Method for determining a velocity of an object in a printing system

FIELD OF THE INVENTION

0001The invention relates to a method for determining a velocity of an object in a printing system. The invention also relates to an apparatus for determining a velocity of an object in a printing system. The invention also relates to a printing system comprising such an apparatus for determining a velocity of an object in the printing system.

BACKGROUND OF THE INVENTION

0002In an inkjet printing system a carriage is moved in a scanning movement over a recording medium. The carriage comprises at least one inkjet print head. Typically the velocity of a moving carriage is determined by measuring the movement of the carriage with respect to a reference pattern, for example a linear code strip. The code strip is fixed to the carriage support frame and extends in the scanning direction of the carriage. Such a code strip is known for example from EP 1674278.

0003The reference pattern comprises a plurality of markers, the markers usually being mutually equidistant. The resolution of the reference pattern is selected in accordance with the criteria of the printing application and in particular in accordance with the printing resolution of the image. Typically for a print resolution in the scanning direction of, for example, 600 dots-per-inch (dpi) a reference pattern is used, which reference pattern has a marker resolution of a division sum of the print resolution divided by an integer number.

0004The markers of the reference pattern are detectable by a sensor. Said sensor is fixed to the carriage and is arranged such that it is able to sense the markers of the reference pattern. The sensor provides a sensor signal based upon the detection of the markers in the reference pattern.

0005The frequency that the sensor senses a marker depends on the movement of the sensor relative to the markers in the reference pattern. The marker frequency is reduced in case the movement of the object is decelerated. As a result the information frequency in the sensor signal is reduced. Consequently the frequency of determining the velocity decreases when the velocity of the carriage decreases.

0006In any position of the object, where the sensor does not sense a marker of the pattern, the actual velocity and position of the object is unknown. In known printing systems the position and velocity of a carriage when moving between subsequent markers is estimated based upon a previous determination of the velocity of the object.

0007Thus the frequency of determining the velocity effects the accuracy of the determination of the velocity. And a lower frequency reduces the accuracy of the determination.

0008The disadvantage of methods for determining a velocity of an object in a printing system is the inaccuracy in determining the velocity of the object, in particular when the object is moving slow relatively to the reference pattern.

SUMMARY OF THE INVENTION

0009It is an object of the present invention to provide a method with an improved accuracy of determining the velocity of the object in a printing system, including for a slow moving object.

0010According to the invention, this object is achieved by a method for determining a velocity of an object in a printing system, the printing system comprising a fixed reference point, a reference pattern, independently movable with respect to said fixed reference point, wherein the object is independently movable with respect to said fixed reference point and with respect to the reference pattern, a first sensor, configured to have substantially the same velocity as said object with respect to said reference pattern and configured such that in operation the reference pattern is sensed by said sensor, comprising the following steps:

0011a) moving said reference pattern relative to said fixed reference point independent of said object,

0012b) determining the velocity of said reference pattern with respect to said fixed reference point,

0013c) sensing said reference pattern using said first sensor,

0014d) providing a sensor signal based on said sensed reference pattern, and

0015e) determining the velocity of said object relative to said fixed reference point based on the determined velocity of said reference pattern with respect to said fixed reference point, and based on said sensor signal.

0016The movement of the reference pattern enables improved accuracy of the determination of the velocity of the object. The reference pattern is moved relative to the fixed reference point or the stationary frame. The object is independently movable with respect to said fixed reference point and with respect to the reference pattern. The first sensor has substantially the same velocity as said object with respect to said reference pattern. The first sensor senses the reference pattern. The frequency at which the sensor senses the reference pattern may be adjusted by moving the reference pattern with a certain direction and velocity. As a result of the moving reference pattern said frequency may become independent of a) the resolution of the markers of the reference pattern and b) the movement of the object. The method provides improved accuracy in determination of the velocity of said object.

0017This method is in particular advantageous in case the object is moving slow relative to a regular operational velocity. This method is also advantageous in case the velocity of the object in operation is changing frequently. Using the method of the present invention the velocity of a scanning carriage in a printing system may be determined accurately for all velocities during operation.

0018The printing system may be any printing system. For example inkjet printing, electro photographic printing, direct inductive printing or any other image forming system. The printing system may also include other means for storage, for transport and/or for finishing of the recording medium.

0019The movable objects may be any movable object in the printing system. For example a carriage, which comprises at least one print head. Another movable object in a printing system is a transport roller, which may be rotatably movable. Yet another typical movable object is an imaging member, such as an imaging belt or imaging drum, which may be rotatable and/or linear movable. Yet another movable object is a receiving medium.

0020The fixed reference point may be, e.g., a fixed point of the frame of the printing system. The fixed reference point may also be a position in an independent axes system (e.g. an orthogonal axes system). In any way the fixed reference point is meant as reference point with respect to both a) the movement of the reference pattern in the printing system, b) the movement of the object in the printing system and c) a position of the printing system. The fixed reference point is as such used as reference position for both movements in order to determine the velocity of the object in the printing system.

0021The reference pattern comprises one marker or a plurality of markers. The plurality of markers may be arranged having a regular pattern in distances between adjacent markers. Preferably the reference pattern comprises a plurality of markers which are arranged at mutually equal distance.

0022The first sensor may be fixed to the object or may be connected to a position of the object. The first sensor may be connected to the object such that the first sensor is moving in the same direction and with the same velocity as the object. The first sensor may also be connected to the object such that the first sensor is moving with the same velocity as the object, however moving with a different direction as the movement of the object.

0023In step a) the reference pattern is moved independent of the movement of the object. The reference pattern may be moved independent of the movement of the object in any direction. For example in a substantial linear direction, in a rotational movement, in a combination of a linear direction and rotational movement, in a scanning movement, in a helical direction, or any other direction.

0024The reference pattern may be moved with a substantially constant velocity. The velocity of the reference pattern may also be changed. The movement direction and/or velocity of the reference pattern may be adjusted in response to the movement direction and/or velocity of the object.

0025In step b) the velocity of the reference pattern with respect to the fixed reference point is determined. Said velocity may be determined by measuring the movement of the markers of the reference pattern. The movement of the reference pattern may be determined by a second sensor. The second sensor in operation is able to sense the markers of the reference pattern. The second sensor is preferably stationary with respect to the fixed reference point.

0026Alternatively the velocity of the reference pattern with respect to the fixed reference point may be determined by accurately controlling the movement of the reference pattern. The movement may be accurately controlled by driving means, which means are configured for accurately moving the reference pattern. Preferably the driving means is configured such that the reference pattern attains a pre-selected velocity. Furthermore in operation the velocity of the reference pattern may be accurately maintained by the driving means at said pre-selected velocity.

0027In step c) the first sensor senses said reference pattern. The first sensor may sense the reference pattern in any way. For example the markers of the reference pattern may be sensed optically, electrically, magnetically, mechanically, or the like. The first sensor and the markers of the reference pattern are selected in order that the markers may be sensed by the first sensor.

0028In step d) the first sensor provides a sensor signal based on said sensed reference pattern. The sensor signal comprises information about the sensed reference pattern. The sensor signal may be any digital signal or analogous signal.

0029The sensor signal may comprise information of the velocity of the sensed reference pattern relative to the first sensor, or may comprise data corresponding to a frequency of at least a part of the plurality of markers of the sensed reference pattern, or may comprise information corresponding to a marker of the sensed reference pattern.

0030In step e) the velocity of said object relative to said fixed reference point is determined. The determination of the velocity of the object with respect to said fixed reference point is based on:

0031a) the determined velocity of the reference pattern with respect to said fixed reference point as determined in step b); and

0032b) the sensor signal as provided in step d).

0033The determination is based on the velocity of the object relative to the reference pattern and on the velocity of the reference pattern relative to said fixed reference point.

0034The velocity of the object relative to the reference pattern may be directly provided in the sensor signal or may be calculated from the information in the sensor signal.

0035The velocity of the object relative to the reference pattern may be calculated based on a known distance between adjacent markers in the reference pattern. Alternatively the calculated may be based on a known relation between a frequency, provided by the sensor signal, and a velocity of the object relative to the reference pattern.

0036In case in step b) the velocity of the reference pattern relative to said fixed reference point is measured by a second sensor, the velocity of the object relative to said fixed reference point may be based on a signal provided by the second sensor; the sensor signal of the second signal comprising information corresponding to the sensing of the reference pattern by the second sensor.

0037In an embodiment of the present invention, step a) comprises moving the reference pattern in a substantial linear movement. This embodiment is especially useful for determining a velocity of a linear moving object, such as a reciprocating scanning carriage in an inkjet printing system.

0038In another embodiment of the present invention, step a) comprises moving the reference pattern in a rotational movement. This embodiment is in particular useful for determining a velocity of a rotationally moving object, such as a transport roller. More in particular the embodiment provides advantages for a stepwise rotatably moving object.

0039In another embodiment of the present invention, step b) comprises providing a second sensor and determining the velocity of said reference pattern with respect to said fixed reference point by said second sensor. This embodiment is in particular useful for determining a velocity of a moving reference pattern. This embodiment provides the advantage that the control of the velocity of the reference pattern by driving means does not have to be very accurate.

0040In another embodiment of the present invention, step b) comprises providing driving means for controlling the velocity of said reference pattern with respect to said fixed reference point, wherein the driving means are configured such that reference pattern attains a pre-selected velocity. This embodiment is in particular useful for determining a velocity of a moving reference pattern. In this embodiment the control of the velocity of the reference pattern by the driving means is very accurate. A desired velocity of the moving reference pattern may be pre-selected. The velocity may be pre-selected based on a pre-estimated velocity of the object with respect to said fixed reference point.

0041In another embodiment of the present invention, step e) comprises determining a time interval between a sub sequential of sensor signals of said first sensor. This embodiment is in particular useful for determining a velocity of an object in case the sensor signal is provided at each time that the first sensor senses a marker of the reference pattern. The velocity of the object with respect to the reference pattern may be based on the time interval between a sub sequential of sensor signals and the known distance between subsequent markers of the reference pattern.

0042In another embodiment of the present invention, step a) comprises adapting the movement of said reference pattern with respect to said fixed reference point relative to the movement of the object. This embodiment is in particular useful for determining a velocity of an object, which object is not moving with a constant velocity during operation. In particular the embodiment is useful for an object which is intended to be stopped at a pre-selected position during operation.

0043In a further embodiment of the present invention, step a) comprises moving the reference pattern in a substantial opposite direction of the movement of the object. This embodiment is in particular useful for accurately determining a velocity of an object. By moving the reference pattern in a substantial opposite direction of the movement of the object the velocity of the object with respect to the reference pattern is increased compared to the velocity of the object with respect to the fixed reference point. As a result the frequency of the sampling of markers of the reference pattern is increased. So the frequency of the sampling of the velocity may also be increased. For slow moving objects with respect to the fixed reference point this embodiment is in particular advantageous.

0044In a further embodiment of the present invention, the method comprises additional steps f) providing a homing point on said reference pattern, g) sensing the homing position by first and second sensor, and h) determining the position of the object with respect to the fixed reference point. This embodiment is in particular useful for an object which is intended to be stopped at a pre-selected position during operation.

0045In another aspect of the present invention, the invention relates to an apparatus for determining a velocity of an object in a printing system, the apparatus system comprising an fixed reference point, a reference pattern, which is independently movable with respect to said fixed reference point and with respect to the object, a first sensor and a control unit, wherein the object is independently movable with respect to said fixed reference point and with respect to the reference pattern, the first sensor being configured to have substantially the same velocity as said object with respect to said reference pattern and configured to sense the reference pattern and provide a sensor signal to the control unit, which control unit is configured to receive and process the sensor signal of the first sensor and thereby determining the velocity of said object relative to said fixed reference point based on the sensor signal of the first sensor and on a determined velocity of the reference pattern with respect to said fixed reference point.

0046Hence, an apparatus configured for performing the method according to the present invention is provided.

0047In the apparatus according to an embodiment, the object is movable in a substantial linear movement and said velocity is the velocity in said linear movement. This embodiment is in particular useful for a scanning carriage, a recording medium or the like.

0048In the apparatus according to another embodiment, the object is rotatable around at least one axes and said velocity is the velocity in said rotational movement. This embodiment is in particular useful for a rotating roller, such as a driven transport roller for transporting a recording medium.

0049In the apparatus according to a further embodiment, said reference pattern is an endless pattern. This embodiment provides the advantage that the reference pattern may be continuously moved in one direction. The endless pattern enables a continuously determining of the velocity of the object.

0050In the apparatus according to another embodiment, a second sensor is provided, which second sensor is stationary with respect to said fixed reference point, and the velocity of said reference pattern with respect to said fixed reference point is determined by said second sensor. This embodiment is in particular useful for determining a velocity of a moving reference pattern. This embodiment provides the advantage that the control of the velocity of the reference pattern by driving means does not have to be very accurate.

0051In the apparatus according to another embodiment, said first sensor is an optical sensor and said reference pattern is an optical readable pattern. This embodiment enables the use of relatively simple and cheap sensors and reference patterns.

0052In the apparatus according to another embodiment, said first sensor is a magnetical sensor and said reference pattern is a magnetically readable pattern. This embodiment enables the use of relatively simple and cheap sensors and reference patterns.

0053In the apparatus according to another embodiment, said reference pattern comprises a homing point, wherein said homing point is distinguishable from said reference pattern for said first and optionally second sensor. This embodiment may be used for determining the position of the object. This embodiment is in particular useful for an object which is intended to be stopped at a pre-selected position during operation.

BRIEF DESCRIPTION OF THE DRAWINGS

0054Hereinafter, the present invention is elucidated with reference to the appended drawings showing non-limiting embodiments and wherein:

0055<figref idref="DRAWINGS">FIG. 1</figref> shows a schematic perspective view of an inkjet printing device in accordance with the present invention;

0056<figref idref="DRAWINGS">FIG. 2</figref> shows a front schematic view of a first embodiment of the method in accordance with the present invention;

0057<figref idref="DRAWINGS">FIG. 3</figref> shows a front schematic view of a second embodiment of the method in accordance with the present invention;

0058<figref idref="DRAWINGS">FIG. 4A</figref> shows a front schematic view of a third embodiment of a method according to the present invention;

0059<figref idref="DRAWINGS">FIG. 4B</figref> shows an enlarged side view along the line II-II in <figref idref="DRAWINGS">FIG. 4A</figref> of the third embodiment of a method according to the present invention;

0060<figref idref="DRAWINGS">FIG. 5A</figref> shows a front schematic view of a fourth embodiment of a method according to the present invention;

0061<figref idref="DRAWINGS">FIG. 5B</figref> shows an enlarged side view along the line II-II in <figref idref="DRAWINGS">FIG. 5A</figref> of the fourth embodiment of a method according to the present invention;

0062<figref idref="DRAWINGS">FIG. 5C</figref> shows an enlarged side view along the line II-II in <figref idref="DRAWINGS">FIG. 5A</figref> of the fifth embodiment of a method according to the present invention;

DETAILED DESCRIPTION OF THE INVENTION

0063In the drawings, same reference numerals refer to same elements. <figref idref="DRAWINGS">FIG. 1</figref> illustrates an inkjet printing system <b>2</b> in accordance with the present invention, wherein a curable hot-melt ink may be applied on a recording medium <b>20</b>. The printing system <b>2</b> comprises a medium advance means <b>8</b> and a recording means <b>5</b>.

0064In the illustrated example, the recording medium <b>20</b>, e.g. paper or any other suitable medium for image-wise receiving ink drops from the inkjet printer <b>2</b>, is movable by means of the medium advance means <b>8</b>. In the illustrated embodiment, the medium advance means comprises a platen <b>7</b>. The medium advance means <b>8</b> are configured to move the medium <b>20</b> with respect to the recording means <b>5</b> in a direction A, which is hereinafter referred to as medium advance direction A.

0065The recording means <b>5</b> comprises four print heads <b>12</b><i>a</i>-<b>12</b><i>d, </i>each comprising a set of nozzles <b>16</b>. The print heads <b>12</b><i>a</i>-<b>12</b><i>d </i>are configured to eject ink drops from the nozzles <b>16</b> such that the ink drops impinge on the medium <b>20</b> at a substantially predetermined position. The four print heads <b>12</b><i>a</i>-<b>12</b><i>d </i>may each be configured to eject ink of a same color, e.g. black ink to generate a black image on the recording medium <b>20</b>, or the print heads <b>12</b><i>a</i>-<b>12</b><i>d </i>may each eject ink of a different color, e.g. cyan, magenta, yellow and black (CMYK), for generating a full color image on the recording medium <b>20</b>.

0066The four print heads <b>12</b><i>a</i>-<b>12</b><i>d </i>are arranged on a carriage <b>11</b> which is movably supported on a guide rail <b>13</b>. Thus, the carriage <b>11</b> is movable in a scanning direction B. Hence, the four print heads <b>12</b><i>a</i>-<b>12</b><i>d </i>are movable with respect to the recording medium <b>20</b> in said scanning direction B. By suitably controlling the movement of the carriage <b>11</b> and the movement of the medium <b>20</b> in the medium advance direction A, while suitably controlling the ejection of ink drops from the nozzles <b>16</b> of the print heads <b>12</b><i>a</i>-<b>12</b><i>d, </i>the printer <b>2</b> is enabled to generate an image on the recording medium <b>20</b>. Such a printing method is well known in the art and is therefore not further elucidated herein.

0067A reference pattern <b>18</b> is arranged in parallel to the guide rail <b>13</b> in the scanning direction B of the carriage <b>11</b>. The reference pattern <b>18</b> is movable arranged with respect to a stationary frame (not shown) of the printing system <b>2</b>. The reference pattern <b>18</b> is moved according to the present invention.

0068The method for determining a velocity of an object in a printing system will be explained in detail in the <figref idref="DRAWINGS">FIGS. 2-5C</figref>.

0069It is noted that the method according to the present invention is not limited to use in an embodiment of a printer according to the exemplary, schematically illustrated printer of <figref idref="DRAWINGS">FIG. 1</figref>, but may as well be employed in any other suitably configured printing system <b>2</b>, such as a toner printing system. The method according to the present invention may as well be employed for paper handling means such as paper transport rollers in paper finishing parts of a printing system.

0070<figref idref="DRAWINGS">FIG. 2</figref> illustrates a front schematic view of a first embodiment of a method in accordance with the present invention for determining the velocity of a carriage.

0071An endless belt <b>32</b>, which comprises a reference pattern <b>34</b>, is arranged in parallel to the scanning direction B of the carriage <b>11</b>. The reference pattern <b>34</b> comprises markers at a fixed and constant distance.

0072A free rotating roller <b>36</b> and a transport roller <b>38</b> supports the endless belt at both sides of the guide rail <b>13</b>. The free rotating roller <b>36</b> is supported by the upper frame part <b>22</b> and the transport roller <b>38</b> is supported by the upper frame part <b>23</b>. Driving means <b>40</b> is connected to the transport roller <b>38</b> and is configured to control the rotating velocity of said transport roller <b>38</b>. By rotating the transport roller <b>38</b> the endless belt <b>32</b> is moved with a velocity corresponding to the rotation velocity of the transport roller <b>38</b>. The endless belt may be constituted of a material suitable for providing the same velocity at substantially all parts of the endless belt. The endless belt may be made of any suitable material, such as (reinforced) plastics materials, metals, ceramics or the like. The endless belt is selected such that the reference pattern <b>34</b> may be reliably transported between free rotating roller <b>36</b> and transport roller <b>38</b> and the distance between the markers in the reference pattern is constant during the movement including acceleration and deceleration of the reference pattern <b>34</b>.

0073In operation the endless belt <b>32</b> is moved with a certain velocity by the driving means <b>40</b>. So the reference pattern <b>34</b> is moved with said velocity between the free rotating roller <b>36</b> and the transport roller <b>38</b> in parallel to the scanning direction of the carriage <b>11</b>. In the first embodiment illustrated the reference pattern is moving in the direction indicated by arrow <b>46</b>.

0074The velocity of the reference pattern <b>34</b> with respect to the stationary frame may be determined by any means. In the embodiment of <figref idref="DRAWINGS">FIG. 2</figref> the velocity of the reference pattern <b>34</b> is determined by accurate control of the velocity by driving means <b>40</b>. The driving means <b>40</b> may be configured such that the reference pattern attains a pre-selected velocity. In this embodiment the driving means may comprise an internal encoder and a sensor (not shown). The control unit may be coupled to the internal encoder and sensor of the encoder for determining the driving velocity of the driving means. The control unit <b>30</b> based on the determined driving velocity of the driving means may predict the velocity of the reference pattern <b>34</b> with respect to the stationary frame <b>10</b>.

0075The first sensor <b>24</b> is supported by the carriage <b>11</b> and is arranged close to the reference pattern <b>34</b>, such that the sensor is able to sense the reference pattern <b>34</b>. The carriage <b>11</b> is movably arranged in a scanning direction B. The carriage <b>11</b> may move in a scanning direction B with a certain velocity by belt drive means <b>44</b>. Consequently the sensor <b>24</b> will move with the same velocity in the scanning direction B. The sensor <b>24</b> in operation senses the reference pattern <b>34</b>. In the illustrated embodiment the carriage <b>11</b> and sensor <b>24</b> move in the opposite direction of the movement of the reference pattern at the position of the sensor <b>24</b>. The relative movement of the sensor <b>24</b> with respect to the reference pattern <b>34</b> is the sum of the movement of the sensor <b>24</b> with respect to the stationary frame in direction +x and the movement of the reference pattern <b>34</b> with respect to the stationary frame in direction −x. So the relative velocity of the sensor <b>24</b> with respect to the reference pattern <b>34</b> is higher than the relative velocity of the sensor <b>24</b> with respect to the stationary frame. So the sensor <b>24</b> senses at a high frequency the markers of the reference pattern <b>34</b>. The sensor signal provided by the sensor <b>24</b> comprises at a corresponding frequency the marker information of the sensed reference pattern <b>34</b>. The sensor <b>24</b> is coupled to a control unit <b>30</b>. The control unit <b>30</b> determines the velocity of the carriage relative to the stationary frame based on the determined velocity of said reference pattern with respect to the stationary frame and based on the sensor signal.

0076The endless belt <b>32</b> of the first embodiment may be moved in operation with a constant velocity and direction by the driving means <b>40</b>. So the velocity is accurately controlled by said driving means <b>40</b>.

0077In an alternative embodiment the movement of the reference pattern <b>34</b> with respect to the stationary frame may be adapted relative to the movement of the carriage <b>11</b> with respect to said stationary frame <b>10</b>. In the illustrated first embodiment the carriage may move in reciprocating scanning movements. The carriage may turn at one end of the guide rail and move in the opposite direction. In response to the turning of the carriage the movement of the reference pattern <b>34</b> with respect to the stationary frame may also be turned in the opposite direction. So in an embodiment the direction of the movement of the reference pattern is adapted such that it moves in a substantial opposite direction of the scanning carriage <b>11</b>.

0078In an alternative embodiment the movement of the reference pattern <b>34</b> with respect to the stationary frame may be adapted relative to the estimated velocity of the carriage <b>11</b> with respect to said stationary frame <b>10</b>. For example in a quality printing mode for providing a high print resolution image the velocity of the carriage <b>11</b> may be decreased in order to enable a more accurate ink drop positioning process. The reference pattern <b>34</b> in this embodiment may move in opposite direction of the moving carriage <b>11</b>. In order to even further improve the accuracy of the determination of the velocity of the carriage <b>11</b> during this printing mode, the velocity of the reference pattern <b>34</b> respect to the stationary frame may be increased and maintained at a higher velocity.

0079<figref idref="DRAWINGS">FIG. 3</figref> illustrates a front schematic view of a second embodiment of the method in accordance with the present invention for determining the velocity of a carriage.

0080In this embodiment a second sensor <b>54</b> is arranged close to the reference pattern <b>34</b>, such that the sensor is able to sense the reference pattern <b>34</b>. The second sensor <b>54</b> is supported by mounting member <b>55</b>, which is fixed to the stationary frame <b>10</b>. The second sensor <b>54</b> is arranged such that in operation the position of the movable arranged first sensor <b>24</b> of the carriage <b>11</b> may not interfere with the position of the second sensor <b>54</b>. The control unit <b>30</b> is connected to second sensor <b>54</b>.

0081In operation sensor <b>54</b> senses the reference pattern <b>34</b> and provides a sensor signal to the control unit <b>30</b>. The control unit <b>30</b> may determine the velocity of the said reference pattern based on the sensor signal of sensor <b>54</b>. As in the first embodiment the control unit <b>30</b> may control the movement of the reference pattern <b>34</b> with respect to the stationary frame <b>10</b> by controlling the driving means <b>40</b>. The advantage of this embodiment is the direct determination of the velocity of the reference pattern <b>34</b>. So the driving means <b>40</b> in the second embodiment may be less accurate in controlling a velocity of the reference pattern <b>34</b>.

0082The second sensor <b>54</b> may be arranged to sense the same markers of the reference pattern as the first sensor <b>24</b>. In an embodiment the first sensor and second sensor may be both an optical sensor and the reference pattern an optical readable pattern. The first sensor and second sensor may be arranged at opposite sides of the reference pattern. The optical readable reference pattern may be optically (semi)-transparent between the markers and both sensors may be configured to sense the same markers of the optical readable reference pattern <b>34</b>. In the second embodiment the second sensor <b>54</b> is positioned at the same side facing the reference pattern as the first sensor <b>24</b>. This arrangement enables an even better combination of both sensors for sensing the reference pattern <b>34</b>.

0083<figref idref="DRAWINGS">FIG. 4A</figref> illustrates a front schematic view of a third embodiment of a method according to the present invention for determining the rotation velocity of a transport roller.

0084A medium advance means <b>8</b> comprises a bearing assembly. The bearing assembly is formed by two bearings <b>50</b>, <b>51</b> which rotatably support a platen <b>7</b> between two frame members <b>48</b>. A sheet support plate <b>54</b> is supported at both ends on the two frame members <b>48</b> and serves to support a sheet of a recording medium <b>20</b> (not shown) which is advanced in a direction A by platen <b>7</b>, referred to as medium advance direction A. A worm-type drive mechanism <b>56</b> for the platen <b>7</b> is arranged near the bearing <b>50</b>. An encoder disc <b>58</b> is rotatably arranged near the right bearing <b>51</b> and is rigidly connected to encoder driving means <b>60</b>. The encoder driving means <b>60</b> is held stationary with respect to the frame <b>10</b>. The encoder disc <b>58</b> may be -in an embodiment- axial supported by bearing <b>51</b>. The encoder disc <b>58</b> comprises a reference pattern <b>59</b>.

0085The encoder driving means <b>60</b> is configured to rotatably move encoder disc <b>58</b>. The encoder driving means <b>40</b> may rotate encoder disc <b>58</b> with a constant pre-selected velocity. A first sensor <b>62</b> is arranged near to the encoder disc <b>58</b>, such that the sensor is able to sense the reference pattern <b>59</b> and is supported by the platen <b>7</b>. So if the platen <b>7</b> rotates, the first sensor <b>62</b> rotates with the same rotational velocity as the platen <b>7</b>.

0086In <figref idref="DRAWINGS">FIG. 4B</figref> illustrates an enlarged side view along the line II-II in <figref idref="DRAWINGS">FIG. 4A</figref> of the third embodiment of a method according to the present invention of the third embodiment of a method according to the present invention for determining the rotation velocity of a transport roller.

0087The encoder disc <b>58</b> according to the third embodiment is constantly rotated in direction of arrow <b>63</b>. The encoder driving means <b>60</b> determines the rotational velocity of encoder disc <b>58</b>. The rotational velocity of the encoder disc <b>58</b> may be maintained at a high rotational velocity compared to the rotational velocity of the platen <b>7</b>. The reference pattern <b>59</b>, comprising markers at fixed angles is rotationally arranged on the encoder disc <b>58</b>.

0088The platen <b>7</b> in operation may mainly move in rotational direction of arrow <b>64</b> for moving a recording medium <b>20</b> in a direction A. The constant rotational movement <b>63</b> is opposite of the direction of the platen <b>7</b>.

0089The platen <b>7</b> rotates with a certain rotational velocity in direction of arrow <b>64</b>. The first sensor <b>62</b>, which is supported by the platen <b>7</b>, rotates with the same rotational velocity. The first sensor <b>62</b> senses the reference pattern <b>59</b> and provides a sensor signal to the control unit <b>30</b>.

0090In the third embodiment shown in <figref idref="DRAWINGS">FIG. 4A</figref> en <b>4</b>B, a control unit as described in relation to <figref idref="DRAWINGS">FIGS. 2 and 3</figref> may be employed for determining the rotational velocity of the platen relative to the stationary frame based on the determined rotational velocity of the encoder disc with respect to the stationary frame and based on the sensor signal.

0091<figref idref="DRAWINGS">FIG. 5A</figref> illustrates a front schematic view of a fourth embodiment of a method according to the present invention for determining the rotation velocity of a transport roller. In this embodiment a second sensor <b>66</b> is arranged close to the reference pattern <b>59</b> such that the sensor is able to sense the reference pattern <b>59</b>. The second sensor <b>66</b> is supported by mounting member <b>68</b>, which is fixed to the stationary frame <b>10</b>. The second sensor <b>66</b> is arranged such that in operation the position of the rotationally arranged first sensor <b>62</b> of the platen <b>7</b> may not interfere with the position of the second sensor <b>66</b>. The second sensor <b>66</b> may be arranged on the opposite side of the encoder disc <b>58</b>. In another embodiment the second sensor <b>66</b> may be arranged at the same side of the encoder disc <b>58</b> at a different distance from the axis of the platen as the first sensor <b>58</b>.

0092The control unit <b>30</b> is connected to second sensor <b>66</b>.

0093In operation sensor <b>66</b> senses the reference pattern <b>59</b> and provides a sensor signal to the control unit <b>30</b>. The control unit <b>30</b> may determine the rotational velocity of the said reference pattern based on the sensor signal of sensor <b>66</b>.

0094<figref idref="DRAWINGS">FIG. 5B</figref> illustrates an enlarged side view along the line II-II in <figref idref="DRAWINGS">FIG. 5A</figref> of the fourth embodiment of a method according to the present invention.

0095The second sensor <b>66</b> may be arranged to sense the same markers of the reference pattern as the first sensor <b>62</b>. In another embodiment the reference pattern <b>59</b> may comprise two sets of markers <b>59</b><i>a </i>and <b>59</b><i>b. </i>The markers <b>59</b><i>a </i>are arranged on the inner circle and comprise a lower resolution than the markers <b>59</b><i>b, </i>which are arranged on the outer circle of the encoder disc <b>58</b>. The first sensor <b>62</b> may sense the markers <b>59</b><i>a </i>and the second sensor <b>66</b> may sense the markers <b>59</b><i>b. </i>This embodiment provides improved accuracy for determining the rotational velocity of the platen <b>7</b> with respect to the encoder disc <b>58</b>. The rotational velocity of platen <b>7</b> changes fast for a stepwise advancing recording medium <b>20</b>.

0096In an embodiment the first sensor and second sensor may be both an optical sensor and the reference pattern an optical readable pattern. In the fourth embodiment the control unit <b>30</b> may be coupled to illumination means (not shown) for illuminating the reference pattern. The illuminating means may improve the accuracy of sensing the reference pattern by the optical sensors.

0097<figref idref="DRAWINGS">FIG. 5C</figref> illustrates an enlarged side view along the line II-II in <figref idref="DRAWINGS">FIG. 5A</figref> of the fifth embodiment of a method according to the present invention.

0098The reference pattern <b>59</b> comprises three sets of markers <b>59</b><i>a, </i><b>59</b><i>b </i>and <b>59</b><i>c. </i>The marker sets <b>59</b><i>a </i>and <b>59</b><i>b </i>both comprise a plurality of markers at fixed angle. The markers <b>59</b><i>a </i>are arranged on the inner circle and comprise a lower resolution than the markers <b>59</b><i>b, </i>which are arranged on the outer circle of the encoder disc <b>58</b>. In the fifth embodiment the first sensor is supported by the sensor member <b>63</b> and is arranged near the outer markers <b>59</b><i>b</i>. The first sensor <b>62</b> may sense the markers <b>59</b><i>b </i>and the second sensor <b>66</b> may sense the markers <b>59</b><i>a. </i>

0099The third set <b>59</b><i>c </i>comprises one marker, marking a fixed angle position of the encoder disc <b>58</b>. The third marker <b>59</b><i>c </i>may be used as a homing point for the determination of a rotational position (i.e. angular position) of the platen <b>7</b>. The first sensor <b>62</b> and second sensor <b>66</b> are configured to distinguish the marker <b>59</b><i>c </i>from the other markers <b>59</b><i>a </i>and <b>59</b><i>b. </i>Both sensor provide a sensor signal based on the sensing of markers <b>59</b><i>c </i>and <b>59</b><i>b</i>, <b>59</b><i>c. </i>The sensor signal provides information on both the sensing of the marker <b>59</b><i>c </i>and the sensing of the markers <b>59</b><i>b, </i><b>59</b><i>c. </i>The control unit <b>30</b> is coupled to both sensors <b>62</b>, <b>66</b>. In the method according to the fifth embodiment the control unit <b>30</b> is configured to determine the rotational position of the platen <b>7</b> based on the sensor signals provided by both sensors <b>62</b>,<b>66</b>. In a first stage at a certain time t<b>1</b> the second sensor <b>66</b> senses the marker <b>59</b><i>c </i>and provides a sensor signal to the control unit <b>30</b>. The marker <b>59</b><i>c </i>rotates with a constant velocity around the axis of the platen <b>7</b>. In a second stage at a certain time, characterized by a delta time t<b>1</b>, the second sensor <b>66</b> senses the next passing of the marker <b>59</b><i>c. </i>The second sensor again provides a sensor signal to the control unit <b>30</b>. The control unit <b>30</b> determines the characteristic time constant, delta time t<b>1</b>, based on the subsequent sensor signal in response to the marker <b>59</b><i>c. </i>The control unit <b>30</b> may determine the rotational velocity of the encoder disc based on the characteristic time constant of the marker <b>59</b><i>c </i>in combination of the sensor signal, indicating the sensing of the marker set <b>59</b><i>a. </i>So the combination of using markers set <b>59</b><i>a </i>and <b>59</b><i>c </i>enables a more accurate determination of the rotational velocity of the encoder disc.

0100Furthermore in an embodiment the control unit <b>30</b> may store the characteristic time constant. The control unit may use the stored time constant to compare next determined time constants. In the fifth embodiment the marker <b>59</b><i>c </i>rotates with a constant velocity. As a result the characteristic time constant delta t<b>1</b> is a periodic time constant.

0101In a third stage at a certain time t<b>2</b> the first sensor <b>62</b> senses the marker <b>59</b><i>c </i>and provides a sensor signal to the control unit <b>30</b>. In a fourth stage the control unit determines a time constant t<b>2</b>, based on the sensor signal of the first sensor <b>62</b> in response to the marker <b>59</b><i>c</i>. The control unit <b>30</b> may determine the time difference delta t<b>2</b> between t<b>1</b> and t<b>2</b> based on the sensor signals provided by first sensor and second sensor. So the delta t<b>2</b> is the time difference between the sensing of the marker <b>59</b><i>c </i>by the second sensor and the first sensor.

0102The control unit may determine the rotational position of the platen <b>7</b> with respect to the position of the frame based on the determined time constant delta <b>1</b> of the rotation of the encoder disc, and based on the time difference delta t<b>2</b>.

0103The method according to the invention enables an accurate step wise movement of the recording medium <b>20</b>. The start position of a stationary platen <b>7</b> is determined by the above indicated steps before starting the rotational movement of the platen <b>7</b>. The platen <b>7</b> is driven by worm-type drive mechanism <b>56</b> such that the recording medium is advanced in direction A. During the rotation of platen <b>7</b> the angular position of the platen <b>7</b> is constantly monitored by using the homing point <b>59</b><i>c </i>using the method of the fifth embodiment. At a certain moment the rotation of platen <b>7</b> is stopped at a desired angular position. After the platen <b>7</b> has been halted the constantly rotating encoder disc <b>58</b> enables the checking of the resulting angular position of the platen <b>7</b>. In an embodiment the control unit may check the resulting angular position of the platen <b>7</b> and, if desired, e.g. in case a mismatch is determined between the actual position and the desired position, may control the worm-type drive mechanism <b>56</b> in order to correct the angular position of platen <b>7</b>. This method for determining the position of the platen <b>7</b> provides an improved step wise advancing of a recording medium <b>20</b>.

0104Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. In particular, features presented and described in separate dependent claims may be applied in combination and any combination of such claims is herewith disclosed.

0105Further, the terms and phrases used herein are not intended to be limiting; but rather, to provide an understandable description of the invention. The terms “a” or “an”, as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two, or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language). The term coupled, as used herein, is defined as connected, although not necessarily directly.