Patent Publication Number: US-6663207-B2

Title: Determining inkjet printer pen turn-on voltages

Description:
This is a continuation of application Ser. No. 09/940,313 filed on Aug. 27, 2001 now U.S. Pat. No. 6,454,376, which is hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to pens for inkjet printers, and more specifically to determining the turn-on voltage for such pens. 
     BACKGROUND OF THE INVENTION 
     Inkjet printers have become increasingly inexpensive and increasingly popular, especially for home computer users. A typical inkjet printer usually has a number of common components, regardless of its brand, speed, and so on. There is usually a print head that contains a number of pens, where each pen has a series of nozzles used to spray drops of ink onto paper. Alternatively, the pens may be separate, and not located on a common print head. Ink cartridges, either integrated into the print head or separate therefrom, supply the ink. There may be separate black and color cartridges, color and black in a single cartridge, a cartridge for each ink color, or a combination of different colored inks in a given cartridge. 
     A print head motor typically moves the print head assembly back and forth horizontally, or laterally, across the paper, where a belt or cable is used to attach the assembly to the motor. Other types of printer technologies use either a drum that spins the paper around, or mechanisms that move the paper rather than the print head. The result is the same, in that the print head is effectively swept across the paper linearly to deposit ink on the paper. Rollers pull paper from a tray, feeder, or the user&#39;s manual input, and advance the paper to new vertical locations on the paper. 
     For the pens to fire their nozzles, resulting in ink sprayed on the paper inserted in the printer, a turn-on voltage is applied. The turn-on voltage causes the nozzles to fire. The turn-on voltage for each pen is desirably precisely known, so that only the exact turn-on voltage is applied to fire the nozzles of a pen. If a greater voltage is applied, the voltage in excess of the turn-on voltage is typically converted into heat. The ink may therefore increase in temperature beyond its recommended setting, which may cause printing quality to decrease. For example, too much ink may be deposited on the paper, or printing artifacts may otherwise be deposited on the paper. Furthermore, excess voltage may result in greater wear-and-tear on the pens and their nozzles, causing them to fail prematurely. 
     Although the inkjet printer pens in theory have a common specified turn-on voltage, in actuality the turn-on voltage for each pen varies depending on a number of different factors. Manufacturing tolerances may cause the turn-on voltages for different pens, as well as for different nozzles within the same pen, to vary. Other voltage errors may result from variations within the inkjet printer itself. For example, the traces and other electrical connections and components within the printer may have electrical resistances less than or greater than specified amounts, such that voltage drops across these connections and components may be less than or greater than what is expected. This means that the actual voltage applied to the inkjet printer pens to turn them on may vary from nozzle to nozzle, from pen to pen, and from printer to printer, effectively causing the turn-on voltages of the pens to vary. 
     These variations mean that using a common theoretical specified turn-on voltage for each pen in each printer will likely cause a decrease in printing quality and result in printing artifacts to develop on the media printed on by the printers. Even determining the turn-on voltages for the pens in isolation, without considering the specific printer in which they are being used, may cause such printing quality degradation. For these and other reasons, therefore, there is a need for the present invention. 
     SUMMARY OF THE INVENTION 
     The invention relates to determining the turn-on voltage for inkjet printer pens. An inkjet printer has a number of pens, and a number of sets of nozzles in each pen. Each set of nozzles of a pen is fired at each of a number of voltages, to obtain a voltage-value curve for each set of nozzles. A nozzle turn-on voltage for each set of nozzles is determined based on a maximum slope of its voltage-value curve. The turn-on voltage for each pen is determined based on the nozzle turn-on voltages of the voltage-value curves for its sets of nozzles. 
     Still other aspects, embodiments, and advantages of the invention will become apparent by reading the detailed description that follows, and by referring to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a representative inkjet printer according to an embodiment of the invention, in which only those components of the printer specified by the embodiment are shown. 
     FIG. 2 is a diagram of a representative inkjet printer pen and its constituent nozzles aimed at a drop detect, according to an embodiment of the invention. 
     FIG. 3 is a flowchart of a method showing generally how one embodiment of the invention determines the turn-on voltages for the pens of an inkjet printer. 
     FIG. 4 is a graph of a representative voltage-value curve that results from the performance of the method of FIG. 3, according to an embodiment of the invention. 
     FIG. 5 is a flowchart of a method showing more specifically how one embodiment of the invention determines the nozzle turn-on voltage for a set of inkjet printer pen nozzles in the method of FIG.  3 . 
     FIG. 6 is a graph of the representative voltage-value curve of FIG. 4 that more specifically results from the performance of the method of FIG. 5, according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized, and logical, mechanical, and other changes may be made without departing from the spirit or scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
     Representative Inkjet Printer 
     FIG. 1 is a block diagram of a representative inkjet printer  100  according to an embodiment of the invention. Only those components specified by the embodiment of the invention are depicted in FIG.  1 . Those of ordinary skill within the art will appreciate that other components are typically present in inkjet printers, such as rollers, print heads, and so on, and such components can be included in printers according to embodiments of the invention. The printer  100  includes printer pens  102 , an electrostatic drop detect  104 , and firmware  106 . The printer pens  102  generally include a pen for each of a number of differently colored ink, as well as one or more pens for black ink. For example, there may be four pens, one for cyan ink, one for magenta ink, one for yellow ink, and one for black ink. The printer pens  102  have nozzles that spray the ink. For purposes of determining the turn-on voltages for the pens  102 , these nozzles are aimed at the electrostatic drop detect  104 . 
     The electrostatic drop detect  104  is one type of drop detect. The electrostatic drop detect  104  detects the charge of the ink drop that is induced upon it by the electrostatic drop detect. This amount of charge that is detected is related to the amount of ink that is deposited on the drop detect and is represented by an electrostatic drop detect score. In this way, the drop detect  104  can be used to determine the amount of ink that has been sprayed by the nozzles of one of the pens  102 . The drop detect  104  may be that which is described in U.S. Pat. No. 6,086,190. 
     The firmware  106  is a computer-readable medium, such as a non-volatile memory or another type of memory, that stores instructions that can be executed by a processor of the printer  100  (not specifically shown in FIG. 1) to cause the printer  100  to operate. In the embodiment of the invention of FIG. 1, the firmware  106  includes instructions to determine the turn-on voltages of the pens  102 , such that the firmware  106  subsequently uses these voltages when operating the printer  100 . The firmware  106  may also include instructions as to how often the turn-on voltage of the pens  102  are determined, such as when new pens  102  and/or new ink cartridges are installed, and periodically over the operating life of the pens  102 . 
     FIG. 2 shows the nozzles of an inkjet printer pen  202 , which can be one of the printer pens  102  of FIG. 1, firing on a drop detect, such as the electrostatic drop detect  104  of FIG. 1, in more detail. An inkjet pen  202  has a number of nozzles divided into sets of nozzles  204 A,  204 B,  204 C, and  204 D. The nozzles are aimed at drop detect  104 , as indicated by the arrow  206 . As used in embodiments of the invention, each set of nozzles  204 A,  204 B,  204 C, and  204 D is individually fired at the drop detect  104 . The number of nozzles shown in FIG. 2 is greatly reduced for illustrative clarity. Typically, there may be more than 200 such nozzles in a given inkjet printer pen. Furthermore, there may be fewer or greater number of sets of nozzles than the four sets that are specifically shown in FIG.  2 . 
     Determining Turn-On Voltages for Inkjet Printer Pens 
     FIG. 3 shows a general method  300  performed by an embodiment of the invention to determine the turn-on voltages for inkjet printer pens, such as those shown in and that have described in conjunction with FIGS. 1 and 2. The method  300  may be performed by the firmware of an inkjet printer, such as the firmware  106  of the printer  100  of FIG.  1 . The method  300  also may more generally be performed by execution of instructions stored in a computer-readable medium by a processor. 
     A current pen is initially set to the first inkjet printer pen of a printer ( 302 ). The nozzles of the current pen are divided into sets of nozzles ( 304 ). A current nozzle set is then set to the first set of nozzles of the current pen of the printer ( 306 ), which are fired at a drop detect at a number of pen firing voltages, from a low voltage to a high voltage ( 308 ). The drop detect may be an electrostatic drop detect, such as the drop detect  104  of FIGS. 1 and 2. 
     The result of the nozzles of the current set being fired against the drop detect is a voltage-value curve ( 310 ). Where the drop detect is an electrostatic drop detect, this curve may be a pen firing voltage-electrostatic drop detect value, or score, curve. A representative such curve is shown in the graph  400  of FIG.  4 . The graph  400  has an x-axis  402  that measures the applied pen firing voltage to the current set of nozzles, and a y-axis  404  that measures the value detected by the drop detect, such as the electrostatic drop detect value. The curve  406  is made up of a number of constituent discrete points  408 A,  408 B, . . . ,  408 I, corresponding to the voltages at which the current set of nozzles were fired. The number of points shown in curve  406  is greatly reduced from the actual number for purposes of illustrative clarity. 
     As can be seen from the graph  400 , for the applied voltages at points  408 A,  408 B,  408 C, and  408 D, the drop detect registers low values, indicating that the inkjet pen nozzles of the current set have not substantially deposited ink on the drop detect. For the voltages at points  408 G,  408 H, and  408 I, the drop detect registers high values, indicating that the inkjet pen nozzles of the current set have substantially deposited ink on the drop detect. The voltages at these points thus indicate that the voltages are greater than the turn-on voltage for the inkjet pen nozzles of the current set. Finally, for the points  408 E and  408 F, the drop detect registers interim values between the low values and the high values, indicating that the inkjet pen nozzles of the current set have not quite turned on completely, and have not deposited a full amount of ink on the drop detect. 
     Referring back to FIG. 3, once the voltage-value curve for the current set of nozzles has been obtained ( 310 ), the turn-on voltage for the current set of nozzles is determined ( 312 ). The turn-on voltage for the current set of nozzles is referred to as the nozzle turn-on voltage, to distinguish this turn-on voltage from the turn-on voltage for the current inkjet printer pen, which is the voltage necessary to turn on all the nozzles of the pen, and not just those of the current nozzle set. The nozzle turn-on voltage is generally determined based on the maximum slope of the curve that has been obtained. In one specific embodiment, the nozzle turn-on voltage can be obtained as shown in the method  312  of FIG.  5 . 
     Referring to FIG. 5, an n-point slope is determined at each voltage of the curve ( 502 ). For example, the n-point slope can be determined at each constituent discrete point of the curve. In particular, a three-point slope can be determined, according to:                  S   i     =               3        (         E   i          V   i       +       E     i   +   1            V     i   +   1         +       E     i   +   2            V     i   +   2           )       -                 (       E   i     +     E     i   +   1       +     E     i   +   2         )          (       V   i     +     V     i   +   1       +     V     i   +   2         )                 3        (       V   i   2     +     V     i   +   1     2     +     V     i   +   2     2       )       -       (       V   i     +     V     i   +   1       +     V     i   +   2         )     2           ,           (   1   )                         
     where E 0 , E 1 , E 2 , . . . , E n  are the values, such as the electrostatic drop detect scores, associated with the voltages V 0 , V 1 , V 2 , . . . , V n  at which the current nozzle set was fired. That is, the constituent discrete points of the curve are (V 0 , E 0 ), (V 1 , E 1 ), (V 2 , E 2 ), . . . , (V n , E n ). The n-point slopes are indicated by S i , iε0,1,2, . . . , n−2. Although a three-point slope has been used in equation (1), more generally an n-point slope from an n-point line fit can be used. For instance, it may be desirable to use a different number of point slope instead, based on the resolution of the discrete pen firing voltage used in the printer. 
     The maximum n-point slope of the curve is then determined, or selected ( 504 ). The maximum n-point slope is the greatest slope of the slopes that were determined. This is the slope 
     
       
           S   max =max { S   i   , iε 0,1,2, . . . ,  n− 2}.  (2) 
       
     
     The point on the curve that has the maximum n-point slope is referred to as the maximum n-point slope point (V max , E max ) 
     Next, the line tangent to the maximum n-point slope point, which corresponds to the maximum n-point slope, is intersected with a horizontal, zero-slope line ( 506 ). In one embodiment, the horizontal line can either be an upper horizontal line, or a lower horizontal line. The upper horizontal line is substantially parallel and corresponds to the values, or scores, of the voltage-value curve at which the current nozzle set has been turned on. Similarly, the lower horizontal line is substantially parallel and corresponds to the values, or scores, of the voltage-value curve at which the current nozzle set has been turned off. The upper horizontal line may be used for black ink, for instance, whereas the lower horizontal line may be used for color ink. 
     More specifically, the upper horizontal line can be determined by averaging the values or scores for the last k points of the voltage-value curve:                E   high     =           E     n   -   k       +     E     n   -   k   +   1       +   …   +     E   n       k     .             (   3   )                         
     The line itself is therefore drawn as a zero-slope line at the value E high . Similarly, the lower horizontal line can be determined by averaging the values or scores for the first k points of the voltage-value curve:                E   low     =           E   1     +     E   2     +   …   +     E   k       k     .             (   4   )                         
     This line is therefore drawn as a zero-slope line at the value E low    
     The nozzle turn-on voltage for the current nozzle set is then determined as the voltage at which the maximum n-point slope intersects with the horizontal line ( 508 ). That is,                  V   ntov     =                E   avg     -     E   max            +       S   max          V   max           S   max         ,       E   avg     ∈     E   high       ,       E   low     .             (   5   )                         
     The voltage V ntov  is the nozzle turn-on voltage for the current nozzle set, which corresponds to the voltage at which the maximum n-point slope intersects with either the upper horizontal or the lower horizontal line. 
     FIG. 6 shows the graph  400  of FIG. 4 in which the maximum n-point slope and the upper and lower horizontal lines have been added. The dotted line  610  corresponds to the maximum n-point slope, which is at the point  612 . The dotted line  610  is tangent to the point  612 . The upper horizontal dotted line  614  intersects with the dotted line  610  at the point  616 , such that the voltage at the point  616  can be selected as the nozzle turn-on voltage for the current nozzle set, where the upper horizontal line is being used. The lower horizontal dotted line  618  intersects with the dotted line  610  at the point  620 , such that alternatively the voltage at the point  620  can be selected as the nozzle turn-on voltage for the current nozzle set, where the lower horizontal line is being used. 
     Referring back to FIG. 3, once the nozzle turn-on voltage has been determined for the current set of nozzles ( 312 ), if there are more sets of nozzles to be fired at the drop detect ( 314 ), then the current set of nozzles is advanced to the next set of nozzles ( 316 ). The new current set is then fired ( 308 ) to obtain a voltage-value curve ( 310 ) from which another nozzle turn-on voltage is determined ( 312 ). Once all the sets of nozzles of the current pen have been fired at the drop detect ( 314 ), the turn-on voltage for the current pen is determined based on the nozzle turn-on voltages for the nozzle sets of the current pen ( 318 ). The nozzle turn-on voltages for the sets of nozzles of the current pen likely do not substantially vary from one another too much. The turn-on voltage for the current pen can be determined in a variety of different manners. For instance, either the median or the average value of the nozzle turn-on voltages may be used as the turn-on voltage for the current pen itself. As another example, the maximum nozzle turn-on voltage may be used as the turn-on voltage for the current pen. 
     The current pen&#39;s operating voltage is next optionally determined from the turn-on voltage for the current pen ( 320 ). The operating voltage may be determined by adding or multiplying an offset factor to the turn-on voltage for the pen. The operating voltage corresponds to the voltage at which the current pen is ultimately maintained during steady operation of the pen&#39;s nozzles. If there are more pens for which operating voltages need to be determined ( 322 ), then the current pen is advanced to the next pen ( 324 ), and the process that has been described is repeated for the new current pen. Once there are no more pens for which operating voltages need to be determined ( 322 ), the method  300  is done ( 326 ). 
     Conclusion 
     The invention provides for advantages within the prior art. In particular, the actual turn-on voltage for each inkjet printer pen is determined with the pens in the printer. This means that the turn-on voltages are precisely determined, given the manufacturing tolerances and environmental and other factors that may affect these voltages for specific pens as installed in specific printers. Furthermore, the turn-on voltages may be determined throughout the lives of the printer pens, to account for changes that may occur over time. The invention substantially decreases printing quality degradation and artifacts that result from overheating inkjet ink by applying voltages in excess of the actual turn-on voltages of the pens. 
     It is noted that, although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement is calculated to achieve the same purpose may be substituted for the specific embodiments shown. For example, other applications and uses of embodiments of the invention, besides those described herein, are amenable to at least some embodiments. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and equivalents thereof.