Patent Publication Number: US-7583920-B2

Title: Electrostatographic single-pass multiple station printer with improved colour registration

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention relates to methods and apparatus for an electrostatographic single-pass multiple station (for example multiple colour) printer for forming an image onto a printing medium, and more in particular to register control means and method for controlling the operation of each of the stations of such electrostatographic single-pass multiple station printer. 
   BACKGROUND OF THE INVENTION 
   Electrostatographic single-pass multiple station printers are known in the art, such as from U.S. Pat. No. 4,977,411. 
   U.S. Pat. No. 5,499,093 describes an electrostatographic single-pass multiple station printer, which comprises a number of consecutive single colour printing stations. Each single colour printing station is adapted to print a specific single colour image to a printing medium passing though the printer. All single colour images printed one over the other provide together the composite image being a multicolour image. In order to generate a multicolour image without colours being in offset one versus the other, it is understood that a perfect timing or synchronisation of the printing actions of the consecutive printing stations is necessary. 
   For obtaining such synchronisation, U.S. Pat. No. 5,499,093 discloses that an encoding means is used to generate a set of pulses, which pulses are indicative of the web displacement that has occurred. The encoding means is driven by one of the rotatable endless surface means onto which a toner image can be formed at one particular printing station. The transformation of the set of pulses into synchronised commands for the consecutive printing stations is performed by a register control means. 
   According to U.S. Pat. No. 5,499,093 the encoder means is preferably coupled to a printing station which is not located as the first, nor as the last printing station in the sequence, but to an intermediate printing station, preferably the central printing station in the sequence of consecutive printing stations. Such a choice of an intermediate printing station is preferred because in that case the web path between the drum carrying the encoder and the drum most remote therefrom is minimised thereby reducing any inaccuracies which may arise from unexpected stretching of the web and of variations induced by the eccentricity of the drums or the rollers defining the wrapping angle ω, caused by e.g. machine vibrations. The high-frequency vibrations may be compensated or filtered with large accuracy by a filtering means being part of the register control means. 
   The register control means may possibly comprise manual adjustment means to compensate deviations of web displacement, measured after printing and analysing prints of calibration images. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide improved electrostatographic single-pass multiple station printers for forming an image onto a printing medium, as well as methods of operating the same. It is further an object of the present invention to provide improved electrostatographic single-pass multiple station printers as well as methods of operating the same being able to avoid displacement of different colour images in general, and more specifically to avoid displacement of different colour images by effects of the toner coverage of the printed pages. 
   An aspect of the present invention is being able to automatically, i.e. without intervention of an operator, compensate for low frequency vibrations. 
   The above objectives are accomplished by an electrostatographic single-pass multiple station printer according to the present invention having the features as set out in the attached claims. 
   In a first aspect, the present invention provides an electrostatographic single-pass multiple station printer for forming an image onto a printing medium, which comprises:
     (i) a plurality of toner image-printing electrostatographic stations comprising
       (ia) rotatable endless surface means onto which a toner image can be formed;   (ib) means for forming an electrostatic toner image on each said surface means; and   (ic) means for transferring the formed toner image onto the printing medium;   
       (ii) means for conveying the printing medium in succession past said stations in synchronism with the peripheral speed of said rotatable endless surface means; and   (iii) register control means for controlling the operation of each of said stations in timed relationship thereby to obtain correct registering of the distinct toner images on the printing medium, wherein said register control means comprises
       (iiia) a main encoder means for producing a first signal indicative of printing medium displacement, and   (iiib) delay means for initiating the operation of subsequent image-printing stations after a predetermined printing medium displacement, as measured by the main encoder means, has occurred.
 
In a printer according to this first aspect, the register control means comprises a further printing medium advancement measurement means for providing a second signal representative of printing medium advancement, said printing medium advancement measurement means being adapted to co-operate with said main encoder means for automatic adjustment of said first signal using said second signal. The adjusted first signal can then be provided to the delay means. The delay means may use this adjusted first signal as being representative of printing medium advancement, in real-time, and use this adjusted first signal for initiating a control signal  911  for each of the printing stations controlled by the register control means  90 .
   
       

   In embodiments of the present invention, the printing medium advancement measurement means may be an optical printing medium advancement measurement means. 
   In other embodiments of the present invention, the printing medium advancement measurement means may be a mechanical printing medium advancement measurement means. In this case, the main encoder means may be mechanically coupled to a first rotatable endless surface means onto which a toner image can be formed. 
   The printing medium advancement measurement means may comprise a second encoder means for producing the second signal indicative of printing medium displacement. The printing medium advancement measurement means may be coupled to said main encoder means by means of a comparator, for obtaining the difference between said second signal and said first signal indicative of printing medium displacement, said difference being used to automatically adjustment said first signal indicative of printing medium displacement. In a first embodiment of the present invention, the second encoder means may be mechanically coupled to a second of said rotatable endless surface means onto which a toner image can be formed. The second rotatable endless surface means preferably is different from the first rotatable endless surface means. Said second of said rotatable endless surface means onto which a toner image can be formed may be the rotatable endless surface means of the toner image-printing electrostatographic station first past by the printing medium. In one other embodiment of the present invention, the second encoder is preferentially mechanically coupled to a high precision roller in contact with the printing medium, preferably upstream the electrostatographic station first past by the printing medium. 
   The main encoder means may be located downstream of said printing medium advancement measurement means. 
   The main encoder means may be mechanically coupled to the rotatable endless surface means onto which a toner image can be formed of an intermediately positioned toner image-printing electrostatographic station. 
   The printing medium advancement measurement means may include measurement means for measuring any of longitudinal printing medium displacement, absolute position of the printing medium, printing medium velocity, printing medium acceleration or a combination of any of these. 
   In a second aspect, the present invention provides a method of operating an electrostatographic single-pass multiple station printer for forming an image onto a printing medium, the printer comprising:
     (i) a plurality of toner image-printing electrostatographic stations comprising
       (ia) rotatable endless surface means onto which a toner image can be formed;   (ib) means for forming an electrostatic toner image on each said surface means; and   (ic) means for transferring the formed toner image onto the printing medium;   
       (ii) means for conveying the printing medium in succession past said stations in synchronism with the peripheral speed of said rotatable endless surface means. The method comprises controlling the operation of each of said stations in timed relationship thereby to obtain correct registering of the distinct toner images on the printing medium, the controlling comprising:
       deriving a first signal indicative of printing medium displacement, and   delaying the operation of subsequent image-printing stations after a predetermined printing medium displacement,   deriving a second signal representative of the printing medium advancement from a measurement performed on said printing medium, and   adjusting said first signal using said second signal.   
       

   The adjusted first signal may thus be used to delay the operation of subsequent image printing stations, more particular to delay the operation of subsequent image printing stations in real-time during displacement of this printing medium. 
   The printing medium advancement may include any of longitudinal printing medium displacement, absolute position of the printing medium, printing medium velocity, printing medium acceleration or a combination of any of these. 
   It is an advantage of embodiments of the present invention to provide accurate signals for the printing medium position as the printing medium travels through the printer, which signals are not or only to a limited extent, e.g. less than in prior-art printers, subject to systematic inaccuracies induced by the toner coverage of the printed pages. It is further an advantage of embodiments of the present invention to be able to provide accurate signals to finishing apparatus such as sheet cutting apparatus, die cutting apparatus for labels, folding apparatus or other apparatus, which benefit from accurate information on the printing medium displacement. 
   Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims. 
   It is understood that electrostatographic single-pass multiple station printers will usually use dry-particulate toner, however the invention is equally applicable where the toner particles are present as a dispersion in a liquid carrier medium or in a gas medium in the form of an aerosol. 
   The electrostatographic single-pass multiple station printers may especially be a colour printer comprising image printing stations-for each of yellow, magenta, cyan and black toner images. Such printing stations being provided to provide images only on one side of the printing medium in a single side printer, or alternatively, of each of such stations one is present to print on each of the sides of the printing medium in a double side printer or so-called “duplex printer”. 
   According to the present invention, the printing medium may be a transfer printing medium, e.g. a temporary support such as a tensioned endless belt, onto which the various colour images are provided by passing the consecutive printing stations. It is understood that in such embodiments, the single pass multiple station printer of the present invention comprises a means for transferring the image formed on the printing medium onto a final printing medium such as a web, e.g. a paper web. The final printing medium may be provided by unwinding the final printing medium from a roll, or as individual parts, e.g. sheets of web. As an alternative, the printing medium may itself be the final printing medium, e.g. a web such as a paper web, onto which the image is to be provided. In such embodiment, the printer may further comprise a roll stand for unwinding a roll of web to be printed in the printer, and a web cutter for cutting the printed web into sheets. 
   The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic representation of a side (planar) view of the apparatus according to a first embodiment of the present invention, showing a single-side electrostatographic single-pass multiple station printer. 
       FIG. 2  is a schematic representation of a side (planar) view of the apparatus according to a second embodiment of the present invention, showing an alternative single side electrostatographic single-pass multiple station printer. 
       FIG. 3  is a schematic representation of a side (planar) view of the apparatus according to a third embodiment of the present invention, showing an alternative single side electrostatographic single-pass multiple station printer. 
       FIG. 4  is a schematic representation of a side (planar) view of a printer station being part of the printer as illustrated in  FIG. 2 , comprising an encoder disc and a sensor means. 
       FIG. 5  shows is a block diagram schematically illustrating the coupling of the main encoder means and a printing medium advancement measurement means under the form of a longitudinal printing medium displacement measurement means, that is based on an additional encoder as in the embodiment of the present invention as shown in  FIG. 2  or  FIG. 3 . 
       FIG. 6  is a schematic representation of a side (planar) view of the apparatus according to a fourth embodiment of the present invention, showing an alternative double side electrostatographic single-pass multiple station printer. 
   

   DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
   The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention. 
   Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. 
   It is to be noticed that the term “comprising”, used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a device comprising means A and B” should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B. 
   Similarly, it is to be noticed that the term “coupled”, also used in the claims, should not be interpreted as being restricted to direct connections only. Thus, the scope of the expression “a device A coupled to a device B” should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. 
   The invention will now be described by a detailed description of several embodiments of the invention. It is clear that other embodiments of the invention can be configured according to the knowledge of persons skilled in the art without departing from the true spirit or technical teaching of the invention, the invention being limited only by the terms of the appended claims. 
   The term ‘printing station’ used hereafter refers to a toner image-printing electrostatographic station. 
   It is understood that electrostatographic single-pass multiple station printers will usually use dry-particulate toner, however the invention is equally applicable where the toner particles are present as a dispersion in a liquid carrier medium or in a gas medium in the form of an aerosol. 
   The electrostatographic single-pass multiple station printers described with reference to the present invention may especially be a colour printer comprising image printing stations for each of a sequence of 3 or more primary colours such as yellow, magenta, cyan as well as other printing stations, e.g. for black toner images or for spot colour toner images. Such printing stations being provided to provide images only on one side of the printing medium in a single side printer, or alternatively, of each of such stations one is present to print on each of the sides of the printing medium in a double side printer. 
     FIG. 1  shows a schematic representation of a side view of a single-side electrostatographic single-pass multiple station printer  10 . The printer  10  illustrated comprises 4 consecutive printing stations labelled A, B, C and D, which are arranged to e.g. print yellow, magenta, cyan and black respectively. It is to be understood that the configuration illustrated is not intended to be limiting for the present invention, and that a configuration with more or less printing stations is included in the present invention as well. The printing stations A, B, C and D are arranged in a substantially vertical configuration, but it is to be understood that a substantially horizontal configuration or any other configuration might apply. The printing medium  12  is unwound from a supply roller  14 , and in the example illustrated is a printing web, such as e.g. a paper web. The printing medium is pulled through the printer  10  by means of a motor driven drive roller  22 . Tension is provided to the printing medium  12  by a brake  11  located at the supply roller  14 . The printing medium  12  is conveyed in upward direction past the printing stations A, B, C, D in turn. The moving printing medium  12  is in face-to-face contact with the surfaces  26  of the drums  24  (see also  FIG. 3 ) of the printing stations A, B, C and D, preferably over an angle α of about 15° of the circumference of the drums  24 , which angle is determined by guiding rollers  36 . After having passed the last printing station D in the row, the printing medium  12  is passed trough an image fixing station  16  and possibly a cooling zone  18 . The printer may furthermore optionally comprise a cutting device  20 . 
   The single-side electrostatographic single-pass multiple station printer  10  further comprises a main encoder means  50 , being coupled to an intermediate printing station, being in the embodiment illustrated in  FIG. 1  either printing station B or C. As shown in  FIG. 1 , the printing station B of the single side electrostatographic single-pass multiple station printer  10  comprises the main encoder means  50  as is described in more detail with regard to  FIG. 4 . The main encoder means  50  generates a set of pulses  501 , which set of pulses  501  is relative to the printing medium displacement, e.g. by its frequency. 
   The main encoder means  50  may provide a first set of pulses which set of pulses is indicative for the printing medium displacement. This means that during the time lapse between consecutive pulses, the printing medium is displaced over a given length ρ. This may be achieved by e.g. mechanically coupling a rotatable encoder disc to one of the rotatable endless surface means onto which a toner image can be formed at a given printing station. The encoder disk carries a number of marks, which may be detected by a sensing means. This sensing means may be stimulated optically, mechanically or magnetically. Measures are taken to prevent slip between the printing medium and the rotatable endless surface means, and between the encoder disc and the rotatable endless surface means. Therefore, as the encoder disc has a substantially constant radius R and rotates around its central axis, the factor in the linear relationship between angle of rotation α and printing medium displacement caused by this rotation is R. The pulses provided by this sensing means, and in particular the frequency of these pulses may be multiplied by a real number by a multiplier means, providing the set of pulses indicative for the printing medium displacement. 
   As preferred, in case the main encoder means  50  is coupled to an intermediate printing station, preferably the central printing station, in the row of consecutive printing stations, most of the high-frequency vibrations will be reflected in the pulses and pulses&#39; frequency and can be compensated or filtered with most accuracy in order to provide an accurate timing of the different printing stations using simple electronic filtering circuits. 
   The term ‘intermediate printing station’ is to be understood as a printing station, which has at least one other station located upstream and one other printing station located downstream in the printing medium moving direction. The term ‘central printing station’ is to be understood as the printing station, which has an equal number of other printing stations located upstream and downstream in the printing medium moving direction in case of an odd number of printing stations. In case of an even number of printing stations, the term ‘central printing station’ is to be understood as the printing station whereby preferably the number of printing stations located upstream along the printing medium moving direction is 1 more compared to the number downstream along the printing medium moving direction. 
   It has been noticed by the Applicant, that, especially in case of high coverage printing output, colour deviations may occur despite a high-frequency compensation being present. Such deviations cannot immediately be compensated by the means present in the electrostatographic single-pass multiple station printers as presently known. For example, in case there is a significant amount of toner present on the printing medium  12  when the printing medium passes the encoder disc  51  of the main encoder means  50  or the device to which the encoder disc  51  is coupled mechanically, which is especially the case when higher coverage printing output is required, the relation between angular rotation α of the encoder disc  51 , and the actual printing medium displacement is no longer R, but is equal to a slightly higher apparent radius R′. The Applicant has found surprisingly that this small difference between R and R′, being the toner thickness present on the printing medium  12  when passing the encoder disc  51  may be sufficient to create a colour offset in the consecutive printing stations. 
   The single-side electrostatographic single-pass multiple station printer  10  according to an aspect of the present invention further comprises a printing medium advancement measurement means  60   a  which may be located in printing medium moving direction before the main encoder means  50 , and in the example of  FIG. 1  is located at the side of the printing medium  12  which is not to be printed. 
   The term ‘printing medium advancement measurement means’ is to be understood as a means for measuring the printing medium&#39;s longitudinal displacement, absolute position, velocity, acceleration or any combination of these. 
   The printing medium advancement measurement means  60   a  may e.g. be any presently known appropriate longitudinal printing medium displacement measurement means, absolute position measurement means, printing medium velocity measurement means, or printing medium acceleration measurement means, or a combination of any of these, such as e.g. an optical or a mechanical measurement means. The printing medium advancement measurement means  60   a  may make contact with the printing medium  12  or may be contactless. 
   In order to have a good effect of the use of a printing medium advancement measurement means  60   a ,  60   b , the printing medium advancement measurement means is preferably a longitudinal printing medium displacement or a means for determining a value related to printing medium advancement such as an average advancement measuring means or or a means for determining a time differential of the advancement such as a printing medium velocity measurement means. More preferred a non-contact measurement means, in order to avoid an influence of any fluctuation in printing medium thickness and/or presence of toner or of frictional variations on the advancement of the printing medium caused by a contact measuring means. In case of mechanical printing medium advancement measurement means, contacting the printing medium, the point of contact between an element of the measurement means and the printing medium is preferably not disturbed by presence of toner. As an example, in case of a single-side electrostatographic single-pass multiple station printer, the contact between printing medium and element of the printing medium advancement measurement means may be provided at the non-printed side. 
   In case of a printing medium velocity measurement means, this means may be a tachometer (not represented in the drawings) comprising a roller in rolling slipless contact with the printing medium  12 . The roller axis is coupled to a DC motor for generating an analog output signal (voltage), which is proportional to the speed of the printing medium  12 . 
   In case of a longitudinal printing medium displacement measurement means, this means to track a longitudinal printing medium displacement typically comprise both mechanical means and optical or magnetic means to accurately account for the synchronous slipless motion of a very precisely machined contacting roller. This means to track a longitudinal printing medium displacement can also directly measure marks or perforations on the printing medium  12  or the advancement of random auto-correlated “texture patterns” on the printing medium  12 . 
   A printing medium advancement measurement means  60   a  carrying out a measurement by contacting the printing medium  12  at locations where no toner is present is preferred. A suitable location in case of single-side printing is to locate the measurement means  60   a  at a non printed side of the printing medium  12 . As alternative embodiments (not shown in the drawings), the printing medium advancement measurement means  60   a  may e.g. be located between printing station D and image fixing station  16 , i.e. after all printing stations A, B, C, D have been passed, at the non printed side of the printing medium  12 , or before the first printing station A at the side of the printing medium  12  which is or is not to be printed. 
   Providing a location before or at the position of the first printing station A is preferred. A specific drum may be provided that can be machined to high precision and that lasts the life of the machine. 
   The printing medium advancement measurement means  60   a  generates a signal  601 , which signal may e.g. be related to and representative for the printing medium displacement or instantaneous velocity at the printing medium advancement measurement means  60   a.    
   By coupling back this printing medium advancement data signal  601  to printing medium displacement data signals  501  obtained from the main encoder means  50 , an adjustment may be made based on the relative drift of the set of pulses of the main encoder means  50  and the signal of the printing medium advancement measurement means  60   a . As will be explained in more detail further, the delay means  91  of the register control means  90  can generate control signals  911  for the different printing stations. The relative drift as resulting from the difference between the actual printing medium advancement determined by the printing medium advancement measurement means  60   a  and the printing medium displacement determined by the main encoder means  50  is thus taken into account. Or more in general, it was found that low frequency vibrations or disturbances may be filtered from the pulse and pulse frequency by taking into account the difference between the actual printing medium advancement, e.g. the actual longitudinal printing medium displacement determined by the printing medium advancement measurement means  60   a  and the longitudinal printing medium displacement determined by the main encoder means  50 . The effect that the higher frequency vibrations, which cause slight variations in the pulse frequency, may be filtered out from the set of pulses as already known in the art, is still applicable. 
   In case the printing medium advancement measurement means  60   a  is a longitudinal printing medium displacement or velocity measurement means, the relative drift of the printing medium displacement data of the two measurement means  50 ,  60   a  can be interpreted as resulting from the time integrated velocity estimations derived from the longitudinal printing medium displacement or velocity measurement means  60   a , and the time integrated velocity estimations derived from the longitudinal printing medium displacement data determined by the main encoder means  50 . The difference in velocity estimation as measured by the two measurement means  50 ,  60   a  may be due to the thickness of toner deposited onto the printing medium  12 . This changes the effective radius of a roller or rotatable endless surface means used for velocity determination and/or longitudinal printing medium displacement determination, and it has been found surprisingly by the inventor that this change in effective radius is non-negligible. 
   As will be described hereinafter as a preferred embodiment of the present invention, in case the printing medium advancement measurement means is a second encoder means producing a second set of pulses indicative for printing medium displacement, and being coupled to a rotatable endless surface means onto which a toner image can be formed at a particular printing station, the printing station to which this second encoder means is mechanically coupled is preferably the first printing station A which is met by the printing medium  12  when passing through the multiple station printer. 
   Best results using the invention are obtained when the main encoder means  50  is located downstream the additional printing medium advancement measurement means  60   a , this is when the printing medium  12  first passes the additional printing medium advancement measurement means  60   a  and then the main encoder means  50 , while passing through the multiple station printer  10 . 
     FIG. 2  schematically shows a side (planar) view of an apparatus of the present invention showing a single side electrostatographic single-pass multiple station printer  100 . The printer  100  is identical to the printer  10  as shown in  FIG. 1  and corresponding numerical references refer to identical means, except from the printing medium advancement measurement means  60   a , e.g. longitudinal printing medium displacement or velocity measurement device, which is now provided as a printing medium advancement measurement means  60   b  comprising a second encoder means, being coupled to the printing station A. The printing medium advancement measurement means  60   b  generates a signal  601  which set of pulses  601  is relative to the printing medium displacement at the printing station A. 
     FIG. 3  shows a schematic representation of a side (planar) view of the apparatus  1000  according to a third embodiment of the present invention, showing an alternative single side electrostatographic single-pass multiple station printer. The printing medium used in this embodiment is a transfer printing medium, e.g. a temporary support such as a tensioned endless belt, onto which the various colour images are provided by passing the consecutive printing stations. The printer  1000  comprises five consecutive printing stations A, B, C, D and E. The printer  1000  is in some aspects similar to the printer  100  as shown in  FIG. 2  and corresponding numerical references refer to identical means, except from the printing medium advancement measurement means  60   b , e.g. longitudinal printing medium displacement or velocity measurement device, which is now provided as a printing medium advancement measurement means  60   c  comprising a second encoder means, being coupled to a reverse roll  1001 . The printing medium  12  is a transfer printing medium implemented as an endless belt, which is tensioned by means of at least two reverse rolls  1001  and  1004 . The printing medium advancement measurement means  60   c  generates however a similar signal  601  as in the embodiment of  FIG. 2 , which signal  601 , preferably a set of pulses, is relative to the printing medium displacement at the reverse roll  1001 . The printer  1000  further comprises a means  1002  for transferring the colour image present on the printing medium  12  onto a final printing medium  1003 , e.g. being a paper sheet. 
   To explain the working of the encoder means, either being the main encoder means  50  at an intermediate printing station C or the second encoder means of the printing medium advancement measurement means  60   b  of the embodiment of  FIG. 2 ,  FIG. 4  shows a printing station to which an encoder means  80  is coupled. An encoder disc  81  is coupled to the shaft of a rotatable endless surface means onto which a toner image can be formed, being the cylindrical drum  24  having a photoconductive outer surface  26  and having a radius R. While the printing medium  12  passes the printing station provided with the encoder means  80 , the drum  24  is rotated over a given angle α. This rotation of the drum  24  causes the encoder disc  81  coupled to it to rotate over the angle α as well. During this rotation, a number of marks  82  will pass an encoder sensor  83 , each mark  82  passing the sensor  83  causing a pulse signal to be generated. Between two pulses, it can be calculated that a paper length being α×(R+T+W/2) has passed, wherein T is the toner thickness being present at the contact zone  27  and W/2 is the distance from the paper surface oriented towards the drum  24  to the neutral line of the paper sheet, assumed to be located at half the thickness W of the paper. 
   A very accurate digital clock generator  86   a  is provided, capable of producing an output clock frequency that is proportional to a digital input value (called direct digital synthesis). This output frequency is compared to the set of pulses from the encoder sensor  83  by means of a comparator  84 . Due to the digital nature of the signals, a very accurate phase comparison between both signals can be made. Using the output of comparator  84 , a loop filter  85  then computes a new value for the input signal of digital clock generator  86   a , such that the output of clock generator  86   a  tracks the encoder input very closely. This setup is well known as a digital PLL. 
   Now, by multiplying the input value of digital clock generator  86   a  by a multiplication constant M using a multiplier  87 , and feeding the multiplied value to a second identical digital clock generator  86   b , a set of pulses  801  is generated which proportionally tracks the encoder input closely, but runs at a much higher frequency. The multiplication constant M does not need to be an integer, but can be any real value, with an accuracy limited only by the resolution of the digital hardware. 
   A benefit of this setup is that the counters inside the digital clock generators actually present a very accurate measure of the position of the encoder disc  81 , and hence of the paper. 
   The time period between each pulse of the pulsed signal  801  after the multiplier  87  now represents a paper displacement of (α×(R+T+W/2))/M. Due to vibrations in the printer, high-frequency vibrations may occur in this pulsed signal, which may be filtered out by means of the loop filter  85 . The output of the encoder means  80  is a set of pulses  801 , which is indicative of the printing medium displacement. 
   It is understood that for the embodiment of  FIG. 3 , the encoder means  80  of  FIG. 4  is to be understood as being coupled to the shaft of the reverse roll  1001  instead as to the shaft of the rotatable endless surface means onto which a toner image can be formed. The set of pulses  601 , which is indicative of the printing medium displacement, is however generated in an identical way, and comprises in the same way information about the printing medium displacement. Turning now back to  FIGS. 1 ,  2  and  3 , the main encoder means  50 , if implemented as the encoder means  80  illustrated in  FIG. 4 , provides a set of pulses  501 , of which the time lapse between two pulses represents a printing medium displacement of (α×(R+T+W/2))/M. In this formula, the toner thickness T may vary over time in view of differences in printing being carried out at the first printing station A, causing low-frequency vibrations or variations. Thus the printing medium displacement at printing station C is no longer uniformly related to the pulse interval. 
   However, according to the present invention, a second printing medium advancement measurement means  60   a ,  60   b  is used, be it an optical or mechanical means, to measure e.g. the printing medium velocity or displacement per time unit. The output signal  601  of such printing medium advancement measurement means  60   a ,  60   b  is now coupled to the main encoder means  50 , in order to automatically adjust the set of pulses  501  being indicative of printing medium displacement. 
   The adjusted set of pulses, being indicative of printing medium displacement during the ongoing printing operation, i.e. in real-time, is provided to a delay means  91 , this delay means  91 , main encoder means  50  and printing medium advancement measurement means  60   a ,  60   b  or  60   c  being part of a register control means  90 , generating control signals  911  being provided to the different printing stations A, B, C and D (as indicated with arrows  911  in  FIG. 1 ,  FIG. 2  and  FIG. 3 ). The delay means  91  uses the real-time adjusted set of pulses as a measure indicative for the printing medium displacement, from which the delay means  91  initiates a control signal  911  for each of the printing stations controlled by the register control means  90 . 
   The accurate signals for the printing medium position as the printing medium travels through the printer, i.e. the adjusted set of pulses, are not or only to a limited extent, subject to systematic inaccuracies induced by the toner coverage and/or image information of the printed pages. This presence of accurate signals for the printing medium position as the printing medium travels through the printer also allows the register control means  90  to provide accurate signals to finishing apparatus such as sheet cutting apparatus, die cutting apparatus for labels, folding apparatus or other apparatus, which benefit from accurate information on the printing medium displacement. 
   It is to be understood that the register control means  90  may further comprise additional means, not described in detail here but within the reach of a person skilled in the art, such as encoder correction means for making corrections for faults caused by incorrect reading of encoder discs, either when such disc is part of the main encoder means of in case the velocity measurement means comprises such encoder disc. 
   Turning to the preferred embodiments as shown in  FIG. 2  and  FIG. 3 , the coupling of the main encoder means  50  and printing medium advancement measurement means  60   b  or  60   c  is schematically shown in  FIG. 5 . 
   The printing medium advancement measurement means  60   b  or  60   c , implemented as a second encoder means  60 , comprises an encoder disc  61  with marks  62  and an encoder sensor  63 , providing a pulsed signal to a comparator  64 . Comparator  64  compares the pulsed signal with the output signal of a digital clock generator  66   a . Using the output of comparator  64 , the loop filter  65  then computes a new value for the input signal of digital clock generator  66   a , such that the output of clock generator  66   a  tracks the set of pulses of the second encoder input very closely. The new value for the input signal of digital clock generator  66   a  as provided by the loop filter  65  is multiplied by multiplier  67  by using a multiplication factor M 6 . This value is now used as input signal of a second identical digital clock generator  66   b , generating a set of pulses  601 , which proportionally tracks the second encoder input closely. 
   The main encoder means  50  comprises an encoder disc  51  with marks  52  and an encoder sensor  53 , providing a pulsed signal to a comparator  54 . Comparator  54  compares the pulsed signal with the output signal of a digital clock generator  56   a . Using the output of comparator  54 , the loop filter  55  then computes a new value for the input signal of digital clock generator  56   a , such that the output of clock generator  56   a  tracks the set of pulses of the encoder input very closely. The new value for the input signal of digital clock generator  56   a  as provided by the loop filter  55  is multiplied by multiplier  57  by using a multiplication factor M 5 . This value is now used as input signal of a second identical digital clock generator  56   b , generating a set of pulses  501 , which proportionally tracks the main encoder input closely. 
   The set of pulses  601  are coupled to the main encoder means  50  by a comparator  111  comparing the signals  501  and  601  from the main encoder  50  and from the second encoder means  60  respectively. By comparing the phase of the set of pulses  501  and  601  from the main encoder  50  and from the second encoder means  60  respectively, a comparison of the position of both encoders is made. Hence, low frequency variations or ‘low frequency drift’ of the imaging drum diameter may be distinguished from high frequency vibrations by this comparison. Compensating means  112 , providing a value for the modification to be applied to the multiplication factor M 5 , in order to compensate this low frequency drift. The multiplication factor M 5  is then modified to M 5 ′ by adding or subtracting A % of the value of M 5 , by means of an adder  113 . Preferably the difference between M 5  and M 5 ′ is not more than 0.1% in both directions. 
   The adjusted set of pulses is provided to the delay means  91  of the register control means  90 , providing control signals  911  being provided to the different printing stations A, B, C and D. 
   In general, the adjusted set of pulses is provided to the delay means  91  for providing control signals  911 , which on their turn are provided to the different printing stations coupled to the delay means. The printing action of each of the printing stations may be triggered by such a control signal  911 . The control signals  911  are preferably used to control the line exposure of line array exposure based printing stations, such as LED-based printing stations such as disclosed in e.g. U.S. Pat. No. 5,499,093 and EP453612A1, Vertical Cavity Surface Emitting Lasers such as disclosed in U.S. Pat. No. 5,940,113, which Lasers are capable of imaging the exposure sensitive medium one line at the time, or exposures based on two-dimensional array based exposure systems that can be based on digital micromirror or liquid crystal 2D-light modulators that are capable of exposing the sensitive media in an overlay scheme as detailed in e.g. U.S. Pat. No. 5,461,411. 
   In case of 1D or line array exposures, an image memory is organised as a two-dimensional array of pixels, where for each line (or row) a stream of pixel values are fed to the writing heads of each of the printing stations which may result in a line-wise exposure of the photoconductive drum surface. After a given number of pulses of the adjusted set of pulses, a next line of pixels is fed to the writing heads, of each of the printing stations. In this way the registration of the different images is not only accurate at the beginning of the image, but it also stays accurate within the image. 
   The control signal  911  may directly, i.e. in real time, determine and start actual line exposure events of the printing station. 
     FIG. 6  shows a schematic representation of a side (planar) view of a printing apparatus  1100  according to a fourth embodiment of the present invention, showing a double side electrostatographic single-pass multiple station printer. The printer  1100  is, in some aspects, similar to the printer  100  as shown in  FIG. 2  and corresponding numerical references refer to identical or analogous means. The printer comprises two sets of five printing stations (A, B, C, D and E, and A′, B′, C′, D′ and E′ respectively), each set being provided at a particular side of the printing medium  12 . The printing medium advancement measurement means  60   b , e.g. longitudinal printing medium displacement or velocity measurement device, is now coupled to a printing station A′, contacting the printing medium  12  at the opposite side as the one which is contacted by the printing station D, to which the main encoder means  50  is coupled. The printing medium advancement measurement means  60   b  generates a similar signal  601  which set of pulses  601  is relative to the printing medium displacement at printing station A′. The skilled man understands that the functioning of this embodiment is analogous to the functioning of the printer  100  of  FIG. 2 . 
   It was found that the adjustment of the pulses  501  by using pulses of a second encoder means according to the present invention is most efficient in case it is used for printers of which the printing medium is the final printing medium. This is especially the case for duplex or double side printers, printing directly on both sides of a final printing medium.