Abstract:
An apparatus for detecting ink droplets ejected from ink drop generators has a target holder and a conductive absorbent target supported by the target holder. The apparatus for detecting ink droplets also has standoffs extending from the target holder. The apparatus for detecting ink droplets further has an actuator for moving the target holder towards the ink drop generators such that the standoffs space the target from the ink drop generators.

Description:
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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     To this end, many inkjet printing mechanisms contain a service station for the maintenance of the inkjet printheads. These service stations may include scrapers, ink-solvent applicators, primers, and caps to help keep the nozzles from drying out during periods of inactivity. Additionally, inkjet printing mechanisms often contain service routines which are designed to fire ink out of each of the nozzles and into a waste spittoon in order to prevent nozzle clogging. 
     Despite these preventative measures, 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. 
     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. 
     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 number of different ink drop detector systems. 
     One type of ink drop detector system utilizes a piezoelectric target surface that produces a measurable signal when ink droplets contact the target surface. Unfortunately, however, this type of technology is expensive and often is unable to detect the extremely small drops of ink used in inkjet printing systems with photographic image quality. 
     Another type of ink drop detector utilizes an optical sensor which forms a measurable signal when an ink droplet passes through a light beam from a sensory circuit. Unfortunately, this method is subject to extremely tight alignment tolerances which are difficult and expensive to setup and maintain. Additionally, an optical ink drop detection system is susceptible to the ink aerosol which results from the firing of the inkjet printhead inside of the printing mechanism. The aerosol coats the optical sensor over time, degrading the optical sensor signal and eventually preventing the optical sensor from functioning. 
     A more effective solution for ink drop detection is to use 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. This drop detection system utilizes an electrostatic sensing element which is imparted with an electrical stimulus when struck by a series of ink drop bursts ejected from an inkjet printhead. The electrostatic sensing element may be made sufficiently large so that printhead alignment is not critical, and the sensing element may function with amounts of ink or aerosol on the sensing element surface which would incapacitate other types of drop detection sensors. 
     In practical implementation, however, this electrostatic sensing element has some limitations. First, 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. Second, as the ink residue dries, it remains conductive and may short out the drop detector electronics as the ink residue grows and spreads. Thus, this dried ink residue may impair the ability of the sensor to measure ink drop characteristics properly. Third, a build-up of dried ink on the sensor may decrease the measurement gap, adversely affecting the drop measurement signal. Fourth, current ink drop sensors may be sensitive to spacing variations, inherent in a printing mechanism, from the printheads to the sensor. 
     Therefore, it is desirable to have an economical method and mechanism for ink drop detection which is less susceptible to waste ink residue build-up and which is able to minimize the measurement spacing variability inherent in current printing mechanisms which utilize ink drop detection systems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmented perspective view of one form of an inkjet printing mechanism, here illustrating a service station which includes an embodiment of an electrostatic ink drop detector. 
     FIG. 2 is an enlarged, fragmented perspective view of the service station of FIG. 1 
     FIG. 3 is an enlarged, fragmented side elevational view of the service station of FIG. 1 shown with a servicing sled in a retracted position. 
     FIG. 4 is an enlarged, fragmented side elevational view of the service station of FIG. 1 shown with a servicing sled in a servicing position. 
     FIG. 5 is an enlarged, fragmented side elevational view of the service station of FIG. 1 shown with an ink drop detection target in a measurement position. 
     FIG. 6 is an enlarged perspective view illustrating a service station similar to the service station in FIG. 2, but having an alternative embodiment of an electrostatic ink drop detector. 
     FIG. 7 is an enlarged side elevational view of the service station of FIG. 6, shown with a servicing sled in a retracted position. 
     FIG. 8 is an enlarged side elevational view of the service station of FIG. 6, shown with a servicing sled in a servicing position. 
     FIG. 9 is an enlarged side elevational view of the service station of own with an ink drop detection target in a measurement position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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 others 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-formt printers, hybrid electrophotographic-inkjet printers, copiers, cameras, video printers, and facsimile machines, to name a few. For convenience the concept introduced herein are described in the environment of an inkjet printer  20 . 
     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 instruction from a host device, such as a computer 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 beign 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. 
     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. 
     In the printzone  30 , the media sheet 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. 
     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 from 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 . 
     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. 
     FIG. 2 shows the service station  48  in detail. A service station frame  50  is mounted to the chassis  22 , and houses a moveable pallet  52 . The moveable pallet  52  may be driven by a motor (not shown) to move in the frame  50  in the positive and negative Y-axis directions. The moveable pallet  52  may be driven by a rack and pinion gear powered by the service station motor in response to the microprocessor  26  according to methods known by those skilled in the art. An example of such a rack and pinion system in an inkjet cleaning service station can be found in U.S. Pat. No. 5,980,018, assigned to the Hewlett-Packard Company, also the current assignee. The end result is that pallet  52  may be moved in the positive Y-axis direction to a servicing position and in the negative Y-axis direction to an uncapped position. The pallet  52  supports a black printhead cap  54  and a tri-color printhead cap  56  to seal the printheads  44  and  46 , respectively, when the moveable pallet  52  is in the servicing position, here a capping position. 
     FIG. 2 also shows an ink drop detector  58  supported by a pivot post  60  which is connected to frame  50 . Interior linkage arm  62  and exterior linkage arm  64  rotate about pivot post  60 . A spring element, such as torsion spring  66  is attached between pivot post  60  and either of the linkage arms  62 ,  64 . The spring  66  imparts a rotational force on the linkage arm  62  or  64  which it is connected to, in a counter-clockwise rotational direction  68 . The linkage arms  62 ,  64  support a target holder  70  at interior target pivot point  72  and exterior target pivot point  74 , respectively. 
     As the rotational angle of the linkage arms  62 ,  64  is varied around pivot point  60 , the target holder  70  is free to rotate on target pivot points  72 ,  74  within a range determined by anti-rotation nubs  76  which extend outward in the positive X-axis direction from target holder  70  on either side of exterior linkage arm  64 . When the target holder  70  reaches certain angles with respect to linkage arm  64 , the anti-rotation nubs  76  interfere with the exterior linkage arm  64  and prevent further rotation of the target holder  70  with respect to the exterior linkage arm  64 . 
     The linkage arms  62 ,  64  rotate in the counter-clockwise direction  68  until interior linkage arm  72  contacts a pallet arm  77  which is supported by the moveable pallet  52 , and which extends outwardly in the positive X-axis direction from the moveable pallet  52 . For illustration purposes, the linkage arms  62 ,  64  are not shown in contact with the pallet arm  77  in FIG. 2 so that the pallet arm  77  may be clearly seen. In normal operation, however, the linkage arms would rotate in a counter-clockwise direction  68  and stop when contact with the pallet arm  77  occurs. 
     Target holder  70  supports a conductive absorbent electrostatic sensing element, or “target”  78 , on the upper side onto which ink droplets may be fired and detected according to the apparatus and method described in U.S. Pat. No. 6,086,190, assigned to the Hewlett-Packard Company, the present assignee. Target  78  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 target  78  could be constructed of polyurethane or a rigid and porous sintered plastic. Conductor  80  connects the target  78  to an electrostatic drop detect printed circuit board assembly (PCA)  82 . The PCA  82  contains various electronics (not shown) for filtering and amplification of drop detection signals received from the target  78  via conductor  80 . An additional electrical conductor  84  links the PCA  82  to controller  26  for drop detection signal processing. Although PCA  82  is illustrated as supported by the service station frame  50 , PCA  82  may be located elsewhere inside of the printer  20  to accommodate design goals such as sharing PCA real estate with other circuitry or removing the PCA  82  from the vicinity of conductive ink residue and ink aerosol. 
     FIG. 3 shows servicing pallet  52  in a retracted position. While the pallet  52  is retracted, the linkage arms  62 ,  64  are positioned against pallet arm  77  such that the linkage arms  62 ,  64  and the target holder  70  are in a non-measurement position which allows printhead carriage  36  to be moved freely along carriage guide rod  32  between the printzone  30  and the servicing region  38 . When the carriage  36  is in the servicing region  38 , it is aligned over the service station  48 , where printheads  44 ,  46  may be serviced, for example, by spitting ink into the service station. Movement in a clockwise direction  86 , is imparted to the linkage arms  62 ,  64  by pallet arm  77  when servicing pallet  52  is moved in the positive Y-axis direction. As the pallet  52  continues to move in the positive Y-axis direction, the servicing pallet  52  moves from the retracted position in FIG. 3 to a servicing position shown in FIG.  4 . When the servicing pallet  52  is in the servicing position, the linkage arms  62 ,  64  are fully rotated in the clockwise direction  86 , holding target holder  70  in a pre-measurement position. 
     When the pallet  52  is moved to the servicing position, the black printhead cap  54  and color printhead cap  56  lift off of the servicing pallet  52  to engage and cap the black printhead  44  and the tri-color printhead  46 , respectively. A servicing mechanism capable of engaging the printheads in this manner is disclosed in U.S. Pat. No. 5,980,018, also assigned to the present assignee, the Hewlett-Packard Company. For simplicity of illustration, caps  54 ,  56  are shown schematically in FIG. 4 as rising up to engage printheads  44 ,  46  when the servicing pallet  52  is in the servicing position. In this manner, the pallet  52  may be moved between the retracted position and the servicing position to perform various printhead  44 ,  46  servicing techniques well-known to those skilled in the art. 
     When printhead  44 ,  46  servicing is complete, the pallet  52  is moved to the retracted position shown in FIG.  3  and the spring  66  rotates the linkage arms  62 ,  64  and the target holder  70  in the counter-clockwise direction  68  into the non-measurement position. At this point, the printhead carriage  36  is free to move in the positive X-axis direction to the printzone  30  for printing if desired. Once the printhead carriage  36  is clear of the servicing region  38 , the target holder  70  may be moved back into the pre-measurement position by moving the servicing pallet  52  from the retracted position back to the servicing position shown in FIG.  3 . At this point, the printhead carriage  36  may be moved back in the negative X-axis direction to align either black printhead  44  or tri-color printhead  46  over conductive absorbent target  78 . Once the printhead  44 ,  46  is properly positioned, the servicing pallet  52  is moved back to the retracted position. As pallet  52  retracts, linkage arms  62 ,  64  and target holder  70  rotate in the counter-clockwise direction  68  until target standoffs  88  engage the printhead  44 ,  46  as is illustrated in FIG.  5 . 
     The standoffs  88  control the spacing from the printheads  44 ,  46  to the electrostatic target  78 , commonly referred to as “Pen to Electrostatic drop detector in the Z-direction (PEZ) spacing” by those in the art. Although four standoffs  88  are illustrated, three or more standoffs  88  could be used. A typical PEZ spacing is on the order of 2.0 millimeters. Targets which may be attached to the printer frame  22 , or the service station frame  50 , and which do not locate to the printheads  44 ,  46  may create a substantial tolerance stack among the many parts between such a non-locating target and the printheads  44 ,  46 . Such a tolerance stack could introduce a variation of plus or minus 1.0 millimeters on top of the desired 2.0 mm PEZ. Such variation threatens printhead reliability on the low end of 1.0 millimeters by increasing the risk of handing off fibers and ink residue from the non-locating target to the printheads  44 ,  46 . At the high end of 3.0 millimeters, although the printhead reliability risk is reduced, ultra-small ink drops, in the range of approximately two to three picoliters, may reach terminal velocity well before they hit this non-locating target. If a drop reaches terminal velocity, then it is possible the drop may be more influenced by convection currents and turbulence to the extent that the ink drops may be driven off course and miss the non-locating target entirely. Therefore, it is advantageous to employ target standoffs  88  in the embodiment of FIG. 5 to control the PEZ spacing with a minimum amount of tolerance variation between the printheads  44 ,  46  and the electrostatic target  78 . 
     Once the printhead  44 ,  46  is properly spaced from the electrostatic target  78 , the controller  26  causes ink droplets  90  to be fired from printhead  44 ,  46  onto the target  78 . An electrical drop detect signal is generated by the ink droplets  90  as they contact the target  78 , and this signal is captured by the electronics of electrostatic drop detector PCA  82 . The drop detect signal is then analyzed by controller  26  to determine whether or not various nozzles of printhead  44 ,  46  are spitting ink properly or whether they are clogged. A preferred method of analyzing signals from an electrostatic target ink drop detector is shown in U.S. Pat. No. 6,086,190, also assigned to the present assignee, the Hewlett-Packard Company. Based on the determination made by the controller  26  as to whether each nozzle is functioning properly, the controller  26  may adjust the print masks to substitute functioning nozzles for any malfunctioning nozzles to provide consistent high-quality printed output while still using a printhead with permanently clogged nozzles. 
     In order to ensure that a reliable measurement may be made by the ink drop detector  58 , it is desirable to prevent the build-up of dried ink deposits on the target  78  after a measurement or series of measurements have been made. Conductive absorbent target  78  is pretreated with a conductive solvent which is selected to dissolve and absorb the ink droplets  90  which contact the target  78 , thereby reducing the likelihood that ink deposits may accumulate over time. Thus, the embodiment of an electrostatic drop detection system illustrated in FIGS. 2-5 may be constructed without additional hardware to clean and scrape the target  78  while still having long life and high reliability. 
     After the desired number of drop detection measurements are taken, the servicing pallet  52  may then be moved in the positive Y-axis direction to the servicing position. The target standoffs  88  disengage the printheads  44 ,  46 , and linkage arms  62 ,  64  and target holder  70  moves to the forward pre-measurement position. The printhead carriage  36  may then be moved in the positive X-axis direction towards the printzone  30 , and then pallet  52  may be moved in the negative Y-axis direction to the retracted position of FIG.  3 . When the pallet  52  is in the retracted position of FIG. 3, the linkage arms  62 ,  64  and target holder  70  are in the non-measurement position, and the printhead carriage  36  is free at this point to move back to the servicing region  38  or to print in the printzone  30 . 
     Clearly, the ink drop detector  58  could be mounted in other locations along the printhead scanning axis  34 , including the right side of the service station frame  50  or the opposite end of the printer from the service station  48 . Additionally, alternate structures for bringing the target standoffs  88  into contact with the printheads  44 ,  46  will be readily apparent to those skilled in the art, such as, for example, a solenoid activated spring mechanism which may translate the target holder  70  substantially parallel to the Z-axis, thereby bringing the standoffs  88  into and out of contact with the printheads when drop detection measurements are desired. 
     FIG. 6 illustrates an alternate embodiment of an electrostatic drop detector  58 , here shown located inside of the service station  48 , and substantially inline with the servicing pallet  52 . The drop detection system  58  has linkage arms  92  which pivot about pivot post  60 . The linkage arms  92  support target holder  94  at target pivot points  96 . The service station  48  has a bonnet  98  which is attached to the top of service station frame  50 , and which covers portions of the service station  48  to protect the servicing elements and to help control the flow of aerosol. The bonnet  98  may additionally be formed to create linkage arm clearance channels  100  on either side of the bonnet  98  between the bonnet  98  and the service station frame  50 . 
     Target holder  94  supports a conductive absorbent electrostatic sensing element, or “target”  102 , on the upper side onto which ink droplets may be fired and detected according to the apparatus and method described in U.S. Pat. No. 6,086,190, assigned to the Hewlett-Packard Company, the present assignee. Target  102 , like target  78 , 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 target  102  could be constructed of polyurethane or a rigid and porous sintered plastic. Conductor  80  connects the target  102  to an electrostatic drop detect printed circuit board assembly (PCA)  82 . The PCA  82  contains various electronics (not shown) for filtering and amplification of drop detection signals received from the target  102  via conductor  80 . An additional electrical conductor  84  links the PCA  82  to controller  26  for drop detection signal processing. Although PCA  82  is illustrated as supported by the service station frame  50 , PCA  82  may be located elsewhere inside of the printer  20  to accommodate design goals such as sharing PCA real estate with other circuitry or to remove the PCA  82  from the vicinity of conductive ink residue and ink aerosol. 
     FIG. 7 shows the service station  48  and electrostatic drop detector  58  of FIG. 6 in a side elevational view. Servicing pallet  52  is shown in a retracted position. The linkage arms  92  and target holder  94  are biased in counterclockwise direction  68  around pivot post  60  by biasing spring element  66 . A hard stop  104  is provided to limit the range of motion of linkage arms  92  when rotating in the counter-clockwise direction  68 . As illustrated in FIG. 7, with linkage arms  92  at rest against a hard stop  104 , the target holder  94  and linkage arms  92  are in a rearward non-measurement position. The linkage arms  92  are able to clear the bonnet  98  by passing through linkage arm clearance channels  100  while in this rearward non-measurement position. 
     If it is only desired to spit ink from the printheads  44 ,  46  into the service station  48 , for example during a print job to make sure all of the nozzles are clear, the print carriage  36  is free to move along carriage guide rod  32  in the negative X-axis direction until the printheads  44 ,  46  are positioned over the service station  48  when the servicing pallet  52  is in the retracted position. In order to be able to service the printheads  44 ,  46  with the servicing pallet  52 , the print head carriage  36  must be moved along carriage guide rod  32 , towards the printzone  30 , in order to provide clearance for the target holder  94  and target standoffs  88  when the servicing pallet begins to move in the positive Y-axis direction into a servicing position. 
     Protruding in the positive Y-axis direction from the front of pallet  52  is a front pallet arm  106 . When the printhead carriage  36  is out of the way, servicing pallet  52  may be moved in the positive Y-axis direction, causing front pallet arm  106  to contact linkage arms  92 . The linear motion force of pallet  92  is greater than the rotational force applied by spring element  66  onto linkage arms  92 , causing linkage arms  92  to rotate in the clockwise direction  86  around the pivot post  60 . The anti-rotation nubs  76  protrude outwardly from the target holder  94  on either side of the linkage arms  92 , but not so far as to interfere with the service station frame  50 . If the target holder  94  is rotated around target pivot point  96  far enough, the anti-rotation nubs  76  will contact the linkage arms  92 , preventing further rotation of the target holder, 94  around the target pivot points  96 . 
     The servicing pallet  52  is momentarily stopped in a pre-servicing position when it has moved far enough in the positive Y-axis direction to have rotated the linkage arms  92  and target holder  94  in the clockwise direction  86  out of the path traveled by the printhead carriage  36 . While the pallet  52  is in this pre-servicing position, the printhead carriage  36  may be moved in the negative X-axis direction until the printheads  44 ,  46  are over the service station  48 . When the printheads  44 ,  46  are in position over the service station  48 , the pallet  52  may be moved further in the positive Y-axis direction. As the pallet  52  moves towards the servicing position shown in FIG. 8, a lower pallet arm  108  comes into contact with the linkage arms  92 , pushing the linkage arms  92  away from the front pallet arm  106  and further down into the service station  48  as linkage arms  92  are rotated around pivot post  60  in the clockwise direction  86 . When the servicing pallet  52  reaches the servicing position of FIG. 8, the linkage arms  92  are fully rotated in the clockwise direction  86 . 
     When the pallet  52  is moved to the servicing position, the black printhead cap  54  and color printhead cap  56  lift off of the servicing pallet  52  to engage and cap the black printhead  44  and the tri-color printhead  46 , respectively. A servicing mechanism capable of engaging the printheads in this manner is disclosed in U.S. Pat. No. 5,980,018, also assigned to the present assignee, the Hewlett-Packard Company. For simplicity of illustration, caps  54 ,  56  are shown schematically in FIG. 8 as rising up to engage printheads  44 ,  46  when the servicing pallet  52  is in the servicing position. In this manner, the pallet  52  may be moved between the retracted position and the servicing position to perform various printhead  44 ,  46  servicing techniques well-known to those skilled in the art. 
     When printhead  44 ,  46  servicing is complete, the pallet  52  may be withdrawn in the negative Y-axis direction and paused in the pre-servicing position to allow the printhead carriage  36  to move in the positive X-axis direction to the printzone  30 . When the printhead carriage  36  clears the service station  48 , the servicing pallet  52  may be completely withdrawn in the negative Y-axis direction until it reaches the retracted position shown in FIG.  7 . The spring element  66  rotates the linkage arms  92  in counterclockwise direction  68  around pivot post  60  as the pallet  52  is withdrawn, thereby also returning the target holder  94  to the rearward non-measurement position. 
     Alternatively, when printhead  44 ,  46  servicing is complete, as shown in FIG. 8, if an electrostatic drop detection measurement is desired, the printhead carriage  36  can be left in position over the service station  48 , and the servicing pallet  52  may then be withdrawn in the negative Y-axis position to a semi-retracted position as shown in FIG.  9 . In moving to this semi-retracted position shown in FIG. 9, the linkage arms  92  and target holder  94  rotate in a counter-clockwise direction  68  around pivot post  60  until standoffs  88  engage the printheads  44 ,  46 . 
     The standoffs  88  control the PEZ (“Pen to Electrostatic drop detector in the Z-direction”) spacing from the printheads  44 ,  46  to the electrostatic target  102 , and minimize the measurement tolerance variation in a similar fashion to the embodiment shown in FIG.  5  and described above. Once the printheads  44 ,  46  are properly spaced from the electrostatic target  102 , the controller  26  causes ink droplets  90  to be fired from printhead  44 ,  46  onto the target  102 . An electrical drop detect signal is generated by the ink droplets  90  as they contact the target  102 , and this signal is captured by the electronics of electrostatic drop detector PCA  82 . The drop detect signal is then analyzed by controller  26  to determine whether or not various nozzles of printhead  44 ,  46  are spitting ink properly or whether they are clogged. A preferred method of analyzing signals from an electrostatic target ink drop detector is shown in U.S. Pat. No. 6,086,190, also assigned to the present assignee, the Hewlett-Packard Company. Based on the determination made by the controller  26  as to whether each nozzle is functioning properly, the controller  26  may adjust the print masks to substitute functioning nozzles for any malfunctioning nozzles to provide consistent high-quality printed output while still using a printhead with permanently clogged nozzles. 
     In order to ensure that a reliable measurement may be made by the ink drop detector  58 , it is desirable to prevent the build-up of dried ink deposits on the target  102  after a measurement or series of measurements have been made. Conductive absorbent target  102  is pretreated with a conductive solvent which is selected to dissolve and absorb the ink droplets  90  which contact the target  102 , thereby reducing the likelihood that ink deposits may accumulate over time. Thus, the embodiment of an electrostatic drop detector  58  illustrated in FIGS. 6-9 may be constructed without additional hardware to clean and scrape the target  78  while still having long life and high reliability. 
     After the desired number of drop detection measurements are taken, the servicing pallet  52  may then be moved in the positive Y-axis direction to the pre-servicing position. The target standoffs  88  disengage the printheads  44 ,  46 , and linkage arms  92  and target holder  94  move clear of the path traveled by the printhead carriage  36  when in motion. The printhead carriage  36  may then be moved in the positive X-axis direction towards the printzone  30 , and then pallet  52  may be moved back in the negative Y-axis direction to the retracted position of FIG.  7 . When the pallet  52  is in the retracted position of FIG. 7, the linkage arms  92  and target holder  94  are in the rearward non-measurement position, and the printhead carriage  36  is free at this point to move back to the servicing region  38  for spitting or to move to the printzone  30  for printing. 
     An electrostatic ink drop detector  58  enables a printing mechanism to reliably gather ink drop detection readings without the need for a cleaning mechanism to clean the target surface, while minimizing the effect of spacing variation due to part tolerances 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 ink drop detector  58  and the service station  48 , various benefits have been noted above. 
     It is apparent that a variety of other structurally equivalent modifications and substitutions may be made to construct an ink drop detector according to the concepts covered herein depending upon the particular implementation, while still falling within the scope of the claims below.