Abstract:
A sensor configuration for use in detecting ink droplets ejected from an ink drop generator is provided. The sensor configuration includes a sensing element configured to receive a biasing voltage which creates an electric field from the sensing element to the ink drop generator. The sensor configuration also includes a sensing amplifier coupled to the sensing element, whereby the sensing element in imparted with an electrical stimulus when at least one ink droplet is ejected in the presence of the electric field, and thereafter passes in close proximity to the sensing element without substantially contacting the sensing element. Sensor configurations with a separate electrically biasing element which may or may not contact the ink droplets are also provided. Additionally, a printing mechanism having such sensor configurations and a method of making ink drop detection measurements are also provided.

Description:
INTRODUCTION  
         [0001]    The present invention relates generally to printing mechanisms, such as inkjet printers or inkjet plotters. Printing mechanisms often include an inkjet printhead which is capable of forming an image on many different types of media. The inkjet printhead ejects droplets of colored ink through a plurality of orifices and onto a given media as the media is advanced through a printzone. The printzone is defined by the plane created by the printhead orifices and any scanning or reciprocating movement the printhead may have back-and-forth and perpendicular to the movement of the media. Conventional methods for expelling ink from the printhead orifices, or nozzles, include piezo-electric and thermal techniques which are well-known to those skilled in the art. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, the Hewlett-Packard Company.  
           [0002]    In a thermal inkjet system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are individually addressable and energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. The inkjet printhead nozzles are typically aligned in one or more linear arrays substantially parallel to the motion of the print media as the media travels through the printzone. The length of the linear nozzle arrays defines the maximum height, or “swath” height of an imaged bar that would be printed in a single pass of the printhead across the media if all of the nozzles were fired simultaneously and continuously as the printhead was moved through the printzone above the media.  
           [0003]    Typically, the print media is advanced under the inkjet printhead and held stationary while the printhead passes along the width of the media, firing its nozzles as determined by a controller to form a desired image on an individual swath, or pass. The print media is usually advanced between passes of the reciprocating inkjet printhead in order to avoid uncertainty in the placement of the fired ink droplets. If the entire printable data for a given swath is printed in one pass of the printhead, and the media is advanced a distance equal to the maximum swath height in-between printhead passes, then the printing mechanism may achieve its maximum throughput.  
           [0004]    Often, however, it is desirable to print only a portion of the data for a given swath, utilizing a fraction of the available nozzles and advancing the media a distance smaller than the maximum swath height so that the same or a different fraction of nozzles may fill in the gaps in the desired printed image which were intentionally left on the first pass. This process of separating the printable data into multiple passes utilizing subsets of the available nozzles is referred to by those skilled in the art as “shingling,” “masking,” or using “print masks.” While the use of print masks does lower the throughput of a printing system, it can provide offsetting benefits when image quality needs to be balanced against speed. For example, the use of print masks allows large solid color areas to be filled in gradually, on multiple passes, allowing the ink to dry in parts and avoiding the large-area soaking and resulting ripples, or “cockle,” in the print media that a single pass swath would cause.  
           [0005]    A printing mechanism may have one or more inkjet printheads, corresponding to one or more colors, or “process colors” as they are referred to in the art. For example, a typical inkjet printing system may have a single printhead with only black ink; or the system may have four printheads, one each with black, cyan, magenta, and yellow inks; or the system may have three printheads, one each with cyan, magenta, and yellow inks. Of course, there are many more combinations and quantities of possible printheads in inkjet printing systems, including seven and eight ink/printhead systems.  
           [0006]    Each process color ink is ejected onto the print media in such a way that the drop size, relative position of the ink drops, and color of a small, discreet number of process inks are integrated by the naturally occurring visual response of the human eye to produce the effect of a large colorspace with millions of discernable colors and the effect of a nearly continuous tone. In fact, when these imaging techniques are performed properly by those skilled in the art, near-photographic quality images can be obtained on a variety of print media using only three to eight colors of ink. This high level of image quality depends on many factors, several of which include: consistent and small ink drop size, consistent ink drop trajectory from the printhead nozzle to the print media, and extremely reliable inkjet printhead nozzles which do not clog.  
           [0007]    Unfortunately, however, there are many factors at work within the typical inkjet printing mechanism which may clog the inkjet nozzles, and inkjet nozzle failures may occur. For example, paper dust may collect on the nozzles and eventually clog them. Ink residue from ink aerosol or partially clogged nozzles may be spread by service station printhead scrapers into open nozzles, causing them to be clogged. Accumulated precipitates from the ink inside of the printhead may also occlude the ink channels and the nozzles. Additionally, the heater elements in a thermal inkjet printhead may fail to energize, despite the lack of an associated clogged nozzle, thereby causing the nozzle to fail.  
           [0008]    Clogged or failed printhead nozzles result in objectionable and easily noticeable print quality defects such as banding (visible bands of different hues or colors in what would otherwise be a uniformly colored area) or voids in the image. In fact, inkjet printing systems are so sensitive to clogged nozzles, that a single clogged nozzle out of hundreds of nozzles is often noticeable and objectionable in the printed output.  
           [0009]    It is possible, however, for an inkjet printing system to compensate for a missing nozzle by removing it from the printing mask and replacing it with an unused nozzle or a used nozzle on a later, overlapping pass, provided the inkjet system has a way to tell when a particular nozzle is not functioning. In order to detect whether an inkjet printhead nozzle is firing, a printing mechanism may be equipped with a low cost ink drop detection system, such as the one described in U.S. Pat. No. 6,086,190 assigned to the present assignee, Hewlett-Packard Company. The nozzle plate of a printhead is inherently near ground potential due to the power supply connections on the printhead. A conductive target may be placed a few millimeters below the nozzle plate, and a biasing voltage may be applied to the target, forming an electric field between the nozzle plate and the target. Upon firing an ink drop, as the ink drop begins to exit the nozzle, a charge accumulates on the protruding tip of the drop, due to the influence of the nozzle-plate-to-target electric field. When drop breakoff occurs, the drop retains this charge. When the drop contacts the target, a small current, in relation to the charge on the drop, is induced from the target to ground. The periodic flow of current from drops striking the target may be converted to a signal voltage by an amplifier which is AC-coupled to the target, and then an analog-to-digital converter may digitize the output signal for processing to determine if a nozzle or group of nozzles are working properly.  
           [0010]    In practical implementation, however, this drop detection system has some limitations. Successive drops of ink, drying on top of one another quickly form stalagmites of dried ink which may grow toward the printhead. Since it is preferable to have the electrostatic sensing element very close to the printhead for more accurate readings, these stalagmites may eventually interfere with or permanently damage the printhead, adversely affecting print quality. Therefore, it is desirable to have a low cost and efficient method and mechanism for ink drop detection which is less susceptible to waste ink residue build-up. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a fragmented perspective view of one form of an inkjet printing mechanism illustrated with one embodiment of an absorbent conductive drop detector.  
         [0012]    FIGS.  2 - 3  are an enlarged, side elevational views illustrating separate embodiments of a drop detector coupled with a paper path support.  
         [0013]    [0013]FIG. 4 is an enlarged, side elevational view of illustrating an embodiment of a drop detector integral with a paper path support.  
         [0014]    FIGS.  5 - 12  are enlarged, partially fragmented perspective views illustrating separate embodiments of non-contact drop detectors.  
         [0015]    FIGS.  13 - 20  are enlarged, partially fragmented perspective views illustrating separate embodiments of non-contact charger drop detectors.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    [0016]FIG. 1 illustrates an embodiment of a printing mechanism, here shown as an inkjet printer  20 , constructed in accordance with the present invention, which may be used for printing on a variety of media, such as paper, transparencies, coated media, cardstock, photo quality papers, and envelopes in an industrial, office, home or other environment. A variety of inkjet printing mechanisms are commercially available. For instance, some of the printing mechanisms that may embody the concepts described herein include desk top printers, portable printing units, wide-format printers, hybrid electrophotographic-inkjet printers, copiers, cameras, video printers, and facsimile machines, to name a few. For convenience the concepts introduced herein are described in the environment of an inkjet printer  20 .  
         [0017]    While it is apparent that the printer components may vary from model to model, the typical inkjet printer  20  includes a chassis  22  surrounded by a frame or casing enclosure  24 , typically of a plastic material. The printer  20  also has a printer controller, illustrated schematically as a microprocessor  26 , that receives instructions from a host device, such as a computer, print server, or personal data assistant (PDA) (not shown). A screen coupled to the host device may also be used to display visual information to an operator, such as the printer status or a particular program being run on the host device. Printer host devices, such as computers and PDA&#39;s, their input devices, such as a keyboards, mouse devices, stylus devices, and output devices such as liquid crystal display screens and monitors are all well known to those skilled in the art.  
         [0018]    A conventional print media handling system (not shown) may be used to advance a sheet of print media (not shown) from the media input tray  28  through a printzone  30  and to an output tray  31 . A carriage guide rod  32  is mounted to the chassis  22  to define a scanning axis  34 , with the guide rod  32  slideably supporting an inkjet carriage  36  for travel back and forth, reciprocally, across the printzone  30 . A conventional carriage drive motor (not shown) may be used to propel the carriage  36  in response to a control signal received from the controller  26 . To provide carriage positional feedback information to controller  26 , a conventional encoder strip (not shown) may be extended along the length of the printzone  30  and over a servicing region  38 . A conventional optical encoder reader may be mounted on the back surface of printhead carriage  36  to read positional information provided by the encoder strip, for example, as described in U.S. Pat. No. 5,276,970, also assigned to the Hewlett-Packard Company, the present assignee. The manner of providing positional feedback information via the encoder strip reader, may also be accomplished in a variety of ways known to those skilled in the art.  
         [0019]    In the printzone  30 , the media sheet is supported by paper path ribs  39  and receives ink from an inkjet cartridge, such as a black ink cartridge  40  and a color inkjet cartridge  42 . The cartridges  40  and  42  are also often called “pens” by those in the art. The black ink pen  40  is illustrated herein as containing a pigment-based ink. For the purposes of illustration, color pen  42  is described as containing three separate dye-based inks which are colored cyan, magenta, and yellow, although it is apparent that the color pen  42  may also contain pigment-based inks in some implementations. It is apparent that other types of inks may also be used in the pens  40  and  42 , such as paraffin-based inks, as well as hybrid or composite inks having both dye and pigment characteristics. The illustrated printer  20  uses replaceable printhead cartridges where each pen has a reservoir that carries the entire ink supply as the printhead reciprocates over the printzone  30 . As used herein, the term “pen” or “cartridge” may also refer to an “off-axis” ink delivery system, having main stationary reservoirs (not shown) for each ink (black, cyan, magenta, yellow, or other colors depending on the number of inks in the system) located in an ink supply region. In an off-axis system, the pens may be replenished by ink conveyed through a conventional flexible tubing system from the stationary main reservoirs which are located “off-axis” from the path of printhead travel, so only a small ink supply is propelled by carriage  36  across the printzone  30 . Other ink delivery or fluid delivery systems may also employ the systems described herein, such as “snapper” cartridges which have ink reservoirs that snap onto permanent or semi-permanent print heads.  
         [0020]    The illustrated black pen  40  has a printhead  44 , and color pen  42  has a tri-color printhead  46  which ejects cyan, magenta, and yellow inks. The printheads  44 , 46  selectively eject ink to form an image on a sheet of media when in the printzone  30 . The printheads  44 ,  46  each have an orifice plate with a plurality of nozzles formed therethrough in a manner well known to those skilled in the art. The nozzles of each printhead  44 ,  46  are typically formed in at least one, but typically a plurality of linear arrays along the orifice plate. Thus, the term “linear” as used herein may be interpreted as “nearly linear” or substantially linear, and may include nozzle arrangements slightly offset from one another, for example, in a zigzag arrangement. Each linear array is typically aligned in a longitudinal direction perpendicular to the scanning axis  34 , with the length of each array determining the maximum image swath for a single pass of the printhead. The printheads  44 ,  46  are thermal inkjet printheads, although other types of printheads may be used, such as piezoelectric printheads. The thermal printheads  44 ,  46  typically include a plurality of resistors which are associated with the nozzles. Upon energizing a selected resistor, a bubble of gas is formed which ejects a droplet of ink from the nozzle and onto the print media when in the printzone  30  under the nozzle. The printhead resistors are selectively energized in response to firing command control signals delivered from the controller  26  to the printhead carriage  36 . It is also possible to implement a page-wide printhead array which does not need to be reciprocated across the printzone  30 .  
         [0021]    Between print jobs, the inkjet carriage  36  moves along the carriage guide rod  32  to the servicing region  38  where a service station  48  may perform various servicing functions known to those in the art, such as, priming, scraping, and capping for storage during periods of non-use to prevent ink from drying and clogging the inkjet printhead nozzles.  
         [0022]    The printer chassis  22  is illustrated as supporting an electrically biased absorbent electrostatic sensing element, or “electrically biased absorbent target”  50 , in the printer&#39;s “inboard” region  52  located on the side of service station  48  near the printzone  30 . The print carriage  36  may be moved along carriage guide rod  32  until printheads  44 ,  46  are positioned over the electrically biased absorbent target  50 . Ink droplets may be fired onto the upper surface of electrically biased absorbent target  50  and detected according to the method described in U.S. Pat. No. 6,086,190, assigned to the Hewlett-Packard Company, the present assignee. Target  50  may be constructed by using a foam pad which is pretreated with a conductive solvent such as glycerol or polyethylene glycol (PEG). Other absorbent materials may similarly be selected depending on design or cost restraints, for example, the electrically biased absorbent target  50  could be constructed of polyurethane or a rigid and porous sintered plastic. Electrically biased sensing conductor  54  applies a biasing voltage to the target  50  while also connecting the target  50  to an electrostatic drop detect printed circuit board assembly (PCA)  56 . The PCA  56  contains various electronics (not shown) for filtering and amplification of drop detection signals received from the target  50  via electrically biased sensing conductor  54 . An additional electrical conductor  58  links the PCA  56  to controller  26  for drop detection signal processing. PCA  56  may be located in various locations inside of the printer  20  to accommodate design goals such as sharing PCA real estate with other circuitry, locating in the proximity of the target  50  to reduce signal noise effects, or to remove the PCA  56  from the vicinity of conductive ink residue and ink aerosol.  
         [0023]    Electrically biased absorbent target  50  does not need a cleaning mechanism, so it is simple to construct and economical, and should prevent the build-up of ink residue stalagmites as ink droplets are fired onto the target  50  because the droplets can be absorbed into the target  50  and preferably kept in solution by the optional ink solvent present in the target  50 . Electrically biased absorbent target  50  may be constructed in varying sizes to accommodate a portion of a printhead&#39;s  44 ,  46  nozzles, an entire printhead&#39;s  44 ,  46  nozzles, or even all of the nozzles for multiple printheads  44 ,  46 . Additionally, electrically biased absorbent target  50  may be located in other locations below the plane defined by printheads  44 ,  46  as they are propelled by the printhead carriage  36  back and forth on carriage guide rod  32  along scanning axis  34 . Examples of alternate locations for electrically biased absorbent target  50  include, for example, the “outboard region”  60  which is on the opposite side of printzone  30  from the service station  48 , the servicing region  38 , and “outside service station region”  62 .  
         [0024]    FIGS.  2 - 4  illustrate embodiments of a non-contact electrically biased sensing target for use with a drop detector system. The printzone paper path ribs  39  support a sheet of printable media  64  as it is moved through the print zone  30 . For clarity of illustration, the printable media  64  is not shown in contact with the paper path ribs  39 , however, is actual practice, the printable media  64  is in contact with and supported by the paper path ribs  39  in the printzone  30 . As FIG. 2 illustrates, a non-contact electrically biased target  66  may be attached to the printzone paper path ribs  39  such that the target  66  rides below, yet does not interfere with, the printable media  64  as it passes over the paper path ribs  39  through the printzone. An electrically biased sensing conductor  54  may connect the non-contact electrically biased sensing target to the drop detector PCA  56  as illustrated in FIG. 1 for signal filtering and amplification. Electrically biased sensing conductor  54  also provides a biasing voltage to the target  66 . The reciprocating printhead carriage  36  may be moved along carriage guide rod  32  until either of the printheads  44 ,  46  or a selected portion of each one is positioned over the non-contact electrically biased target  66 . The biasing voltage present on the target  66  creates an electric field between the target  66  and the ground plane present at the nozzle plate of the printheads  44 ,  46 . Selected printhead  44 ,  46  nozzles may then be fired in response to commands from controller  26  to eject ink droplets  68  onto the print media  64  over the non-contact electrically biased target  66 . As each droplet  68  begins to exit the printhead  44 ,  46  nozzle, a charge accumulates on the protruding tip of the drop, due to the influence of the printhead  44 ,  46  nozzle-plate-to-target  66  electric field. When drop breakoff occurs, the drop retains this charge. When the drop contacts the print media  64 , a small capacitive current, in relation to the charge on the ink droplet  68 , is created from the target  66  to ground. The periodic flow of capacitive current, from ink droplets  68  striking the print media  64  over target  66 , may be converted to a digitized signal voltage by PCA  56  which is coupled to the target  66  via electrically biased sensing conductor  54 . Processor  26  may then receive the digital signal from PCA  56  via conductor  58  for processing to determine if a nozzle or group of nozzles are working properly.  
         [0025]    [0025]FIG. 3 illustrates another embodiment of a non-contact electrically biased sensing target for use with a drop detector system. Similar to the target  66  in FIG. 2, the embodiment of FIG. 3 has a non-contact electrically biased target  70 , however the target  70  of FIG. 3 may be coated or attached over the entire length of the paper path ribs  39  in the printzone  30 . The printable media  64  passes over target  70 , supported by target  70  and paper path ribs  39  as the print media  64  is moved through the print zone. Since the target  70  is full-width with respect to the printzone  30 , drop detection measurements may be taken at any location ink droplets  68  are fired onto the print media  64 , by examining the digital signal created by the capacitive current as done for the embodiment in FIG. 2. The embodiment illustrated in FIG. 3 may be used with reciprocating printheads  44 ,  46 , or with a full-width printhead array  72 .  
         [0026]    [0026]FIG. 4 illustrates another embodiment of a non-contact electrically biased sensing target for use with a drop detector system. Similar to the target  70  in FIG. 3, the embodiment of FIG. 4 has a full-width non-contact electrically biased target  74 , however the target  74  of FIG. 4 is integrally constructed with the paper path ribs  39  as opposed to the coated or attached target  70 . A conductive material such as, for example, copper, gold, palladium, stainless steel, or conductive plastic may be used to form the target  74  as illustrated in FIG. 4. Since the target  74  also performs the functions of paper path ribs  39  in FIG. 2, the target  74  naturally rides below, and does not interfere with, the printable media  64  as it passes over the target  74  through the printzone. Since the target  74  is full-width with respect to the printzone  30 , drop detection measurements may be taken at any location ink droplets  68  are fired onto the print media  64 , by examining the digital signal created by the capacitive current as done for the embodiment in FIG. 2. The embodiment illustrated in FIG. 4 may be used with reciprocating printheads  44 ,  46 , or with a full-width printhead array  72 . Additionally, drop detection measurements taken using the sensors illustrated in FIGS.  2 - 4  may be taken while printing a calibration or test page, or even while printing any print job.  
         [0027]    FIGS.  5 - 10  illustrate embodiments of a non-contact electrically biased sensing target for use with a drop detector system. In each of the embodiments illustrated in FIGS.  5 - 10 , a pen, such as black pen  40 , may be positioned such that the printhead  44  nozzles are aligned over the opening defined by the target loop  76 . It is intended that target loop  76  not be limited to the sizes and shapes shown in FIGS.  5 - 10 . Rather, the intent of illustrating various possible designs for the target loop  76  is to show that many shapes may be good candidates to select for a given application, such as, for example, circles, ovals, squares, rectangles, triangles, trapezoids, and other multi-sided or curved shapes, based on factors such as the size of the printheads, the electric field desired, and manufacturing concerns. The target loop  76  may be constructed by stamping it from a sheet of metal, forming it out of a conductive plastic, coating a plastic of the desired shape with a conductive material, bending a wire, or using a printed circuit board. Other methods of construction will be readily apparent to those skilled in the art, and are intended to be covered within the scope of this description.  
         [0028]    An electrically biased sensing conductor  54  may connect the non-contact target loop  76  to the drop detector PCA  56  as illustrated in FIG. 1 for signal filtering and amplification. Electrically biased sensing conductor  54  provides a biasing voltage to the target loop  76 . The biasing voltage present on the target loop  76  creates an electric field between the target loop  76  and the ground plane present at the nozzle plate of the printhead  44 . Selected printhead  44  nozzles may then be fired in response to commands from controller  26  to eject ink droplets  68  through the opening defined by target loop  76 . As each droplet  68  begins to exit the printhead  44  nozzle, a charge accumulates on the protruding tip of the drop, due to the influence of the printhead  44  nozzle-plate-to-target loop  76  electric field. When drop breakoff occurs, the droplet  68  retains this charge. When the droplet  68  approaches and passes through the opening defined by the target loop  76 , a small current is induced from the target loop  76 , in relation to the charge on the ink droplet  68 , to ground. The periodic flow of this induced current from ink droplets  68  passing through the target loop  76  may be converted to a digitized signal voltage by PCA  56  which is coupled to the target  56  via electrically biased sensing conductor  54 . Processor  26  may then receive the digital signal from PCA  56  via conductor  58  for processing to determine if a nozzle or group of nozzles are working properly. Despite ink aerosol which may be present, target loop  76  does not substantially come into contact with the ink droplets  68 , so it should not need to be cleaned. A spittoon  78  may be provided below the target loop  76  to collect the ink droplets  68  which are fired through the target loop  76 . The spittoon  78  may be appropriately sized to have capacity to hold enough ink droplets  68  for the intended life of the printing mechanism which employs the target loop  76 . The ink droplets  68  may form stalagmites, but the surface of the spittoon where the ink droplets  68  impact can be designed to be far enough away from the printhead  44  to avoid most concerns for stalagmite crashes with the printhead  44 . If stalagmites are still a concern, an absorbent pad  80 , made from such materials as foam or felt, may be fitted into the bottom of spittoon  78  and optionally pretreated with a solvent such as glycerol or polyethylene glycol (PEG). The solvent tends to dissolve the ink droplets  68 , and the absorbent pad  80  tends to absorb the dissolved ink, thereby decreasing the likelihood of stalagmites.  
         [0029]    FIGS.  11 - 12  illustrate embodiments of a non-contact electrically biased sensing plate  82  for use with a drop detector system. In each of the embodiments illustrated in FIGS.  11 - 12 , a pen, such as black pen  40 , may be positioned such that the printhead  44  nozzles may be energized causing ink droplets  68  to pass through an electric field created between the electrically biased sensing plate  82  and the ground plane defined by the printhead  44  nozzles. As FIG. 12 illustrates, multiple electrically biased sensing plates  82  may be used. It is intended that electrically biased sensing plates not be limited to the configurations shown in FIGS.  11 - 12 . Rather, the intent of illustrating possible designs for the electrically biased sensing plates  82  is to show that many plate orientations may be good candidates to select for a given application. The electrically biased sensing plates  82  may be constructed from metal, from conductive plastic, by coating a plastic of the desired shape with a conductive material, or by using a printed circuit board. Other methods of construction will be readily apparent to those skilled in the art, and are intended to be covered within the scope of this embodiment.  
         [0030]    An electrically biased sensing conductor  54  may connect the non-contact electrically biased sensing plates  82  to the drop detector PCA  56  as illustrated in FIG. 1 for signal filtering and amplification. Electrically biased sensing conductor  54  provides a biasing voltage to the electrically biased sensing plates  82 . The voltage present on the electrically biased sensing plates  82  creates an electric field between the sensing plates  82  and the ground plane present at the nozzle plate of the printhead  44 . Selected printhead  44  nozzles may then be fired in response to commands from controller  26  to eject ink droplets  68  through the electric field. As each droplet  68  begins to exit the printhead  44  nozzle, a charge accumulates on the protruding tip of the drop, due to the influence of the printhead  44  nozzle plate-to-electrically biased sensing plates  82  electric field. When drop breakoff occurs, the droplet  68  retains this charge. As the droplet  68  approaches and passes by the electrically biased sensing plates  82 , a small current is induced from the sensing plates  82 , in relation to the charge on the ink droplet  68 , to ground. The periodic flow of this induced current from ink droplets  68  passing by the sensing plates  82  may be converted to a digitized signal voltage by PCA  56  which is coupled to the target  56  via electrically biased sensing conductor  54 . Processor  26  may then receive the digital signal from PCA  56  via conductor  58  for processing to determine if a nozzle or group of nozzles are working properly. Despite ink aerosol which may be present, electrically biased sensing plate  82  does not substantially come into contact with the ink droplets  68 , so it should not need to be cleaned. A spittoon  78  may be provided below the sensing plates  82 , inline with the droplets spit from printhead  44 , to collect the ink droplets  68  which are fired past the sensing plate  82 . The spittoon  78  may be appropriately sized to have capacity to hold enough ink droplets  68  for the intended life of the printing mechanism which employs the biased sensing plate  82 . The ink droplets  68  may form stalagmites, but the surface of the spittoon where the ink droplets  68  impact can be designed to be far enough away from the printhead  44  to avoid most concerns for stalagmite crashes with the printhead  44 . If stalagmites are still a concern, an absorbent pad  80 , made from such materials as foam or felt, may fitted into the bottom of spittoon  78  and optionally pretreated with a solvent such as glycerol or polyethylene glycol (PEG). The solvent tends to dissolve the ink droplets  68 , and the absorbent pad  80  tends to absorb the dissolved ink, thereby decreasing the likelihood of stalagmites.  
         [0031]    FIGS.  13 - 18  illustrate embodiments of a non-contact electrically biased loop in conjunction with a contact sensing target for use with a drop detector system. In each of the embodiments illustrated in FIGS.  13 - 18 , a pen, such as black pen  40 , may be positioned such that the printhead  44  nozzles are aligned over the opening defined by the electrically biased loop  84 . It is intended that electrically biased loop  84  not be limited to the sizes and shapes shown in FIGS.  13 - 18 . Rather, the intent of illustrating various possible designs for the electrically biased loop  76  is to show that many shapes may be good candidates to select for a given application, such as, for example, circles, ovals, squares, rectangles, triangles, trapezoids, and other multi-sided or curved shapes. The electrically biased loop  84  may be constructed by stamping it from a sheet of metal, forming it out of a conductive plastic, coating a plastic of the desired shape with a conductive material, bending a wire, or using a printed circuit board. Other methods of construction will be readily apparent to those skilled in the art, and are intended to be covered within the scope of this embodiment.  
         [0032]    Electrically biased conductor  86  provides a biasing voltage to the electrically biased loop  84 . The voltage present on the electrically biased loop  84  creates an electric field between the electrically biased loop  84  and the ground plane present at the nozzle plate of the printhead  44 . Selected printhead  44  nozzles may then be fired in response to commands from controller  26  to eject ink droplets  68  through the opening defined by electrically biased loop  84 . As each droplet  68  begins to exit the printhead  44  nozzle, a charge accumulates on the protruding tip of the drop, due to the influence of the printhead  44  nozzle-plate-to-electrically biased loop  84  electric field. When drop breakoff occurs, the droplet  68  retains this charge. Droplet  68  passes through the opening defined by the electrically biased loop  84  and contacts conductive target  88 . A sensing conductor  90  connects the target  88  to the drop detector PCA  56  as illustrated in FIG. 1 for signal filtering and amplification. When the droplet  68  contacts the conductive target  88 , a small current is created from the target  88 , in relation to the charge on the ink droplet  68 , to ground. The periodic flow of the current from ink droplets  68  contacting the target  88  may be converted to a digitized signal voltage by PCA  56 . Processor  26  may then receive the digital signal from PCA  56  via conductor  58  for processing to determine if a nozzle or group of nozzles are working properly. Despite ink aerosol which may be present, electrically biased loop  84  does not substantially come into contact with the ink droplets  68 , so it should not need to be cleaned. The target  88  may be placed relatively far from the printhead  44  when compared to the electrically biased loop  84 , reducing the likelihood that stalagmites from the ink droplets  68  may be a problem for the printhead  44 . A spittoon  78  may be provided around target  88  to contain the ink residue incident on the target  88 . Additionally, the conductive target  88  may be constructed of an absorbent pad which is pretreated with a conductive solvent such as glycerol or polyethylene glycol (PEG). Other absorbent materials may similarly be selected depending on design or cost restraints, for example, the conductive target  88  could be constructed of polyurethane or a rigid and porous sintered plastic. The solvent tends to dissolve the ink droplets  68 . The absorbent pad version of conductive target  88  tends to absorb the dissolved ink, thereby decreasing the likelihood of stalagmites.  
         [0033]    FIGS.  19 - 20  illustrate embodiments of a non-contact electrically biased plate  92  in conjunction with a contact sensing target  88  for use with a drop detector system. In each of the embodiments illustrated in FIGS.  19 - 20 , a pen, such as black pen  40 , may be positioned such that the printhead  44  nozzles may be energized causing ink droplets  68  to pass through an electric field created between the electrically biased plate  92  and the ground plane defined by the printhead  44  nozzles. As FIG. 20 illustrates, multiple electrically biased plates  92  may be used. It is intended that electrically biased plates  92  not be limited to the configurations shown in FIGS.  19 - 20 . Rather, the intent of illustrating possible designs for the electrically biased plates  92  is to show that many plate orientations may be good candidates to select for a given application. The electrically biased plates  92  may be constructed from metal, molded of a conductive plastic, coated on a plastic of the desired shape with a conductive material, or fabricated by using a printed circuit board. Other methods of construction will be readily apparent to those skilled in the art, and are intended to be covered within the scope of this embodiment.  
         [0034]    Electrically biased conductor  86  provides a biasing voltage to the electrically biased plates  92 . The voltage present on the electrically biased plates  92  creates an electric field between the electrically biased plates  92  and the ground plane present at the nozzle plate of the printhead  44 . Selected printhead  44  nozzles may then be fired in response to commands from controller  26  to eject ink droplets  68  through the electric field. As each droplet  68  begins to exit the printhead  44  nozzle, a charge accumulates on the protruding tip of the drop, due to the influence of the printhead  44  nozzle-plate-to-electrically biased plates  92  electric field. When drop breakoff occurs, the droplet  68  retains this charge. A sensing conductor  90  connects the target  88  to the drop detector PCA  56  as illustrated in FIG. 1 for signal filtering and amplification. When the droplet  68  contacts the conductive target  88 , a small current is created from the target  88 , in relation to the charge on the ink droplet  68 , to ground. The periodic flow of the current from ink droplets  68  contacting the target  88  may be converted to a digitized signal voltage by PCA  56 . Processor  26  may then receive the digital signal from PCA  56  via conductor  58  for processing to determine if a nozzle or group of nozzles are working properly. Despite ink aerosol which may be present, electrically biased plates  92  do not substantially come into contact with the ink droplets  68 , so the plates  92  should not need to be cleaned. The target  88  may be placed relatively far from the printhead  44  when compared to the electrically biased plates  92 , reducing the likelihood that possible stalagmites from the ink droplets  68  may be a problem for the printhead  44 . A spittoon  78  may be provided around target  88  to contain the ink residue incident on the target  88 . Additionally, the conductive target  88  may be constructed of an absorbent pad which is pretreated with a conductive solvent such as glycerol or polyethylene glycol (PEG). Other absorbent materials may similarly be selected depending on design or cost restraints, for example, the conductive target  88  could be constructed of polyurethane or a rigid and porous sintered plastic. The solvent tends to dissolve the ink droplets  68 . The absorbent pad version of conductive target  88  tends to absorb the dissolved ink, thereby decreasing the likelihood of stalagmites.  
         [0035]    In each of the embodiments illustrated in FIGS.  13 - 20 , the non-contact loops  84  and the non-contact plates  92  have been described as supplied with a biasing voltage by conductor  86 . Additionally, the targets  88  in FIGS.  13 - 20  have been described as connected to the drop detector PCA  56  by conductor  90 . It is also possible, however, to switch the connectors  86  and  90  so that the loops  84  and plates  92  are used exclusively as non-contact sensing elements for ink drop detection and the targets  88  are used exclusively for electrically biasing. In this set of embodiments, As each droplet  68  begins to exit the printhead  44  nozzle, a charge accumulates on the protruding tip of the drop, due to the influence of the printhead  44  nozzle-plate-to-target  88  electric field. When drop breakoff occurs, the droplet  68  retains this charge. When the droplet  68  passes by the loop  84  or plates  92 , a small current is induced from the loop  84  or the plates  92 , in relation to the charge on the ink droplet  68 , to ground. The periodic flow of this induced current may be converted to a digitized signal voltage by PCA  56 . Processor  26  may then receive the digital signal from PCA  56  via conductor  58  for processing to determine if a nozzle or group of nozzles are working properly.  
         [0036]    Various non-contact electrically biasing and sensing electrostatic drop detect target configurations, as well as absorbent target configurations have been illustrated with example embodiments to enable a low cost and efficient method and mechanism for ink drop detection which is less susceptible to waste ink residue build-up. Each of the target and electrically biasing element embodiments illustrated in FIGS.  1 - 20  may be constructed in varying sizes to accommodate a portion of a printhead&#39;s  44 ,  46  nozzles, an entire printhead&#39;s  44 ,  46  nozzles, or even all of the nozzles for multiple printheads  44 ,  46 . Additionally, target and electrically biasing element embodiments illustrated in FIG. 1 and FIGS.  5 - 20  may be located in many locations below the plane defined by printheads  44 ,  46 . Examples of locations for the target and electrically biasing element embodiments illustrated in FIG. 1 and FIGS.  5 - 20  include, for example, the “inboard region”  52  between the printzone and the service station, the “outboard region”  60  which is on the opposite side of printzone  30  from the service station  48 , the servicing region  38 , and “outside service station region”  62 .  
         [0037]    Non-contact electrically biasing and sensing electrostatic drop detect target configurations, as well as absorbent target configurations enable a printing mechanism to reliably and economically gather ink drop detection readings, without the need for a cleaning mechanism to clean the target surface, in order to provide users with consistent, high-quality, and economical inkjet output despite printheads  44 ,  46  which may clog over time. In discussing various components of the non-contact electrically biasing and sensing electrostatic drop detect target configurations, as well as absorbent target configurations, various benefits have been noted above.  
         [0038]    It is apparent that a variety of other structurally equivalent modifications and substitutions may be made to construct non-contact electrically biasing and sensing electrostatic drop detect target configurations, as well as absorbent target configurations, according to the concepts covered herein depending upon the particular implementation, while still falling within the scope of the claims below.