Abstract:
A printing apparatus having a plurality of printing devices (e.g., printheads or firing chambers) that permits registration of the printing devices so as to achieve a uniform output of print ink. Logic that adjusts the length of a firing signal (which is related to drop size) and timing of a firing signal is disclosed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to printers and, more specifically, to registration of multiple printheads and/or multiple firing chambers in a printer. 
     BACKGROUND OF THE INVENTION 
     Many types of printers are known and they include ink jet, laser and various thermal and impact printers. Ink jet printers include those that are thermally actuated (e.g., resistive element) and those that are mechanically actuated (e.g., piezo-electric element). Representative ink jet printers include those made by Hewlett Packard, Canon and Epson, etc. The multiple printhead and/or multiple firing chamber registration techniques of the present invention are applicable to all printers and particularly to ink jet printers. 
     The present invention is directed towards the registration of printing devices within a printer. This registration may be manifested in many ways. A few of these ways are now presented for pedagogical purposes. Multiple printheads or printheads with multiple firing chambers are utilized as a way of delivering color printing (one primary or equivalent color per printhead) and of increasing printing speed and coverage area, amongst other features. 
     In a thermally actuated multiple printhead printer, the ink expulsion elements are usually formed as resistors in a semiconductor substrate. Due to variations in the semiconductor fabrication process, the size of these resistors may vary between wafers and even between dies from the same wafer. Since different size resistors have different power draws, the same firing current delivered to two different resistors will produce different magnitude thermal pulses and hence different sized ink drops. The different sized ink drops in turn result in printed images that are more strongly of one color than another, for example. 
     Another relevant registration consideration arises when multiple printheads (or multiple firing chambers in a single printhead) are used in a printing apparatus that prints on a curved surface, e.g., a plotter or other printer with a roll bar. A flat printing device surface used in conjunction with a curved printing surface will normally result in some of the printheads or firing chambers being situated closer to the print media than others. Since it will take longer for ink from the more distant printheads or chambers to arrive at the print media, ink drops will not align appropriately, thus resulting in reduced image quality. 
     A need thus exists to provide registration in multiple printhead or firing chamber printers so that similarly sized ink drops are delivered in a timely manner to print media. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a printing apparatus that provides registration of printhead and/or firing chambers to achieve improved image quality. 
     It is another object of the present invention to provide a printing apparatus that increases the uniformity of the size of ink drops delivered to print media. 
     It is also an object of the present invention to achieve such registration and related benefits by modifying the timing and/or strength of firing signals delivered to ink expulsion elements. 
     These and related object of the present invention are achieved by use of a printhead registration apparatus and method as described herein. 
     The attainment of the foregoing and related advantages and features of the invention should be more readily apparent to those skilled in the art, after review of the following more detailed description of the invention taken together with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a printer having multiple printing devices in accordance with the present invention. 
     FIG. 2 is a schematic diagram of a firing signal adjustment circuit in accordance with the present invention. 
     FIG. 3 is another embodiment of a firing signal adjustment circuit in accordance with the present invention. 
     FIG. 4 is a diagram of a firing signal time modification circuit in accordance with the present invention. 
     FIG. 5 is a timing diagram that corresponds to the circuit of FIG.  4 . 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a schematic diagram of a printer  10  having multiple “printing devices”  20  in accordance with the present invention is shown. In the description of FIG. 1 that follows, the printing devices  20  are described as being printheads. It should be recognized, however, that the term printing device as used herein also includes firing chamber (a multiplicity of which may be provided in a single printhead) and is generally intended to include any ink expulsion arrangement that may benefit from the firing signal control taught herein. 
     Printer  10  includes a plurality of printheads  20  that are individually identified as printheads  20 A- 20 D. These printheads are preferably formed in a semiconductor substrate. The printheads are preferably provided in a movable carriage  21 , though they may be provided in a stationary manner, e.g., formed wide enough to cover a sheet of paper or other print media. Two headed arrow A illustrates transverse movement of carriage  21 . Printer  10  also preferably includes a controller  15  (to which the printheads are preferably coupled), print media input/output (I/O) unit  12 , power supply  16  and an ink supply  18 . 
     Suitable controller logic for performing typical printer/printhead functions is known in the art. It should be recognized, however, that logic for performing functions of the present invention may be provided in the controller, though such embodiment may result in increased EMI and require a sufficiently fast master clock. 
     The print media I/O unit preferably includes paper input and output trays, guides, and appropriate sensors and transport mechanisms, etc. Power supply  16  provides regulated DC at appropriate voltage levels. 
     Ink supply  18  preferably provides an appropriate ink supply  18 A- 18 D, respectively, for printheads  20 A- 20 D. Individual ink supply components may be formed integrally with their corresponding printheads or formed separately therefrom. Ink supplies  18 A- 18 D may be provided in a refillable or replaceable manner. Ink level detection logic  19  is preferably provided with the ink supplies to indicate an ink volume level. Suitable ink supply arrangements are known in the art. 
     Printer  10  preferably receives print data from a host machine  5  which may be a computer, facsimile machine, Internet terminal, camera, plotter or other device that is capable of propagating print data to printer  10 . 
     Each of printheads  20 A- 20 D preferably include a plurality of firing chambers that each have an associated ink expulsion point or nozzle  44 . The nozzles may be grouped into primitives  45  which are subsets of nozzles in which only one nozzle (or less than all nozzles) is fired per firing interval. While FIG. 1 illustrates four nozzles per primitive, more or less than this number may be provided. The use of primitives may decrease power consumption and lead interconnects and may address fluidic concerns. 
     Fire signal generation logic  30  preferably generates a firing signal for each nozzle. Control logic  25  preferably propagates control signals that indicate which nozzles are to fire during a given firing interval. Adjustment logic  40  preferably adjusts a firing signal (e.g., the output of firing signal logic  30 ) such that an ink drop expelled as a result of that firing signal is of an appropriate size and happens at an appropriate moment in time. Additional adjustment logic  40 ′,  40 ″, etc., may be provided on the printhead dies to provide more localized firing signal adjustment. 
     While the firing signal logic and adjustment logic, etc., are shown in FIG. 1 as being within the printheads, it should be recognized that one or more of these items may be provided in controller  15 . To implement these or related features within controller  15  a suitably fast master clock is preferably provided. EMI suppression is also preferably provided. 
     Embodiments of adjustment logic  40  that provide adjustment of the length of the firing signal and when the firing signal occurs are now provided. 
     Referring to FIG. 2, a schematic diagram of a firing signal adjustment circuit in accordance with the present invention is shown. The circuit of FIG. 2 preferably includes an input  61 , a configuration register  62 , a counter  63  and a gate (an AND gate)  64  for gating passage of the firing signal. The embodiment of FIG. 2 is a “front trimming” circuit because the circuit trims the time from the front end of the input firing signal. A “back trimming” circuit is shown in FIG.  3 . 
     A value indicative of the amount of time to subtract from the front of the firing signal is loaded into configuration register  62 . It should be recognized that configuration value for each printhead (or for particular regions of a printhead as discussed below) can be determined during test (for example, compared to a standard) and then programmed into configuration register  62  using known techniques. This loaded value is then provided as an initial value to countdown counter  63 . 
     An input firing signal is propagated to both AND gate  64  and counter  63 . When the firing signal is low, the counter is loaded with the configuration value and does not count. When the firing signal transitions high, the firing signal begins to count. During this count, the output of the counter is such that gate  64  does not pass the firing signal. After counter  63  has counted down the loaded value, gate  64  enables passage of the firing signal for the remainder of its duration. For example, if the original firing signal is 20 clock cycles in duration and the value loaded in register  62  is indicative of a delay of five clock cycles, then the firing signal will not pass AND gate  64  until after a delay of five clock cycles has been counted down by the counter and thus, the duration of the passed firing signal is only 15 clock cycles in length. 
     Referring to FIG. 3, another embodiment of a firing signal adjustment circuit in accordance with the present invention is shown. The circuit of FIG. 3 is similar to that of FIG. 2, but it is a back trimming circuit. The circuit of FIG. 3 preferably includes an input  71 , a configuration registration  72 , a counter  73  and a gate (an AND gate)  74 . In circuit  70 , a value equal to the desired duration of the firing signal is loaded in configuration register  72  and provided to counter  73  for comparison. An input firing signal is passed to counter  73  and through gate  74  initially. The counter keeps gate  74  enabled for the amount of time indicated in configuration register  72 . When the count expires, gate  74  is effectively disabled and the firing signal is terminated. Similar to the above example, if a firing signal of 15 clock cycles is desired, then this value is loaded into configuration register  72  and the firing signal is terminated at this number of clock cycles due to the expiration of the count in counter  73 . 
     Referring to FIG. 4, a diagram of firing signal time modification circuit  80  in accordance with the present invention is shown. Referring to FIG. 5, a timing diagram for circuit  80  of FIG. 4 is shown. Circuit  80  preferably includes a configuration register  82 , a counter  83  (which in the embodiment of FIG. 4 happens to be an up counter, hence the signal equal), an exclusive NOR (exNOR)  86  and a flip-flop (a clocked latch)  87 . The firing signal is input to exNOR  86  and flip-flop  87 . The output delayed firing signal is fedback to the exNOR. Counter  83  receives a reset signal from the exNOR, a clock signal and a count value from configuration register  82 . The output of the counter is coupled to the clock input of flip-flop  87 . 
     As indicated by the firing diagram, when the firing signal goes high, RESET goes inactive and the count begins. When count equals the configuration value, EQUAL goes high, latching the value at flip-flop  87 . EQUAL is high for as long as it takes for the latch signal to feedback and reset counter. When the firing signal goes low, RESET is again deactivated, causing a delay equal to the count value to expire before the firing signal end is latched through by flip-flop  87 . 
     With respect to determination of configuration value for the embodiment of FIGS. 2-4, these may be determined in a plurality of ways, including, but not limited to, the following. Each printhead can be tested, compared to a standard and then provided with a value that compensates for differences from the standard. In the case of a curved printing surface, the value indicative of the requisite offset can be determined and loaded in each of the appropriate printing devices. Other scenarios include providing a user with a variety of test prints and permitting the user to select the test print they prefer. Logic in the host for controller  15 , then propagates value corresponding to the selected print to the configuration register(s), etc. 
     While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification, and this application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth, and as fall within the scope of the invention and the limits of the appended claims.