Abstract:
A bulldozer-type cleaning system is provided for removing ink residue from an electrostatic drop detecting sensor which detects ink droplets contacting the detector. A scraper head scrapes the ink residue from the sensor, and then contacts a flexible, compliant cleaning member, illustrated as a coil spring. The spring is secured at each end and is stretched when pushed by the scraper head. This stretching flexation allows the spring to trap the ink residue between the coils. As the scraper head retracts, the resulting contracting flexation of the spring squeezes the ink residue from between the coils. Any ink residue remaining on the coils dries and then flakes off the coils when the spring is stretched again during the next cleaning stroke of the scraper head. An inkjet printing mechanism having such a cleaning system, and a method of cleaning a sensor are also provided.

Description:
INTRODUCTION  
         [0001]    The present invention relates generally to inkjet printing mechanisms, and more particularly to a bulldozer-type cleaner for removing ink residue from an electrostatic drop detector which detects ink droplets contacting the detector.  
           [0002]    Inkjet printing mechanisms use pens which shoot drops of liquid colorant, referred to generally herein as “ink,” onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. 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, Hewlett-Packard Company. In a thermal 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 energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).  
           [0003]    To clean and protect the printhead, typically a “service station” mechanism is mounted within the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. To facilitate priming, some printers have priming caps that are connected to a pumping unit to draw a vacuum on the printhead. During operation, partial occlusions or clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a clearing or purging process known as “spitting.” The waste ink is collected at a spitting reservoir portion of the service station, known as a “spittoon.” After spitting, uncapping, or occasionally during printing, most service stations have a flexible wiper, or a more rigid spring-loaded wiper, that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead.  
           [0004]    To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, more waterfast printing with darker blacks and more vivid colors, pigment based inks have been developed. These pigment based inks have a higher solids content than the earlier dye-based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to use plain paper.  
           [0005]    Unfortunately, occasionally a printhead nozzle becomes permanently damaged or blocked, so the nozzle is no longer able to eject ink. A missing nozzle cannot eject ink when directed to do so by the printer controller, leaving bare spots in the resulting image. Most earlier inkjet printers had no way of knowing when a nozzle was missing from the array, and the only way to improve print quality was to replace the defective printhead, often while the pen still contained a good supply of ink. Thus, there was a need to know when a particular nozzle was no longer functioning, and to fill this need a low cost ink drop detector was invented, as described in U.S. Pat. No. 6,086,190 to Schantz et al., currently assigned to the present assignee, the Hewlett-Packard Company. Use of the electrostatic drop detector provides a mechanism for communicating to the printer controller when a particular nozzle is out. Knowing this information, the printer controller may substitute a nozzle which is in good working order for the bad nozzle so print quality is unaffected by the missing nozzle. There are a variety of different ways this may be done, for instance using multi-pass print modes various shingling or mask routines, or other schemes known to those skilled in the art.  
           [0006]    While several different types of electrostatic drop detectors are discussed in the Schantz et al. patent, several of the illustrated embodiments use an ink absorbing pad, such as a foam material, in conjunction with the electrostatic drop detector. The purpose of this foam is to absorb liquid components of the ink being spit onto the detector. However, as mentioned above, the current preferred electrostatic drop detector has a relatively smooth spit target surface, with no ability to absorb liquid components of the ink, or to dispel particulate matter of the ink composition. Indeed, droplets which are fired from functioning nozzles onto the drop detector may eventually build up over time, causing the detector to give inaccurate readings. In an extreme case, the ink residue may actually build up and form stalagmites. These ink stalagmites may eventually grow to a height where they could hit and damage the printhead, clogging nozzles or permanently destroying the printhead. Thus, it is apparent that an inkjet printing mechanism using such an electrostatic drop detection system needs some manner of addressing the ink residue build-up on the detector. 
       
    
    
     DRAWING FIGURES  
       [0007]    [0007]FIG. 1 is a fragmented, partially schematic, perspective view of one form of an inkjet printing mechanism including a servicing station having an electrostatic drop detector and a bulldozing cleaner system for removing ink residue left by ink droplets contacting the detector.  
         [0008]    [0008]FIG. 2 is a perspective view of one form of a service station of FIG. 1.  
         [0009]    [0009]FIGS. 3 and 4 are enlarged, side elevational views of the service station of FIG. 1, with the bulldozing cleaner system of:  
         [0010]    [0010]FIG. 3 showing a retracted rest position; and  
         [0011]    [0011]FIG. 4 showing a cleaning position.  
         [0012]    [0012]FIG. 5 is an enlarged side elevational view of one form of a scraper head for the bulldozing cleaner system of FIG. 1, including a waste ink container portion of the service station.  
         [0013]    [0013]FIG. 6 is a partially fragmented, perspective view of the scraper head of FIG. 5 shown during a cleaning operation.  
         [0014]    [0014]FIG. 7 is a fragmented top plan view showing another portion of the cleaning operation. 
     
    
     DETAILED DESCRIPTION  
       [0015]    [0015]FIG. 1 illustrates an embodiment of an inkjet printing mechanism, here shown as an inkjet printer  20 , constructed in accordance with the present invention, which may be used for printing for business reports, correspondence, desktop publishing, and the like, 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 present invention include plotters, portable printing units, copiers, cameras, video printers, and facsimile machines, to name a few. For convenience the concepts of the present invention are illustrated in the environment of an inkjet printer  20 .  
         [0016]    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 housing or casing enclosure  24 , typically of a plastic material. Sheets of print media are fed through a printzone  25  by an adaptive print media handling system  26 , constructed in accordance with the present invention. The print media may be any type of suitable sheet material, such as paper, card-stock, transparencies, mylar, and the like, but for convenience, the illustrated embodiment is described using paper as the print medium. The print media handling system  26  has a feed tray  28  for storing sheets of paper before printing. A series of conventional motor-driven paper drive rollers (not shown) may be used to move the print media from tray  28  into the printzone  25  for printing. After printing, the sheet then lands on output tray portion  30 . The media handling system  26  may include a series of adjustment mechanisms for accommodating different sizes of print media, including letter, legal, A-4, envelopes, etc., such as a sliding length and width adjustment levers  31  and  32  for the input tray, a sliding length adjustment lever  33  for the output tray, and an envelope feed slot  34 .  
         [0017]    The printer  20  also has a printer controller, illustrated schematically as a microprocessor  35 , that receives instructions from a host device, typically a computer, such as a personal computer (not shown). Indeed, many of the printer controller functions may be performed by the host computer, by the electronics on board the printer, or by interactions therebetween. As used herein, the term “printer controller  35 ” encompasses these functions, whether performed by the host computer, the printer, an intermediary device therebetween, or by a combined interaction of such elements. The printer controller  35  may also operate in response to user inputs provided through a key pad (not shown) located on the exterior of the casing  24 . A monitor coupled to the computer host may be used to display visual information to an operator, such as the printer status or a particular program being run on the host computer. Personal computers, their input devices, such as a keyboard and/or a mouse device, and monitors are all well known to those skilled in the art.  
         [0018]    A carriage guide rod  36  is mounted to the chassis  22  to define a scanning axis  38 . The guide rod  36  slideably supports a reciprocating inkjet carriage  40 , which travels back and forth across the printzone  25  and into a servicing region  42 . One suitable type of carriage support system is shown in U.S. Pat. No. 5,366,305, assigned to Hewlett-Packard Company, the assignee of the present invention. A conventional carriage propulsion system may be used to drive carriage  40 , including a position feedback system, which communicates carriage position signals to the controller  35 . For instance, a carriage drive gear and DC motor assembly may be coupled to drive an endless belt secured in a conventional manner to the pen carriage  40 , with the motor operating in response to control signals received from the printer controller  35 . To provide carriage positional feedback information to printer controller  35 , an optical encoder reader may be mounted to carriage  40  to read an encoder strip extending along the path of carriage travel.  
         [0019]    Housed within the servicing region  42  is a service station  44 . The service station  44  includes a translationally movable pallet  45 , which moves forward in the direction of arrow  46 , in rearwardly in the direction of arrow  47  when driven by a motor  48  operating in response to instructions received from the controller  35 . While a variety of different mechanisms may be used to couple the drive motor  48  to the pallet  45 , preferably a conventional reduction gear assembly drives a pinion gear which engages a rack gear formed along the undersurface of the pallet  45 , for instance as shown in U.S. Pat. Nos. 5,980,018 and 6,132,026, both currently assigned to the present assignee, the Hewlett-Packard Company.  
         [0020]    In the printzone  25 , the media sheet receives ink from an inkjet cartridge, such as a black ink cartridge  50  and/or a color ink cartridge  52 . The cartridges  50  and  52  are also often called “pens” by those in the art. The illustrated color pen  52  is a tri-color pen, although in some embodiments, a set of discrete monochrome pens may be used. While the color pen  52  may contain a pigment based ink, for the purposes of illustration, pen  52  is described as containing three dye based ink colors, such as cyan, yellow and magenta. The black ink pen  50  is illustrated herein as containing a pigment based ink. It is apparent that other types of inks may also be used in pens  50 ,  52 , such as thermoplastic, wax or paraffin based inks, as well as hybrid or composite inks having both dye and pigment characteristics.  
         [0021]    The illustrated pens  50 ,  52  each include reservoirs for storing a supply of ink. The pens  50 ,  52  have printheads  54 ,  56  respectively, each of which have an orifice plate with a plurality of nozzles formed therethrough in a manner well known to those skilled in the art. The illustrated printheads  54 ,  56  are thermal inkjet printheads, although other types of printheads may be used, such as piezoelectric printheads. Indeed, the printheads  54  and  56  may be constructed as illustrated by printhead P in the prior art drawing of FIG. 8, including nozzles N and a pair of encapsulant beads E, as described in the Introduction section above; however, it is apparent that other printheads may be constructed without encapsulant beads. These printheads  54 ,  56  typically include a substrate layer having a plurality of resistors which are associated with the nozzles. Upon energizing a selected resistor, a bubble of gas is formed to eject a droplet of ink from the nozzle and onto media in the printzone  25 . The printhead resistors are selectively energized in response to enabling or firing command control signals, which may be delivered by a conventional multi-conductor strip (not shown) from the controller  35  to the printhead carriage  40 , and through conventional interconnects between the carriage and pens  50 ,  52  to the printheads  54 ,  56 .  
         [0022]    Preferably, the outer surface of the orifice plates of printheads  54 ,  56  lie in a common printhead plane. This printhead plane may be used as a reference plane for establishing a desired media-to-printhead spacing, which is one important component of print quality. Furthermore, this printhead plane may also serve as a servicing reference plane, to which the various appliances of the service station  45  may be adjusted for optimum pen servicing. Proper pen servicing not only enhances print quality, but also prolongs pen life by maintaining the health of the printheads  54  and  56 . To hold the pens,  50 ,  52  in place securely against alignment datums formed within carriage  40 , preferably the carriage  40  includes black and color pen latches  57 ,  58  which clamp the pens  50 ,  52  in place as shown in FIG. 1.  
         [0023]    [0023]FIG. 2 shows one form of the service station  44 , constructed in accordance with the present invention. The pallet  45  may carry a variety of different servicing members for maintaining the health of the printheads  54 ,  56 , such as printhead wipers, primers, solvent applicators, caps and the like. These various servicing members are represented in the drawing figures as black and color caps  60 ,  62  for sealing the printheads  54 ,  56  of pens  50 ,  52 , respectively. Preferably, the pallet  45  is housed between a lower frame portion  64 , and an upper frame portion  66  of the service station  44 . As mentioned above, the motor  48  drives the pallet  45  in the forward and reverse directions of arrows  46  and  47  to bring the various servicing components into contact with the printheads  54 ,  56 , preferably using a gear assembly, such as a rack and pinion gear (omitted for clarity). The frame lower portion  64  preferably defines a waste ink reservoir or spittoon  68 , which receives ink purged from the printheads  54 ,  56  in a spitting routine. In the view of FIG. 2, the pallet  45  has been retracted to expose the spittoon  68  for a spitting operation.  
         [0024]    The service station  44  includes an electrostatic drop detection system  70 , here shown as being mounted along an inboard wall  72  of the lower frame  64 . As used herein, the term “inboard” refers to items facing toward the printzone  25 , and the term “outboard” refers to items facing away from printzone. The electrostatic drop detector system  70  communicates with the controller  35 , such as via an electrical conductor  74  which is attached to an electronics portion (not shown) of system  70 , with this electronic portion preferably being located at least in part under a spit target  75  of the system. Preferably the spit target  75  is constructed of a conductive plate which is electrically isolated from the electrical ground plane of the chassis  22 , such as a plate having a conductive surface, currently gold plated, which is chemically durable with respect to the ink compositions employed, as well as having a corrosion resistance to various other environmental factors encountered by the printer  20 . The spit target  75  and the associated electronics, which may be fashioned as a printed circuit assembly (“PCA”), or as an application specific integrated circuit (“ASIC”), in accordance with the teaching of U.S. Pat. No. 6,086,190 to Schantz, et al., discussed in the Introduction section above.  
         [0025]    In the illustrated embodiment, the spit target  75  is located in line with the main spittoon  68 , allowing the target  75  to receive ink droplets from printheads  54  and  56  upon entering or exiting the spittoon  68 . Only when the carriage  40  is held stationary over the spittoon  68  is the pallet  45  then moved in the forward direction of arrow  46  to accomplish servicing using the various servicing members supported by pallet  45 . Referring briefly to FIG. 3, we see the color printhead  56  ejecting ink droplets  76  from one nozzle  78 .  
         [0026]    The tri-color pen  52 , preferably has three pairs of linear nozzle arrays, with one pair ejecting cyan ink, the second pair ejecting yellow ink, and the third pair ejecting magenta ink. In the illustrated embodiment, each color linear array contains 32 nozzles, resulting in 64 nozzles being available for dispensing each color, so that in total, the color printhead  56  has 192 nozzles. As mentioned above, the black cartridge  50  contains a pigment-based ink, whereas the color pen  52  contains dye-based inks. For the black pen  50 , preferably printhead  54  has 300 nozzles, arranged in two linear arrays of 150 nozzles each. These dye-based color inks and the black pigment-based ink are relatively incompatible, and thus require separate servicing components within the service station  44 . While two spit targets  75  may be used, one for the color inks and one for the black ink, preferably to minimize the overall width of printer  20 , a single spit target  75  is used for both types of ink. The incompatibility of the dye-based inks and the pigment-based inks assists in preventing bleeding of the color inks into the black region and vice versa when laid down on a sheet of media, such as paper, to print a desired image. However, the incompatibility of these inks requires special cleaning of the electrostatic drop detector target  75  to allow the system  70  to function properly, and to avoid build-up of ink residue on the target to the point where it could possibly contact and damage the printheads  54 ,  56 , in a phenomenon known as “a printhead crash.” 
         [0027]    To keep the electrostatic drop detector target  75  clean, the service station  44  includes an electrostatic drop detector cleaning system, such as a bulldozing cleaner system  80 , constructed in accordance with the present invention. The illustrated cleaning system  80  includes a slider housing  82  projecting upwardly from the inboard frame wall  72 , and which may include a cover portion  83  extending inboardly from the frame upper portion  66 . Housed within the slider housing  82 ,  83  is a slider member or arm  84 . In the illustrated embodiment, the slider arm  84  slides back and forth in the direction of arrows  46  and  47  over a smooth portion of a PCA circuit board  85 , which carries drop detector electronics (not shown) underlying at least a portion of the drop detect target  75 . The PCA board  85  preferably has electrical conductors or traces running along its undersurface, opposite the slider arm  84 , to carry signals from the electronics under target  75  to the conductor  74  for communication with the controller  35 .  
         [0028]    Preferably, the slider arm  84  is biased in the rearward direction  47  by a biasing member, such as a coil spring  86  which is attached to a stationary location on the service station frame, such as post  88  projecting inboardly from the upper frame portion  66 . The slider arm  84  terminates in a bulldozing scraper head  90  which traverses over target  75 . To move the bulldozing head  90  from the rest position of FIGS. 2 and 3, and through a scraping stroke shown terminating in FIG. 4, preferably the pallet  45  includes an activation member, such as the upwardly projecting activation member or finger  92 , which engages an activatable member or latch  94  projecting downwardly or outboardly from the slider arm  84 . From the unengaged position in FIG. 3, the service station pallet  45  is driven in the forward direction  46  by motor  48  until the activation finger  92  engages latch  94  and begins pulling the slider arm  84  forward, allowing the scraper head  90  to remove ink residue from the target  75 .  
         [0029]    Preferably, the PCA board  85  terminates at the opening of a waste ink debris collection reservoir or bin  95 , which may funnel ink residue removed from target  75  into the spittoon  68 . The opposite side of the waste bin  95  is bounded by an absorptive deposition surface  96 , which absorbs liquid ink residue cleaning to the scraper head  90 . Preferably, the deposition surface  96  is fluidically coupled to a main absorber  98 , so through capillary action, liquid ink residue flows from the deposition surface  96  to the main absorber body  98 . In the main absorber body  98 , the liquid residue eventually evaporates, leaving only solid particles from the ink compositions stored within the main absorber  98 . Of course, any liquid ink residue falling into bin  95 , and then into spittoon  68 , may also be absorbed by an absorbent liner  99  layin along the bottom surface of the spittoon  68 .  
         [0030]    To further assist in removing ink residue from the scraper head  90 , preferably a flexible, compliant, scraper head cleaner, such as a metallic coil spring  100 , is suspended between two support posts  102  and  104  at or over the entrance to the debris bin  95 . FIG. 5 shows an enlarged view of the scraper head  90  as having a concave interior surface defining a cavity  105 , defined in part by a bottom portion of the scraper head  106 , and in part by an upper hook portion of the scraper head  108 . The head lower portion  106  rides along the target surface  75  and the upper surface of the PCA board  85  to scrape off ink residue  109 . Preferably, the lower head portion  106  has a concave shape also, which facilitates in removing highly viscous ink accumulation from the target surface  75 . This concave shape of the lower head portion  106  acts like a snow shovel, or, for those who are not familiar with colder climates, like an ice cream scoop, curling up the ink residue as it is removed from the target  75  and gathering the ink residue within the interior of the shovel cavity  105 . As the ink residue  109  accumulates along the inside surface of the bulldozer cavity  105 , the upper hook portion  108  of the head prevents the ink residue  109  from leaving the interior  105  of the head  90 .  
         [0031]    To remove ink residue  109  from inside the head  90 , FIG. 6 shows the cleaner spring  100  received inside the scraper head cavity  105 , and beginning to impact ink residue  109  therein. FIG. 5 also shows an alternate embodiment, where a second spring  110  is coiled inside the main spring  100 . The secondary internal spring  110  may also be attached on each end to the support posts  102  and  104 .  
         [0032]    [0032]FIG. 7 shows that as the spring  100  is stretched, it rolls and twists, capturing the ink residue  109  between the coils of spring  100 . The spring  100  is stretched and flexed as the scraper head  90  moves beyond the support posts  102 ,  104 , allowing ink residue  109  trapped between the coils to drop from the spring into the waste bin  95 . As the head  90  retracts, the spring  100  flexes again back into a neutral state between the support posts  102  and  104 , with this return flexing action causing more ink residue to drop from the spring coils and land in the bin  95 , as shown for residue  112  in FIG. 6, forming an ink residue accumulation  114  along the bottom of the waste bin  95  and spittoon  68 . The upper hooked portion  108  of the scraper head  90  limits the ink residue from growing vertically to impact the printheads  54 ,  56 . Moreover, the head hooked portion  108  secures the cleanout spring  100  inside cavity  105  during the cleaning action of FIGS. 6 and 7.  
         [0033]    Any liquid ink residue clinging to the spring cleaner  100  may be captured on the absorbent deposition surface  96 , where the liquids are later absorbed through capillary action into the main absorber  98 . In an earlier design, it was suggested to increase the height of the deposition surface  96  to totally fill the interior of the scraper head  90 , but it was believed that foam lacked enough compliance to flex, particularly after becoming coated and saturated with ink residue. It was believed that this lack of compliance of a foam absorber might have caused the service station motor  48  to stall. Furthermore, other manufacturing tolerance accumulations may not have allowed such an oversized foam deposition surface  96  to provide thorough cleaning of head  90 . Thus, the spring cleaner  100 , with or without the optional secondary spring  10 , is presently preferred for its greater compliance, as shown in FIG. 7, where the spring cleaner flexes and yields, without causing any stalling of the service station motor  48 .  
         [0034]    The bulldozer cleaner  100  has a multitude of coils which provide voids therebetween for the ink residue  109  to enter. The residue  109  is then trapped between the spring coils as the scraper head  90  retracts, or the ink residue fall away from the spring into the bottom of the waste bin  95  then into the main spittoon  68 . By varying the pitch of the coils of spring  100  and/or sprint  110 , as well as the initial or “rest” tension between support posts  102  and  104 , the bulldozer cleaner  100  may be adjusted to offer primarily a wicking path between adjacent coils for the liquid ink residue to enter, and/or coil surfaces which have a surface tension that attracts ink residue and sludge away from the bulldozer interior  105 . Additionally, the natural deflection of the spring  100 ,  110  shown in FIG. 7 causes the spring to wipe the interior surface of the scraper head cavity  105 . Furthermore, any ink residue which does not fall from the spring  100 ,  110  but instead remains attached to the coils sits on the coils and dries. Then during the next cleaning stroke, this dried ink residue clinging to the coils flakes off the coils as the spring is deflected. Thus, any dried ink clinging to the coils is not reintroduced onto the target  75  or PCA board  85  as the slider  84  retracts under the urging of the retraction spring  86 .  
         [0035]    The scraper head cleaning stroke of FIGS. 6 and 7 is a unidirectional stroke, so during retraction of the cleaned head  90  over the target  75  and the PCA board  85  no ink residue is reintroduced by the head onto these surfaces. Since all the ink residue was cleaned from the target and PCA board during the cleaning stroke, during the retraction stroke the head lower hooked portion  106  traverses smoothly over a clean surface. Additionally, use of the spring head cleaner  100 , with or without the optional secondary spring  110 , forms a compliant cleaning system which is economical, easy to assemble, and robust enough to last the lifetime of printer  20 . Use of the secondary spring  110  advantageously provides additional wicking paths between the coils of spring  110  to trap liquid ink residue, and the flexing of the internal spring  110  against the main spring  100  assists in cleaning ink residue from the interior of spring  100  during the deflection of FIG. 7. To avoid having the coils of spring  110  get trapped between the coils of spring  100 , these springs may be oriented with their twists going in opposite directions.  
         [0036]    While the concepts of the bulldozing cleaner system  80  for removing ink residue  109  from the inkjet electrostatic drop detector  70  have been described with respect to two embodiments, one with a single spring  100  and one with multiple springs  100  and  110 , it is apparent that these concepts may be employed in a variety of equivalent manners, depending upon the particular implementations employed, while still falling within the scope of the claims below. For example, the multiple spring embodiment may not only have one spring embedded inside the other, instead the springs may be arranged side-by-side or on top of each other.  
         [0037]    As another example, while in the illustrated embodiment the target  75  is held in a fixed position and the scraper head  90  moves over the target, in some implementations it may be preferred to have the scraper head  90  remain in a fixed position, and the electrostatic drop detector target  75  move, or both the scraper head and target may move. Relative motion between the target  75  and scraper  90  cleans the target; relative motion between the scraper  90  and the cleaner spring  100 ,  110  cleans the scraper; and flexion of the cleaner  100  cleans the scraper cavity  105 , as well as the cleaner. For instance, the target  75  may carry a latch member similar to latch  94  to be activated by motion of the pallet finger  92 , with the target advancing out to a drop detecting position as shown in FIG. 2, and then withdrawing under a stationarily mounted scraper head  90 . During this withdrawal stroke, if the cleaning spring  100 ,  110  were mounted at the front end (positive Y-axis direction) of the target, the head would be cleaned during this withdrawal process, leaving a clean target stored in a retracted position underneath the slider arm  84 . In such an implementation, the waste bin  95  may be relocated to a more rearward position to collect debris from the head as the target is withdrawn under the scraper head and the spring  100  is also withdrawn into engagement with the scraper head  90 .  
         [0038]    Additionally, while coil springs  100 ,  110  are illustrated, in some implementations it may be desirable to stretch other flexible compliant members like an elastomeric member, such as a group of rubber band-like members, between the support posts  102  and  104 , either instead of or in addition to the springs  100 ,  110 ; however the illustrated metallic coil springs are preferred for their durability. Furthermore, other enhancements may be made to the head cleaner, such as to weave bristles between the spring coils, providing additional cleaning surfaces for removing residue  109  from the head interior  105 . Such variations and modifications of the concepts described herein fall within the scope of the claims below.