Abstract:
The invention provides a printing apparatus for printing an image having a spatial resolution onto a print medium. The printing apparatus includes a printhead having a plurality of printing elements for ejecting ink droplets having a mass onto the print medium, and a temperature sensor for sensing a temperature of the printhead, producing a temperature signal. The apparatus also includes a media sensor for determining what type of print medium is loaded in the printer and producing a print medium signal based on the type of print medium installed in the printer. A plurality of heating devices are located adjacent the printhead for adjusting the temperature the printhead. The apparatus includes a printer controller for receiving the temperature signal from the temperature sensor and the print medium signal from the media sensor, and determines the ink temperature based on the print medium signal, and activates the heating devices to maintain the temperature of the printhead within a desired temperature range that is dependent upon the print medium signal.

Description:
TECHNICAL FIELD 
     The invention relates generally to print quality control within an ink jet printer. More particularly, the invention relates to adjusting the print quality of an ink jet printer based on the print head temperature and the media type used during printing. 
     BACKGROUND OF THE INVENTION 
     Ink jet printers are becoming much more common as the printer of choice because of their relatively low cost compared to laser printers and the ability of ink jet printers to produce multi-color images on a variety of media types at reasonable costs per printed sheet. Recent improvements in ink jet printers include improvements in the print heads and the ink cartridges and improved or specialized ink formulations. These improvements have led to improved print quality which results in the ability to produce high quality and/or photographic images. As the use of ink jet printers continues to expand, the need to produce images on a variety of print media has also expanded. 
     For many applications, the type of print media used in an ink jet printer has little effect on the usefulness of the resulting printed product. However, for certain applications, it is important to identify the media being used for the printing operation. That is, ink spread varies depending on the type of media. For example, plain paper allows the ink to wick into the paper fibers which tends to create a larger spot. On the other hand, coated and glossy types of paper tend to keep the ink spot well rounded, not allowing the ink to spread. As a result, spots tend to be smaller on higher quality media than on plain paper for the same ink drop mass. As drop mass increases, spot size also increases. Drop mass can be controlled within limits by adjusting the ink temperature before firing. It is desirable to optimize the spot sizes so that they are substantially the same regardless of the media type used in the printing application. 
     Ink jet printers have the capability of printing on a variety of media types, from glossy paper to overhead projection media to plain paper. The variety of printable media places a demand on ink jet printer manufacturers and designers to print quality images on the full gamut of media types without losing print quality or speed. As described above, ink spreads differently depending on the media type used in the printer, which makes ink spot size optimization difficult when using media of varying characteristics. Furthermore, ink droplet misdirection may be exacerbated due to the media type used during the printing application. Ink droplet misdirection is a function of the ink temperature which determines the ink drop exit velocity. Faster exiting ink drops tend to be less misdirected when printed. 
     What is needed, therefore, is a method and apparatus to control ink drop properties based on temperature and media. 
     SUMMARY OF THE INVENTION 
     The foregoing and other needs are met by a printing apparatus and method for controlling the ink droplet mass of the ink ejected from the nozzles of a printhead in an ink jet printer. The printing apparatus is used for printing a desired image having a spatial resolution onto a print medium. The printing apparatus includes a printhead having a plurality of printing elements for ejecting ink droplets having a mass onto the print medium, and a temperature sensor for sensing a temperature of the printhead and producing a temperature signal. The apparatus also includes a media sensor for determining what type of print medium is loaded in the printer and producing a print medium signal based on the type of print medium installed in the printer. A plurality of heating devices are located adjacent the printhead for adjusting the temperature of the printhead. The apparatus includes a printer controller for receiving the temperature signal from the temperature sensor and the print medium signal from the media sensor, for determining the temperature of the ink based on the print medium signal, and for activating the heating devices to maintain the temperature of the printhead within a desired temperature range that is dependent upon the print medium signal. 
     In another aspect, the invention provides a method for adjusting the characteristics of ink droplets ejected from an inkjet printhead to form dots on a print medium. The dots printed during multiple passes of the printhead across the print medium form a printed image on the print medium. The method includes sensing a print medium type and sensing a printhead temperature. Moreover, the printer controller determines, based on the print medium type, an optimal temperature range in which to maintain the printhead temperature and maintains the printhead temperature within the optimal temperature range. The image is printed by ejecting the ink droplets from the printhead as the printhead passes across the print medium to form the printed image. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages of the invention will become apparent by reference to the detailed description when considered in conjunction with the figures, wherein like reference numbers indicate like elements through the several views, and wherein: 
     FIG. 1 is a functional block diagram of a printer utilizing two sensor inputs according to a preferred embodiment of the invention; 
     FIGS. 2A-2C depict a method for controlling the ink droplet mass and exit velocity according to a preferred embodiment of the invention; and, 
     FIG. 3 is a graphical representation depicting ink spot size varying with temperature. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Depicted in FIG. 1 is block diagram of a printer  10  which is controlled by a printer controller  12  for printing images onto a print medium  14 . The printer controller  12  receives image data or print data from a data source, preferably a host computer, such as an IBM personal computer (PC) or a network host. According to the invention, the printer controller  12  utilizes inputs from a media sensor  16  and a printhead temperature sensor  18 , to control the amount of ink output from a plurality of nozzles located adjacent to a silicon substrate in a printhead  20 , thereby printing a desired image onto the print medium  14 . 
     The media sensor  16  is preferably an optical sensor, for example, an infra-red (IR) light-emitting diode (LED) and optical detector set to reflect off of the print medium  14 . The optical sensor  16  may be a light diode sensor, wherein light is transmitted from the sensor by an LED, for example, reflected by the print medium  14  and received by the sensor. Preferably, the optical sensor  16  is a near IR optical sensor transmitting an IR signal, such as the QEC  113  (GaAs Infrared Emitting Diode), and utilizing a two channel detector, such as the QSC  113  (Silicon Phototransistor), both manufactured by QT Optoelectronics of Sunnyvale, CAlif., for determining amounts of spectral and diffuse reflectivities reflected from the print medium  14 . As used herein, spectral reflectivity is referred as a band of frequencies within the spectrum of transmitted light. The received spectral and diffuse reflectivities are compared to a standard, preferably in the form of a look-up table. The look-up table or standard is generated by calibrating the media sensor  16  with a variety of media types. 
     Preferably, the printhead temperature sensor  18  is a thermistor-type temperature sensing device. In a preferred embodiment, the temperature sensor  18  is a thermal sense resistor (TSR) embedded on the silicon substrate. Moreover, it is preferred that the TSR is formed in a serpentine fashion on the periphery of the silicon substrate. The resistance of the TSR varies with temperature at about 100 ohms per 10 degrees. Since the ink is located adjacent to the substrate, the TSR is operable to determine the ink temperature. 
     Firing resistors are located adjacent to the nozzles which transfer heat into the ink when it is time to print, thereby forming a bubble in the ink, expelling ink from the nozzles onto the print medium  14 . The ink firing resistors are preferably a layer of material, such as tantalum, aluminum, or silicon oxide, deposited on a silicon substrate. The printer controller  12  controls the amount of energy supplied to the firing resistors which is a factor in determining the ink bubble size and heat generation. The printer  10  may contain one monochromatic printhead  20  or a plurality of multicolored printheads  20 . 
     The printhead  20  is attached to a printhead carriage  22  which translates the printhead  20  across the print medium  14  as ink is ejected from the nozzles. Accordingly, the print resolution of printer  10  is defined as the number of ink droplets laid down per unit area as the carriage  22  translates across the print medium  14 . Generally, as the print mode changes from a lower to a higher resolution, the number of shingling passes that the carriage  22  makes increases, printing a greater number of droplets per unit area. Moreover, more shingling passes are made for the higher resolution print modes to reduce banding effects. According to the invention, by adjusting the printhead temperature to control the ink droplet mass, the print quality is optimized regardless of the number of shingling passes. The invention adjusts spot size to be roughly equivalent for all media types and printing modes by matching drop mass to the wicking properties of the media. That is, the resulting ink dot size on the print medium is optimized according to the sensed print medium and adjusted printhead temperature. 
     Preferably, a number of printhead heaters  24  are located adjacent to the printhead  20  for heating the printhead  20 . The printhead heaters  24  may include a plurality of resistive heating elements selectively located on the printhead  20 . In a preferred embodiment, the printhead heaters  24  are formed of the same material as the ink firing resistors, and may be formed on the same substrate layer. The printhead heaters  24  are operable to regulate the temperature of the printhead  20 , thereby controlling the temperature of the ink contained in the printhead  20 . Since the ink droplet mass is a function of the ink temperature, the printhead temperature is a determining factor of the ink droplet mass. Therefore, according to the invention, ink droplet mass is optimized by controlling the printhead temperature according to the current print medium. 
     According to the preferred embodiment of the invention, the printer controller  12  continually maintains the temperature of the printhead  20  using a closed loop thermal control routine, thereby controlling the size and mass of ink droplets being ejected from the nozzles of the printhead  20  onto the print medium  14 . Additionally, by adjusting the temperature of the printhead  20 , the ink droplet exit velocity is also controlled. By increasing the temperature of the printhead  20 , the ink contained within the printhead  20  is energized, and as a result, the ink droplets&#39; nozzle exit velocity increases proportionally with the printhead temperature. One contributor to ink droplet misdirection is ink having a low energy content, where energy content is proportional to the temperature of the printhead  20 . A lower energy content tends to produce lower droplet exit velocities, contributing to the misdirection. As the ink droplet energy and resulting exit velocity increases with increasing printhead temperature, ink droplet misdirection tends to be correspondingly decreased. Consequently, the printer  10  is operable to control the ink droplet mass and exit velocity, thereby printing images on a various types of media  14  without having to reconfigure the nozzle geometry, heater size, or ink chemistry to vary the ink droplet mass or nozzle exit velocity. Each of these latter alternatives for varying the ink droplet mass and exit velocity are expensive and inefficient practices for a particular printing application. Furthermore, according to the invention, the ink droplet mass and exit velocity is continually optimized for the current print medium  14 . 
     Referring now generally to FIGS. 2A-2C, the operation of a preferred embodiment of the invention will be described. When it is desired to print an image, the host computer sends a signal to printer  10 , and the printer  10  is initialized in preparation for printing (step  100 ). The optical media sensor  16  senses the type of print medium  14  and produces a corresponding print medium signal which is input into the printer controller  12  (step  102 ). The optical sensor  16  determines the medium type  14  by transmitting an optical signal towards the print medium  14  and measuring the amount of reflectivity based on the reflected signal received by the optical sensor  16  from the print medium  14 . Based on the reflected signal, the printer controller  12  determines the amount of light reflected by the print medium  14  and determines the print medium type based on the amount of reflection. As discussed below, the printer controller  12  configures the printer  10  to print according to the sensed medium type  14 . 
     Based on the print medium signal from the optical sensor  16 , the printer controller  12  enables a corresponding print mode which defines the resulting print resolution until the optical sensor  16  senses a different medium type  14 . In a preferred embodiment of the invention, before enabling the printhead heaters  24  and thereby heating the ink in the printhead  20 , the printer controller  12  takes into account the amount of heat generated by the firing resistors during a print swath by predicting their contribution to the printhead temperature. The predicted value is used in calculating minimum and maximum temperature targets for each print mode. At step  104 , the optical sensor  16  senses whether a plain type paper is the current print medium  14 . Used herein, plain type paper generally refers to paper having ordinary finish properties of paper typically sold for general printing and copying. Correspondingly, the printer controller  12  sets a minimum and a maximum printhead temperature target range for the plain type paper. For plain paper, the minimum temperature target is between about 20° C. and about 25° C. and the maximum temperature target is between about 30° C. and about 35° C. (step  106 ). The plain paper temperature target range is a preferred range of printhead temperatures at which to maintain the printhead  20  for optimal print quality when printing on plain type media. 
     Because ink has a tendency to wick into the fibers of a plain paper type, the plain paper tends to absorb more ink than a glossy type medium. FIG. 3 depicts the differing ink spot size when printing on plain versus glossy paper. For the same temperature, the plain paper has an ink spot size ranging from 22% to 35% larger than for the glossy paper. Accordingly, since the ink spreads more readily on the plain paper, a lesser amount of ink ejected onto the plain type paper will create substantially the same ink spot size as a greater amount of ink ejected onto the glossy type medium. Additionally, the ink droplet exit velocity is proportional to the ink temperature. As a result of the lower printhead temperature, any misdirection of ink droplets which may occur due to the lower printhead temperature is not as critical when printing on plain paper, since the ink tends to spread out more on the plain paper and mask any ink droplet misdirection. Therefore, a correspondingly lower printhead temperature may be used for the plain type medium than for other mediums, without sacrificing print quality. 
     When the optical sensor  16  detects a glossy type medium (step  108 ), a corresponding input to the printer controller  12  will set the minimum and a maximum printhead temperature target based on the glossy type medium. The minimum temperature target is between about 35° C. and about 40° C. and the maximum temperature target is between about 45° C. and about 50° C. (step  110 ). The printhead temperature targets are a preferred range of printhead temperatures at which to maintain the printhead  20  for optimal print quality with the same spot size as was achieved on plain paper. The glossy type paper tends to require more ink to print substantially the same spot size as for plain paper. Glossy type paper refers to glossy or coated type paper having clay or chemical coating. The glossy type paper requires more ink to be ejected from the printhead  10  since the ink droplet spread on the glossy type paper tends to be minimal. Additionally, ink droplet misdirection may be exacerbated when printing on glossy type paper due to the minimal ink droplet spread. The higher printhead temperature tends to lessen any associated misdirection due to the higher ink temperature and resulting higher ink droplet exit velocity from the printhead  20 . 
     Preferably, if the optical sensor  16  does not sense plain paper or glossy type paper as the current print medium  14 , the printer controller  12  sets the print targets to an intermediate temperature range (step  112 ). An example of an intermediate temperature range would have a minimum temperature target of between about 30° C. and about 35° C. and the maximum temperature target of between about 35° C. and about 40° C. 
     Once the printer controller  12  has set the minimum and maximum printhead temperature targets based on the current print media, the printer controller  12  sends a signal to the host computer instructing the host computer to send the image data to the printer  10  (step  114 ). The host computer then transmits the image data to the printer  10 , wherein the printer controller  12  stores the image data in memory in preparation for printing. Alternatively, the printer  10  may already have stored print data in memory, awaiting instructions for further processing from the printer controller  12 . 
     The printer controller  12  is structured to control the operation of the printer  10  according to an internal timer and associated interrupt structure. When the printer controller  12  determines that it is time to print, the printer controller  12  enables the closed loop thermal control routine (step  116 ). According to a preferred embodiment of the invention, once the printer  10  begins printing, the closed loop thermal control routine executes about every twenty-five (25) milliseconds according to the internal timer interrupt. Preferably, the printhead temperature is adjusted at about 5 millisecond to about 50 millisecond intervals. For a printer carriage  22  requiring 0.50 seconds to translate across the print medium  14 , and a control loop cycle time of 0.25 milliseconds, the closed loop thermal control routine cycles through at least twenty times during a single translation of the carriage, regulating the printhead temperature as required. The printer  10  is not limited to the above described carriage  22  translation speed or control loop interval, but each may be configured according to the preferred printing requirements. 
     The printer controller  12  utilizes the media sensor input  16  and the temperature sensor  18  input to regulate the temperature of at least one printhead  20  using the closed loop thermal control routine. As a result, the printer  10  is operable to control the ink mass ejected from the nozzles of the printhead  20  by controlling the temperature of the printhead  20  using the closed loop thermal control routine and printhead heaters  24 . It is preferred that the printer controller  12  sends a number of voltage pulses to the printhead heaters  24  which, in turn, controls the heat output from the printhead heaters  24 . The printer controller  12  is operable to control the duty cycle and number of the pulses to control the heater output and printhead temperature. Since the ink droplet mass varies according to the temperature of the printhead  20 , the printer  10  thereby controls the print density of the ejected ink droplets onto the print medium  14  by continually regulating the printhead  20  temperature. Table 1 depicts the effects on drop volume and spot diameter for a population of printheads as a result of using the closed loop thermal control routine utilizing the sensed media type input from the media sensor  16  for a glossy type medium  14 . 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Drop 
                   
                   
                   
                 Mis- 
                   
               
               
                 Tem- 
                 Vol- 
                 Standard 
                 Spot 
                 Standard 
                 direc- 
                 Standard 
               
               
                 perature 
                 ume 
                 Deviation 
                 Diameter 
                 Deviation 
                 tion 
                 Deviation 
               
               
                   
               
             
             
               
                 +22° C. 
                 23.2 
                 1.5 
                 64.8 
                 2.1 
                 18.6 
                 3.6 
               
               
                 +37° C. 
                 28.7 
                 1.7 
                 79.0 
                 8.5 
                 14.5 
                 4.9 
               
               
                 +47° C. 
                 34.6 
                 1.4 
                 91.1 
                 6.4 
                 10.8 
                 3.8 
               
               
                   
               
             
          
         
       
     
     The first column of Table 1 lists a number of temperatures at which the population of printheads were tested. For the population, columns 2 and 3 show the drop volume average and standard deviation at those temperatures (units of picoliters). As the operating temperature is increased from +22° C. to +47° C., the drop volume increases by 50%. The fourth and fifth columns show the resulting average single drop spot size and the corresponding standard deviation created by these printheads at different temperatures on the same media (units of microns). As the operating temperature is increased from +22° C. to +47° C., the spot diameter increases with increasing drop volume by about 40%, which means that the average paper area covered by a spot has approximately doubled. The sixth and seventh columns show the resultant average misdirection of the drops and the corresponding standard deviations (units of microns). Accordingly, drops fired at higher temperatures have more energy and as a result, the drop velocity increases. Correspondingly, the higher ink drop velocities result in less misdirection of the drops. 
     Moreover, the ink droplet exit velocity is also controlled according to the invention, since one aspect of the ink droplet exit velocity is determined by the temperature of the printhead  20 , which corresponds to the temperature of the ink contained therein. Therefore, based on the sensed medium type  14 , the printer controller  12  controls the printhead  20  temperature to achieve the desired print resolution and density without sacrificing print speed or quality. 
     At step  118 , the printer controller  12  receives the temperature sensor signal corresponding to the temperature of printhead  20  from the temperature sensor  18 . The printer controller  12  compares the sensed printhead temperature signal to the minimum temperature target according to the current print medium (step  120 ). For plain paper, if the sensed printhead temperature is greater than the minimum temperature target of between about 25° C. and about 30° C., the printer controller  12  disables the printhead heaters  24  (step  122 ). For intermediate paper type printing operations, if the sensed printhead temperature is greater than the minimum temperature target of between about 30° C. and about 35° C., again, the printer controller  12  disables the printhead heaters  24  (step  122 ). Likewise, for glossy or coated paper types, if the sensed printhead temperature is greater than the minimum temperature target of between about 35° C. and about 40° C., the printer controller  12  also disables the printhead heaters  24  (step  122 ). 
     If the sensed printhead temperature is less than the minimum temperature target of between about 25° C. and about 30° C. for plain paper types, the printer controller  12  enables the printhead heaters  24  (step  124 ). For intermediate paper type printing operations, if the sensed printhead temperature is less than the minimum temperature target of between about 30° C. and about 35° C., again, the printer controller  12  enables the printhead heaters  24  (step  124 ). Similarly, for glossy or coated type paper. if the sensed printhead temperature is less than the minimum temperature target of between about 35° C. and about 40° C., the printer controller  12  enables the printhead heaters  24  (step  124 ). 
     In a preferred embodiment of the invention, if the sensed printhead temperature is less than the minimum temperature target for each print mode, the printer controller  12  calculates a printhead heater value based on the temperature difference from the target, and the cooling characteristics of the printhead design. Heat is enabled by varying the voltage pulses sent by the printer controller  12  to the heaters  24 , as described above. In the preferred embodiment, this is an 80% duty cycle for the temperatures below 2° C. below target, and a linear interpolation between 2° C. below and the target. 
     For a printer  10  having more than one printhead  20 , the printer controller  12  cycles through each printhead  20  (steps  126  and  128 ), receiving the current temperature signal from the temperature sensor  18  located adjacent to each printhead  20 . The sensed printhead temperature signal is compared to the minimum and maximum temperature targets, as described above and the printhead heaters  24  are enabled or disabled. Once the printer controller  12  has cycled through each printhead  20  (steps  126  and  128 ), the printer controller  12  determines whether the sensed temperature signal for each printhead  20  is greater than the minimum temperature target and less than the maximum temperature target (step  130 ). If the printhead temperature is not within the minimum and maximum printhead temperature target range, then the printer controller  12  again cycles through the closed loop thermal control routine at the next timer interrupt (steps  130  and  116 ). On the other hand, if the printhead temperature is within the maximum and minimum temperature target range, the printer controller  12  instructs the printer  10  to print a swath across the print medium  14  by ejecting ink from the printhead  20  as the carriage  14  translates across the print medium  14  (step  132 ). The printer controller  12  then reverts back to step  100  and cycles through the print control routine until the image data is completely printed. 
     Since the printhead temperature is continually sensed and processed by the printer controller  12  to control the printing process, the ink droplet mass and nozzle exit velocity is thereby continually and effectively regulated during printing. As a result, the printer  10  is able to produce a quality printed image on any medium type  14  without having to reconfigure the nozzle geometry, heater size, or ink chemistry. Accordingly, the printer controller  12  is operable to continually regulate at least one printhead temperature based on a sensed medium type  14  and sensed printhead temperature. 
     Having described various aspects and embodiments of the invention, and several advantages thereof, it will be recognized by those of ordinary skills that the invention is susceptible to various modifications, substitutions and revisions within the spirit and scope of the appended claims. For example, the invention is not limited to the print mode being determined by the optical sensor  16  input. A user may prefer to print an image having a higher resolution and may therefore override the optical sensor  16  input through a menu interface, either on the host computer or on the printer  10  itself. Furthermore, the present invention is not limited to three print modes, but may have more or less depending on the printer  10 .