal, with which a control signal for triggering the start of printing is delayed compared to the time at which the sensor output signal occurs. The known monitoring device is impressively simple, but should therefore only fulfill the monitoring function to a certain extent, which does not yet meet high demands on the print quality.

Furthermore, EP-A3-0 144 824 discloses a method and a device for maintaining a predetermined edge distance of the print on recording media in an electrophotographic printer, in which the start for the generation of a latent charge image on a charge storage drum as a function of the lateral placement of one to be printed recording medium is controlled with respect to a reference position in the transport path. For this purpose, the side edge of the conveyed recording medium is scanned before it enters the transfer printing station, and corresponding scanning signals are converted in a printer controller into a shift factor which defines the beginning of the line of the charge image on the charge storage drum. This known monitoring device only relates to the determination of the lateral storage of a funded recording medium and says nothing about it from how the beginning of the line of the printed image is determined at a distance from the front edge of the recording medium.

Furthermore, from DE-A1-36 22 972 and Patent Abstracts of Japan, Volume 11, No. 226 (P-598) (2673) from July 23, 1987 an electrostatically operating copying machine is known in which an optical scanning unit for determining the presence of a funded record carrier and for determining the size of its lateral offset perpendicular to the direction of transport is provided. Furthermore, a comparator arrangement can optionally be provided, which is used to determine the actual transport speed of the conveyed recording medium. If this deviates from a specified transport speed, then the optical scanning of the original document to be copied is adjusted accordingly. To determine the actual transport speed, the rotating movement is monitored by alignment rollers which are operated by a pulse motor. The number of motor pulses from the start of the rotation of the pair of rollers to the reaching of the optical scanning unit with the front edge of the conveyed recording medium is counted in order to measure an actual feeding interval. This measured value can be compared with a predetermined interval.

With the mechanical measures known hitherto, the sheet-shaped recording medium can be aligned substantially parallel to the direction of transport, in particular lateral trays, as explained, can only be further reduced with mechanical guides only with very complex constructions.

The invention is therefore based on the object of providing a device of the type mentioned at the outset which allows the transport of the sheet-shaped recording medium, in particular in the area of the transfer printing station of the electrophotographic printer, to be monitored with simple means and reliably in order to monitor the toner image on the sheet-shaped To transfer record carriers as true to format as possible.

This object is achieved in a device of the type mentioned in the invention with the features described in the characterizing part of the main claim.

The solution according to the invention is based on the fact that a certain alignment of the sheet-shaped recording medium with respect to the transport path is possible with the mechanical measures hitherto customary, that the tolerances still remaining, particularly in electrophotographic printers with high printing performance, are very difficult to narrow further using known measures . The solution according to the invention therefore also assumes that a coarse alignment of the sheet-shaped recording medium parallel to the transport direction is entirely possible with relatively simple means, even if this is still not sufficient for the intended purpose. For this reason, additional measures are provided which, in spite of a not exactly exact alignment of the recording medium, enable the toner image to be re-printed onto the recording medium with a closely tolerated vertical and horizontal distance of the printed image from the front edge or a side reference edge of the sheet-shaped recording medium.

This is achieved in that the front edge of the recording medium transported in the direction of the transfer printing station of the electrophotographic printer is detected with a light barrier located in the transport path when passing through a position which is precisely aligned with the geometry of the charge storage drum and the arrangement of a character generator for generating the latent charge image is. For a given transport speed, the feed of the sheet-like recording medium can thus be precisely synchronized with the rotation of the charge storage drum, so that the front edge of the recording medium is already at a defined vertical distance from the transfer printing area when the first line of the toner image enters the transfer printing area.

In addition, at a certain distance dependent on the process speed, an optical measuring device is provided in front of this light barrier that detects the leading edge, which measures the position of the lateral edge of an incoming sheet-shaped recording medium exactly and forwards this measured value to a printer controller. Since known electrophotographic printers also have a relatively complex printer control, which in particular prepares the print information in such a way that it is available point-to-point synchronized in the character generator of the electrophotographic printer, it does not pose any particular difficulty to adapt the printer control so that it adjusts the lateral orientation of the printer corrected pressure information provided in the character generator using this measured value.

The invention therefore does not attempt, for example, to further improve the alignment of the sheet-like recording medium with respect to the direction of transport, which would be difficult to carry out for reasons of complexity. Instead, the toner image to be re-printed is shifted along the surface lines of the charge storage drum in a tolerance range of a few millimeters in such a way that the lateral deposition of the recording medium caused by mechanical tolerances is corrected from a standard position. Conversely, in the solution according to the invention, the charge image to be re-printed is "aligned" in order to correct side deposits of the record carrier.

As can be seen from further developments of the invention described in the subclaims, this can be achieved with simple means by means of a further light barrier which can be displaced transversely to the transport direction and whose linear displacement path is controlled by a stepper motor. In this lick barrier, the transmitter and receiver are arranged opposite one another at the ends of a fork which can be moved transversely to the transport direction and which encompasses the side edge to be detected. The direction of displacement of this light barrier is controlled so that it is moved from a zero position towards the side edge to be detected, whereby the incremental steps required to detect the side edge are counted. This number of steps is thus a measure of the lateral storage of the sheet-shaped recording medium which is just arriving. This value is processed in binary coding in the printer controller in order to provide the print information in the character generator for the corresponding print page in an exactly horizontally aligned manner on the recording medium. Other developments of the invention are described in the remaining subclaims, their function and properties are explained in more detail in the context of the description of an exemplary embodiment.

• FIG 1 in einer Seitenansicht eine schematische Darstellung der Transportbahn eines elektrofotografischen Druckers im Bereich der Umdruckstation, in der mehrere Lichtschranken vor der Umdruckstation angeordnet sind,
• FIG 2 in einer Ansicht von oben eine schematische Darstellung der Position dieser Lichtschranken in bezug auf die Umdruckstation und die Transportbahn für die blattförmigen Aufzeichnungsträger,
• FIG 3 eine Prinzipdarstellung für den mechanischen Aufbau einer dieser Lichtschranken, die quer zur Transportrichtung der blattförmigen Aufzeichnungsträger zur Detektion einer Seitenbezugskante des blattförmigen Aufzeichnungsträgers mit Hilfe eines Schrittmotors verschiebbar ausgebildet ist und
• FIG 4 ein elektrisches Schaltbild für die Ansteuerung dieses Schrittmotors sowie die Ermittlung der Schrittanzahl bis zur Detektion der Seitenkante als Maß für die seitliche Ablage des abgetasteten Aufzeichnungsträgers, das als Korrekturwert einer Steuerung des elektrofotografischen Druckers zugeführt wird.

The embodiment of the invention is described below with reference to the drawing. It shows:

• 1 shows a schematic side view of the transport path of an electrophotographic printer in the area of the transfer printing station, in which a plurality of light barriers are arranged in front of the transfer printing station,
• 2 shows a view from above of a schematic representation of the position of these light barriers in relation to the transfer printing station and the transport path for the sheet-shaped recording media,
• 3 shows a schematic diagram for the mechanical structure of one of these light barriers, which is designed to be displaceable transversely to the transport direction of the sheet-shaped recording medium for detecting a side reference edge of the sheet-shaped recording medium with the aid of a stepping motor, and
• 4 shows an electrical circuit diagram for the control of this stepping motor and the determination of the number of steps up to the detection of the side edge as a measure for the lateral storage of the scanned recording medium, which as Correction value is supplied to a controller of the electrophotographic printer.

1 shows schematically at 1 a transport path for sheet-shaped recording media 2 of an electrophotographic printer in the vicinity of a charge storage drum 3. The charge storage drum 3 rotates clockwise, as indicated by an arrow 4, past a character generator 5, which transmits the pressure information line by line to the charge storage drum 3 in the form of point-controlled radiation energy. During the further circulation, the charge storage drum 3 passes through a transfer printing station 6, in which the charge image generated by the character generator 5 is transferred line by line to the recording medium 2 in the form of appropriately configured toner particles. The electrophotographic principle is generally known, so that it does not have to be elaborated on here.

A number of light barriers 7 to 9 are arranged in front of the charge storage drum 3 in the course of the transport path 1, but the illustration in FIG. 1 is not to scale and only shows the principle. The transport speed of the recording media 2 is necessarily identical to the peripheral speed of the charge storage drum 3. Under this condition, the first of these light barriers, which is referred to as vertical light barrier 7, lies at a distance a from the transfer printing station. This distance a corresponds, as indicated in FIG. 1, to the handling of the circumference of the charge storage drum 3 between the transfer station 6 and the character generator 5. The vertical light barrier thus has the task of determining the front edge of the record carrier 2 transported along the transport path 1, so that the start of printing in the transfer station can be controlled so that the first print line on the recording medium 2 lies exactly with a predetermined vertical distance from its front edge.

In front of the first light barrier 7 - viewed in the direction of transport - lies at a distance b the second light barrier, which is therefore referred to as horizontal light barrier 8, because although this light barrier also detects the passage of the front edge of a recording medium 2, it triggers a process that causes the horizontal The position of the recording medium 2 with respect to the transport path 1 is determined. This process is carried out with the aid of the third light barrier, which is designated as an edge light barrier 9 and detects the position of a side edge of the recording medium 2 with respect to the transport path 1.

In FIG. 2, another view for the arrangement described with reference to FIG. 1 is shown schematically as a view from above. In the transport path 1, the location of the transfer printing station is indicated by (6). The vertical light barrier 7 lies at a distance a therefrom, which - as described - is defined by the geometrical arrangement of the character generator 5 in relation to the transfer printing station 6. This light barrier, like the horizontal light barrier 8 adjacent at a distance b, lies in the middle of the transport path 1, in which an incoming recording medium 2 is shown schematically. The edge light barrier 9, which is assigned to a side edge of the transport path 1, lies at a distance c from the horizontal light barrier 8. For this edge light barrier 9, it is indicated schematically by a double arrow 10 that it is arranged to be able to be displaced transversely to the transport path in order to detect the side edge of the recording medium 2.

In Figure 3, the mechanical arrangement for moving the edge light barrier 9 is shown schematically. The edge light barrier 9 with its transmitting diode 90 and its receiving transistor 91 is arranged vertically to the transport path 1 below or above this transport path. In order to obtain a good signal-to-noise ratio, a slit diaphragm 92 is assigned to the light entry side of the receiving transistor 91. This edge light barrier 9 is with its transmitting or receiving part fixed in a fork 11, which includes the transport path 1 on both sides. The fork 11 is mechanically coupled to a stepper motor 12, which is designed such that it converts the incremental rotary movement into a linear movement with an internal gear so that the edge light barrier 9 can be displaced bidirectionally in the direction of the double arrow 10 transversely to the transport path 1.

Another light barrier, a zero position detector 13, is provided for monitoring the linear movement of the edge light barrier 9. Its position in relation to the transport path 1 results from the basic function of the edge light barrier 9. To determine the current position of the side edge of the recording medium 2, the edge light barrier 9, controlled by the stepping motor 12, is first moved into the transport path 1 until it is one predetermined end position reached. This end position is selected such that the edge light barrier 9 then lies within the side edge 200 of the recording medium 2 to be detected in any case, even if the recording medium 2 is still permitted to be deposited laterally. This end position defined in this way is referred to as the zero position. Accordingly, a zero position detector 13 is arranged transversely to the transport path 1 in such a way that in this zero position it detects the edge of a flag 110 of the fork 11.

In order to determine the actual position of the side edge 200 of the recording medium 2 to be detected, the edge light barrier 9 is moved out of the transport path 1 incrementally. The edge light barrier 9 changes the signal state at its output at the moment the side edge overflows. The number of steps of the stepping motor 12, which are necessary from the zero position until the position of the side edge 200 is recognized, is thus a measure of the actual position of the side edge 200 with respect to the zero position. As will be explained, this measured value is fed to a controller 23 of the printer and processed there. in order to then supply the character generator 5 with printing information with a correspondingly adapted value for the horizontal start of the printing lines.

The step-by-step linear movement of the edge light barrier 9 or of the stepping motor 12 is expediently adapted to the raster width of the character generator 5. This will be explained using an example. Is the resolution of the character generator 5 z. B. 240 dpi (dots per inch), the raster width is the reciprocal of this number size. The linear displacement path of the stepping motor 12 is expediently chosen to be just as large per step and accordingly, expressed in millimeters, is 0.158 mm. Under the assumed conditions, this value is essentially the absolute error of this mechanical optical measuring device. If one additionally assumes that parallel displacements of the recording medium 2, which can occur along the transport path, amount to approximately ± 3 mm, this would result in a maximum linear displacement path of the edge light barrier 9 or of the stepping motor 12 of approximately 6 mm or approximately 60 incremental Steps. For reasons of digital processing of the measured variable, a linear displacement path of the stepping motor is assumed with a total of 63 steps, so that lateral displacements of the side edges 200 of the recording media of approximately 3.3 mm, viewed in absolute terms, can be compensated for.

After this explanation of the basic function of the edge light barrier and its geometric configuration, it can also be illustrated how large the distance b of the horizontal light barrier 8 from the vertical light barrier 7 is to be expediently selected. If, as presupposed, the horizontal light barrier 8 determines the start of the measuring process explained above, then its distance from the vertical light barrier 7 must be at least so large that the measuring process described and the subsequent processing of the measured value into control signals for the character generator 5 can be carried out in the period of time that the front edge of the recording medium 2 needs to cover the distance b.

This distance b depends first of all on the transport speed v of the recording medium 2. This transport speed may be 0.2 m / s, for example. In addition, a minimum time period t _min must be set, which is required to transmit a determined measured value to the printer controller, to have it processed there and to provide the adapted print information in the character generator 5. For this period t _min , for example, 50 ms are set. Finally, the third influencing variable is the maximum time period for the course of the measuring process itself. It is influenced by the start / stop operating frequency of the stepping motor 12 and the number of maximum steps required in the direction of displacement 10. With the above assumptions, the distance b is then, for example, 35 mm and more. The distance c of the horizontal light barrier 8 from the edge light barrier 9 is less determined by such functional boundary conditions and can be chosen relatively freely in terms of construction, for example 10 mm.

The function described in principle above can be implemented in different ways. Such a possibility is shown in the block diagram of FIG. 4. This shows schematically the light barriers 7, 8, 9 and 13 described above, the signal outputs VER, HOR, EDGE and POS0̸ are fed in parallel to a programmable controller 14. This essentially generates the control signals for controlling the stepper motor 12. Stepper motor controls as such are known from a large number of applications. They are used so frequently that specific integrated circuits are also customary on the market. Two such circuits, a drive circuit 15 and a power driver circuit 16, in Commercially available as circuits L297 or L298 from SGS, are shown in FIG. 4 in a typical, conventionally customary configuration.

The control circuit 15 receives control signals generated by the programmable controller 14, an activation signal MOEN, a clock signal MOCLK and a direction signal MODIR. The enable signal MOEN activates the control of the stepper motor 12. The clock signal MOCLK triggers an incremental step of the stepper motor 12 with its rising edges. With its two signal states, the direction signal MODIR defines one of the possible directions of rotation of the stepping motor 12. The two integrated circuits 15 and 16 are connected to one another in accordance with the known regulations of the circuit manufacturer, so that a detailed explanation does not appear to be necessary here. At the outputs of the power driver circuit 16 there is a bridge circuit, made up of eight diodes D1 to D8, via which windings 121 and 122 of the stepping motor 12 are selectively connected to the operating voltage (24 V) in order to trigger the stepping movements.

In the present exemplary embodiment, it is now assumed that the zero position of the edge light barrier 9 represents the initial state. Therefore, the corresponding output signal POS0̸ of the zero position detector 13 should set the measuring device in a defined initial state. This is illustrated in the block diagram of FIG. 4 so that in the programmable controller 14 a reset signal RES for the connected circuit is derived, for example, from the output signal POS0̸ of the zero position detector 13 supplied to it.

If the front edge of a recording medium 2 then runs over the horizontal light barrier 8, the measuring process is started, ie the edge light barrier 9, controlled by the stepping motor 12, is moved out of the transport path 1.

In order to determine the offset of the side edge 200 of the recording medium 2 from the zero position, the incremental stepping movements of the stepping motor 12 must be counted. A binary counter 17 can be used for this purpose, which is clock-controlled by the clock signal MOCLK. Now, however, only the steps of the stepping motor 12 are to be counted, which the edge light barrier 9 executes from the zero position up to the side edge 220 of the recording medium 2. Therefore, the binary counter is reset by the reset signal RES as soon as the zero position of the edge light barrier 9 is reached.

The binary counter 17 is activated as soon as the measuring process is triggered by the change in the state of the output signal HOR of the horizontal light barrier 8. This is illustrated in FIG. 4 in such a way that this output signal HOR is fed to the set input of an RS flip-flop 19 via a first inverter 18. This is set as soon as the front edge of a record carrier 2 is detected by the horizontal light barrier 8. The output signal of the RS flip-flop 19 is fed to an AND gate 20, which also receives the enable signal MOEN at a further input. A signal derived from the output signal EDGE of the edge light barrier 9 is fed to a third input of the AND gate 20 via a further inverter 21. Thus, the AND gate switches the enable signal MOEN for the stepper motor 12 on the condition that the horizontal light barrier 8 has triggered the start of the measurement process and that, on the other hand, the edge light barrier 9 has not yet detected the side edge 200 of the record carrier.

The output signal of the AND gate 20 forms the enable signal EN for the binary counter 17. As long as this enable signal is applied to the binary counter 17, the positive edges of the clock signals MOCLK are added up in the binary counter. In the final state, the counter reading corresponds to the binary counter 17 the number of steps which the edge light barrier 9 has covered from the zero position until the side edge 200 of the recording medium 2 has been reached. This value is transferred to an intermediate register 22 so that it can be transferred to the printer controller 23 regardless of the reset of the binary counter 17. In the case of electrophotographic printers, the printer control 23 supplies, as is known, bit by bit the print information for the character generator 5, which is converted there into individual optical signals corresponding to a raster point of a microprint line.

The exemplary embodiment described illustrates that it is entirely possible, using measures known per se and using conventional control principles in electrophotographic printers, to determine vertical and horizontal starting points for the start of printing on a recording medium with great accuracy even when the recording medium is transported along the Transport path results in vertical or horizontal tolerances.

The exemplary embodiment described here represents only one of several possible forms for realizing a reference scale for the printed image, which is determined again and again individually for the recording medium. For the principle on which the invention is based, it is, for example, of only minor importance in what way the measuring process in individual expires. In the exemplary embodiment described above, it has been assumed that the zero position is definitely always within the record carrier, so that the edge to be detected is always sought in a specific direction. The zero position could, however, also be chosen to be identical to the nominal position of the side edge of the recording medium. The direction of displacement during the measuring process would then depend on the output signal of the edge light barrier at the beginning of the measuring process.

Furthermore, it does not necessarily have to be assumed that the edge light barrier is already in the zero position when the front edge of the record carrier is detected by the horizontal light barrier. With a correspondingly adapted geometric arrangement of the vertical light barrier in relation to the horizontal light barrier, it would also be possible to return the edge light barrier to its zero position only at the beginning of the actual measuring process. In the exemplary embodiment described, however, it was assumed that there is sufficient time for the edge light barrier to be returned to its zero position while an already detected recording medium passes by, in order to return the edge light barrier to its initial position after the detection of the side edge.

Finally, it should be pointed out that even when using conventional control circuits for the stepper motor, it is not absolutely necessary to record the measured value in the manner described. Known principles of programmable controls offer the person skilled in the art further possibilities for implementation, which are not excluded by the exemplary embodiment described.

Claims (5)

1. Device for monitoring the position of recording carriers (2) in sheet form with respect to a transport path (1) in an electrophotographic printer having a rotating charge-storage drum (3), a transfer-printing station (6), assigned to the latter, as well as optical scanning means (7 and 9) arranged upstream in the transport path of the transfer-printing station (6) for detecting the leading edge as well as the position of the side edge of an in-running recording carrier, by the output signal of which means the charge-reversing and transfer-printing operation at the charge-storage drum can be synchronised with the position of the side edge and the transport of the recording carrier, characterised in that the optical scanning means include a first optical sensor (7), the distance (a) of which from the transfer-printing station (6) on the circumferential surface of the charge-storage drum corresponds to the distance between the transfer-printing station and the location of the character generator (5), so that by the output signal (VER) of the said sensor the function of the character generator can be synchronised with the transport of the recording carrier, in that the optical scanning means further comprise an optical measuring device (9 to 13), which is arranged upstream, seen in the direction of transport, of the first optical sensor (7), is designed to be displaceable linearly from a defined starting position for detecting a side edge (200) of the recording carrier transversely to the transport path and generates a measured value, corresponding to the displacement travel up to detection of the side edge, for horizontal setting of the toner image to the beginning of a line at a predetermined horizontal distance from the detected side edge, and in that between the first optical sensor (7) and the optical measuring device (9 to 13) there is arranged in the transport path (1) a further optical sensor (8), the output signal (HOR) of which controls the timing of the measuring operation in the measuring device and the distance of which from the first optical sensor (7) is chosen in such a way that the measured-value acquisition and processing is concluded at a predetermined process rate before the leading edge of the monitored recording carrier (2) reaches the first optical sensor (7).
2. Device according to Claim 1, characterised in that the optical measuring device (9 - 13) has a fork (11) of an edge light barrier (9) as well as a stepping motor (12), which are mechanically coupled to each other for the linear displacing of the light barrier in incremental steps and in that in addition, apart from a motor control device (15, 16, D1 to D8), a measured-value acquisition device (14, 17 to 22) is provided, which delivers the acquired measured value, corresponding to a lateral offset of the recording carrier (2), to a printer control (23), assigned to the printer, for further processing.
3. Device according to Claim 2, characterised in that the measured-value acquisition device (14, 17 to 22) is designed for counting the steps of the stepping motor (12) executed during the linear displacement from the starting position to the detection of the side edge (200) of the recording carrier (2) and provides the summed-up value as a measured variable in binary-coded form.
4. Device according to one of Claims 1 to 3, characterised in that the measuring device (9 to 13) has as zero-position detector a further optical sensor (13), which detects the return of the measuring device into its starting position and emits a corresponding control signal (POS0̸) for stopping the measuring device.
5. Device according to one Claims 2 to 4, characterised in that the stepping motor (12) of the measuring device (9 to 13) is equipped with an internal gear mechanism for converting the motor rotary movement into a linear stroke movement and is coupled directly to the fork (11) of the edge light barrier (9).

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