Abstract:
The present system counteracts the distorting effect of a bi-directional print head by reducing the visual effect of a satellite drop displacement when the print head is traveling in a direction that increases the satellite displacement from a main drop. The system can also counteract the distorting effect of the bi-directional print head by increasing the visual effect of a satellite drop misplacement when the print head is traveling in a direction that decreases the satellite misplacement from the main drop. The satellite drop displacement can be adjusted by correcting a measured ink temperature, and thereby, an amount of pre-pulsing prior to ejection of the ink.

Description:
This Non-Provisional Application claims the benefit of U.S. Provisional Application No. 60/200,875 entitled “METHOD AND APPARATUS FOR PREVENTING SATELLITE INDUCED BANDING IN AN INK JET PRINTER USING PRE-PULSE COMPENSATION” which was filed on May 1, 2000 and is hereby incorporated by reference in its entirety. The Applicants of the Provisional Application are Roger G. Markham and Yonglin Xie. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to ink jet printers. 
     2. Description of Related Art 
     A thermal ink jet print head selectively ejects droplets of ink from a plurality of drop emitters to create a desired image on an image receiving member, such as a sheet of paper. The print head typically comprises an array of the drop emitters that convey ink to the image receiving member. In a carriage-type ink jet print head, the print head moves back and forth relative to the image receiving member to print the image in swaths which are many pixels tall. 
     An ink jet print head typically comprises a plurality of ink passageways, such as capillary channels. Each channel has a nozzle and is connected to an ink supply manifold. Ink from the manifold is retained within each channel until, in response to an appropriate signal applied to a resistive heating element in each channel, the ink and a portion of the channel adjacent to the heating element is rapidly heated and vaporized. Rapid vaporization of some of the ink in the channel creates a bubble that causes a quantity of ink, i.e., an ink droplet, to be ejected from the emitter to the image receiving member. When an ink droplet is ejected at a velocity greater than approximately 5 m/s, the ink droplet is usually accompanied by one or more small sub-drops called satellites. 
     When a quantity of ink in the form of a ink droplet is ejected from the ejector to a receiving member, the resulting spot and any satellite drops become part of a desired image. Uniformity in spot size of a large number of droplets is very important for image quality. If the volume of droplets ejected from the print head over the course of producing a single document are permitted to vary widely, the lack of uniformity will have noticeable effects on the quality of the image. The most common and important cause of variance in the volume of droplets ejected from the print head is variations in the temperature of the print head over the course of use. The temperature of the liquid ink, before vaporization by the heating element, substantially affects both the bubble growth behavior and the viscosity of the ink. These two properties substantially influence the ejection volume of the ink droplets, and thus the resulting spot size on the image receiving member. The viscosity of the ink also directly affects the resulting spot size, by affecting the spread rate of the droplet after it contacts the image recording member. Control of temperature of the print head has long been of primary concerns in the art. 
     In order to maintain a constant spot size from an ink jet print head, various strategies have been attempted. One example is U.S. Pat. No. 4,899,180 to Elhatem et al., which is incorporated herein by reference in its entirety. In the′180 patent, the print head has a number of heater resistors and a temperature sensor that operate to heat the print head to an optimum operating temperature and maintain that temperature regardless of local temperature variations. 
     U.S. Pat. No. 4,791,435 to Smith et al., which is incorporated herein by reference in its entirety, discloses an ink jet system where the temperature of the print head is maintained by using the heating elements of the print head not only for ink ejection but also to maintain the temperature. The print head temperature is compared to thermal models of the print head to provide information for controlling the print head temperature. At low temperature, low energy pulses, i.e., pre-pulses, are sent to each channel, or nozzle, at a voltage that is below the voltage threshold that causes a ink droplet to be ejected. Alternatively, the print head can be warmed by firing some droplets of ink into an external chamber or “spittoon,” rather than onto the image receiving member. 
     SUMMARY OF THE INVENTION 
     As described above, the satellite drops are usually smaller than the main drop and travel at a lower speed than the main drop. As a result, the satellite drops can land on different locations of the image receiving member relative to the main drop during printing when the drop ejector and the image receiving member are moving relative to each other. Furthermore, the satellite drop can land at different locations from the main drop because the satellite drops generally have a velocity vector that is slightly different from that of the main drop. This satellite drop displacement can increase or decrease with increasing print head speeds and with an increased gap between the print head and the image receiving member. 
     Because the satellite drops occasional travel in slightly different directions relative to the main drop, the effect of the satellite drops on the image quality can differ based on a traveling direction of the print head. In one exemplary embodiment of a conventional print head, the print head transfers ink onto the image receiving member as the print head travels from right-to-left and left-to-right relative to the image receiving member, i.e., a bi-directional print head. Furthermore, while in a stationary position, the nozzles of this print head create satellite drops having velocity vectors which direct satellite drops to land slightly to the left of a main drop on the image receiving member. 
     As the print head passes from right-to-left and transfers ink onto the image receiving member, the velocity vector of the print head in conjunction with the velocity vectors of the satellite drops cause the satellite drops to land further away from the main drop on the image receiving member relative to that of the satellite drops of the print head when stationary. In other words, the satellite drops will lag even further behind the main drop due to the additional print head velocity vector. 
     On a return swath, as this print head moves from left-to-right and transfers ink onto the image receiving member, the velocity of the print head causes the satellite drops to land closer to, or actually within, the main drop on the image receiving member. The differing positions of the satellite drops relative to the main drop causes variations in the appearance between swaths formed by a left-to-right pass versus a swath formed by a right-to-left pass. Accordingly, this effect causes bidirectional banding in the final image because the patterns formed by drops printed in one direction are different from those printed in the other direction. 
     This invention counteracts the distorting effect of the bi-directional print head by reducing the volume of emitted ink when the print head is traveling in a direction that increases the satellite misplacement from the main drop. The emitted ink volume can be reduced by reducing an amount of pre-pulsing prior to ejection of the ink. Reducing the amount of pre-pulsing prior (generally a period of several micro-seconds) to the ejection of the ink can reduce the ink&#39;s temperature causing a smaller, less forceful, bubble to eject a more viscous ink from the channel. This combination of a smaller bubble and a more viscous ink in the channel can result in a smaller spot being formed on the receiving member and it can also affect the satellite displacement. 
     This invention can also counteract the distorting effect of the bi-directional print head by increasing the emitted ink volume when the print head is traveling in a direction that decreases the satellite displacement from the main drop. The emitted ink volume can be increased by increasing the amount of pre-pulsing prior to ejection of the ink. Increasing the amount of pre-pulsing prior to ejection of the ink can cause a larger, more forceful, bubble to eject a less viscous ink from the channel. This combination of larger bubble and less viscous ink can result in a larger spot being formed on the receiving member and it can also affect the satellite displacement. 
     As a result of these two effects, the overall appearance of right-to-left swaths more closely matches that of left-to-right swaths. Other objects, advantages and salient features of the invention will become apparent from the following detailed description taken in conjunction with the drawings, which disclose preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described with reference to the following drawings in which like reference numerals refer to like elements, and wherein: 
     FIG. 1 is a schematic view of a printing system in accordance with the present invention; 
     FIG. 2 is a cross-sectional view of a single ejector channel for a prior art ink jet print head; 
     FIG. 3 is an exemplary data structure for storing pre-pulsed sequence patterns corresponding to temperatures; 
     FIGS. 4-6 are exemplary spot patterns formed on a receiving member; 
     FIG. 7 is a portion of an exemplary image formed on a receiving member; 
     FIG. 8 is an exemplary image formed on a receiving member having a banding pattern; 
     FIG. 9 is an exemplary block diagram of the print controller of FIG. 1, according to the present invention; and 
     FIG. 10 is a flowchart of an exemplary process of the satellite induced banding prevention system according to the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows an exemplary carriage-type ink jet printing device  2 . A vertically linear array of droplet producing channels is housed in the print head  4  of a reciprocal carriage assembly  5 . Ink droplets  6  are propelled to a recording medium  8 , such as a sheet of paper, that is stepped by a motor  10  a preselected distance (often equal to the size of the array) in a direction of arrow  12  each time the print head  4  traverses across the recording medium  8  in the directions indicated by arrow  14 . The recording medium  8  can be stored on a supply roll  16  and stepped onto takeup roll  18  by stepper motor  10  or other structures, apparatuses or devices well known to those of skill in the art. 
     The print head  4  is fixedly mounted on the support base  20 , which is adapted for reciprocal movement using any well known structure, apparatus or device, such as two parallel guide rails  22 . The reciprocal movement of the print head  4  may be achieved by a cable  24  and a pair of pulleys  26 , one of which is powered by a reversible motor  28 . The print head  4  is generally moved across the recording medium  8  perpendicularly to the direction the receiving member  8  is moved by the motor  10 . Of course, other structures for reciprocating the carriage assembly  5  are possible. 
     The ink jet printing device  2  is operated under the control of a print controller  100 . The print controller  100  transmits commands to the motors  10  and  28  and the print head  5  to produce an image on the image recording medium  8 . Furthermore, the print head controller  100  receives information from the various components of the ink jet printing system  2 . For example, the print head controller  100  receives measurements of ink temperature from a temperature sensor located in print head  5 . 
     FIG. 2 shows one exemplary embodiment of an ink droplet emitter, or ejector  30 , of the ink jet print head  4 . The ink droplet ejector  36  is one of a large plurality of such emitters found in a typical ink jet print head. While FIG. 2 shows a side-shooter emitter, other emitters such as roof-shooter emitters may similarly be used with this invention. Typically, such emitters are sized and arranged in linear arrays of 300 to 600 emitters per inch, although other arrangements are known to one skilled in the art. A silicon member having a plurality of channels for ink droplet emission is known as a “die module” or “chip”. Each die module typically comprises hundreds of emitters, spaced 300 or more to the inch. An ink print head may have one or more die modules extending the effective size of the array. In print heads with multiple die modules, each die module may include its own ink supply manifold, or multiple die modules may share a common ink supply manifold. 
     Each emitter  30  includes a capillary channel  32  terminating in an orifice or nozzle  34 . The channel  32  holds a quantity of ink  36  maintained within the capillary channel  32  until such time as a droplet of ink is to be emitted. Each capillary channel  32  is connected to a supply of ink from an ink supply manifold (not shown). The upper substrate  38  abuts a thick film layer  40 , which in turn abuts a lower substrate  42 . 
     Sandwiched between the thick-film layer  40  and the lower substrate  42  are electrical heating elements  46  used to eject ink droplets from the capillary channel  32  in a well-known manner. The heating element  46  may be located within a recess  44  formed by an opening in a thick film layer  40 . The heating element  46  is directly or indirectly electrically connected to an addressing electrode  50 . Each of the ejectors  30  in the print head  4  may have its own heating element  46  and an individual addressing electrode  50 . The addressing electrode  50  may be protected by a passivation layer  52 . Each addressing electrode  50  and corresponding heating element  46  may be selectively controlled by control circuitry. Other embodiments of the ink jet print head are well known to one skilled in the art and are also within the scope of this invention. 
     As is well as known in the art, when a print pulse is applied to the addressing electrode  50 , the heating element  46  is energized. The print pulse is a signal that is of a sufficient magnitude and/or duration so that the heat from the resistive heating element  46  will cause the liquid ink immediately adjacent to the heating element  46  to vaporize, creating a bubble  54  of vaporized ink. The force of the expanding bubble  54  ejects an ink droplet  56 , which may include a main droplet and smaller satellite drops, from the nozzle  34  onto the surface of the recording medium  8 . 
     Furthermore, under the control of the print controller  100 , thermal ink jet print heads may apply a plurality of pre-pulses to the heating element  46  prior to ejecting each ink droplet  56 . Typically, one or more pre-pulses may be applied by the heating element  46  prior to a print pulse in order to warm the ink prior to ejection. The amount and timing of the pre-pulse can vary as a function of the temperature of the ink. 
     For example, FIG. 3 shows a pre-pulse table  300  having an ink temperature in a field  302  and a corresponding pre-pulse sequence in a field  304 . As shown in this example, for an ink temperature of 40° C. the pre-pulse sequence is absent while at 25° C. there is an extended pre-pulse sequence. For temperatures above 40° C., the adjustment (if any) is contained in the main (ejecting) pulse itself. The pre-pulse signals may be used to raise the temperature of the ink adjacent the heating element  46 , and additionally may be used to control the volume of ink droplet  56 . The pre-pulse signals do not contain enough energy to cause the ink droplet  56  to be emitted before the main (last) pulse. 
     Subsequent to any pre-pulse sequences, the print pulse, i.e., the drive pulse, firing pulse or main pulse, may be applied to the heating element  46 . As described above, the print pulse causes the ink droplet  56  to be ejected on to the image receiving member  8 . 
     FIGS. 4,  5  and  6  show three different spot patterns  400 ,  410  and  420  formed by a typical nozzle  34  of a print head  4 . For simplicity of explanation, each of the spot patterns  400 ,  410  and  420  includes a main spot portion  402 ,  412  and  422 , respectively, and a satellite spot portion  404 ,  414  and  424 , respectively. As described above, the spot portions  402 ,  412  and  422  describe where the majority of ink ejected from the nozzle  34  lands upon the image recording medium  8 . The satellite spot portions  404 ,  414  and  424  are portions of the ink spots formed by satellite drops having slightly different velocity vectors than the main drops that form the main spot portions  402 ,  412  and  422 . As a result, the satellite spot portions  404 ,  414  and  424  land upon the recording medium  8  at slightly different positions from the main spot portions  402 ,  412  and  422 . The farther the satellite drop lands away from the main spot portions  402 ,  412  and  422 , the more elongated the spot patterns  400 ,  410  and  420  become. 
     FIG. 4 shows a spot pattern  400  formed by a nozzle  34  that is held in a stationary position relative to the image recording medium  8 . As shown in FIG. 4, the satellite spot portion  404  is positioned slightly to the left of the main spot portion  402  on the image recording medium  8 . This is because the velocity vector of the satellite drop as it is ejected from the nozzle  34  is slightly different from the velocity vector of the main drop. As a result, as the ink is ejected from the nozzle  34  and travels towards the image recording medium  8 , the satellite drop travels slightly to the left in accordance with the difference in the velocity vectors. 
     FIG. 5 shows a spot pattern  410  formed by a nozzle  34  of the print head  4 , as the print head  4  travels from right-to-left relative to the image recording medium  8 . As shown in FIG. 5, the satellite spot portion  414  now extends a greater distance to the left relative to the satellite spot portion  404  of FIG.  4 . The “stretching” of the spot pattern  410  is due to the additional velocity vector imparted to the satellite drop by the print head  4  traveling in a right-to-left direction relative to the image recording medium  8 . In other words, as the print head  4  travels from right-to-left, the difference between the velocity vectors of the main drop and the satellite drop increases. The overall effect of the increased difference in the velocity vectors is to increase the distance between the landing positions of the satellite drop forming the satellite spot portion  414  and the main drop forming the main spot portion  412 . 
     FIG. 6 shows a spot pattern  420  formed by the same print head  4  moving in a left-to-right direction relative to the image receiving member  8 . As shown in FIG. 6, the satellite spot portion  424  of the spot pattern  420  is positioned relatively closer to the main spot portion  422  relative to the spot pattern  400 . The “compression” of the spot pattern  420  is due to the velocity of the print head when moving in the left-to-right direction decreasing the difference between the velocity vectors of the main drop and the satellite drop. In other words, the satellite drop forming the satellite spot portion  424  now lands in a position closer to that of the main drop forming the main spot portion  422  on the image recording medium  8 . 
     FIG. 7 shows a recording medium  8  having a series of spot patterns  410  and  420  formed on the recording medium  8  in a “checker-board” pattern. For illustration purposes only, a swath having only a single spot is shown. However, it is to be understood that generally print heads produce a swath path that is several spots in height. The checker-board pattern has a pixel distribution where every other pixel in a row has an ink spot, and every other row is offset from an adjacent row by one pixel. As shown in FIG. 5, the pattern includes rows  1 , 3  and  5  formed by a print head  4  moving in the right-to-left direction. The checker-board pattern also includes rows  2  and  4  formed by a print head  4  moving in the left-to-right direction. 
     As shown in FIG. 7, when the print head is traveling in the right-to-left direction to form rows  1 , 3  and  5 , the spot patterns  410  formed at each pixel position are elongated. In contrast, the spot patterns  420 , formed by the print head moving in the right-to-left direction when forming rows  2  and  4 , are compressed. 
     The aggregate effect of the directional distortion of the spot patterns  410  and  420  is shown in FIG.  8 . FIG. 8 illustrates the banding effect that occurs in an image  600  due to the directionally distorted spot patterns  410  and  420  formed on the recording medium  8 . In FIG. 8, the swaths  602  formed by the print head  4  moving in the left-to-right direction appears slightly different than the swath  604  formed by the print head  4  moving in the right-to-left direction. This distortion effect is commonly referred to as banding because the final image  600  formed on the recording medium  8  appears to be formed by a series of differing horizontal bands  602  and  604 . 
     FIG. 9 is a block diagram of one exemplary embodiment of the printer controller  100 . The printer controller  100  includes a controller  102 , a print head interface  104 , a memory  106 , and a general interface  108 , connected together by a control/signal bus  110 . In operation, the controller  102  communicates with the print head  4  and the printer motors  8  and  28  through the print head interface  104  and the general interface  108 , respectively, to create an image from image data received from a data source (not shown). 
     Prior to printing each swath  602  or  604 , the controller  102  determines the travel direction of the print head  4 . Furthermore, the controller  102  determines the temperature of the ink in the print head  4 , in response to a signal from a temperature sensor (not shown) received through the print head interface  104 . Based on the determined travel direction of the print head  4 , the controller  102  adds an adjustment constant to, or subtracts an adjustment constant from, the determined ink temperature prior to selecting a pre-pulse sequence from the memory  106 . The adjustment constant is called a satellite temperature difference adjustment value between right and left printing. 
     The adjustment value can be a negative or positive number depending on the travel direction of the print head  4  and the orientation or behavior of the satellite drops generated by the print head  4 . In one exemplary embodiment, if the controller  102  is only compensating during single direction of print head travel, then the absolute value of the adjustment value may be the amount to be added to or subtracted from the ink temperature (prior to pre-pulse determination) for motion in only one direction. Alternatively, if determined ink temperature is to be adjusted for motion in both directions, then half of the adjustment value will be used to adjust the determined ink temperature for each direction. 
     Which technique is used can depend on the ink temperature at the start of a page. However, for the best image quality, the technique chosen should remain the same for the duration of the page. The technique selection is made based on consideration of the finite range of control available using the pre-pulse technique. The table of FIG. 3 contains entries for temperatures of 25 to 55 degrees. To make the most of the invention, the adjusted temperatures should stay within that range. If the temperature at the top of page is high, then only temperature reducing adjustments should be made. Conversely, if the temperature at the top of page is low, only temperature increasing adjustments should be made. For intermediate top-of-page temperatures, both reducing and increasing adjustments might be made without exceeding the range of the table. 
     By increasing or decreasing the apparent ink temperature using the adjustment value prior to selecting a pre-pulse sequence, the selected pre-pulse sequence either heats the ink more or less than would normally be expected for the (actual) determined ink temperature. By doing so, the ink ejected from the nozzle  34  when traveling in a direction that tends to enlarge the effect the satellite drops have on the size of the spot pattern will be heated less than ink ejected from the nozzle  34  when traveling in a direction that tends to reduce the effect the satellite drops have on the size of the spot pattern. 
     For example, in an exemplary embodiment, assume that the determined ink temperature is 35° C. Furthermore, assume that the adjustment value is 10° C. and that the ink temperature is to be adjusted in both the left-to-right and the right-to-left travel directions of the print head  4 . Accordingly, during a first pass of the print head  4  moving in a left-to-right direction, the measured temperature of the ink is decreased by one-half of the adjustment value, i.e., 5° C. from the actual determined temperature. Therefore, the adjusted ink temperature is 30° C. and the pre-pulsing sequence corresponding to the 30° C. entry in the table  300  is used to heat the ink prior to the ink print pulse. By doing so, the ink will be hotter and the size of the spot pattern of the ink will be increased relative to ink pre-pulsed based on the actual determined temperature of 35° C. 
     During a second pass of the print head  4  moving in a right-to-left direction, the adjusted desired temperature of the ink is increased by one-half of the adjustment value, i.e., 5° C. from the actual determined temperature. Therefore, the adjusted ink temperature is 40° C., and the pre-pulsing sequence corresponding to the 40° C. entry in the table  300  is used to heat the ink prior to the ink print pulse. By doing so, the ink will be cooler and the size of the spot pattern of the ink would be decreased in order to counteract the enlarging influence the print head  4  traveling in the right-to-left direction has on the effect of the satellite drop. 
     The adjustment value can be in a constant selected based on an average satellite placement of a particular print head  4  design. For example, if a print head  4  design is known, on average, to position a satellite drop slightly to the left of a main drop, then a certain adjustment value can be selected to counteract the placement for all printers using this print head  4 . 
     Alternatively, the adjustment value may be selected using a trial and error method for each individual print head  4 . This may either be done by the manufacturer of the print head or by the user of the ink jet printer that includes the print head  4 . If it is done by the manufacturer, then the print head  4  may be tested to see where the nozzles  34  are placing satellite drops in relation to the main drop. Once the satellite drop placement is known, the manufacturer may select and store a corresponding adjustment value in the controller  102  and/or memory  106  to counteract the satellite misplacement while the print head  4  is operating. 
     Alternatively, or in addition to the manufacturer selecting the adjustment value, a user may select an optimum adjustment value by a trial and error process during the printing of images or during a set-up procedure. In this case, the user may vary the adjustment value while printing a series of the same images, and select the adjustment value which produces the best image (i.e., the image having the least amount of banding). In this manner, the adjustment value can be varied in accordance with the user&#39;s preference. 
     FIG. 10 is a flowchart outlining one exemplary embodiment of a method for reducing satellite-induced banding in an image. In step S 800 , the process begins when a controller receives a request to print a swath. Next, in step S 802 , the temperature of the ink in the print head is determined. The temperature of the ink in the print head may be measured directly or may be determined to be a temperature close to that of the print head. Next, in step S 804 , a determination is made whether the print head is traveling in an enlarging direction or a compressing direction. If the print head is traveling in the compressing direction, control proceeds to step S 806 . Otherwise, control jumps to step S 808 . 
     In step S 806 , a satellite temperature difference adjustment value is subtracted from the determined ink temperature to create an adjusted ink temperature. Control then jumps to step S 810 . Alternatively, in step S 808 , the controller adds an adjustment value to the determined ink temperature to create the adjusted ink temperature. Control then proceeds to step S 810 . 
     In step S 810 , a pre-pulsing sequence is selected from a pre-pulsing sequence table which corresponds to the adjusted ink temperature. Then, in step S 812 , the pre-pulsing sequence is sent by the controller to the print head interface. Next, in step S 814 , the printing swath is started and the print head interface uses the stored prepulse sequence at each jet firing during the swath. Control proceeds to step S 818  where the controller process ends, only to be restarted when the next swath is ready to be printed. 
     As shown in FIG. 9, the print controller  100  and print head interface  104  is preferably implemented using an application specific integrated circuit (ASIC). However, both  100  and  104  can also be implemented using any other known or later developed integrated circuit, such as a micro-processor, a micro-controller, a digital signal processor, a hardwired electronic or logic circuit such as a discrete element circuit, a programmable logic device such as a PLD, PLA, FPGA or PAL, or the like. In general, any integrated circuit or logic device capable of implementing a finite state machine that is in turn capable of implementing the flowchart shown in FIG. 10, can be used to implement the print controller  100 . 
     Thus, it should be understood that the block diagram shown in FIG. 9 can be implemented as portions of a suitably designed ASIC or of a system that includes a micro-processor, a micro-controller or a digital signal processor. Alternatively, the block diagrams shown in FIG. 9 can be implemented as physically distinct hardware circuits using a FPGA, a PDL, a PLA or a PAL, or using discrete logic elements or discrete circuit elements. The particular form of the circuitry used to implement the system shown in FIG. 9 will take is a design choice and will be obvious and predictable to those skilled in the art. 
     While the systems and methods of this invention have been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to the those skilled in the art. Accordingly, the exemplary embodiments of the systems and methods of this invention, as set forth above, are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention.