GB2136733A - Asynchronous printing - Google Patents

Abstract

A position or velocity encoder 52 with a digital pulse output is used to synchronize the timing of a printing device with the motion of the photoreceptor or copy sheet 50. The medium may be xerographic, and the printing device may be a full line optical printer. The printer may be an ink-jet printer. <IMAGE>

Description

SPECIFICATION Asynchronous printing The invention relates to the synchronization of a raster output scanner and a moving light sensitive medium and, more particularly, to an asynchronous printing system where the moving medium is provided with a position encoder, the output of which varies the rate of the raster output scanner to match the varying speed of the medium.

Most copier/duplicator systems in use today achieve printing of a two dimensional output by printing one line at a time, and then moving the printing medium in the orthogonal direction to produce a raster. The line printing is done either by linearly scanning a point across the line, such as is done with a laser scanner and xerogrpahy, or by printing a full line at once, as might be done with ink jet. In either case, if the timing of the scan line is such that a constant time interval occurs between line starts (a synchronous printer), velocity variations in the printing medium or photoconductor will result in nonuniform spacing of the print lines on the output, thus giving rise to a banding structure in the output image.An example of a synchronous printing mechanism is polygon ROS printer, where the line timing is set by the polygon rotational speed rather than controlled by the electronics.

This is especially critical in the digital printing of halftone patterns where one of the major difficulties is the inherently high sensitivity of these patterns to dot location errors in the printer. A very small positional error on any printed dot will show up in the halftone, particularly if it is constant along a print line, as occurs for line banding errors caused by nonuniform velocity errors in the transport which moves the paper or photocnductor past the printing mechanism.

One solution to this problem is to accurately control the polygon and photoconductor speeds with servo systems. In the less costly versions, there is a small but significant low frequency lead or lag to be expected, while more accurate servo systems are large and expensive. However, even with a more powerful servo system, transient fluctuations may still occur. This would commonly happen in a copier, for instance, as the paper handling machinery starts and stops, creating mechanical vibrations and electrical fluctuations in the unit.

This invention uses a position or velocity encoder to adjust the line scan timing in an asynchronous printer to compensate for speed fluctuations of the photoreceptor to achieve high quality printing.

The photoreceptorofthe printer may be in the form of a drum, plate or belt.

For the case of a drum photoconductor, the drum drives an angular position encoder, the digital output of which provides the printer and associated circuitry with information on the timing of the scan.

The cost of the addition to the system of a position encoder and its associated mechanical and electrical parts is less than the cost of a drum rotation servo system, in addition to providing higher quality by removing transients. The result is superior copy quality and a reduced unit cost.

The details of this invention will be more clearly seen in reference to the following drawings, in which: Figure lisa simplified diagram of the system.

Figure2 is a schematic side view of an electron optic line printer including a proximity coupled TIR multi-gate light valve which is one embodiment of the present invention.

Figure 3 is a schematic bottom plan view of the electro-optic line priner shown in Figure 2.

Figure 4 is an enlarged side view of a TIR light valve for the electro-optic line priner of Figures 2 and 3.

Figure 5 is an enlarged cutaway bottom view of the TIR light valve of Figure 4 showing a pattern of individually addressable electrodes.

Figure 6 is a simplified block diagram of a system for applying differentially encoded serial input data to the individually addressable electrodes of the electrode pattern shown in Figure 5.

Figure 7 is an enlarged and fragmentary schematic end view of the TIR light valve shown in Figure 4 to better illustrate the proximity coupling of the eletrodes to the electro-optic element and the interaction which occurs within the electro-optic element between the light beam and the electric fringe fields.

Figure 8 is an enlarged and fragmentary schematic plan view of the electrode pattern of Figure 5 as embodied on a silicon integrated circuit in accordance with this invention.

Figure 9 is an enlarged cutaway bottom view of a TIR light valve having an alternative electrode pattern.

In Figure 1, the photoconductive drum 50 may be any electrostatic or xerographic drum rotating as shown. The shift 51 is mechanically coupled to a rotary position encoder 52 which generates digital position information and transmits it to suitable electronics 53 which converts the counts into line start information. This information is then used to initiate a scan line using an asynchronous print head of any kind. In this way, equally spaced scan lines will be formed on the photoconductor even in the presence of low frequency and transient fluctuations of the speed of photoconductor drum 50.

Various position encoders 52 are available off-theshelf from vendors. One example is the Incremental Optical Encoder, Type L25, supplied by BEI Electronics, Inc, 7230 Hollister Ave., Goleta, California 93017. This particular encoder features electronic multiplication to increase the precision of the output by the interpolation.

Interpolating encoders are not suited for velocity servos since there is a reduced pattern regularity and a lack of an absolute rotational reference point.

However, they are ideal for generating position information because of the greater degree of rotational position resolution.

In the referenced encoder, there are 50,800 counts per shaft rotation, a multiplication factor of 5, a transition accuracy of +1 count, and a frequency response of 500 Khz.

As an alternative, the Model 8635 Rotary Encoder manufactured by Teledyne Gurley, 514 Fulton St., Troy, New york 12181, could be used.

An alternative to a shaft encoder is any encoder which monitors the photoconductor angular speed directly.

The output of the position encoder 52 is then sent to a counting circuit 53 which produces a Line Start pulse for each predetermined number of encoder pulses. Thus, the spacing between scan lines on the drum 50 will be equal regardless of the velocity variations of the drum 50.

Afull line print head suitable for this use is described in Figures 2 through 9.

In Figures 2 and 3, there is an electro-optic line printer 11 comprising a multi-gate light valve 12 for exposing a photosensitive recording medium 13 in an image configuration. The recording medium 13 is depicted as being a photoconductively coated xerographic drum 14 which is rotated (by means not shown) in the direction of the arrow. It nevertheless will be evident that there are other xerographic and non-xerographic recording media that could be used, including photoconductively coated xerographic belts and plates, as well as photosensitive film and coated paper in web or cut sheet stock form.

The recording medium 13 should, therefore, be visualized in the generalized case as being a photosensitive medium which is exposed in an image configuration while advancing in a cross line or line pitch direction relative to the light valve 12.

As shown in Figures 4 and 5, the light valve 12 includes an electro-optic element 17 and a plurality of individually addressable electrodes 1 8a - 1 8i. For a total internal reflection (TIR) mode of operation as illustrated, the electro-optic element 17 typically is a y cut crystal of, for example, LiNbO3 having an optically polished reflecting surface 21 which is integral with and disposed between optically polished input and output faces 22 and 23, repectively. The electrodes 18a - 18i are intimately coupled to the electro-optic element 17 adjacent the reflecting surface 21 and are distributed across essentially the full width thereof.Typically, the electrodes 1 spa - 1 8i are 1-30 microns wide and are on centers which are more or less equidistantly separated to provide a generally uniform interelectrode gap spacing of 1-30 microns. In this particular embodiment, the electodes 18a - 18i extend generally parallel to the optical axis of the electro-optic element 17 and have projections of substantial length along that axis.

Alternatively, the electrodes 18a - 18i could be aligned at the so-called Bragg angle relative to the optical axis of the electro-optic element 17. As will be appreciated, if the electrodes 18a - 18i are aligned parallel to the optical axis of the electro-optic element 17, the light valve 12 will produce a diffraction pattern which is symmetrical about the zero order diffraction component. If, on the other hand, the electrodes 18a - 18i are atthe Bragg angle relative to the optical axis of the electro-optic element 17, the light valve 12 will produce an asymmetrical diffraction pattern.

Briefly reviewing the optical of the line printer 11 depicted in Figures 2 - 5, a sheet-like collimated beam of light 24 from a suitable source, such as a laser (not shown), is transmitted through the input face 22 of the electro-optic element 17 at a grazing angle of incidence relative to the reflecting surface 21. The light beam 24 is brought to a wedge shaped focus (by means not shown) at approximately the center line of the reflecting surface 21 and is totally internally reflected therefrom for subsequent transmission through the outut face 23. As will be seen, the light beam 24 illuminates substantially the full width of the electro-optic element 17 and is phase front modulated while passing therethrough in accordance with the differentially encoded data samples applied to the electrodes 18a - 18i.

More particularly, as shown in Figure 6, serial input data samples, which represent picture elements for successive lines of an image, are applied to a differential encoder 25 at a predetermined data rate. The encoder 25 differentially encodes the input samples on a line-by-line axis in response to control signals from a controller 26, and a multiplexer 27 ripples the encoded data samples onto the electrodes 18a - 18i art a ripple rate which is matched to the data rate in response to further control signals from the controller 26. The input data may, of course, be buffered (by means not shown) to match the input data rate to any desired ripple rate.

Additionally, the input data may be processed (by means also not shown) upstreamof the encoder 25 for text editing, formatting or other purposes, provided that the data samples for the ultimate image are applied to the encoder 25 in adjacent picture element sequence.

Referring to figure 7, the electrode to electrode voltage drops create localized fringe fields 28 within an interaction region 29 of the electro-optic element 17, and the fringe fields 28 cause localized variations in the refractive index of the electro-optic element 17 widthwise of the interaction region 29. The voltage drop between any adjacent pair of electrodes, such as 18b and 18e or 18e and 18d,determinesthe refractive indexforthe portion of the interaction region 29 which bridges between thoe two electrodes. Hence, the refractive index variations within the interaction region 29 faithfully represent the input data samples appearing on the electrodes 18a 1 8i in differentially encoded form at any given point in time.It therefore follows that the phase front of the light beam 24 (Figure 4) is sequentially spatially modulated in acordance with the data samples for successive lines of the image as the light beam 24 passes through the interaction region 27 of the electro-optic element 17.

Returning for a moment to Figures 2 and 3,to expose the recording medium 13 in an image configuration, there suitably are Schlieren central dark field imaging optics 31 which are optically aligned between the electro-optic element 17 and the recording medium 13 for imaging the light beam 24 onto the recording medium 13. The imaging optics 31 convert the spatial phase front modulation of the light beam 24 into a correspondingly modulated intensity profile and provide any magnification that is required to obtain an image of a desired width.To accomplish that, the illustrated imaging optics 31 include a field lens 34 for focusing the zero order diffraction components 32 of the phase front modulated light beam 24 onto a central stop 35 and an imaging lens 36 for imaging the higher order diffraction components onto the recording medium 13; i.e., the image plane for the light valve 12. The field lens 34 is optically aligned between the electrooptic element 17 and the stop 35 so that substantially all of the zero order components 32 of the light beam 24 are blocked by the stop 35. The higher order diffraction components of the light beam 24 scatter around the stop 35 and are collected by the imaging lens 36 which, in turn, causes them to fall onto the light valve image plane defined by the recording medium 13.

To summarise, as indicated in Figure 3 by the broken lines 39, each neighbouring pair of electrodes, such as 1 8b and 1 8c (Figure 7), cooperates with the electro-optic element 17 and with the p-sensitive readout optics 31 to effectively define a local modulator for creating a picture element at a unique, spatially predetermined position along each line of the image. Accordingly, the number of electrodes 1 8a - 1 8i determines the number of picture elements that can be prined per line of the image. Moreover, by sequentially applying successive sets of differentially encoded data samples to the electrodes 18a - 18i while the recording medium 13 is advancing in a cross line direction relative to the light valve 12, successive lines of the image are printed.

As best shown in Figure 8, the electrodes 18a - 18i are defined by a suitably patterned elelctrically conductive layer, generally indicated by 30, which is deposited on and a part of an integrated electrical circuit 31, such as a LSI (large scale integrated) silicon circuit, to make electrical contact to the integrated drive electronics 32b - 329. For example, as illustrated, the multiplexer 27 is embodied in the integrated circuit 31, and the electrodes 18a - 18i are an extension of the metalization of polysilicon layer 30 which is used to make electrical corrections to the output transfer gates or pass transistors 32b - 32g and other individual components (not shown) of the multiplexer 27.The pass transistors 32b - 329 and the other components of the multiplexer 27 are formed on the integrated circuit 31 by using more or less standard LSl components fabrication techniques, and the metalization or polysilicon layer 30 is thereafter deposited on the outer surface 33 of the integrated circuit 31. An etching process or the like is then used to pattern the electrically conductive layer 30 as required to provide electrically independent connections to the electrically independent components of the multiplexer 27 in keeping with standard practices and to form the electrically independent electrodes 18a - 18i (only the elecrrodes 18b - 18g can be seen in Figure 8) in keeping with this invention.For instance, the data transfer lines 34b 34g for the pass transistors 32b - 32g are defined in the metalization or polysilicon layer 30 by the same etching process which is used to define the electrodes 18a - 18i.

Referring again to Figure 7, the electrodes 18a - 18i are proximity coupled to the electro-optic element 17 to enable the light valve 12 to perform as previously described. To carry out the proximity coupling, a bonding agent, such as the clamp schematically represented by the arrows 42 and 43, is engaged with the electro-optic element 17 and with the silicon integrated circuit 31 to urge the electrodes 18a - 18i into pressure contact with the reflecting surface 23 of the electro-optic element 17. An adhesive or suction might be used as the bonding agent in place of or in combination with the clamp 42 and 43.Regardless of the bonding agent selected, a small gap 44 is likely to exist over an appreciable portion of the interface between the electrodes 18a - 18i and the reflecting surface 23 due to unavoidable imperfections in the flatness of those elements and to the presence of any foreign matter, such as dust particles (not shown), which may be entrapped in the gap 44. The interelectrode gap spacing of the electrodes 18a - 18i must be sufficiently large relative to the maximum width of the gap 44 to ensure that the fringe fields 28 span the gap 44 and penetrate the electro-optic element 17 to interact with the light beam 24 as previously described.

If a significant portion of the overall surface area of the electrodes 1 8a - 18i is in direct contact with the reflecting surface 21 of the electro-optic element 17, the light beam 24 may experience an unacceptable level of spurious phase and amplitude modulation under quiescent conditions (i.e., in the absence of any voltage drops across the electrodes). To avoid that, a thin dielectric layer 49 of, say, SiO2, may be overcoated either on the reflecting surface 21, as shown, or on the electrode bearing surface of the integrated circuit 31 (not shown), thereby isolating the electrodes 18a - 18i from the reflecting surface 21.The dielectric layer 49 is selected to have an index or refraction which is less than the quiescent index of refraction of the electro-optic element 17, and the thickness of the dielectric layer 49 is controlled so that it is substantially less than the interelectrode gap spacing (e.g. an SiO2 layer 49 having a thickness on the order of 1000 angstroms will provide ample isolation to avoid electrode induced spurious modulation of the light beam 24).

Thus, the fringe fields 28 may be coupled into the electro-optic element 17 via the dielectric layer 49 without suffering an unacceptable degree of attenuation.

As shown in Figure 9, the present invention may use an alternting pattern of individually addressable electrodes 18a1 - 18i1 and ground plane electrodes 19a1 - 1911. As is known, such an electrode pattern may be used if the input data samples are not differentially encoded.

Claims (7)

1. An asynchronous line printer comprising: an encoder for producing digital output pulses, the frequency or number of which is a function of the change in photosensor or sheet position; counter means for producing one line start pulse for a predetermined number of digital output pulses produced by said encoder; and a device for exposing or printing a line on the photosensor or sheet on the receipt of a line start pulse to produce an exposed or printed line which is spaced a predetermined distance from the previous exposed or printed line.

2. The system of Claim 1 wherein the device is a raster output scanner.

3. The system of Claim 1 wherein the device is a full line printer.

4. The apparatus of Claim 1 wherein the device is an electro-optic element which exposes said photosensor a full line at one time.

5. The apparatus of Claim 1, wherein the device is an ink jet printer.

6. The apparatus of any preceding Claim, wherein said photosensor is a drum, belt, plate or film.

7. The apparatus of any preceding Claim, wherein said position encoder provides increased positional resolution by using electronic multiplication.