Abstract:
A printing system includes an inkjet printhead for selectively depositing ink drops on print media. An ink reservoir stores ink to be provided to the inkjet printhead. An ink level sensing circuit provides an ink level sense output that is indicative of a sensed volume of ink in the ink reservoir. A memory device stores sensor compensation information. A processor responsive to output of the memory device and the ink level sense output generates a compensated ink level sense output. The processor provides an estimate of available ink based on the compensated ink level sense output.

Description:
THE FIELD OF THE INVENTION 
     The present invention relates to printers and to ink supplies for printers. More particularly, the invention relates to a pressure ink level sensing system including a digital compensation system for an ink supply. 
     BACKGROUND OF THE INVENTION 
     The art of inkjet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, and facsimile machines have been implemented with inkjet technology for producing printed media. Generally, an inkjet image is formed pursuant to precise placement on a print medium of ink drops emitted by an ink drop generating device known as an inkjet printhead assembly. An inkjet printhead assembly includes at least one printhead. Typically, an inkjet printhead assembly is supported on a movable carriage that traverses over the surface of the print medium and is controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to a pattern of pixels of the image being printed. 
     Inkjet printers have at least one ink supply. An ink supply includes an ink container having an ink reservoir. The ink supply can be housed together with the inkjet printhead assembly in an inkjet cartridge or pen, or can be housed separately. When the ink supply is housed separately from the inkjet printhead assembly, users can replace the ink supply without replacing the inkjet printhead assembly. The inkjet printhead assembly is then replaced at or near the end of the printhead life, and not when the ink supply is replaced. 
     For some hard copy applications, such as large format plotting of engineering drawings and the like, there is a requirement for the use of much larger volumes of ink than can be contained within inkjet cartridges housing an inkjet printhead assembly and an ink supply. Therefore, relatively large, separately-housed ink supplies have been developed. 
     In an inkjet device, it is desirable to know the level of the ink supply so that the inkjet printhead assembly is not operated in an out-of-ink condition. Otherwise, printhead damage may occur as a result of firing without ink, and/or time is wasted in operating a printer without achieving a complete printed image, which is particularly time consuming in the printing of large images which often are printed in an unattended manner on expensive media. 
     Some existing systems provide each ink container with an on-board memory chip to communicate information about the contents of the container. The on-board memory typically stores information such as manufacture date (to ensure that excessively old ink does not damage the print head,) ink color (to prevent misinstallation,) and product identifying codes (to ensure that incompatible or inferior source ink does not enter and damage other printer parts.). Such a chip may also store other information about the ink container, such as ink level information. The ink level information can be transmitted to the printer to indicate the amount of ink remaining. A user can observe the ink level information and anticipate the need for replacing a depleted ink container. 
     In one prior art ink level sensing (ILS) technique, a coil is positioned on each side of the ink reservoir. One coil acts as a transmitter, and the other coil acts as a receiver. As the ink in the ink reservoir is used up, the reservoir collapses and the coils come closer together. Signal level in the receiver provides a measure of the ink level in the ink reservoir. The coils function as a non-contacting inductive transducer that indirectly senses the amount of ink in the ink reservoir by sensing the separation between the opposing walls of the reservoir. An AC excitation signal is passed through one coil, inducing a voltage in the other coil, with a magnitude that increases as the separation decreases. The change in voltage in the coil results from the change in the mutual inductance of the coils with change in the separation between the coils. The output voltage is readily related to a corresponding ink volume. The use of this ILS technique is relatively expensive, however, and typically results in about 60 cc of stranded ink. 
     In a second technique, a pressure ink level sensing (P-ILS) system is used to sense ink level. A P-ILS system has the potential advantage of 50% less cost, and typically strands about 50% less ink than the coil ILS technique. However, P-ILS systems require a compensation system to compensate or correct the output of a pressure sensor. Existing compensation systems use resistors or similar means to set compensation values. The resistors are typically laser trimmed or mechanically trimmed to provide the desired compensation values, which is a relatively complex process. In addition, the compensation resistors require space on the integrated assembly, making it more difficult to reduce the size of the assembly 
     There is a need for a pressure ink level sensing (P-ILS) system that includes a compensation system without the disadvantages of prior compensation systems. 
     SUMMARY OF THE INVENTION 
     The present invention provides a printing system that includes an inkjet printhead for selectively depositing ink drops on print media. An ink reservoir stores ink to be provided to the inkjet printhead. An ink level sensing circuit provides an ink level sense output that is indicative of a sensed volume of ink in the ink reservoir. A memory device stores sensor compensation information. A processor responsive to output of the memory device and the ink level sense output generates a compensated ink level sense output. The processor provides an estimate of available ink based on the compensated ink level sense output. 
     One aspect of the invention is directed to an ink container for an inkjet printing system having an inkjet printhead that selectively deposits ink drops on print media. The ink container includes an ink reservoir for storing ink to be provided to the inkjet printhead. A sensor provides an ink level sense signal that is utilized by a controller. An information storage device stores sensor compensation information that is utilized by the controller to provide a compensated ink level sense signal. 
     Another aspect of the invention is directed to a method for determining an amount of ink remaining in an ink container installed in a printing system having an inkjet printhead for receiving ink from the ink container and selectively depositing ink drops on print media. An ink level sense signal is provided that is indicative of a sensed volume of ink in the ink container. Digital compensation values are also provided. Compensated ink level sense values are generated based on the ink level sense signal and the digital compensation values. The amount of ink remaining in the ink container is calculated based on the compensated ink level sense values. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a block diagram of a printer/plotter system in which the present invention can be incorporated. 
     FIG. 2 illustrates a block diagram depicting major components of one of the print cartridges of the printer/plotter system of FIG.  1 . 
     FIG. 3 illustrates a block diagram depicting major components of one of the ink containers of the printer/plotter system of FIG.  1 . 
     FIG. 4 illustrates a simplified isometric view of an implementation of the printer/plotter system of FIG.  1 . 
     FIG. 5 illustrates a typical pressure sensor output, showing offset and non-linear response characteristics. 
     FIG. 6 illustrates a P-ILS system with an analog compensation system. 
     FIG. 7 illustrates a preferred P-ILS system according to the present invention, with a digital compensation system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
     The P-ILS system of the present invention will be discussed in the context of a printer/plotter with an ink supply housed separately from an inkjet printhead assembly. However, it will be understood by those of ordinary skill in the art that the techniques described herein are also applicable to other devices employing inkjet technology with ink supplies housed either separately from or together with inkjet printhead assemblies, including, but not limited to, computer printers and facsimile machines. 
     FIG. 1 illustrates a block diagram of a printer/plotter  50  in which the present invention can be employed. Such a printer/plotter is described in commonly-assigned U.S. Pat. No. 6,151,039 to Hmelar, which is hereby incorporated by reference. The Hmelar patent also discloses a technique for ink level estimation using an ink level sensor. In one embodiment, the ink level sensor in Hmelar is a two-coil sensor, which was described above in the Background of the Invention section. 
     As shown in FIG. 1, a scanning print carriage  52  holds a plurality of printer cartridges  60 - 66 , which are fluidically coupled to an ink supply station  100  that supplies pressurized ink to printer cartridges  60 - 66 . In one embodiment, each of the cartridges  60 - 66  comprises an inkjet printhead and an integral printhead memory, as schematically depicted in FIG.  2 . As shown in FIG. 2, printer cartridge  60  includes an inkjet printhead  60 A and an integral printhead memory  60 B. The ink provided to each of the cartridges  60 - 66  is pressurized to reduce the effects of dynamic pressure drops. 
     Ink supply station  100  contains receptacles or bays for accepting ink containers  110 - 116 , which are respectively associated with and fluidically connected to respective printer cartridges  60 - 66 . Each of the ink containers  110 - 116  includes a collapsible ink reservoir, such as collapsible ink reservoir  110 A that is surrounded by an air pressure chamber  110 B. An air pressure source or pump  70  is in communication with air pressure chamber  110 B for pressurizing the collapsible ink reservoir  110 A. In one embodiment, one pressure pump  70  supplies pressurized air for all ink containers  110 - 116  in the system. Pressurized ink is delivered to the printer cartridges  60 - 66  by an ink flow path that includes, in one embodiment, respective flexible plastic tubes connected between the ink containers  110 - 116  and respectively associated printer cartridges  60 - 66 . 
     In one embodiment, each of the ink containers  110 - 116  comprises an ink reservoir  110 A, an ink level sensor  110 C, and an integral ink cartridge memory  110 D, as schematically depicted in FIG. 3 for ink container  110 . 
     Referring again to FIG. 1, scanning print carriage  52 , printer cartridges  60 - 66 , and ink containers  110 - 116  are electrically interconnected to printer microprocessor controller  80 . Controller  80  includes printer electronics and firmware for the control of various printer functions, including analog-to-digital (A/D) converter circuitry for converting the outputs of the ink level sensing circuits  110 C of ink containers  110 - 116 . In one embodiment, each one of the ink containers  110 - 116  includes its own A/D converter for converting the output of ink level sensing circuit  110 C to digital values. Controller  80  controls the scan carriage drive system and the printheads on the print carriage to selectively energize the printheads, to cause ink droplets to be ejected in a controlled fashion on the print media  40 . Printer controller  80  further estimates remaining ink volume in each of the ink containers  110 - 116 , as described more fully herein. 
     A host processor  82 , which includes a CPU  82 A and a software printer driver  82 B, is connected to printer controller  80 . In one embodiment, host processor  82  comprises a personal computer that is external to printer  50 . A monitor  84  is connected to host processor  82 , and is used to display various messages that are indicative of the state of the inkjet printer. Alternatively, the printer can be configured for stand-alone or networked operation wherein messages are displayed on a front panel of the printer. 
     FIG. 4 shows in isometric view of a large format printer/plotter  120  in which the present invention can be employed. Printer/plotter  120  includes four off-carriage ink containers  110 ,  112 ,  114 ,  116 , which are shown positioned in an ink supply station  100 . The printer/plotter  120  of FIG. 4 further includes a housing  54 , a front control panel  56 , which provides user control switches, and a media output slot  58 . While this exemplary printer/plotter  120  is fed from a media roll, it should be appreciated that alternative sheet feed mechanisms can also be used. 
     Ink level sensor  110 C (shown in FIG. 3) is a preferably a pressure ink level sensor (P-ILS). In one embodiment, ink level sensor  110 C uses a piezo-resistive strain gauge bridge to measure pressure. Such bridges, while low-cost and reliable, require compensation to produce a desired output. The compensation processes typically include offset correction, slope or gain adjustment, linearization correction, and temperature compensation. 
     FIG. 5 illustrates a typical pressure sensor output  508  showing offset  514  and non-linear response characteristics. Compensation is used to produce a linear response, so that a given output voltage from ink level sensor  110 C can be related to a predictable pressure value. FIG. 5 shows two examples of linearization approximations, which are a “Best Straight Line Fit” approximation represented by line  510  and a “Straight Line Fit” approximation represented by broken line  512 . 
     Pressure sensor compensation has previously been accomplished by an analog compensation system as shown in FIG.  6 . P-ILS system  600  includes strain gauge bridge  602 , differential amplifier  604 , electronic correction system  606 , and analog-to-digital (A/D) converter  608 . The pressure applied to strain gauge  602  produces a differential output that is amplified by differential amplifier  604 . The output from amplifier  604  is provided to electronic correction system  606 . Electronic correction system  606  includes corrective inputs for offset, slope or gain, and linearization coefficients. Electronic correction system  606  modifies the uncompensated, amplified output from strain gauge  602  based on the offset, slope and linearization inputs to produce an analog compensated output. 
     The offset, slope and linearization inputs of correction system  606  are typically implemented using variable resistors. The variable resistors are set mechanically or trimmed automatically with lasers during manufacturing. The compensation resistors are trimmed to appropriate values based on characteristics of the sensor. The compensation resistors are then included as part of the pressure sensor assembly  600 . 
     The analog compensated output from correction system  606  is converted to digital values by A/D converter  608  for use by printer controller  80  (shown in FIG.  1 ). Each digital value output by A/D converter  608  is proportional to an associated pressure measurement. Printer controller  80  uses the digital values output by A/D converter  608  to estimate the ink level in the associated one of ink containers  110 - 116 . 
     FIG. 7 illustrates a preferred P-ILS system  700  according to the present invention. Strain gauge bridge  702  and amplifier  704  function the same as described with respect to FIG.  6 . Instead of modifying the amplifier output by a correction system  606  as in I-ILS system  600 , P-ILS system  700  provides the output from amplifier  704  directly to A/D converter  708 . Thus, the digital output produced by A/D converter  708  reflects uncorrected values with all of the offset, gain and non-linearization dependencies typically found in this sensor system. 
     During manufacture, the offset, gain and non-linearization correction components of P-ILS system  700  are determined based on characteristics of the sensor, just as in the analog system  600  of FIG.  6 . Instead of requiring correction factors to be stored in hardware resistor values, the correction factors of P-ILS system  700  are determined and stored in the associated memory  706 , which is integrated with the P-ILS system  700 . Since memory  706  is an integral part of the ILS system, storing compensation values in memory  706  costs nothing in terms of physical space within the system, as the values are stored along with the traditional values associated with the ink container. In one embodiment, memory  706  is an EEPROM. In one embodiment, selected compensation values are determined and stored in memory  706  after manufacture of the device. As one example, the offset compensation value can be stored in memory  706  after insertion of the ink container in the printer. By storing the compensation values after manufacture of the device, any changes in the sensor characteristics that occur during or after manufacture of the device will be taken into account and corrected by the digital compensation system. 
     The positioning of memory  706  depends upon the particular printer configuration. In a system where the inkjet printhead assembly and the ink supply are separately housed, such as the system shown in FIG. 1, a memory  706  is preferably positioned with each one of ink containers  110 - 116  (e.g., positioned like memory  110 D shown in FIG.  3 ). In a system where the inkjet printhead assembly and the ink supply are housed together in an inkjet cartridge, memory  706  is positioned with the inkjet cartridge. 
     In use, printer controller  80  addresses the integrated P-ILS system  700  digitally, and reads the digital output from the P-ILS system  700  and the compensation values stored in memory  706 . Printer controller  80  compensates the digital output from A/D converter  708  using the compensation values obtained from memory  706 , thereby producing a corrected pressure value for each sampled uncompensated pressure value. Printer controller  80  then estimates the ink level in the associated one of ink containers  110 - 116  based on the corrected pressure values. In one embodiment, the calculated ink level is output from printer controller  80  back to memory  706 , where it is stored. Thus, even if the ink container with memory  706  is removed from the printer and put in a second printer, the ink level in the ink container is easily obtainable by the second printer. 
     The digital compensation system of the present invention provides several advantages over the analog compensation system shown in FIG.  6 . Digital compensation values can be stored in memory  706  easier than analog resistors can be trimmed mechanically or automatically by laser trimmers. The cost of storing digital compensation values in memory  706  is less expensive than using on-board resistors or other on-board compensation components. Further, more elaborate compensation factors (such as a least-squares line fit) do not appreciably increase the cost of compensation. 
     Although specific embodiments have been illustrated and described herein for purposes of description of the preferred embodiment, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent implementations calculated to achieve the same purposes may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. Those with skill in the chemical, mechanical, electro-mechanical, electrical, and computer arts will readily appreciate that the present invention may be implemented in a very wide variety of embodiments. This application is intended to cover any adaptations or variations of the preferred embodiments discussed herein. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.