Printing device

Abstract: A printing device and a method in a printing device in which image data are fed to an image forming device (13) to form an image on a photoconductive drum (10) moving at substantially constant speed, said image being developed and transferred to a receiving sheet (18), wherein a synchronisation value is derived from the displacement of the receiving sheet during image transfer and the supply of image data to the image forming device (13) is controlled by means of said synchronisation value. In this way, it is possible to correct for slow degradation in the synchronisation between the receiving sheet (18) and image formation on the photoconductive drum (10).

Description: The invention relates to a method in a printing device in which image data are fed to an image forming device to form an image on an image forming medium moving at substantially constant speed, said image being developed and transferred to a receiving sheet moving at substantially constant speed. The invention also relates to a printing device for performing this method in accordance with the preamble of claim 5. A device of this kind is known from JP-A-55-43536, in which the circumferential speed of a photoconductive drum is detected and in which the supply of data to a laser scan module is controlled in connection with this speed. The speed of the receiving sheet to which the toner image is transferred is also controlled on the basis of the detected circumferential speed of the drum near the image transfer station. Although in this way a correction is made for the photoconductive drum being out of round, all kinds of unwanted effects occur nevertheless particularly in the transfer station and prevent accurately synchronised image transfer. For example, the receiving sheet, which is fed on a belt, will always have some slip in the transfer zone, thus resulting in image distortion. In addition, if the image is transferred to a thick receiving sheet, the surface speed of said sheet in the transfer zone will be greater than that of a thin receiving sheet. Particularly in the case of printing devices used in drawing offices, it is a requirement that a print on a receiving sheet should differ very little from the original. The length of the image on the copy may deviate by 0.3% maximum. This requirement applies to all kinds of receiving sheets processed in the printing device, in formats A0 to A4, thicknesses from 0.06 to 0.25 mm, transparent films, etc. In order to satisfy this very high requirement for a 1:1 image, some known printing devices comprise a control element which can be adjusted by the operator and by means of which, for example, the speed of the receiving material can be adjusted by means of a fine control. To this end, the operator measures the dimensions of the resulting image on the print and compares them with the dimensions of the original. This operation must be repeated for the subsequent prints until an exact 1:1 image is obtained. Satisfying this high imaging accuracy becomes even more difficult if use is made of transfer devices of the kind described, for example, in US-A-4 541 709. Here, the powder image on a photoconductive belt is first transferred to an intermediate, such as a belt having a top layer of silicone rubber. This belt is heated and the toner image thus softened is transferred to a receiving sheet. The top layer is pressed in somewhat as a result of the pressure at the contact surface of the intermediate and the photoconductive belt, so that the toner image transferred to the intermediate is some percent longer than the original image. An opposite effect occurs on transfer to the receiving sheet, but complete compensation never occurs owing to the differences in pressure between the two contact surfaces. The deformation (image lengthening or shortening) is to a high degree pressure-dependent. For example, on transfer to a thick receiving sheet the deformation will be different from that applying in the case of transfer to a thin receiving sheet. The same kind of variation occurs on transfer to a large format (A0) or a small format (A4) receiving sheet. Another disadvantage of the known systems is that the transfer rollers and the like exhibit some wear during their life and thus decrease in thickness. This in turn has the effect that for an originally correctly adjusted speed of rotation the circumferential speed of the rollers decreases, so that, for example, the receiving sheets are supplied at a reduced speed, so that there is again deformation of the transferred image. The object of this invention is to obviate the disadvantages described. In a method according to the preamble, this object is attained in that at least one synchronisation value is derived from the displacement of the receiving sheet during image transfer and the supply of image data to the image forming device is controlled by means of said synchronisation value. In a printing device according to the preamble, this object is attained in that the printing device comprises a device for determining the displacement of the receiving sheet during at least a part of the image transfer and a device for deriving at least one synchronisation value therefrom, and a device with which the supply of image data to the image forming device is controlled by means of the synchronisation value. As a result, accurate synchronisation is always obtained and is controlled by the medium (the receiving sheet) on which the accurate imaging is required. This imaging accuracy is independent of any deformation occurring during the use of an intermediate. In another method according to the invention, for receiving sheets having different dimensions or properties an associated synchronisation value is derived and stored in a memory. As a result, an accurate 1:1 print is obtained for each receiving material without the operator being required to take any extra action. The invention will be explained with reference to the accompanying drawings, and further advantages will be apparant therefrom. Fig. 1 represents a printing device according to the invention, Fig. 2 is another representation of an embodiment of a printing device according to the invention, Fig. 3 diagrammatically illustrates an electronic circuit for a printing device according to the invention, and Fig. 4 is a flow diagram of a method according to the invention. Fig. 1 represents a printing device. A drum 10 provided with a photoconductive layer 11 rotates by means of a motor (not shown) at a substantially constant speed in the direction of arrow 12. The layer 11 is given a uniform electric charge by means of a corona device 27 and is then exposed image-wise line-by-line by the image forming device 13, which is shown here in the form of an LED array which extends parallel to the axis of rotation of the drum 10. The latent image obtained from exposure is provided image-wise with a toner layer by means of developing device 14 and said toner image is transferred to a receiving sheet 18 fed past at substantially constant speed in the transfer device 16. The receiving sheet 18 is fed from a tray 20 containing a stack 19 of receiving sheets, between rollers 17, and is deposited in collecting tray 21. Before the receiving sheet is deposited the toner image is fused on said sheet by means of device 15. The transfer device 16 comprises a metal roller which can be given a voltage adapted to transfer the powder image from layer 11 to the receiving sheet 18. The roller is driven by a motor (not shown) at a speed substantially equivalent to the speed of the drum 10 which is applied against the roller with some pressure. The receiving sheet 18 passing through thus receives a speed which is substantially equivalent to the circumferential speed of drum 10. A device 22,23 for determining the displacement of the receiving sheet 18 comprises a pick-up 22 which detects the linear displacement of the receiving sheet 18 during a period when the receiving sheet 18 is situated in the transfer device 16. Pick-up 22 is a very wear-resistant roller made of tungsten carbide, which makes non-slip contact with the receiving sheet 18 and which is provided with a calibrated disc. On a displacement of the receiving sheet 18 over a distance of exactly 100.0 mm the disc performs one complete revolution and delivers one pulse to a circuit 23 which derives a synchronisation value therefrom. This synchronisation value is a measurement of the line frequency at which the image data 26 must be fed to the LED array. Let it be assumed that an image is to be depicted on the receiving sheet with a resolution of 10 lines per mm. If the receiving sheet travels a distance of 100.0 mm in exactly 1 second, the time required for a displacement over one line spacing is equal to 1 msec. This means that a pulse must be fed to circuit 25 every 1 msec, whereupon circuit 25 feeds, from the supplied image data, a complete line of image data to the LED array 13 every 1 msec. If the receiving sheet were to cover the distance of 100.0 mm in 1.10 second, the time required for a displacement over 1 line spacing is equal to 1.10 msec and this means that a pulse must be fed to circuit 25 every 1.10 msec, circuit 25 feeding a complete line of image data to the LED array every 1.10 msec from the image date supplied. The actual speed of revolution of the drum 10 is unimportant to form an exact 1:1 image on the receiving sheet. The only important feature is that the drum is moved at a substantially constant speed as is the receiving sheet. Since, in a device as shown diagrammatically in Fig. 1, an image must first be formed over approximately half the circumference of the drum 10 before a toner image can be transferred to the receiving sheet 18, a delayed control system is preferably used, in which, during transfer to a first receiving sheet, a synchronisation value is derived from the linear displacement of the receiving sheet and said value is stored in a memory. This stored synchronisation value is now fed to circuit 25 during the supply of image data of a second image to be formed to the LED array 13, so that the formation of the second image is controlled with the synchronisation value obtained during the printing of the preceding image. During the transfer of this second image to a second receiving sheet a new synchronisation value is derived from the linear displacement of said second receiving sheet, and this synchronisation value is now stored in the memory, so that it can control the image data supply of a third image. Since this second synchronisation value will differ only very little from the first synchronisation value, since the speed of the drum and the speed of the receiving sheets are kept constant, a good synchronisation and 1:1 imaging will be maintained. During the life of the printing device, the roller in the transfer device 16 will, for example, wear to some extent, so that the speed of displacement of the receiving sheet 18 will decrease. This means that an increasingly higher synchronisation value will be derived and the supply of image data to the LED array will always slower, so that the 1:1 ratio is accurately maintained. The fact that a synchronisation value is always derived afresh makes any new adjustment or re-adjustment by an operator or service engineer unnecessary. Fig. 2 represents another printing device to the invention. A drum 30 provided with a photoconductive layer 31 rotates by means of a motor (not shown) at a substantially constant speed in the direction of arrow 32. The layer 31 is provided with a uniform electric charge by means of a corona device 35 and is then exposed image-wise line-by-line in the image forming device, which is here shown in the form of an LED array 34 extending parallel to the axis of rotation of the drum 30. The latent image obtained from exposure is provided with a toner layer image-wise by means of developing device 33 and said toner image is transferred to a receiving sheet fed past at substantially constant speed in the transfer device 44-48. The transfer device comprises a roller 44 provided with a top layer 45 of silicone rubber. The toner image is received by the silicone rubber 45 from the drum 30 and, since roller 44 is heated, the toner image will be fed in a somewhat softened state to a transfer zone between roller 44 and biasing roller 46. A receiving sheet is now fed between the two rollers 44, 46 and the softened toner image will be transferred to the receiving sheet and cool to give a permanent toner image. The rollers 47 and 48 are operative to protect the relatively thin biasing roller 46 against deflection. The printing device has a roll dispenser 38 in which three rolls of receiving material 39, 40, 41 are disposed in such manner that one of the rolls 39, 40, 41 is selected at any time in response to an input command via a control panel. The rolls may contain receiving materials which differ from one another in respect of width and thickness or properties. For example, roll 39 may contain paper of a width of about 30 cm, roll 40 paper of a width of about 60 cm and roll 41 paper of a width of about 80 cm. In Fig. 2 roll 40 has been selected and the path travelled by the web of paper through the printing device is denoted by line 55. The web of paper can be cut off by means of knife 37 to give A1 size sheets for example. The web of paper from roll 40 is fed via tensioning rollers 42, knife 37 and roller 49 between rollers 50, 51, 53 to the transfer device 44-48 and then deposited in the sheet receiving device 43 via rollers 54. Pick-up 52 makes non-slip contact with the paper web opposite roller 50 and detects the linear displacement of a receiving sheet passing through as described for Fig. 1. The detection signal is fed to a circuit 56 and from this a synchronisation value is derived to control the supply of image data 57 to the LED array 34. It has been found in practice that a different linear speed of displacement occurs for each separate width of the receiving sheet using the above printing device. This is probably due to non-uniform wear of biasing roller 46 or 44, so that a somewhat different slip occurs with different formats. Differences in thickness between the receiving sheets from the rolls 39, 40, 41 also result in differences in speed of displacement as a result of the pressure changing between rollers 44 and 46 in consequence. In order that an exact 1:1 image may be obtained on any receiving material, however, a separate synchronisation value is derived for each type of receiving material and stored in a memory in circuit 56, together with an indication of said material, e.g. the position of the material on one of the rolls 39, 40, 41. One of the receiving material rolls 39, 40, 41 is selected by the operator during printing, or else such selection may be automatic, in dependence on the size of the original to be printed. At the same time, the last determined synchronisation value associated with that roll is called up from the memory and the flow of image data controlled therewith. In the device according to Fig. 2, any desired kind of receiving sheet can be supplied via rollers 58. Via the control panel the operator inputs a code for the sheet fed in. If a receiving sheet of this type has already previously been processed by the device, a synchronisation value for that material is already stored in the memory and the image formation on drum 30 is synchronised therewith. If the material has not been used previously, then a synchronisation value is allocated during the inputting of a code for the same. During printing on this material, a synchronisation value is again derived and stored with the associated code in the memory, so that the next print on the same material gives an exact 1:1 image. Fig. 3 is a diagram showing an electronic circuit for a printing device according to the invention. A tungsten carbide roller 52 having a rough surface is pressed against a receiving sheet 59 and measures the displacement thereof, which is transmitted to a revolution pick-up 60. Roller 52 has a circumference of 100 mm. In response to a signal from the control device 66 fed via line 71 to the pick-up 60, the latter delivers a first pulse to a flip-flop when the roller 52 is in a specific position. The output of this flip-flip thus goes high. When the roller 52 has performed exactly one revolution and the receiving sheet 59 has thus moved over a distance of 100 mm, the revolution pick-up 60 delivers a second pulse to the flip-flop, so that the output 61 again goes low. This output signal from the flip-flop is used as an enable signal for a counter 62. During the time that this enable signal is high on output 61, counter 62 counts the number of pulses fed by a 1 MHz clock pulse generator 63 to counter 62. If the enable-signal is exactly 1 sec high, the counter 62 will have counted 1 x 10⁶ pulses. Given a required resolution of 10 lines per mm, i.e. a line spacing of 0.1 mm, the number of pulses generated on displacement of the receiving sheet over one line spacing is equal to 1000. The count of counter 62 is divided by 1000 in the control device 66 to give the synchronisation value. In the example described this value is 1000. This synchronisation value is stored in a memory in the control device 66 together with a code for the type of receiving sheet being used. As described above, this code can be input by the operator via a keyboard 67, or automatically via sensors 68 disposed at the stock rolls of receiving material. The synchronisation value is then fed as a pre-set value to a pre-settable counter 64. This counter 64 is fed with the same clock pulses from clock pulse generator 63 as are fed to counter 62. In the example described counter 64 is pre-set to 1000. After exactly 1000 clock pulses have been fed to counter 64 the latter delivers a synchronisation pulse to circuit 65 which comprises a serial-in parallel-out shift register in which exactly one image line of image data fed via 70 is stored. The synchronisation pulse starts the parallel output of image data via lines 72 to the image forming device. After counter 67 has delivered a synchronisation pulse, counter 64 is again pre-set to 1000 and the cycle. is repeated until a complete image has been printed on the receiving sheet. The circuit described above is intended only as an example and can be implemented in many other ways, for example depending on the displacement pick-up used, or on the image forming device, which can be controlled in parallel or in series. Of course a microcomputer can be used for this circuit. Fig. 4 represents a flow-chart of a possible print cycle of the kind that can be carried out by means of a microcomputer. After the start 74 of the printing device, step 75 searches the material, the type and/or format of the receiving sheet on which a print is to be made. This can be done automatically via sensors at the roll device or, as described above, be input by the operator via the control panel. If paper cassettes are used the input can be carried out via known methods as a detection of marks or the like. The associated synchronisation value is searched in step 76 in a memory table. Step 77 checks whether this value is present and, if so, the program goes on to step 79. If there is no associated synchronisation value present, a provisional synchronisation value is allocated in step 78. This can be done in various ways, e.g. by always selecting a fixed value, but preferably a value is generated which is equal to the average of all the synchronisation values present in the memory. In step 79 the synchronisation value is fed as a pre-set value to counter 64. Image formation can be started in step 80. After image formation has started, step 81 determines the moment at which a receiving sheet is present in the image transfer zone by repeatedly scanning a sensor present there. If a sheet is present, the measuring cycle is started in step 82. In step 83 the revolution pick-up 60 is accessed to determine the first zero position, whereon counter 62 is started in step 84. In step 85 the pick-up is always accessed again to determine the moment at which the pick-up disc has completed exactly one revolution. When this moment has been detected, the synchronisation value is determined in step 86 in the manner described above from the count of counter 62 and stored in the memory in a table at the code of the material input in step 75. The program described is terminated in step 87. In the above description it has always been assumed that the drive for both the image forming medium and for the receiving sheet remains substantially constant and that only a very slow change occurs between the two speeds as a result of wear during the life of the printing device. However, the invention can also be applied if the receiving sheet does not pass through the transfer device with a constant displacement per unit of time during the image transfer. A situation of this kind can occur if the pressure between the transfer rollers is so great that a receiving sheet passing therethrough causes a considerable change in the speed of transit. Such a change may also occur if the receiving sheets used have a thickened binder edge at the leading or trailing edge. In these cases, a different displacement pick-up can be used, which delivers a pulse on each displacement of the receiving sheet over a distance equivalent to the distance between two image lines to be printed. The counter 62 in Fig. 3 then always counts the number of clock pulses from the clock pulse generator 63 occurring in each period between two image lines. A synchronisation value is derived from each of these values in the manner described above and all these synchronisation values derived for all the image lines of a complete receiving sheet are stored in the memory. When a print is made on an identical following receiving sheet all these stored synchronisation values are fed, in the manner described above, successively as a pre-set value for the counter 64 to the latter and each time the number of clock pulses supplied corresponds to the pre-set value a synchronisation pulse is delivered to circuit 65, after which a following synchronisation value is fed to the counter 64 as a pre-set value. In this way an exact 1:1 image is obtained in those cases in which the speed of the receiving sheet does not remain constant during image transfer. The invention has been described with reference to an electrophotographic printing device in which image formation is effected by means of an LED array, but any other exposure device is usable, e.g. a laser or an array of switchable light valves. The invention can be used in all systems in which an image is depicted on a medium other than the receiving sheet and then transferred to a receiving sheet. Thus the invention can be used in those systems in which a latent electrostatic image is first transferred to an intermediate and developed and then transferred from the intermediate to a receiving sheet. The invention can also be used correspondingly in magnetographic systems in which image formation is carried out by means of a number of magnet heads on a magnetisable drum and in which the magnetic image is developed and transferred to a receiving sheet. In addition to the 1:1 imaging described, any other fixed imaging ratio can be obtained according to the invention, for example, by dividing or multiplying the determined synchronisation value by an enlargement factor. If, for example, a synchronisation value of 1000 is determined and a value of 1000:2 = 500 is supplied in Fig. 3, then image formation will take place at a double frequency and a print having exactly half the size of the original as considered in the direction of sheet transport will be obtained on the receiving sheet.

Claims: A method in a printing device in which image data are fed to an image forming device to form an image on an image forming medium moving at substantially constant speed, said image being developed and transferred to a receiving sheet moving at substantially constant speed, characterised in that at least one synchronisation value is derived from the displacement of the receiving sheet during image transfer and the supply of image data to the image forming device is controlled by means of said synchronisation value. A method according to claim 1, characterised in that the synchronisation value or values is/are stored in a memory and in that the image data for a subsequent receiving sheet are controlled with the said synchronisation value or values. A method according to claim 2, characterised in that at least one associated synchronisation value is derived for each receiving sheet of different dimensions or properties, and is stored in a memory. A method according to claim 2 or 3, characterised in that a number of synchronisation values corresponding to the number of image lines to be printed is derived for a receiving sheet. A printing device comprising an image forming device (13) to which image data can be supplied to form an image on an image forming medium (11) moving at substantially constant speed, a developing device (14) for developing the resulting image, and a transfer device (16) with which the developed image is transferred to a receiving sheet (18) moving at substantially constant speed, characterised in that the printing device comprises a device (22) for determining the displacement of the receiving sheet (18) during at least a part of the image transfer and a device (23) for deriving at least one synchronisation value therefrom, and a device (25) with which the supply of image data to the image forming device (13) is controlled by means of the synchronisation value. A printing device according to claim 5, comprising a memory for the storage of synchronisation values for receiving sheets (18) having different dimensions or properties.

Inventors: HEEREN THEODORUS ANTONIUS GERA, HEEREN, THEODORUS ANTONIUS GERARDUS
Applicants: OCE NEDERLAND BV, OCE-NEDERLAND B.V.
Application year: 1991
Main classification: G06K15
IPC codes: B41J2, G03G21, G06K15, H04N1