Abstract:
A method and system is provided for more accurately determining ink in-flow and out-flow to a reservoir in a solid-to-liquid ink phasing delivery system for supplying ink to a printer. The printer throughput is safely maximized with a software algorithm that measures the ink available in the printer reservoir for printing. The algorithm is based on the known amount of ink in the reservoir when a level sensor probe is tripped and then calculates additional changes in ink volume. The process is done until the algorithm determines the reservoir volume is below a predetermined minimum level when the level sense probe senses ink. The algorithm calculates the ink leaving the reservoir using an out-flow model based on pixel counting and calculates ink entering the reservoir using an in-flow model based on a minimum guaranteed amount of ink delivered from the melt heater. A time out period is further calculated in which the reservoir should be refilled, and if not, the system is checked for an ink stick jam.

Description:
BACKGROUND 
     The present exemplary embodiments relate to printing systems and, in particular, printing devices which utilize a supply of colored inks to be communicated to a print head for document printing. More particularly, the present embodiments utilize solid ink sticks as the supply ink, which must be heated to a liquid form before being capable of communication to the print head. Such systems are commercially available under the PHASER® mark from Xerox Corporation. 
     The present embodiments concern the structure, control system and operation methods of the heater element for causing a phase change in the solid ink supply to a liquid form capable of fluid communication to a print head for document printing. 
     The basic operation of such phasing print systems comprises the melting of a solid ink stick, its communication to a reservoir for interim storage, and then a supply process from the reservoir to a print head for printing of a document. One object of the control strategy is to avoid the printing system running out of ink while trying to print, because such an event can be a catastrophic failure to the system. Prior known systems will typically supply a measuring device in the reservoir to monitor ink levels therein. When the ink drops below a certain level due to normal usage, then the ink supply control system would melt more of the solid ink supply until the reservoir would refill to the desired level. The steps of asking for more ink, turning on the heater to melt the solid ink, delivering the ink to the reservoir to a desired level and then turning the heater off is commonly referred to as an “ink melt duty cycle.” 
     Conventional systems used a fixed applied power supply to the heater that was intended to provide a desired melt rate for the ink into a reservoir that was relatively large (approximately twenty-two grams of ink could be held therein). The ink level detector would initiate an ink melt duty cycle when the measuring device indicated that the ink level had dropped below a predetermined level. In the situation where an ink stick jam has occurred, i.e., the solid ink stick is obstructed from sliding down the ink loader tray to engagement with the heater, the continued supply of energy to the heater would not be able to melt the solid ink stick, because the stick was spaced from the heater itself. If the reservoir were to actually run dry, the printing system would suffer a catastrophic failure and would be unable to print. In addition, the continued application of the power to the elements of the heater could cause high temperature damage to the heater itself and to adjacent componentary. The print head could become clogged requiring an expensive maintenance repair with significant printer down time. 
     In order to avoid the possibility of running out of ink after it had dropped below a certain level in the reservoir, conventional systems employed a timer which would time out after a preselected amount of time that was assumed would not be enough time to let the reservoir run out, even for maximum printing usage of that color. If the measuring device did not indicate a refill of the reservoir during the time out period, the controller would disable the application of energy to the heater, thus assuming an ink stick jam. The system would disable further printing and heating after the elapse of the timer time-out cycle. This time out was calculated as the number of seconds of usable ink remaining in the reservoir based on the print image and mode that uses the maximum amount of ink. After the time out, the software would disallow printing until the ink level in the reservoir increased and was sensed by the ink level sense probe. Since the time out is based on the maximum possible ink usage, the printer was frequently not allowed to print which caused the printing rate to fall below specifications. Lastly, after the time out, the ink stick jam could then be identified and corrected, and the reservoir would then have to be refilled before printing could recommence. 
     The present exemplary embodiments are intended to employ a smaller reservoir of approximately five to six grams of ink. Smaller reservoirs present an advantage of not having to heat larger ink portions to remain liquid in the print head. In a maximum fill printing operation, the smaller reservoirs can be drained relatively quickly so that a time-out operation before assessing an ink stick jam presents an unacceptable risk of a reservoir going dry and consequential damage to the print head and the jets therein. There is a need for a system which can provide a more accurate indication of an ink stick jam to provide for improved operating control of system operation and improved safety against a catastrophic failure of a dried out reservoir occurring during a print operation. There is also a need for more efficiency in controlling the melt duty cycle to improve the overall printing rate. The present exemplary embodiments satisfy these needs as well as others to provide an improved method and assembly for detecting an ink stick jam in a phasing printing system. However, it is to be appreciated that the present exemplary embodiments are also amenable to other like applications where the supply of power to the heating element needs to be interrupted relatively soon due to the failure to supply an item intended to be heated by the heater element. 
     BRIEF DESCRIPTION 
     Printer throughput is safely maximized with a software algorithm that estimates the ink available in the print head reservoir for printing. This algorithm is based on the known amount of ink in the reservoir when the level sense probe is tripped and then calculates additional changes in ink volume. This process is done until the algorithm determines that the reservoir volume is below a predetermined minimum level or when the level sense probe senses ink. The algorithm calculates the ink leaving the reservoir using an out-flow model based on pixel counting and calculates the ink entering the reservoir using an in-flow model based on a minimum guaranteed amount of ink delivered from the melt heater. 
     A method and system is provided for controlling an ink melt heater in a solid-to-liquid ink phasing delivery system for supplying ink to a printer. The phasing system includes a heater disposed to engage a solid ink stick and heat an engaging portion of ink stick to a liquid phase for communication to a reservoir associated with a print head. The reservoir includes an ink level detector. A controller selectively supplies a predetermined amount of power to the heater. The ink level detector measures an amount of liquid ink in the reservoir. When the amount of liquid ink is measured to be at a predetermined level, the controller calculates an amount of ink thereafter delivered from the reservoir for printing. When the calculating indicates that the amount of ink delivered from the reservoir approximates an amount of ink stored therein at the predetermined level, and the level detector indicates that the ink in the reservoir remains above the predetermined level, the controller halts the supply of power to the heater. The calculating preferably comprises counting pixels printed by the printer with ink from the reservoir. Additionally, the controller can time the period during which the ink in the reservoir is below the predetermined level and can compare that timing with an estimated time for refilling the reservoir to the predetermined level with a minimum guaranteed amount of ink delivered from the melt heater. When the comparing indicates that the amount of ink in the reservoir should have been refilled to the predetermined level, and the measuring indicates that the ink in the reservoir still remains below the predetermined level, the controller halts the supply of power to the heater and the delivery system is checked for an ink stick jam. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view in partial section of a print head, ink stick and ink loader assembly, and power supply and control system therefor; 
         FIG. 2  is an end view of one embodiment of a heater melt plate; 
         FIG. 3  is a diagramatic view of a print head reservoir including an ink level sense detector; and 
         FIG. 4  is a flow chart illustrating the overall operating steps of heater control in one embodiment; and 
         FIG. 5  is a flow chart for particularly illustrating the control steps for ink stick jam detection using a variable timer with printer pixel counting. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , the basic elements of an ink supply system in an ink “phase-changing” printing system can be seen. Ink loader assembly  10  includes a tray  12  for holding a solid phase ink stick  14 . An ink melt heater  16  is disposed at an open end  18  of the tray to contact a proximate portion of the ink stick and to allow for egress of liquid phase ink during heating from the tray  10 . The heating plate  16  receives its heating energy from a power supply and control system  20 . The heating element includes an assembly with resistance traces thereon so that electrical energy supplied thereto can be converted to heat energy. 
       FIG. 1  is intended to illustrate an accurate positional disposition of the ink stick in the tray  11  to illustrate that the ink stick is urged against the heater plate  16  by both gravity and some other applied force means such as a spring bias (not shown) or the like. If, as the ink stick  14  is urged towards the heating plate  16 , some obstruction causes it to be unable to slide into engaging contact, the heater plate  16  can rise to a temperature substantially in excess of the desired melt rate temperature due to the absence of a cooling effect of a melting ink stick against it. 
     Ink stick jams can occur due to the cracking of the ink stick itself over time and the falling of particles from the stick on to the glide surfaces of the tray  12 . Alternatively, the stick  14  could somehow be moved out of the track path or become skewed in the path to limit its ability to slide down the tray. The door (not shown) which allows the refilling of a solid ink stick into the tray could be detached and also could obstruct the ink stick&#39;s movement. Other causes could be dirt falling into the tray or any other causes of friction between the tray glide surface and the stick. However, whatever the cause, the failure of the ink stick to engage the heater plate  16  can cause overheating damage to the plate, and when such a lack of ink supply causes the print head assembly to run out of ink, the failure can be catastrophic. 
       FIG. 1  shows an ink drip  40  falling from the tray  10  and the heating element  16  assembly from ink drip point  36  into a print head assembly  42 . Print head assembly  42  comprises a reservoir  44  to receive the melted ink and to communicate with the ink through nozzles (not shown) within the print head assembly for printing on a document. The reservoir is intended to hold approximately five to six grams of melted ink and is accordingly heated to maintain the ink stored therein in liquid form. 
     With particular reference to  FIG. 2 , power pads  30  connectwires (not shown) from the power supply to the heating plate  16 . The plate includes a first portion  32  disposed to engage the ink stick and phase change the solid ink stick to a liquid. A heated liquid ink zone  34  then allows the liquid ink to flow to an ink drip point  36 . It should be appreciated that the embodiment shown in  FIG. 2  comprises the side of the heater element having the heat traces shown. The ink stick will actually contact the element comprising a metallic heat plate on a back side from that shown in  FIG. 2 . A rivet hole  38  is used to attach the assembly of heat traces to the metallic plate. A temperature sensing device  50  is associated with the heating plate  16  for detecting a temperature thereof. Although numerous temperature sensing devices are available, such as thermometers, electrical sensors, chemical sensors, or the like, in this presently preferred embodiment a thermistor  50  mounted on a depending portion  51  and in direct communication with the control system  20  effectively detects a signal representative of the temperature of the heater. 
     The present preferred embodiment comprises an algorithm that monitors the amount of ink in the reservoir  44  to accomplish the overall objective of controlling the heater  16  to provide ink to the print head  42 , but also implement such overall control in an intelligent method which tracks the amount of usable ink in the reservoir  44  and provides an ink jam detection system to reliably maximize printing speed and avoid print head ink starvation. The present embodiment comprises an improved algorithm to significantly improve the printer controller&#39;s knowledge of the ink level in the print head reservoir, which reduces the amount of time that printing is disallowed, thereby increasing printing speed. The algorithm also prevents starvation in the print head caused by an inadequate ink level in the reservoir  44 . 
     With particular reference to  FIG. 3 , a diagramatic view of a reservoir  44  shows an ink level detector  70  of the type that measures ink level by a closed circuit wherein an ink meniscus line  72  extending between the detector  70  and the side wall  74  of the reservoir forms a closed circuit. When the meniscus  72  snaps to form an open circuit, the controller  20  knows that the ink has fallen to a certain level. More particularly, the detector  70  indicates two ink levels within the reservoir  44 . The first level indicated by line  76  is the position of the detector  70  within the reservoir  44 . A second level  78  represents an ink level at which the meniscus  72  is likely to snap as the ink level continues to fall from the reservoir. In particular, in the present embodiment, the reservoir is assumed to hold approximately 6 grams of ink when the ink volume is at least at the level indicated by line  76 . The maximum meniscus volume, i.e., the ink volume between level  76  and  78 , is about 1.8 grams and the nominal volume of ink when the meniscus snaps, i.e., the volume below level  78 , is approximately 3.3 grams. Accordingly, in a printing operation, as ink is supplied from the reservoir to the print head, the ink volume will continue to flow out until the level detector indicates an open circuit, at which point the controller  20  will consider that the remaining usable volume of ink in each reservoir in the system is approximately 3.3 grams. Upon refilling, with melted ink supplied from the heater, the detector  70  will not function as part of a closed circuit until ink volume has risen again to level  76 , i.e., approximately 6 grams. In addition, the control strategy of the present embodiments assumes corrections for nominal volume variances comprising reservoir cavity tolerances, printer tilt or level sense tolerances. 
     With particular reference to  FIG. 4 , an overall general control strategy for controlling ink flow into the reservoir is shown. As the printer is performing a print job  80 , there is no need to initiate a melt duty cycle by applying power to the heater while the ink is touching  82  the detector  70  indicating that the volume of ink in the reservoir is at least the nominal usable volume (i.e., 3.3 grams). The selective applying of power to the heaters based upon detected volume of ink in the reservoir  40  is referred to as a “melt-on-demand” operation. When the printing has depleted the volume of ink in the reservoir to below the nominal usable volume, i.e., the meniscus no longer forms a closed circuit with the sensor  70 , power is supplied  86  to the heater  16  to cause an ink in-flow into the reservoir  44 . While printing during the melt duty cycle, the control strategy is to calculate the approximate volume of ink in the reservoir by assuming a preselected amount of in-flow into the reservoir from the heater and by monitoring the amount of ink flow out of the reservoir by counting the number of pixels printed with ink from that reservoir. If the ink in-flow from the melting, less the out-flow for the printing, does not refill the reservoir within a certain amount of time determined to be appropriate for the refill, then the controller  20  stops the print job and indicates that a detection should be made for an ink jam within the loader tray.  FIG. 4  indicates that the inquiries made at steps  82  and  88  are accomplished every 0.4 seconds, for essentially a continuous monitoring of system operation. 
     With reference to  FIG. 5 , a more detailed flow chart for monitoring ink in-flow versus out-flow over a limited time is shown. 
     After printing every single page in a print job and before printing the next page of the job, the controller  20  needs to be satisfied that there is enough ink in the print head to complete the printing of that page.
 
Print If(Minimum ink volume+Ink in-flow−Ink out-flow)&gt;ink mass of 100% fill print   (1)
 
     By “100% fill print” is meant the amount of ink which would be drawn from a reservoir if all of the jets for that reservoir were printing continuously for the entire page. A single page 100% fill print is thus considered the reservoir ink safety margin. As noted above, the prior art system of having a fixed timer based on the highest possible ink out-flow and the lowest reservoir volume heavily penalizes the print speed of a user who might have light fill jobs, which is the majority of users for most jobs. The subject embodiment therefore satisfies the demand for a printing and ink delivering algorithm that optimizes the print speed for the majority of the users and avoids catastrophic ink starvation failures. 
     As noted above, the maximum meniscus ink volume is approximately 26% of the print head reservoir volume, or about 1.8 grams in a 6 grams reservoir. Certain compensating factors, such as printer tilting and tolerance factors for the reservoir capacity and the level sense probe are also included as adjustments when considering the usable volume of ink in accordance with the present embodiments. The following equation defines the maximum available volume of ink as used in the subject algorithm and which comprises the nominal volume of the reservoir (approximately 6 grams) minus the maximum meniscus volume (approximately 1.8 grams) and minus certain selected tolerance factors (1.5×RSS (RSS represents reservoir cavity tolerances, printer tilt factors and level sense probe tolerances). Accordingly, the available ink can be represented by the following equation:
 
Mavail — for — pixel — ct=nom — usable — vol−meniscus−(1.5×RSS (reservoir cavity, printer tilt, level sense probe)   (2)
 
     Pixel counting starts after time zero  92  ( FIG. 5 ) and the timer increments  94  to calculate  96  the ink in the reservoir  44 . “Time zero” is defined as the event in the print head when ink in the reservoir  44  drops below the level sense probe  70 , i.e., level  78 , snapping the ink meniscus  72 . At time zero, a minimum guaranteed volume of ink is present. Ink usage accounting comprising the ink used in the printing process is deducted from this minimum volume. In the exemplary embodiment described above, the ink volume in the reservoir at time zero should be about 3.3 grams and from this volume at time zero, the volume out (V o ) for printed pixels is subtracted. 
     After a print job has been completed, real time ink jetting data is available. This data includes a number of pixels printed of each color. Total volume used of each color can be calculated by using the maximum drop mass and subtracting totals from the estimated volume of ink for each color&#39;s reservoir channel. The print head would usually have three different color channels. The maximum drop mass is calculated by taking a preselected drop size for the print head and adding an adjustment factor to account for variation in drop size caused by life of the print head and dither (for example, 11.5% volume may be added). After the job has been printed, the volume of ink in the reservoirs is updated based on actual pixels printed in accordance with the following equation:
 
New volume of ink=old volume−[(pixels printed of that color)×(maximum dropmass)]  (3)
 
     Additionally, prior to allowing a print job, the system ensures that there is adequate ink volume in the reservoir  44  to print a page. Since ink volume used per print is only available after a job has been printed, the printer must make certain that enough ink is present in each of the four reservoir channels for a single color print at 100% fill prior to allowing any print job. The paper size and print type is available in the preprint command. Using that information the algorithm calculates a maximum ink potentially required for the given print type at maximum drop mass, i.e., the volume of ink in the reservoir, Vres must be greater than or equal to the volume required for safely completing the job, V sf . The “type” of print job is important for example because transparencies are dual pass and may use twice as much ink as paper print. If there is not enough ink in the reservoir for a “highest demand” print, the subject algorithm does not allow printing until the ink level reaches the ink level sense probe  70 , i.e., level  76  in reservoir  44 . When the ink reaches this level, the ink loader has melted enough ink to refill the reservoir channel and the minimum mass available for pixel counting is reset. Accordingly, after this system has calculated  100 , the volume of ink in the reservoir left after the pixel counting process, a time is computed  102  necessary to fill the reservoir up to the probe, i.e., level  76 . A predetermined minimum amount of ink is assumed to flow into the reservoir from the heater for this refill process. If the refill does not occur  104  within the minimum time (T a ) so computed, the system will check for an ink jam  106 . 
     The exemplary embodiment has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the exemplary embodiment be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.