WO2004088969A1

WO2004088969A1 - Dynamic position adjusting device

Info

Publication number: WO2004088969A1
Application number: PCT/IL2003/000269
Authority: WO
Inventors: Haim Livne; Craig Breen; Michael Plotkin
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2003-03-31
Filing date: 2003-03-31
Publication date: 2004-10-14
Also published as: AU2003215883A1

Abstract

Apparatus for optical imaging onto a moving surface comprising: a photosensitive surface movable in a first, cross-scan direction and having associated encoder markings indicating position in the cross-scan direction; a scanner that scans at least one scanning light beam across the photosensitive surface in a scan direction, approximately perpendicular to the cross-scan direction; an encoder light beam, whose position in the cross-scan direction is linked to the position of the scanning beam, that illuminates the encoder markings; a detector that receives the encoder light beam as modified by the encoder markings, to produce a position signal indicating the position of the encoder light beam with respect to the encoder markings;a control unit that receives the position signal and adjusts the relative position of the scanning light beam on the photosensitive surface in the cross-scan direction.

Description

DYNAMIC POSITION ADJUSTING DEVICE

FIELD OF THE INVENTION

The present invention relates generally to optical imaging on a moving surface and more particularly to optical imaging in electrophotography. BACKGROUND OF THE INVENTION

Many devices incorporate systems for example laser printers and photocopiers, which use optical imaging methods to write on a moving substrate. Typically such devices use a focused light beam (e.g. laser beam) to imprint an image on a rotating photo-conductive drum. It has been found that distortions occur in the resulting image due to uneven velocity of the photo-conductive drum during imaging, or a lack of perfect synchronization between the data being written and the rotation. This problem is even more noticeable in multi-color multipass systems, which require accurate alignment between passes.

U.S. patent 5,268,687 to Peled et al., the disclosure of which is incorporated herein by reference, describes apparatus to reduce distortion due to non-uniform drum speed. In the described apparatus, the light is focused on the drum using a servo controlled relay mirror with a detector to control the angle of the mirror. An encoder is coupled to the drum to report drum position relative to a desired position. A control circuit calculates the discrepancy between the drum position and the desired position and rotates the mirror to focus the light and compensate for variations in the drum speed. In general, in such systems, a beam or beams scan the surface in a scan direction that is generally perpendicular to a direction of motion of the surface. The direction of motion is generally referred to as the cross-scan direction. It is understood that since the surface moves during the scan, the scan lines written on the surface are only approximately perpendicular to the cross-scan direction. SUMMARY OF THE INVENTION

An aspect of some embodiments of the invention relates to a system that writes a latent image on a photosensitive surface while the surface is in motion.

In an exemplary embodiment of the invention, encoding markings at an edge (or other suitable portion) of the photosensitive surface or an extension thereof are illuminated by an encoder light beam whose position is related to the actual position of a writing beam on the surface being written. The encoder light beam is reflected from the encoder toward a detector. The detector detects variations in the reflected encoder light beam caused by the movement of the encoding markings relative to the illuminating encoder light beam. The detector outputs a signal to a control unit, which encodes the position of the surface, in the cross-scan direction, relative to the encoder light beam and thus relative to the writing light beam. The control unit compares the position to a base position to produce a signal to control the position of the writing light beam (and also the position of the related encoder beam) to compensate for the errors in movement of the surface. In an exemplary embodiment of the invention, a relay mirror transmits the writing light beam to the surface and adjustment of an angular position of the relay mirror adjusts the position of the writing beam to overcome errors caused from non-uniform motion of the surface.

In an exemplary embodiment of the invention, the light beam that illuminates the encoder is reflected from the same relay mirror that is used to adjust the position of the writing beam. Thus, adjusting the writing beam automatically adjusts the position of the encoder light beam.

In some embodiments of the invention, since corrections in the position of the mirror cause the encoder beam to move, the system is a closed loop system, as compared with the open loop system of U.S. patent 5,268,687.

In some embodiments of the invention, the photosensitive surface is shaped as a cylindrical drum that rotates around its central axis. Alternatively or additionally, the surface can be a semi flat surface, for example a conveyer belt system or flat surface.

There is thus provided, in accordance with an embodiment of the present invention, apparatus for optical imaging onto a moving surface comprising: a photosensitive surface movable in a first, cross-scan direction and having associated encoder markings indicating position in the cross-scan direction; a scanner that scans at least one scanning light beam across the photosensitive surface in a scan direction, approximately perpendicular to the cross-scan direction; an encoder light beam, whose position in the cross-scan direction is linked to the position of the scanning beam, that illuminates the encoder markings; a detector that receives the encoder light beam as modified by the encoder markings, to produce a position signal indicating the position of the encoder light beam with respect to the encoder markings; a control unit that receives the position signal and adjusts the relative position of the scanning light beam on the photosensitive surface in the cross-scan direction.

Optionally, the surface is shaped as a cylindrical drum.

In an embodiment of the invention, the encoder light beam continuously illuminates the encoding markings. In an embodiment of the invention, the encoder light beam illuminates the encoder intermittently. Optionally, the encoder light beam is comprised of an extension of the scan of the scanning light beam.

Optionally, the angular correction of said adjustment is greater than the spacing of a scan row.

Optionally, the encoder markings are arranged to measure position at a precision of at least 1/300 inches, at least 1/600 inches, at least 1/800 inches, at least 1/1200 inches, at least 1/1600 inches or at least 1/2400 inches.

In an embodiment of the invention, the position of the encoder light beam and the scanning light beam are reflected by a same mirror prior to impinging on the encoder marking and the photosensitive surface. Optionally, the control unit adjusts a coordinate of the mirror to adjust said relative position. Optionally, the coordinate is an angular coordinate of the mirror.

In an embodiment of the invention, the position of the encoder light beam and the scanning light beam are adjusted by a same prismatic element prior to impinging on the encoder marking and the photosensitive surface. Optionally, the control unit adjusts a coordinate of the prismatic element to adjust said relative position. Optionally, the coordinate is an angular coordinate of the prismatic element.

In an embodiment of the invention, the position of the encoder light beam and the scanning light beam are adjusted by a same lens prior to impinging on the encoder marking and the photosensitive surface. Optionally, the control unit adjusts a coordinate of the lens to adjust said relative position.

Optionally, the control unit is operative to correct the relative position to an accuracy and precision better than a scan line spacing. Optionally, the encoder selectively reflects the encoder light beam to detector.

Alternatively, the encoder selectively attenuates the encoder light beam prior to its detection by the detector.

BRIEF DESCRIPTION OF THE DRAWINGS Particular non-limiting embodiments of the invention will be described with reference to the following description of embodiments in conjunction with the figures. Identical structures, elements or parts which appear in more than one figure are preferably labeled with a same or similar number in all the figures in which they appear, in which:

Fig. 1 is a schematic illustration of a system for optical imaging onto a moving surface, in accordance to an exemplary embodiment of the invention; Fig. 2 is a schematic block diagram illustration of a control for implementing the apparatus according to an exemplary embodiment of the invention; and

Fig. 3 is a flow diagram of the process performed by a control unit according to an exemplary embodiment of the invention. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Fig. 1 is a schematic illustration of an imaging system 100 for optically imaging onto a moving surface according to an exemplary embodiment of the invention. In the illustrated embodiment, an optical image source 10 provides a scanning light beam 5 (shown in several position, A, B and C as will be described below) to sweep across a photoreceptive surface and create one or more rows of a latent image. In some embodiments of the invention the light source comprises a stationary laser beam or beams, reflected from a rotating polygon to provide the scanning beam or beams, as is known in the art. Alternatively or additionally, other light sources and/or scanning methods can be used.

In some embodiments of the invention, the scanning beam is directed toward a relay mirror 55, which reflects the light beam onto a photoreceptive surface for example a photoreceptive drum 30 as shown in Fig. 1. Alternatively or additionally, other shapes and surfaces can be used, for example a semi flat surface controlled by a conveyer belt system or a flat surface. For the illustrated embodiment, photoreceptive drum 30 comprises a cylinder with a central axis 90. In some embodiments of the invention photoreceptive drum 30 rotates at a constant angular speed around its central axis to allow the scanning beam to create sequential rows (or rows of rows, when multiple parallel rows are scanned together) on the circumference of the drum producing a latent image. hi an exemplary embodiment of the invention, relay mirror 55 is mounted on a rotatable axis, which allows adjusting the angle of the relay mirror 55 relative to the light beams 5 from the optical image source 10. Optionally, one end of the relay mirror axis is mounted on a ball bearing 45 and the other end is mounted on a servo-motor 25, which controls the angle of the mirror. Changing the angle of the relay mirror 55 changes the position on photoreceptive drum 30 that is illuminated by the scanning light beam. In some embodiments of the invention, the angular correction due to adjusting the relay mirror can be greater than the scan row spacing or even greater than several scan rows.

In an exemplary embodiment of the invention, an end of drum 30 is marked with a column of encoding markings 35. Optionally, the encoding markings are evenly positioned, liked a ruler, on the circumference of photoreceptive drum 30. Alternatively, position coding is used. In some embodiments of the invention, the number of encoding markings is similar to the image resolution, for example 300, 600 or 800 per inch. Alternatively or additionally, a smaller or greater number of encoding markings, such as 300, 600, 800, 1200, 1600 or 2400 per inch can be used to allow finer or less fine adjustments relative to the writing accuracy. For example the encoding markings can be at a density of 2400 marks per inch with an image resolution of only 800 dots per inch, this will allow adjusting the drum by one third dot increments. In some embodiments of the invention, the adjustment accuracy is not proportional to the image resolution, for example with encoding markings at an accuracy of 500 marks per inch relative to an image accuracy of 600 dots per inch.

In an exemplary embodiment of the invention, an encoder light beam 15 is transmitted via relay mirror 55 to impinge on column 35. A detector 40 (e.g. photo-diode) is positioned near column 35 to detect the reflection of encoder light beam 15 from the encoding markings.

Alternatively, the encoder is a transmission type encoder and the detector is position within the periphery of the drum, such that it detects light that passes through the encoder markings. It is noted that both writing light beams 5 and encoder light beam 15 are reflected from the relay mirror. Thus, if the relay mirror is rotated, any correction in the position of the writing light beams will also adjust the position of the encoder light beam by a similar amount. This results in a closed loop system, without hysteresis.

As mentioned above, in Fig. 1, scanning light beam (or multiple light beams) 5 is shown in three positions:

A starting position, position A;

An image beginning position, position B; and

An image end position, position C.

Between position B to position C scanning light beam 5 is reflected from relay mirror 55 on to photoreceptive drum 30 in order to form a latent image. Beams 5 are modulated by a modulator (not shown) in optical image source 10, to impress a desired image on drum 30.

In an exemplary embodiment of the invention, just prior to reaching the start of the scan on the drum, scanning light beam 5, in position 5A is detected by a start of scan (SOS) detector 60. In some embodiments of the invention, SOS detector 60 comprises for example a photo-diode. Alternatively or additionally, other light detectors can be used, for example a light relay which passes on the light signal. Optionally, SOS detector 60 is positioned to receive the start of scan beam 5 A and produce a signal (e.g. an electronic signal), which serves to indicate to the system the start of each scan, for synchronization of the data modulated onto beam 5 with the start of scan and for indicating a reference time for determining rotational position of the drum.

In an exemplary embodiment of the invention, the signal from detector 40 is transferred over line 75 to a control unit 50. Additionally, the signal from SOS detector 60 is transferred over line 70 to control unit 50. In some embodiments of the invention, line 70 and line 75 are electric wires. Alternatively, line 70 and line 75 may be fiber optic wires for a light signal or other materials for transferring other types of signals. In some embodiments of the invention, the signals may be transferred by wireless methods, for example Bluetooth or other methods. In some embodiments of the invention, control unit 50 accepts the signals from SOS detector 60 and from detector 40 and outputs control instructions on line 80 to a mirror control

20. Mirror control 20 controls servo motor 25, which adjusts the angle of relay mirror 55.

Fig. 2 is a schematic block diagram of a control unit 200 for implementing the optical imaging apparatus according to an exemplary embodiment of the invention. In an exemplary embodiment of the invention, control unit 50 comprises two signal counters:

1. A counter 210, which is incremented by the SOS signal from SOS detector 60; and

2. A counter 230, which is incremented by the signal from detector 40 representing the rotation of photoreceptive drum 30. In some embodiments of the invention, counter 210 and counter 230 are compared by a comparator 220, for example, by adding counter 210 to the negative of counter 230 as shown in Fig. 2, in order to determine if the scanning position is in alignment with the position of photoreceptive drum 30. The results of the comparison are optionally converted from a digital signal to an analog signal by a D/A converter 240. The analog signal is output on line 80 to mirror control 20. In an exemplary embodiment of the invention, mirror control 20 comprises:

1. An analog filter 250 to smooth the signal to be amplified and remove the pulse transients.

2. An amplifier 260 to amplify the signal to control relay mirror 55 using servo motor 25.

In an exemplary embodiment of the invention, relay mirror 55 adjusts the positions of impingement of the light beams on the photoreceptive surface of drum 30 and directly affects the light beam detected by detector 40 as described above.

In some embodiments of the invention, control unit 50 receives an initialization signal via line 65 from image source 10. The initialization signal is optionally, transmitted at the beginning of a separation and/or at the beginning of a page. Optionally, when receiving an initialization signal control unit 50, resets counters 210 and 230 and repositions relay mirror 55 at its initial position. Resetting control 50 prevents relay mirror 55 from incrementally reaching an extreme position, where it will not be able to adjust the scanning light beam 5 in a specific direction.

Fig. 3 is a flow diagram 300 of A process performed by control unit 50 according to an exemplary embodiment of the invention.

In an exemplary embodiment of the invention, for each scan sweep of scanning light beam 5, light beam 5 starts in position A (310) and sends a signal to counter 210. Counter 210 accepts the signal (320) and increments its value by a preset number. Optionally the preset number is equal to the number by which counter 230 is incremented during the time of a single scan sweep for an ideal uniformly rotating photoreceptive drum 30.

In an exemplary embodiment of the invention, photoreceptive drum 30 is continuously rotating (330) at an approximately constant speed. Counter 230 is incremented (340) by the signal from detector 40 according to the encoding marks on photoreceptive drum 30. At any specific time, counter 230 holds a value representing the position of photoreceptive drum 30. In some embodiments of the invention, the two counters are compared (350) once per scan sweep, for example at the beginning of the row (e.g. position A) in order to adjust (360) relay mirror 55 and overcome any latency due to non uniform or slightly incorrect rotation speed of photoreceptive drum 30.

In the embodiment of the invention described above, the deviation between the position of light beam 5 from its actual position to the required position as determined by control unit 50, is based on a direct measurement of encoder light beam 15 following a similar path as scanning light beam 5. This method can give more accurate measurements since a relative deviation is determined based on the actual position, whereas the angular disposition of relay mirror 55 does not need to be measured. While the invention has been described in an embodiment in which the controlled element is an adjustable relay mirror, other possible methods of controlling the circumferential positions of the writing and encoder beams so that their relationship is constant or known can be used. Alternatively to using a relay mirror, the beam may be positioned in the cross-scan direction rotating a reflecting or transmitting prism or by off axis movement of a lens in the light path. Such rotation or movement can take place in any appropriate position in the light path and may take place either before or after a scanning mechanism (conventionally a polygon, but can be other apparatus known in the art) that converts an input light beam into a scanned beam. While the present invention has been described in an embodiment in which the encoder marking are continuously illuminated by a beam whose cross-scan position is linked to that of the scanning beam, an intermittent beam may also be used. For example, the scarrning beam may be configured to scan past the image portion of the photoreceptive surface to the encoder markings. This provides an intermittent light signal from the encoder markings. In this embodiment of the invention, encoder markings that provide a continuous variation of signal with position are optionally used. For example encoder markings that provide a triangular or sinusoidal variation of signal with angular position can be used to provide indications of position which can be used, using methods known in the art, to determine positions, despite the signal being intermittent. Optionally, an encoder with such markings can be used with continuous illumination to provide more accurate tracking.

While the present invention has been described in terms of writing a latent image on a photoreceptor, the invention is also applicable to writing an image on photographic film or to other imaging uses. The present invention has been described using non-limiting detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. It should be understood that features and/or steps described with respect to one embodiment may be used with other embodiments and that not all embodiments of the invention have all of the features and/or steps shown in a particular figure or described with respect to one of the embodiments. Variations of embodiments described will occur to persons of the art.

It is noted that some of the above described embodiments may describe the best mode contemplated by the inventors and therefore include structure, acts or details of structures and acts that may not be essential to the invention and which are described as examples. Structure and acts described herein are replaceable by equivalents which perform the same function, even if the structure or acts are different, as known in the art. Therefore, the scope of the invention is limited only by the elements and limitations as used in the claims. When used in the following claims, the terms "comprise", "include", "have" and their conjugates mean "including but not limited to".

Claims

1. Apparatus for optical imaging onto a moving surface comprising: a photosensitive surface movable in a first, cross-scan direction and having associated encoder markings indicating position in the cross-scan direction; a scanner that scans at least one scanning light beam across the photosensitive surface in a scan direction, approximately perpendicular to the cross-scan direction; an encoder light beam, whose position in the cross-scan direction is linked to the position of the scanning beam, that illuminates the encoder markings; a detector that receives the encoder light beam as modified by the encoder markings, to produce a position signal indicating the position of the encoder light beam with respect to the encoder markings; a control unit that receives the position signal and adjusts the relative position of the scanning light beam on the photosensitive surface in the cross-scan direction.

2. Apparatus according to claim 1, wherein said surface is shaped as a cylindrical drum.

3. Apparatus according to claim 1 or claim 2, wherein the encoder light beam continuously illuminates the encoding markings.

4. Apparatus according to claim 1 or claim 2 wherein the encoder light beam illuminates the encoder intermittently.

5. Apparatus according to claim 4 wherein the encoder light beam is comprised of an extension of the scan of the scanning light beam.

6. Apparatus according to any of the preceding claims, wherein the angular correction of said adjustment is greater than the spacing of a scan row.

7. Apparatus according to any of the preceding claims, wherein the encoder markings are arranged to measure position at a precision of at least 1/300 inches.

8. Apparatus according to any of the preceding claims, wherein the encoder markings are arranged to measure position at a precision of at least 1/600 inches.

9. Apparatus according to any of the preceding claims, wherein the encoder markings are arranged to measure position at a precision of at least 1/800 inches.

10. Apparatus according to any of the preceding claims, wherein the encoder markings are arranged to measure position at a precision of at least 1/1200 inches.

11. Apparatus according to any of the preceding claims, wherein the encoder markings are arranged to measure position at a precision of at least 1/1600 inches.

12. Apparatus according to any of the preceding claims, wherein the encoder markings are arranged to measure position at a precision of at least 1/2400 inches.

13. Apparatus according to any of the preceding claims wherein the position of the encoder light beam and the scanning light beam are reflected by a same mirror prior to impinging on the encoder marking and the photosensitive surface.

14. Apparatus according to claim 13 wherein the control unit adjusts a coordinate of the mirror to adjust said relative position.

15. Apparatus according to claim 14 wherein the coordinate is an angular coordinate of the mirror.

16. Apparatus according to any of claims 1-12 wherein the position of the encoder light beam and the scanning light beam are adjusted by a same prismatic element prior to impinging on the encoder marking and the photosensitive surface.

17. Apparatus according to claim 16 wherein the control unit adjusts a coordinate of the prismatic element to adjust said relative position.

18. Apparatus according to claim 17 wherein the coordinate is an angular coordinate of the prismatic element.

19. Apparatus according to any claims 1-12 wherein the position of the encoder light beam and the scanning light beam are adjusted by a same lens prior to impinging on the encoder marking and the photosensitive surface.

20. Apparatus according to claim 19 wherein the control unit adjusts a coordinate of the lens to adjust said relative position.

21. Apparatus according to any of the preceding claims wherein the control unit is operative to correct the relative position to an accuracy and precision better than a scan line spacing.

22. Apparatus according to any of the preceding claims wherein the encoder selectively reflects the encoder light beam to detector.

23. Apparatus according to any of the preceding claims wherein the encoder selectively attenuates the encoder light beam prior to its detection by the detector.